1 www.irf.com
Servo Motor Driver Application
Davide Giacomini, Enrico Bianconi, Luca Martino, Marco Palma
International Rectifier Corporation
corso Novara, 17, Venaria, TO 10078 (Italy)
Abstract
Today’s solution for Motor Control Applications
require more flexibility and a higher level of in-tegration that could be achieved only through
specialized products. In this paper a new family
of Intelligent Power Modules developed for Servo
Motor Drive Application is presented. The
module’s architecture includes several key de-sign features needed in these applications, such
as motor phase current sensing, DC bus voltage
control and short circuit protections while the
embedded DSP allows easy realization of the
current loop and even speed loop at the module
level without any exte rnal components.
INTRODUCTION
Today’s Servo Motor Driver application require
an increasing level of performances in a usually
limited space, integrated motor-control systems
(so called IMD i.e. Integrated Motor Drivers) are
becoming the real challenge for the future; the
benefit of this new approach has to be seen in a
lower system cost, by eliminating separated pack-aging and long cables, as well as in a cost effec-tive performances improvement thanks to the
easier matching of the motor – driver system and
EMI reduction due to the elimination of long and
noisy connections between the two parts.
Power products today available on the mar-ket give motor driver manufacturer the opportu-nity to design their own driver, though still leave
the designer all control electronics to be defined
and designed, so called IPMs (i.e. Intelligent
Power Modules) often integrate IGBTs, Diode
and Gate Drivers while only few of them also
provide Current Sensing and fault/protection
feedback.
The PI-IPMTM here presented is a new gen-eration Intelligent Power Module designed spe-cifically to implement itself a complete motor
driver system, the device contains all peripher-als needed to control a six IGBTs inverter, in-cluding voltage, temperature and current output
sensing, completely interfaced with a 40Mips
DSP, the TMS320LF2406A from Texas Instru-ments. All communications between the DSP and
the local host, including DSP software installing
and debugging, is realized through an asynchro-nous isolated serial port, either a CAN bus or an
isolated port for incremental encoder inputs is
also provided making this module a complete user
programmable solution connected to the system
only through a serial link cable.
In the following we will describe the first de-vices realized and tested in our laboratories, Table
1 lists all features and Power rating of these first
samples.
SYSTEM DESCRIPTION
The PI-IPM
TM
is realized in two distinct parts:
the Power Module “EMP
TM”
(“EcoManyPack
TM
,”
to recall the idea of different configurations avail-ability) and the Embedded Driving Board
“EDB,” these two elements assembled together
Table 1: PI-IPM Features and Power Rating
IGBTs Collector-Emitter voltage
IGBTs continuous collector current
@T
C
=85 C
Diodes Continuous Fwd Current
@T
C
=85 C
Sensing resistors
Embedded DSP
Interfaces
Diodes reverse breakdown voltage
TMS320LF2406A
SCI
Incremental Encoder
1200 V
50 A
1200 V
50 A
2 mOhm
constitute the complete device with all
performances described in the following.
Before analyzing those two sections we
glance at the total block schematic showing all
function implemented in the product, as
represented in fig. 1. The new module concept
includes everything depicted within the dashed
line, the power module only includes IGBTs,
Diodes and Sensing Resistors while all remaining
electronics is assembled on the EDB that is fitted
on the top of it as its cover with also a mechanical
protection function.
Connections between the two parts are realized
through a standard connector from AMP, this
particular solution has been chosen not only to
speed up and ease the final product assembly in
the manufacturing line but also to maintain a
certain flexibility, always considered a big plus in
the motor driver market. In fact the EDB only,
without disassembling the power module from the
system mechanic, could be easily substituted “on
site” for an upgrade, a system configuration
change (different control architecture) or a board
replacement. Also software upgrades are possible
but this does not even require any hardware
changes thanks to the DSP programmability
through the serial or JTAG port.
THE “EMP
TM
” POWER MODULE
This module contains six IGBTs + HexFreds
Diodes in a standard inverter configuration. IGBTs
used are the new NPT 1200V-50A (current rating
measured @ 100C), irgc50b120kb generation V
from International Rectifier; the HexFred diodes
have been designed specifically as pair element for
these power transistors and the coupled part is the
hf50d120ace. Thanks to the new design and
technologic realization, this gen V devices do not
need any negative gate voltage for their complete
turn off and the tail effect is also substantially
reduced compared to competitive devices of the
same family. This feature simplifies the gate
driving stage as will then be described in a
dedicated chapter. Another not standard feature in
this type of power modules is the presence of
sensing resistors in the three output phases, for
precise motor current sensing and short circuit
protections, as well as another resistor of the same
value on the DC bus minus line, needed only for
device protections purposes.
Complete schematic of the EMP
TM
module is
shown in fig. 2 where sensing resistor have been
clearly evidenced, a thermal sensor is also
embedded and directly coupled with the DSP
inputs.
The package chosen is mechanically
compatible with the well known EconoPack
outline, also the height of the plastic cylindrical
nuts for the external PCB positioned on its top is
the same, so that, with the only re-layout of the
main motherboard, this module could be swapped
into the same mechanical fixing of the standard
Econo II package thus speeding up the device
evaluation in an already existing driver.
An important feature of this new device is the
presence of Kelvin points for all feedback and
command signals between the board and the
module, because of this the standard EconoPack
pin out couldn’t be used and a total redesign of the
package has been done. The cost of a non-common
configuration is however balanced by the
advantage of having all emitter and resistor
sensing points independent from the power path
giving the tremendous result of having all low
power signal from/to the controlling board
unaffected by parasitic inductances or resistances
inevitably present in the module power layout.
Fig. 2: EMP
TM
Power Module Schematic
Fig. 1: PI-IPM
TM
block diagram
Without this effort the realization of the system
would have been seriously compromised because
of false voltages spikes and voltages levels seen at
the DSP input pins though filtered by conditioning
circuitry.
The new package outline is depicted on fig. 3,
where the signal and power pins are clearly shown
and recognizable.
Fig. 4 shows the module layout, clearly
evident are the sensing resistors on the three output
phases and the DC bus minus, note that, because
of high current spikes on those inputs, the DC bus
power pins are doubled in size comparing to the
other power pins. Module technology uses the
standard and well know DBC: over a thick Copper
base an allumina (Al
2O3
) substrate with a 300?m
copper foil on both side is placed and IGBTs and
Diodes dies are directly soldered, through screen
printing process. These dies are then bonded with
a 15 mils aluminum wire for power connections
and 6 mils wire for signal connections. All
components are then completely covered by a
silicone gel with mechanical protection and
electrical isolation purposes.
THE “EDB” EMBEDDED DRIVING BOARD
This is the core of the device intelligence, as
previously described all control and driving
functions are implemented at this level, the board
finds its natural placement as a cover of the
module itself and has a double function of
mechanical enclosure and intelligent interface.
DSP and all other electronics are here assembled,
on fig. 5 the board schematic is presented and all
connection pins clearly shown.
Looking at the schematic, all diamond shaped
pins are signal connections, some belonging to the
RS485 port interface and some to the IEEE 1149.1
(JTAG) connector. All other pins are used for
communication between the board and the module,
they are positioned laterally in the board so that
the module doesn’t have any pins in the middle of
Fig. 3: the EMP
TM
package
Fig. 4: the inverter configuration layout
its body, in this way all PCB internal area is
available for components mounting and copper
lines layouting, to ease the board realization.
From the top left, in anti-clockwise direction
we identify the following blocks that will be then
described in details:
1. DSP and opto isolated serial and JTAG
ports
2. Flyback Power Supply
3. Current Sensing interfaces, over-current
protections and signal conditioning
4. Gate drivers
5. DC bus and Input voltage feedback
1. DSP and opto isolated serial and JTAG
ports.
This block is shown in fig. 6. The DSP used in
this application is the new TMS320LF2406A from
TI, it is a improvement of the well known in the
motor driver market “F240” used in many motor
driver applications. If we compare this new device
with the predecessor, the new DSP has some
Table 2: TMS320LF2406A vs TMS320F240
‘F2406 ‘F240
MIPS
RAM
Flash
ROM
Boot ROM
Ext. Memory I/F
Event manager
• GP timers
• CMP/PWM
• CAP/QEP
Watchdog timer
10-bit ADC
• Channels
• Conv. time (min)
SPI
SCI
CAN
Digital I/O pins
Voltage range
40
2.5Kw
32Kw
—
256w
—
Yes
4
10/16
6/4
Yes
Yes
16
500ns
Yes
Yes
Yes
37
3.3V
20
544w
16Kw
—
—
Yes
Yes
3
9/12
4/2
Yes
Yes
16
6.6?s
Yes
Yes
—
28
5 V
Fig. 5: Embedded Driving Board Block Schematic
added features that let the software designer
significantly improve the system control
performances, tab. 2 shows a list of relevant data,
for all other information the reader should refer to
the related device datasheet. Just to be noted here
is the increased number of instruction per second,
(40MIPS) and of I/O pins, the availability of a
boot ROM and a CAN, a much faster ADC and the
reduced supply voltage from 5V down to 3.3V, to
follow the global trend for this type of products.
The choice of the DSP has been done looking at
the high number of applications already existing in
the market using devices of this family, however it
is clear that the same kind of approach could be
followed using products from different suppliers to
let the customer work on its preferred and well
known platform.
The “2406A” has three different serial interfaces
available: SCI, SPI, and CAN bus. In this first
prototypes the serial communication is made
through the asynchronous port while three other
opto-isolated lines are occupied by the hall effect
sensor interface fed directly to the first three DSPs
input capture ports. Maximum bit rate for this
asynchronous serial port is 2.5Mbps while the SPI
(synchronous) could reach 10Mbps. The choice of
the SCI has been taken for easy interfacing with a
standard computer serial port, the only component
needed is a line driver to adapt the RS232 voltage
standard with the RS485 at 3.3V used on this
application. In this case the 100kbps PC RS232
port speed is not enough to handle the system
control properly but somewhat adequate for
demonstration purposes in a simple V/F control for
induction motors. In a real application with a
Brushless motor usually 1Mbps are far enough to
transmit all information needed for the torque
reference updates and other fault and feedback
signals at a maximum frame rate of 10kHz
(100bits/frame), in this way the on-board line
driver let us use long connecting wires instead of
short copper lines in a PCB, leaving the user the
possibility of having the PI-IPM
TM
displaced near
the motor, e.g. in its connecting box, avoiding long
ad noisy three phase cables between driver and
load.
The JTAG port is the standard one, neither
isolation nor signal conditioning are provided here
and all signal, except the Tck-ret, are directly
connected from the related DSP pins to the
connector; however, due to the limited board
space, the connector used in not the standard 14
pins at two rows header, then an adaptor has to be
realized to connect it to the JTAG adapter interface
provided by Texas Instruments.
Last but not least is the ADC speed and load
characteristic: as the table shows the conversion
time is 500ns, in fact this DSP has a single ADC
handling, in time sharing, all 16 inputs, then, if all
inputs are used and need to be converted, the total
conversion time for all inputs, which is a fixed
delay to wait for before having all data updated, is
around 8?s. The other characteristic to be carefully
taken into account is the capacitive load that the
input pin shows when the conversion is being
performed: accordingly to the device datasheet this
load is around 20pF, then, with an easy
calculation, whatever feeding this pin has to be
able to provide (within 500ns) the needed current
to this capacitor without showing a voltage
dropout higher than ½ LSB. Because of this in
many cases a further capacitor was inserted
between the ADC input pin and ground with a
value calculated as follows:
nF pF C 41
2
10 * 125 . 3
)
2
10 * 125 . 3
20 . 3 (
* 20 min
3
3
?
?
?
?
?
We used in all this cases a 47nF capacitor; the
added benefit is also that, in this way, we reduced
the local impedance towards ground thus
increasing the immunity to noise of all these pins.
Fig. 6: DSP and opto-isolated serial and JTAG ports
2. Flyback Power Supply
Though the board space is pretty limited (only
80mm X 44mm ca.) we found enough space to
accommodate a power supply for the floating
stages. The real limit however was the footprint
dimension, we couldn’t allocate more than a 10
pins (2 row of 5) transformer thus limiting the
maximum number of outputs to four. As the block
schematic in fig. 7 shows, we have three 15V
outputs for the floating stages, isolated from each
other at 1.5kV minimum, and a single 5V and
3.3V output.
The 5V supplies all low voltage electronics
and a 3.3V linear regulator is used to feed the DSP
and some analog and logic interfaces to it. This 5V
and 3.3V are directly referred to the DC bus
minus, so that all control circuitry is alternately at
one of the input lines potential, isolation (as
previously stated) is provided at the DSP serial
link level, then avoiding all delays due to opto
couplers insertion between DSP and control logic.
Note that also the required 15V input voltage is
referred to the same DC bus minus and directly
supplies the low side gate drivers stages, this is
necessary because of the already mentioned
limited space on the transformer footprint, then the
user should pay some attention on how this supply
line is realized in his application. Just for
completeness, fig. 8 gives a possible solution to
that that doesn’t impact heavily on the user
application: normally a 5V power supply is
already present, for displays, electronics and micro
processor, the same 5V could be used for the 5V
iso supply of opto-couplers and line driver, the
15V could be realized as an added winding in the
secondary side of the flyback transformer, the only
care that should be taken is in keeping its isolation
from the above mentioned 5V at the required level
(at least 1.5kV).
To avoid noise problems in the measuring
lines due to the commutating electronics during
normal functioning of the system, references are
kept separated. A 5V linear regulator, directly
supplied from the 15V input, is used to provide the
reference voltage to the current sensing amplifying
and conditioning components while a precise op-amp, configured as a voltage follower, acts as a
buffer of the partition at 3.20V created down the
5V reference. This 3.20V is used as a reference for
the DSP A/D converter. It is to be noted that in the
schematic we are using the same linear regulator
as a starting point for all reference voltages. In fact
if the 5V linear regulator derives in temperature or
time, then all references (even the 3.20V being this
a simple partitioning) follow in track and still keep
the overall chain precision. The trimming is then
done only once, in a single point of the measuring
chain, that is the conditioning op-amp collecting
the current sensing ICs signal as will be described
in the following chapter.
3. Current sensing interfaces, over-current
protections and signal conditioning.
This block is the real critical point of the
system. Current measuring performances directly
impact on motor control performances in a servo
application: errors in current evaluation, delay in
its measuring chain or poor overall precision of the
system, such as scarce references or lower number
of significant A/D bits, inevitably results in
unwanted trembling and unnatural noise coming
from the motor while running at lower speed or at
blocked shaft conditions.
Fig. 8: examples of power supply for PI-IPM
TM
15V and 5V iso inputs
Fig. 7: PI-IPM
TM
embedded flyback power supply
To avoid all these afore mentioned problems the
current sensing chain has to comply with usually
good performances that could be easily listed:
s s latency
kHz bandwidth
? ? 20 15
5
? ?
?
A common mistake is to look only at the band-with without taking too much care at the delay that
the information carries with it. This inevitably
leads to close the current loop with lower gains
than expected (to avoid instability and work with a
decent phase margin) and consequent poor
performances due to the limited system precision
in torque control. Most used devices for current
measuring are hall effect sensor, their precision
and bandwidth is far enough for a servo motor
driver application, closed loop ones have usually
100kHz BW measured at a phase lag of only
10deg, however their cost is pretty high and in
many cases also their dimensions does not fit in
the limited space available in the driver or in the
motor connection box.
Because of these simple reasons another
viable solution for current sensing is gaining
popularity: though not used for high current level,
because of power dissipation constraints, the
sensing resistor dropout measurement is still
adequate up to around 100A – 150A, with the
benefit of a lower area and somewhat a lower cost.
This is the solution chosen in the PI-IPM
TM
with
the added value of having the shunts element
embedded in the power module and all Kelvin
connections available.
As the block schematic in fig. 9 shows, the
voltage across each sensing resistor is applied,
through an anti-aliasing 400kHz filter, at the input
of a current sense IC. In this application we use the
HCPL788J because of their bandwidth, datasheet
shows 20kHz and we measured from these devices
a latency of 10?s at a frequency of around 3kHz.
As previously stated these performances are good
enough for servo-amplifier applications, these
integrated circuits also provide level shifting and
galvanic isolation through the internal high speed
opto-coupler; while the first function is essential
the second one is not really needed and useless in
this new type of intelligent power module where
isolation lies at the serial link level.
International Rectifier also has some current
sensing devices in its portfolio, however those
devices (e.g. IR2271 or IR2273) have been
developed for AC induction motor control and
their performances are not suitable for servo motor
driver application, driving a BLDC motor.
Signal outputted from the current sensing has a
0 to +5V dynamic, with a sensing resistor of
2mohms the input measured current range is +/-
100A then we have a situation as follows:
V A
V A
V A
0 . 5 100
5 . 2 00 . 0
0 . 0 100
? ?
?
? ?
The DSP ADC inputs have an external reference
of 3.20V then we must map this signal in a 0 –
3.20V frame and also filter the information to
recover the average value of the current flowing
to the motor phase. This is done in a single step:
though the block schematic shows a op-amp plus
an external passive filter this is simply realized
implementing a VCVS cell (i.e. a Constant Gain or
Sallen – Key cell) configured so that the offset and
gain could be trimmed by three on board resistors.
This is the setting point of all current measurement
chain: in a single easy step we set the precision
and recover the system offset through the
soldering of two SMD resistors in the op-amp
input lines, while the third one is used to re-center
the damping factor 2???and the resonant angular
frequency ?
?
of the second order filter.
The Sallen - Key cell is also pretty flexible and let
Fig. 9: Current Sensing and protections
us implement any type of second order (or even a
simple single order) low pass filter. We chose for
this first prototypes a second order Bessel filter
with 5kHz pole frequency, the reason for this is
that this type of polynomials are calculated with
the aim of having a constant group delay within
the pass-band frequencies, thus giving the
minimum waveform distortion to the output signal
up to almost twice the filter pole. In other hands
we could also say that the group delay of the signal
chain from the sensing resistor up to the ADC
input of the DSP is constant from 0 to 5kHz.
Summing up our current measurements
performances are shown in table 3.
By changing some resistors on PCB all parameters
could be changed: current range, type of filter,
pole frequency, leaving the flexibility of using the
same circuit for module of different power levels.
The “2406” DSP has a 10 bit ADC, that means
that, in our prototypes, the minimum appreciable
current step is approximately:
? ? ? 1953 . 0
2
100 * 2
10
LSB
That is
mA LSB 195 1 ?
Of course, reducing the maximum current range or
simply changing the sensing resistor value (for
lower power modules) this minimum current step
could be sensibly reduced.
Another used feature of the HCPL788J is the over-current output signal. The related fault pin goes
low when a 250mV voltage across sensing pins is
detected, this means an over-current detection
level of approximately 25%. Though the internal
opto-coupler the delay of this line is around 3?s
and fast enough to let the DSP react within the
10?s IGBTs short circuit rating, thus providing
full device protection for any phase-to-ground and
phase-to-phase short circuits. The only failure not
covered in this way is the cross through, where
high current levels are not seen externally the
module rather internally between two IGBTs of
the same leg. In this case the protection is
implemented by means of a fourth sensing
element, with the same resistive value of the other
shunts present in the power module, inserted in
series to the DC bus minus. The related dropout
voltage is then filtered by a 22kHz passive filter to
avoid false fault detections due to unwanted
induced voltage spikes and finally applied to an
operational amplifier configured as a comparator.
4. Gate Drivers
This is another critical part of the system, gate
drivers are responsible for correct turn on and turn
off of all inverter IGBTs, avoiding cross
conductions, false turn on during commutations
and dangerous propagation delays between input
signal and output commands. Devices used to
perform this task are the well-known IR2213,
capable of 2A sink and 2A source maximum gate
driving current, in a SO16W package; fig. 10
shows the block schematic of the gate driving
section of the module.
As previously described, the IGBTs used in
the PI-IPM
TM
(genV NPT 1200V - 50A from IR)
do not need any negative gate drive voltage for
their complete turn off, this simplifies the flyback
power supply design avoiding the need of center
tapped transformer outputs or the use of zener
diodes to create the central common reference for
the gate drivers floating ground. This is a big plus
Table 3: PI-IPM
TM
Current sensing
chain typical performances
value units
current range +/- 100 A
precision 0.3 %
bandwidth 5 kHz
latency time 10 ?s
?
Fig. 10: Gate Drivers
remembering that the board space is very limited
and that drawing three electrically floating lines
instead of two (for each output phase) all across
the board is a potential source of increased noise
and isolation problems. Though the IR2213 do
have +/- 2A of gate current capability, an easy
calculation based on IGBTs gate charge, supported
by instrumental observation in the laboratories, led
us to use different gate resistor values for turn on
and turn off as follows:
ohm off turn
ohm on turn
6 . 7
33
? ?
? ?
Commonly realized through a diode-resistor series
in parallel with a single resistor used in turn on
only. Observed rise and fall times are around
250ns – 300ns depending on the output current
level and we considered this values pretty
adequate for a 50A application at 16kHz
symmetric PWM carrier, space vector modulation.
Contrarily to the current sensing devices, that are
opto-coupled, these gate drivers do provide levels
shifting without any galvanic isolation that is no
opto-couplers are built inside. This turns out to be
a major benefit in this stage where the usual 1?s
delay of optos impacts on the system control as a
systematic and fastidious delay. Only drawback of
these gate drivers is the 5V logic compatible inputs
that cannot be driven directly by the 3.3V logic
level DSP. This forced us to insert a level shifter
interface, 74HCT541 with the only purpose of
logic level adaptation. New gate drivers, a
substantial improvement of the IR2213, are
presently in development and will soon be
available to overcome this problem and also
simplify other aspect of the here described
schematic.
5. DC bus and Input voltage feedback
The purpose of this block is to continuously
check the voltage of the two supply lines of the
system: Vin and DC bus; it is shown in fig. 11.
Vin is the only external power supply needed for
all electronics in the EDB. The internal flyback
regulator has its own under-voltage lockout to
prevent all electronics from start working when an
insufficient supply voltage is present; this UVLO
is internally set at a typical voltage of 8.4V with an
hysteresys of approximately 0.8V, when this level
is reached the internal flyback turns on and the
board is alive. Remembering what described in the
previous chapter dedicated to the internal flyback
Power Supply, low side gate drivers are directly
fed from the Vin line and there is no further
control to this voltage than their own under-voltage lockout. This is typically set at 8.5V and
this level could be not sufficient to properly drive
the IGBT gates, then, through a standard resistor
divider, we check the Vin voltage and impose that
the system could start switching only when the Vin
voltage is between 10V and 18V thus providing
also an over-voltage control.
The DC bus voltage is also important for the
system functioning and needs to be continuously
kept under control, this is done through another
resistor divider that unfortunately is at high
voltage and had to be layouted on the EDB with a
particular care, unfortunately occupying some
precious space. The divider provides a partition
coefficient of 3.47mV/V, having the ADC
reference at 3.20V this yield a maximum mapped
voltage of:
V VDCbus 922
10 * 47 . 3
20 . 3
max _
3
? ?
?
As the block schematic shows, it has to be taken
into account that, to avoid false detections due to
voltage spikes inevitably present on the partitioned
voltage, a 1kHz passive filter has been inserted
between the divider and the voltage follower
buffer whose output is connected to one of the
ADC inputs. An easy calculation shows that the
precision reached in reading the DC bus voltage is:
V LSB 9 . 0
2
922
1
10
? ?
Which is more than enough for this purpose.
Fig. 11: DC bus and Input voltage feedback
PRINTED CIRCUIT BOARD REALIZATION
AND COMMENTS
This so far described PI-IPM
TM
is a kind of
PEBB (i.e. Power Electronic Building Block) for
many motor driver applications and it is suitable
for any type of motor: Brush-less, Induction, and
Switched Reluctance. Because of the “on board”
presence of a high intelligence component such as
a DSP, it is also well positioned to become an
interesting component in many sensor-less
applications. Depending on the type of system in
which it is inserted, the performances level and
target cost should change to encounter the major
number of possible sockets in the market.
As already stated, one of the most important
features of this device is in fact flexibility, the
EDB could be designed and tailored to the
customer application, still leaving to the customer
the benefit of developing and downloading its own
proprietary software. However this is not such an
easy task and many critical factors challenge the
engineer in developing a good driving board for
this power module.
Fig. 12 and 13 show one of the first prototypes
realized and tested in our laboratories, their
performances evaluation and first experimental
results will be the subject of a further paper.
During the realization we faced two major
problems: isolation and noise of the environment.
The first was solved only through introducing
enough spacing between components and high
voltage copper lines, it was also in some cases
very helpful to add more layers in the PCB
stratification. The EDB is in fact a six layers
board: two internal layers are planes for ground
and supply voltages Vin and V5-iso; top, bottom
and a third layer are instead for low power signal
interconnections; the other remaining internal
layer is used for the high voltage floating lines of
gate drivers and current sense devices.
Components positioning is also a major issue,
some devices are noise and temperature sensitive
and cannot be placed in the board bottom side
while others are not so critical. Integrated circuits
such as the DSP and the current sensing have been
placed on top, following the above-mentioned rule,
while gate drivers and the flyback controller found
their socket on the bottom side.
Fig. 12: PI-IPM
TM
first prototypes
Not only the noise of the environment (IGBTs
switching in the underneath power module) is the
cause of potential problems but also the printed
circuit board itself. Some copper traces are a
relevant source of troubles, for example the
floating power supply lines of the high side gate
driver stages that are, in the system normal
operation, commutating between DC bus minus
and DC bus plus with a voltage swing of around
500V. In this case any inductive or capacitive
coupling, though very limited in absolute value,
could be enough to induce a considerable noise in
any analog high impedance line, thus inferring in
feedback and protection signals. Those 6 lines had
to be carefully drawn in the PCB layout, confined
in a single internal layer, and any crossing with
other lines in different layers was intentionally
forced to be perpendicular to reduce likely cross-talk of any type.
Again talking about the flyback power supply, also
its position in the printed circuit board is critical.
The controller is switching at 200kHz and
consequently is a relevant source of noise, having
sensitive devices (such as the current sensing and
the DSP) in the area is not advisable, then we
placed this block in one corner, at the opposite side
of the DSP and as much as possible away from any
other analog circuitry.
CONCLUSIONS
A new Intelligent Power Module, so called PI-IPM
TM
, using the well-known DBC technology has
been developed specifically for high performances
servo motor driver application. Thanks to its
considerable limited dimensions, it is particularly
suited for space saving solutions where the space
is a real constraint, such as Integrated Motor
Drivers or Motor Drivers of the new generation.
To include all functions and reach high-level
performances a new package, named EMP
TM
, has
been designed to accommodate the printed circuit
board on its top, with all needed connections for a
proper communication between the two parts.
With this new device a motor driver designer can
easily design and debug its system with an
excellent level of performances and a considerably
improved time to market.
Fig. 13: PI-IPM
TM
module and driving board
International Rectifier Corporation
corso Novara, 17, Venaria, TO 10078 (Italy)
Abstract
Today’s solution for Motor Control Applications
require more flexibility and a higher level of in-tegration that could be achieved only through
specialized products. In this paper a new family
of Intelligent Power Modules developed for Servo
Motor Drive Application is presented. The
module’s architecture includes several key de-sign features needed in these applications, such
as motor phase current sensing, DC bus voltage
control and short circuit protections while the
embedded DSP allows easy realization of the
current loop and even speed loop at the module
level without any exte rnal components.
INTRODUCTION
Today’s Servo Motor Driver application require
an increasing level of performances in a usually
limited space, integrated motor-control systems
(so called IMD i.e. Integrated Motor Drivers) are
becoming the real challenge for the future; the
benefit of this new approach has to be seen in a
lower system cost, by eliminating separated pack-aging and long cables, as well as in a cost effec-tive performances improvement thanks to the
easier matching of the motor – driver system and
EMI reduction due to the elimination of long and
noisy connections between the two parts.
Power products today available on the mar-ket give motor driver manufacturer the opportu-nity to design their own driver, though still leave
the designer all control electronics to be defined
and designed, so called IPMs (i.e. Intelligent
Power Modules) often integrate IGBTs, Diode
and Gate Drivers while only few of them also
provide Current Sensing and fault/protection
feedback.
The PI-IPMTM here presented is a new gen-eration Intelligent Power Module designed spe-cifically to implement itself a complete motor
driver system, the device contains all peripher-als needed to control a six IGBTs inverter, in-cluding voltage, temperature and current output
sensing, completely interfaced with a 40Mips
DSP, the TMS320LF2406A from Texas Instru-ments. All communications between the DSP and
the local host, including DSP software installing
and debugging, is realized through an asynchro-nous isolated serial port, either a CAN bus or an
isolated port for incremental encoder inputs is
also provided making this module a complete user
programmable solution connected to the system
only through a serial link cable.
In the following we will describe the first de-vices realized and tested in our laboratories, Table
1 lists all features and Power rating of these first
samples.
SYSTEM DESCRIPTION
The PI-IPM
TM
is realized in two distinct parts:
the Power Module “EMP
TM”
(“EcoManyPack
TM
,”
to recall the idea of different configurations avail-ability) and the Embedded Driving Board
“EDB,” these two elements assembled together
Table 1: PI-IPM Features and Power Rating
IGBTs Collector-Emitter voltage
IGBTs continuous collector current
@T
C
=85 C
Diodes Continuous Fwd Current
@T
C
=85 C
Sensing resistors
Embedded DSP
Interfaces
Diodes reverse breakdown voltage
TMS320LF2406A
SCI
Incremental Encoder
1200 V
50 A
1200 V
50 A
2 mOhm
constitute the complete device with all
performances described in the following.
Before analyzing those two sections we
glance at the total block schematic showing all
function implemented in the product, as
represented in fig. 1. The new module concept
includes everything depicted within the dashed
line, the power module only includes IGBTs,
Diodes and Sensing Resistors while all remaining
electronics is assembled on the EDB that is fitted
on the top of it as its cover with also a mechanical
protection function.
Connections between the two parts are realized
through a standard connector from AMP, this
particular solution has been chosen not only to
speed up and ease the final product assembly in
the manufacturing line but also to maintain a
certain flexibility, always considered a big plus in
the motor driver market. In fact the EDB only,
without disassembling the power module from the
system mechanic, could be easily substituted “on
site” for an upgrade, a system configuration
change (different control architecture) or a board
replacement. Also software upgrades are possible
but this does not even require any hardware
changes thanks to the DSP programmability
through the serial or JTAG port.
THE “EMP
TM
” POWER MODULE
This module contains six IGBTs + HexFreds
Diodes in a standard inverter configuration. IGBTs
used are the new NPT 1200V-50A (current rating
measured @ 100C), irgc50b120kb generation V
from International Rectifier; the HexFred diodes
have been designed specifically as pair element for
these power transistors and the coupled part is the
hf50d120ace. Thanks to the new design and
technologic realization, this gen V devices do not
need any negative gate voltage for their complete
turn off and the tail effect is also substantially
reduced compared to competitive devices of the
same family. This feature simplifies the gate
driving stage as will then be described in a
dedicated chapter. Another not standard feature in
this type of power modules is the presence of
sensing resistors in the three output phases, for
precise motor current sensing and short circuit
protections, as well as another resistor of the same
value on the DC bus minus line, needed only for
device protections purposes.
Complete schematic of the EMP
TM
module is
shown in fig. 2 where sensing resistor have been
clearly evidenced, a thermal sensor is also
embedded and directly coupled with the DSP
inputs.
The package chosen is mechanically
compatible with the well known EconoPack
outline, also the height of the plastic cylindrical
nuts for the external PCB positioned on its top is
the same, so that, with the only re-layout of the
main motherboard, this module could be swapped
into the same mechanical fixing of the standard
Econo II package thus speeding up the device
evaluation in an already existing driver.
An important feature of this new device is the
presence of Kelvin points for all feedback and
command signals between the board and the
module, because of this the standard EconoPack
pin out couldn’t be used and a total redesign of the
package has been done. The cost of a non-common
configuration is however balanced by the
advantage of having all emitter and resistor
sensing points independent from the power path
giving the tremendous result of having all low
power signal from/to the controlling board
unaffected by parasitic inductances or resistances
inevitably present in the module power layout.
Fig. 2: EMP
TM
Power Module Schematic
Fig. 1: PI-IPM
TM
block diagram
Without this effort the realization of the system
would have been seriously compromised because
of false voltages spikes and voltages levels seen at
the DSP input pins though filtered by conditioning
circuitry.
The new package outline is depicted on fig. 3,
where the signal and power pins are clearly shown
and recognizable.
Fig. 4 shows the module layout, clearly
evident are the sensing resistors on the three output
phases and the DC bus minus, note that, because
of high current spikes on those inputs, the DC bus
power pins are doubled in size comparing to the
other power pins. Module technology uses the
standard and well know DBC: over a thick Copper
base an allumina (Al
2O3
) substrate with a 300?m
copper foil on both side is placed and IGBTs and
Diodes dies are directly soldered, through screen
printing process. These dies are then bonded with
a 15 mils aluminum wire for power connections
and 6 mils wire for signal connections. All
components are then completely covered by a
silicone gel with mechanical protection and
electrical isolation purposes.
THE “EDB” EMBEDDED DRIVING BOARD
This is the core of the device intelligence, as
previously described all control and driving
functions are implemented at this level, the board
finds its natural placement as a cover of the
module itself and has a double function of
mechanical enclosure and intelligent interface.
DSP and all other electronics are here assembled,
on fig. 5 the board schematic is presented and all
connection pins clearly shown.
Looking at the schematic, all diamond shaped
pins are signal connections, some belonging to the
RS485 port interface and some to the IEEE 1149.1
(JTAG) connector. All other pins are used for
communication between the board and the module,
they are positioned laterally in the board so that
the module doesn’t have any pins in the middle of
Fig. 3: the EMP
TM
package
Fig. 4: the inverter configuration layout
its body, in this way all PCB internal area is
available for components mounting and copper
lines layouting, to ease the board realization.
From the top left, in anti-clockwise direction
we identify the following blocks that will be then
described in details:
1. DSP and opto isolated serial and JTAG
ports
2. Flyback Power Supply
3. Current Sensing interfaces, over-current
protections and signal conditioning
4. Gate drivers
5. DC bus and Input voltage feedback
1. DSP and opto isolated serial and JTAG
ports.
This block is shown in fig. 6. The DSP used in
this application is the new TMS320LF2406A from
TI, it is a improvement of the well known in the
motor driver market “F240” used in many motor
driver applications. If we compare this new device
with the predecessor, the new DSP has some
Table 2: TMS320LF2406A vs TMS320F240
‘F2406 ‘F240
MIPS
RAM
Flash
ROM
Boot ROM
Ext. Memory I/F
Event manager
• GP timers
• CMP/PWM
• CAP/QEP
Watchdog timer
10-bit ADC
• Channels
• Conv. time (min)
SPI
SCI
CAN
Digital I/O pins
Voltage range
40
2.5Kw
32Kw
—
256w
—
Yes
4
10/16
6/4
Yes
Yes
16
500ns
Yes
Yes
Yes
37
3.3V
20
544w
16Kw
—
—
Yes
Yes
3
9/12
4/2
Yes
Yes
16
6.6?s
Yes
Yes
—
28
5 V
Fig. 5: Embedded Driving Board Block Schematic
added features that let the software designer
significantly improve the system control
performances, tab. 2 shows a list of relevant data,
for all other information the reader should refer to
the related device datasheet. Just to be noted here
is the increased number of instruction per second,
(40MIPS) and of I/O pins, the availability of a
boot ROM and a CAN, a much faster ADC and the
reduced supply voltage from 5V down to 3.3V, to
follow the global trend for this type of products.
The choice of the DSP has been done looking at
the high number of applications already existing in
the market using devices of this family, however it
is clear that the same kind of approach could be
followed using products from different suppliers to
let the customer work on its preferred and well
known platform.
The “2406A” has three different serial interfaces
available: SCI, SPI, and CAN bus. In this first
prototypes the serial communication is made
through the asynchronous port while three other
opto-isolated lines are occupied by the hall effect
sensor interface fed directly to the first three DSPs
input capture ports. Maximum bit rate for this
asynchronous serial port is 2.5Mbps while the SPI
(synchronous) could reach 10Mbps. The choice of
the SCI has been taken for easy interfacing with a
standard computer serial port, the only component
needed is a line driver to adapt the RS232 voltage
standard with the RS485 at 3.3V used on this
application. In this case the 100kbps PC RS232
port speed is not enough to handle the system
control properly but somewhat adequate for
demonstration purposes in a simple V/F control for
induction motors. In a real application with a
Brushless motor usually 1Mbps are far enough to
transmit all information needed for the torque
reference updates and other fault and feedback
signals at a maximum frame rate of 10kHz
(100bits/frame), in this way the on-board line
driver let us use long connecting wires instead of
short copper lines in a PCB, leaving the user the
possibility of having the PI-IPM
TM
displaced near
the motor, e.g. in its connecting box, avoiding long
ad noisy three phase cables between driver and
load.
The JTAG port is the standard one, neither
isolation nor signal conditioning are provided here
and all signal, except the Tck-ret, are directly
connected from the related DSP pins to the
connector; however, due to the limited board
space, the connector used in not the standard 14
pins at two rows header, then an adaptor has to be
realized to connect it to the JTAG adapter interface
provided by Texas Instruments.
Last but not least is the ADC speed and load
characteristic: as the table shows the conversion
time is 500ns, in fact this DSP has a single ADC
handling, in time sharing, all 16 inputs, then, if all
inputs are used and need to be converted, the total
conversion time for all inputs, which is a fixed
delay to wait for before having all data updated, is
around 8?s. The other characteristic to be carefully
taken into account is the capacitive load that the
input pin shows when the conversion is being
performed: accordingly to the device datasheet this
load is around 20pF, then, with an easy
calculation, whatever feeding this pin has to be
able to provide (within 500ns) the needed current
to this capacitor without showing a voltage
dropout higher than ½ LSB. Because of this in
many cases a further capacitor was inserted
between the ADC input pin and ground with a
value calculated as follows:
nF pF C 41
2
10 * 125 . 3
)
2
10 * 125 . 3
20 . 3 (
* 20 min
3
3
?
?
?
?
?
We used in all this cases a 47nF capacitor; the
added benefit is also that, in this way, we reduced
the local impedance towards ground thus
increasing the immunity to noise of all these pins.
Fig. 6: DSP and opto-isolated serial and JTAG ports
2. Flyback Power Supply
Though the board space is pretty limited (only
80mm X 44mm ca.) we found enough space to
accommodate a power supply for the floating
stages. The real limit however was the footprint
dimension, we couldn’t allocate more than a 10
pins (2 row of 5) transformer thus limiting the
maximum number of outputs to four. As the block
schematic in fig. 7 shows, we have three 15V
outputs for the floating stages, isolated from each
other at 1.5kV minimum, and a single 5V and
3.3V output.
The 5V supplies all low voltage electronics
and a 3.3V linear regulator is used to feed the DSP
and some analog and logic interfaces to it. This 5V
and 3.3V are directly referred to the DC bus
minus, so that all control circuitry is alternately at
one of the input lines potential, isolation (as
previously stated) is provided at the DSP serial
link level, then avoiding all delays due to opto
couplers insertion between DSP and control logic.
Note that also the required 15V input voltage is
referred to the same DC bus minus and directly
supplies the low side gate drivers stages, this is
necessary because of the already mentioned
limited space on the transformer footprint, then the
user should pay some attention on how this supply
line is realized in his application. Just for
completeness, fig. 8 gives a possible solution to
that that doesn’t impact heavily on the user
application: normally a 5V power supply is
already present, for displays, electronics and micro
processor, the same 5V could be used for the 5V
iso supply of opto-couplers and line driver, the
15V could be realized as an added winding in the
secondary side of the flyback transformer, the only
care that should be taken is in keeping its isolation
from the above mentioned 5V at the required level
(at least 1.5kV).
To avoid noise problems in the measuring
lines due to the commutating electronics during
normal functioning of the system, references are
kept separated. A 5V linear regulator, directly
supplied from the 15V input, is used to provide the
reference voltage to the current sensing amplifying
and conditioning components while a precise op-amp, configured as a voltage follower, acts as a
buffer of the partition at 3.20V created down the
5V reference. This 3.20V is used as a reference for
the DSP A/D converter. It is to be noted that in the
schematic we are using the same linear regulator
as a starting point for all reference voltages. In fact
if the 5V linear regulator derives in temperature or
time, then all references (even the 3.20V being this
a simple partitioning) follow in track and still keep
the overall chain precision. The trimming is then
done only once, in a single point of the measuring
chain, that is the conditioning op-amp collecting
the current sensing ICs signal as will be described
in the following chapter.
3. Current sensing interfaces, over-current
protections and signal conditioning.
This block is the real critical point of the
system. Current measuring performances directly
impact on motor control performances in a servo
application: errors in current evaluation, delay in
its measuring chain or poor overall precision of the
system, such as scarce references or lower number
of significant A/D bits, inevitably results in
unwanted trembling and unnatural noise coming
from the motor while running at lower speed or at
blocked shaft conditions.
Fig. 8: examples of power supply for PI-IPM
TM
15V and 5V iso inputs
Fig. 7: PI-IPM
TM
embedded flyback power supply
To avoid all these afore mentioned problems the
current sensing chain has to comply with usually
good performances that could be easily listed:
s s latency
kHz bandwidth
? ? 20 15
5
? ?
?
A common mistake is to look only at the band-with without taking too much care at the delay that
the information carries with it. This inevitably
leads to close the current loop with lower gains
than expected (to avoid instability and work with a
decent phase margin) and consequent poor
performances due to the limited system precision
in torque control. Most used devices for current
measuring are hall effect sensor, their precision
and bandwidth is far enough for a servo motor
driver application, closed loop ones have usually
100kHz BW measured at a phase lag of only
10deg, however their cost is pretty high and in
many cases also their dimensions does not fit in
the limited space available in the driver or in the
motor connection box.
Because of these simple reasons another
viable solution for current sensing is gaining
popularity: though not used for high current level,
because of power dissipation constraints, the
sensing resistor dropout measurement is still
adequate up to around 100A – 150A, with the
benefit of a lower area and somewhat a lower cost.
This is the solution chosen in the PI-IPM
TM
with
the added value of having the shunts element
embedded in the power module and all Kelvin
connections available.
As the block schematic in fig. 9 shows, the
voltage across each sensing resistor is applied,
through an anti-aliasing 400kHz filter, at the input
of a current sense IC. In this application we use the
HCPL788J because of their bandwidth, datasheet
shows 20kHz and we measured from these devices
a latency of 10?s at a frequency of around 3kHz.
As previously stated these performances are good
enough for servo-amplifier applications, these
integrated circuits also provide level shifting and
galvanic isolation through the internal high speed
opto-coupler; while the first function is essential
the second one is not really needed and useless in
this new type of intelligent power module where
isolation lies at the serial link level.
International Rectifier also has some current
sensing devices in its portfolio, however those
devices (e.g. IR2271 or IR2273) have been
developed for AC induction motor control and
their performances are not suitable for servo motor
driver application, driving a BLDC motor.
Signal outputted from the current sensing has a
0 to +5V dynamic, with a sensing resistor of
2mohms the input measured current range is +/-
100A then we have a situation as follows:
V A
V A
V A
0 . 5 100
5 . 2 00 . 0
0 . 0 100
? ?
?
? ?
The DSP ADC inputs have an external reference
of 3.20V then we must map this signal in a 0 –
3.20V frame and also filter the information to
recover the average value of the current flowing
to the motor phase. This is done in a single step:
though the block schematic shows a op-amp plus
an external passive filter this is simply realized
implementing a VCVS cell (i.e. a Constant Gain or
Sallen – Key cell) configured so that the offset and
gain could be trimmed by three on board resistors.
This is the setting point of all current measurement
chain: in a single easy step we set the precision
and recover the system offset through the
soldering of two SMD resistors in the op-amp
input lines, while the third one is used to re-center
the damping factor 2???and the resonant angular
frequency ?
?
of the second order filter.
The Sallen - Key cell is also pretty flexible and let
Fig. 9: Current Sensing and protections
us implement any type of second order (or even a
simple single order) low pass filter. We chose for
this first prototypes a second order Bessel filter
with 5kHz pole frequency, the reason for this is
that this type of polynomials are calculated with
the aim of having a constant group delay within
the pass-band frequencies, thus giving the
minimum waveform distortion to the output signal
up to almost twice the filter pole. In other hands
we could also say that the group delay of the signal
chain from the sensing resistor up to the ADC
input of the DSP is constant from 0 to 5kHz.
Summing up our current measurements
performances are shown in table 3.
By changing some resistors on PCB all parameters
could be changed: current range, type of filter,
pole frequency, leaving the flexibility of using the
same circuit for module of different power levels.
The “2406” DSP has a 10 bit ADC, that means
that, in our prototypes, the minimum appreciable
current step is approximately:
? ? ? 1953 . 0
2
100 * 2
10
LSB
That is
mA LSB 195 1 ?
Of course, reducing the maximum current range or
simply changing the sensing resistor value (for
lower power modules) this minimum current step
could be sensibly reduced.
Another used feature of the HCPL788J is the over-current output signal. The related fault pin goes
low when a 250mV voltage across sensing pins is
detected, this means an over-current detection
level of approximately 25%. Though the internal
opto-coupler the delay of this line is around 3?s
and fast enough to let the DSP react within the
10?s IGBTs short circuit rating, thus providing
full device protection for any phase-to-ground and
phase-to-phase short circuits. The only failure not
covered in this way is the cross through, where
high current levels are not seen externally the
module rather internally between two IGBTs of
the same leg. In this case the protection is
implemented by means of a fourth sensing
element, with the same resistive value of the other
shunts present in the power module, inserted in
series to the DC bus minus. The related dropout
voltage is then filtered by a 22kHz passive filter to
avoid false fault detections due to unwanted
induced voltage spikes and finally applied to an
operational amplifier configured as a comparator.
4. Gate Drivers
This is another critical part of the system, gate
drivers are responsible for correct turn on and turn
off of all inverter IGBTs, avoiding cross
conductions, false turn on during commutations
and dangerous propagation delays between input
signal and output commands. Devices used to
perform this task are the well-known IR2213,
capable of 2A sink and 2A source maximum gate
driving current, in a SO16W package; fig. 10
shows the block schematic of the gate driving
section of the module.
As previously described, the IGBTs used in
the PI-IPM
TM
(genV NPT 1200V - 50A from IR)
do not need any negative gate drive voltage for
their complete turn off, this simplifies the flyback
power supply design avoiding the need of center
tapped transformer outputs or the use of zener
diodes to create the central common reference for
the gate drivers floating ground. This is a big plus
Table 3: PI-IPM
TM
Current sensing
chain typical performances
value units
current range +/- 100 A
precision 0.3 %
bandwidth 5 kHz
latency time 10 ?s
?
Fig. 10: Gate Drivers
remembering that the board space is very limited
and that drawing three electrically floating lines
instead of two (for each output phase) all across
the board is a potential source of increased noise
and isolation problems. Though the IR2213 do
have +/- 2A of gate current capability, an easy
calculation based on IGBTs gate charge, supported
by instrumental observation in the laboratories, led
us to use different gate resistor values for turn on
and turn off as follows:
ohm off turn
ohm on turn
6 . 7
33
? ?
? ?
Commonly realized through a diode-resistor series
in parallel with a single resistor used in turn on
only. Observed rise and fall times are around
250ns – 300ns depending on the output current
level and we considered this values pretty
adequate for a 50A application at 16kHz
symmetric PWM carrier, space vector modulation.
Contrarily to the current sensing devices, that are
opto-coupled, these gate drivers do provide levels
shifting without any galvanic isolation that is no
opto-couplers are built inside. This turns out to be
a major benefit in this stage where the usual 1?s
delay of optos impacts on the system control as a
systematic and fastidious delay. Only drawback of
these gate drivers is the 5V logic compatible inputs
that cannot be driven directly by the 3.3V logic
level DSP. This forced us to insert a level shifter
interface, 74HCT541 with the only purpose of
logic level adaptation. New gate drivers, a
substantial improvement of the IR2213, are
presently in development and will soon be
available to overcome this problem and also
simplify other aspect of the here described
schematic.
5. DC bus and Input voltage feedback
The purpose of this block is to continuously
check the voltage of the two supply lines of the
system: Vin and DC bus; it is shown in fig. 11.
Vin is the only external power supply needed for
all electronics in the EDB. The internal flyback
regulator has its own under-voltage lockout to
prevent all electronics from start working when an
insufficient supply voltage is present; this UVLO
is internally set at a typical voltage of 8.4V with an
hysteresys of approximately 0.8V, when this level
is reached the internal flyback turns on and the
board is alive. Remembering what described in the
previous chapter dedicated to the internal flyback
Power Supply, low side gate drivers are directly
fed from the Vin line and there is no further
control to this voltage than their own under-voltage lockout. This is typically set at 8.5V and
this level could be not sufficient to properly drive
the IGBT gates, then, through a standard resistor
divider, we check the Vin voltage and impose that
the system could start switching only when the Vin
voltage is between 10V and 18V thus providing
also an over-voltage control.
The DC bus voltage is also important for the
system functioning and needs to be continuously
kept under control, this is done through another
resistor divider that unfortunately is at high
voltage and had to be layouted on the EDB with a
particular care, unfortunately occupying some
precious space. The divider provides a partition
coefficient of 3.47mV/V, having the ADC
reference at 3.20V this yield a maximum mapped
voltage of:
V VDCbus 922
10 * 47 . 3
20 . 3
max _
3
? ?
?
As the block schematic shows, it has to be taken
into account that, to avoid false detections due to
voltage spikes inevitably present on the partitioned
voltage, a 1kHz passive filter has been inserted
between the divider and the voltage follower
buffer whose output is connected to one of the
ADC inputs. An easy calculation shows that the
precision reached in reading the DC bus voltage is:
V LSB 9 . 0
2
922
1
10
? ?
Which is more than enough for this purpose.
Fig. 11: DC bus and Input voltage feedback
PRINTED CIRCUIT BOARD REALIZATION
AND COMMENTS
This so far described PI-IPM
TM
is a kind of
PEBB (i.e. Power Electronic Building Block) for
many motor driver applications and it is suitable
for any type of motor: Brush-less, Induction, and
Switched Reluctance. Because of the “on board”
presence of a high intelligence component such as
a DSP, it is also well positioned to become an
interesting component in many sensor-less
applications. Depending on the type of system in
which it is inserted, the performances level and
target cost should change to encounter the major
number of possible sockets in the market.
As already stated, one of the most important
features of this device is in fact flexibility, the
EDB could be designed and tailored to the
customer application, still leaving to the customer
the benefit of developing and downloading its own
proprietary software. However this is not such an
easy task and many critical factors challenge the
engineer in developing a good driving board for
this power module.
Fig. 12 and 13 show one of the first prototypes
realized and tested in our laboratories, their
performances evaluation and first experimental
results will be the subject of a further paper.
During the realization we faced two major
problems: isolation and noise of the environment.
The first was solved only through introducing
enough spacing between components and high
voltage copper lines, it was also in some cases
very helpful to add more layers in the PCB
stratification. The EDB is in fact a six layers
board: two internal layers are planes for ground
and supply voltages Vin and V5-iso; top, bottom
and a third layer are instead for low power signal
interconnections; the other remaining internal
layer is used for the high voltage floating lines of
gate drivers and current sense devices.
Components positioning is also a major issue,
some devices are noise and temperature sensitive
and cannot be placed in the board bottom side
while others are not so critical. Integrated circuits
such as the DSP and the current sensing have been
placed on top, following the above-mentioned rule,
while gate drivers and the flyback controller found
their socket on the bottom side.
Fig. 12: PI-IPM
TM
first prototypes
Not only the noise of the environment (IGBTs
switching in the underneath power module) is the
cause of potential problems but also the printed
circuit board itself. Some copper traces are a
relevant source of troubles, for example the
floating power supply lines of the high side gate
driver stages that are, in the system normal
operation, commutating between DC bus minus
and DC bus plus with a voltage swing of around
500V. In this case any inductive or capacitive
coupling, though very limited in absolute value,
could be enough to induce a considerable noise in
any analog high impedance line, thus inferring in
feedback and protection signals. Those 6 lines had
to be carefully drawn in the PCB layout, confined
in a single internal layer, and any crossing with
other lines in different layers was intentionally
forced to be perpendicular to reduce likely cross-talk of any type.
Again talking about the flyback power supply, also
its position in the printed circuit board is critical.
The controller is switching at 200kHz and
consequently is a relevant source of noise, having
sensitive devices (such as the current sensing and
the DSP) in the area is not advisable, then we
placed this block in one corner, at the opposite side
of the DSP and as much as possible away from any
other analog circuitry.
CONCLUSIONS
A new Intelligent Power Module, so called PI-IPM
TM
, using the well-known DBC technology has
been developed specifically for high performances
servo motor driver application. Thanks to its
considerable limited dimensions, it is particularly
suited for space saving solutions where the space
is a real constraint, such as Integrated Motor
Drivers or Motor Drivers of the new generation.
To include all functions and reach high-level
performances a new package, named EMP
TM
, has
been designed to accommodate the printed circuit
board on its top, with all needed connections for a
proper communication between the two parts.
With this new device a motor driver designer can
easily design and debug its system with an
excellent level of performances and a considerably
improved time to market.
Fig. 13: PI-IPM
TM
module and driving board
No comments:
Post a Comment