Design Proposal for Low Power Drives
L. Lorenz, K. Kanelis
Industrial Electronics System Engineering
Infineon Technologies in cooperation with eupec
Germany
1 Introduction
Although most small drives cover the power range between 40 W and 2.2 kW, their configuration is
quite different depending on the application. The compressor of an air conditioner, the drum drive of a
washing machine and the spindle drive of a numeric controlled lathe may all have the same power
ratings of about 750 W. But they strongly differ in their control algorithms and inverter set-ups assuming
the same electrical motor, either and induction machine or a brushless DC type. While the operation of
a fan or a compressor might be already sufficient with an open loop voltage-frequency control, the
washing machine requires a closed loop current control for avoiding current overshoot at accelerations
and even a feedback speed control for sensing unalignment conditions. A high performance servo drive
requires an accurate current control acquiring all phase currents and an exact speed control to follow
sudden variations of the set value.
Therefore, the feed forward control scheme for a fan drive can be implemented on an 8-Bit
microcontroller. The simplified vector control for the drum drive, reconstructing the phase currents from
the DC-link current of the minus bar, requires an 8-Bit microcontroller. The complete field oriented
control for a dynamic servo drive demands an improved 16-Bit microcontroller or a DSP microcontroller,
especially for realising a control scheme without mechanical sensor. Microcontrollers require a rather
constant supply voltage. Integrated voltage regulators keep the voltage constant independent of the
input voltage and ambient temperature. They also provide useful auxiliary functions in one IC.
IGBTs are available in different packages, as discrete devices and modules. They should be selected
according to the power and the assembly capabilities. Principally only one heat sink is desired for the
semiconductor switches, mainly IGBT devices. This requires a suitable assembly concept for the power
stage. In the lower power range, up to 400 W, the relative low losses allow a cheaper concept than the
power range up to 750 W or even up to 2.2 kW. The comparison of three concepts, one with SMD
IGBTs on IMS, another with fullpack IGBTs on PCB and another with SMD IGBTs on PCB, indicates,
that each concept is suitable for a different power range, as demonstrated in fig. 1.
Up to 250 W the IGBT losses can be dissipated already by the FR4 PCB and SMD IGBTs are
recommended. Higher power values up to about 500 W require thermal paths through the PCB but also
an isolating foil between the PCB and the heat sink. The power range up to 750 W can be covered by
fullpack IGBTs. The power limitation results from the plastic coating of 0.3 mm according to the
specifications. The upper power range up to 2.2 kW can be served by special isolating coatings of heat
conductors, such as the isolating metal substrate IMS. The IMS enforces the automated production
with SMD IGBTs, available up to 30 A. A new low power, low cost IGBT module family, the easyPIM,
combines convenient assembly and industrial reliability standards with compact and low cost inverter
design covering the power range from 500 W up to 2.2 kW.
2
Key components of a low cost drive
The high volume market of low power drives is extremely cost sensitive requiring semiconductor devices
exactly fitting to the given application. The IGBTs and the microcontrollers are themselves components
with a considerable cost part in the drive system. Their proper selection helps optimising the drives
performance at minimum costs.
2.1 NPT IGBT
IGBTs are basically manufactured according to two concepts, the PT and the NPT technology. In the
market of IGBT devices the NPT concept is well established. But up to now this concept was restricted
to the voltage range of 1200 V devices and above due to the limitations in processing wafers of
thickness far below 200 µm. However, if it becomes possible to reduce thin wafers of approximately
100 µm, new 600 V NPT IGBTs may become an interesting alternative to a conventional 600 V epi or
PT IGBT. The technological way to the manufacturability of such ultrathin 100 µm IGBT wafers as well
as further improvements of the NPT structure itself are state of the art at Infineon Technologies. The
600 V NPT IGBT is characterised by a superior trade off relationship between on-state and switching
losses compared to a 600 V epi IGBT [1]. As there have been many studies and efforts during the
recent years on improving especially the conventional 600 V PT IGBTs, e.g. by trench transistor cells or
by optimisation of the buffer structure, this field of 600 V devices is of course is very interesting also for
an NPT IGBT. A huge effort has been achieved by applying the NPT structure to the 600 V device along
with a cell optimisation.
2.2 CoolSET
CoolSET combines a control chip with a power MOSFET in a single package for the purpose of
controlling a switched mode power supply. A two-chip solution was chosen because this allows the
most effective use to be made of the latest technologies in the high voltage and low voltage areas, thus
offering the customer an excellent price / performance ratio.
In the low voltage control part, CoolSET combines state of the art bipolar technology with a latest
generation power MOSFET, implemented in CoolMOS technology as a high voltage power switch. This
combination enables the realisation of the IC in a P-DIP-8 package. Even in this small package, the
operation of a switched mode power supply with an output power of 40 W is possible. CoolSET
therefore satisfies the demands of the innovative trend towards “Silicon instead of heat sink”.
The circuit diagram in fig. 2 shows a flyback converter with two output voltages, 15 V directly regulated
by the circuit and 7 V being regulated down to 5 V by a voltage regulator. The transformer is based on
an E20 core. The proposed flyback converter is capable of providing 150 mA.
Figure 2: Utilisation of CoolSET in a flyback converter.
2.3 Fast switching high voltage diodes as free-wheeling diodes for motor control systems
Conventional diode manufacturing processes use a base material on which the n—zone layer is grown
epitaxially with the proper doping on a highly doped carrier layer. In contrary Infineon uses the EmCon
diode concept. The main properties of this diode are soft switching behaviour even at extremely high
switching frequencies, low reverse recovery charge, low on-state losses, positive temperature
coefficient.
2.4 Microcontrollers for variable speed drives
The Infineon microcontroller family with 8-bit and16-bit architectures are designed with a stress on
drives control and are therefore equipped with sufficient memory space, several timers, peripheral
devices and PWM-units.
2.4.1
8-bit microcontrollers
Several low cost motor drive applications applying a feedforward speed control can be served by 8-Bit
microcontrollers offering the possibility of on-line parameter adjustment, communication and status
display.
An interesting microcontroller for the high volume market of home appliances and general purpose drive
is the C508 with the following main features.
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Enhanced 8-bit C500-CPU (fully software/toolset compatible to Standard 8051 microcontrollers).
300 ns instruction cycle time at 20 MHz CPU clock (SDIP: 375 ns at 16 MHz).
Built-in PLL (x2).
19 interrupt vectors with 4 priority levels selectable (10 external Interrupts).
32 Kbyte on-chip ROM/OTP. ROM protection available.
256 byte on-chip internal RAM, 1024 byte on-chip extended RAM (XRAM).
Supports external address range up to 64 Kbyte program and data memory.
8 datapointers with 16 bits for indirect addressing of program and data memory.
Three 16-bit timer/counters, Timer 2 with 4 channel PWM.
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Three-channel 16-bit capture/compare unit dedicated for AC/DC motor control applications.
1 channel 10-bit compare unit.
8-channel 10-bit A/D converter, 6 µs conv. time at 16MHz.
U(S)ART with programmable baudrate generator.
40 multifunctional I/O pins, 8 input pins, 2 ports 10mA sinking current (total max. 100mA).
Power saving modes: slow down mode, idle, interrupt wakeable.
P-MQFP-64, P-SDIP-64 packages.
Temperature ranges: 0ºC to +70ºC and -40ºC to +85ºC.
2.4.2
16-bit microcontrollers
The microcontrollers of the Infineon 16-bit family have been designed to meet the high performance
requirements of real-time embedded control applications. Complex tasks, such as pretentious drive
control, demand the processing of an enormous flood of digital and analogous input signals and the
response with the evaluation and output of set values with minimum delay. Embedded control
applications require for their realisation microcontrollers, which offer a high level of system integration,
eliminate the need for additional peripheral devices and the associated software overhead, provide
system security and fail-safe mechanisms and provide effective means for the control and reduction of
the power consumption of the device.
The high flexibility of the 16-bit microcontroller architecture has been optimised for high instruction
throughput and minimum response time to external interrupt sources. Intelligent peripheral subsystems
have been integrated to reduce the need for CPU intervention to a minimum extent. This also minimises
the necessity for communication via the external bus interface. The core of the 16-bit family has been
developed with the background of a modular family concept. Fig. 8 shows schematically the structure of
the 16-bit microcontroller C164CI.
Many drive applications require the integration of specific peripheral modules in addition to the standard
on-chip peripherals. This integration is supported by the XBUS concept, an internal representation of
the external bus interface that opens and simplifies the integration of peripherals by standardising the
required interface. One representative peripheral taking advantage of this technology is the integrated
CAN module.
Two microcontrollers important for drive applications are the C164 and the C167. Both combine high
CPU performance (up to 10 million instructions per second) with high peripheral functionality and means
for power savings. Both are equipped with a high performance 16-bit CPU with four-stage pipeline. The
interrupt structure offers 32 nodes with separate interrupt vectors. The interrupt latency counts to 300 /
500 ns typical / maximum in case of internal program execution.
The two Capture / Compare Units of the C164 with independent time bases each and the very flexible
PWM unit and event recording unit with different operating modes exhibit powerful instruments for high
performance control of all kinds of electric machine drives. The control schemes also require A/D
converters and timers. The C164 operates with an 8-channel 10-bit A/D converter with programmable
conversion time (9.7 µs minimum), with auto scan modes and channel injection mode. The
multifunctional General Purpose Timer Unit includes three 16-bit timers / counters with 400 ns resolution
at a clock rate of 20 MHz. Additional useful devices are the bootstrap loader allowing flexible system
initialisation and the on-chip CAN module.
2.5 Voltage regulators
Voltage regulators are integrated circuit commonly providing constant 5 V at the output independent of
the ambient temperature and the input voltage. Infineon offers the low drop voltage regulators TLE 426x
and TLE 427x. Their main functions and features are: Inhibit, Watchdog, Reset active until output
voltage falls below 1 V, Adjustable reset time, Extended operation range (up to 40 V, Excellent
temperature behaviour, Reduction of external components, Easy PCB design, Safe EMC behaviour,
very high RF noise immunity, Output voltage tolerance of 2 %, Low-drop voltage of only, 0.35 V at 550
mA load current, Low quiescent current consumption, Outputs protected against short-circuits,
Protection against excess temperature, Protection against polarity reversal of supply voltage, Voltage
surge protection up to 65 V for less than 400 ms, ESD protection for more than 4 kV, Wide
temperature range from -40 °C to 125 °C. As an example the usage of a TLE4268G in conjunction with
a C164CI is described in fig. 3.
At power on the TLE4269 provides an acitve RESET# signal until a voltage level of higher than 4.6V is
reached. At power down the RESET# will be activated if the voltage drops below 4.6V again The
threshold and the reset level can be adjusted externally by a voltage divider. Short spikes on the input
voltage of the TLE4269 are ignored and therefore a stable operation of the microcontroller unit is
guaranteed even under rugged conditions such as a drive environment. As the C508 has an active high
reset the reset signal has to be inverted.
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Schematics
The low cost IGBT module easyPIM1 for 600 V, 10 A, the 16-Bit microcontroller C164CI, the CoolSET
TDA16832G and the voltage regulator TLE4268G have been utilised in a reference inverter design for 1
kW. The driving stage unit with separate Gate resistances for optimally switching on and off the IGBTs
is demonstrated in fig. 4. Acquiring the DC-link current of the low side bar and incorporating the PWM
pulse patterns of the microcontroller the phase currents can be derived. Evaluating the motor speed by
counting the pulses generated by a tacho generator or hall sensors, a simplified vector control can be
implemented. The utilised circuit arrangement wit the corresponding filters for current and speed signals
is described in fig. 5.
Figure 3: 16-Bit microcontroller C164CI enabled by voltage regulator TLE4268G.
Figure 4: Three-phase driver with bootstrap circuit for a low cost IGBT module including 600 V, 10 A
IGBTs, single-phase rectifier and NTC.
Figure 5: Current control circuit utilising DC-link current of low side bar.
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Conclusion
Low cost motor drive systems require components exactly specified and designed for the given high
volume application. Infineon Technologies has responded to this challenge providing dedicated products
for the power stages, the data processing and the communication. These semiconductor devices cover
a wide range of applications. The IGBTs are available in all common discrete packages. The product
palette has been enriched by new low cost IGBT modules, the easyPIM family, with rectifier, inverter
bridge, brake chopper and NTC. Even if the 8-Bit microcontroller C508 is capable of serving most low
cost drive, there are already home appliances, such as washing machines, with complex demands. The
necessary feedback control for current and speed as well as the communication ports for the control
panel require the features of the C164CI. This 16-Bit microcontroller with excellent peripherals is
capable of carrying out a vector control for induction and synchronous machines with mechanical
sensor, managing in parallel the power factor correction of the inverter input current and performing the
communication through CAN-Bus. Although being initially understood as low cost devices, several home
appliances including vacuum cleaners, show performance characteristics close to servo drives.
Semiconductor devices are called to follow this trend with the constraint of a given system price. And
this is achieved by system integration as already messaged by the Infineon Industrial Electronics
products.
Reference
[1]
T. Laska et al.
“Ultrathin wafer technology for a new 600 V – NPT – IGBT” (1997),
Proceedings ISPSD 1997, pp. 361-364