MOTOROLA AN1524 User Manual

MOTOROLA

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Prepared by: Ken Berringer Motorola Inc., Hybrid Power Module Operation, Phoenix, Arizona

Abstract

The AC induction motor is the workhorse of modern industry. Worldwide about 50 million motors are installed every year that have greater than 1/2 hp. Today only a small percentage of these motors utilize variable speed drives. Almost half of the variable speed AC drives sold today are in the 1 to 5 hp range. Companies producing this range of drives are under a great deal of pressure to reduce costs. Lower system cost will result in higher volumes as more applications use variable speed. The power semiconductors are a significant portion of the cost of these drives. A new module called an Integrated Power Stage may be used to reduce the

cost and complexity of the power semiconductors. A functional demo board has been developed using this module for a 1 to 2 hp AC motor drive.

INDUCTION MOTOR SYSTEM

A systems approach can be taken to reduce the overall cost of an AC drive in the 1 to 3 hp power range. The system can be partitioned by semiconductor technologies and power dissipation requirements. A block diagram of a basic AC drive system is shown in Figure 1.

Figure 1. System Block Diagram

THREE PHASE

AC INPUT

INTEGRATED POWER STAGE

INTEGRATED POWER STAGE AC DRIVE

THREE PHASE

AC OUTPUT

PRECHARGE

CIRCUIT

+ +

CAPACITORS

VOLTAGE

DIVIDER

TEMPERATURE

AMPLIFIER

CURRENT

AMPLIFIER

GATE DRIVE

POWER SUPPLY

+18 V

+5 V

+12 V ISOLATED

CONTROL

FUNCTIONS

+18 V

+5 V +12 V

MCU BOARD

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SEMICONDUCTOR APPLICATION NOTE

 Motorola, Inc. 1996

REV 1

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The power semiconductors for an AC drive consist of a three phase rectifier and a three phase inverter. The three phase rectifier converts a three phase AC power source into a DC supply. A single phase power source may also be connected to any two of the three inputs. Electrolytic capacitors are used to provide energy storage for the DC supply.

A three phase inverter consisting of six IGBTs and six soft recovery diodes is used to drive the motor. Three phase AC is then generated by using sine wave pulse width modulation. The voltage and frequency of the AC output can be easily controlled using an embedded microcontroller unit. The most common constant V/F (volts per hertz) drives provide sine frequencies of 1 to 120 Hz and voltages of 0 to 240 V AC or 480 VAC. The three phase inverter is very flexible and can also be used for most three phase motors and modulation or commutation schemes. This includes brushless DC motors.

Because AC motors under sine wave excitation are also capable of generating, a dynamic brake circuit is often used. When the speed command is reduced in an AC drive, the motor will regenerate. The real part of the three phase power flows from the motor to the inverter. This results in current flowing from the inverter into the DC buss capacitors. Because this energy cannot flow back into the AC supply it must be stored in the capacitors or dissipated. If too much energy is

pumped into the capacitors the voltage will rise to unacceptable levels. A dynamic brake resistor and a seventh IGBT are often used to dissipate this energy. Because large resistors used for dynamic braking are inductive, a clamp diode is used to limit the transistor voltage during turnoff.

INTEGRATED POWER STAGE MODULE

There are a total of 20 power semiconductor devices in this AC drive system. Using single transistor discrete packages is very cumbersome. Often a rectifier module and IGBT six pack is used with a discrete transistor for the brake. This simplifies matters a little, reducing the number of components that must be mounted to a heatsink and soldered to three.

The “Integrated Power Stage” Module is a comprehensive solution for small motor drives. A schematic of the Integrated Power Stage is shown in

Figure 2 and the module is illustrated

Figure 3. These modules contain all the power electronics for an AC drive. By combining all the power devices into a single package mounting use is greatly simplified. A single module is fewer parts than two modules. This should reduce manufacturing cost and improve reliability. The Integrated Power Stage also contains a 1% sense resistor and a temperature sense diode. These components can be used to provide status and control functions as well as protection features.

Figure 2. Integrated Power Stage Schematic

D8 D10 D12

D9 D11 D13

0.01

Ω

I+

T+

T–

I–

Q1 Q5

Q4Q2 Q6

D3 D5

D2 D4 D6

G2 G4 G6

16 17 14

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Figure 3. Integrated Power Stage Module Illustration

There is a family of six modules for 1, 2 and 3 hp motor drives for both low voltage (up to 240 VAC) and high voltage (up to 480 VAC). Table 1 contains the part numbers and the

voltage and current ratings for the Integrated Power Stage modules.

Table 1. Integrated Power Stage Ratings

ББББББББББББББББББББББББББ

IC (Amps)

ББББББ

15–16

CES

600

MHPM7A15A60A

MHPM7A20A60A

MHPM7A30A60B

CES

(Volts)

1200

MHPM7A8A120A

MHPM7A12A120A

MHPM7A16A120B

IPS AC MOTOR DRIVE DEMO BOARD CIRCUITRY

A demo board has been developed for the Integrated Power Stage family of modules. A photograph of this demo board is included at the end of the text as shown in Figure 13.

The purpose of this demo board is to provide a vehicle for evaluating the performance of these modules and to enable customers to rapidly develop proof of concept systems. The board contains the integrated power stage module, passive power components and gate drive. It also contains a current amplifier, temperature amplifier and voltage divider for monitoring drive status. Over–current, over–voltage comparators and a programmable logic device provide instantaneous fault protection.

Power Circuit

The power electronics for the demo board is shown in Figure 4. Three phase or single phase AC power is connected to the input terminals R, S, and T. The order of the input connections makes absolutely no difference. The Integrated Power Stage (U1) rectifies the AC and provides a DC output at P1 (1) and N1 (25). Negative temperature coefficient thermistors (R1 and R2) are used to limit the inrush current and protect the rectifier diodes from excessive surge current. Two thermistors with a cold resistance of 1 Ω are suf ficient to

protect the low voltage systems. Alternatively, a relay and precharge resistor could be used to limit inrush current. Both upper and lower branches of the DC buss are split between the rectifier and brake for flexibility.

Capacitor C1 is a polypropylene capacitor for high frequency performance. Capacitors C2 and C3 are the DC buss capacitors. Jumpers are provided to configure these capacitors in series or parallel. The PC board is designed for both high voltage and low voltage systems. All the values shown are for the low voltage configuration.

The output terminals U, V, and W are connected to the motor. Exchanging the order of the connections will reverse the direction of the motor rotation. The motor rotation can also be reversed by software control. A connector is also provided for dynamic brake connection.

The heatsink of U1 is earth grounded using a small screw through one of the module standoffs. The heatsink should be earth grounded for safety reasons. The clearance and creepage distances for the printed circuit board are designed to meet the spacing requirements of UL 840 and IEC 335 for 480 VAC drives. The AC drive standards UL 508, UL 508C, VDE 0110, and VDE 0160 refer to these standards or use similar clearance tables.

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Figure 4. Power Electronics

HEATSINK

1.0

Ω

1.0

Ω

INTEGRATED POWER STAGE

N1 R S T U V WB

P1 N2 P2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

1819202122232425

R S T U V WDB1 DB2

C3 470

0.033

240

480

C2 470 µF

Gate Drive

The gate drives for the high side IGBT s use optocouplers for

level shifting. The circuit is shown in

Figure 5. The HCPL0453 optocoupler (U8) was chosen for its high dv/dt capability and speed of operation. The 480 V AC drives must operate with DC link voltages as high as 850 volts under regeneration

conditions. Thus, a 600 volt HVIC solution is not suitable for 480 VAC drives. It is estimated that high voltage drives account for more than 1/3 of 1 hp drives and about three–fourths of the 5 hp drives. Optocouplers provide a universal solution for both voltage ranges.

Figure 5. High Side Gate Drive

C18 10

HCPL0453

C19

0.1

G1 (U1 PIN 9)

E1 (U1 PIN 8)

(FROM PLD)

U11 MC33151D

D10

+18 V

R43

180

Ω

R46

2.2

Ω

R53

Ω

R52

100

Ω

R47

5.6 k

Ω

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PlusI

(ANALOG OUTPUT)

U4b

MC33272D

R31

2.2 k

Ω

R29

12.1 k

Ω

R28

3.01 k

Ω

R30

13.0 k

Ω

R25

13.0 k

Ω

R27

1.10 k

Ω

R26

1.10 k

Ω

U3b

LM293AD

+5 V

C10 120 pF

+2.5 V

–

(4.0 V)

Power is supplied by a bootstrap circuit consisting of a high voltage diode (D4), a small resistor (R46), and capacitor (C18). When the lower IGBT is turned on, the capacitor charges through the diode and resistor. The lower transistor is then turned off and the upper transistor is turned on. The charge stored in the capacitor supplies the gate drive energy . Actually, the bias current of the IC and optocoupler pull–up resistor are the primary current drains on the bootstrap capacitor. The 10 µF capacitor can provide a holdup time of several milliseconds. An 18 volt supply is recommended because about 3 volts are lost in the lower transistor on voltage and the bootstrap diode. The small resistor limits the peak current under transient conditions.

The gate drive current is supplied by an MC33151D (U11) high current MOSFET driver. This is an economical LVIC with

1.5 amps of source and sink capability . Separate resistors are used for turn–on and turn–off. A low turn–off impedance is essential to minimize turn–off losses and shoot–through current. The turn–on resistor is selected to limit the turn–on dv/dt to an acceptable level. The values for R52 and R53 thus vary according to the specified power module/development system.

The dv/dt limitations necessitate some additional power losses in the IGBTs, particularly for the high voltage drives. The dv/dt applied to the motor is also a consideration as this stresses the motor insulation. The typical dv/dt measured on the demo board is 5 to 10 V/ns during IGBT turn–on. This value can be lowered by adding output filter inductors or using

larger gate drive resistors. However, using larger gate drive resistors will greatly increase power dissipation in the IGBTs.

The low side gate drive uses a similar circuit without the optocoupler. Two non–inverting MC33152Ds are used for the three low side transistors and the brake transistor.

Status Circuitry

The sense resistor in the integrated power stage is placed between the brake and inverter. This allows the resistor to sense both positive motor current and dynamic braking currents. A resistor placed between the rectifier and brake transistor would not sense the regenerative current when the brake is turned on. The DC link current may be used in many different control and protection schemes. The DC current in this resistor multiplied by the DC voltage gives the real power output of the inverter. The real power divided by 3 and divided by the output phase voltage gives the real output phase current. Unfortunately, the output power factor is not known. However, the peak current in the resistor is indicative of the peak phase current. Thus, a DC measurement and a peak over current detect provides a cost effective means of control and protection.

The current amplifier and over–current comparator are shown in

Figure 6. A differential amplifier senses current in both directions. A 2.5 volt reference provides an offset for the differential amplifier. The conditioned output varies from 0 to 5 volts with 2.5 volts representing zero current. The over–current comparator trips at 12.7 amps.

(U1 PIN 4)

MinusI

(U1 PIN 5)

OC (TO PLD)

C9 120 pF

C11

120 pF

Figure 6. Current Amplifier and Over–Current Comparator

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The temperature amplifier is shown in Figure 7. The circuit consists of an inverting amplifier with a 0.69 volt offset. The output provides a temperature signal of approximately

26.5 mV/°C. The compensation capacitor is essential to avoid oscillations with such a large feedback resistor. Resistor R20 biases the temperature sense diode at 1 mA. The bias current can be increased if necessary to minimize noise problems. However, the gain will have to be readjusted.

(ANALOG OUTPUT)

R20

4.42 k

Ω

R24

1.30 k

Ω

R23

3.40 k

Ω

R22

10.0 k

Ω

R21

137 k

Ω

1000 pF

U4a

MC33272D

PlusT

(U1 PIN 3)

+5 V

0.1

+2.5 V

–

(0.69 V)

The voltage divider and over–voltage comparator are shown in

Figure 8. The voltage divider provides a 0 to 5 volt signal, 1 volt per 100 volts on the DC buss. Four resistors in series are used to ensure the voltage rating of the resistors is not exceeded for the high voltage system. The over–current comparator trips at 4.25 volts. Some hysteresis is also provided to prevent the brake from oscillating between the on and off states. The resistive divider also serves as a bleeder resistor to slowly discharge the buss capacitors. The high voltage drive uses resistors twice the value shown for R1 1–14.

Logic Circuit

The input logic of the demo board uses a programmable

logic device for flexibility, see

Figure 9. The 22V10 PLD has 12 input pins and 10 I/O pins. Seven of the outputs are dedicated to the seven IGBT gate drives. Seven of the inputs are configured for use with the seven transistor inputs. Seven resistors are connected to the inputs and may be configured as pull–up or pull–down resistors. The polarity of the input signals may be changed by modifying the device equations and setting the polarity jumper. One of the inputs is dedicated as a hardware enable. The three remaining I/O pins are available as programmable I/O signals to the ribbon connector.

Complete Schematic & Bill of Materials

(ANALOG OUTPUT)

R17

2.2 k

Ω

R18

51.1 k

Ω

R16

9.09 k

Ω

(TO PLD)

R19

150 k

Ω

+5 V

U3a

LM293AD +

–

(4.25 V)

Appendix I contains a complete board level schematic of the demo board. All of the component values listed in the schematic are for the 1 hp 240 VAC configuration. Appendix II is a bill of materials for the demo board. Again the values listed are for the 1 hp 240 VAC configuration. The 2 hp 240 VAC configuration requires changing several resistor component values. The 1 & 2 hp 480 VAC configurations require changing J1 and using 1000 volt bootstrap diodes, in addition to changing several resistor values. Alternate bill of materials are available on special request.

USING THE IPS AC MOTOR DRIVE DEMO BOARD

PWM Signals

The demo board requires a PWM signal source. The PWM signals can be generated by a microcontroller. Two microcontrollers suitable for this purpose are the MC68332 and the MC68HC16Y1. Basic code for PWM AC motor control has been written for the MC683321 and the MC68HC16Y12. Both of these microcontrollers include a Time Processor unit. The Time Processor unit is an autonomous timer which may be micro–programmed for various time functions. A special microcode primitive has been written called Multi–Channel PWM3 that can generate center aligned PWM signals with dead time. A motor control development system has been developed for the ’HC16Y1. The integrated power stage demo board was designed specifically to interface directly to the ’HC16Y1 motor control development system. However, the PLD provides added flexibility for use with different microcontrollers.

R11

82.5 k

R12

82.5 k

R13

82.5 k

R14

82.5 k

Ω

Figure 7. Temperature Amplifier

R15

3.32 k

Ω

Figure 8. Voltage Divider and Over–voltage

Comparator

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Figure 9. Input Logic

BRK

VO TO

I9 I10

I2 I1 I0 VCCI/O9 I/O8

I/O7

I/O6

I/O5

I/O4

I/O3

I/O2

I/O1I/O0

GND I11

22V10 NC

28234

12 13 16 17 18

26271

BRK

I/O3

I/O2

I/O1

GND

+5 V

CN7

R10

4.7 k

Ω

+5 V

R3–R9

4.7 k

Ω

The PWM signals should be center aligned with dead time. The demo board does not provide any dead time. The microcontroller can provide accurate digitally generated dead time far better than an analog circuit. Appropriate PWM signals are illustrated in Figure 10.

Figure 10. PWM Timing

2 µs

The recommended dead time for the demo board is at least 2 µs. Less than 2 µs of dead time will allow some shoot through current during switching, resulting in higher switching

losses. Inadequate dead time may also result in damage to the power transistors.

PLD Programming

The demo board is shipped with a preprogrammed PLD. This PLD provides only two basic essential functions, half–bridge lockout and a master enable. A half–bridge lockout ensures that both upper and lower transistors are not simultaneously turned on. The master enable function enables or disables all six PWM signals according to the position of SW1 (U2 pin I0).

The standard PLD is programmed for active high logic signals. Therefore jumper J1 should be positioned to configure the input resistors as pull downs. The three general purpose I/O pins are programmed for compatibility with the HC16Y1 development system. Connector CN7 pin 6 (I/O3) is connected to the active low fault pin of the development system. It is set high to enable the PWM signals. The other two I/O pins are tri–stated. The logic equations for the standard PLD were compiled using ABLE. The PLDs were

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