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Set Up and Operation
1.4.8
1.4.8.1
Firing Signal Isolation and Low Voltage Power Supplies
INTRODUCTION
The choice of isolation strategy and how the low voltage power supplies are to be
derived are two of the major decisions that determine the architecture of a motor drive
controller. For this design, the requirement was for flexibility while maintaining user
safety. It was essential that the system could work off a wide range of input voltages.
1.4.8.2
FIRING SIGNAL ISOLATION (APPENDIX A, SHEET 5)
Either optical or transformer-based isolation strategies can be used for transmitting
gate firing commands. For this design, it was decided to use optocouplers, as it was
possible that a particular firing command may be used for commutation as opposed to
high frequency modulation. This could lead to transformer saturation at low speeds.
The hardware implemented is described below:
? U6-U13 – HCPL4503 optocouplers. These parts were chosen to provide good
noise immunity, while requiring low current consumption on the output side.
Speed was also a consideration, so that too much delay or pulse distortion was
not introduced.
? R153-R160 – Pull-up resistors for the open collector outputs of the optocouplers.
? R162-169 – Series resistors to ensure at least 18 mA flows through the emitter
stage of the optocouplers. Note that the ground return is via a transistor (Q15) that
allows shutdown during detection of an over-current from one of the isolated Hall
effect current transducers.
? U32 and U33 – Schmitt-triggered inverters which clean up the edges from the
outputs of the optocouplers.
1.4.8.3
LOW VOLTAGE POWER SUPPLIES (APPENDIX A, SHEET 5 AND 8)
The requirement for operation over a wide range of input voltage resulted in using the
+5V supply coming from the control board as the power source for all the low voltage
power supplies. This was true for both the isolated and live sides of the system. The
power supply circuitry on the live side is described below:
? U18 – An isolating (3 kV rated) 1W unregulated DC-DC converter with 5V input
and 15V (nominal) output. The +15V supply is used for the gate drive of the power
devices and the comparators on the live side of the isolation barrier.
? L4 and C83: A filter to reduce the reflected ripple on the +5V supply from the
control board caused by the DC-DC converter.
? L3 and C77: A filter to reduce the magnitude of the ripple on the live +15V supply.
? U5 – A 5V linear regulator for the live control circuit supplies.
? D4 – A 1A diode to protect U5 against reverse bias during power-down.
? R61 – A 0 ? resistor linking the low voltage power supply star point to the -DC bus.
? 2003 Microchip Technology Inc.
DS70096A-page 27
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