Posted on January 19, 2015 at 23:34I'm attempting to drive a servo motor from a dsPIC33 and a STGIPS20K60. This module, a 20kHz three-phase IGBT bridge, has an overcurrent fault input (where 0.5V instantaneous on pin 16 immediately shuts down the module) and a time constant (set by an RC network on pin 15) which keeps the module shut down for a configurable amount of time after a shutdown/fault.
The overcurrent protection is implemented entirely in hardware (the module is rated to survive a four-microsecond output short circuit, but the micro would never respond that quickly in firmware) and is fed from a 1/40th ohm power resistor in the negative supply line through an RC circuit with a three-microsecond time constant (as a PWM drive and motor are noisy and will otherwise false-trip the circuit during switching transitions).
I'm a little unsure about the RC network (pin 15) that keeps the module shut down after a fault. On the demo board, this appears to be 330pF and 10k ohms, which would give a time constant on the order of several microseconds at best. What happens if the motor drive hits a wall or similar locked-rotor mechanical obstruction? It trips on overcurrent, is re-enabled relatively rapidly (within some fractional sliver of the width of a single 20kHz drive pulse), only to trip again? Repeatedly?
At some point, these retries will damage the module.
Is there any particular rationale behind these component selections? Is the intention that pin 15 be fed back to a hard-wired ''PWM fault'' input on the micro, which would shut down the logic outputs for all three phases? It would seem that a short time constant here is just going to keep retrying until something breaks.
#stgips20k60
