Synchronous rectification in IGBT

Question

^{simulate this circuit – Schematic created using CircuitLab}

In above standard 3-ph IGBT based inverter circuit suppose Diode D4 is carrying current because of inductive nature of load. If corresponding igbt Q4 gets gate pulses in this freewheeling interval then whether current will be transferred to igbt from diode as it happens in MOSFET called as "synchronous rectification" (which is used to minimize conduction loss because of diode)? In short if PWM signal is applied to an igbt whose freewheeling diode is conducting will that igbt get destroyed/overheated? second doubt is if collector of a high side igbt is not connected to voltage source while bootstrap capacitor is fully charged then upon applying gate pulse to high side igbt it will get turned ON?

third doubt is if an igbt is rated for 600V 10A then can i operate that igbt at down to 1A or even below 500mA? also can i turn on it for collector to emitter voltage of 25V?

Answer 1

Addressing your questions in order:

1. The IGBT will not be impacted in any way. IGBTs do not conduct current in reverse; this is why they must be coupled with a separate diode [1]. Even with the IGBT on, its free-wheeling (also called 'anti-parallel') diode will conduct the current. MOSFETs, on the other hand, can conduct in both directions. In the MOSFET, the current can reverse-conduct through either its channel or its body diode, depending on the current, channel resistance and body diode drop. The body diode is inherently built into the MOSFET.

2. It will indeed be turned on, but no current would flow since the collector would be floating. An IGBT, like a MOSFET, turns on gradually. It begins to turn on when the gate voltage exceeds a threshold, relative to the emitter.

3. Yes to both. The ratings you mention (10A, 600V) are the maximum ratings. You can use the IGBT with 25V bus and 500mA, no problem.

[1] See an example of a TO-247 device which is effectively an IGBT and diode co-packaged: http://www.power-eetimes.com/imf/c/eyJtYXNrIjoiNDMyeDI4OCJ9/images/01-edit-photo-uploads/2014/2014-06-june/onsar2662_fig-1.jpg.