Purpose of piece of circuit in Online UPS?

[Atlantis]

Hello, I am repairing on-line UPS. It had short-circuit on input (but from batteries works fine and also charges them when "off") but when connected to mains it trips circuit breaker after energizing relays connecting AC to power section. Anyway - I found shorted IGBT in interesting piece of circuit I really don't understand what it does, here is photo:

Bad transistor is "G20N60"...what I found it is IGBT. This circuit is connected just to AC and transistor Gate is connected to some resistors and opto-coupler. Nothing more

It looks power goes from AC to rectifier bridge and from here negative goes to IGBT E. Positive to IGBT C and to diode parallel connected to 10ohm, 10W resistor (not in photo, just schematic drawing). After that is is just bridged with negative through 10nF cap. I am newbie but can someone explain purpose of this circuit? is it some intelligent load?
In order to repair - should I replace just IGBT or also optocoupler and diode bridge just to be sure?

[SeanB]

Check the diode bridge with the IGBY out and the AC disconnected, it should be detectable as 4 diodes. If one is shorted it will show. This is most likely a transfer switch, and probably was destroyed by a short across the output, which killed the IGBT. It is designed to apply the AC from the mains to the output, and to disconnect the inverter from feeding back into the mains.

Check the relays have not got shorted contacts as well, and try running it with the IGBT out, which will tell you if the bridge is breaking down with voltage.  The diode, resistor and capacitor are a snubber across the IGBT, to keep spikes from damaging it.

[Atlantis]

Thanks for answer. I checked relays as first thing when I recieved UPS - they are fine. Diode bridge seems fine too.  I desoldered it and checked it separately.

Do you think the IGBT just died and simple replace will work? I am going to buy one tomorrow but it is quite expensive (5USD)

You mean running UPS without IGBT? Won't it harm anything? How critical is this circuit to UPS operation? Becuase I still don't understand what it does - it looks it is just loading/short-circuiting input AC. 

[NiHaoMike]

Could it be an active snubber for inductive loads? An old APC UPS had something like that directly across the output.

[Atlantis]

I examined it in more detail and discovered that from optocoupler's diode leads to small board named "PFC DC"...so it looks like some sort of power factor correction. But I don't understand how it works? I thought PFC should be boost converter in series with load...this looks like it is parallel to load.
I just hope there is nothing bad in PFC circuit because it looks complicated

[Mangozac]

There also exists buck-boost style PFC (among several other configurations), but from your description this doesn't sound like the transistor's function. Plus from my understanding MOSFETs are generally used for the buck switch rather than the more expensive IGBTs.

There are other complicated startup functions that are sometimes implemented so perhaps it's being used for that...

[Atlantis]

Do you think it is possible it's purpose is to keep input loaded even when there is no current draw? I mean in the sine wave phase when voltage on input in lower than voltage on capacitors so there is no current flowing.
That is stupid idea right? Because it would mean massive waste of energy just to keep input power factor look good...

EDIT: I just replaced just the shorted IGBT with new one and UPS works fine