How do FET's work in a charger/inverter

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I have an old inverter/charger from circa 1990. Built using a massive transformer and FET switches - 32 in total with some control logic boards. When in invert mode the 12 volt input gets 'fired' at the correct frequency and produces a quasi sine output (120 volts) across the transformer! Well that it what I presume.

But what happens in the charger mode, 120 v is applied across the xfrmr and 14 v is the output. What I would like to know is do the FET's act like a passive diode to rectify the output or do they need to be 'fired' in a controlled way with a timed signal to the gate or even a constant gate current/voltage.

Any help appreciated here.

 

[Diver300]

Without the circuit I can't be sure, but I think that the body diodes in the FETs will rectify passively.

The power loss can be reduced if the FETs are turned on actively at the same time.

 

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Trying to upload the circuit diagram but it rejects it on size so a section showing just the FETs is shown at the moment.

The output at 12 v is up to 120 amps so quite large currents. pin 11 and 12 from the gate goes to an SG 3524 which is (or was) a pretty standard control chip.. I can try and get that whole section loaded as well.

The charger board is separate and also has an SG 3524 controller on board that drives a triac to control AC amps and that part works OK now. There is a single connection from the charger pcb to the inveter pcb and I am trying to figure out if this is turning the FETs on.

 

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Here is a much better image of the circuit

 

[alec_t]

The transformer has two terminals numbered '7' . I hope they're not galvanically connected.