Paralleling MOSFETs or MOSFET and IGBT to increase capacity in QRSMPS

Question

Is there a relatively simple way to increase the load capacity of a integrated SMPS/MOSFET controller by adding a parallel device?

This is the device: Fairchild FSCQ1265RT

The method has been successfully used: MOSFET-Assisted Soft-Switching of IGBTs

However, a potential problem is that since the main device is integrated, there is no way to connect to the gate signal. The logic to turn the assisting device on and off would have to come from some kind of sensing circuit, thus introducing possibly undesirable delays.

The required addition in switching current is about 3A, across a frequency range of 24kHz-~100kHz.. The least amount of additional components, besides the helper device, is desirable. Essentially, the capacity of the FSCQ1565RP device (which has been discontinued) is required.

Answer 1

I haven't searched BUT it is quite likely that these parts are functionally equivalent and possibly pin for pin to ones from a range of other manufacturers. This is the sort of thing that eg TopSwith do - and various others.

The lack of access to the gate makes paralleling hard. You could look at current in the Drain lead as a trigger signal for a parallel FET.

The functionally easiest method is probably to use the on-chip FET as a driver only for an external MOSFET. In order to use an N Channel lowside device you'll need a gate driver to invert the signal. Conceptually a high side PNP or small P-Channel FET with emitter to a say 12V supply, base driven by IC-Drain and new MOSFET gate driven from PNP collector. You then need a 12V supply for gate drive and some glue parts. New FET gate drive is slow and will probably need a jelly bean 2 transistor driver. Parts count starts to add up. Not vast but also not zero.

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Answer 2

If you are going to parallel devices, make sure that they have a positive temperature-voltage coefficient at high currents (the on-state voltage plot on the datasheet will show this). If not, then you'll have devices go into thermal runaway and pop in sequence as each takes all the current in turn.

Answer 3

Using a better heatsink will increase your current handling. Of course the increase depends on the current design and how much can you afford to pay for additional cooling.