High idle power revisited

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Murphy's friend


Joined: 04/10/2019
Location: Australia
Posts: 612
Posted: 08:07am 06 Sep 2023      

A while back I posted about my 6KW inverter having an unexpectedly high standby power.

This I 'cured' by increasing the dead time capacitor. This inverter is now powering my house and working fine but wiseguy and others were not convinced I made the right 'cure'.

Now, experimenting with my 3 KW inverter, which is very similarly built, just a smaller toroid transformer, I notice it too had an unacceptable high idle power. Worse than my big inverter had in fact.

Sooo, having time on my hand, I started checking parts. Nothing faulty was found, dead time capacitor changes made little difference and in the end the problem was pinned to the mosfets alone.

They are mounted on carrier boards so they can be exchanged without resorting to messy soldering fixes (one of my better ideas ).
Here is how they look, there are four identical boards:

This board is fitted with IRFP4468PbF mosfets. All 4 boards test faultless when driven by a 20KHz, 50% duty cycle square wave. As load I used LED car brakelights with suitable dropping resistors so testing could be done at 55V DC.

But, driving the mosfets from the symmetrical control board output (either NANO or EG8010 derived), the inverter takes too much idle power.

Now the really strange thing, if I replace those carrier boards with ones fitted with new HY4008 mosfets the idle power is where it should be.

As the IRFP4468's had been in use for some time previously I wonder if these things change in some way. Still switching 20KHz square waves but not what the control board feeds them.
Or is it something entirely different?