High Vce(on) with an IGBT - 100kHz smps

I'm currently using a pair of IGBTs in my prototype smps ( UC3525
controller - half bridge configuration with IR2110 driver ).

It's up and running nicely now after the kind help here.

I've been testing it with some dummy loads to check device dissipation
and current waveforms ( just a large R across the bridge directly or
coupled by a 1:1 TX ).

At a load of ~ 600W I was surprised by the temp rise in the IGBTs. Makes
no difference if the load is transformer coupled or not btw.

Looking at the Vce of the lower device whilst on, I saw about 8V.
According to the data*** ( it's an Infineon SGW30N60 btw ) with the 4A
or so it's carrying, Vce should be < 1.5V. High side device appears to
dissipate the same. Temp rise seems to correlate with the heatsink
thermal resistance ( 3C/W) and the calculated dissipation.

Gate drive voltage is ~ 14 V. Waveforms look clean to me.

Now the puzzling bit. I removed the big load but a small load ( actually
33k ) remains. Vce(on) remained at 8V !

It should be *very close* to ground. Any clues why not anyone ?

Device on-time is ~ 4 us and dead time ~ 1us currently.

Graham

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