Re: Switching losses in half bridge

John Popelish wrote:

> Pooh Bear wrote:
> > I have a fairly classic half-bridge 'forward converter style' ( but no
> > feedback loop - it runs open-loop ) SMPS with a Vbus =~ 320V and
> > operating frequency ~ 100kHz.
> >
> > You may recall I posted about his a while back - notably in connection
> > with the UC3525.
> >
> > It's working and delivering load but I've got to the point where I
> > need to look closely at various aspects of its operation and refine
> > them. In particular for example I've been looking at switching losses.
> >
> > I'd noticed that my mosfets were running warmer than I'd like. Part of
> > the problem was that some snubber Rs were actually contributing to the
> > problem, so I took a step back, removed them completely and made some
> > measurements from new.
> >
> > I can run the bridge into a simple 33kohm load which just gives me a
> > basic load so that I can look at the no-load switching waveforms. With
> > this puny load I saw a heatsink temp of ~50C on the mosfets.
> >
> > I assumed switching losses on account of the Coss. A quick calc showed
> > a calculation on that basis to be in the right ball park with the
> > observed temp rise, so I substituted some other mosfets I had handy
> > with about 60% of the original Coss value.
> >
> > To my surprise they actually ran about 1C hotter !
> >
> > Anyone got any bright ideas why that should be ?
> >
> > Mosfets are Infineon SPW20N60C3 ( originally )
> > http://www.farnell.com/datasheets/33657.pdf
> >
> > and Fairchild FQA28N50 ( substituted )
> > http://www.farnell.com/datasheets/31981.pdf
> >
> > Ton is ~ 4us and Tdead is ~ 1.5us ( big but I have a trimpot on it )
> > btw.
>
> Is your approximately 5 mA load current (320/2/33000) changing the
> drain voltage much, during the dead time?

Not visibly. You mean as in dragging the output voltage towards Vbus/2 ? That
point has intruiged me in its own right.


> Has the gate voltage of the turning off fet gone through almost all of
> its swing during the 1.5 us dead time, before the other gate swings on?

I don't recall looking at it recently but it damn well should be ! It's got
an IR2013 driver and about 10 ohms Rg.


> Does it stay in the off state when the other fet turns on, or does the
> gate voltage bounce back into the transition region during the drain
> voltage swing?

Erk ! I don't think so. You're thinking of the 'Miller capacitance ' effect
? The IR2013 ought to keep that sorted.


> Do the neighbors complain about TV interference during your testing? ;-)

No - lol. No neighbours. The office is in an industrial park. No adverse
signs locally either.

Incidentally when driving the transformer - there's no noticeable temp rise.
And the output waveform looks truly lovely. Nice slow transition from 'rail
to rail' almost perfectly matched to the dead time. Lmag ~ 900uH btw.

Graham


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