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


Terry Given wrote:

> Fritz Schlunder wrote:
> > "Pooh Bear" <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote in message
> > news:42CC6F0F.8BE9DEDA@xxxxxxxxxxxxxx
> >
> >>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.
> >>
>
> drop the switching frequency down to, say, 1kHz where you can completely
> ignore the switching losses (Lmag may need to be cranked up, or Vdc
> reduced to compensate).

No problem there. Excellent idea. I can hang the load directly across the bridge
anyway since the tranformer ratio is 1:1 anyway.


> >>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.
>
>
> thats a seriously crap Vce.

Indeed !


> there cant be too many reasons:
>
> - measurement is wrong

I've been musing over that. And how - why.


> - Vge is way too low

14V seems ok to me.


> forget 1kHz, do it at DC - turn the bottom IGBT on, and slam some
> serious current thru it. Easy to measure Vce then, esp. when Vdc is low.

I can do that too I guess.


> > Are you saying the thermal dissipation isn't outrageously high, but the
> > Vce(on) appears very high even so?
> >
> > Make sure your oscilloscope isn't lying to you. When off the IGBT blocks a
> > huge amount of voltage, but when on the voltage becomes very small. On the
> > small scale needed to measure Vce(on) the oscilloscope may not work properly
> > due to the extreme high voltage at other times.
>
> good point - scope thermal tails due to overdrive.....

Hadn't explicitly considered scope issues but had a nagging feeling. Interesting
one. I can see the sense in that.


> > Try measuring Vce(on) through say a resistor and a low voltage zener diode
> > to clamp the maximum peak voltage to something small like 10V. Keep in mind
> > the resistor plus zener and probe capacitance make a low pass filter which
> > will degrade measurement bandwidth.

I'll check the probe C. A 10:1 probe should be fairly ok. I could even use a 100:1
probe. That's about 4pF IIRC.


>
> i've built compensated dividers clamped with 4148s for this job.

Ahhh - ok I got it. Clamp the voltage you're measuring to something that's not
over deflecting the scope basically.

Cheers guys, Graham

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