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

Pooh Bear wrote: 
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


bingo. kind of crap, but better than nothing. I forget the vendor, but a Tektronix spin-off makes beautiful broadband diff probes for exactly this job. $$$$ :(

linear tech's AN47 talks about it in detail

Cheers
Terry
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