"N_Cook" wrote in message
...
Arfa Daily wrote in message
...


"Jon Elson" wrote in message
...
Arfa Daily wrote:


IGBTs are my least favourite power switching devices. I've always
found
them hugely over-priced compared to FETs, and extremely fussy about
their
operating conditions.

The big problem with IGBTs is they are a parallel structure of many
bipolar transistors, with POSITIVE temperature coefficient. So, they
do not balance current across the die well. When driven hard into
saturation, they balance better. So, the secret an IR app engineer
told me years ago is you have to drive them hard into saturation,
and never allow them to stay in the linear region for more than
a few ns, if possible. (You can only do so much of this during
turnoff, however.)

Jon



What I've never really understood, is what supposed advantages IGBTs have
over FETs. A while ago, I had the misfortune to work on a switcher that
was
in a Yammy powered speaker. It used a pair of IGBTs that were about eight
quid apiece from memory. Much like the OP's induction heater, it would
run
fine for a while, and then the stupid things would just destroy
themselves
again - and I mean violently, legs actually blown off, and all that good
stuff. I mend a lot of switch mode power supplies, and for the most part,
their failure modes and what needs to be done to obtain a reliable fix,
hold
no mysteries for me. But after about the fourth set of devices that I put
in
the Yammy, along with just about every other component in the surrounding
area, I gave up on it. Many switchers of similar size and ratings, use an
almost identical topology, but with a pair of FETs. Typically a couple of
quid apiece, and in my estimation, *much* more robust. So with power FETs
rated to 800 volts and staggering amounts of amps being readily available
almost for pence, why would anyone actually design with IGBTs ?

Arfa



Is there any rules for substituting powerFETs for IGBTs in such situations
that you had there, assuming you are not averse to going against the
designer's wishes?


I don't know. I must admit that at the time, I *did* consider trying exactly
that, but you know what it's like when you get one of these soul-destroying
jobs. I wrestled for several days with the idea of giving it one more go
with FETs in place of the IGBTs, but in the end, the owner said that if it
was going to be a lot of trouble, and absolute future reliability couldn't
be reasonably guaranteed, then he would just scrap it. I must admit that I
then drew a deep breath of relief, bundled it back together, and cheerfully
gave him it back. Next time, maybe ...

Arfa