Joel Kolstad wrote:

Good evening Joerg,

"Joerg" wrote in message
...

As Harry mentioned check that negative going peak on the FET drive. I don't
particularly like gate resistors but you might need 22ohms or such if the
chip can't stomach this.


The Schottky diode Harry suggested appears to have made it happy. I tried a
4.7 ohm gate resistor as well, and it didn't make a difference.


Quite suspicious if an IC misfires because it's outputs are tortured a
bit. Not s'posed to happen, IMHO. This is one reason I rarely use PWM
chips in switchers. Had a similar thing with a stepper driver IC at a
client, kicked them all out.

4.7ohm ain't much. It would need to be higher.


I'm thinking the source of the negative voltage on the gate drive pin is...
capacitve coupling from a negative spike on the drain from leakage inductance
in the primary?


Cgd is what couples back. But it's only a real issue for HV apps, where
the drain has to switch tens of volts. On 15V max this shouldn't cause a
problem in a PWM driver. After all, it is supposed to live with real
world conditions. One of the few PWM chips I did use is the LM3478 and
it had no problem with this and in my case the drain had to muscle 100V
around.


Also, probe around all the pins of the chip to see if there is anything
suspicious.


The rest looked OK, although I'd have to admit it was with a digital scope --
I'll haul out the 2465 tomorrow and check again for good measure.


Take a look at R2/C1. R2 is way too high IMHO.


Agreed; it's actually changed to 150k/1nF at the moment. Tomorrow -- if
things go well -- I'll drop the capacitors and re-compute the compensation
network, which off-the-cuff I believe will reduce R1/increase C1 without
unduly restricting frequency response.


--
Regards, Joerg

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