Posted on March 12, 2014 at 17:43A new application is producing problems with very high spike
currents.
We are driving 4 STGP20H60DF using two L6390 in two half bridge
setups, for a specialty single phase motor (not a standard motor, I
can't talk about it, but inrush is not an issue).
230V input, very standard virtually app note circuit with 0.025 ohm
sense resistor, common to both half bridges. Gate resistors in the
47 to 100 ohm area for turn on, 22 ohm turn-off. About 10A rms,
similar to starting currents in prior designs to drive PSC type
single phase motors. This one won't drive those motors, despite very
similar circuit design and layout.
The IGBTs are low gate charge, and should be driveable by this chip,
although they are a very fast type.
We get very high current spikes of very short duration at switching
(80A and 100-200 ns), that we have not seen in any prior design. The
spikes seem to actually shut down the chip, although we cannot see
any voltage changes or pulses on the overcurrent input. In fact we
have shorted the current sensing, and still see the problem.
UVLO is not the problem, at least the 15V input is very stable with
reference to the chip common pin, and VBOOT is also stable.
Current sense has even been shorted at the pin, and no change.
The chip heats up a bit more when the problem occurs. and the problem
is current sensitive, more current = more problem.
The only things which have made any large difference are freezing the
chip, which fixes it, and also inserting a fairly large resistor in
the 15V supply to the chip, in the 100 ohm area, which substantially
reduces it.
Older date code devices seemed to be far less sensitive, not showing
the problem until much higher currents.
The problem is noticed on the lower output, which is normally on for
a half cycle (upper is PWM'd). It shuts off for several ms partway
into the half cycle, then comes back on. WAY shorter than the /SD
timing. And the /SD pin is not affected. With time, the shutdown
becomes shorter, but does not go away.
Very low switching frequency... around 5 kHz, audible sound is not an
issue, and there is just no need for anything higher.
It seems that we have managed to do something undesirable, but it is
not at all obvious exactly what. I have never seen this before,
with this or any other IC driver.
Anyone seen this problem? Have a solution?
