Half bridge current shoot through... Why? - Page 1
Forum section: electronics / repair

author: EugenioN
author_rank: Contributor
timestamp: July 25, 2022, 09:02:36 pm
content: Hi all.I'm repairing a faulty ICEPOWER ICE1000A audio amplifier (datasheet herehttps://icepoweraudio.com/download/2408/)It's a H bridge with IR2010 high and low side gate drivers, four FDP2532 mosfet (datasheet herehttps://www.onsemi.com/pdf/datasheet/fdi2532-d.pdf) and a PNP bjt gate turn off:

author: Andy Watson
author_rank: Super Contributor
timestamp: July 25, 2022, 09:57:19 pm
content: Are the two PNP transistors functioning correctly? They should have gain, however, if the collector is not sinkng the required current the turn-off time will be impaired. There is a current path to discharge the gate via the emitter-base diode - so the turn-off could still appear to be rapid but not as fast as it should be with the extra collector current.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 25, 2022, 10:14:16 pm
content: Did you replace the IR2010? Transistors?Think about what you said: one failed as a three-way short. Meaning, at least momentarily, some fraction of the +120V was applied to the gate driver and associated components. This often fries the driver, and could fry stuff further upstream of that (say by failing as a multi-way short in turn).Shoot-through isn't necessarily problematic; it needs to be designed in, however. If used, it's likely a very small amount, 10s of ns, and switching loop inductance likely will be modified to support this (snubbers).Tim

author: David Hess
author_rank: Super Contributor
timestamp: July 25, 2022, 10:16:07 pm
content: That bottom waveform looks like it has a snivet displayed at the rising edge of the gate waveform on a DSO where aliasing occurred. It does *not* look like poor probe grounding because the same thing is not shown at other fast edges. A snivet is a type of negative resistance oscillation, and would easily explain high shoot through and heating. This is consistent with your report of solving the problem by increasing the gate resistance.Ideally the gate resistance is close to the body of the MOSFET, so how is the layout?Take a close look at the waveform in analog mode or peak detection mode to see if the rising edge is oscillating at a high frequency.I might add a ferrite EMI beat at the MOSFET gate lead to try and suppress the oscillation if that is what is going on.

author: langwadt
author_rank: Super Contributor
timestamp: July 25, 2022, 10:42:54 pm
content: Trr of the body diodes?

author: EugenioN
author_rank: Contributor
timestamp: July 26, 2022, 08:44:41 pm
content: Quote from: langwadt on July 25, 2022, 10:42:54 pm105ns for the original mosfets, 146ns for the replacements.+40ns of recovery time can be the culprit?

author: EugenioN
author_rank: Contributor
timestamp: July 26, 2022, 08:53:12 pm
content: Quote from: Andy Watson on July 25, 2022, 09:57:19 pmI'm wondering why the mosfets failed in the first place. Turn-off bjts losing some gain maybe caused the fireworks: Now I changed the gate resistors and a slower turn-on is "hiding under the carpet" this issue. I'll get some replacement bjts.Thank you!

author: langwadt
author_rank: Super Contributor
timestamp: July 26, 2022, 09:00:24 pm
content: Quote from: EugenioN on July 26, 2022, 08:44:41 pmone is max the other typical, but who knows

author: EugenioN
author_rank: Contributor
timestamp: July 26, 2022, 09:13:56 pm
content: Quote from: T3sl4co1l on July 25, 2022, 10:14:16 pmWishful thinking: gate drivers survived that explosive event.I'll try to replace both of them: I've got plenty of IR2110S: they seems good enough for the jobQuote from: T3sl4co1l on July 25, 2022, 10:14:16 pmInteresting: do you have any reference/application? I'm curious now!Looking at the "good" bridge leg I can measure ~100ns of deadtime, and a small/negligible drain current spike during the gate voltage plateau: everything as expected, so I assume no black voodoo magic.Side curiosity: this class D amplifier has a bridge tied speaker with a custom coupled inductor (common mode) and a capacitor (differential) as LC output filter. Quite uncommon, I've always seen two independent LC filter, one for each leg.

author: EugenioN
author_rank: Contributor
timestamp: July 26, 2022, 09:31:51 pm
content: Quote from: David Hess on July 25, 2022, 10:16:07 pmI must confess my ignorance: I don't quite get what "snivet" means!When properly probed with the ground clip connected to the source, the gate voltage is perfect in both legs: nice turn-on with barely visible voltage plateau during turn on, steady 10V voltage during on state and a fast turn-off ramp. No ringing.I'm sorry for this poor scope screenshot, but it was the only leftover in the waveform memory. My faultQuote from: David Hess on July 25, 2022, 10:16:07 pmWell, actually one leg is working fine as is. No special care can be observed in pcb design: maybe it works because the gate driving circuitry dimensions are less than a couple of squared centimeters, so even sparse components cannot be that harmful.I don't know: ony the leg with new mosfets is shooting through, and the pcb design is a cut&paste of the working one. Before the explosions everything was fine.