[SOLVED] Xenon Flash Lamp Circuit: IGBT failure - Page 1
Forum section: electronics / projects

author: arduino_guy
author_rank: Contributor
timestamp: July 30, 2018, 03:17:21 pm
content: Hi Guys,I have a Xenon Flash lamp flash circuit based on a reference design from ONSEMI:http://www.onsemi.com/pub/Collateral/ANDTIG065E8-D.PDFIf you look at the attached pic, in my circuit, cap CM is charged to around 300V and discharged through the Xenon Flash tube via the IGBT(TIG065E8-TL-H). The design worked well for a day and then it started failing.The problem was traced to the IGBT, which seemed shorted to GND every time.On further analysis, I think that the transformer connected to the IGBT should have a freewheeling diode connected to it. Could it be the inductive kickback that's killing the IGBT?What do you guys thing? I went through other reference designs and none of them seem to have a freewheeling diode.I have also attached my original schematic, C3 is CM in my design.

author: ConKbot
author_rank: Super Contributor
timestamp: July 30, 2018, 04:50:43 pm
content: If be worrying more about the layout and the wiring inductance, assuming the trigger transformer is relatively small, and so is the current though it when the IGBT is shutting off.Assuming this is on a PCB and not lashed up on a breadboard, there is a few gotchas to look out for:Return current path of the gate current. Since the current increase will be sharp, and large, if the voltage increases at the emitter compared to the gnd of the gate driver (pcb inductance/resistance) the actual voltage seen by the gate can be reduced, starting to bring it out of saturation, and the high dissipation can kill it. Make sure the gate current and strobe current are maintained as separate as you can keep them.Lead inductance of the output leads can be surprisingly large, make sure they are twisted. You can also go for a higher voltage rated IGBT, add an RC snubber, or a MOV to clamp the voltage. Though a MOV will wear out with repeated hits.The easy way get an idea as to which is the culprit, is to test it with long discharges where the tube shuts itself off, not the IGBT. If it dies, desaturation may be the problem. If it doesn't, inductance causing a spike on turn off may be more likely.Pictures of the setup or a view of the board layout would probably yield more advice from others.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 31, 2018, 04:06:28 am
content: I seem to recall a catch diode probably being used on the camera flash I took apart (a Canon S1), but it was a multilayer board (quite thin, at that) and I didn't probe to how things were connected.We're talking an operating range of 300V, 100A and a few microseconds turn-off, so inductance is definitely a consideration, one you need to be on the correct side of.Tim

author: arduino_guy
author_rank: Contributor
timestamp: July 31, 2018, 04:33:01 am
content: Quote from: ConKbot on July 30, 2018, 04:50:43 pmYup, the design is on a PCB. Unfortunately I can't share the layout or PCB setup since it's kinda of against the company policies. I'll have a look at things based on your feedback.Quote from: ConKbot on July 30, 2018, 04:50:43 pmI guess you're referring to the output leads of the transformer. I don't get what you mean by "make sure they're twisted".

author: ConKbot
author_rank: Super Contributor
timestamp: July 31, 2018, 08:05:35 am
content: Quote from: arduino_guy on July 31, 2018, 04:33:01 amThe leads from the pcb to the flash tube. (If it's not a small one soldered to the board) With a large loop area, it's possible to get more than a few uH of inductance. Trying to shut off a few uH with a dozens or hundreds of amps going though it can make for a nasty spike, way worse than the trigger transformer. No harm in trying a freewheeling diode, and dropping the trigger cap to minimum size on a test board, but got feel says other parasitic inductance in the circuit warrants just as much attention.

author: StillTrying
author_rank: Super Contributor
timestamp: July 31, 2018, 01:57:11 pm
content: Is the C3 300V cap in a good position with no connections or components in the high current path between the top of the tube and the IGBT's emitter.I don't think you'd get anywhere with a diode near the trigger transformer, the 4-5kV has to oscillate for a few us before the tube lights.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 31, 2018, 05:49:10 pm
content: Quote from: T3sl4co1l on July 31, 2018, 04:06:28 amI just looked at an old flash board and it did have a freewheeling diode along with a series inductor. I suspect the inductor is to reduce the peak current.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 31, 2018, 10:45:50 pm
content: I saw a really old Polaroid flash that did that, a little multilayer air core coil. No IGBT turn-off back then though.Tim

author: arduino_guy
author_rank: Contributor
timestamp: August 01, 2018, 03:48:11 am
content: What about a freewheeling diode across the primary and an RC snubber across the switch?

author: arduino_guy
author_rank: Contributor
timestamp: August 01, 2018, 03:50:34 am
content: You mean freewheeling diode?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 01, 2018, 07:03:54 am
content: From collector to ground. An RCD clamp snubber or TVS diode would be preferred, although the values required may be prohibitive, who knows.Tim

author: arduino_guy
author_rank: Contributor
timestamp: August 08, 2018, 12:58:57 pm
content: Hi Guys,I think I have solved the issue. I was triggering the gate driver using a switch (for testing purpose). Replaced it with a pulse from a signal generator and things have been working fine since then. I assume that possible switch bounce when the secondary trigger coil is at high voltage created a high voltage at the collector of the IGBT due to inductive kickback. Now the gate driver is driven by a 2second pulse with a high time of about 1.8s. I haven't dared to tweak further and try with lower high time.Thanks you all for the inputs.