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| IGBT full bridge issues |
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| TmaxElectronics:
I have an IGBT full bridge as the primary driver in a tesla coil, but am having some, let's just say issues :P with it. The gate drive ICs i am using (FAN7390) keep dying and i don't know why. :-// The circuit i am using is a one to one copy of one in an induction cooker, where it worked flawlessly for five years. Also the IGBTs (IXXN110N65) blew up for seemingly no reason at all, i have got a current limiting circuit in the controller that was set to stop excitation at 150A and was operating at a pulse width of only 20us, so there should not have been thermal issues. However i think that this might have been caused by a gate driver failure causing shoot through, the resulting explosion then pushed 350V down the gate driver output, blowing it up and spiking the control lines to cause shoot through on the other side as well. I am also aware of the very common drive circuit by Steve Ward but decided not to use it as i wanted to learn by designing one from the ground up. Attached are pictures of the gate driver output waveform (keep in mind that the frequency is NOT the actual drive frequency when operating) and the layout of the driver PCB. Are there any issues with the design that could cause such trouble, or is the design perhaps flawed altogether? Sorry btw if this post is hard to understand, i am quite honestly not sure how i should explain what is going on properly :/ EDIT: i have already made two changes to the design since: i have increased spacing between low and high side traces and added clamping diodes across the outputs of each channel. |
| T3sl4co1l:
Show the connection to, and layout of, the H-bridge. Tim |
| Etesla:
Nothing looks wrong at a glance. In general I think it's difficult to determine weather the IGBT's or gate drivers failed first. One failing can easily cause the other to fail. Have you done a test where you run the circuit with dummy capacitors around 5000 pF in place of the IGBT gates and see if the drivers fail on their own? |
| TmaxElectronics:
--- Quote --- Show the connection to, and layout of, the H-bridge --- End quote --- Unfortunately i only have the attached image, as the destruction was rather violent and it disassembled itself :-BROKE , because of those jucy capacitors in the foreground (the bridge with the IGBTs is in the background). I hope the image is shows enough. The length of the chinch cables used for the gates is quite significant, but I couldn't think of a better solution for connecting the drivers. I will try the drivers out with a dummy load once new ones arrived, the old ones are now a part of the atmosphere. Also I will try to mount the capacitors directly onto the primary bridge power rails next time to reduce inductance, I didn't have usable spacers yet. |
| T3sl4co1l:
BWAHAHA RCA cables, is that what they are! :-DD But anyway yeah, you've got a good probably 300nH supply inductance, about 100 of which is between the transistors. Not clear if you have other ground paths between DC link and control. Check the DC and transient voltage limits on that gate driver, particularly between the bootstrap common and VSS. Consider switching speed i.e. dI/dt with respect to that inductance. Oh yikes, and a big fat output cable running right over all the control signals, that can't be helping either. If you're looking for a slam dunk: Make a 4-layer PCB that screws onto the transistors. Put DC+/- on inner layers. OUT1 and OUT2 on bottom. Gates on top. OUT1/2 can be taken out to screw terminals without having to cross anything. Use an inline current transformer maybe. Likewise DC+/- can be poured in the middle and taken out to screw terminals without having to overlap the drivers. Use ground plane under the drivers. Probably use somewhat larger capacitors, too. Use fat wide pours for all transistor connections, including the gates. Add a few film caps to bypass the supply, 1-4.7uF, e.g. TDK B32672 family. Keep the Kelvin gate connections, don't short the two emitter terminals together. Put the gate drivers along the board edges, within a few cm of their respective transistors. This at least has the possibility of getting stray inductance down below 20nH, and eliminates the resistance and inductance of the gate driver cables. You can use smaller gate resistors (mfg recommends 2.2 ohms; plots up to 10 ohms). The board cost will be only a few times what your existing control board was, probably the same or less if you include the labor of cutting and shaping the bus bars. Can also consider TO-247 or TO-264 transistors, soldered into the board. Cheaper than SOT-227s. Tied to planes at the leads, the stray inductance is about the same either way, no performance loss. Obviously, if you already have a tube of the latter, this doesn't help much, that's fine. Tim |
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