Unwanted body diode conduction in full-bridge design - Page 1
Forum section: electronics / projects

author: paszczakojad
author_rank: Newbie
timestamp: July 27, 2018, 09:45:58 pm
content: Hi,I'm trying to make a DC-AC inverter with 400V DC input, 3 kW output power. As my MOSFETs were blown all the time I limited design to a H-bridge (removing half of the circuit and connecting it to GND - as on attached schematic) and tried to check what's going on. The lower MOSFET should be constantly OFF, the higher MOSFET is driven by 4 kHz PWM. I used IPW60R125P6 as both high and low switch. At lower voltages everything worked, but after increasing DC input to 120-150V both MOSFETs got blown. I suspected that Q4 turns on when it should not, I've read about parasitic turn-on, but the circuit behaved in the same way when I shorted gate to source and when I connected -12V to the gate (relative to the source of course).To check currents going through Q4 I placed a 0R5 resistor between Q4 drain and the rest of the circuit. And I noticed - as you can see on attached traces - that the voltage at the drain isn't "square" in the middle of the PWM sequence (it was square before 0R5 resistor), so apparently Q4 was conducting in these places. The higher the DC input, more PWM pulses are "rounded", and the rounding is deeper.I suspect something is going on with the body diode during reverse recovery, but I can't figure out what.What I checked:- increasing Rg of Q3 to 120R didn't help- increasing output load lowers the amount of rounded pulses- lowering the inductance of L1 and L2 lowers the amount of rounded pulses- using a diode (IDW40E65D2) instead of lower MOSFET allows higher DC input, like 200-300V- using a IGBT (IKW40N120H3) allows to go with DC input up to 400VI could use that IGBT as the lower switch, because it works, but I'd prefer to understand why it works. Both MOSFET and IGBT body diode have 10 times longer recovery time than standalone diode (which didn't do the best work).I think that the inductor enters someting like CCM during the middle PWM pulses (i.e. it doesn't discharge all it's energy while the high switch is OFF), but why the result is like that at the Q4 drain? And why the MOSFET (IPW60R125P6) keeps failing in this place, while IGBT works (at least with 300-400W output, I didn't check more yet)?Best regards,Piotr

author: jbb
author_rank: Super Contributor
timestamp: July 27, 2018, 10:35:41 pm
content: Ah yes, the higher voltage MOSFET H bridge. Good times...In general, the fastest way to destroy MOSFETs seems to be over voltage (i.e. voltage spikes). This can happen through ringing or through trying to turn the device off too fast when too much current is flowing.Device will also fail through straight-up overheating, too, but this takes some time to happen.So, here are some possibilities:Body diode reverse recoveryOver voltage stress due to leakage inductance. Leakage inductance can lead to big,fastspikes.Over voltage stress due to excessive turnoff speed. Maybe add some resistance in series with D5 and D7.Shoot through (a.k.a. Cross Conduction) seems unlikely with D5 and D7 there.Ground loop destruction of gate driver chips.Never mind, those are opto-isolated.Cross conduction caused by the gate drivers. Vishay has anApplication Noteon this subject and offers some circuit improvements (see Figure 25 onwards for suggestions).For further testing, I recommend you cook up adouble pulse testcircuit (can be as simple as a function generator) so that you can test under known conditions (Vdc, current level). (Semikron has anapp note on calculating switching losses, too.)One the topic of measurement, how fast is your 'scope? You need to check that you are measuring the circuit itself and not the 'scope (or probes). I blew up an H bridge once because I had the 20MHz BW limit turned on and completely missed a big nasty spike at around 70 MHz bandwidth. The devices actually detonated and fired plastic around the place.Finally, what safety precautions are you taking? A 400V DC link can kill you dead (and may remain live for some time on stored energy!), or blow a MOSFET apart hard enough to fire plastic chips into your eyes.

author: Circlotron
author_rank: Super Contributor
timestamp: July 27, 2018, 10:52:21 pm
content: Does C8 & C10 stay sufficiently charged under all pulse widths?

author: paszczakojad
author_rank: Newbie
timestamp: July 28, 2018, 12:03:52 am
content: Quote from: Circlotron on July 27, 2018, 10:52:21 pmYes, the gate signal is correct. Even the Q3 source is OK. Only after 0R5 resistor there are "roundoffs".Piotr

author: paszczakojad
author_rank: Newbie
timestamp: July 28, 2018, 12:26:35 am
content: Quote from: jbb on July 27, 2018, 10:35:41 pmScope is up to 1 GHz and... yeah, see attachment. Switch side output, without 0R5. These oscillations have 40-50 MHz and some spikes are 3 times the DC input voltage. Maybe IGBT survived 400V input, because it was a 1200V device, while MOSFET was 600V only (so failed at 150-200V input). I'll try to move input capacitor closer and make a snubber circuit. I don't think traces can be shortened anymore.But I'm still woried about these "rounded" traces after 0R5 resistor. Mentioned above ringing was on all PWM periods, so I don't think it's related. If the measurement was correct, there is 20V drop on 0R5 resistor - that's 40A current. Or perhaps it's a measurement error - that voltage drop is relatively smaller, when I switch to 50V/div... But why?I'll sacrifice one more transistor - if after dampening oscillations it will burn, then there's something to do with that unwanted current.Perhaps these oscillations somehow make the body diode of Q4 conduct and that happens only when there is charge in the inductor? But I don't see any way this could happen.Quote from: jbb on July 27, 2018, 10:35:41 pmThanks for care, I really appreciate this. I test circuit using scope at lower voltages, up to 100V and I move away from the circuit when I switch to 400V. And wear protective glasses - I already had a MOSFET blown into pieces. I have input and output voltages displayed on a LCD, so after disconnecting power supply I can see if everything went to 0. I already had cases, where capacitors were left charged to 400V for minutes, with no discharge current flowing.Piotr

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 28, 2018, 02:22:55 am
content: Layout?Tim

author: paszczakojad
author_rank: Newbie
timestamp: July 28, 2018, 07:38:29 am
content: Quote from: T3sl4co1l on July 28, 2018, 02:22:55 amHere's part of the layout with Q3 and Q4, the other half of the bridge was above. GND_HV trace goes on bottom side, directly under NetC4_2 and HV+ to make ground loop smaller, so it's invisible here.Piotr

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 28, 2018, 09:03:01 am
content: Looks like a good, what, 15cm trace length, from the southern transistor to the nearest bypass cap? Guessing trace impedance is something like 30 ohms, so, around 20nH? Not clear how fast you're switching but that turn-off could be much sharper than you were expecting.I don't get the sloppy waveforms. Am I seeing shoot through? L/R time constant? Inductive resistor? No idea.There's a trick to that. Rather than trying to get the absolute least inductance (which is impossible, it can never be actually zero -- inductance is proportional to length!), set up some mutual inductance with the current path, and subtract it out of your measurement. This looks like so:Here, a metal strip type current shunt has considerable inductance. The sense lead (blue) comes up from the board, makes two loops beside the one loop of the resistor, then goes off to a connector. By varying the position of the loop within the resistor, the inductance can be nulled from the reading.Adjustment does require a clean waveform to start with, like a load resistor, preferably with a short pulse test rather than a blast of full wave operation. The null is apparent when the waveform has no curvature -- just like setting probe compensation.Definitely, do as much checkout and evaluation as you can at lower voltages. Change out components to run at much higher current, if you need to. You can test different corners of operation separately: full voltage at low current, full current at low voltage, then finally full voltage and current.The transistors have an aggressive C(V) curve, so mind that you will not see representative switching at low voltages (<50V, say). Things will get sharp, and ringy, at higher voltages. Those transistors are more than happy to start squealing at 200MHz if you don't have gate resistance (ferrite beads are applicable here), and they can do ordinary things quite fast still as well (say, 10s of ns drain voltage risetime).Tim

author: Benta
author_rank: Super Contributor
timestamp: July 28, 2018, 03:19:54 pm
content: MOSFETs? IKW40N120 is an IGBT (as your schematic also shows). IGBTs don't have a body reverse diode, but rather a separate high speed, soft recovery diode die.