For my master thesis, I am designing a wireless power transfer using magnetic resonance. For this purpose, I designed a full bridge inverter which is connected to an autotransformer whose output is rectified (this way I can gradually increase my DC input voltage).
The bridge is driven by two IR2110 drivers which are connected to a dspic30F1010 microcontroller. The microcontroller generates four PWM signals of constant 50% duty cycle. (edit: with a dead time of 50 ns, so not exactly 50% duty cycle).
The system works fine and I was able to transmit about 700 W during 30 minutes without any problems. Yet, when I increase my input voltage to a certain value (a bit less than 300 V), one leg of my bridge fails.
The right leg of the bridge works fine and keeps a nice 50% duty cycle, but the other leg shows the following features:
Figure 1: Vds of the TOP MOSFET of the failing leg
Figure 2: Vds of the BOTTOM MOSFET of the failing leg
The first picture shows Vds of the top MOSFET and the second one shows Vds of the bottom MOSFET. One can see that the top MOSFET gets turned off every 4 PWM cycles.
On the second picture, one can see that the current keeps flowing via the bottom MOSFET when the top one fails (I think it flows through the body diode, because Vgs remains low during that moment).
The following picture shows a picture of Vgs of the failing MOSFET (the top one):
Figure 3: Vgs of the failing MOSFET (TOP MOSFET of the failing leg)
First, one can see that it is really spiky and not stable. I'm not sure whether this kind of behaviour is normal with a bootstrap system or not. Then, one can see that during the glitch, Vgs almost remains zero all the time.
It also seems that the measurement itself impacts the phenomenon, as it happens every 4 cycles when measuring Vds and is much less regular when measuring Vgs (it seems to be every 5 cycles on the picture, but it is not always the case though).
I also checked the control signals coming from my microcontroller. These stay 4 nice PWM signals as expected.
The following pictures may help you to better understand my layout. The first one shows the PCB itself, in case it might be linked to the layout:
Figure 4: PCB of the inverter
Then the following two are diagrams of the circuit and the components that I used for the design.
The 100 ohm - 100 pF is used to filter out high frequency glitches at the output of the microcontroller and the 4.7 kohm resistor is a pull-down resistor to avoid short-circuiting the bridge at start-up.
The MOSFETs are STW28N60M2, the gate diode is V2P6X and the bootstrap diode is GB01SLT06.
[edit: I changed the following picture to remove the ambiguity on the capacitor value]
Figure 5: Microcontroller circuit
Figure 6: H-bridge & drivers circuit
This behaviour is something I never met before and I really can't find any explanation. It is even more strange to me because it occurs only in one leg and only after a given voltage value.
I tried changing the bootstrap capacitor value (lower & higher), I tried changing the gate resistor value, I tried to replace the IR2110 and the MOSFETs, nothing solved the problem.
Did anyone ever meet a similar kind of behavior, or does anyone have a clue on how to solve this?