mosfet highvoltage full bridge driver

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aaronmawunze99

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Hie, is it OK to use mosfets with breakdown voltage of 400V on 350V. I am using 4 irf740 mosfets in full bridge mode with 2 ir2110 ICs driven by atmega 328chip. They work just fine for quite some time, about 30Minutes or so and no heating observed but the mosfets just blow any time, even under light loads.What could be the problem. The mosfet full bridge chops down 350V dc to 250V modified sinewave ac
 

What is the peak-to-peak voltage of your 250Vac modified sine wave? I calculate just over 700 volts peak to peak for a standard sine wave (assuming your 250Vac is and rms voltage measurement.
 
I measured the dc voltage and it indicated 350V dc on my multimeter. Upon measuring the chopped down ac modified sinewave, it indicated 285V open circuit which droppef down to 255V ac when loaded
 
Can you post your schematic? I know it is hard but we are just guessing with out more information. Also pictures of the board/parts.
 
Here is the picture of the board. Its a 24V DC to 240V ac inverter with ferrite core transformers. The output from the ferrites is first rectified to 350V DC which is then chopped down to 240 V ac by a four irf 740 mosfets arranged in full bridge and driven by a full bridge driver circuit. Now the original full bridge driver circuit was no longer working so I removed it and designed one using atmega 328p microcontroller and two ir2110 ICs. Each ir2110 IC drives a half bridge of the four mosfets. There is a dead time of 4milliseconts. Now the board works without a problem for some time then, randomly, the irf 740 mosfets just blow. It occurs irregardless of the size of load. During normal operation, no heating is observed on the mosfets
 
Here is the image of the board. Its a 24Vdc inverter with an output of 240V ac. It uses high frequency ferrite core transformers. The output is rectified to 350V dc which is then chopped down to 240V ac by 4mosfets arranged in bridge mode. The original circuit stopped working and I designed this one using atmega 328p and two ir2110 ICs, one for each half bridge
 
Can't see the images. Use the 'Attach files' button and make sure the uploaded file size is under 1meg.
 
Thats the picture I posted above. I have removed the burnt irf740 mosfets. Those are 1k gate-source resistors just visible on the main board where i removed the mosfets
 
Thatz the schematic. Q1 and Q4 switch on for 6milliseconds and the switch of. There is a dead time of 4milliseconds before Q2 and Q3 switch on for 6milliseconds and then off followed by another 4ms dead time and the cycle repeats again
 
aaronmawunze99 said:
Thats the picture I posted above. I have removed the burnt irf740 mosfets. Those are 1k gate-source resistors just visible on the main board where i removed the mosfets
Click to expand...
I have used a 7812 regulator ic to bring the voltage down to 12V for the ir2110 ICs and a 7805 regulator IC for the atmega chip. Thr bootstrap capacitors for the ir2110 are 47uf
 
A few points :-
1) None of your added ICs have supply decoupling capacitors (typically 100nF) connected very close to the IC supply pins, so they could be prone to erratic operation as the result of supply transients/interference.
2) In the image below :-
(a) Hard to tell from the pic, but the two FETs highlighted look to be in danger of shorting to adjacent metalwork/components. Perhaps vibration did cause a short?
(b) The highlighted long wire to the FET gate is prone to interference pickup.
 
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