MOSFET heating problem in AC-DC converter

[justerr]

Hi everyone. I have built this AC-DC converter, but can't get it working properly. I tested it in simulator and it worked just fine, but on a breadboard (or on PCB) I have had problems. The biggest problem is that the pulse at MOSFET's (Q12) gate gets rounded, which heats up the MOSFET. Or at least I think that's the main problem why the MOSFET heats up.

The idea here is that the LTC1771 gets its power initially through R69&R71 and then starts controlling the MOSFET with the help of the transistors. The pulse at PGATE of LTC1771 is really sharp, but gets rounded after T2. Driving the transistors and the MOSFET should be strong enough, so it shouldn't cause this, right? Or what am I missing here? Otherwise the circuit is working fine. Just that you can't run it very long without blowing up the MOSFET

Any ideas? Or does anyone has any good application notes, etc.. about this subject, I could look at?

 

[goldsmith]

Hi everyone. I have built this AC-DC converter, but can't get it working properly. I tested it in simulator and it worked just fine, but on a breadboard (or on PCB) I have had problems. The biggest problem is that the pulse at MOSFET's (Q12) gate gets rounded, which heats up the MOSFET. Or at least I think that's the main problem why the MOSFET heats up.

The idea here is that the LTC1771 gets its power initially through R69&R71 and then starts controlling the MOSFET with the help of the transistors. The pulse at PGATE of LTC1771 is really sharp, but gets rounded after T2. Driving the transistors and the MOSFET should be strong enough, so it shouldn't cause this, right? Or what am I missing here? Otherwise the circuit is working fine. Just that you can't run it very long without blowing up the MOSFET

Any ideas? Or does anyone has any good application notes, etc.. about this subject, I could look at?

Hi justerr
A mosfet in linear region deals with dissipation . investigate of what waveform is across the GS . as i can understand you didn't drive your mosfet as fully on . the PWM signal must be across the GS with very low rise and fall time . check it out !

Good Luck
Goldsmith

 

[FvM]

What's the actual rise and fall time of Vgs? Does the drive voltage speed increase if you disconnect the gate?

I see this points:
- R47 is ruining T2 switching speed and should be omitted, the emitter resistor can be adjusted respectively
- T2 may be still too slow for the application
- T3 and T4 should be transistors with higher current rating so that they have sufficient current gain at the expectable gate currents

If you don't manage to increase the gate signal speed as necessary, think about an active pulse shaping circuit.

The final reason for bad efficiency is the large Vin/Vout voltage ratio. Switching losses of switched mode converter are always generated relative to the switched power product, in this case Vin*Iout.

 

[justerr]

Thanks, guys. Disconnecting the gate increased the drive voltage speed a bit but not much. It's still way too slow compared to the voltage at PGATE of LTC1771. That's why the problem must be transistors T2, T3 and/or T4, as you said. I'll try next to find faster transistors and try with those next. I'll let you know how that went.