MOSFET switching issues with Push Pull converter

[zappyton]

Hi everyone,

I'm building a push pull converter, and my MOSFETs aren't switching properly when given the signal to. I'm assuming that it's some kind of snubber problem, but I don't know. Maybe a TVS diode would help?

The circuit is attached; the MOSFETS are IXFX30N100Q2 and they are being given a signal from a MC34152P driver chip. I've also attached a photo of the waveforms without the RC snubber across the transformer coils to Vin; I appear to have not taken with the snubber (i.e. the 6,8nF cap, and the 80R resistor), but the sharp peak was exactly the same, but the oscillation before the circuit goes high again was reduced in frequency. The trasnformer is one big transformer; all the secondary coils are the same, ad the job of the circuit is to charge the capacitor to 3500V volts, which is why I've done it with a split secondary like that.

In the scope picture, channel one is the input to the MOSFET gate, channel 2 is the drain voltage. I was expecting them to be almost opposing square waves, but this is clearly not the case. Can anyone offer any suggestions?

Oh, and although the diagram shows a 50V input, as does the scope, it will need be running off of mains, i.e. 320V; at the moment I have it connected to a variac.

It so nearly works, but not quite! Any help would really be appreciated.

 

[MicrosiM]

It would be great idea if you can show the setup ( the PCB ) of the driver circuit.

Also show waves at S+G

Witch PWM chip you are using?

 

[zappyton]

I'm using veroboard at the minute, so there isn't really a circuit board as of yet, but I can do the Gate and Source (i.e. top of the current sense) waveforms.

I'm using a SG3524 control chip.

 

[MicrosiM]

I'm using veroboard at the minute, so there isn't really a circuit board as of yet, but I can do the Gate and Source (i.e. top of the current sense) waveforms.

I'm using a SG3524 control chip.

I am not sure if your image shows BOTH waves or single wave? the upper one is really bad

 

[zappyton]

The upper one is the point where the both sources join. And yes, it is really bad, can you offer any suggestions?

BTW, I removed the output inductors, and that improved the drain voltages I was getting quite a bit.

 

[MicrosiM]

The upper one is the point where the both sources join. And yes, it is really bad, can you offer any suggestions?

BTW, I removed the output inductors, and that improved the drain voltages I was getting quite a bit.

Please show the driver circuit in full

 

[MicrosiM]

Any plans to show the schematic?

Or things got solved?


Regards

 

[zappyton]

Sorry about the delay!

I don't actually have a schematic in any kind of presentable form (it's on my to do list!), but I managed to find a solution. I had the squares correct to the gates, which is why thought the drive circuit was OK, but someone suggested that it might be some kind of interference between the two coils of the primary. The actual solution in the end was I reversed the polarity of one of the primaries, and then the circuit just worked. I wasn't sure which way the two primaries should be connected, whether in phase or anti phase, and to be honest, I'm still not sure which way they are connected. But before was obviously the wrong way, and now it's the right way, so hurray!