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Mosfet won't mirror the signal from the PWM controller in a flyback SMPS

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Aether32

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Greetings,

I am working on a flyback smps.

Currently I am having a problem with the mosfet switching, the mosfet itself won't mirror the drive
signal from the PWM controller, only a distorted pulse appear. I am sure the PWM controller is able
to drive the mosfet without a problem as the input capacitance of the mosfet is max. 750 pF and
the allowed capacitance for the PWM controller is 1 nF.

Input from mains: 230~ V AC -> 325~ DC
Switching frequency: 450 KHz
output voltage: 19 V
output current max.: 4.74 A

parts:
PWM controller: UC3842
Mosfet: 2SK2700

The PWM controller receives power from an external power supply.

Circuit schematic and signal captures from the circuit are attached.


Any pointers are welcome
:D
 

Re: SMPS mosfet problem

The zero mains voltage waveform is more or less meaningless, but it doesn't seem wrong anyhow. The 10V AC waveform shows the
output transistor in operation. You must consider, that you won't see a squarewave at this high operation frequency
rather than soft, resonant switching. Most likely the waveform shows regular behaviour.
 

    Aether32

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Re: SMPS mosfet problem

FvM said:
The zero mains voltage waveform is more or less meaningless, but it doesn't seem wrong anyhow. The 10V AC waveform shows the
output transistor in operation. You must consider, that you won't see a squarewave at this high operation frequency
rather than soft, resonant switching. Most likely the waveform shows regular behaviour.

Yeah, I guess the "distorted" wave at 0 V AC doesn't tell much, but check the huge spike over the transistor @10 V AC, it's like 50 V, shouldn't be so high, it looks like the transistor is operating wrong and the spike from the 0 V screen gets amplified.
The voltage between drain and source shouldn't be higher than 10 V * sqrt(2) (+ possible spikes).

I've tried several transistors now, still this strange issue, the GS voltage and everything seems to be correct...

Offtopic: I think this topic got posted in the wrong category, please move it to "analog design" -> "power electronics"
 

Re: SMPS mosfet problem

The voltage between drain and source shouldn't be higher than 10 V * sqrt(2) (+ possible spikes).
The operation principle of the SMPS is flyback converter, not simple rectifier. I think, the discussion isn't actually wrong among Electronic Elementary Questions.
 

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