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Old 30th August 2007, 03:12 PM   #1
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Join Date: Jun 2006
Default Vds oscillation at low duty cycle

Hello,

I have been busy with my medium current smps(24 Volts-20Amps) for the last few weeks. I made some progress but still having problems with the output regulation.

I decided to use push-pull topology and finally I got 240 Watts output with paralleled two IRF 150N. When the PWM is almost 49% the mosfet drain source voltage looks OK. But if I lower the duty cycle below than that of 49% the drain source voltage oscillates. These time interval corresponds to where the both outputs of the double ended drive (UC2825) are off due to regulation. When the outputs are fully on (i.e close to 49% duty cycle) it looks OK.

The gate drive signals look OK too. There is no ringing in the gate drive circuit.What can cause to these oscillations?
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Old 31st August 2007, 06:15 AM   #2
mzzj is offline mzzj  Finland
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Location: 65N 25E
Default Re: Vds oscillation at low duty cycle

Quote:
Originally posted by ml342001
Hello,

I have been busy with my medium current smps(24 Volts-20Amps) for the last few weeks. I made some progress but still having problems with the output regulation.

I decided to use push-pull topology and finally I got 240 Watts output with paralleled two IRF 150N. When the PWM is almost 49% the mosfet drain source voltage looks OK. But if I lower the duty cycle below than that of 49% the drain source voltage oscillates. These time interval corresponds to where the both outputs of the double ended drive (UC2825) are off due to regulation. When the outputs are fully on (i.e close to 49% duty cycle) it looks OK.

The gate drive signals look OK too. There is no ringing in the gate drive circuit.What can cause to these oscillations?
Cause is LC-oscillator, L is the leakage inductance of your transformer and C is parasitic capacitance of mosfets/winding. You can reduce that oscillation (often called as a ringing) with snubber.
Check out this: http://focus.ti.com/lit/an/slup100/slup100.pdf

BTW, have you find that russian girl to be the penpal?
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