ringing of forward converter

[franticET]

Hi i have a problem with this circuit:



on output of the first transformer (OUT1) i measure an important ringing in corrispondence of the closing of the mos as in figure.



The problem is that i want to measure a DC voltage of 4 Volts (result of transformer ratio) where i draw a red arrow (OUT4), but i measure the voltage given by the peaks of the square wave.

The switch is controlled by microcontroller: when open through an A/D tre micro reads 0 of pull-down 10k resistor, when closed 7.5 Volts.

I tried to solve the problem putting a SNUBBER circuit on primary of both transformers but it doesn't delete the disturbance.

How can I do in order to avoid this problem?

 

[Mike odom]

 

[franticET]

--es1d--

I've just tried with snubber on primary (it didn't solve the problem), but i've not try with only a diode. Which kind of diode you think is the better choice?

 

[Mike odom]

1n4007 (1n4004, 1n4005, 1n4006)

 

[franticET]

1n4007 (1n4004, 1n4005, 1n4006)

I try your advise but it doesn't solve the problem....

Tha waveform on the Drain of the mos, so on the primary winding of the transformer, is quite good and doesn't show any oscillation.
So i put the probe on the cathode of the diode of recovery winding and there is a little oscillation that becomes bigger on the secondary.

I try to make some simulations and i noticed that i can replicate this behaviour if i put a capacitor (200-300pF) in parallel with this diode....

Any idea? PCB? Thanks