Forward Converter problems ( voltage form and peak voltage)

Hello,

I designed a forward converter for university needs. Now I am testing the converter and I have some difficulties in seeing where my problem comes from.
First of all.

Uin=12V Uout=100V Iout=1A
tr=128/5
UC3845 atm no control, it is running at full 50% duty cycle
L=approx 5,8mH
Lprim= 100mH ΔI=0.25A


The problem is:
After the Mosfet turned of, the voltage across the Mosfett should be 24V until the demagnitizing winding is demagnitized. Then the voltage across should be 12V. My measurement display a little simimlarity. First there is a voltage peak (which i tried to get rid of with a snubber...omg it is not working correctly), then the voltage breaks down and comes back to 12V. I don't understand why the voltage breaks down after the peak. it should stay around 24V.



Caused by the peak and "oscillation" my Mosfets gets thermally destroyed after a while. My snubber consists out of a fast rectifier diod in Series with a capacitor 0,47µF and a 5 parallel 82Ω resistors. The peak is about 52V high, therefore ΔU=28V which should be absorbed by the Snubber.

Can anybody help me with:

1. Why is the voltage breaking down after turning of the Mosfet?
2. What do I have to do that my Snubber is working correct?

Cheers

On the image you see 1. the clock (50kHz), 2. Umosfet (voltage across mosfet), 3. I1 ( current through primary winding rising up to around 28A)

The 52V peak is being limited at that by the Vds breakdown voltage of the MOSFET perhaps ? i've seen exactly that in my SMPS experiments too....
And the current then rings a bit.
Leakage inductance is the cause maybe?
Try decreasing the air-gap in the core.

To me it looks like the snubber is doing its job, but there is a efficiency decision to be made, the better the snubber works, the more it dissipated => lost energy => heat.
Have you tried a reset winding (instead/in addition?) ?

I'm not sure why you expecting the voltage across the mosfet to be 24V ?
your spec is saying Uin=12V

QUOTE=xaccto;784564]
The 52V peak is being limited at that by the Vds breakdown voltage of the MOSFET perhaps ?
[/QUOTE]
I am using the IRFB4110 which has a breakdown voltage of 100V!

xaccto said:

I'm not sure why you expecting the voltage across the mosfet to be 24V ?
your spec is saying Uin=12V

Click to expand...

During the turn off phase of a forward converter, the voltage across the Transistor is 2xUin, as the voltage of the demagnetizing winding and the voltage of the primary winding are added. So I am expecting directly 24V after the voltage peak. On the image it is hard to see, but after a while the voltage across T is 24V. But I think it should be 24V directly.


Thanks for helping me, but still the question is how to do the Snubber correctly?

From your snubber values it looks your snubber is very lossy & is eating up all of the Dmag current which is the reason why you haven't seen the 24V, Try reducing your snubber value to 10nF~47nF, 0.47uF looks too big for an switching frequency over 20Khz(I assume the converter is runing at more than 20Khz)

switching frequency is 50kHz. I will try and see if reducing the values to 10-47nF will help. thanks