Forward Converter problems ( voltage form and peak voltage)

[lowrainer]

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)

 

[xaccto]

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

 

[lowrainer]

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!

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

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?

 

[rvionics]

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)

 

[lowrainer]

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