reducing big spike on collector in the switching power supply with snubber circuit

Thread Starter

hamedtahamtan

Joined Sep 22, 2017
5
hi
ich trying to make a dc-dc isolated switching power supply
i try to do it with a uc3843
i used a circuit in the datasheet

kfghj.png

i changed it in my way like the below picture
2.png

ths cicuit is working but ther is a problem
the drain voltage is lik below
WhatsApp Image 2020-07-22 at 13.21.17.jpeg

it has a big >100v spike and the snubber circuit that is in the picture ( i make a circute on it ) did not cut it
i need a good snubber desighn to reduce the spike
 

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Papabravo

Joined Feb 24, 2006
14,428
I guess the TS thinks he can adapt the design without understanding it. If it works at 115 VAC, why can't he just lower the AC voltage input and get the same result. Easy-peazy right! ah...NOT.
Oh and BTW, a MOSFET does not have a collector, just so you know.
 
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Thread Starter

hamedtahamtan

Joined Sep 22, 2017
5
Thank you for your remark

I was a little tired at the time of writing this text and made a few small mistakes
As I stated in the text, I mean the presence of a spike on the drain "not collector"( i cant change it now. admin please change the topic if possible "
as i mentioned i changed the desighn in my way and it is working properly . i undrestanding most of the desighn
the only problem that i need to solve it is the big spikes on Drain and so so on the input power
i think the problem is with the snubber circuit and im not undrestanding this part .
 

DickCappels

Joined Aug 21, 2008
6,593
Don't worry we all understand what you meant in your first post.

This is a flyback (also called boost) converter, and these need a load to work properly. Does this supply have a load on its output?
 

Thread Starter

hamedtahamtan

Joined Sep 22, 2017
5
on the output i used a 1k resistor
but what will happened when i have no output??
because some times i need to turn the device off while the power supply is on
 

DickCappels

Joined Aug 21, 2008
6,593
A lot of the energy stored in the magnetization of the core has no where to go but back out through the primary. That is why your turn-off spike is huge compared to what would be considered normal.

Try using a much smaller load resistor if the spike worries you. The energy has to somewhere its best if it goes to a load on the secondary.
 

Papabravo

Joined Feb 24, 2006
14,428
Do you want to preserve the energy in the spikes or are they of no interest? The reason I ask is because you can arrange to "dump" the energy where it won't affect anything else, or you can try to do something useful with it.
 
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