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snubber heat problem in smps circuit using uc3843B

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Mithun_K_Das

Mithun_K_Das

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I have made a smps power supply as per the uc3843B datasheet indicates. the circuit is similar as the image given below:
yy.jpg

The main problem is the snubber resistor (R5). I tried with 3K3/2W,5K6/2W,47K/2W,56K/2W,100K/2W,220K/2W,270K/2W. but all of them shows smoke within a few seconds. what is the problem and what is the solution here? please suggest.

C10=102/2KV
 

If MOSFET is off, transformer current is flowing from R5, right? Adjust MOSFET and transformer on timing.

---------- Post added at 12:32 ---------- Previous post was at 12:29 ----------

Why don't you reduce R5 value to about 2k-ohm
 

Why don't you reduce R5 value to about 2k-ohm
Click to expand...
A good way to "smoke" more resistors. Did you notice, that R5 is connected in parallel to the primary winding?

It's a RCD clamp "snubber" to limit the swicth-off overvoltage caused by the leak inductance. The charge is primary absorbed by the capacitor, the resistor must however be able to discharge it during transistor on-time. In a special circuit variant, a slow diode is used to recover part of the capacitor energy.

A usual dimensioning is several nF and 50 to 200 k, see e.g. Powerint data sheets. In this range, resistor overload shouldn't be an issue. If a 2W, 220 k resistor burns in your circuit, there's most likely something fundamentally wrong with it.
 

A good way to "smoke" more resistors. Did you notice, that R5 is connected in parallel to the primary winding?
Click to expand...
I apologize if I suggested wrong. I thought in the simple way as current passing through Diode and R5 will increase linearly, and power loss across R5 will be I^2*R5. That made me idea of reducing R5 and hence power loss across resistor.

But now I realize your point, as my suggestion could be a big mistake.
 

This shouldn't happen, if the voltage regulation is working correctly. Actually, the energy stored in leakage inductance should be considerably lower with no load, because the peak current is also lower. There's something wrong with your circuit, I guess. May be windings polarity or some other detail.
 
I think must replace 1N4007 diodes with some faster models.
 

The original circuit uses UF4007, which is a fast rectifier. Slow rectifier diodes in this place may however serve a purpose, as already mentioned. In any case, excessive resistor losses aren't related to this point.
 

I think these diodes are too slow. Yes, slowly diodes are usefull in sucjh snubber but not too slow...
 

I managed that problem solved. The problem was in wire gauge used for primary side, I used larger than required first. That's why that snubbers became heated. when I replaced with less thick wire in primary than before it is solved.
 
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