Problem with Clamp circuit in top250y design

Posted by: thangaraju on 06/18/2014

I have desiged 230VAC to 24V dc 80W using top250Y. Everything is good expect the clamp circuit getting too hot (Zener Diode, Resistor).as per design VOR around 180V, but it measures around 240V. So my clamp circuit getting failure. Please find the schematic in the attachement. Max Drain voltage at 230V ac is 520V DC. Max peak inverse voltage across output diode is 58V dc. Leakgae inductance is in the limit. Please suggest next step want to proceed?

Files

TOPSwitch-GX_PIDesign1 24v 100w.uds (277 KB)

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I do not understand why you said your VOR=180. I see in the spreadsheet it is 135.

NS=5, NP=27, VOUT=24, Drop in output diode = 1 V

So VOR = 25*27/5 =135

Am I right?

Please check your transformer construction. If you do not have the same VOR as the spreadsheet (when you have 24VOUT), then it means the transformer is not properly built. If you like , please post your DRAIn voltage and DRAIN current waveforms for me to take a look

Sorry for the wrong data given to you. it is right VOR is 135V. for your reference find the attachement of drain waveform. Actually yesterday i tried with 66KHz i found clamp zener getting less hot but for 132KHZ i got extreme level of heating in the clamp circuit. i go through tha AN14 notes and made the remedies given for clamp zener hot but i did not get the result. Do the best.

If your VOR is correct, then you have to check your transformer construction. In particular you need to check the transformer leakage inductance. The energy transferred to the DRAIN snubber is given by the formula W=(1/2)*(L_leakage)*(Ipeak^2). The power dissipated in the snubber will be P= F(switching Frequency) * W. When you set up your controller to run at half switching frequency you see the snubber is cooler.

Check your DRAIN current to be in specifications as calculated by PI Expert.

If your transformer construction is correct, the DRAIN current is a s calculated by the progam, and the leakage inductance is in range, then you will need to adjust the snubber for proper temperature

I see your schematic D1 is a fast recover diode. In my opinion is not a good choice for such power level Try to change it to a UF4007. If it is still hot , then use a 3 amps ultra fast rectifier.