Use of Top249yN

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Hello sir,

Currently I am using Top249yN in my design to have a various out out of 20v, 15v, -15v and 5v respectively. But the problem I am facing right now is that whenever I am switching ON the power supply it gets burn out the TOP SWITCH. 

I am attaching my design lay out with this mail. 

Would you mind providing me the more detail regarding the transformer design please. As of now these are the primary and secondary voltage in my circuit.

P1-110v   S(12-11) - 20v

             S(9-8-10) - +15v and -15v

             S(6-7) - 5v

             S(4-5) - 15v (This is the more confusing part to me bcz the control switch is driven by the MOC8102 optocoupler)

             S(14-15) - 5v

Please have a look on my design and rectify it if needed. Waiting for yours positive reponse. 

Custom DCDC.pdf31.85 KB

Hi rnzn1712

Thank you for your interest of using Power Integrations' part.

TopGX family which is you are currently using is not recommended anymore for new design.  TopJX is the new recommended replacement for the  same application with ehnanced performance.  Based on your schematic, I can't find any problem that can cause the Topswitch to burn.  I just have some recommendation, put cap across C pin and ground for decoupling.  On Lpin you can remove D2010, R2026 and R2029 then adjust R2030 and R2034 accordingly for your desired line UV and OV protection.  The only thing that can possibly cause your topswitch to burn is when your transformer is saturating.  Can your share your spreadsheet design?  Can you also capture your Drain current for us to check if it is saturating.  Thanks.

Dear Sir,

Thank you for your kind response. I had already tried with the capacitor with control pin C of 0.1 ufd but it’s all gone in vain.

Here I am attaching my spreadsheet with this for yours reference. We had used EI40 core transformer with 26 AWG insulated CU wire for winding of it. However we had not given any parallel turns of any kind (like 2 wires / 3wire) as it can reduce the transformer winding area. Is it the reason for transformer saturation??


Our power requirements are very small as we need only 1.5 amp current across the entire output voltage source. 

Hi rnzn1712

Based on your calculation, I think it is ok and will not saturate.  How many turns is your bias winding (pin4 to pin5).  You also have 2 47uF/450V input caps in series.  You don't need to put it in series since 450V is enough for you 220Vac max input.  We actually need higher capacitance so we need to have it in parallel instead.  If it is not your concern it will just lower your power factor.  Thanks.


Again I recommand to put blocking zener diode in parallel with RC clamp circuit. I always prefer to use blocking diode even when there is RC clamp.

best regards,

Hi Allahyar


If you still want to put the blocking diode (D2010, R2026, R2029) for protection, the end of R2029 should be connected to pin7 instead of pin2 for additional MOSFET protection.  Pin 2 connection is just basically clamping across R2030 and R2034.  Thanks you and best regards.