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I m Design smps using TOP257yn.op are Sec1 57v/1.4A & Sec2 15v/0.1A (Sec2 are currently un use
my problem is that when smps on with out load gives 57v out put check contineous 8 hrs. but when connect the full load,1/2 load & 1/4 load,after 2 or 3 mintes smps shut down automatically
all components are use as per PI EXPERT7.1 Results pls give solution urgently

It would appear that the part is going into thermal shutdown. To confirm this, monitor the source pin temperature using a T type thermocouple. If the source pin reaches approximately 120-130 degrees C and then the part shuts down you have hit thermal shutdown.

You must increase the heatsink size to correct this.

Hope this helps.


Sorry for delay to replying.Temp of tab of TOP257yn approx 80 degree with 1/4th load .my Alluminium plate size are(L X WX T) 300x200x10mm.& we are mounting TOP257yn ic (fitting) on the alluminium body.i observe that pin 1 of tl431c have is adjust to 2.4v , Here attach the schematic of my circuit.
in sch when R41 10k vref=2.24v & vout-53v,8.3k vref=2.3v & vout-51v,7k vref-2,4v & vout-49v


The heatsink dimensions sound reasonable for the power. Try running a fan over the heatsink and see if that enables the supply to operate continuously at full load. This will confirm that you need an even larger or more effective extruded aluminum heatsink.

Experiment with the heatsink size by first choosing a heatsink that is oversized and easily allows the supply to operate continuously under full load, at low line and high line, at the highest ambient temperature necessary. Then begin to reduce the heatsink size until the source pin is at about 100 C under worst case line, load and ambient temperature conditions.

I hope this helps!


Ok i will In previeous comment i attach schematic of smps . i have a question is that Top 257yn are shutdown if Regulation circuit are not working properly.TL431 reg ic Vref pin have 2.3-2.4v on without load .Tl431 Vref pin working range are 2.45-2.55 so Pls check that.

It is possible that the supply will go into auto-restart if it loses regulation. However this would occur quickly and not over time. I would be sure you are measuring the reference pin voltage carefully and your meter is properly calibrated.

It may be possible that you are above the TL431's maximum Anode-cathod voltage specification. Place a low test current (5 mA) zener diode with a 25 V or higher rating. This may help things.