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Thermal Shutdown TOP259EN-TOP261EN

Posted by: Tugay on Wed, 08/25/2021

Hi frends,

I have designed 32V 7A SMPS.I designed using PIExpert. I wrapped the transformer as it was said in PIExpert.Output values are correct, i see 32 volts.

PROBLEM: Top259EN-Top261EN turns off after running for a few seconds. This shutdown is most likely caused by thermal shutdown

QUESTION 1: What should I do for the circuit to work normally without going into thermal shutdown?

I installed a fairly large heatsink but it only increased the runtime by 1-2 minutes. As the load connected to the output increases, the closing time becomes shorter.

QUESTION 2: Vds voltage is 640-650V, sometimes even over 700V. It changes according to the load connected to the output. How can I lower the maximum vds voltage?

I tried making changes to the snubber circuit but top259EN-top261EN exploded. When I checked the circuit after it exploded, I saw that the tvszener diode (P6KE350A) was burned.

QUESTION 3: Is it 640-650V, 700-750V vds_max voltage high for circuit or normaly?

I attached the all needed documents. Real Schematic, Waveforms, Pıexpert project, transformer test results..

In the real schematic design, the values of some components were chosen different from the values that PIExpert said, since we do not have the same value component.

Transformer primary inductance: 250uH 

Transformer leakage inductance: 5uH 


Attachment Size
PIExpert File 1.26 MB
Real Schematic Design 276.61 KB
Vds Waveform1 285.65 KB
Vds Waveform2 252.51 KB


Submitted by PI-csbabu on Sat, 08/28/2021

Hi Tugay,

      I didnt find any attachments added to this post. Please share Pixpert sheet, layout, schematic and transformer details to analyze the problem. All the problems listed above related to transformer design and layout.



Submitted by PI-csbabu on Fri, 09/03/2021

Hi Tugay,

       I am able to check all the attached files. The design looks good and this is high power design using flyback, I recommend to check layout carefully. May I know the leakage inductance of built transformer. The higher spike is due to leakage inductance of the transformer and try to reduce high frequency loop area. Please follow layout guidelines in the attached file. If you need further help, please share layout file.   

Attachment Size
an14_TOpSwitch Tips _Techniques.pdf 358.61 KB
Submitted by Tugay on Tue, 09/07/2021

Hi PI-csbabu,

Thank you for reply. 

Pi expert says that the leakage inductance should be 3.75uh, but I was able to reduce the leakage inductance to between 5 - 7 uH.

The transformer values I am using now are as follows

Transformer primary inductance:  250uH

Transformer leakage inductance: 5uH