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TOP264EG randomly gets off.

Posted by: niteens1 on Fri, 08/06/2021


We use TOPSWITCH-JX (TOP265EG & TOP264EG) about 2 years.

Now days I have  problem. I have two TOPswitch in one SMPS (TOP265EG & TOP264EG in single design but different outputs).

TOP265EG= 12V@1.5A 

TOP264EG= 5V@0.500A & 5V@1.5A

Same input line supply but only TOP264 gets off in SMPS.

After Off & On; SMPS works fine. after that no problem founds.

Problem not found in all SMPS but it happens in random SMPS at full load.

Please help if anything you suggest.

Is this fault of false triggering at V pin?

Can i add Output over-voltage shutdown circuit.i.e. 5k1 resistor & 1N4148 diode between V pin & bias winding o/p.

Also i check over-voltage condition upto 390VAC. No problem found.


Please help if anything you suggest.


Submitted by PI-csbabu on Fri, 08/06/2021

Hi ,

Is it running for some time and shutting down? How much load on the converter during shutdown? Please share schematics. Thank you.

Submitted by niteens1 on Sun, 08/08/2021

Yes, it runs for some time & shuts down but after off & on SMPS no issue found.

I run test for 4-5 days no issue found.


As mentioned in above comment (TOP264EG= 5V@0.500A & 5V@1.5A) load is not greater than this.

Also i tested using over loading but no issue found.

attached schematic.

Sheet 1 is for reference; it contains input AC supply circuit & TOP265EG which has no issue. And sheet 2 contains TOP264EG

which make trouble to us.

Attachment Size
TOP265EG 65.88 KB
TOP264EG 60.94 KB
Submitted by niteens1 on Sun, 08/08/2021

Is this fault of false triggering at V pin?

Submitted by PI-csbabu on Mon, 08/09/2021


  I didn't find any issue on the schematics. Do you have waveform during shutdown event. If possible try to capture VDS and comp pin voltages during shutdown.


Submitted by niteens1 on Tue, 08/10/2021

Sorry but we can't capture waveform because its random problem and not occurs after resetting AC (i.e. OFF & ON SMPS).

We tested same SMPS after problem found with my unit for more than 400 cycles but no problem found. 

Submitted by PI-csbabu on Wed, 08/11/2021


  Based on problem description, unit was latching occasionally and need to restart the unit to deliver the power. Unit latch happens only with output over voltage, when primary bias voltage fed to Vpin through zenar diode and resistor connected in series . This can be ruled out , since there is no output over voltage protection circuit. 

   Only other way is due to noise on Vpin as you mentioned and  Vpin current is above latch current limit for more than 100us. The noise triggering can happen due to line surges and poor layout.

In the schematic, I didn't see any varistor to protect against line surges. Add a varistor at input and test unit.

The line sense resistors (R13 &R15) to be placed near to the Vpin of TOpswitch. Please share the layout for verification.  One more question, in the schematic Mosfet_S node connected to bulk capacitor (C8) return or not?  


Submitted by niteens1 on Wed, 08/11/2021

Thanks for reply.

I already figured it out & initiated to change layout. layout is not proper. R13 & R15 are not close to V pin.

also V pin trace is close proximity to High DC line.

And in other section resistors are close to V pin &  V pin trace is not close proximity to High DC line so that section has no problem.

Also varistor is already added externally.

Yes;  Mosfet_S node connected to bulk capacitor (C8) return but through external over voltage circuit.

Thanks for support.