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TOP246YN replacement with TOP256YN?

Posted by: samsam on 07/23/2022

I have one old power board with failed TOP246YN. Contacting manufacturer of the board I found that failing of this element is the most common issue with those of their boards and because currently TOP246YN is out of stock at my distributors, plus is marked as "Not for new designs" (all GX family), so I am wondering if there is better substitution and from my research TOP256YN is the best fit, but not direct replacement:

- first 3 pins V,X,C have to be re-bend/bridged in order to mimic the TOP246YN pinouts and fit on the existing PCB, line sensing/voltage detect resistor should be changed from 2 to 4 MOhms.

Is there any other change needed that I am missing or any obstacles that make TOP256YN  impossible or worse substitution than original old failing TOP246YN?





Submitted by PI-Wrench on 07/28/2022

What criteria did you use to select  the TOP256YN? Was this based on the output power requirement?  Also make sure which operating frequency is selected  for the old TOP246YN (66 or 132 kHz). This will make a great deal of difference for the operation of the supply - you should use the same operating frequency that the old TOPSwitch used.

Also, the resistor in series with the TL431  and the optocoupler in the secondary circuit will likely need to be changed to a higher value.

Submitted by PI-Wrench on 08/01/2022

One  thing I forgot to mention is that you should use the same value of Xpin resistor for the TOP265 design that was used for the TOP246. 

Submitted by samsam on 08/01/2022

Hi Pi-Wrench,

Highly appreciate your comments:.

In original design X,F and S are tight together so frequency is 132 kHz and no resistor on X. (But I was considering to add resistor on X if eventually use TOP247YN or TOP257YN). So, to answer your your question what criteria I used to select TOP256YN - I was looking which one is the closest safe substitution with better performance of TOP246YN and minimum changes that I could DIY with regular tools, but will appreciate if you hint me for better solution.


Submitted by PI-Wrench on 08/02/2022

Since X pin is not utilized, you will need to choose a part that has a peak current limit close  to that of the TOP246.  If you choose a device with a substantially higher current limit, you may run the risk of saturating the transformer at startup, a situation not conducive to the health of the supply. .