Power limitation using ic power integration topswitch

4 posts / 0 new

hi every body,

So i find this circuit (flyback power supply 24 V 1 A) which use the ic top255yn from top-switch HX and the transformer MYRRA 74030 so i would to change it with the ic top245yn from top-switch GX,i know for that i need to change R3,R4 and R5 (R6 and R7 i know how to change them) so how i can calculate the values of R3,R4 and R5 to get the same features (Vout,Iout,Pmax).


FullSchema_1.gif82.19 KB

Hi hicham,

the resistors R3, R4, and R5 are used to control KI and peak drain currents for TOPSwitch. The curves for their selection are given on p39 of TOPSwitch-HX datasheet and p40 of TOPSwitch-GX datasheet. They are slightly different, so as you correctly said you will have to slightly change the resistors to get the same behavior.

I wasn't able to reproduce your design in PI Expert just by looking at the schematic, so I can't tell what settings your design uses.

First of all, I am assuming that you are looking for a drop-in replacement?

Do you have the original design in PI Expert? If so, would you mind attaching the design file?

If you don't have it, could you provide any target parameters for the desired controller operation (such as KI), so I can reproduce it?


thanks for your email, so l read the page but the curve is only for R5,so how we can get the value of R3 and R4.

you find attached the designed file


24-1.pdf2.06 MB
24-1_Copy00.uds1.12 MB

To adapt the design to TOP-GX, it looks like you would need to change R6 and R7 to 499k from 2M. Calculating these values is mentioned in the TOP-GX datasheet. The X-pin curves on GX and HX look identical, so R5 is the same for both designs. However, I think that the values shown in the schematic you provided are off by a factor of 10. 1.43M rather than 14.3M each for R3 and R4 would be far more appropriate values, and approximately in line with the values shown in the example circuits for both the GX and HX data sheets. Where did you get the original TOP-HX circuit?