SMPS Restart Problem

[Thivanka]

I have designed a 24v SMPS(switch mode power supply), but it is restarting what is the reason for that, I have used viper38hd IC.

Here I attached my schematic, ic datasheet, and transformer datasheet.

[Peter BENSCH]

Welcome @Thivanka, to the community!

Well, you use the transformer winding N4 to supply the VIPER38HD. You also connect N2 and N3 in series for the 24V output voltage. However, your transformer has the winding ratio N2:N3:N4 = 1:1:0.55, so if you connect N2 and N3 in series, you can assume 2:0.55 for (N2+N3):N4. In relation to 24V, this results in a VDD of approx. 24V*0.55/2=6.6V, which is definitely below the startup threshold.

Please switch from N4 to N5 as a test by connecting the transformer with pin 4 instead of pin 6 to D1/D6.

Regards
/Peter

[Thivanka]

Thank you Mr peter for replying, now it is working in no load condition it gives stable 24v output but when I connect it to the load voltage drops to nearly 8-volt why is it

[Peter BENSCH]

There can be many reasons for this, e.g. auto-restart, unintentional burst mode or overloading of the 24V. However, since you have not mentioned much apart from the 24V voltage at the output, it is not yet possible to assess this You have also not shown the layout, which also makes it difficult to assess.

What mains voltage is present and what output current do you allow to flow or what load resistor is connected?

[Thivanka]

Now the circuit is working properly, how do I set the current limit for the IC? i want to set 500mA over current limit.

[Peter BENSCH]

The current limit can be set via Rlim, which is also shown in the data sheet, table 9 and fig 13.

Note that the tolerance range of IDlim increases with decreasing RLIM, as can be seen from the MIN and MAX marks in fig 13. Rlim is therefore only given down to 10kohms, which corresponds to an IDlim of 60%, i.e. typ. 0.69A. In addition, IDlim depends on the junction temperature TJ (fig 6).

Hope that helps?

Good luck!
/Peter