DI-124 won't start under load

Posted by: fyxemiqad on 07/23/2013

My power supply won't start under load, it stays in auto restart mode. I'm using the design in DI-124, but changed the secondary voltage to 24V instead of 12V. I believe the problem is the supply isn't regulating within 50mS after power up because of the higher output voltage. AN-37 suggests to add capacitor across the feedback resistor to enable a soft start. However, DI-124 uses an optocoupler in the feedback loop. How does one implement a softstart in this case?

Comments

Hi,

Thanks for contacting PI, if you want to add soft starter circuit for the feedback with optocoupler, you can refer to the reference design from our TOP switch.

http://www.powerint.com/sites/default/files/PDFFiles/rdr142.pdf

You could apply the same soft starter circuits when you use LNK switch TN. Hope this will be helpful for you.

That helps, thank you. I did some more debugging and found that the 24V output capacitor only makes it to 20V in the 50 ms period it is allowed otherwise the switcher shuts off and tries again in 800ms. I reduced the output capacitor from 470uf to 220uf to increase the rise time and that solved the problem. However, this will increase the ripple.

Now that I have a solution, I am unclear how a soft start will solve the problem as was suggested in AN-37. Won't a soft start make it worse?

Hi,

I am glad that you solved the issue.

You are right, the soft start capacitor will make the start time longer, that is why we called it soft start. Actually this cap also create a path that inject current to the FB pin. With this current, the switching of the mosfet will be enabled in 50ms, even the whole start time will be longer compared to the situation without this capacitor.

Hopeful this is explain your concern.