reverse current during load removal - synch buck converter

hi,

I have read the article "Reverse Recovery Phenomenon in Synchronous Rectifiers".

https://www.google.co.in/url?sa=t&r...8MyKAw&usg=AFQjCNEuenns2K5XzbOBdxuWuBEDz8zpYQ


When the load is fully removed from 100% to no load, the inductor current can go to a very large negative value.

This large reverse current will not cause any issue if dc dc converter is of NON isolated synchronous current mode converter.

But it will affect ISOLATED synchronous current mode converter.

- Please refer paragraph 4 on page number 6 of the article.


1. Why does it affect isolated converter but not the Non isolated converter (though both have top MOSFET and down MOSFET)? -Please clarify.

Well he states his reasoning clearly once:

During turn off at no-load or light-load condition, when the inductor current is negative, if both MOSFETs are turned off, this negative current can only find a way through the body diode of Qrect to the input dc source. In an isolated converter, this produces voltage stress on the MOSFET as this current cannot find an immediate path for the current to continue to flow. The result is higher voltage stress on the devices. This reverse current eventually charges up the MOSFET output capacitance and then this reverse current can get reflected to primary side and can cause voltage stress on the primary-side MOSFET, eventually leading to its failure.

Click to expand...

And I agree with him up to a point. If IL is negative before the synchronous FET turns off, that negative inductor current will have nowhere to go. But the author says that it will conduct through Qrect's body diode, but this can't be right, since Qrects body diode will be reversed biased by negative IL. So they only way for that current to affect the primary side of things is for that diode to reverse breakdown. And at that point you've already got a big problem...

So yes I can see why negative current is bad in a synchronous isolated converter, but I don't think the problem is exactly where the author says it is. It should put voltage stress on the secondary side, not the primary side.

hi,

Thanks for your assistance.

Please clarify the effect of reverse current on Non isolated synchronous buck converter. It has both the MOSFETS. The problems faced by isolated converter should be faced by non isolated converter also. Am I right?

balamanikandan said:

The problems faced by isolated converter should be faced by non isolated converter also. Am I right?

Click to expand...

No, because in a nonisolated buck converter the rectifying FET will have its body diode pointed in the opposite direction, allowing the reverse current to flow back to the DC source even when the FET is off.