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Synchronous buck converter

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usernam

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How exactly does a buck converter work. I've read the previous posts in the forum but I still can't figure out how a mosfet in parallel with the diode reduces losses.
Its only in reverse current flow that the mosfet comes into play. For current that is positive (i.e similar in direction to a normal buck) the diode conducts and not the low side switch so how does it reduce losses?
 

Actually, the MOSFET can conduct current in both directions. That is how it reduces losses. It is turned on shortly after the other MOSFET is turned off.
And in a synchronous buck the lower MOSFET only conducts current "in reverse".
 

VVV said:
Actually, the MOSFET can conduct current in both directions. That is how it reduces losses. It is turned on shortly after the other MOSFET is turned off.
And in a synchronous buck the lower MOSFET only conducts current "in reverse".

The MOSFET conducts current in both directions??? How is that achieved?
From what I remember of MOSFETs if its an nMOS, current goes from drain to source and in a pMOS, current goes from source drain. Maybe there is a slight reverse leakage current but that's hardly significant.

In the lower MOSFET when current is "in reverse" i.e as it would be in a normal buck, the MOSFET's body diode does the conduction. So in this case we are back to the same old buck topology with attendant losses and lower efficiency.
 

The MOSFET conducts in both directions and exhibits the same low on resistance between source and drain. That is where the savings come from. Instead of seeing the on voltage drop of a diode, you see the low Rdson voltage drop of a MOSFET.

As to how the MOSFET conducts in both directions, look at the internal MOSFET structure and you will see that there is nothing special about conduction "in reverse", since there is only one channel between the source and the gate. This channel is of one type of semiconductor only, either P or N, at the ends of which you have semoconductors of the opposite polarity. The point is it can conduct current in either direction, if the gate voltage is such that the channel is open, shorting out its own body diode.

For more on that see this appnote, especially page 3. www.onsemi.com/pub/Collateral/AN1520-D.PDF
 

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