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I'm trying to understand how synchronous rectification works.

Here is an example in boost topology:

enter image description here

The function of D1 is to allow current from left to right and block right to left.

M3 is a PMOSFET which conducts from source to drain when the gate is driven on. The conduction direction is from right to left which is the opposite to the diode.

From what I understand, the MOSFET has body diode that conducts in reverse of the channel direction, and the body diode cannot be turned off. The body diode has much worse performance than a power Schottky diode in terms of forward voltage and recovery time.

Does synchronous rectification just use the body diode of M3 to replace the D1? If so, why bother driving the gate of M3 since the body diode cannot be controlled by the gate anyway?

When M3 is driven on, the channel allows current flow from right to left, which is undesirable, so again why bother driving the gate of M3?

The whole point of synchronous rectification is to avoid forward voltage. I don't see how that could happen when the MOSFET conduction direction is in the opposite direction of the rectifier.

Answer: Misconception about MOSFET

False: MOSFET conducts only in the charge carrier direction (S->D for P cahnnel, D->S for N channel) 
True: MOSFET conducts in both directions, and behave like a resistor with R = Rds(on) when fully driven.
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Why MOSFET are connected in reverse for synchronous rectification?

MOSFETs have an internal bulk diode that is usually wired to the source and, it makes the synchronous boost circuit become this: -

enter image description here

It's pretty much the same as your asynchronous boost circuit but, the diode can be "shorted" out by the p-channel MOSFET and yes, MOSFETs do conduct in reverse when activated by the appropriate gate-source voltage. 4 quadrant MOSFET operation: -

enter image description here

And, of course, it's quite often an n-channel MOSFET that is used for both devices: -

enter image description here

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  • \$\begingroup\$ So the body diode only behave like a diode when the gate is fully closed? When gate is open the body diode behave like a resistor? \$\endgroup\$
    – 7E10FC9A
    Commented Jan 9, 2023 at 18:46
  • \$\begingroup\$ I don't know what you mean by "fully-closed" in this context. Ditto "When gate is open" @7E10FC9A. The gate is either activated with sufficient voltage with respect to the source to obtain triode conduction (i.e. the channel's "on" or fully conducting) or it's not activated i.e. has 0 volts or a negative voltage with respect to source. \$\endgroup\$
    – Andy aka
    Commented Jan 9, 2023 at 18:48
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    \$\begingroup\$ @7E10FC9A The body diode always behaves like a diode, it is a diode. When the FET is conducting, it doesn't matter how the body diode behaves, as it's shorted. When the FET is not conducting, then you can see the diode behaviour (doesn't conduct in one direction, conducts with a forward drop in the other). \$\endgroup\$
    – Neil_UK
    Commented Jan 9, 2023 at 18:49
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    \$\begingroup\$ so my misconception is that MOSFET conducts only in the charge carrier direction (S->D for PMOSFET, D->S for NMOSFET) instead of both directions? \$\endgroup\$
    – 7E10FC9A
    Commented Jan 9, 2023 at 18:56
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    \$\begingroup\$ To the OP: If "open" and "closed" are confusing, I recommend you say "on" and "off." For us, "open" means no circuit (off) and "closed" means there is a circuit (on). \$\endgroup\$
    – user57037
    Commented Jan 9, 2023 at 19:04

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