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Why are half bridge circuits (2 MOSFETs in a buck converter for example) often referred to as synchronous rectifiers?

I understand they are called synchronous because the two FETs have to be driven in sync to avoid shoot-through. But why rectification? What's the AC to DC operation here?

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    \$\begingroup\$ Because the diode has been replaced with a MOSFET to allow said synchronous rectification. A standard high side MOSFET and low side diode will always be asynchronous. \$\endgroup\$
    – winny
    Aug 28, 2021 at 20:34

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The entire half-bridge configuration is not called a synchronous rectifier- the low-side MOSFET (which replaces a diode) is the synchronous rectifier. The gate of that MOSFET must be driven synchronously for it to work.

There are other kinds of synchronous rectifiers, for example this full-wave mechanical synchronous rectifier (home-built at a time when high-voltage silicon rectifier stacks were prohibitively expensive):

enter image description here

Image from C.L. Stong , Scientific American Amateur Scientist. Created for a homebuilt HeNe laser used in DIY hologram experiments.

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    \$\begingroup\$ I very much enjoy seeing this kind of reminder. +1 \$\endgroup\$
    – jonk
    Aug 29, 2021 at 6:08
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    \$\begingroup\$ That thing looks totally insane! So it's basically arcing 9 kV through that moving 2 mm gap, and making and breaking the arc 120 times per second? That must look (and sound) crazy. Somebody (not me!) ought to build it and film it for YouTube. Heck, might as well build the laser too… \$\endgroup\$ Aug 29, 2021 at 17:16
  • \$\begingroup\$ Add Nitrogen flame extinguisher and whisky rotor cleaner \$\endgroup\$ Aug 30, 2021 at 18:02
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    \$\begingroup\$ TBF anyone with an old car has something similar bolted onto the engine- the 'distributor'- even with a fancy automatic mechanical/pneumatic phase adjustment ('spark advance'). \$\endgroup\$ Aug 30, 2021 at 20:10
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First off, I am not aware that half-bridge circuits are always called synchronous rectifiers.

The way I understand it, synchronous rectifiers replace the traditional diodes in a half-wave or full wave bridge configuration with transistors (usually MOSFETs nowadays). These transistors are turned on and off synchronously with respect to the incoming AC or chopped DC waveform. This requires additional control circuitry.

The advantage here lies in the fact that the traditional diodes need ~0.7V across them to begin conducting. This 0.7 V represents a switching loss. MOSFETS can conduct with a few mV across them. Therefore synchronous rectifiers built with MOSFETS are more efficient than diode bridges, particularly when generating low voltages like 3.3 V or 1.8 V that a lot of digital systems operate at.

Synchronous rectifiers can be used in a half wave, full wave, or bridge configuration just like traditional diodes.

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    \$\begingroup\$ I would not categorize the loss in a conventional diode rectifier (the one caused by forward voltage drop) nominally as a switching loss, but otherwise I heartily agree with your explanation, you have answered the bold question. You have my upvote :-) \$\endgroup\$
    – frr
    Aug 29, 2021 at 5:37
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The inductor becomes an AC source when driven this way. From the standpoint of the inductor, its polarity is reversing. If this isn't clear, have a look at illustrations in this answer.

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I understand they are called synchronous because the two FETs have to be driven in sync to avoid shoot-through. But why rectification? What's the AC to DC operation here?

Definition: convert (alternating current) to direct current.

There are different types of DCDC converters which are "synchronous converters"

  1. Externally clocked with PFM, PWM, missing pulse
  2. Self clocked ; Resonant and Quasi-resonant

When Diodes are use to Rectify, they do not need to be synchronously controlled. They conduct whenever a forward potential drop develops.

The design conversion from flyback diode or forward diode to switching transistor requires synchronization to rectify the AC into DC.

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