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