Synchronous rectification for SMPS theory

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#12

Joined Nov 30, 2010
18,224
Having picked up an idea in another thread, I ask: What is the purpose and method for synchronous rectification?

I suspect that the theory is to use a mosfet to replace a diode by using the proper switching time. This pretty much eliminates power loss in a rectifier diode.

Just outline the intent and a bit about how it is achieved.

I attach 2 data sheets to point you in the right direction.

Thank you,
Number Twelve
 

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BobTPH

Joined Jun 5, 2013
8,804
I suspect that the theory is to use a mosfet to replace a diode by using the proper switching time. This pretty much eliminates power loss in a rectifier diode.
That is the way I understand it. In a buck converter, for instance, you would normally use a diode to conduct during the off portion of the PWM cycle. Replacing it with a MOSFET that is turned on when the main switch is turned off gives you less losses, and possibly even faster switching.

Bob
 

wayneh

Joined Sep 9, 2010
17,496
That is the way I understand it.
Me too. I looked into it quite a bit when I was looking at wind power, i.e. rectifying low voltage AC waveforms with minimal power loss. The chip companies like IR make dedicated MOSFET drivers for this that include the comparator function that looks for voltage across the MOSFET's body diode. As soon as a voltage develops (in the positive direction), it turns on the MOSFET to bypass the body diode. Note that the arrangement intends current flow from source to drain in parallel with the body diode. Opposite the normal orientation.
 

THE_RB

Joined Feb 11, 2008
5,438
You can use synchronous recification from any push-pull type driver (like a hbridge or even a CMOS pin like a PIC output pin), heres one example using a PIC as a complete SMPS buck, which relies on the PIC pin having complimentary FETs in push-pull;
http://www.romanblack.com/smps/pic-smps.htm

Synchronous rectification also gives the ability for power to flow either way, ie a buck circuit that can simultaneously convert 24v to 12v and also convert 12v to 24v (like a battery matching SMPS).

That means it also works for regenerative motor braking, as the current can either power the DC motor from the battery or charge the battery from the spinning DC motor, the only difference is the PWM duty cycle.
 
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