Synchronous rectification for SMPS theory

Discussion in 'General Electronics Chat' started by #12, Oct 30, 2013.

  1. #12

    Thread Starter Expert

    Nov 30, 2010
    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
    Last edited: Oct 30, 2013
  2. BobTPH

    Active Member

    Jun 5, 2013
    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.

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  3. wayneh


    Sep 9, 2010
    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.
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  4. #12

    Thread Starter Expert

    Nov 30, 2010
    Thank you. That's enough to answer my question.
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
    Another use of synchronous rectification is in high end (expensive) motor control.
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    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;

    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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