Can any SMPS run bidirectionally?

Discussion in 'General Electronics Chat' started by ithinkican, Jun 22, 2013.

  1. ithinkican

    Thread Starter New Member

    Apr 6, 2013
    So -

    as i continue learning, i have come up with a question i can;t get answered easily via my own research.

    Can an SMPS, for example a flyback, run bidirectionally?

    I can see how a standard line frequency transformer will run in either direction just fine, so why can't a high frequency SMPS do the same?

    My feeling is that it can, and the surrounding circuit will be the difference.

    So, with a flyback example.... say we have a 50VDC to 250VDC flyback .....
    you switch the 50VDC side in order to get 250VDC on the secondary via the multiplication of the turns ratio.

    If you switched 250V on the secondary, and i'm sure the duty cycle would have to change by the turns ratio or something, would you get 50V on the primary?

    What got me started thinking on this was reading about synchronous rectification.... so if now we have a flyback with 50v on one side and 250v on the other, and controllable switches on both sides of the transformer, what stops up from switching power in either direction???

    Maybe it's as simple as not having any practical application! I can't really think why my iphone charger would need to have this ability, so i suppose the application would be pretty specific :)
  2. ithinkican

    Thread Starter New Member

    Apr 6, 2013
    OK, i just though of an application.

    a 12vdc to 24vdc converter that will also do 24vdc to 12vdc, the conversion just depending on a mode switch. That way one product could be made that converts truck 24v input to car 12v output or vice versa.

    I bet that when i now search on google i will find such a thing.....

    So my question is really - am i right in thinking that bidirectional SMPS is easy and practical, but not found often because there are few applications?
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    I bet you will not.

    A bidirectional SMPS is practically impossible, even though there would be many applications for such a device. The output is a filter with no switch element back to the magnetics.

    Now if you keep the input as the input and the output as the output you can get converters to make 12V to 24V and 24V to 12V.

    Those are called boost buck converters.
    absf likes this.
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Sorry ErnieM but you are totally wrong.

    There are a number of commercial 12v<->24v bidirectional SMPS converters on the market.

    They don't use a transformer, all it requires is a buck regulator with synchronous recification (switched FET as a diode).

    The principles are well understood, and current will be fed in either direction, from whichever voltage is higher than the transfer ratio.
    kubeek likes this.
  5. EB255GTX

    Active Member

    Apr 30, 2011
    Cheers - that was what i was really asking, that IF you have a controllable switch on the secondary (like SR in a buck) is there any reason you can't run bidirectionally...

    For a flyback it's slightly less clear to me.... imagine that you have a switch on the secondary and so you can switch the secondary winding, would the flyback action work via the transformer and get energy to the primary ?
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Yes, that works with a buck converter. That's because if you draw it backward it's a boost converter too (provided both semi devices are switched).

    With a transformer based SMPS it will be harder, to have bidirectional operation both sides of the transformer need to be synchronously modulated. And the high voltage rectifier is a synchronous rect (FET). Maybe it's possible? I have not seen one.