Regulation in flyback converter ...HOW??

Discussion in 'General Electronics Chat' started by supermankid, Apr 27, 2014.

  1. supermankid

    Thread Starter Member

    May 26, 2013
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    Flyback converter can be used to create multiple output using multiple secondary transformers. I am wondering how is the regulation maintained....for eg.....if we have 5V, 3V3, 1V8 and 1V5 as output from 50V input. If the load changes on one of the output...does this not effect the other rail....I don't understand how the controller maintains the reulation on all the rails if load on all rails is changing....or is there a clever idea I might me missing?:confused:
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Only one rail is regulated usually. The other rails are assumed to track the regulated rail. If the tracking isn't accurate enough for your purpose you might consider using individual post-converter regulators for the rails which need it.
     
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  3. ian field

    Distinguished Member

    Oct 27, 2012
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    A really good PSU will have a chopper transformer for every rail, otherwise there are various methods, the cheap ones have tightly coupled secondaries so as long as the regulation sampled rail is within spec, the others won't be far off.

    Common in TV set top boxes, several rails are sampled and summed together in a resistor network.
     
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  4. #12

    Expert

    Nov 30, 2010
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    The basic principle is about voltage being proportional in every winding. You pretty much can't have one winding with a different volts per turn ratio than the other windings in series with it. If the load makes any single voltage sag, they all sag and the drive is increased until that voltage is right. The proportionality of the transformers windings forces voltage tracking even if the current loads are different.
     
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  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Many multi-rail PSUs have secondary regulators on some of the lower power rails.
     
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  6. ian field

    Distinguished Member

    Oct 27, 2012
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    They're usually high frequency switching, so the secondary windings will have few turns and very little DC resistance - but you can't count it out altogether when you go down to 3.3, 2.7 and 1.8V outputs.

    At those voltages, having to add extra turns to compensate for rectifier Vf doesn't help.
     
  7. supermankid

    Thread Starter Member

    May 26, 2013
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    Hi, could you please explain me what it means by this can't count it out altogether when you go down to lower voltage.......thanks in advance
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    At very low voltage *ALL* unwanted resistances in the circuit become significant.

    At very low voltage; the Vf of the rectifier amounts to a significant percentage of the intended output voltage, so the winding length on the secondary has to be increased to compensate for it.

    Its only a few milli-Ohms here and there, but at several amperes or more the volt drops add up.
     
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