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Thread: Q about secondary side.

  1. #1

    Q about secondary side.

    I have been doing electronics on and off (mostly off ) since about 90'. I have been studying SMPS for a while now. Reading the guides, watching the videos etc. Something has been bugging me. Many of these people are not native English speakers. They always seem to say that AC comes out of the secondary side and gets rectified through the Schoytkey Diodes and smoothed etc. Isn't it really pulsating DC? That is what gets fed into the transformer after all in this kind of Power Supply. I assumed that is why the diodes only go one direction coming out of the secondary side because the pulses only ever come from that direction anyways? However, that creates another question. Why have diodes at all then? Why not just capacitors? It must be AC after all? Confused...
    Thanks

  2. #2
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    Quote Originally Posted by jarrod0987 View Post
    I have been doing electronics on and off (mostly off ) since about 90'. I have been studying SMPS for a while now. Reading the guides, watching the videos etc. Something has been bugging me. Many of these people are not native English speakers. They always seem to say that AC comes out of the secondary side and gets rectified through the Schoytkey Diodes and smoothed etc. Isn't it really pulsating DC? That is what gets fed into the transformer after all in this kind of Power Supply. I assumed that is why the diodes only go one direction coming out of the secondary side because the pulses only ever come from that direction anyways? However, that creates another question. Why have diodes at all then? Why not just capacitors? It must be AC after all? Confused...
    Thanks
    Regarding the secondary voltages. The diode/s are there to rectify the usual square wave pulse. it is also used as a one way valve between pulses so that during the off time the output inductor can release the energy stored in it.

  3. #3
    I'm sorry, I still don't understand. Is the square wave True AC? Does it peak at + and - or is it just Pulsing DC..Say +7 and +2 or some such?

  4. #4
    Diodes are there to freewheel after the pulse is over. If the diode was not there when the square wave dropped it would take the capacitors charge with it, or try to, and something wouldn't be happy in a short period of time.

  5. #5
    Yes square waves are ac.

  6. #6
    Hmmmm... So It looks like the output rectifier diodes on the secondary side are half wave rectifiers then? Isn't that pretty wasteful? I know SMPS are famous for being efficient. Sorry for the newb questions. I'm used to AC transformers.

    I'm used to AC in the primary and AC out of the secondary but 180 out of phase. I always thought they used a full wave or bridge rectifier when you had Positive and negative alternations so that both could be used. Half wave just threw one away basically.
    In this case it's a square wave and not a sine wave but your saying it still has a + and - alternation? So why does the secondary side not need full wave rectification on each line? Trying to visualize it.

    UPDATE: I think I figured it out. The output filter capacitor only charges if the output voltage is higher then the capacitors current charge voltage. So the Diode stops it from discharging back into the wrong direction again. Yes?
    Last edited by jarrod0987; 07-06-2017 at 09:06 PM.

  7. #7
    You are correct.

    If your secondary is center tapped, you could use a rectifier bridge if you want. Or you could wind two center tapped secondaries so you could use standard common cathode to-220 diodes.

  8. #8
    Something like these in the pic

    Untitled.jpg

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