smps feedback circuit -help understanding

Discussion in 'General Electronics Chat' started by mikcatta, Jan 1, 2015.

  1. mikcatta

    Thread Starter New Member

    Oct 12, 2014
    5
    0
    Hi

    I have a question on SMPS feedback circuits and would appreciate a bit of guidance. The circuits I have found are using discrete components and both very similar, however the explanations seem to contradict.

    In this blog post (1), the author describes the pulse from the feedback winding causes the FET to switch OFF and hence induce current in the secondary winding. Fair enough I thought.

    However in circuit (2), the feedback pulse is described as turning the FET ON harder, and it's voltage developed over the resistor in the source, via an NPN transistor, that switches the gate to ground.

    (1) http://martin-jones.com/2014/03/03/denon-dvd-1720-dvd-player-power-supply-schematic-and-repair/
    (2) http://homemadecircuitsandschematics.blogspot.co.uk/2014/03/12v-5-amp-transformerless-battery.html

    Thanks
    Mike
     
  2. MrAl

    Well-Known Member

    Jun 17, 2014
    2,423
    490
    Hi,

    It is hard to tell what kind of opto coupler the sloppy drawing circuit is using. Its output is drawn as a light sensitive diode.

    These circuits operate by storing energy in the core and then turning the drive off and then the output shoots up. They dont show the transformer winding polarities either so it's hard to tell if the sloppy drawing is connected the same way.
     
  3. mikcatta

    Thread Starter New Member

    Oct 12, 2014
    5
    0
    Hi

    It's the "theory" of how the FET is driven that I would like to understand. These are the only ones I've found using discrete components rather than an IC.

    As a side issue on the SMPS I have here from a dud DVD player, there is a shorted diode on the secondary. Would this prevent the the whole power supply from operating?

    Thanks for taking the time to reply to my post and happy new year.

    Rgds
    Mike
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    this is not a smps, it is a power oscillator. the little feedback winding provides positive feedback to the gate from teh output transformer. there is a little feedback from the rectified output to control the gain of the oscillating fet.
     
  5. tindel

    Active Member

    Sep 16, 2012
    568
    193
    It can make the whole thing not work... because it's not rectifying the voltage properly. replace the diode with another to find out. Diodes are cheap and can often be found in other hardware that's been thrown away if you're on a tight budget.

    I personally wouldn't work too hard on fixing a DVD player unless it is something very simple... maybe replace the diode if that doesn't do the trick finding a new one on craigslist or at walmart is a pretty cheap solution these days too.
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    yes, that would make it not work. the voltage rating on the diode would not be very high on the secondary side. it might be a schotkey diode, they were used a lot because of low forward voltage drop.
     
  7. mikcatta

    Thread Starter New Member

    Oct 12, 2014
    5
    0
    Thanks guys. It's definitely a schottky diode, popping out to pick up a replacement today.

    I find fixing these items a better way to learn than just trying to study the theory and there is no shortage of faulty electricals to work on these days. Even if this dvd never comes back to life, I've learnt quite a bit a long the way.

    As a side issue, can I use a Variac to bring the power supply input voltage up slowly (and safely) or is it likely it would only work with the 300v DC ?

    Mike
     
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