fly-back converter with a push pull transformer

Discussion in 'General Electronics Chat' started by high voltage123, Nov 9, 2012.

  1. high voltage123

    Thread Starter New Member

    Feb 23, 2012
    14
    0
    hii all
    am looking for ur urgent help in explaining to me this circuit which is a flyback converter with a push pull transformer.. am wondering why there are 2 switches connected to the transformer and also the full wave bridge rectifier, why he ddint use a half wave rectifier.
    thnx in advance
     
    Last edited: Nov 9, 2012
  2. ramancini8

    Member

    Jul 18, 2012
    442
    118
    Split primary doubles the output voltage. Full-wave bridge halves the ripple voltage.
     
  3. high voltage123

    Thread Starter New Member

    Feb 23, 2012
    14
    0
    thanx man for the help..but can u answer in detail please..thanxx
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    The pwm chip has two outputs complimentary, (when one is high the other is low) these pulse the transistors on and off which pulse the transformer on and off on both sides, to give a symmetrical wave form from the transformer.

    The output is monitored by the Tl431 variable zener, to turn on the opto coupler when 2.5v is reached this will slow down the pwm, and reduce the voltage , then the opto stops conducting and the pwm increases output, and so the cycle continues...
     
    Last edited: Nov 9, 2012
  5. high voltage123

    Thread Starter New Member

    Feb 23, 2012
    14
    0
    Dodgydave can u please explain this cycle of the PWM u mentioned in detail.. thank you
     
  6. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    He described it fairly well already, I think. Do you know what PWM is and how it works?
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    The pwm is a square wave oscillator giving out two square waves on the output pins 6 & 14, these are opposite in polarity, each turns on the transistor Q5 then Q9 which fires the mosfets Q6 then Q10 to drive the pulse transformer T2, this produces a higher voltage at its output secondary winding, which is rectified by diodes D3,8,9,10 to produce a dc voltage across the capacitor C7.

    This voltage is monitored by the Tl431 to switch on the opto coupler U2, which stops the pwm from working, so the output decreases and the opto stops working, the the pwm starts again,and the voltage rises again then the opto stops the pwm all this happens very fast...so by altering the voltage across the Tl431 using R15,you can alter the output voltage to suit your needs

    Do I need to go further....?
     
    Last edited: Nov 9, 2012
  8. high voltage123

    Thread Starter New Member

    Feb 23, 2012
    14
    0
    no need, i got the idea thank you so much.
     
  9. high voltage123

    Thread Starter New Member

    Feb 23, 2012
    14
    0
    Mr dodgydrave one more question if u dont mind :) can i call this converter a flyback converter?? if yes what it is operation??
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    Last edited: Nov 9, 2012
  11. high voltage123

    Thread Starter New Member

    Feb 23, 2012
    14
    0
    ahaaa..actually i am going to build this circuit and introduce it as a fly-back converter..because the only deference in the transformer, where in flyback uses a one output pulsed as u said but here it uses 2... do u think there is a big deference between them??
     
  12. JC13

    New Member

    Jul 5, 2015
    16
    0
    Hi Dogydave...I see that you know much about this topic....can you help me about my problem....

    The main problem is that I don't know is there positive or negative voltage in circuit.

    MOSFET 1....it is connected like this on the picture below. He is connected on the input of the board.

    MOSFET 2....pins 1,2,3 on transformer make me confuse and also connections to the voltage regulator (IC6). Diodes D9 and D10 are connected to the ground and then to Vin of the voltage regulator.
    Also the pins 10,11,12....only I know it can be is that negative voltage goes through D6 to
    Emitter of T5 and on emitter is negative voltage. T5 is giving 15V to ICs on the board.

    And connection of T5 MOSFET, is it really like this, is it good connection.

    Can you explain me how those two circuits are working from the pictures below.

    I have drawn a schematic from the board and really, drain and source are connected to the ground, I checked 100 times.

    Component values are important but not in this case. I just want to know how this circuit works and what is his purpose.

    I have tested this circuit on breadboard and first I have connected 100VDC without any voltage between drain and source and then I had Vgs=18V. But when I connected voltage between D and S (30V) I lost the voltage Vgs.

    And maybe it is really negative voltage on input, because drain and source are connected to the ground. And this circuit is connected EXACTLY on the input from the board. It must have some purpose.

    Second thing....rectifier diodes (D9, D10) are connected also to the ground and from there to input from voltage regulator ?....sorry but I have seen first time circuit like this one.

    I really hope that you can give me an answer and can explain the purpose of this circuit.

    Best regards

    Jovan
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    Push-pull tends to be first choice for higher power converters.

    Flyback is simpler and cheaper but has all sorts of downsides.

    You can get big current from the flyback converter by reversing the rectifier and filter caps on the secondary and using it as a forward converter, but you then have to have more turns on the secondary - which massively increases the flyback amplitude that the rectifier has to be rated for.
     
  14. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    That is not true. Pay attention. There is a very big difference because a flyback converter and a forward converter and very different. Calling your circuit a flyback converter is wrong. It is not a flyback converter. It is a half-bridge forward converter. The way the transformer behaves in a flyback converter is completely different from the one in your circuit.

    ak
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    There can be a grey area - some monitor manufacturers used a forward conversion supply for the low voltage/high current heater circuit on what was otherwise, mostly a flyback PSU - but the rectifier had to be rated for a much higher reverse voltage.
     
  16. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    I don't think it is a grey area. I don't think you can mix flyback and forward converter magnetics on a single transformer core. Unless you can do that, they are two separate converters no matter what else they share.

    ak
     
  17. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    Well a few monitor manufacturers did - and seemed to have got away with it.

    Some muppet at Philips circulated a memo claiming that stable modern SMPSUs meant that CRT heaters should be run at 6.15V instead of 6.3V - this invariably resulted in cathode poisoning.

    Some engineers risked X-ray increase by turning up the PSU voltage preset to get the heater voltage right, my solution was to replace the heater rectifier with a Shottky-barrier type - I couldn't get SB rectifiers with high enough reverse voltage for the occasional monitor that had forward conversion heater supply.

    Of course some of the normal type already had SB heater rectifiers - there was no way to correct the problem on those either.
     
Loading...