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HV flyback psu

Discussion in 'Power Electronics' started by vitom001, May 15, 2016.

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  1. vitom001


    Aug 12, 2015
    I'm trying to build a HV power supply consisting of a mains-driven control circuit and a flyback transformer - the circuit is exemplified in the following link:

    the problem I have is that the output fromt he power-transistor BU-508D is not as it's supposed to be. To illustrate, the following link shows the output I am aiming for, and the attached image shows what I'm currently getting.

    As you can see the peak is not of the correct shape and has a significant DC offset for some reason. I have tested all parts of the circuit and they are working well - including flyback transformer itself. The pulse frequency is ~15kHz as supposed to.

    THE ONLY THING I have modified is the potentiometer (freq.adj) in the oscillator circuit where i used a 10K instead of 4.7K since I did not have any of those at hand. I thought this might be the source of my troubles but I am not sure. If anybody knows what might be going on I'd be glad to hear it.


    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    1) where is the probe ground connected?

    2) Is your 12v rail stable?

    3) the part of the circuit around the 3055 seems poorly designed and will not be able to deliver much current. In addition, the pot is likely to let the smoke out. Check the stability of your 0-70v rail.
  3. 73's de Edd

    73's de Edd

    Aug 21, 2015

    Sir vitom001. . . . . . .

    Can you try two things ? :

    On that Pin 2 of the flyback transformer put about a 10 ufd 100 VDC from that terminal to ground . . .with + to 2 and negative to ground and additionally, hi freq shunt THAT cap with a .1 ufd ceramic cap.
    Give a short try out with C6 being shorted across.

    Currently looking at the resultant output waveform, suggests that you are getting far less drive level, than is needed to fully saturate that stage . . .or its drive pulse width is being of too great of a width.
    If there was too much DC bleed through from pin 3 . . . then sub in a .1 ufd ceramic or paper/poly cap for C6.
    Can you scope the generated drive signal waveform at 555 pin 3 and see just how narrow it is being ?
    Key us in on the scope referencing . . . as I am currently assuming your last scoping was being at:
    200V per . . . . vertical and 50 us per . . . . hoz.

    Looking at Naudin's waveform and comparing to your's, seems like you are being closer to the optimal 15Khz ( 62 us ) operation of the flyback, than he was .

    So you have no worry on your 5K versus his 10K pot since the freq trim range currently falls within your adjustment range . . . that R4 1 K series limiter still prevents any boo-boo's.
    A C4 capacitance change can also tweak freq if required . . .more cap lower freq . . . less cap higher freq.

    Had to look up that old BU(colic) 508 to remember if it has a built in damper diode . . . .and it does.

    73's de Edd

  4. dorke


    Jun 20, 2015
    Please probe the base of Q2.
    (if possible in the same time with the collector,using 2 scope channels).
    And show the voltages and time on the pic.
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