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Thread: 0~500V SMPS based on SG3525

  1. #1

    0~500V SMPS based on SG3525

    Hi ,

    I am designing a 0~500V(25mA) power supply based on the SG3525.The input is a 12V battery.
    The transformer was designed using the software available from Vladimir Denisenko, available on this site.
    The problem I am facing is that I am getting no output from 0~100V (0~0.5V setting) and then it maxes out at 400V.
    I checked the voltage at the set point,it varies from 0~2.5 as per the pot. setting.The voltage at the feedback point also varies as per expectation from 100~400V.
    Can anyone suggest any check points on why there is no output from 0 ~ 100v ?
    I could probably adjust my turns ratio to get the 500V max. , but i do not know why there is not output voltage from 0~100V.
    schematic has been attached FYI.
    Attached Images Attached Images

  2. #2
    I think you should follow the wave form from the sg3525 to the gates, then primary, and then secondary to see if the duty cycle is changing smoothly with your pot.

  3. #3
    Thank you for replying.
    At higher values >100V,the duty cycle is very less ( < 5%),but when i go below 100V setting there are no pulses coming to the gate at all.Is it because I am not using a gate driver ?
    I am planning to put a TC427 between SG3525 and the IRF540 and check if there is any effect.
    Please do chime in with any other suggestions.

  4. #4
    Find a way to remove the wire to the gates of the FET’s or to remove the fet’s. Then check the waveform coming out of the 3525.

  5. #5
    Find a way to remove the wire to the gates of the FETís or to remove the fetís. Then check the waveform coming out of the 3525.

  6. #6
    I tried that.The 3525 releases a square wave but only after the set voltage @ pin2 crosses 0.5V.Since there is no feedback back voltage,the 2 waveforms are of about but less than 50% (to include deadband ) of the full cycle.
    This is the same behavior when the transformer is connected,viz. I get a voltage only after 100V (corresponding to 0.5V).So it does not seem like a gate drive issue after all.
    Could you suggest any reason for this ?

  7. #7
    Here is a pin for pin description of how your circuit should work

  8. #8
    Well I had seen that before too.Does not show anything which could address my issue,except for the network on the compensation pin.I could not figure that one out.

  9. #9
    With the wrong compensation your 3525 could be confused and not know what to do. Try using the values in the blog if you can read them, i could not. Also your switching frequency seems very low to me, around 16khz at the transformer.

  10. #10
    diysmps Senior Member
    Join Date
    Oct 2012
    there is often only a relatively narrow range of Vin, Vout and Iload through which a converter will work, beyond this you might be pushing the limits of duty cycle or pushing the bipolar transistors in the IC beyond their limits etc. also these things are generally designed and compensated for a fixed output, making this variable complicates things but at least Vin is relatively fixed. there's also the added issue that high voltage means standard schottky rectifier is out of the question and uf4007 are much slower.

    so I see a push pull converter under voltage mode control with a classic industry standard PWM IC. What I don't see is the most important component of a dc dc converter, an output inductor, not sure why it's not there unless I'm missing something. push pull is based on a buck converter, so it should have one. not sure why you used a push pull converter if you are going to have a non-isolated feedback path, but I do prefer using buck derived rather than boost or flyback derived topologies you'd normally use for high voltage, especially if you want its output to be widely variable. the compensation pin is floating, so your error amplifier has no local feedback, I.e. it's operating as a comparator not an opamp, that needs sorting. once you have an output inductor you can design the compensation, remembering the complex conjugate double LC pole will be a bit of a headache, but at least it's all in the left hand plane, RHP (being much worse).

    ps. apologises for any errors, writing on a phone without looking at the IC datasheet.

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