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SMPS Troubles

Discussion in 'General Electronics Discussion' started by michan, Feb 15, 2016.

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  1. michan

    michan

    22
    1
    Feb 21, 2015
    Hi guys,

    Just having problems with a switch mode power supply that I'm trying to design based off a transformer that I'v already wound. The main problem is that the circuit when modelled in LTspice has a strange wobble on the falling edge of the ne555 timer signal to the mosfet. This wobble is then causing some weird voltage peaks and troughs in the rest of the circuit on the primary side of up to 900V roughly. The output voltage of the smps is also a bit of a mess because of this and reaches negative 20KV which is not at all what I'm looking for.

    Im new to simulating my own circuits and also to smps circuits so I really don't know what is causing this wobble, the extremely high voltage spike or how to fix it. How ever I am aware of leakage inductance which can cause voltage spikes but I don't think this is due to that as a snubber circuit did not help remove this voltage spike or even reduce it unless I'v done that wrong as well which may be the case haha. Any help is appreciated, thanks!

    In the pictures the dark blue line is the ne555 output voltage to the mosfet, the light green line is the mosfet gate current from the ne555, and the red line is the voltage on the primary side of the transformer.
     

    Attached Files:

  2. Alec_t

    Alec_t

    2,939
    799
    Jul 7, 2015
    Your L1 value is rather low for the frequency you're generating and that may be causing the ringing you see, via capacitive feedback to the gate. You need a back-emf protection diode across L1 to kill the voltage spike. The output needs a cap to stabilise the voltage.
    Try this mod:
    SMPSmod.PNG
    The asc file is attached.
     

    Attached Files:

    michan and Arouse1973 like this.
  3. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    As far as I can see, your circuit is doing exactly what I would expect.

    Agree with Alec_T that the inductor is far too small. Should be on the order of 10s or 100s of uH.

    Check the current through the coil, I think you will find it to be 100s of Amps.

    Bob
     
  4. michan

    michan

    22
    1
    Feb 21, 2015
    Thank you both so much! Your circuit really helps too, really appreciate it!
     
  5. michan

    michan

    22
    1
    Feb 21, 2015
    Quick question about your circuit, I thought the current on the primary of the 'transformer' was meant to have a sawtooth shape due to the inductance resisting any changes in current but it seems to show a square wave on your circuit? Also I am trying to get an output voltage of 630v roughly with the same circuit but different inductances on the primary and secondary (20uH on primary, 59mH on secondary) but this seems to draw a current of about 360A for the first 8ms before tapering off to 33A o the primary, would you happen to know why this occurs?

    Thanks again
     
  6. Alec_t

    Alec_t

    2,939
    799
    Jul 7, 2015
    It happens because the 20uH value is too small. The current in the primary does ramp up, but with such a low inductance that ramp occurs in the first microsecond or so of the 'on' pulse. (Zoom in on the waveform to see). For the remainder of the pulse the current is limited only by the inherent resistance of the inductor and the FET Rds(on) (a few milliOhms).
     
    Last edited: Feb 18, 2016
    michan likes this.
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