+ Post New Thread
Page 2 of 2 FirstFirst 1 2
Results 21 to 22 of 22
  1. #21
    Newbie level 6
    Points: 64, Level: 1

    Join Date
    Jun 2017
    Posts
    12
    Helped
    0 / 0
    Points
    64
    Level
    1

    Re: Switching power supply much lower output than LTSPICE model

    Quote Originally Posted by mtwieg View Post
    So the start of your waveform matches up fairly well, but at 10ms instead of plateauing around 0.6V (0.27A), the input current decreases and starts ringing until stopping completely at 22ms. Try probing other points in the circuit and see if you see any odd behaviors correlating with that.
    L3 is probably saturating. Its saturation current is 270mA. LTSpice's default inductor model doesn't model saturation at all. When the current drops low enough that the inductor comes out of saturation, the ringing starts.

    I tried shorting out R18 and L3. That's just noise filtering to prevent switch noise from getting back into the USB power source. That eliminates L3 saturation as a factor, and increases final output a bit, but it still takes about 45ms to charge up the 2uF caps. With R18 shorted out, there's no good place to measure a voltage drop and calculate current.

    (This is all on a surface mount board, so it's hard to tap in for current. Voltage, no problem,)

    Now, if the core is not fully resetting during the "off" time
    (and I see nothing explicitly making that so) maybe there
    is flux walk / staircasing that leads to eventual saturation.
    But flyback converters are kinda outside my scope. Just
    a thing to consider.
    Is that a possibility? Do I need a diode somewhere to drain out the transformer primary current on each cycle?

    The default transformer model in LTSpice is oversimplified for this sort of thing. No saturation, and no hysteresis. If the model diverges badly from the real world, that's the most likely reason. Everything else has a reasonable model. Especially with L3 out of the picture, as noted above.



    •   Alt22nd June 2017, 01:05

      advertising

        
       

  2. #22
    Advanced Member level 5
    Points: 18,207, Level: 32

    Join Date
    Jan 2011
    Posts
    3,045
    Helped
    1073 / 1073
    Points
    18,207
    Level
    32

    Re: Switching power supply much lower output than LTSPICE model

    Quote Originally Posted by nagle View Post
    L3 is probably saturating. Its saturation current is 270mA. LTSpice's default inductor model doesn't model saturation at all. When the current drops low enough that the inductor comes out of saturation, the ringing starts.
    It's possible that L3 saturating is contributing to the strange behavior... nonlinearity is hard to predict.

    I tried shorting out R18 and L3. That's just noise filtering to prevent switch noise from getting back into the USB power source. That eliminates L3 saturation as a factor, and increases final output a bit, but it still takes about 45ms to charge up the 2uF caps. With R18 shorted out, there's no good place to measure a voltage drop and calculate current.
    With those components shorted out, does the input current waveform change much? R2 should still work fine for measuring input current.

    The sudden change in input current at 22ms is also very strange. I can't tell what the trace offset is on your scope, but it looks like current goes to zero, which should not happen.

    Is that a possibility? Do I need a diode somewhere to drain out the transformer primary current on each cycle?

    The default transformer model in LTSpice is oversimplified for this sort of thing. No saturation, and no hysteresis. If the model diverges badly from the real world, that's the most likely reason. Everything else has a reasonable model. Especially with L3 out of the picture, as noted above.
    Staircase saturation only affects push-pull type converters, flyback not included. If saturation is happening in your transformer, it would be at the beginning of the charging cycle, when Vout is low. Your simulations do show this happening, so it may be a factor. Hard to tell, since coilcraft does not explain how much saturation occurs.

    If you were to simulate the circuit with L3 and R18 shorted, you get a much faster charging time, especially at the beginning, and a much higher peak input current (Ip is almost 5A, supply current peaks at 0.76A). But with your real circuit, an overcurrent limit will likely trip before then. If it were me, I would try to implement a soft start so that the duty cycle ramps from zero at the start, in order to keep the inrush current lower.
    Last edited by mtwieg; 22nd June 2017 at 15:54.



--[[ ]]--