Why is my circuit oscillating

Discussion in 'General Electronics Chat' started by Rock Slate, Aug 1, 2015.

  1. Rock Slate

    Thread Starter New Member

    Jul 22, 2015
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    Hi guys

    I am trying to build a power transfer circuit using an inductor and a capacitor. When I try to use the PULSE function(it is a constant frequency with a really short of time) , the circuit seems to be oscillating. Instead if I supply a DC voltage of 0V , it seems to reach a response of 12V, and if I supply an input voltage to the switch of 12V, it reaches 0V. I cant seem to understand when I supply a pulse with a large on time(almost the same as a DC voltage), it seems to oscillate.

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  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Is this supposed to be a buck regulator circuit?
     
    Last edited: Aug 2, 2015
  3. Rock Slate

    Thread Starter New Member

    Jul 22, 2015
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    Just one part of the debugging circuit, but yes.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Check the current being delivered by V3
     
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  5. Rock Slate

    Thread Starter New Member

    Jul 22, 2015
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    Yes. There is something funny going on. The current through it seems to be in the order of kilo amp. Maybe I have to increase the input resistance of the switch. but this still doesnt explain the oscillation.

    Second, the more ideal the switch, i though the better the circuit was. Is this because of the surges when the switch turns on?
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Dont understand why you have two switches?

    As configured, there is a short period when both switches are turned on at the same time (called shoot-through) due to overlap in the control signals.
     
  7. Rock Slate

    Thread Starter New Member

    Jul 22, 2015
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    Is that not the way any standard circuit is implemented? There is a HSS(which is usually a PMOS) and the LSS.
     
  8. MikeML

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    No, only one switch is typically used. Read post #7 in this thread.

    Note the positioning of the inductor. Note how the diode is used as the other switch...
     
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  9. Rock Slate

    Thread Starter New Member

    Jul 22, 2015
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    Yes. I didnt want to use a diode . That is why I used the NMOS instead. But thanks for linking to that thread. SOme really cool information which I didnt know.
     
  10. MikeML

    AAC Fanatic!

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    You could use a second MOS switch in place of the Schottky, but you would have to generate non-overlapping gate drive signals...
     
  11. Rock Slate

    Thread Starter New Member

    Jul 22, 2015
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    For the SW1, it is on whenever the input is low. It is an ideal switch so I would assume it would transition immediately and there is no rise time or fall time . I changed the circuit to generate non overlapping signals so that both the switches are not on simoultaneously. However I still see the huge currents. I have attached the circuit. Could you take a look? I forgot to split the source, so I am just specifying what I did instead . I split the voltages source V2, and changed the on time to a shorter value. However , it does not seem to work.
     
    Last edited: Aug 1, 2015
  12. crutschow

    Expert

    Mar 14, 2008
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    What is the odd TRIGGER statement at the end of the V2 pulse command supposed to do?
     
  13. Rock Slate

    Thread Starter New Member

    Jul 22, 2015
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    It is for current mode control of the buck converter. I was playing around with the one-shot values, however ideally it should be around 0.3-0.5V. I forgot to check it before uploading
     
  14. crutschow

    Expert

    Mar 14, 2008
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    How did you arrive at your inductor and output capacitor values?
    Those values give a resonant frequency for the output L and C of about 13MHz, which is the oscillation you are seeing.
    Normally the output resonant frequency of a switching regulator is in the low kHz range, not the MHz range.
     
    Last edited: Aug 2, 2015
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  15. Rock Slate

    Thread Starter New Member

    Jul 22, 2015
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    Last edited: Aug 3, 2015
  16. crutschow

    Expert

    Mar 14, 2008
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    You are unlikely to get a real switching regulator to operate at 81MHz. :eek:
    If you want to operate at that frequency, why is you simulation switching frequency 6.7MHz?
     
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  17. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Where did you get the 81 MHz?
    The given PDF is talking only of 400 kHz, wich is far more realistic.

    Bertus
     
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  18. Rock Slate

    Thread Starter New Member

    Jul 22, 2015
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    Okay. I think I will change the values and check if they work. Thanks for the suggestions
     
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