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PLL un-equal square wave?

Discussion in 'Electronic Repair' started by SRG, May 31, 2005.

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  1. SRG

    SRG Guest

    Hello Group,

    I have been playing around with a 4046 Phase lock loop!
    I am using it to drive two mosfet drivers one inverted and the other non
    inverted which then drive a gate transformer connected to a Half Bridge.

    Now when I view the square wave output with a scope from the PLL vcoout it
    appears slightly un-equal.
    The negative portion of the wave appears slightly shorter than the positive
    portion. This is exaggerated even more when viewing the output(s) of the
    mosfet gate driver (tc4421 / tc4422). I believe this is causing one side of
    the half bridge to work harder which creates much more heat in one mosfet
    than in the other (Well that's what I think is happening as one mosfet
    becomes much hotter than the other) .

    Additinal info :-
    I am using a current sense transformer located around a parallel tank
    circuit to send the correct phase signal to the Phase Lock Loop. This
    appears to lock the PLL / VCO at the self resonant frequency of the tank
    circuit, which should then drive the Half Bridge mosfets at ZCS / ZVS
    causing the desired affect of soft switching.

    The output on the scope suggests that soft switching is almost taking place
    all though there is slight ringing which at this point I am unable to
    minimalise any further and which may be connected to the above waveform
    issue?

    Does anyone have any idea why the initial waveform could be un-even even
    when just running from the fixed VCO (ie with no PLL feedback)?

    Hope this makes some degree of sense! ;-)

    Regards
    Steve.
     
  2. Can you run the PLL 2 times as fast and use a divide by 2 to get the
    original frequency with perfect duty cycle?
    GG
     
  3. SRG

    SRG Guest

    Thanks for the feedback!
    Not sure if that would work I could be wrong ofcourse as this is the first
    time I have played with a PLL.

    The VCO on the 4046 is currently set to around 150khz with a shift allowance
    from aprox 100 - 200 khz. The output drives a parallel tank circuit
    consisting of 1.5uh inductor and 564nf worth of capacitors giving a
    frequency of around 171khz. Additionally the output of the inverter is
    connected to DC blocking cap then a 45uh inductor in series(for impedance
    matching though having trouble finding the correct value for this!) this
    then connects to the aforementioned Parallel tank circuit.
    I have connected a ferrite (Clip on type) with aprox three turns which acts
    as a current sense transformer (CT) which passes its output to the PLL
    input. I then tune the VCO until I get a perfect phase alignment (well
    almost) for both current and voltage so as to achieve the golden Zero
    Current Switching for the mosfets in the inverter (half bridge). That's the
    plan anyway ;-)

    So in effect the VCO (on the 4046) acts as a starting frequency to drive the
    inverter which in turn feeds the tank circuit AC that acts like a filter ie
    only producing significant power when the inverter is tuned to the tanks
    resonant frequency. The current transformer feeds back voltage and phase to
    the PLL which follows any resonant frequency change that's produced from the
    tank circuit thus theoretically keeping everything running sweetly in tune
    and consequently soft switching mosfets at ZCS / ZVS which should make them
    run cooler and more efficiently with the exception of inherent mosfet
    losses.

    So I guess by upping the frequency would not help in this case?
    I still am at a loss why the square wave output from the VCO and gatedriver
    is not evenly spaced for the positive side compared to the negative side of
    the waveform?

    My God did I just type that? LOL scratching head......
     
  4. Jamie

    Jamie Guest

    MosFet's have Capacitance on the gate verses what you normally
    see on bipolar, you maybe current sinking ?
    you may want to test the output unloaded to see if you are
    still getting the problem? if not, then the problem is most likely
    current sinking due to external cap.
    you could try a current driver Tansistor (emitter flower)? or
    some high speed switching device to output more current with much
    less input current and capacitance.

    the ringing you see may just be the effects of your scope that is
    common, also you may want to experiment a bit with your scope probe
    in 1:1 mode and 1:10 mode, you may not hage the cap set correctly in the
    probe.
     
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