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Phase detector

Discussion in 'Electronic Design' started by Tim Williams, Mar 25, 2005.

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  1. Tim Williams

    Tim Williams Guest

    Okay, I've got a Rube Goldberg thing going in my mind...

    First of all, I have an SG3524 based driver circuit, running a half bridge
    that runs an induction coil.
    Driver schematic:
    Half bridge:
    (Cmatch + Ctank = 6.4uF, Lmatch = 35uH, Ltank = 1uH, supply = 110V.)

    Well the circuit works as good as I can expect it to at this power level...
    what I'm racking my brain on is control. At the moment I manually adjust it
    to resonance for maximum power (or above resonance, giving the inverter a
    lighter, inductive phased load). As you can imagine... frequency varies
    quite a bit depending on load resistance, dimensions and permeability.

    What kind of circuit would work here? I was thinking of detecting phase
    between the instant the tank reaches peak voltage and the time when the
    inverter switches off. At 50-100kHz (the larger model will be lower), that
    requires fractional-microsecond pulses, which aren't impossible but I'm not
    familiar with the circuitry (storage time doesn't like me..) to handle it.
    Then some sort of circuit to measure the time between the pulses and
    translate that to a current which controls the 3524's frequency (current
    draw from pin 6, which probably goes to a current mirror inside the chip).

    Or if you have recommendations for chip(s) I have on hand that do all this
    automatically...I'm all ears...(which is basically limited to an MC1391 I
    pulled from a monitor, and various 74[LS] chips I have on hand).

  2. John  Larkin

    John Larkin Guest

    XOR gate!

  3. John Walton

    John Walton Guest

  4. Jim Thompson

    Jim Thompson Guest

  5. John Walton

    John Walton Guest


    Yeah, but I was able to snag a bag of TC5081's on EBay for next to nothing.

    Intersil has a very nice phase-detector circuit on their website (AN9637) --
    and it is very acurate. I built it using some TI cfb opamps (the intersil
    part was discontinued -- the transistor array is a bit tough to find). CFB
    opamps with disable function are preferred for ease of gain switching.

    Basically, the CFB amps drive 1n4148's through 200R -- this clips the sine
    wave of both reference and signal to be measured -- each circuit feeds one
    side of the transistor array used as a comparator. Phase is proportional to
    the voltage averaged over time.
  6. Tim Williams

    Tim Williams Guest

    Hm, so I can add a current loop toroid to the work coil supply, clip the
    signal with two 1N4148's in antiparallel, amplify (hm, rather one 1N4148 and
    a B-E junction) to get a "full height" square wave, and send this to a LTP
    against clock from the 3524? Does it matter in the closed loop that the
    3524 isn't ever at 50% duty cycle?

    And that makes a PWM error signal, which can then be averaged as you say and
    compared to a control voltage (power level) and sent to drive the frequency
    pin, no?

    I like... almost uses fewer pins (as discrete transistors) than that 9-pin
    SIP alone. ;-)

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