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LDC1000 in induction heater controller

Ralph Stirling
Prodigy 30 points

I am interested in using an LDC1000 in a small induction heater (~10W power level).  Is there any way to drive some power mosfets with the LDC1000 to enable higher coil currents?  I'm thinking of something analogous to using a power transistor with an opamp.  If I could use the LDC1000, I think I could infer temperature of the object being heated by the measured inductance.  If I cannot drive power devices with the LDC1000, is there a way to have a "sense coil" in the same magnetic circuit to monitor impedance while the circuit is being driven by another circuit?

  • 0
    TI__Intellectual 2790 points

    Hi Ralph,

    The sense coil is the way to go but I would be concerned about both the low frequency of induction heating and the temperature of the LDC1000 and sensor coil.   The sensor LC tank must be designed at a frequency close to that of the frequency of induction, but still meet the Rp range requirements of 800-4MOhm.  The temperature of the sensor coil and LDC1000 must be constant stable or digitally compensated.  The key data parameter is frequency counter data, which can be converted to L.

    Thanks

  • 0 in reply to Charles Cheung*
    Prodigy 30 points

    Thanks, Charles.  My induction heater is not low frequency (~15-20kHz).  Perhaps I could phase lock the power switching circuit to the LDC1000 frequency.  I would like to use the LDC1000 to determine the temperature, as the rest of the properties that affect Rp in my application do not change during operation.  I wish there was more information on the LDC1000 front end published.

    -- Ralph

  • 0 in reply to Ralph Stirling
    TI__Intellectual 2790 points

    Typically induction heating frequencies are low enough to penetrate at a considerable depth through the material.  Let me redact my prior statement:  It is possible, and, actually, significantly better to run at a much higher frequency if the eddy current loss can be seen closer to the surface.  In this case, phase locking is not required; the eddy current loss is averaged over the sensor frequency. 

    The biggest problem is the mutual coupling between the induction heating element and the sensor coil.  It's possible for high voltages that the sensor coil voltage will be saturated and possibly INA / INB will be damaged.  The issue is physical placement of the coil- it has to be near the source of heat for the part, which makes it prone to damage from the heating element.

  • 0 in reply to Charles Cheung*
    Prodigy 30 points

    Again, more info on the INA/INB pins would be helpful.  Can I protect them with back to back diodes? TVS devices?  My induction heater works with other circuitry, I'm just looking to see if I can simplify my system and incorporate indirect temperature sensing.  If it starts getting complicated I'll just stick with what I have.  I'm not worried about heat issues, but excess voltage on INA/INB could be a problem if I can't put protection circuitry on the pins.

  • 0 in reply to Ralph Stirling
    TI__Intellectual 2790 points

    Hi Ralph,

    There is an internal 5V clamp and an absolute maximum of 8mA on INA/INB.  It is possible to put external clamps on each pin to common.  To avoid damaging the internal clamp, we recommend lower voltage clamps/TVS at 4V.  Keep in mind that these will have to be fast to avoid affecting the frequency count.

    Thanks,