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# Impedance Matching

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#### MOSFET KILLER

##### New Member
How do I calculate the source impedance of a MOSFET inverter. The inverter uses four IRFP460 MOSFETs in parallel. These MOSFETs have a drain to source on-resistance of 0.27Ω. The MOSFETs are in parallel so they will have a total resistance of 67.5 miliohms. Would this be the source impedance? The output is an LCLR tank circuit. How would I calculate the load impedance of this output circuit.

If by source impedance you mean the MOSFET output impedance, then that would be 67.5 milliohms for the four in parallel when fully turned on, as you calculated.

The tank circuit impedance depends upon how close to resonance you are operating, and whether you are operating in parallel resonance or series resonance mode

The MOSFET on-resistance isn't the whole story when you're driving a resonant load. It's complicated, more closely related to the V/I that you expect to load the stage with.

A schematic of what you plan to do would help.

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Here is the inverter schematic, the gate drivers and the power supply are part of another schematic. The drive frequency is phase locked to the LCLR output.

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• Inverter2.jpg
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Let's say that the inverter is tuned directally to the resonant frequency of 100 kHz. The load in the coil will affect both the load resistance and the tank frequency. How would I calculate the load resistance for a load in the output coil? Should I find the impedance of the power supply too? How can I find the impedance of the tank circuit with a load in the coil?

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