This is what I found on the website I mentioned above,

Impedance matching

Or simply "Matching". This refers to the electronics that sits between the source of high frequency power and the work coil we are using for heating. In order to heat a solid piece of metal via induction heating we need to cause a TREMENDOUS current to flow in the surface of the metal. However this can be contrasted with the inverter that generates the high frequency power. The inverter generally works better (and the design is somewhat easier) if it operates at fairly high voltage but a low current. This allows common switch mode MOSFETs to be used. The comparatively low currents also make the inverter less sensitive to layout and stray inductance. It is the job of the matching network and the work coil to transform the high voltage/low current from the inverter to the low voltage/high current required to heat the workpiece efficiently.

We can think of the tank circuit incorporating the work coil (Lw) and its capacitor (Cw) as a parallel resonant circuit.

This has a resistance (R) due to the lossy workpiece coupled into the work coil due to the magnetic coupling between the two conductors.

See the schematic below.





In practice the resistance of the work coil, the resistance of the tank capacitor, and the resistance of the workpiece all introduce a loss into the tank circuit and damp the resonance. Therefore it is useful to combine all of these losses into a single "loss resistance." In the case of a parallel resonant circuit this loss resistance appears directly across the tank circuit. This resistance represents the only component that can consume power, and therefore we can think of this loss resistance as the load that we are trying to drive power into as efficiently as possible.


When driven at resonance the current drawn by the tank capacitor and the work coil are equal and opposite in phase and therefore cancel each other out as far as the source of power is concerned. This means that the only load presented to the power source at the resonant frequency is the loss resistance across the tank circuit. (Note that, when driven either side of the resonant frequency, there is an additional "out-of-phase" component to the current caused by incomplete cancellation of the work coil current and the tank cap current. This reactive current increases the total magnitude of the current being drawn from the source but does not contribute to any useful heating in the workpiece.)

The job of the matching network is simply to transform this relatively large loss resistance across the tank circuit down to a lower value that better suits the inverter attempting to drive it. There are many different ways to achieve this impedance transformation including tapping the work coil, using a ferrite transformer, a capacitive divider in place of the tank capacitor, or a matching circuit such as an L-match network.

In the case of an L-match network it can transform the relatively high load resistance of the tank circuit down to something around 10 ohms which better suits the inverter. This figure allows the inverter to run from several hundred volts whilst keeping currents down to a reasonable level so that standard switch-mode MOSFETs can be used to perform the switching operation.

The L-match network consists of components Lm and Cm shown below.


The L-match network also has another highly desirable property. The L-match network provides a progressively rising inductive reactance to all frequencies higher than the resonant frequency of the tank circuit. This is very important when the work coil is to be fed from an inverter that generates a squarewave voltage output. Here is an explanation of why this is so…

The squarewave voltage generated by most half-bridge and full-bridge circuits is rich in high frequency harmonics as well as the wanted fundamental frequency. Direct connection of such a voltage source to a parallel resonant circuit would cause excessive currents to flow at the harmonics of the drive frequency! This is because the tank capacitor in the parallel resonant circuit presents a progressively lower capacitive reactance to increasing frequencies. This is potentially very damaging to a voltage-source inverter. It results in large current spikes at the switching transitions as the inverter tries to rapidly charge and discharge the tank capacitor on rising and falling edges of the squarewave. The inclusion of the L-match network between the inverter and the tank circuit negates this problem. Now the output of the inverter sees the inductive reactance of Lm in the matching network first, and all harmonics see a gradually rising inductive impedance.

In summary, the inclusion of an L-match network between the inverter and the parallel resonant tank circuit achieves two things.

1. Impedance matching so that the required amount of power can be supplied from the inverter to the workpiece,
2. Presentation of a rising inductive reactance to high frequency harmonics to keep the inverter safe and happy.

Looking at the previous schematic above we can see that the capacitor in the matching network (Cm) and the tank capacitor (Cw) are both in parallel. In practice both of these functions are usually accomplished by a single capacitor. Most of its capacitance can be thought of as being in parallel resonance with the work coil, with a small amount providing the impedance matching action with the matching inductor (Lm.) Combing these two capacitances into one leads us to arrive at the LCLR model for the work coil arrangement, which is commonly used in industry.


Regards,
Prahlad Purohit