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The 'T' Network
The T network finds use where the load resistance is extremely low, even below 1Ω. This is not common in plasma systems but can occur in any application where there is poor coupling between the load material and the electric field generated. This is not unusual in industrial heating. The T network is not able to cope with large variations in load capacitance, unless the inductor can be replaced or adjusted. One benefit of this network is that the bulk of the load current passes through the inductor and load capacitor but not through the tune capacitor. A T network can achieve an RF match into its own circuit losses, risking severe overheating. For this reason components may need to be over-specified compared to an L network and a degree of water cooling may be required. The T network can be used to weld some plastics such as polyurethane which have low dielectric losses and present a very small load to the network. An L network would do the job but only with an oversized load capacitor. It is a poor choice for industrial applications where the more predictable 'L' network would be preferable unless some means can be found to prevent it tuning into its own losses in an empty load condition. |
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