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By Richard Frey, P.E.

Click here to download the entire application note in pdf format.

With the improvement in high power MOSFETs of late – lower gate charge, low loss gate structures, and much improved frequency capability – it has become more possible to employ these "switchmode" devices in rf generators at medium hf. The objective is to improve the ‘power density' (W/m3) and efficiency of the equipment. The most common problem associated with this application is getting the matching right. For the gate drive side it is matter of obtaining an efficient and repeatable match to a very small impedance. On the drain side it is a matter of choosing a proper operating class and power goal for the device and then designing a network to provide repeatable and cost effective realization. The discussion below outlines the procedure for both.

The Input
The gate of a power MOSFET can be modeled as a series RLC circuit. The C is the specified C_iss from the data sheet at the desired operating supply voltage. The R is the effective series resistance, ESR, of C_iss and includes dielectric losses, gate metalization (or polysilicon conductor), and bond wire conduction losses. The L is essentially bond wire inductance, generally package specific. The resonant frequency of this RLC for large switchmode devices is usually in the middle hf range, 10-30 MHz, and the Q is fairly high.

As an example, the APT 5010LLC data sheet C_iss is 5200pF at Vds of 25V. In practice, a higher drain voltage is employed and this value will be somewhat smaller. On data sheets where the C values are graphed, it is common for the graph to stop at 50 or 100 volts. When graphed on a log-log scale, the C can be extrapolated linearly to the proper operating voltage. The effective gate C in an amplifier is increased through the action of the Miller capacitance. Generally the 50 or 25 volt value is a good place to start.

Click here to download the entire application note in pdf format.

Microsemi Corporation