Gate Voltage Rating — Never exceed the gate voltage

damage to the oxide layer in the gate region.

Gate Termination — The gates of these devices are es-

sentially capacitors. Circuits that leave the gate open–cir-

cuited or floating should be avoided. These conditions can

result in turn–on of the devices due to voltage build–up on

the input capacitor due to leakage currents or pickup.

Gate Protection — These devices do not have an internal

monolithic zener diode from gate–to–source. The addition of

an internal zener diode may result in detrimental effects on

the reliability of a power MOSFET. If gate protection is re-

quired, an external zener diode is recommended.

IMPEDANCE CHARACTERISTICS

Device input and output impedances are normally obtained

by measuring their conjugates in an optimized narrow band test

circuit. These test circuits are designed and constructed for a

number of frequency points depending on the frequency cover-

age of characterization. For low frequencies the circuits consist

of standard LC matching networks including variable capacitors

for peak tuning. At increasing power levels the output imped-

ance decreases, resulting in higher RF currents in the matching

network. This makes the practicality of output impedance mea-

surements in the manner described questionable at power lev-

els higher than 200–300 W for devices operated at 50 V and

150–200 W for devices operated at 28 V. The physical sizes

and values required for the components to withstand the RF

currents increase to a point where physical construction of the

output matching network gets difficult if not impossible. For this

reason the output impedances are not given for high power de-

vices such as the MRF154 and MRF157. However, formulas

like

for

a

single

ended

design

for a push–pull design can be used to

obtain reasonably close approximations to actual values.

MOUNTING OF HIGH POWER RF

POWER TRANSISTORS

The package of this device is designed for conduction

cooling. It is extremely important to minimize the thermal re-

sistance between the device flange and the heat dissipator.

If a copper heatsink is not used, a copper head spreader is

strongly recommended between the device mounting sur-

faces and the main heatsink. It should be at least 1/4

″ thick

and extend at least one inch from the flange edges. A thin

layer of thermal compound in all interfaces is, of course, es-

sential. The recommended torque on the 4–40 mounting

screws should be in the area of 4–5 lbs.–inch, and spring

type lock washers along with flat washers are recommended.

For die temperature calculations, the

∆ temperature from a

corner mounting screw area to the bottom center of the

flange is approximately 5

°C and 10°C under normal operat-

ing conditions (dissipation 150 W and 300 W respectively).

The main heat dissipator must be sufficiently large and

employed.

CIRCUIT CONSIDERATIONS

At high power levels (500 W and up), the circuit layout be-

comes critical due to the low impedance levels and high RF

currents associated with the output matching. Some of the

components, such as capacitors and inductors must also

withstand these currents. The component losses are directly

proportional to the operating frequency. The manufacturers

specifications on capacitor ratings should be consulted on

these aspects prior to design.

Push–pull circuits are less critical in general, since the

ground referenced RF loops are practically eliminated, and

the impedance levels are higher for a given power output.

High power broadband transformers are also easier to de-

sign than comparable LC matching networks.

EQUIVALENT TRANSISTOR PARAMETER TERMINOLOGY

Collector

Drain

. . . . . . . . . . . . . . . . .

Emitter

Source

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Base

Gate

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. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

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6

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