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MRF154 Datasheet, PDF (5/8 Pages) Motorola, Inc – N-CHANNEL BROADBAND RF POWER MOSFET | |||
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RF POWER MOSFET CONSIDERATIONS
MOSFET CAPACITANCES
The physical structure of a MOSFET results in capacitors
between the terminals. The metal oxide gate structure deter-
mines the capacitors from gateâtoâdrain (Cgd), and gateâtoâ
source (Cgs). The PN junction formed during the fabrication
of the RF MOSFET results in a junction capacitance from
drainâtoâsource (Cds).
These capacitances are characterized as input (Ciss), out-
put (Coss) and reverse transfer (Crss) capacitances on data
sheets. The relationships between the interâterminal capaci-
tances and those given on data sheets are shown below. The
Ciss can be specified in two ways:
1. Drain shorted to source and positive voltage at the gate.
2. Positive voltage of the drain in respect to source and zero
volts at the gate. In the latter case the numbers are lower.
However, neither method represents the actual operat-
ing conditions in RF applications.
Cgd
GATE
Cgs
DRAIN
Cds
SOURCE
Ciss = Cgd + Cgs
Coss = Cgd + Cds
Crss = Cgd
LINEARITY AND GAIN CHARACTERISTICS
In addition to the typical IMD and power gain data pres-
ented, Figure 5 may give the designer additional information
on the capabilities of this device. The graph represents the
small signal unity current gain frequency at a given drain cur-
rent level. This is equivalent to fT for bipolar transistors.
Since this test is performed at a fast sweep speed, heating of
the device does not occur. Thus, in normal use, the higher
temperatures may degrade these characteristics to some ex-
tent.
DRAIN CHARACTERISTICS
One figure of merit for a FET is its static resistance in the
fullâon condition. This onâresistance, VDS(on), occurs in the
linear region of the output characteristic and is specified un-
der specific test conditions for gateâsource voltage and drain
current. For MOSFETs, VDS(on) has a positive temperature
coefficient and constitutes an important design consideration
at high temperatures, because it contributes to the power
dissipation within the device.
GATE CHARACTERISTICS
The gate of the RF MOSFET is a polysilicon material, and
is electrically isolated from the source by a layer of oxide.
The input resistance is very high â on the order of 109 ohms
â resulting in a leakage current of a few nanoamperes.
Gate control is achieved by applying a positive voltage
slightly in excess of the gateâtoâsource threshold voltage,
VGS(th).
Gate Voltage Rating â Never exceed the gate voltage
rating. Exceeding the rated VGS can result in permanent
damage to the oxide layer in the gate region.
Gate Termination â The gates of these devices are es-
sentially capacitors. Circuits that leave the gate openâcir-
cuited or floating should be avoided. These conditions can
result in turnâon of the devices due to voltage buildâup on
the input capacitor due to leakage currents or pickup.
Gate Protection â These devices do not have an internal
monolithic zener diode from gateâtoâsource. If gate protec-
tion is required, an external zener diode is recommended.
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.
Since the device mounting flange is made of soft copper, it
may be deformed during various stages of handling or during
transportation. It is recommended that the user makes a final
inspection on this before the device installation. ±0.0005Ⳡis
considered sufficient for the flange bottom.
The same applies to the heat dissipator in the device
mounting area. If copper heatsink is not used, a copper head
spreader is strongly recommended between the device
mounting surfaces and the main heatsink. It should be at
least 1/4â³ thick and extend at least one inch from the flange
edges. A thin layer of thermal compound in all interfaces is,
of course, essential. The recommended torque on the 4â40
mounting screws should be in the area of 4 â 5 lbs.âinch, and
spring type lock washers along with flat washers are recom-
mended.
For die temperature calculations, the â 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
have low Rθ for moderate air velocity, unless liquid cooling is
employed.
MOTOROLA RF DEVICE DATA
MRF154
5
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