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MRF154_17 Datasheet, PDF (7/10 Pages) M/A-COM Technology Solutions, Inc. – Broadband RF Power MOSFET | |||
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MRF154
Broadband RF Power MOSFET
600W, to 80MHz, 50V
Rev. V1
RF POWER MOSFET CONSIDERATIONS
The physical structure of a MOSFET results in capacitors
between the terminals. The metal oxide gate structure de-
termines the capacitors from gateâtoâdrain (Cgd), and gate
âtoâsource (Cgs). The PN junction formed during the fabri-
cation of the RF MOSFET results in a junction capacitance
from drainâtoâsource (Cds).
These capacitances are characterized as input (Ciss),
output (Coss) and reverse transfer (Crss) capacitances on
data sheets. The relationships between the interâterminal
capacitances 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 oper-
ating conditions in RF applications.
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 openâ
circuited 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.
LINEARITY AND GAIN CHARACTERISTICS
In addition to the typical IMD and power gain data pre-
sented, Figure 5 may give the designer additional informa-
tion on the capabilities of this device. The graph represents
the small signal unity current gain frequency at a given
drain current level. This is equivalent to fT for bipolar tran-
sistors. 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 extent.
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
under specific test conditions for gateâsource voltage and
drain current. For MOSFETs, VDS(on) has a positive tem-
perature coefficient and constitutes an important design
consideration at high temperatures, because it contributes
to the power dissipation within the device.
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
resistance between the device flange and the heat dissipa-
tor.
Since the device mounting flange is made of soft copper,
itmay 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 installa-
tion. ï±0.0005ï¬ is 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 de-
vice mounting surfaces and the main heatsink. It should be
at least 1/4ï¬ thick and extend at least one inch from the
flange edges. A thin layer of thermal compound in all inter-
faces is, of course, essential. The recommended torque on
the 4â40 mounting screws should be in the area of 4â5 lbs.
âinch, and spring type lock washers along with flat washers
are recommended.
For die temperature calculations, the ï temperature from a
corner mounting screw area to the bottom center of the
flange is approximately 5ï°C and 10ï°C under normal operat-
ing conditions (dissipation 150 W and 300 W respectively).
The main heat dissipater must be sufficiently large and
have low Rï± for moderate air velocity, unless liquid cooling
is employed.
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M/A-COM Technology Solutions Inc. (MACOM) and its affiliates reserve the right to make changes to the product(s) or information contained herein without notice.
Visit www.macom.com for additional data sheets and product information.
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