MRF157
Linear RF Power MOSFET
600W, to 80MHz
Designed primarily for linear large signal output stages
to 80 MHz.
Specified 50 volts, 30 MHz characteristics
Output power = 600 watts
Power gain = 21 dB (typ.)
Efficiency = 45% (typ.)
Rev. V1
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1
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MRF157
Linear RF Power MOSFET
600W, to 80MHz
Rev. V1
2
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.
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MRF157
Linear RF Power MOSFET
600W, to 80MHz
Rev. V1
3
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.
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MRF157
Linear RF Power MOSFET
600W, to 80MHz
Rev. V1
4
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.
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MRF157
Linear RF Power MOSFET
600W, to 80MHz
Rev. V1
5
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.
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MRF157
Linear RF Power MOSFET
600W, to 80MHz
Rev. V1
6
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MRF157
Linear RF Power MOSFET
600W, to 80MHz
Rev. V1
RF POWER MOSFET CONSIDERATIONS
MOSFET CAPACITANCES
The physical structure of a MOSFET results in capacitors
between the terminals. The metal oxide gate structure determines the capacitors from gate–to–drain (Cgd), and gate
–to–source (Cgs). The PN junction formed during the fabrication of the RF MOSFET results in a junction capacitance
from drain–to–source (Cds).
These capacitances are characterized as input (Ciss),
output (Coss) and reverse transfer (Crss) capacitances on
data sheets. The relationships between the inter–terminal
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 operating conditions in RF applications.
LINEARITY AND GAIN CHARACTERISTICS
In addition to the typical IMD and power gain data presented, 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 current 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 extent.
DRAIN CHARACTERISTICS
One figure of merit for a FET is its static resistance in the
full–on condition. This on–resistance, VDS(on), occurs in
the linear region of the output characteristic and is specified
under specific test conditions for gate–source 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 — on the order of 109
ohms
— resulting in a leakage current of a few nanoamperes.
Gate control is achieved by applying a positive voltage
slightly in excess of the gate–to–source threshold voltage,
VGS(th).
Gate Voltage Rating — 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 — The gates of these devices are essentially capacitors. Circuits that leave the gate open–
circuited 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. If gate protection is required, 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 coverage 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 impedance decreases,
resulting in higher RF currents in the matching network.
This makes the practicality of output impedance measurements in the manner described questionable at power levels 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 devices such as the MRF154 and
MRF157.
However, formulas like
for a single ended
design
or for a push–pull design can be
used to obtain reasonably close approximations to actual
values.
7
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MRF157
Linear RF Power MOSFET
600W, to 80MHz
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 dissipator.
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 installation. 0.0005 is considered sufficient for the flange bottom.
The same applies to the heat dissipator in the device
mounting area. If copper heat sink is not used, a copper
head spreader is strongly recommended between the device mounting surfaces and the main heat sink. 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, essential. 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 5C and 10C under normal operating conditions (dissipation 150 W and 300 W respectively).
The main heat dissipater must be sufficiently large and
have low R for moderate air velocity, unless liquid cooling
is employed.
Rev. V1
CIRCUIT CONSIDERATIONS
At high power levels (500 W and up), the circuit layout
becomes 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 design than comparable LC matching networks.
8
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MRF157
Linear RF Power MOSFET
600W, to 80MHz
Rev. V1
9
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.
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https://www.macom.com/support
MRF157
Linear RF Power MOSFET
600W, to 80MHz
Rev. V1
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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.
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