Communication Concepts MRF154 - Manual (Page 7)

7
Broadband RF Power MOSFET
600W, to 80MHz, 50V
M/A-COM Products
Released - Rev. 07.07
MRF154
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typical. Mechanical outline has been fixed. Engineering samples and/or test data may be available.
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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 gate–to–drain (Cgd), and
gate–to–source (Cgs). The PN junction formed during the
fabrication of the RF MOSFET results in a junction capaci-
tance 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 oper-
ating conditions in RF applications.
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
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 tem-
perature 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 es-
sentially 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 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
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.
±0.0005, 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 1/4
, 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 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
Δ 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 dissipater must be sufficiently large and
have low R
θ for moderate air velocity, unless liquid cooling
is employed.