Communication Concepts AN762 Application Note - Manual (Page 6)

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C
HIVE INF
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RMA
TI
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PRODUCT TRANSFERRED T
O
M/A
–
COM
AN762
6 RF Application Reports
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Loops can be provided for current probe measurements.
L3 L4
T3
C6 C6
C C d a
c b
E
E
T2
E Q2 Q1
E
B
B
L5
C10
C11
R4
R4
R2
R2 R3
R3
R1
R1
L1 L2
Q3
R12
T1
R5
R10
R6
R9 R7
C12
R8
MC1723G
D2
R11
C8 C9
C3 C2
D1
B C E
C4
C1
Board Stand Off’s
Terminal Pins and Feedthroughs
Feedthrough Eyelets
OUTP GND
NEG POS
Figure 4. Component Layout of tthe Basic Amplifier
The thermal design (determining the size and type of a
heat sink required) can be accomplished with information
in the device data sheet and formulas presented in
references 5 and 6. As an example, with the 180 W unit using
MRF421’s, the Junction-to-Ambient Temperature (R
θJA ) is
calculated first as
P
T
J
– T
A
R θJA =
where:
T
J
= Maximum Allowed Junction Temperature
(150
°C).
T
A
= Ambient Temperature (40 °C).
P = Dissipated Power (180/ η) x (100 – η)
η = Collector Efficiency (%).
If the worst case efficiency at 180 W CW is 55%, then
P = 148 W, and
= 1.49 °C/W (for one device).
(148/2)
150 – 40
R θJA =
The Heat Sink-to-Ambient Thermal Resistance, R θSA =
R θJA – (R θJC + R θCS ) where: R θJC = Device Junction-to-
Case Thermal Resistance, 0.60 °C/W* (from data sheet).
R θCS = Thermal Resistance, Case-to-Heat Sink, 0.1 °C/W
(from table in reference 5). Then:
= 0.395 °C/W
2
1.49 – (0.60 + 0.1)
R θSA =
* The R θJC figure of 0.85 °C/W given for the MRF421 is in error, and
will be corrected in the future prints of the data sheet.