Communication Concepts AN762 Application Note User Manual
Page 6
AR
C
HIVE INF
O
RMA
TI
O
N
PRODUCT TRANSFERRED T
O
M/A
–
COM
AN762
6
RF Application Reports
ЗЗЗЗ
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ЗЗЗ
ЗЗЗ
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ЗЗЗ
ЗЗЗ
ЗЗЗ
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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.