AN10714_1
© NXP B.V. 2010. All rights reserved.
Application note
Rev. 01 — 26 January 2010
18 of 21
NXP Semiconductors
AN10714
Using the BLF574 in the 88 MHz to 108 MHz FM band
Example:
Assuming the device is running at 600 W with the RF output power at 70 %
efficiency on a heatsink (e.g. 40
°
C). T
j
can be determined based on the operating
efficiency for the given heatsink temperature:
•
Dissipated power (P
d
) = 257 W
•
Temperature rise (T
r
) = P
d
×
R
th
= 257 W
×
(0.4
°
C/W) = 103
°
C
•
Junction temperature (T
j
) = T
h
+ T
r
= 40
°
C + 103
°
C = 143
°
C
Based on this, the TTF can be estimated using a device greased-down heatsink as
follows:
•
The operating current is just above 17 A
•
T
j
= 140
°
C
The curve in
intersects the x-axis at 17 A. At this point, it can be estimated that
it would take 100 years for 0.1 % of the devices to fail.
(1) T
j
= 100
°
C.
(2) T
j
= 110
°
C.
(3) T
j
= 120
°
C.
(4) T
j
= 130
°
C.
(5) T
j
= 140
°
C.
(6) T
j
= 150
°
C.
(7) T
j
= 160
°
C.
(8) T
j
= 170
°
C.
(9) T
j
= 180
°
C.
(10) T
j
= 190
°
C.
(11) T
j
= 200
°
C.
Fig 14. BLF574 time-to-failure
001aal311
10
2
10
10
4
10
3
10
5
TTF
(y)
1
I
dc
(A)
0
20
16
8
12
4
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
