Communication-concepts FM-1KW Broadcast Application Note Manuale Utente Scaricare il pdf (Pagina 8)

RF Reference Design Data

Freescale Semiconductor

MRFE6VP61K25H MRFE6VP61K25HS FM Broadcast

Reliability

Mean time to failure (MTTF) is defined as a 10% reduction

in current handling capability on 50% of the devices within a

given sample size. The primary factor in device failure is due

to metal electromigration on the die surface. Once the

average operating conditions for a given application are

determined, then the MTTF can be calculated using the

thermal resistance R

value given in the MRFE6VP61K25H

product data sheet.

Example: If the desired operating output power is 1100 W,

with 80% drain efficiency.

 I

Drain

= 1100 W / (80%  50 V) ~ 27.5 A

 MRFE6VP61K25H R

=0.15C/W, case temperature =

80C

 Dissipated power = P

-- P

out

 Dissipated power = 50 V  27.5 A -- 1100 W + 4 W = 279 W

 Temperature rise = 279 W  0.15C/W = 42C

 T

rise

=42C+80C = 122C

Utilizing Figure 12 which calculates MTTF versus I

Drain

and T

Drain

= 27.5 A, MTTF for this example is 2700 years.

250

100000

, JUNCTION TEMPERATURE (C)

Figure 12. MTTF versus Junction Temperature

1000

100

110 130 150 170 190

MTTF (YEARS)

210 230

10000

24 Amp

32 Amp

28 Amp

THERMAL MEASUREMENTS

Thermal images of the MRFE6VP61K25H FM broadcast

reference circuit were taken using a FLIR Infrared(IR) T360

camera. The hottest point observed was located at the

Coax1 and Coax2 junction point with the C4 mica capacitor

at 108 MHz. The recorded temperature was 100C after

10 minutes of operation to reach steady state temperature.

Due to the high efficiency achieved by the FM broadcast

reference fixture, the overall baseplate temperature remains

relatively cool at around 60C, with forced air cooling at

25C.

87.5 MHz, 1100 W CW, 79% Drain Efficiency

Figure 13. IR Images of the Output Matching Network

98 MHz, 1100 W CW, 78% Drain Efficiency

108 MHz, 1100 W CW, 80% Drain Efficiency

1 2 3 4 5 6 7 8 9 10 11 12 13 14

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Communication-concepts FM-1KW Broadcast Application Note Manuale Utente Pagina 8

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