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TPS40054 Datasheet, PDF (27/33 Pages) Texas Instruments – WIDE INPUT SYNCHRONOUS BUCK CONTROLLER
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DESIGN EXAMPLE
TPS40054
TPS40055
TPS40057
SLUS593B − DECEMBER 2003 − REVISED APRIL 2004
4. Calculate the power losses
Power losses in the high-side MOSFET (Si7860DP) at 24-VIN where switching losses dominate can be
calculated from equation (51).
IRMS + IO Ǹd + 8 Ǹ0.135 + 2.93 A
(51)
substituting (34) into (33) yields
PCOND + 2.932 0.008 (1 ) 0.007 (150 * 25)) + 0.129 W
(52)
and from equation (33), the switching losses can be determined.
PSW(fsw) + ǒVIN IO tSWǓ fSW + 24 V 8 A 20 ns 300 kHz + 1.152 W
(53)
The MOSFET junction temperature can be found by substituting equation (35) into equation (34)
TJ + ǒPCOND ) PSWǓ qJA ) TA + (0.129 ) 1.152) 40 ) 85 + 136OC
(54)
5. Calculate synchronous rectifier losses
The synchronous rectifier MOSFET has two (2) loss components, conduction, and diode reverse recovery
losses. The conduction losses are due to IRMS losses as well as body diode conduction losses during the dead
time associated with the anti-cross conduction delay.
The IRMS current through the synchronous rectifier from (38)
IRMS + IO Ǹ1 * d + 8 Ǹ1 * 0.135 + 7.44 ARMS
(55)
The synchronous MOSFET conduction loss from (33) is:
PCOND + IRMS2 RDS(on) + 7.442 0.008 (1 ) 0.007(150 * 25)) + 0.83 W
(56)
The body diode conduction loss from (39) is:
PDC + 2 IO VFD tDELAY fSW + 2 8.0 A 0.8 V 100 ns 300 kHz + 0.384 W (57)
The body diode reverse recovery loss from (40) is:
PRR + 0.5 QRR VIN fSW + 0.5 30 nC 24 V 300 kHz + 0.108 W
(58)
The total power dissipated in the synchronous rectifier MOSFET from (41) is:
PSR + PRR ) PCOND ) PDC + 0.108 ) 0.83 ) 0.384 + 1.322 W
(59)
The junction temperature of the synchronous rectifier at 85°C is:
TJ + PSR qJA ) TA + (1.322) 40 ) 85 + 139oC
(60)
In typical applications, paralleling the synchronous rectifier MOSFET with a Schottky rectifier increases the
overall converter efficiency by approximately 2% due to the lower power dissipation during the body diode
conduction and reverse recovery periods.
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