TPS40056

SLVS612 − APRIL 2006

22

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DESIGN EXAMPLE

4. Calculate the power losses

Power losses in the high-side MOSFET (Si7860DP) at 14.4-VIN where switching losses dominate can be

calculated from equation (27).

I

RMS + IO

d

+ 8

0.086

+ 2.35 A

substituting (27) into (26) yields

P

COND + 2.35

2

0.008

(1

) 0.007

(150

* 25)) + 0.083 W

and from equation (28), the switching losses can be determined.

P

SW(fsw) + VIN

I

O

t

SW

f

SW + 14.4 V

8A

20 ns

170 kHz

+ 0.39 W

The MOSFET junction temperature can be found by substituting equation (30) into equation (29)

T

J + PCOND ) PSW

q

JA ) TA +

(0.083

) 0.39)

40

) 85 + 90OC

5. Calculate synchronous rectifier losses

The synchronous rectifier MOSFET has two 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 (31)

I

RMS + IO

1

* d + 8

1

* 0.126 + 7.48 A

RMS

The synchronous MOSFET conduction loss from (26) is:

P

COND + 7.48

2

0.008

(1

) 0.007(150 * 25)) + 0.83 W

The body diode conduction loss from (32) is:

P

DC + 2

I

O

V

FD

t

DELAY

f

SW + 2

8.0 A

0.8 V

100 ns

170 kHz

+ 0.218 W

The body diode reverse recovery loss from (33) is:

P

RR + 0.5

Q

RR

V

IN

f

SW + 0.5

30 nC

14.4 V

170 kHz

+ 0.037 W

The total power dissipated in the synchronous rectifier MOSFET from (34) is:

P

SR + PRR ) PCOND ) PDC + 0.037 ) 0.83 ) 0.218 + 1.085 W

The junction temperature of the synchronous rectifier at 85

°C is:

T

J + PSR

q

JA ) TA +

(1.085)

40

) 85 + 128oC

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.

(44)

(45)

(46)

(47)

(48)

(49)

(50)

(51)

(52)

(53)