TPS51113 datasheet(13/26 Pages) TI

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SW 1

ö æ

÷ ç

ø è

(11)

O UT

RIP P LE

D 2

O UT

RIP P LE

and

(12)

HFET _ Loss

COND1

SW1

(13)

(

)

(

) (

)

RIPPLE

1 - D

RMS2

OUT

(14)

(

)

( )

COND2

RMS2

DS ON 2

(15)

COND3

OUT

´ f

(16)

´ f

(17)

SR _ Loss

COND2

COND3

(18)

TPS51113, TPS51163

www.ti.com........................................................................................................................................................................................................ SLUS864 – MAY 2009

Also, the switching loss can be approximately described as

where

• I

D1 and ID2 are the current magnitudes at the time instance when the MOSFETs switch

where

• t

s1 is the MOSFET switching-on time

• t

s2 is the MOSFET switching-off time

Therefore, the total power loss of the high-side MOSFET is estimated by the sum of the above power losses,

Synchronous Rectifier MOSFET Power Loss

Power loss associated with the synchronous rectifier (SR) MOSFET mainly consists of RDS(on) conduction loss,

body diode conduction loss and reverse recovery loss.

Similarly to the high-side MOSFET, the conduction loss of the SR MOSFET is also the I2R loss of the MOSFET’s

on-resistance, RDS(on)2. Since the switching on-time of the SR MOSFET is (1-D)

, where T is the duration of one

switching cycle, the RMS current of the SR MOSFET can be calculated as follows.

The symchronous rectifier (SR) MOSFET conduction loss is

The body diode conduction loss is

where

• V

F is the forward voltage of the MOSFET body diode

• t

D is the total conduction time of the body diode in one switching cycle

The body diode recovery time – the time it takes for the body diode to restore its blocking capability from forward

conduction state, determines the reverse recovery losses.

where

• Q

RR is the reverse recovery charge of the body diode

Therefore, the total power loss of the SR MOSFET is estimated by the sum of the above power losses.

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