TPS40051PWPG4 Datasheet, PDF (20/37 Pages) Texas Instruments – IDE-INPUT SYNCHRONOUS BUCK CONTROLLER

TPS40050
TPS40051
TPS40053
SLUS540F − DECEMBER 2002 − REVISED JUNE 2004
APPLICATION INFORMATION
The maximum allowable power dissipation in the MOSFET is determined by equation (34).
P_T
+
ǒ_{TJ * TA}Ǔ
q_JA
(Watts)
where:
P_T + P_COND ) P_SW(fsw) (Watts)
and θ_JA is the package thermal impedance.
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(34)
(35)
SYNCHRONOUS RECTIFIER MOSFET POWER DISSIPATION
The power dissipated in the synchronous rectifier MOSFET is comprised of three components: R_DS(on)
conduction losses, body diode conduction losses, and reverse recovery losses. R_DS(on) conduction losses can
be found using equation (29) and the RMS current through the synchronous rectifier MOSFET is described in
equation (36).
I_RMS + I_O ^Ǹ1 * d ^ǒAmperes_RMS^Ǔ
(36)
The body-diode conduction losses are due to forward conduction of the body diode during the anti−cross
conduction delay time. The body diode conduction losses are described by equation (37).
P_DC + 2 I_O V_F t_DELAY f_SW (Watts)
(37)
where:
D V_F is the body diode forward voltage
D t_DELAY is the delay time just before the SW node rises
The 2-multiplier is used because the body diode conducts twice during each cycle (once on the rising edge and
once on the falling edge). The reverse recovery losses are due to the time it takes for the body diode to recovery
from a forward bias to a reverse blocking state. The reverse recovery losses are described in equation (38).
P_RR + 0.5 Q_RR V_IN f_SW (Watts)
(38)
where:
D Q_RR is the reverse recovery charge of the body diode
The Q_RR is not always described in a MOSFET’s data sheet, but may be obtained from the MOSFET vendor.
The total synchronous rectifier MOSFET power dissipation is described in equation (39).
P_SR + P_DC ) P_RR ) P_COND (Watts)
(39)
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