Datasheet

UDG−02139
∆I
ANTI−CROSS
CONDUCTION
SYNCHRONOUS
RECTIFIER ON
BODY DIODE
CONDUCTION
BODY DIODE
CONDUCTION
HIGH SIDE ON
I
D1
I
D2
I
O
SW
0
}
d 1−d
TPS40055-EP
www.ti.com SGLS310D –JULY 2005–REVISED FEBRUARY 2012
Figure 16. Inductor Current and SW Node Waveforms
The maximum allowable power dissipation in the MOSFET is determined by Equation 34.
(34)
where:
(35)
and θ
JA
is the package thermal impedance.
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 31 and the RMS current through the synchronous rectifier MOSFET is described in Equation 36.
(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.
(37)
where:
V
F
is the body diode forward voltage
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.
(38)
where:
Q
RR
is the reverse recovery charge of the body diode.
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