Datasheet
P
T
+
T
J
* T
A
q
JA
(Watts)
q
JA
+ 36.515
O
CńW
P
T
+
ǒ
2 Q
g
f
SW
) I
Q
Ǔ
V
IN
(Watts)
P
T
+
ǒ
2 P
D
V
DR
) I
Q
Ǔ
V
IN
(Watts)
P
D
+ Q
g
V
DR
f
SW
(Wattsńdriver)
P
SR
+ P
DC
) P
RR
) P
COND
(Watts)
P
RR
+ 0.5 Q
RR
V
IN
f
SW
(Watts)
P
DC
+ 2 I
O
V
F
t
DELAY
f
SW
(Watts)
I
RMS
+ I
O
1 * d
Ǹ
ǒ
Amperes
RMS
Ǔ
TPS40054
TPS40055
TPS40057
SLUS593H –DECEMBER 2003–REVISED JULY 2012
www.ti.com
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 defined
using Equation 31 and the RMS current through the synchronous rectifier MOSFET is described in Equation 37.
(37)
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 38.
where
• V
F
is the body diode forward voltage
• t
DELAY
is the delay time just before the SW node rises (38)
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 recover
from a forward bias to a reverse blocking state. The reverse recovery losses are described in Equation 39.
where
• Q
RR
is the reverse recovery charge of the body diode (39)
The Q
RR
is not always described in a MOSFET data sheet, but may be obtained from the MOSFET vendor. The
total synchronous rectifier MOSFET power dissipation is described in Equation 40.
(40)
TPS4005x POWER DISSIPATION
The power dissipation in the TPS4005x is largely dependent on the MOSFET driver currents and the input
voltage. The driver current is proportional to the total gate charge, Qg, of the external MOSFETs. Driver power
(neglecting external gate resistance, ( refer to PowerPAD Thermally Enhanced Package
[2]
) can be calculated
from Equation 41.
(41)
And the total power dissipation in the TPS4005x, assuming the same MOSFET is selected for both the high-side
and synchronous rectifier, is described in Equation 42.
(42)
or
where
• I
Q
is the quiescent operating current (neglecting drivers) (43)
The maximum power capability of the PowerPad package is dependent on the layout as well as air flow. The
thermal impedance from junction to air, assuming 2 oz. copper trace and thermal pad with solder and no air flow,
(44)
The maximum allowable package power dissipation is related to ambient temperature by Equation 45.
(45)
Substituting Equation 38 into Equation 43 and solving for f
SW
yields the maximum operating frequency for the
TPS4005x. The result is described in Equation 46.
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