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P
QSW
+ P
QSW(CON)
) P
QSW(SW)
) P
QSW(GATE)
(25)
P
QSW(CON)
+ R
DS(on)
I
D
2
+ R
DS(on)
V
O
V
IN
ǒ
I
LOAD
2
)
DI
2
12
Ǔ
(26)
P
QSW(SW)
+ V
IN
f
SW
ȧ
ȱ
Ȳ
ǒ
I
LOAD
)
DI
2
Ǔ
ǒ
Q
gs1
) Q
gd
Ǔ
I
g
)
Q
OSS(SW)
) Q
OSS(SR)
2
ȧ
ȳ
ȴ
(27)
P
QSW(GATE)
+ Q
g(TOT)
V
g
F
SW
(28)
TPS40075
SLUS676A – MAY 2006 – REVISED SEPTEMBER 2007
Equation 25 through Equation 28 can be used to calculate the power loss, P
QSW
, in the switching MOSFET
where
• P
QSW(CON)
= conduction losses
• P
QSW(SW)
= switching losses
• P
QSW(GATE)
= gate drive losses
• Q
gd
= drain source charge or miller charge
• Q
gs1
= gate source post threshold charge
• I
g
= gate drive current
• Q
OSS(SW)
= switching MOSFET output charge
• Q
OSS(SR)
= synchronous MOSFET output charge
• Qg
(TOT)
= total gate charge from zero volts to the gate voltage
• Vg = gate voltage
If the total estimated loss is split evenly between conduction and switching losses, Equation 25 and Equation 26
yields preliminary values for R
DS(on)
and (Q
gs1
+ Q
gd
). Note output losses due to Q
OSS
and gate losses have been
ignored here. Once a MOSFET is selected these parameters can be added.
The switching MOSFET for this design should have an R
DS(on)
of less than 9 m Ω . The sum of Q
gd
and Q
gs
should
be approximately 4 nC.
It is not always possible to get a MOSFET which meets both these criteria so a comprise may have to be made.
Also by selecting different MOSFETs close to this criteria and calculating power loss the final selection can be
made. It was found that the PH6325L MOSFET from Philips semiconductor gave reasonable results. This device
has an R
DS(on)
of 6.3 m Ω and a (Qgs1+Qgd) of 5.9 nC. The estimated conduction losses are 0.178 W and the
switching losses are 0.270 W. This gives a total estimated power loss of 0.448 W versus 0.5 W for our initial
boundary condition. Note this does not include gate losses of approximately 10 mW and output losses of less
than 1 mW.
3.1.5 Rectifier MOSFET, QSR
Similar criteria can be used for the rectifier MOSFET. There is one significant difference. Due to the body diode
conducting, the rectifier MOSFET switches with near zero voltage across its drain and source so effectively with
near zero switching losses. However, there are some losses in the body diode. These are minimized by reducing
the delay time between the transition from the switching MOSFET turn off to rectifier MOSFET turn on and vice
versa. The TPS40075 incorporates TI's proprietary predictive gate drive which helps reduce this delay to
between 10 ns and 20 ns.
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