Datasheet


SLVS612 − APRIL 2006
16
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APPLICATION INFORMATION
dv/dt Induced Turn−On
MOSFETs are susceptible to dv/dt turn-on particularly in high-voltage (V
DS
) applications. The turn-on is caused
by the capacitor divider that is formed by C
GD
and C
GS
. High dv/dt conditions and drain-to-source voltage, on
the MOSFET causes current flow through C
GD
and causes the gate-to-source voltage to rise. If the
gate-to-source voltage rises above the MOSFET threshold voltage, the MOSFET turns on, resulting in large
shoot-through currents. Therefore, the SR MOSFET should be chosen so that the C
GD
capacitance is smaller
than the C
GS
capacitance.
High Side MOSFET Power Dissipation
The power dissipated in the external high-side MOSFET is comprised of conduction and switching losses. The
conduction losses are a function of the I
RMS
current through the MOSFET and the R
DS(on)
of the MOSFET. The
high-side MOSFET conduction losses are defined by equation (26).
P
COND
+
ǒ
I
RMS
Ǔ
2
R
DS(on)
ǒ
1 ) TC
R
ƪ
T
J
* 25
ƫ
Ǔ
(Watts)
where:
D TC
R
is the temperature coefficient of the MOSFET R
DS(on)
The TC
R
varies depending on MOSFET technology and manufacturer but is typically ranges between
.0035 ppm/_C and .010 ppm/_C.
The I
RMS
current for the high side MOSFET is described in equation (27).
I
RMS
+ I
O
d
Ǹ
ǒ
Amperes
RMS
Ǔ
The switching losses for the high-side MOSFET are descibed in equation (28).
P
SW(fsw)
+
ǒ
V
IN
I
OUT
t
SW
Ǔ
f
SW
(Watts)
where:
D I
O
is the DC output current
D t
SW
is the switching rise time, typically < 20 ns
D f
SW
is the switching frequency
Typical switching waveforms are shown in Figure 8.
(26)
(27)
(28)