IRF7821PbF
Power MOSFET Selection for Non-Isolated DC/DC Converters
Control FET
Special attention has been given to the power losses
in the switching elements of the circuit - Q1 and Q2.
Power losses in the high side switch Q1, also called
the Control FET, are impacted by the R
ds(on)
of the
MOSFET, but these conduction losses are only about
one half of the total losses.
Power losses in the control switch Q1 are given
by;
Synchronous FET
The power loss equation for Q2 is approximated
by;
*
P
loss
=
P
conduction
+
P
+
P
output
drive
P
loss
=
I
rms
×
R
ds(on)
+
(
g
×
V
g
×
f
)
Q
(
2
)
P
loss
= P
conduction
+ P
switching
+ P
drive
+ P
output
This can be expanded and approximated by;
⎛
Q
⎞
+ ⎜
oss
×
V
in
×
f
+
(
Q
rr
×
V
in
×
f
)
⎝
2
⎠
*dissipated primarily in Q1.
For the synchronous MOSFET Q2, R
ds(on)
is an im-
portant characteristic; however, once again the im-
portance of gate charge must not be overlooked since
it impacts three critical areas. Under light load the
MOSFET must still be turned on and off by the con-
trol IC so the gate drive losses become much more
significant. Secondly, the output charge Q
oss
and re-
verse recovery charge Q
rr
both generate losses that
are transfered to Q1 and increase the dissipation in
that device. Thirdly, gate charge will impact the
MOSFETs’ susceptibility to Cdv/dt turn on.
The drain of Q2 is connected to the switching node
of the converter and therefore sees transitions be-
tween ground and V
in
. As Q1 turns on and off there is
a rate of change of drain voltage dV/dt which is ca-
pacitively coupled to the gate of Q2 and can induce
a voltage spike on the gate that is sufficient to turn
the MOSFET on, resulting in shoot-through current .
The ratio of Q
gd
/Q
gs1
must be minimized to reduce the
potential for Cdv/dt turn on.
P
loss
=
(
I
rms
×
R
ds(on )
)
2
⎛
Q
gs 2
Q
gd
⎞ ⎛
⎞
+⎜
I
×
×
V
in
×
f
⎟ + ⎜
I
×
×
V
in
×
f
⎟
i
g
i
g
⎝
⎠ ⎝
⎠
+
(
Q
g
×
V
g
×
f
)
+
⎛
Q
oss
×
V
in
×
f
⎞
⎝
2
⎠
This simplified loss equation includes the terms Q
gs2
and Q
oss
which are new to Power MOSFET data sheets.
Q
gs2
is a sub element of traditional gate-source
charge that is included in all MOSFET data sheets.
The importance of splitting this gate-source charge
into two sub elements, Q
gs1
and Q
gs2
, can be seen from
Fig 16.
Q
gs2
indicates the charge that must be supplied by
the gate driver between the time that the threshold
voltage has been reached and the time the drain cur-
rent rises to I
dmax
at which time the drain voltage be-
gins to change. Minimizing Q
gs2
is a critical factor in
reducing switching losses in Q1.
Q
oss
is the charge that must be supplied to the out-
put capacitance of the MOSFET during every switch-
ing cycle. Figure A shows how Q
oss
is formed by the
parallel combination of the voltage dependant (non-
linear) capacitances C
ds
and C
dg
when multiplied by
the power supply input buss voltage.
8
Figure A: Q
oss
Characteristic
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