irlr7821cpbf International Rectifier Corp., irlr7821cpbf Datasheet - Page 8

• Manufacturer Part Number: irlr7821cpbf
• Description: 30v Single N-channel Hexfet Power Mosfet In A D-pak Package Designed For The Consumer Market
• Manufacturer: International Rectifier Corp.

Control FET
P
P
Power MOSFET Selection for Non-Isolated DC/DC Converters
loss
8
loss
This can be expanded and approximated by;
= I
+ I ×
+ Q
+
= P
(
⎛
⎜
⎝
(
⎛
⎝
Q
rms
conduction
g
2
oss
× V
2
Q
× R
i
× V
g
gd
g
× f
ds(on )
in
× V
+ P
× f
)
in
)
switching
× f
⎞
⎠
⎞
⎟ + I ×
⎠
+ P
⎛
⎜
⎝
drive
Q
+ P
i
gs 2
g
× V
output
in
× f
⎞
⎟
⎠
Synchronous FET
by;
*dissipated primarily in Q1.
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
verse recovery charge Q
are transfered to Q1 and increase the dissipation in
that device. Thirdly, gate charge will impact the
MOSFETs’ susceptibility to Cdv/dt turn on.
of the converter and therefore sees transitions be-
tween ground and V
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
potential for Cdv/dt turn on.
Figure A: Q
P
P
loss
loss
The power loss equation for Q2 is approximated
For the synchronous MOSFET Q2, R
The drain of Q2 is connected to the switching node
= P
= I
+ Q
+
(
(
⎛
⎜
⎝
conduction
rms
Q
oss
g
2
oss
2
× V
Characteristic
gd
× R
/Q
× V
g
gs1
+ P
ds(on)
× f
in
in
must be minimized to reduce the
. As Q1 turns on and off there is
drive
× f
)
)
rr
⎞
⎠
+ P
both generate losses that
+ Q
output
(
*
rr
× V
www.irf.com
in
ds(on)
× f
oss
is an im-
and re-
)