irlr3717 International Rectifier Corp., irlr3717 Datasheet - Page 8

• Manufacturer Part Number: irlr3717
• Description: Hexfet Power Mosfet
• Manufacturer: International Rectifier Corp.

Control FET
P
P
IRLR/U3717
Power MOSFET Selection for Non-Isolated DC/DC Converters
loss
8
loss
This can be expanded and approximated by;
gs2
oss
gs2
 I u
 Q

= P
oss
§
¨
©
§
©
I
Q
rms
conduction
g
2
oss
u V
2
Q
u R
dmax
i
u V
g
gd
g
u f
ds(on )
in
u V
+ P
u f
in
switching
u f
·
¹
ds
gs1
·
¸  I u
¹
+ P
gs2
§
¨
©
dg
gs2
oss
drive
Q
+ P
i
gs 2
g
u V
output
ds(on)
in
u f
·
¸
¹
gs2
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
 Q

P
§
¨
©
I
conduction
rms
Q
oss
g
2
oss
2
u V
Characteristic
gd
u R
/Q
u V
g
gs1
 P
ds(on)
u f
in
in
must be minimized to reduce the
. As Q1 turns on and off there is
drive
u f
rr
·
¹
 P
both generate losses that
 Q
output
*
rr
u V
www.irf.com
in
ds(on)
u f
oss
is an im-
and re-