www.irf.com4
IRF7807/APbF
5V Supply : Q1=Q2=IRF7807
89
90
91
92
93
94
95
11.522.533.544.55
Load Current (A)
Efficiency (%)
Vin = 10V
Vin = 14V
Vin=24V
Typical Mobile PC Application
The performance of these new devices has been tested
in circuit and correlates well with performance predic-
tions generated by the system models. An advantage
of this new technology platform is that the MOSFETs
it produces are suitable for both control FET and syn-
chronous FET applications. This has been demon-
strated with the 3.3V and 5V converters. (Fig 3 and
Fig 4). In these applications the same MOSFET IRF7807
was used for both the control FET (Q1) and the syn-
chronous FET (Q2). This provides a highly effective
cost/performance solution.
3.3V Supply : Q1=Q2=IRF7807
84
85
86
87
88
89
90
91
92
93
11.522.533.544.55
Load Current (A)
Efficiency (%)
Vin = 10V
Vin = 14V
Vin = 24V
Figure 3 Figure 4
Figure 2: Q
oss
Characteristic
For the synchronous MOSFET Q2, R
ds(on)
is an im-
portant characteristic; however, once again the impor-
tance 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.
Spice model for IRF7807 can be downloaded in ma-
chine readable format at www.irf.com.
