IRF7805 Datasheet, PDF (4/8 Pages) International Rectifier – Chip-Set for DC-DC Converters

IRF7805/IRF7805A
For the synchronous MOSFET Q2, R is an im-
ds(on)
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 and re-
oss
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 . As Q1 turns on and off there is
in
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 IRF7805 can be downloaded in ma-
chine readable format at www.irf.com.
Figure 2: Q_oss Characteristic
4
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