IRF7805/IRF7805A
Power MOSFET Selection for DC/DC
Converters
4
Drain Current
Control FET
1
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 Rds(on) of the MOSFET,
but these conduction losses are only about one half of
the total losses.
Gate Voltage
t2
t3
t1
VGTH
t0
Power losses in the control switch Q1 are given by;
2
Drain Voltage
Ploss = Pconduction+ Pswitching+ Pdrive+ Poutput
This can be expanded and approximated by;
Figure 1: Typical MOSFET switching waveform
2
Synchronous FET
P
I
R
×
ds(on)
=
+
(
)
loss
rms
The power loss equation for Q2 is approximated
by;
Qgd
ig
Qgs2
ig
I
V
f
I
V
f
×
×
×
×
+
×
×
in
in
P
P
P
P*
+
output
=
=
+
loss
conduction
drive
Q
(
V
f
×
+
+
×
)
g
g
2
P
Irms
R
×
ds(on)
loss
( )
Qoss
2
V
f
×
×
in
Q
(
V
f
×
+
×
)
g
g
This simplified loss equation includes the terms Qgs2
and Qoss which are new to Power MOSFET data sheets.
is a sub element of traditional gate-source charge
Qoss
2
V
f
Q
V
×
in
f
×
+
×
×
+
(
)
in
rr
Q
gs2
that is included in all MOSFET data sheets.The impor-
tance of splitting this gate-source charge into two sub
elements, Qgs1 and Qgs2, can be seen from Fig 1.
Qgs2 indicates the charge that must be supplied by
the gate driver between the time that the threshold volt-
age has been reached (t1) and the time the drain cur-
rent rises to Idmax (t2) at which time the drain voltage
begins to change. Minimizing Qgs2 is a critical factor in
reducing switching losses in Q1.
*dissipated primarily in Q1.
Qoss is the charge that must be supplied to the output
capacitance of the MOSFET during every switching
cycle. Figure 2 shows how Qoss is formed by the paral-
lel combination of the voltage dependant (non-linear)
capacitance’s Cds and Cdg when multiplied by the power
supply input buss voltage.
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