IRF7807PBF Datasheet, PDF (3/8 Pages) International Rectifier – HEXFET® Chip-Set for DC-DC Converters

IRF7807/APbF
Power MOSFET Selection for DC/DC
Converters
Control FET
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 R_ds(on) of the MOSFET,
but these conduction losses are only about one half of
the total losses.
Power losses in the control switch Q1 are given by;
P =P
+P +P +P
loss
conduction
switching
drive
output
This can be expanded and approximated by;
( ) P_loss =
I2
rms
×
R_{ds(on )}
+
⎛
⎜I
⎝
×
Q_gd
i_g
× V_in
×
f ^⎞⎟
⎠
⎛
+⎜ I
⎝
×
Q_{gs 2}
i_g
× V_in
×
f ^⎞⎟
⎠
( ) + Q_g × V_g × f
+
⎛
⎝
Q_oss
2
× V_in
×
f
⎞
⎠
This simplified loss equation includes the terms Q_gs2
and Q_oss which are new to Power MOSFET data sheets.
Q_gs2 is a sub element of traditional gate-source charge
that is included in all MOSFET data sheets. The impor-
tance of splitting this gate-source charge into two sub
elements, Q_gs1 and Q_gs2, can be seen from Fig 1.
Q_gs2 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 I_dmax (t2) at which time the drain voltage
begins to change. Minimizing Q_gs2 is a critical factor in
reducing switching losses in Q1.
Q_oss is the charge that must be supplied to the output
capacitance of the MOSFET during every switching
cycle. Figure 2 shows how Q_oss is formed by the paral-
lel combination of the voltage dependant (non-linear)
capacitance’s C_ds and C_dg when multiplied by the power
supply input buss voltage.
4
Drain Current
t2 _t3
t1
V_GTH
t0
1
Gate Voltage
2
Drain Voltage
Figure 1: Typical MOSFET switching waveform
Synchronous FET
The power loss equation for Q2 is approximated
by;
P_loss
=
P_conduction
+
P_drive
+
P*
output
( ) ^Ploss ⁼
I2
rms
×
R_ds(on)
( ) + Q_g × V_g × f
( ) + ⎛⎜ Qoss
⎝2
× V_in ×
f
⎞
⎠
+
Q_rr × V_in × f
*dissipated primarily in Q1.
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