IRF7807VPBF Datasheet, PDF (3/8 Pages) International Rectifier – HEXFET® Power MOSFET

IRF7807VPbF
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_loss = P + _conduction P + _switching P_drive+ P_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 importance 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
voltage has been reached (t1) and the time the drain
current rises to I_dmax (t2) at which time the drain volt-
age begins to change. Minimizing Q_gs2 is a critical fac-
tor in reducing switching losses in Q1.
Q_oss is the charge that must be supplied to the out-
put capacitance of the MOSFET during every switch-
ing cycle. Figure 2 shows how Q_oss is formed by the
parallel combination of the voltage dependant (non-
linear) capacitance’s C_ds and C_dg when multiplied by
the power supply input buss voltage.
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t1
V_GTH
t0
4
Drain Current
t2 _t3
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 ^{= I}rms ^{2 × R}ds(on)
( ) + Q_g × V_g × f
( ) + ⎛⎜ Qoss
⎝2
× V_in
×
f^⎞
⎠
+
Q_rr × V_in × f
*dissipated primarily in Q1.
3