MIC4605 Datasheet, PDF (18/25 Pages) Micrel Semiconductor – 85V Half-Bridge MOSFET Drivers

Micrel, Inc.
Gate Driver Power Dissipation
Power dissipation in the output driver stage is mainly
caused by charging and discharging the gate to source
and gate to drain capacitance of the external MOSFET.
Figure 11 shows a simplified equivalent circuit of the
MIC4605 driving an external high-side MOSFET.
E
=
1
2
× C_ISS
×
V_GS ² ,
but
Q = C × V,
so
E
=
1
2
× Q_G
×
V_GS
MIC4605
Eq. 6
Where:
C_ISS = Total gate capacitance of the MOSFET
Figure 11. MIC4605 Driving an External MOSFET
Dissipation during the External MOSFET Turn-On
Energy from capacitor C_B is used to charge up the input
capacitance of the MOSFET (C_GD and C_GS). The energy
delivered to the MOSFET is dissipated in the three
resistive components, R_ON, R_G and R_{G_FET}. R_ON is the on
resistance of the upper driver MOSFET in the MIC4605.
R_G is the series resistor (if any) between the driver IC and
the MOSFET. R_{G_FET} is the gate resistance of the
MOSFET. R_{G_FET} is usually listed in the power MOSFET’s
specifications. The ESR of capacitor C_B and the
resistance of the connecting etch can be ignored since
they are much less than R_ON and R_{G_FET}.
The effective capacitances of C_GD and C_GS are difficult to
calculate because they vary non-linearly with Id, V_GS, and
V_DS. Fortunately, most power MOSFET specifications
include a typical graph of total gate charge vs. V_GS.
Figure 12 shows a typical gate charge curve for an
arbitrary power MOSFET. This chart shows that for a
gate voltage of 10V, the MOSFET requires about 23.5nC
of charge. The energy dissipated by the resistive
components of the gate drive circuit during turn-on is
calculated as noted in Equation 6:
Figure 12. Typical Gate Charge vs. VGS
The same energy is dissipated by R_OFF, R_G, and R_{G_FET}
when the driver IC turns the MOSFET off. Assuming Ron
is approximately equal to R_OFF, the total energy and
power dissipated by the resistive drive elements is
illustrated in Equation 7:
E_DRIVER = Q_G × V_GS
and
P_DRIVER = Q_G × V_GS × f_S
Eq. 7
Where:
E_DRIVER = Energy dissipated per switching cycle
P_DRIVER = Power dissipated per switching cycle
Q_G = Total gate charge at V_GS
V_GS = Gate to source voltage on the MOSFET
f_S = Switching frequency of the gate drive circuit
November 11, 2013
18
Revision 1.0