MIC4100_11 Datasheet, PDF (13/18 Pages) MIC GROUP RECTIFIERS – 100V Half Bridge MOSFET Drivers

Micrel, Inc.
MIC4100/1
external
diode
C_B
Vin
Vdd
HB
Level
HI
shift
HO
HS
LI
LO
Vss
Figure 6
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 7 shows a simplified equivalent circuit of the
MIC4100 driving an external MOSFET.
Vdd
HB
External
FET
Dissipation during the external MOSFET Turn-On
Energy from capacitor C_B is used to charge up the input
capacitance of the MOSFET (Cgd and Cgs). The energy
delivered to the MOSFET is dissipated in the three
resistive components, Ron, Rg and Rg_fet. Ron is the on
resistance of the upper driver MOSFET in the MIC4100.
Rg is the series resistor (if any) between the driver IC and
the MOSFET. Rg_fet is the gate resistance of the
MOSFET. Rg_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 Ron and Rg_fet.
The effective capacitance of Cgd and Cgs is difficult to
calculate since they vary non-linearly with Id, Vgs, and
Vds. Fortunately, most power MOSFET specifications
include a typical graph of total gate charge vs. Vgs. Figure
8 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:
E
=
1
2
× Ciss ×V_gs ²
but
Q = C×V
so
E = 1/2 × Qg ×V_gs
where
Ciss is the total gate capacitance of the MOSFET
Ron
Cgd
C_B
HO
Rg Rg_fet
Roff
Cgs
Gate Charge
10
V_DS = 50V
8 ^{ID = 6.9A}
6
4
HS
Figure 7
2
0
0
5 10 15 20 25
Qg - Total Gate Charge (nC)
Figure 8
March 2006
The same energy is dissipated by Roff, Rg and Rg_fet
when the driver IC turns the MOSFET off. Assuming Ron
is approximately equal to Roff, the total energy and power
dissipated by the resistive drive elements is:
13
M9999-031506