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Micrel, Inc.
Figure 6 shows a simplified equivalent circuit of the
MIC4102 driving an external MOSFET.
External
MIC4102
Vdd
Ron
HB
Cgd
FET
C B
HO
Roff
HS
Rg
Rg_fet
Cgs
Figure 7. Typical Gate Charge vs. V GS
Figure 6. MIC4103 Driving an External MOSFET
The same energy is dissipated by Roff, Rg and Rg_fet
when the driver IC turns the MOSFET off.
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
E driver =
and
1
2
× Qg × V gs
P driver =
× Qg × V gs × fs
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
MIC4102. 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 7 shows a typical gate charge curve for an
arbitrary power MOSFET. This chart shows that for a
1
2
where
E driver is the energy dissipated during turn - on or turn - off
P driver is the power dissipated during turn - on or turn - off
Qg is the total gate charge at Vgs
Vgs is the gate to source voltage on the MOSFET
fs is the switching frequency of the gate drive circuit
The power dissipated inside the MIC4102 equals the
ratio of Ron & Roff to the external resistive losses in Rg
and Rg_fet. The power dissipated in the MIC4102 due
to driving the external MOSFET is:
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
Pdiss drive = P driver ×
Ron
Ron + Rg + Rg _ fet
+ P driver ×
Roff
Roff + Rg + Rg _ fet
calculated as:
E =
but
1
2
× Ciss × V gs 2
Supply Current Power Dissipation
Power is dissipated in the MIC4102 even if is there is
nothing being driven. The supply current is drawn by the
Q = C × V
so
E = 1/2 × Qg × V gs
where
Ciss is the total gate capacitanc e of the MOSFET
bias for the internal circuitry, the level shifting circuitry
and shoot-through current in the output drivers. The
supply current is proportional to operating frequency and
the Vdd and Vhb voltages. The typical characteristic
graphs show how supply current varies with switching
frequency and supply voltage.
November 2006
12
M9999-112806
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