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Micrel
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 MIC4103.
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 trace can be ignored since they are
much less than R ON and R G_FET .
The effective capacitance of C GD and C GS is difficult to
calculate since they vary non-linearly with I D , V GS , and V DS .
Fortunately, most power MOSFET specifications include a
typical graph of total gate charge vs. V GS . 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:
MIC4103/4104
The same energy is dissipated by R OFF , R G , and R G_FET
when the driver IC turns the MOSFET off. Assuming R ON is
approximately equal to Roff, the total energy and power
dissipated by the resistive drive elements is:
E driver = Q G × V GS
and
P driver = Q G × V GS × fs
where
E driver is the energy dissipated per switching cycle
P driver is the power dissipated by switching the MOSFET on and off
Q G is the total gate charge at Vgs
V GS is the gate to source voltage on the MOSFET
fs is the switching frequency of the gate drive circuit
The power dissipated inside the MIC4103/4 is equal to the
ratio of R ON & R OFF to the external resistive losses in R G
and R G_FET . Letting R ON =R OFF , the power dissipated in the
MIC4103 due to driving the external MOSFET is:
E =
1
2
× Ciss × V gs 2
Pdiss drive = P driver
R ON
R ON
+ R G + R G _ FET
but
Q = C × V
so
E = 1/2 × Qg × V gs
where
Ciss is the total gate capacitanc e of the MOSFET
Supply Current Power Dissipation
Power is dissipated in the MIC4103 even if there is nothing
being driven. The supply current is drawn by the 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 V DD and V HB
voltages. The typical characteristic graphs show how
supply current varies with switching frequency and supply
voltage.
The power dissipated by the MIC4103 due to supply
current is:
Pdiss sup ply = V DD × I DD + V HB × I HB
Total Power Dissipation and Thermal Considerations
Total power dissipation in the MIC4103 or MIC4104 is
equal to the power dissipation caused by driving the
external MOSFETs, the supply current, and the internal
bootstrap diode.
Pdiss total = Pdiss supply + Pdiss drive + Pdiode total
Figure 8. Typical Gate Charge vs. VGS
November 2010
14
M9999-110910-B
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