MIC4103 Datasheet, PDF (12/17 Pages) Micrel Semiconductor – 100V Half Bridge MOSFET Drivers 3/2A Sinking/Sourcing Current

Micrel
and gate to drain capacitance of the external MOSFET.
Figure 7 shows a simplified equivalent circuit of the
MIC4103 driving an external MOSFET.
MIC4103/4104
Vdd
HB
External
FET
Ron
Cgd
C_B
HO
Rg Rg_fet
Roff
Cgs
HS
Figure 7. MIC4103 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 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:
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
Figure 8. Typical Gate Charge vs. V_GS
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:
Pdiss_drive
= P_driver
R_ON
R_ON
+ R_G + R_{G _ FET}
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_supply = V_DD × I_DD + V_HB × I_HB
October 2007
12
M9999-100107-B