MIC44F18_11 Datasheet, PDF (11/17 Pages) Micrel Semiconductor – 6A, 13V High Speed MOSFET Drivers with Enable Input

Micrel, Inc.
MIC44F18/19/20
Application Information
Power Dissipation Considerations
Power dissipation in the driver can be separated into two
areas:
• Output driver stage dissipation
• Quiescent current dissipation used to supply the
internal logic and control functions.
Output Driver Stage 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 4 shows a simplified equivalent circuit of the
MIC44F18 driving an external MOSFET.
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 4. Output Driver Stage Power Dissipation
Dissipation during the External MOSFET Turn-On
Energy from capacitor C_VDD 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 MIC44F18.
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 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 5 shows
a typical gate charge curve for an arbitrary power
MOSFET. This illustrates that for a gate voltage of 10V,
the MOSFET requires about 23.5nC of charge.
Figure 5. GATE Charge
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:
E_DRIVER = Q_G × V_GS
and
P_DRIVER = Q_G × V_GS × f _S
Where
E_DRIVER is the energy dissipated per switching
power
P_DRIVER is the power dissipated by switching the
MOSFET on and off
Q_G is the total GATE charge at V_GS
V_GS is the GATE to SOURCE voltage on the
MOSFET
f_S is the switching frequency of the GATE drive
circuit
February 2011
11
M9999-020111