MIC4600 Datasheet, PDF (14/26 Pages) Microchip Technology – 28V Half-Bridge MOSFET Driver

MIC4600
6.5 Power Dissipation Considerations
Power dissipation in the driver can be separated into
two areas:
• Quiescent current dissipation
• Internal driver dissipation
6.6 Quiescent Current Power
Dissipation
Power is dissipated in the MIC4600 even if nothing is
being driven. The quiescent current is drawn by the
bias for the internal circuitry and the level shifting
circuitry. The quiescent current is proportional to
operating frequency. The typical characteristic graphs
show how quiescent current varies with switching
frequency.
The power dissipated due to quiescent current is
calculated in Equation 6-5.
EQUATION 6-5:
P_{DISS _IQ} = V _DD  I _DD
6.7 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 6-1 shows a simplified equivalent circuit of the
MIC4600 driving an external high-side MOSFET.
VDD
BST
EXTERNAL FET
R_ON
C_GD
C_B
DH
R_OFF
MIC4600
HIGH-SIDE DRIVER
SW
R_G
R_{G_FET}
C_GS
FIGURE 6-1:
MIC4600 Driving an
External MOSFET.
6.7.1
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
DS20005584A-page 14
MIC4600. 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 capacitances of C_GD and C_GS are difficult
to calculate because 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 6-2 is a typical gate charge
curve for a power MOSFET. This chart shows that for a
gate voltage of 4.5V, the MOSFET gate is charged up
to 25 nC of total gate charge. The energy dissipated by
the resistive components of the gate drive circuit during
turn-on is calculated as noted in Equation 6-6 through .
EQUATION 6-6:
E
=
1--
2

C
ISS

V
2
GS
but:
EQUATION 6-7:
Q = CV
so:
EQUATION 6-8:
E
=
1--
2

Q_G

V
GS
Where:
C_ISS = Total gate capacitance of the MOSFET
FIGURE 6-2:
V_GS.
Typical Gate Charge vs.
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 R_OFF, the total
energy and power dissipated by the resistive drive
elements is illustrated in Equation 6-9.
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