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Micrel, Inc.
MIC4223/MIC4224/MIC4225
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 driver’s output stage is mainly
caused by charging and discharging the gate to source
and gate to drain capacitance of the external MOSFET.
Figure 3 shows a simplified circuit of the MIC4223 driving
an external MOSFET.
Figure 4. MOSFET Gate Charge vs. V GS
The energy dissipated during turn-on is calculated as:
E = 1 2 × C iss × V GS
2
where C iss is the MOSFET' s total gate capacitanc e
but :
Q = C × V
so
E = 1/2 × Q G × V GS
An equivalent amount of energy is dissipated in the
Figure 3. Functional MOSFET/Driver Diagram
Dissipation Caused by Switching the External
MOSFET
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 upper driver
MOSFET and Bipolar impedances. 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 4 shows a
typical MOSFET gate charge curve. The graph illustrates
that for a gate voltage of 10V, the MOSFET requires about
23.5nC of charge.
driver’s sink circuit when the MOSFET turns off. The total
energy and power dissipated by the drive components 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 cycle
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 MOSFETs Gate to Source voltage
f S is the switching frequency of the Gate drive
circuit
June 2009
10
M9999-061109-A
(408) 944-0800
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