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Micrel, Inc.
Applications Information
Power Dissipation Considerations
Power dissipation in the driver can be separated into
three areas:
? Internal diode dissipation in the bootstrap circuit
MIC4102
The total diode power dissipation is:
Pdiode total = Pdiode fwd + Pdiode RR
An optional external bootstrap diode may be used
instead of the internal diode (Figure 5). An external
diode may be useful if high gate charge MOSFETs are
being driven and the power dissipation of the internal
?
?
Internal driver dissipation
Quiescent current dissipation used to supply the
internal logic and control functions.
diode is contributing to excessive die temperatures. The
voltage drop of the external diode must be less than the
internal diode for this option to work. The reverse
voltage across the diode will be equal to the input
Bootstrap Circuit Power Dissipation
Power dissipation of the internal bootstrap diode
primarily comes from the average charging current of the
C B capacitor times the forward voltage drop of the diode.
Secondary sources of diode power dissipation are the
reverse leakage current and reverse recovery effects of
the diode.
The average current drawn by repeated charging of the
high-side MOSFET is calculated by:
I F ( AVE ) = Q gate × f S
where : Q gate = Total Gate Charge at V HB
f S = gate drive switching frequency
The average power dissipated by the forward voltage
drop of the diode equals:
Pdiode fwd = I F ( AVE ) × V F
voltage minus the Vdd supply voltage. A 100V Schottky
diode will work for most 72V input telecom applications.
The above equations can be used to calculate power
dissipation in the external diode, however, if the external
diode has significant reverse leakage current, the power
dissipated in that diode due to reverse leakage can be
calculated as:
Pdiode REV = I R × V REV × ( 1 ? D )
where : I R = Reverse current flow at V REV and T J
V REV = Diode Reverse Voltage
D = Duty Cycle = t ON / f S
fs = switching frequency of the power supply
The on-time is the time the high-side switch is
conducting. In most power supply topologies, the diode
is reverse biased during the switching cycle off-time.
where : V F = Diode forward voltage drop
The value of V F should be taken at the peak current
through the diode, however, this current is difficult to
calculate because of differences in source impedances.
The peak current can either be measured or the value of
V F at the average current can be used and will yield a
good approximation of diode power dissipation.
The reverse leakage current of the internal bootstrap
external
diode
Vdd
Level
shift
HB
C B
HO
Vin
diode is typically 11uA at a reverse voltage of 100V and
125C. Power dissipation due to reverse leakage is
typically much less than 1mW and can be ignored.
Reverse recovery time is the time required for the
injected minority carriers to be swept away from the
PWM
Q
FF
_
Q
HS
LO
depletion region during turn-off of the diode. Power
dissipation due to reverse recovery can be calculated by
computing the average reverse current due to reverse
recovery charge times the reverse voltage across the
diode. The average reverse current and power
dissipation due to reverse recovery can be estimated by:
I RR ( AVE ) = 2 × I RRM × t rr × f S
Pdiode RR = I RR ( AVE ) × V REV
where : I RRM = Peak Reverse Recovery Current
t rr = Reverse Recovery Time
Vss
Figure 5. Optional Bootstrap Diode
Gate Drive 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.
November 2006
11
M9999-112806
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