mic4100 Micrel Semiconductor, mic4100 Datasheet - Page 12

• Manufacturer Part Number: mic4100
• Description: 100v Half Bridge Mosfet Drivers
• Manufacturer: Micrel Semiconductor

Application Information
Power Dissipation Considerations
Power dissipation in the driver can be separated into three
areas:
Bootstrap Circuit Power Dissipation
Power dissipation of the internal bootstrap diode primarily
comes from the average charging current of the C
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:
The average power dissipated by the forward voltage drop
of the diode equals:
The value of V
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
approximation of diode power dissipation.
The reverse leakage current of the internal bootstrap 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 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
where
Pdiode
where
Micrel, Inc.
March 2006
F
F
at the average current can be used and will yield a good
(
AVE
•
•
•
)
:
:
=
fwd
Q
V
f
Internal diode dissipation in the bootstrap circuit
Internal driver dissipation
Quiescent current dissipation used to supply the
S
internal logic and control functions.
Q
F
gate
=
=
gate
=
gate
I
=
Diode
F
×
Total
(
F
AVE
f
should be taken at the peak current
drive
S
)
×
forward
Gate
V
switching
F
Charge
voltage
frequency
at
V
drop
HB
B
12
The total diode power dissipation is:
An optional external bootstrap diode may be used instead
of the internal diode (Figure 6). An external diode may be
useful if high gate charge MOSFETs are being driven and
the power dissipation of the internal 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 voltage minus the Vdd supply
voltage. A 100V Schottky diode will work for most 72Vinput
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:
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
Pdiode
I
Pdiode
where
RR
(
I :
AVE
REV
V
fs
D
R
REV
Pdiode
=
I :
)
=
RR
=
t
=
=
switching
rr
RRM
Duty
Reverse
=
I
0
=
=
R
5 .
Diode
I
Reverse
×
=
RR
total
×
V
Cycle
Peak
(
I
REV
AVE
RRM
=
current
Reverse
frequency
)
Pdiode
×
×
×
Reverse
=
Recovery
1 (
V
t
t
rr
REV
−
ON
×
D
flow
/
fwd
Voltage
)
f
f
S
S
of
Recovery
+
at
Time
Pdiode
the
V
REV
power
M9999-031506
and
RR
Current
MIC4100/1
supply
T
J