MIC4100_11 Datasheet, PDF (12/18 Pages) MIC GROUP RECTIFIERS – 100V Half Bridge MOSFET Drivers

Micrel, Inc.
MIC4100/1
Application Information
Power Dissipation Considerations
Power dissipation in the driver can be separated into three
areas:
• Internal diode dissipation in the bootstrap circuit
• Internal driver dissipation
• Quiescent current dissipation used to supply the
internal logic and control functions.
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
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 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 _{RR( AVE)} = 0.5 × 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
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 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:
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.
March 2006
12
M9999-031506