MIC4103_10 Datasheet, PDF (13/18 Pages) Micrel Semiconductor – 100V Half Bridge MOSFET Drivers 3/2A Sinking/Sourcing Current

Micrel
Application Information
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
Vdd
Level
HI
shift
C_B
HB
HO
HS
MIC4103/4104
Vin
The total diode power dissipation is:
LI
LO
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 V_DD 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
Vss
Figure 6. Optional Bootstrap Diode
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 7 shows a simplified equivalent circuit of the
MIC4103 driving an external MOSFET.
Vdd
HB
External
FET
Ron
Cgd
C_B
HO
Rg Rg_fet
Roff
Cgs
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.
HS
Figure 7. MIC4103 Driving an External MOSFET
November 2010
13
M9999-110910-B