MIC4604 Datasheet, PDF (10/18 Pages) Micrel Semiconductor – 85V Half Bridge MOSFET Drivers with up to 16V Programmable Gate Drive

Micrel, Inc.
MIC4604
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:
Eq. 3
Pdiode _total = Pdiode _fwd + Pdiode_RR
Eq. 4
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 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. 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:
Figure 5. 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 6 shows a simplified equivalent circuit of the
MIC4604 driving an external MOSFET.
Pdiode _REV = I_R × V_REV × (1 − D)
Eq. 5
Where:
I_R = reverse current flow at V_REV and T_J
V_REV = diode reverse voltage
D = duty cycle = t_ON × f_S
The on-time is the time the high-side switch is conducting.
In most topologies, the diode is reverse biased during the
switching cycle off-time.
Figure 6. MIC4604 Driving an External MOSFET
Dissipation during the External MOSFET Turn-On
Energy from capacitor C_B is used to charge up the input
capacitance of the MOSFET (Cgd and Cgs). The energy
delivered to the MOSFET is dissipated in the three
resistive components, Ron, Rg and Rg_fet. Ron is the on
resistance of the upper driver MOSFET in the MIC4604.
June 25, 2013
10
Revision 1.0