MIC4605 datasheet(18/34 Pages) MICROCHIP

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MIC4605

DS20005853E-page 18

 2018-2019 Microchip Technology Inc.

FIGURE 7-4:

Negative HS Pin Voltage.

7.3

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.

7.4

Bootstrap Circuit Power Dissipation

Power dissipation of the internal bootstrap diode primar-

ily comes from the average charging current of the CB

capacitor, multiplied by 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:

EQUATION 7-1:

The average power dissipated by the forward voltage

drop of the diode equals:

EQUATION 7-2:

The value of VF should be taken at the peak current

through the diode; however, this current is difficult to cal-

culate because of differences in source impedances.

The peak current can either be measured or the value of

VF at the average current can be used, which will yield a

good approximation of diode power dissipation.

The reverse leakage current of the internal bootstrap

diode is typically 3 µA at a reverse voltage of 85V at

+125°C. Power dissipation due to reverse leakage is

typically much less than 1 mW 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:

EQUATION 7-3:

EQUATION 7-4:

The total diode power dissipation is:

EQUATION 7-5:

An optional external bootstrap diode may be used

instead of the internal diode (Figure 7-5). 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:

EQUATION 7-6:

F AVE



GATE



Where:

QGATE = Total Gate Charge at VHB – VHS

fS = Gate Drive Switching Frequency

DIODEfwd

F AVE





Where:

VF = Diode Forward Voltage Drop

RR AVE



0.5 I

RRM



Where:

IRRM = Peak Reverse Recovery Current

tRR = Reverse Recovery Time

DIODErr

R R AVE



REV



DIODEtotal

DIODEfwd

DIODErr

DIODErev

REV



1 D

–





Where:

IR = Reverse Current Flow at VREV and TJ

VREV = Diode Reverse Voltage

D = Duty Cycle (tON × fS)

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