MIC4102 datasheet(15/28 Pages) MICROCHIP

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DS20005575A-page 15

MIC4102

6.0

APPLICATION INFORMATION

6.1

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.

6.2

Bootstrap Circuit Power

Dissipation

Power dissipation of the internal bootstrap diode

primarily comes from the average charging current of

the CB 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:

EQUATION 6-1:

The average power dissipated by the forward voltage

drop of the diode equals:

EQUATION 6-2:

The value of VF should be taken at the peak current

through the diode. However, this current is difficult to

calculate

because

differences

source

impedances. The peak current can either be measured

or the value of VF 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 11 µA at a reverse voltage of 100V

and 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 multiplied by the reverse

voltage across the diode. The average reverse current

and power dissipation due to reverse recovery can be

estimated by:

EQUATION 6-3:

EQUATION 6-4:

The total diode power dissipation is:

EQUATION 6-5:

An optional external bootstrap diode may be used

instead of the internal diode (Figure 6-1). 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 72V input telecom

applications. The equations above 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 6-6:

FAVE



gate



Where:

Qgate

Total Gate Charge at VHB

Gate Drive Switching Frequency

Pdiode

fwd

FAVE





Where:

Diode Forward Voltage Drop

RR AVE



RRM



Where:

IRRM

Peak Reverse Recovery Current

trr

Reverse Recovery Time

Pdiode

RR AVE



REV



Pdiode

total

Pdiode

fwd

Pdiode

REV



–





Where:

Reverse Current Flow at VREV & TJ

VREV

Diode Reverse Voltage

Duty Cycle = tON/fS

Switching Freq. of Power Supply

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