Micrel, Inc.

MIC4100/1

March 2006

12

M9999-031506

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

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:

frequency

switching

drive

gate

V

at

Charge

Gate

Total

Q

:

where

HB

gate

)

(

=

=

×

=

S

S

gate

AVE

F

f

f

Q

I

The average power dissipated by the forward voltage drop

of the diode equals:

drop

voltage

forward

Diode

V

:

where

F

)

(

=

×

=

F

AVE

F

fwd

V

I

Pdiode

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

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:

Time

Recovery

Reverse

t

Current

Recovery

Reverse

Peak

I

:

where

5

.

0

rr

RRM

)

(

)

(

=

=

×

=

×

×

×

=

REV

AVE

RR

RR

S

rr

RRM

AVE

RR

V

I

Pdiode

f

t

I

I

The total diode power dissipation is:

RR

fwd

total

Pdiode

Pdiode

Pdiode

+

=

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:

supply

power

the

of

frequency

switching

fs

/

t

Cycle

Duty

D

Voltage

Reverse

Diode

V

T

and

V

at

flow

current

Reverse

I

:

where

)

1

(

ON

REV

J

REV

R

=

=

=

=

=

−

×

×

=

S

REV

R

REV

f

D

V

I

Pdiode

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.