Micrel

MIC4103/4104

November 2010

13

M9999-110910-B

Application Information

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

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:

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.

HS

HB

HO

Vdd

C

B

LO

Level

shift

HI

LI

Vss

Vin

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.

HS

HB

HO

External

FET

Vdd

C

B

Rg

Rg_fet

Ron

Roff

Cgd

Cgs

Figure 7. MIC4103 Driving an External MOSFET