Micrel, Inc.

MIC4100/1

March 2006

13

M9999-031506

HS

HB

HO

Vdd

C

B

LO

Level

shift

HI

LI

Vss

Vin

external

diode

Figure 6

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

MIC4100 driving an external MOSFET.

HS

HB

HO

External

FET

Vdd

C

B

Rg

Rg_fet

Ron

Roff

Cgd

Cgs

Figure 7

Dissipation during the external MOSFET Turn-On

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 MIC4100.

Rg is the series resistor (if any) between the driver IC and

the MOSFET. Rg_fet is the gate resistance of the

MOSFET. Rg_fet is usually listed in the power MOSFET’s

of the connecting etch can be ignored since they are much

less than Ron and Rg_fet.

The effective capacitance of Cgd and Cgs is difficult to

calculate since they vary non-linearly with Id, Vgs, and

Vds. Fortunately, most power MOSFET specifications

include a typical graph of total gate charge vs. Vgs. Figure

8 shows a typical gate charge curve for an arbitrary power

MOSFET. This chart shows that for a gate voltage of 10V,

the MOSFET requires about 23.5nC of charge. The energy

dissipated by the resistive components of the gate drive

circuit during turn-on is calculated as:

MOSFET

the

of

e

capacitanc

gate

total

the

is

Ciss

Qg

1/2

E

so

V

C

Q

but

2

2

1

where

V

V

Ciss

E

gs

gs

×

×

=

×

=

×

×

=

10

8

6

4

2

0

0

5

10

15

20

25

Gate Charge

Qg - Total Gate Charge (nC)

Figure 8

The same energy is dissipated by Roff, Rg and Rg_fet

when the driver IC turns the MOSFET off. Assuming Ron

is approximately equal to Roff, the total energy and power

dissipated by the resistive drive elements is: