Micrel, Inc.

MIC4604

June 25, 2013

11

Revision 1.0

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 capacitances of Cgd and Cgs are difficult to

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

Vds. Fortunately, most power MOSFET specifications

include a typical graph of total gate charge versus Vgs.

Figure 7 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:

gs

2

gs

2

1

V

Qg

1/2

E

so

V

C

Q

but

V

Ciss

E

×

×

=

×

=

×

×

=

Eq. 6

Where

Ciss = total gate capacitance of the MOSFET

10

8

6

4

2

0

0

5

10

15

20

25

Gate Charge

Qg - Total Gate Charge (nC)

Figure 7. Typical Gate Charge vs. VGS

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:

fs

V

Qg

P

and

V

Qg

E

gs

driver

gs

driver

×

×

=

×

=

Eq. 7

Where:

Qg = total gate charge at Vgs

Vgs = gate to source voltage on the MOSFET

fs = switching frequency of the gate drive circuit

The power dissipated inside the MIC4604 is equal to the

ratio of Ron and Roff to the external resistive losses in Rg

and Rg_fet. Letting Ron = Roff, the power dissipated in the

MIC4604 due to driving the external MOSFET is:

fet

_

Rg

Rg

Ron

Ron

P

Pdiss

driver

drive

+

+

=

Eq. 8

Supply Current Power Dissipation

Power is dissipated in the MIC4604 even if nothing is

being driven. The supply current is drawn by the bias for

the internal circuitry, the level shifting circuitry, and shoot-

through current in the output drivers. The supply current is

proportional to operating frequency and the VDD and VHB

voltages. The typical characteristic graphs show how

supply current varies with switching frequency and supply

voltage.

The power dissipated by the MIC4604 due to supply

current is

IHB

VHB

IDD

VDD

Pdiss

ply

sup

×

+

×

=

Eq. 9

Total Power Dissipation and Thermal Considerations

Total power dissipation in the MIC4604 is equal to the

power

dissipation

caused

by

driving

the

external

MOSFETs, the supply current and the internal bootstrap

diode.

total

drive

ply

sup

total

Pdiode

Pdiss

Pdiss

Pdiss

+

+

=

Eq. 10