MIC4100 datasheet(13/18 Pages) MIC

13 / 18 page

Micrel, Inc.

MIC4100/1

March 2006

M9999-031506

Vdd

Level

shift

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.

External

FET

Vdd

Rg_fet

Ron

Roff

Cgd

Cgs

Figure 7

Dissipation during the external MOSFET Turn-On

Energy from capacitor CB is used to charge up the input

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

specifications. The ESR of capacitor CB and the resistance

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

capacitanc

gate

total

the

Ciss

1/2

but

where

Ciss

Gate Charge

Qg - Total Gate Charge (nC)

VDS = 50V

ID = 6.9A

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:

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