MIC44F20 datasheet(11/17 Pages) MICREL

11 / 17 page

Micrel, Inc.

MIC44F18/19/20

February 2011

M9999-020111

Application Information

Power Dissipation Considerations

Power dissipation in the driver can be separated into two

areas:

•

Output driver stage dissipation

•

Quiescent current dissipation used to supply the

internal logic and control functions.

Output Driver Stage 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 4 shows a simplified equivalent circuit of the

MIC44F18 driving an external MOSFET.

Figure 4. Output Driver Stage Power Dissipation

Dissipation during the External MOSFET Turn-On

Energy from capacitor CVDD 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 MIC44F18.

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 5 shows

a typical gate charge curve for an arbitrary power

MOSFET. This illustrates 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

Figure 5. GATE Charge

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:

DRIVER

and

Where

EDRIVER is the energy dissipated per switching

power

PDRIVER is the power dissipated by switching the

MOSFET on and off

QG is the total GATE charge at VGS

VGS is the GATE to SOURCE voltage on the

MOSFET

fS is the switching frequency of the GATE drive

circuit

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