MIC4606 datasheet(25/40 Pages) MICROCHIP

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DS20005604D-page 25

MIC4606

7.6

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

MIC4606. RG is the series resistor (if any) between the

driver 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 capacitances of CGD and CGS are difficult

to calculate because they vary nonlinearly with ID, VGS

and VDS. Fortunately, most power MOSFET specifica-

tions include a typical graph of total gate charge versus

VGS. Figure 7-8 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

23.5 nC of charge. The energy dissipated by the

resistive components of the gate drive circuit during

turn-on is calculated as:

EQUATION 7-4:

but

EQUATION 7-5:

EQUATION 7-6:

FIGURE 7-8:

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:

EQUATION 7-7:

and

EQUATION 7-8:

---CISS



VGS



Where:

CISS = Total Gate Capacitance of the MOSFET

QC V



---QG



VGS



G - TOTAL GATE CHARGE (nC)

EDRIVER

QG VGS



Where:

EDRIVER = Energy Dissipated per Switching

Cycle

PDRIVER

QG VGS



Where:

PDRIVER = Power Dissipated per Switching

Cycle

QG = Total Gate Charge at VGS

VGS = Gate-to-Source Voltage on the

MOSFET

fS = Switching Frequency of the

Gate Drive Circuit

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