AN-7502 Datasheet, PDF(6/9 Page) Fairchild Semiconductor

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AN-7502 Datasheet, PDF (6/9 Pages) Fairchild Semiconductor – Power MOSFET Switching Waveforms

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Application Note 7502

For peak gate voltages other than 10 volts, and load resis-

tances other than BVDSS/ID(MAX), the equations of Table 1

may be used in conjunction with slope estimates from the

characterization curves for CX and CGS + CX(1 + gM/gMJ) at

the appropriate drain-current level.

Characterization-Curve Limits

The switching-time range over which the characterization can be

applied is very impressive. For gate currents of the order of

microamperes, device dissipation is the limiting factor. For gate

currents of the order of amperes, the device response will be

slowed by gate propagation delay. This delay, of course,

degrades the linear switching relationship to gate current. How-

ever, as Figure 12 graphically shows, the characterization is valid

across five decades of gate current and switching time, allowing

all but a very few switching applications to be described by the

characterization curves of Figure 9.

104

RFM15N15

tD(OFF)

103

tD(ON)

102

101

100

10-1

10-2

100

101

102

103

104

105

106

GATE CURRENT (IG) - MICROAMPERES

FIGURE 12. FIVE DECADES OF LINEAR RESPONSE

Conclusions

The viability of the proposed characterization curves using con-

stant current has been demonstrated and the limits of applica-

tion defined. The existence of a vertical JFET in a power

MOSFET makes data-sheet capacitances of little use for esti-

mating switching times. The classical method of defining

switching time by 10% and 90% is a poor representation for

power MOSFETs because of the dual-slope nature of the drain

waveforms. Switching influences are masked because the 10%

level is controlled by one mechanism and the 90% level by

another. Device comparisons based on the classical switching

definition can be very misleading.

Appendix A - Analysis for Resistive Step

Voltage Inputs

Step Voltage Gate Drive

To obtain the necessary relationships, six device switching

states must be examined using the same device equivalent

circuit as was used for the constant-gate-current case, but

with the forcing function replaced wIth a step voltage with

internal resistance RO, Figure A-1.

GATE

RO VGS

CX VX gMJ VX VD

DRAIN

CGS

gM VG

CDS

SOURCE

LEGEND

VGS

CGS

- Gate Voltage

CDS

- JFET Driving Voltage gM

- Drain Voltage

gMJ

- Gate Source

Capacitance

CX - MOSFET Feedback IG

Capacitance

- Drain Source Capacitance

- MOSFETTransconductance

- JFET Transconductance

- Drain Load Resistance

- Constant Current Amplitude

FIGURE A-1. POWER MOSFET EQUIVALENT CIRCUIT

State 1: Mos Off, JFET Off

As before, both current generators are open circuits, reducing

the equivalent circuit to simply charging CISS through RO.

t = ROCISSIn(1/(1 - VGS(TH)/VG)]

State 2: Mos Active, JFET Active

Before proceeding, it is wise to examine an actual device

response and make use of available simplifications. Figure A-2

shows iG(t) and iD(t) for a typical power MOSFET driven by a

step gate voltage. For truly resistive switching, realize that these

waveforms are only mirror images of their voltage counterparts

vG(t) and vD(t). Using Figure A-2, applicable gate currents for

each of the device states may be listed.

IPK1

IPK2

IPK3

iG(t)

iD(t)

TIME

IPK4

IPK6

IPK5

FIGURE A-2. iG(t) AND iD(t) FOR A TYPICAL POWER MOSFET

DRIVEN BY A STEP GATE VOLTAGE

Application Note 7502 Rev. A1

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