# Characterization-curve limits, Conclusions – Fairchild SEMICONDUCTOR AN-7502 User Manual

## Page 6

©2002 Fairchild Semiconductor Corporation

Application Note 7502 Rev. A1

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

tances other than BV

DSS

/I

D(MAX)

, the equations of Table 1

may be used in conjunction with slope estimates from the

characterization curves for C

X

and C

GS

+ C

X

(1 + g

M

/g

MJ

) 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.

**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 R

O

, Figure A-1.

**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 C

ISS

through R

O

.

**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 i

G

(t) and i

D

(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

v

G

(t) and v

D

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

each of the device states may be listed.

**FIGURE A-2. i**

**G**

**(t) AND i**

**D**

**(t) FOR A TYPICAL POWER MOSFET **

**DRIVEN BY A STEP GATE VOLTAGE**

**10**

**4**

**10**

**2**

**10**

**0**

**10**

**-2**

**10**

**0**

**10**

**2**

**10**

**4**

**RFM15N15**

**t**

**D**

**(OFF)**

**t**

**R**

**t**

**F**

**t**

**D**

**(ON)**

**GATE CURRENT (I**

**G**

**) - MICROAMPERES**

**T**

**IME**

**(t) - **

**M**

**IC**

**R**

**OS**

**E**

**C**

**ON**

**D**

**S**

**10**

**3**

**10**

**1**

**10**

**-1**

**10**

**1**

**10**

**3**

**10**

**5**

**10**

**6**

**LEGEND**

V

GS

- Gate Voltage

C

DS

- Drain Source Capacitance

V

X

- JFET Driving Voltage g

M

- MOSFET Transconductance

V

D

- Drain Voltage

g

MJ

- JFET Transconductance

C

GS

- Gate Source

Capacitance

R

L

- Drain Load Resistance

C

X

- MOSFET Feedback

Capacitance

I

G

- Constant Current Amplitude

t =

R

O

C

ISS

In(1/(1 - V

GS(TH)

/V

G

)]

**GATE**

**C**

**DS**

**SOURCE**

**DRAIN**

**C**

**X**

**V**

**GS**

**V**

**G**

**g**

**MJ**

** V**

**X**

**R**

**L**

**C**

**GS**

**g**

**M**

** V**

**G**

**V**

**D**

**V**

**X**

**R**

**O**

**I**

**PK1**

**I**

**PK2**

**I**

**PK3**

**I**

**PK4**

**I**

**PK5**

**I**

**PK6**

**CURRENT**

**TIME**

**i**

**D**

**(t)**

**i**

**G**

**(t)**

**Application Note 7502**