# Using the characterization curve, figure 9 – Fairchild SEMICONDUCTOR AN-7502 User Manual

## Page 5 Application Note 7502 Rev. A1

Experimental Verification

Since the switching equations for step currents and voltages
differ only by gate-current magnitudes for the same device
type, one would expect a plot of switching time versus 1/R

O

to be of the same form as those obtained for a step current
drive. This is exactly the case, as Figure 10 is merely a vari-
ation of Figure 8. Using the relationships of Table 1, the
observed differences between Figures 7 and 9 can be pin-
pointed. The two sets of experimental curves confirm that,
on the basis of the short-circuit drive current V

G

/R

O

equal-

ling the constant I

G

, t

D

(on), t

R

, t

D

(off), and t

F

will all be

longer, as predicted by the ratios of the gate drive currents of
Table 1. Notice also that t

R

, t

F

switching symmetry is dis-

rupted by the use of a step voltage with source resistance
R

O

. For states 2 and 6 the time ratio is:

For states 3 and 5 the time ratio is:

Utilization of available maximum gate drive voltage and cur-
rent can be optimized for fastest power MOSFET switching
speed through the use of constant-current gate drive at the
expense of increased gate-drive circuit complexity.

FIGURE 10.

CONSTANT GATE VOLTAGE SWITCHING TIME

Using the Characterization Curve,
Figure 9

To estimate the switching times for an RFM15N15 power
MOSFET under the conditions V

G

= 10V, V

DD

= 75V, R

O

=

100 ohms, and R

L

= 10 ohms, precedes as follows:

State 1: MOS Off, JFET Off

This time can be estimated without recourse to the curves

State 2 & 6: MOS Active, JFET Active

State 3: MOS Active, JFET Saturated

State 4: MOS Saturated, JFET Saturated

State 5: MOS Active, JFET Saturated

Figure 11 shows RFM15N15 waveforms using the conditions
specified in the example.

FIGURE 11.

STEP GATE VOLTAGE INPUT TO AN RFM15N15

t

TURN-ON

=

V

G(SAT)

t

TURN-OFF

V

G

- V

GS(TH)

t

TURN-ON

=

V

G(SAT)

t

TURN-OFF

V

G

- V

G(SAT)

10

1

0.1

0.01

10

-4

10

-3

10

-2

10

-1

DATA

t

D(OFF)

t

R

t

F

t

D(ON)

(t) -

M

IC

R

OS

E

C

ON

D

S

THEORY

1/R

O

RFM15N15
V

DD

= 75V

I

D

V

G

= 7.5A
= 10V

t =

100(1200 x 10

-12

) ln [1/(1 - 4/10)]

t =

61 ns

I

G

=

(10 - 4)/100 = 60mA

t =

(curve divisions) x I

T

µ

s

=

9

=

150ns

60

60

I

G

=

(10 - 7)/100 = 30mA

t =

(curve divisions) x I

T

µ

s

=

14

=

467ns

30

30

C

GS

+ C

x

=

(gate voltage slope)(test current)

=

(1.5 x 10

-6

s/5 volts)(10mA)

=

3000pF

t =

100(3000 x 10

-12

) ln [10/6.6]

t =

125ns

I

G

=

6.6/100 = 66mA

t =

(curve divisions) x I

T

µ

s

=

8

=

121ns

66

66

STATE

CALCULATED

TIME

MEASURED

TIME

RATIO

(t

C

, ns)

(t

M

, ns)

(t

C

/t

M

)

1

61

60

1.02

2 + 3

617

670

0.92

4

125

137

0.91

5 + 6

271

375

0.72

TIME - MICROSECONDS

DRAIN V

O

L

T

A

G

E

-

V

O

L

T

S

RFM15N15
V

DD

= 75 VOLTS

R

L

= 10 OHMS

V

G

= 10 VOLTS

R

O

= 100 OHMS

V

D

V

GS

0

75

0

1.5

3

Application Note 7502