# Step-voltage gate drive – Fairchild SEMICONDUCTOR AN-7502 User Manual

## Page 4

Application Note 7502 Rev. A1

FIGURE 9. NORMALIZED RFM15N15 SWITCHING WAVE-

FORMS FOR CANSTANT GATE-CURRENT DRIVE.

Step-Voltage Gate Drive

The majority of power MOSFET applications employ a step
gate-voltage input with a finite source resistance R

O

. Often

R

O

for turn-on is not the same as R

O

for turn-off. How can

switching times for these situations be estimated using the
switching characterization curves just described? The analy-

sis for resistive step voltage inputs, which is complex
because the gate current is no longer constrained to be con-
stant, but is a function of device gate-voltage response, is
covered in Appendix A. (A second, shorter appendix, B, has
been added to illustrate the estimation of R

O

for some practi-

cal gate drive circuits.) Table 1 summarizes the common
switching equations, and indicates the appropriate 1

G

to be

used in each state for relating step voltage drives to the char-
acterization curves.

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:

100

75

50

25

0

20I

T

/I

G

40I

T

/I

G

60I

T

/I

G

80I

T

/I

G

RFM15N15
I

T

= 1mA

V

G

= 10 VOLTS

R

L

= V

DSS

/I

D(RMS)

% RA

T

ES V

DSS

TIME - microseconds

TABLE 1.

COMMON SWITCHING EQUATIONS

T

U
R
N
O
N

CONSTANT CURRENT

STATE 1: MOS OFF, JFET OFF

CONSTANT VOLTAGE

t =

C

ISS

V

GS(TH)

t = R

O

C

ISS

In

[1]

I

G

[1 - V

GS(TH)

/V

G

]

I

G

= I

T

STATE 2: ACTIVE, ACTIVE

I

G

= (V

G

- V

GS(TH)

)/R

O

t =

[V

DD

- V

DK

] [C

GS

+ C

x

(1 + g

M

/g

MJ

)]

g

M

R

L

I

G

I

G

= I

T

STATE 3: ACTIVE, SATURATED

I

G

= (V

G

- V

G(SAT)

)/R

O

t =

(V

DK

- V

D(SAT)

)C

X

I

G

T

U
R
N
O

F
F

I

G

= I

T

STATE 4: SATURATED, SATURATED

I

G

= -V

G

/R

O

t =

(C

GS

+ C

X

)(V

G

- V

G(SAT)

)

t = R

O

(C

GS

+ C

X

) In (V

G

/V

G(SAT)

)

I

G

I

G

= I

T

STATE 5: ACTIVE, SATURATED

I

G

= (V

G

- V

G(SAT)

)/R

O

t =

(V

DK

- V

D(SAT)

)C

X

I

G

I

G

= I

T

STATE 6: ACTIVE, ACTIVE

I

G

= (V

G

- V

G(SAT)

)/R

O

t =

[V

DD

- V

DK

] [C

GS

+ C

X

(1 + g

M

/g

MJ

)]

g

M

R

L

I

G

Application Note 7502