AN-6069

APPLICATION NOTE

© 2007 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.3 • 1/6/10

2

The circuit waveforms for a MOSFET turning on into a

clamped inductive load are illustrated in Figure 2.

V

DD

V

PL

V

TH

I

L

t4

t3

t2

t1

V

DS

I

DS

V

GS

I

G

time

V

O

I

PK

I

PL

Figure 2.

MOSFET Turn on with Inductive Load

Figure 3 indicates the gate current paths active during the

individual intervals of the MOSFET turn -on process.

Figure 3.

Current Paths During MOSFET Turn on

internal gate resistance along with any series gate resistor.

effective value changes throughout the switching interval.

combining information presented in references [1] and [2].

the MOSFET carries no inductor current.

As interval t2 begins, the MOSFET starts to conduct current

in the linear mode as:

)

V

V

(

g

I

TH

GS

m

D

−

=

(1)

through the current paths shown in Figure 3(b). The parallel

voltage to a plateau level given by

TH

m

D

PL

V

g

I

V

+

=

(2)

needed during this transition and can be determined from the

MOSFET datasheet characteristic curves, as illustrated in

allows calculation of the time required for this transition as:

G

2

GS

rise

,

IDS

I

Q

t

2

t

=

=

(3)

and the diode commutates.

As interval t3 commences, the gate current flows through

G

GD

fall

,

VDS

I

Q

t

3

t

=

=

(4)

Figure 3(d). During t4, the gate-source voltage rises from

the MOSFET has simultaneous high voltage across it and

high current flowing through it, so the instantaneous power

loss during the turn on interval is:

()

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

⎟

⎠

⎞

⎜

⎝

⎛

×

=

3

t

,

G

GD

2

t

,

G

2

GS

SW

LOAD

IN

ON

,

SW

I

Q

I

Q

f

2

I

V

P

(5)

in relation to the switching losses. Unfortunately, there are

no formal equations to calculate the current available from a

given driver as the output voltage swings throughout its

different driver output voltage levels and are presented in

the section “Evaluating Drivers on the Bench” below.