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TN1156

Basic concepts

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1

Basic concepts

In linear zone, the Power MOSFET can be subjected to a thermal run-away process that

could lead the device to fail. Failures are experimentally explained as a drain current

focusing phenomenon (hot spots). Failures are generally localized in the center of the die,

close to the source wire bonding. Owing to that, standard theoretical FBSOA does not

exactly describe the safe area in forward-biased conditions since the red area portion is lost

(see

Figure 1). An analytical approach has been performed to describe this phenomenon. In

linear zone (saturation region), I

D

can be written as:

Equation 1

c

OX

is the gate oxide capacitance by unit area, W is the perimeter of the device, L is the

length of the channel, μ the mobility of the carriers in the channel and V

TH

is the threshold

voltage. Introducing the term K as:

Equation 2

Equation 1 can be rewritten as:

Equation 3

Figure 1. Example of Power MOSFET FBSOA

During the linear zone operating condition, when a power pulse is applied, the device

warms, the junction temperature increases and I

D

changes its value because the mobility of

I

D

1

2

---

μW

L

----- c

OX

V

GS

V

TH

–

=

K

1

2

---c

OX

W

L

-----

μ

=

I

D

KV

GS

V

TH

–

=

GIPG141120131502FSR