AN2842

Approach to the study

Doc ID 15110 Rev 1

13/22

2.4

Temperature imbalance between devices

As explained in the previous paragraph, an important parameter for current imbalance in a

parallel configuration is working temperature. Analytical proof of thermal instability is derived

MOSFET device. In this region, the drain current is independent of the drain-source voltage:

characteristics and is given by:

where:

●

W: channel perimeter

●

L: channel length

●

●

The current temperature coefficient at low drain current is related to geometrical and

physical parameters of the device by:

since both (

∂V

2.5

Influence of the differences between the power circuit

components

The principal factors linked to the temperature for this analysis are the threshold voltage and

effects of the two parameters are opposite. In fact, if the threshold voltage decreases, the

greater losses and consequently a higher temperature. This phenomenon is iterative (known

as thermal runaway) and could cause the failure of the device. Losses linked to this

parameter are evident during switching and in particular during turn-on and turn-off

operations, and in a systems with high frequency they are highly important. When the

temperature changes, the contribution linked to the RDS(on) parameter is different. This

factor plays a fundamental role during the conduction phase. In fact, during this period, if the

the conduction operation. So, while in a first phase the conduction losses are more

significant than switching losses; when the temperature increases, the second become

predominant over the first. In the end, the working temperature is a factor that plays an

important role in the parallel configuration, and for this reason if the devices are mounted to

2

Th

GS

'

D

)

V

V

(

K

I

−

⋅

=

OX

c

'

C

)

L

W

(

2

1

K

⋅

µ

=

T

K

)

V

V

(

T

V

)

V

V

(

k

2

T

I

'

2

th

GS

th

th

GS

'

D

∂

∂

⋅

−

−

∂

∂

⋅

−

⋅

=

∂

∂