488

The failure rate

λ

is determined experimentally or by statistical analysis of a current

production. A defined number of samples N of the same type is operated under defined

conditions with rated load. The number n of components failed within the period of

observation

∆

t will be counted. In a current production then, the ratio of all products

manufacturedmay be related to the returns from the field.

t N

n

∆⋅

=λ

If, for example, n = 4 failures in a total of N = 8000 components in the time of

∆

t =

5000h, then the failure rate results as follows:

h10

1

h 5000

8000

4

7

=

⋅

=λ

If the failure rate is to be specified inFIT, then:

FIT 100

h 10

h 10

1 9

1 7

=

=λ

− −

− −

Overall, it is desirable to minimise the FIT rate, both at component level and at system

level. FIT rates of complex systems are calculated by simply adding the FIT rates of its

individual components, as far as there is no inherent redundancy. If for example, an

IGBT module has a FIT rate of 100 and has 24 internally parallel connected chips, it

follows that each chip is limited to aFIT rate of 4.2. This statement relates, however, not

to a particular module, but is a statistical statement for modules in large production

quantities. Manufacturers today achieve FIT rates for IGBTmodules in the range of 100

and lower.

If the FIT rates of individual components are known, the MTBF value of an existing

system of several of these components may be calculated. These components do not

have to be IGBT modules. MTBF stands for "Mean Time Between Failures" and is

defined as:

∑

λ

=

1

MTBF

The relationship is illustratedwith an example. Having 40 components, 20 of which have

a FIT rate of 125 FIT, 15 a value of 350 FIT and 5 a value of 300 FIT. TheMTBF value

is thus:

h

129000

h 10 300 5 h 10 250 15 h 10 125 20

1

FIT 300 5 FIT 250 15 FIT 125

20

1

1

MTBF

1 9

1 9

1 9

40

36 n

n

35

21 n

n

20

1n

n

≈

⋅

⋅ +

⋅

⋅

+

⋅

⋅

=

⋅ +

⋅

+

⋅

=

λ +λ +λ

=

− −

− −

− −

=

=

=

∑∑∑

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