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AN4870 Application Note

www.dynexsemi.com

80

100

120

140

160

180

200

Thyristor junction temperature - (˚C)

0

20

40

60

80

100

Fig.1 Thyristor de-rating curves

The junction temperature ( T

particular situation profoundly affects its performance and

reliability. During its working life a thyristor can experience a wide

range of temperatures.

Operating at –40˚C is not damaging but allowance must be made

by the user for increased gate trigger current, latching current

and holding current as well as slow turn-on (see application note

AN4840 Gate Triggering and Gate Characteristics). Working in

the range between room temperature and 125˚C gives the best

compromise between ease of operation and operational life.

T

this, blocking current starts to increase rapidly, thus degrading

voltage rating, see fig.1.

The device becomes much more susceptible to over-voltage

transients , high dv/dt, di/dt and surge current. In the case of the

forward blocking junction there is an increasing chance of

forward breakover triggering.

For special applications it is

possible to select devices to operate continuously with low

leakage at T

characterised and rated on other parameters at 140˚C.

Many applications involve infrequent current overloads for short

periods and it is possible to allow T

such situations. A typical situation is during a load short circuit

when the device is protected by a fuse. In 50Hz circuits the

thyristor may often have to carry short circuit current for up to

10ms. During this time T

without the junction being damaged. Peak temperature lags

peak current by typically 2 or 3 milliseconds and, although falling,

is still high at the end of the current pulse. If current is interrupted

by a fuse, little or no reverse voltage appears across the device.

However, the re-application of reverse voltage at such a high

temperature can result in very high reverse recovery power

dissipation. This escalates the junction temperture further and

the subsequent high blocking current leads to reverse voltage

failure by thermal runaway.

Limit case surge currents are determined by experimental means

using a 50Hz half sine of current and published in the data sheet.

These I

( Using I

current loop ( Using I

are then taken as the limit temperatures for the particular device.

If temperatures in other applications are kept below these, then

the condition will be safe.

The method of calculating overload T

currents and other overload conditions is discussed below.

The overload above assumed a high speed fuse or circuit

breaker will interrupt the supply before forward blocking voltage

appears. Some overloads require that the device survives with

AN4870

Effects Of Temperature On Thyristor Performance

Application Note

Replaces September 2000 version, AN4870-3.0

AN4870-3.1 July 2002