AN4870 Datasheet, PDF (1/5 Pages) Dynex Semiconductor – Effects Of Temperature On Thyristor Performance

AN4870 Application Note
AN4870
Effects Of Temperature On Thyristor Performance
Replaces September 2000 version, AN4870-3.0
Application Note
AN4870-3.1 July 2002
The junction temperature ( T ) of a power semiconductor in any
j
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_j = 125˚C is chosen as the design maximum value since above
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_j = 140˚C but such devices may need to be fully
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 to rise well above 125˚C in
j
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_j can rise transiently to 300 - 500˚C
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_TSM limit values are used to determine the peak temperature
( Using I_TSM for V_R=0 ) and the temperature at the end of the
current loop ( Using I_TSM for V_R = 50% V_RRM ). These temperatures
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 for the published I
j
TSM
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
100
80
60
40
20
0
80
V_RRM
V_DRM
100
120
140
160
180
200
Thyristor junction temperature - (˚C)
Fig.1 Thyristor de-rating curves
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