AN4870 Application Note
AN4870
Effects Of Temperature On Thyristor Performance
Application Note
Replaces September 2000 version, AN4870-3.0
AN4870-3.1 July 2002
The junction temperature ( T
j
) of a power semiconductor in any
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
j
to rise well above 125˚C in
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
j
for the published I
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
Percentage of voltage grade
80
60
V
RRM
40
V
DRM
20
0
80
100
120
140
160
Thyristor junction temperature - (˚C)
Fig.1 Thyristor de-rating curves
180
200
1/5
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AN4870 Application Note
1
Failure rate / 1000hrs
0.1
100%
75%
50%
25%
0.01
0.001
40
50
60
70
Junction temperature - (˚C)
80
90
100
Fig.2 Thyristor failure rate vs applied voltage as a percentage of V
DRM
(rated) and junction temperature due
to ion migration in junction passivation
Circa 250˚C continuous: Rubber locaters and organic
passivation material starts to disintegrate; some annealing-
out of electron irradiation.
Above 600˚C. The metal of the surface contacts starts to
penetrate into the silicon causing eventual short circuit. This
is probably a factor in di/dt failure.
1100 to 1300˚C. This is the temperature reached at non-
repetitive di/dt limits. The high local thermal stress causes
cracking of the silicon.
1415˚C - Melting point of silicon.
Another important temperature limit is the magnitude of
temperature excursions (
∆T
j )
which relates strongly to the
operating life of the device. Slow temperature changes stress the
various mechanical parts of the device and cause the movement
of one component relative to another due to differential expansion
and contraction.
both reverse and then forward voltage being reapplied. For
forward blocking two possible failure modes apply:-
1) Failure to turn-off because of the high turn-off time,t
q
value at
elevated temperature.
2) Breakover due to high blocking current alone.
The most likely is 1). Variation of t
q
with temperature for a range
of other conditions must be determined experimentally.
Other important temperatures are:
Temperatures below T
j(max)
where ion migration on the silicon
surface under the passivation can lead to long term
degradation. (See fig.2)
Continuous T
j
permitted before thermal runaway occurs.
This is likely to be important only with high leakage
thyristors and when very small heatsinks are used.
250
Temperature excursion - (˚C)
200
150
100
50
0
1.00E + 03
30mm
38mm
50mm
75mm
100mm
1.00E + 04
1.00E + 05
1.00E + 06
No. of cycles
1.00E + 07
1.00E + 08
Fig.3 Thermal fatigue life expectancy
2/5
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AN4870 Application Note
Rapid temperature changes associated with highdi/dt can cause
micro cracking. It has been shown that, in silicon, micro cracking
occurs with
∆T
between 300 and 350˚C. Somos et al have shown
how the value of
∆T
relates the expected life time of the device
measured in numbers of operations and device diameter. (Fig.3).
Although continuous operation at 250 to 300˚C will destroy PN
junction characteristics it is possible to operate transiently in this
region if allowance is made for reduced device life. Such is the
philosophy behind surge current protection when roughly 100
operations up to I
TSM
values are allowed in the life time of a device.
When any overload current wave shape is more complex than a
simple sine wave a method of calculating end-of-pulse
temperature has to be used. Calculation of steady state T
j
takes
account of the device case temperature, average current/power
loss and steady state thermal resistance. However, for short
term overloads it is necessary to include variation of device
thermal resistance with time and the device on-state volt drop
with temperature. A method of calculating junction temperature
using a computer program is described for overloads lasting 1 to
about 100ms:
The information on the overload current is inputted as a
series of instantaneous current values with corresponding
time points.
5
Z( t ) =
Σ
A(i).exp[ -t / B ( i ) ]
i=1
where B = 0.001,0.01,0.1,1.0 and 2 seconds.
Associated with each component is a constant and each device
type has its own unique set of 5 constants.
The variation of on-state voltage with forward current is also
represented by a polynomial with 5 components.
V( I, T
j
) = Vφ ( 1 + BT x T
j
) + Rφ + I (1 + AL x T
j
)
+ Eφ( 273 + T
j
) Log
10
(i) + 2.3025
The curve is determined experimentally using a 10ms half sine
pulse which goes to currents which are almost 90% of I
TSM
. The
resultant heating effect is noticeable by the V
F
increase on the
falling edge of the current pulse. An example of such a “surge
loop” is shown in fig.5.
Notice that the surge loop equation includes a temperature term
which the normal data sheet V
TM
curve does not. In other words,
the “surge loop” model calculates change in V
TM
due to junction
temperature increase.
The device transient thermal impedance curve is represented
as a polynomial with 5 components, fig.4
0.1
Thermal Impedance - junction to case - (˚C/W)
Anode side cooled
0.01
Conduction
Effective thermal resistance
Junction to case ˚C/W
Double side
0.022
0.024
0.026
0.027
Anode side
0.038
0.040
0.042
0.043
d.c.
Halfwave
3 phase 120˚
6 phase 60˚
0.001
0.001
0.01
0.1
Time - (s)
1.0
10
Voltage - (V)
Fig.5 Surge loop
Fig.4 Maximim (limit) transient thermal impedance
- junction to case
Current - (A)
Double side
cooled
3/5
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AN4870 Application Note
These three input items are then used to calculate instantaneous
power and temperature rise at specified time intervals, e.g. 1ms.
the procedure using the superposition thereom is as follows:
1. Take the initial T
j
at the start of the first 1ms period as T
j
(1).
2. Use this in the “surge loop” equation to calculate average
power in the first interval. (P1).
3. From the average period power and transient thermal
resistance at 1ms calculate temperature rise in the first
period and hence starting temperature for second period,
T
j
(2) where T
j
(2) = T
j
(1) + T rise (1).
4. Proceed to the second time period and use T
j
(2) to calculate
appropriate volt drop values and power in this period.
5. Use the average power in period 2 (P2) and the change in
thermal resistance between 1ms and 2ms to calculate the rise
in the second interval. This then gives the temperature at the
end of the second interval, T
j
(3).
6. Continue this procedure for as many intervals as necessary.
The procedure is more clearly explained by considering a
waveform with 5 intervals.
T
j(6)
=
P1 [ Z (T
6
-T
1
) - Z( T
6
-T
2
) ]
+ P2 [ Z (T
6
-T
2
) - Z( T
6
-T
3
) ]
+ P3 [ Z (T
6
-T
3
) - Z( T
6
-T
4
) ]
+ P4 [ Z (T
6
-T
4
) - Z( T
6
-T
5
) ]
+ P5 [ Z (T
6
-T
5
) ]
We are using the calculated results as a measure of device
survivability so how reliable are the results?
The main assumption is that current flow is uniform across the
device area so that temperature is also assumed uniform. This
means that current pulses must be wide enough to allow the
thyristor to reach full conduction. For small thyristors of a few mm
diameter this is easily achievable for pulses of less than 1ms.
With larger diameter devices e.g. 30 to 100mm, pulses of several
milliseconds are required. For most converter applications this
presents no restriction.
Another possible source of error is the potential inaccuracy of the
transient thermal impedance curve, particularly at times of 1 to
10ms. It is very difficult to measure this part of the curve so
calculation is used. A transmission line model is assumed but
since it is difficult to assign accurate values to the various contact
thermal resistances between metallic parts conservative values
are used. Values depend on surface finishes and clamping
forces.
For times longer than about 100ms heat generated at the
junction starts to pass into the cooling fin. This is not accounted
for in this particular model.
Calculation of temperature rise for short pulses requires more
complex 2 and 3 dimensional analysis, possibly involving finite
element analysis techniques. Device turn-on behaviour and its
dependency on voltage, temperature, di/dt and gate drive has to
be taken into account.
P1
T
1
= O
6
T
j(1)
T
2
T
j(2)
P2
T
3
T
j(3)
P3
T
4
T
j(4)
P4
T
5
T
j(5)
P5
T
6
T
j(6)
Time
Junction temperature
Fig.6
4/5
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