AN4999 Application Note
Turn-On Performance Of Thyristors In Parallel
Application Note
Replaces September 2000 version, AN4999-4.0
AN4999-4.1 July 2002
The selection of thyristors for connection in parallel in high power
circuits follows many of the same rules as used for rectifier
diodes. The basic problem is to ensure that the devices share
the load current as evenly as possible. The sharing calculations
have to take account of the need to operate over a range of
currents and device heatsink temperatures and with devices with
different on-state characteristics. In the discussion below it is
assumed that the diodes and thyristors are used in a mains
rectification role, typically at 50 or 60Hz.
For thyristors, this brings the additional problem of needing to
trigger into conduction every mains cycle. Variations in turn-on
times can cause late firing of some of the paralleled group,
effectively reducing the average current in those thyristors.
Another problem arises when one thyristor turns on much faster
than the rest, hogging all the current and thus preventing the
turn-on of the remainder.
The well known solution to paralleling problems both at turn-on
and in the fully-on state is to use reactors in series with each
Device 1 Device 2
Tj = 25˚C
device. Unfortunately, reactors are bulky and often expensive
so designers usually prefer ‘hard paralleling’ i.e. direct connection
to the common busbars without reactors. However, even short
busbars have some inductance and this has to be taken into
The basic rules for paralleling rectifier diodes and thyristors in
the continuous current operating mode are given in standard
power electronics textbooks and elsewhere. Because devices
must always be initially selected for continuous operation the
essential rules are re-stated here.
Stage 1 - On-state voltage banding
Fig 1 shows the idealised on-state characteristics at room
temperature of 2 rectifier diodes connected in parallel. Assuming
zero impedance in the interconnection the voltage across each
device will be the same. At V2 the current in device (1) is I1 and
in (2) is I2. The total load current Iload is (I1 + I2). Clearly, this
current mis-sharing could overload one device and underload
the other. The ideal solution is to have devices with identical
characteristics but production tolerances do not allow this.
However, special selections can match device Vf values into
defined bands. A band width at normal operating current of
200mV is typical but a width as low as 50mV is possible if lower
yield and higher cost is acceptable.
Note that matching is usually done at room temperature,
approximately 20 to 25˚C.
Stage 2 - Heatsinking
Fig. 1 Idealised on-state characteristics at room
temperature of two rectifier diodes in parallel
The shape of the V f characteristic varies with junction
temperature so that the close Vf banding described above has
no value if operating junction temperatures are not nearequalised. The major determinant here is the heatsink
performance. Every effort should be made to ensure that the
heatsink temperature is the same for each device. If possible,
mount all the devices on the same heatsink close to each other.
Mounting more than 2 devices in a vertical column of air-natural
cooled fins can sometimes cause problems.
Fig 2 shows the Vf curves for a particular device at two junction
temperatures. Notice that at low currents, below the ‘crossover’
point, Vf decreases with increase in temperature and above the
AN4999 Application Note
Measured under pulse conditions
Instantaneous on-state current, IT - (A)
1: Tj = 25˚C min
2: Tj = 125˚C min
3: Tj = 25˚C max
4: Tj = 125˚C max
Instantaneous on-state voltage, VT - (V)
Fig. 2 Thyristor limit on-state characteristics
Anode terminal
Fig. 3 Thyristor cylindrical arrangement
Copper busbar
Copper strips
Gate trigger
Copper busbar
Cathode terminal
Fig. 4 Typical busbar arrangement - 3 clamped thyristors in parallel (heatsink not shown)
AN4999 Application Note
crossover point the reverse is true. The maximum and minimum
of the production spread are shown.
Thus, at low currents without good control, the hotter devices
will tend to become even hotter . Fortunately, at high currents
the opposite is true and a self sharing effect takes over.
Stage 3 - Busbar connections
Gate pulse length has some effect on sharing performance
but only when the thyristor load current source is low voltage.
This is particularly true for amplifying gate devices. For example,
a thyristor switching a 50Hz, 10V rms ac current source is unlikely
to switch on until about 700us from voltage zero.
t0 t1 t2
The least considered part of the paralleling exercise is usually
the interconnections. Unless the conductors linking each device
to the common power connections are of equal lengths the
devices will not share. All this in spite of the careful parallel
banding in stage 1, above. Fig 3 shows diagramatically a
cylindrical arrangement with equal length connections and,
clearly, this is not easy to achieve for high current devices. More
typical is the non-symmetrical arrangement of Fig 4. If tightly
banded devices are used in such an arrangement poor current
sharing is the most likely result! Some positions in the connection
will be seen to be current ‘hoggers’ and others current ‘shunners’.
Finger voltage
Device B
Low voltage supply
Thyristor forward voltage
When more than 2 devices are paralleled, experience has shown
that it is better to have devices spread across a fairly wide Vf
band. 200mV is about right. The user then fits high Vf devices
into current ‘hogger’ positions and low Vf ones into current
‘shunner’ positions. This approach usually gives a satisfactory
Finger voltage
Device B
Low voltage supply
Thyristor forward voltage
Unequal conduction due to delay time variations and gate
drive performance:
Even if all the thyristor gates are triggered at the same time,
variations in delay time will lead to variations in turn-on time.
Experience has shown that, provided high gate currents are
used, delay time differences are small for a particular thyristor
type. Selection on delay time is only necessary for sensitive
At low gate drives the devices turn on adequately but there are
significant variations in the delay times. As gate currents are
increased the delay time variations become smaller until an
optimum gate current is reached where there are no further
reductions. Similar comments apply regarding gate current rise
time where further reductions in rise time have no effect.
Happily, a high current, fast rising gate pulse also gives a good
thyristor di/dt rating.
On-state current
All the procedures described above are also essential for good
sharing of thyristors at turn-on.The additional requirement is for
thyristors to turn on as near together in time as possible,
otherwise the average current in a late turning on thyristor is
less than in the other (normal speed) devices. Note that turn-on
problems only occur in a minimum of situations and only for
some device types.
Finger voltage points
On-state voltage Device Device V
Fig. 5 Finger voltages
AN4999 Application Note
The other extreme condition is where one thyristor in the
paralleled batch has an exceptionally low spike voltage. This
device turns on early and can hog all the current.
Generally, the lower the supply voltage the worse are the
paralleling problems.
In practice, a small increase in busbar inductance in series with
each thyristor can usually overcome the turn-on problem.
Fig. 6
This corresponds to about 3V anode to cathode which is the
minimum needed for turn on of a large thyristor. Clearly, a gate
pulse width of 10 to 15us is OK for a switched load of several
hundred volts but a 1ms pulse is needed for a 10V source.
Unequal conduction due to transient on-state voltage
As indicated above, thyristors need a minimum anode to cathode
voltage to turn on. This is indicated by the well known thyristor
switching characteristic, Fig 5. The minimum value is sometimes
known as the ‘finger voltage’
It is clear that a group of thyristors connected in parallel with
different finger voltages and different delay times could present
a problem for turning on.
The problem is sometimes worse when amplifying gate devices
are used. The amplifying gate thyristor is effectively a main
thyristor, darlington driven by a pilot thyristor connected between
anode and gate, Fig 6.
When an amplifying gate thyristor is triggered current initially
flows in the pilot thyristor until enough current is flowing to trigger
the main device. Then the circuit current transfers to the main
thyristor. Usually, the initial voltage across the pilot thyristor must
reach a minimum level to initiate transfer. This minimum level
appears as a ‘spike’ on the Vf waveform. Fig 7 is an example of
a high spike voltage. ‘Spike’ voltage values can vary across a
batch of devices.
In a paralleled system, if one of the thyristors has a high spike
voltage it may be late to turn on and so conduct for only part of
the half sine wave period. The consequence is a low average
current. In the worse case the device may fail altogether to turn
How can devices be selected to overcome paralleling problems?
The easy solution is to include large series reactors so that missharing is negligable, even with unselected devices.
If this is not possible the following procedures should be adopted:
Maximum effort should be made to equalise busbar lengths.
This will help both steady state and turn-on sharing.
Ensure that as near as possible the device case temperatures
are equalised.
Ensure that a high gate drive current is used (similar to that
recommended for good di/dt rating).
Finally, determine the equipment operating current at which
current sharing is most important. Usually, this is the full load
operating current but sometimes overload current is more
At this stage a preliminary choice of thyristor should be made
for continuous operation, assuming a mis-sharing of current
between devices of, say, 15%:
Define mis-sharing factor, ‘m’ as:
m = (Imax - Imin) x 100%
Where Imax is the current flowing in the highest current device
and Imin is the current flowing in the lowest current device.
From the thyristor supplier determine the available Vf
selections for paralleling at the Tj and current you require.
Calculation procedures can then be used which input device
maximum and minimum Vf curves and the device busbar
connection inductance to determine steady state mis-sharing
Finally consider turn-on mis-sharing due to turn-on spike
voltage variations. This is unusual unless very short busbar
lengths are used. Also, the problem only affects a few device
types but if a problem is likely it should be mentioned to the
supplier in case a selection is needed.
AN4999 Application Note
The possibility of turn-on mis-sharing of large thyristors in
parallel is sometimes forgotten. However, if a problem exists it
can usually be overcome by using slightly longer than normal
busbars. As a final resort a supplier can select to eliminate
extreme values of spike voltage. Use of high output gate drive
is also important.
Fig. 7 Example of high spike voltage
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