AN4999 Datasheet, PDF (1/6 Pages) Dynex Semiconductor – Turn-On Performance Of Thyristors In Parallel

Replaces September 2000 version, AN4999-4.0
AN4999 Application Note
AN4999
Turn-On Performance Of Thyristors In Parallel
Application Note
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
I_C
T_j = 25˚C
I1
Device 1 Device 2
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
account.
BASIC REQUIREMENTS FOR PARALLELING
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 I_load 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 V_f 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.
I2
V2
V_F
Fig. 1 Idealised on-state characteristics at room
temperature of two rectifier diodes in parallel
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Stage 2 - Heatsinking
The shape of the V_f characteristic varies with junction
temperature so that the close V_f banding described above has
no value if operating junction temperatures are not near-
equalised. 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 V_f curves for a particular device at two junction
temperatures. Notice that at low currents, below the ‘crossover’
point, V_f decreases with increase in temperature and above the
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