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AN4999: DescriptionTurn-On Performance Of Thyristors In Parallel; File Size101KB，6 Pages; ManufacturerDynex; Websitehttp://www.dynexsemi.com/

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Turn-On Performance Of Thyristors In Parallel

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AN4999 Application Note

AN4999

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. 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 I

and

in (2) is I

. The total load current I

load

is (I

+ I

). 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

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.

= 25˚C

Device 1 Device 2

Stage 2 - Heatsinking

The shape of the V

characteristic varies with junction

temperature so that the close V

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

curves for a particular device at two junction

temperatures. Notice that at low currents, below the ‘crossover’

point, V

decreases with increase in temperature and above the

1/6

Fig. 1 Idealised on-state characteristics at room

temperature of two rectifier diodes in parallel

www.dynexsemi.com

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