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Series and Parallel SCR Connections

SCRs are connected in series and parallel to extend voltage and current ratings.

For high-voltage, high-current applications, series-parallel combinations of SCRs are

used.

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SCRS IN SERIES Unequal distribution of voltage across two series SCRs.

Two SCRs do not share the same supply voltage. Maximum voltage that SCRs can block is V1+V2, not 2VBO. 3

Resistance equalization

Voltage equalization

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RC equalization for SCRs connected in series.

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SCRS IN PARALLEL Unequal current sharing between two SCRs is shown:

Total rated current of parallel connection is I1+I2, not 2I2.

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With unmatched SCRs, equal current sharing is achieved by adding low value resistor or inductor in series with each SCR, as shown below.

Value of resistance R is obtained from: R=V1-V2

I2-I17

Current sharing in SCRs with parallel reactorsEqualization using resistors is inefficient due to

Extra power loss Nonc ompansation for unequal SCR turn-on and turn-off times. Damage due to overloading

SCRs with center-tapped reactors is shown below.

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When the load current exceeds the rating of a single thyristor, thyristors are connected in parallel to increase the overall current capability. Contrary to what might be expected, the load current is not shared equally between the thyristors, because thyristors are not perfectly matched. Figure shows the V-I characteristics of two thyristors T1 and T2 which are connected in parallel.

Current sharing in SCRs with parallel reactors

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The thyristor carrying the higher current would dissipate more power which in turn will increase the junction temperature and hence decrease the internal resistance. This in turn will increase its current sharing capacity and maybe damage the thyristor. This process is termed thermal runaway and is not common only to thyristors. Thermal runaway may be prevented by using one common heat sink to ensure that both thyristors are operating at the same temperature. Equal current sharing could be accomplished with the use of a small resistor or inductor in series with each thyristor as shown in above figure.

When a resistor is used to produce equal current sharing, the losses in the series resistor is very high and may be unacceptable. When magnetically coupled inductors are used for the purpose of current sharing, if thyristor T1 current increases, a voltage of opposite polarity to that of the coil in series with T1 will be induced in the coil in series with thyristor T2. The polarity of this voltage is as such to increase the anode potential of thyristor T2, thereby increasing the current flow through this thyristor. 11

SCR di/dt Calculationdi/dt depends on the type of component being used and it appears in datasheets.

Snubber circuit in dv/dt

The circuit used to limit the dv/dt (specified in datasheet) across SCR is called snubber circuit.