The maximum power that can be controlled by a single SCR is determined by its rated forward current and rated forward blocking voltage. To maximize one of these two ratings, the other must be reduced. Although SCR’s are currently available with very high voltage ratings, in many applications, such as transmission lines, the required voltage rating exceeds the voltage that can be provided by a single SCR. Then it is necessary to connect two or more SCR’s in series. Similarly, for very high current applications, SCR’s must be connected in parallel. For high-voltage, high-current applications, series-parallel combinations of SCR’s are used.
SCRs in Series
In cases where the input voltage exceeds the voltage rating of a single SCR, multiple SCR's can be connected in series to increase their forward-blocking capability. However, it's important to note that the characteristics of two SCRs, even those of the same make and rating, may differ. When two SCRs are connected in series, the voltage is divided between them inversely proportional to their leakage currents. This results in an uneven voltage distribution across the two SCRs. Figure 1 displays the leakage current of two identical SCRs, SCR1 and SCR2.
Each has a forward-blocking voltage VBO. When two such SCR’s are connected in series, the same current flows through the two devices; however, the voltage across SCR1.
(V1) is higher than that across SCR2 (V2) because the leakage current for SCR1 is smaller than that of SCR2 for the same voltage. Therefore, the two SCR’s do not share the supply voltage equally. The maximum voltage that the SCR’s can block is only V1 + V2, not 2VBO. To use the full forward blocking capabilities of each SCR1 the forward blocking voltage must be equally distributed.
A nearly equal distribution of voltages during blocking is easily accomplished by connecting voltage-equalizing resistors R1 and R2 (Figure 2(a)) in parallel with each SCR such that each parallel combination has the same resistance. However, when several SCR’s are connected in series, this method becomes uneconomical. A second approach, which permits a reasonably uniform distribution of voltages, is to use the same value resistance in parallel with each SCR. This allows a different but fixed voltage to appear across each SCR. In this arrangement (Figure 2(b)), the SCR with the lower leakage current will have a greater portion of the blocking voltage than the SCR with the higher leakage current.
Let us assume that the leakage current of SCR1 (ISCR1) is greater than the leakage current of SCR2 (ISCR2). SCR2 will be required to have the larger voltage (V2).
V2 = I2R
The voltage across the series combination is
Vm = V2 + I1R
Applying Kirchhoff’s current law (KCL) to the middle node
ISCR1 + I1 = ISCR2 + I2
ISCR1 + ISCR2 = I1 - I2 = ΔI
I1 = I2 – ΔI
Vm = V2 + (I2 – ΔI) R
= V2 + I2R – ΔIR
= V2 + V2 – ΔIR
= 2 V2 – ΔIR
R = (2 V2 – Vm) /ΔI
Unequal voltage distribution among SCR’s in series also occurs during turn-ons and off. One SCR may turn ON or turn OFF before the other. The OFF SCR will then be subjected to the full source voltage. Shunt capacitors are very effective in equalizing voltages during switching. The capacitor also forces voltage sharing during sudden changes in supply voltage. A resistor is added in series with the capacitor to limit the current and di/dt (due to discharge of the capacitor through SCR) during turn ON. This CS and RS combination. Shown in Figure 3, is essentially a snubber circuit. A diode (D) connected across RS shunts it for forward voltages.
The voltages across two SCR’s connected in series are 200V and 180V. calculate the value of the required equalizing resistor if the SCR’s have a maximum difference of 1mA in latching current. Also, find the power dissipated by the blocking resistor.
R = (2V2 – Vm) / ΔI
= (2(200) – 380) / 1(10-3) = 20 kΩ
PR = (V21 / R) / (V22 / R)
= (1802 + 2002) / (20 (103) = 3.62 W
Applications of silicon controlled rectifiers (SCRs) connected in series
- High voltage DC supplies - Series SCR strings are used to control very high DC voltages for applications like X-ray generators, electrostatic systems, etc.
- AC voltage regulators - Phase control using series SCRs allows regulation of AC voltage for applications like light dimmers, and heater controls.
- Soft starters - A series SCR string enables gradual start-up and reduced inrush current for large motors and transformers.
- Snubber circuits - Small capacitors across each device in a series string provide snubbing to limit voltage transients during switching.
- Voltage multiplier circuits - Series diode-SCR ladder circuits act as voltage multipliers to generate high DC voltages from an AC supply.
- Frequency converters - Configurations like the McMurray inverter use SCRs in series to convert power frequencies in motor drives.
- Harmonic filters - Series SCRs can dynamically adjust filter properties to eliminate harmonics in power systems.
- Industrial furnaces - Very high voltages for induction heating and melting applications use SCRs in series switching configurations.
- Lighting - Series SCR strings switched on and off at line frequency are used in some lamp ballasts to control brightness.
- Negative resistance - The nonlinear I-V curve of series SCRs allows the creation of negative resistance used in oscillators.
Series SCR connections are commonly used for high-voltage DC generation, AC voltage control, variable frequency supplies, and harmonic reduction in power electronics.