How to protect an SCR using protection circuits ?
SCRs are sensitive to high voltage, over-current, and any form of transients. For satisfactory and reliable operation they are required to be protected against such abnormal operating conditions. Because of complex and expensive protection, usually some margin is provided in the equipment by selecting devices with ratings higher (3 or 4 times higher) than those required for normal operation. But it is always not economical to use devices of higher ratings, hence their protection is imperative.
High forward voltage protection is inherent in SCRs. The SCR will breakdown and start conducting before the peak forward voltage is attained so that the high voltage is transferred to another part of the circuit (usually the load). The turn-on of SCR causes a large current to flow and poses a problem of over-current protection.
Over-current protection can be provided by connecting a circuit breaker and a fuse in series with the SCR, as usually done for the protection of any circuit. However, there are some reservations to their use. A semiconductor device is capable of taking overloads for a limited period, so the fuse used should have high breaking capacity and rapid interruption of current. There must be a similarity of SCR and fuse I2t rating without developing high voltage transients which endanger those SCRs in the off or infinite impedance condition. These are contradictory requirements necessitating voltage protection when fast-acting fuses are employed. Fuses when used, their arc voltages are kept below 1.5 times the peak circuit voltage. For small power applications it is pointless to employ high speed fuses for circuit protection because it may cost more than the SCR. Current magnitude detection can be employed and is used in many applications. When an over-current is detected the gate circuits are controlled either to turn-off the appropriate SCRs, or in phase commutation, to reduce the conduction period and so the average value of the current.
If the output to the load from the SCR circuit is alternating current, LC resonance provides over-current protection as well as filtering. A current limiting device employing a saturable reactor is shown in figure. With normal currents the saturable reactor L1 offers high impedance and C and L are in series resonance to offer zero impedance to the flow of current of the fundamental harmonic. An over-current saturates L1 and so gives negligible impedance. There is LC parallel resonance and hence infinite impedance to the flow of current at the resonant frequency.
Protection against Voltage Surges.
There are many types of failure due to voltage surges as SCRs do not really have a safety factor included in their ratings. External voltage surges cannot be controlled by the SCR circuit designer. Voltage surges often lead to either malfunctioning of the circuit by unintentional turn-on of SCR or permanent damage to the device due to reverse breakdown. SCR can be protected against voltage surges by employing shunt connected non-linear resistance devices. Such protective devices register a fall in resistance with the increase in voltage and so develop a virtual short-circuit across the SCR when a high voltage is applied. An over-voltage protection circuit employing thyrector-diode, which has low resistance at high voltage and vice-versa, is shown in figure. Inductor L and capacitor C provides protection to SCR against large dV/dt and dI/dt.