To obtain satisfactory and reliable SCR operation, it is necessary to provide protection against over voltages and over currents. Under overload or other abnormal operating conditions an SCR may carry current many times greater than as rated value and can be destroyed permanently. Overvoltage’s, which arise from non-ideal operation of the devices and from transients, may exceed the rated voltage of the SCR. The SCR should be protected against any unusual transient conditions. Selecting an SCR rating based on abnormal operating conditions is not economical. Usually some safety margin is provided by choosing devices with ratings larger than those satisfactory for normal operation.
Overvoltage is one of the most important causes of SCR failure. An overvoltage is a voltage that exceeds the rated peak value of the supply voltage. Overvoltage’s are caused mainly by switching disturbances. usually due to the energy stored in the inductive components. The effect is to produce a transient voltage whose peak may exceed the rated forward blocking voltage of the SCR. Depending on the intensity of the overvoltage and the energy that it represents the SCR may experience false triggering or may he permanently destroyed due to reverse breakdown.
To protect an SCR against oven voltages a diode is connected in series with the SCR. The reverse voltage will be shared between the diode and the SCR. However, this method is inefficient since the voltage drop across the diode when conducting will introduce power loss. Another simple method is to choose an SCR with a higher voltage rating by introducing a voltage safety factor (VF).
VF = VRRM (PIV) of the SCR / peak value of the voltage applied to the SCR
Semiconductor devices are normally rated for economic consideration. With a voltage safety factor of 1.5. Any overvoltage’s must be limited to less than this value. Protection against overvoltage’s can also be achieved by using an RC snubber circuit across the overvoltage source. The RC circuit will suppress the transient overvoltage by absorbing its energy. Another method is to connect a nonlinear resistor called a varistor across the SCR. The varistor provides a low resistance path to the transient voltage and thus bypasses the device.
Find the PIV rating of an SCR connected across a 220 V AC source. Use a safety factor of 1.5.
An overcunent can occur due to source failure during inversion, overload, or a short circuit. The SCR is usually protected by using conventional protective devices such as overcunent relays, fast acting fuses, and high-speed circuit breaker. The proactive device is required to carry continuously the normal current that the SCR is rated to carry but to open the circuit in case of a short circuit or fault before the SCR is overheated and damaged.
The energy that must be dissipated by a fuse comes from either the power source or the energy stored in the inductive components of the circuit. The heat energy to be dissipated by the fuse is equal to I2t. Therefore, the I2t rating defines the thermal capacity of fuses and is used in the protection of SCRs. During overloads, faults, and short circuits, the SCR must withstand conditions leading to high function temperatures. The I2t rating for the SCR allows selection of the correct protection to avoid overheating the junction.
Manufacturers of fuses and SCRs specify in their data the I2t ratings of their devices. The I2t rating of the SCR is based on the device Operating at maximum rated current and maximum junction temperature. Because fuses and SCRs are rated on a common basis, it is only necessary to select a fuse with an I2t rating less than the I2t Rating of the SCR for proper protection. I is the RMS value of the current and t is the time in seconds.
Gate Circuit Protection
The gate pulse generating circuit must be protected against induced voltage transients it is also necessary to separate it from the power wiring as widely as possible Electrical isolation between the SCR and the gate circuit is usually provided by a pulse transformer or an optocoupler. The pulse transformer, which has a ferrite core, has a primary winding and one or more secondary winding's, which permit simultaneous gate pulses to be applied to SCRs in series or parallel connections. The most commonly used optocouplers consist of a combination of a light-emitting diode (LED) and a photo-transistor assembled in a single package. This arrangement allows coupling of signal from one circuit to another, while providing almost complete electrical isolation between the two circuit.