SCR Firing / Triggering

- details of thyristor or SCR triggering along with circuits and additional components to ensure accurate repeatable triggering or firing.

Triggering or firing is one of the key issues associated with SCRs or thyristors.

Accordingly SCR triggering methods need to be considered when designing a circuit using one of these components.

The performance of the circuit and the very way it operates will be largely dependent upon the SCR triggering or firing method chosen.

Within the triggering of SCRs, various aspects including the gate drive requirements if gate triggering is used, trigger time where the time of the trigger stimulus applied needs to be maintained for the circuit to latch and others are all important. The importance of the various parameters being dependent upon the form of SCR triggering used.

SCR triggering methods summary

There are several ways in which an SCR can be triggered or fired. These methods include the following which are discussed below:

  • Gate triggering:   This form of SCR triggering is the one that is most commonly seen in the different circuits used. It is simple, reliable, efficient and also easy to implement for most applications - a simple trigger signal can be applied, with suitable processing if required. This means that other electronic circuits can be used to derive a suitable trigger signal and this can then be applied to the SCR.

    For gate SCR triggering to be used, the SCR must operate below its breakdown voltage, and a suitable safety margin also allowed to accommodate any transients that may occur. Otherwise forward voltage or breakdown triggering may occur.

    To turn-on of an SCR, a positive gate voltage between gate and cathode. This gives rise to a gate current where charges are injected into the inner p layer of the device. This effectively reduces the voltage at which forward break-over occurs. It can be gathered that the gate current determines the forward voltage at which the device switches to its conducting state. Higher the gate current, the lower the forward break-over voltage.

    There are many simple methods of applying the trigger signal. Possibly one of the simplest arrangements is shown in the diagram below.

    SCR gate circuit
    Gate resistors in SCR circuit

    Here is can be seen that there are two resistors. The first is R1 which is included to limit the gate current to an acceptable level. This resistor is chosen to provide sufficient current to trigger the SCR while maintaining it within safe limits for the device. It can easily be calculated using the device ratings and Ohms law.

    The second resistor, R2 is the gate cathode resistor. This is sometimes denoted as RGK and it is included to prevent spurious triggering. The action of the resistor can be seen with respect to the two transistor analogy of the SCR. It shows that a low external resistance between the gate and cathode bypasses some current around the gate junction. Accordingly a higher anode current is required to initiate and maintain conduction. It is particularly found that low current high sensitivity SCRs are triggered at very low current levels and therefore an external gate-cathode resistance is required to prevent triggering by thermally generated leakage current in the gate region. However the gate cathode resistance bypasses some of the internal anode current caused by the rapid rate of change of the anode voltage (dv/dt). It also raises the forward break-over voltage by reducing the efficiency of the NPN transistor region thus requiring a somewhat higher avalanche multiplication effect to initiate the triggering. The current that bypasses the gate junction also affects the latching and holding currents.

    It can therefore be seen that the effects of using the gate cathode bypass resistor include:
    • Increase the dv/dt capability.
    • Retain gate damping to assure the maximum repetitive peak off-state voltage VDRM capability.
    • Lower the turn-off time, tq.
    • Raise latching and holding current levels
    Although the simple circuit shown above is adequate for many applications, where a more controlled triggering mechanism is required, account needs to be taken of the gate characteristics before triggering, during triggering and afterwards. This is required because the gate characteristics change as a result of the current changes within the device.
  • Anode cathode forward voltage SCR triggering:   This form of SCR triggering or firing occurs when the voltage between the anode and cathode causes avalanche conduction to take place. The way in which this occurs can be seen in conjunction with the SCR structure.

    SCR structure
    SCR Structure

    When the anode to cathode forward voltage is increased, diode junction, J2, comes under increasing stress as it is reverse biased. Ultimately the voltage gradient will increase beyond the breakdown point and avalanche breakdown will occur triggering the SCR. The voltage at which this occurs is known as the forward break-over voltage VB0.

    As the junction J2 breaks down, current will flow and triggering the SCR to its conducting state. The junctions J1, J3 are already forward biased, and therefore the breakdown of junction J2 allows the flow of carriers across all three junctions enabling the load current to flow. As with other forms of triggering the SCR, the device remains in its conducting condition.

    The use of this method of turning the device on is not advised because exceeding the value of VB0 could destroy the device. Any circuit should be designed to avoid this method of triggering, noting the maximum of any likely voltage spikes.
  • Temperature triggering:   This form of SCR triggering may occur under some circumstances. It may give rise to unexpected responses and therefore its effects should be noted as part of any design process.

    Temperature triggering of SCRs occurs as the voltage across the junction J2 and any leakage current may raise the temperature of the junction. The increase in temperature further increases the temperature which will in turn increase the leakage current. This cumulative process may be sufficient to trigger the SCR, although it tends to only occur when the device temperature is high.
  • Light triggering:   This form of SCR triggering or firing is often used with high voltage systems. Here an electrical connection is not required from the firing mechanism, and an isolated light source can be used.

    Where light SCR triggering is to be used, specially manufactured SCRs are available. The light triggering occurs within the inner P-type later. When this area is irradiated by light, free charge carriers are generated and just like applying a gate signal, the SCR is triggered.

    To achieve the maximum light absorption, specialised SCR structures are used, often having a recess in the inner P-type later to enable maximum access to the light.

    To enable the light triggering to take place, light is often directed to the correct point in the SCR using optical fibre. Once the light exceeds a certain intensity, switching occurs. An SCR of this type is often referred to as a Light-activated SCR or LASCR. These LASCRs have been used in high voltage power distribution switching centres. The optical switching enables very high levels of isolation to be achieved while still being able to switch with low level circuitry.
  • dv/dt triggering:   SCR triggering can also occur without any gate current if the rate of rise of anode to cathode voltage exceeds certain limits for the particular device.

Although there are several forms of SCR triggering or SCR firing, not all are applicable for use. However it is necessary to be aware of SCR triggering mechanisms such as forward breakdown in order to know they exist and should be avoided..

By Ian Poole

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