What is Breakover Voltage of an SCR?

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Editorial Team - everything PE

Jun 5, 2024

The breakover voltage of a Silicon Controlled Rectifier (SCR) is the minimum forward voltage at which the SCR switches from the off state (high impedance) to the on state (low impedance) without any gate current. This is an important parameter for understanding SCR operation, as it specifies the voltage level needed to trigger the SCR into conduction solely through the application of the anode-to-cathode voltage, without any gate signal.

Silicon Controlled Rectifiers

SCR is a four-layer, three-terminal device with three junctions J1, J2 and J3. These junctions are forward-biased or reverse-biased based on the voltage applied between the anode and cathode. When the SCR is forward-biased, that is when the anode is positive with respect to the cathode, and when the gate terminal is kept open, junctions J1 and J3 will be forward-biased and J2 will be reverse-biased.

 

When the voltage applied between the anode and cathode is less, the junction J2 which is already reverse biased will have more width. When the voltage applied between the anode and cathode is increased, the width of the junction J2 decreases. Now when the voltage is increased to a higher level, a stage occurs when the junction J2 completely vanishes due to avalanche breakdown. As a result large current flows from the anode to the cathode. The voltage at which the junction Jbreaks down and a current flows through the device is known as the forward breakover over the voltage of an SCR.

Thus, the breakover voltage is the voltage above which the SCR enters the conduction region. If the supply voltage is less than the breakover voltage, the SCR remains in the forward blocking (OFF) state. Applying a forward voltage between the anode and cathode with the gate open is not the normal way to turn on an SCR, as the breakover voltage is usually much greater than the supply voltage. The SCR is normally turned on by applying a small positive voltage to the gate, which reduces the breakover voltage considerably.

Significance of Breakover Voltage of SCR

  1. Turning ON: The breakover voltage is the minimum forward voltage at which the SCR starts conducting heavily, i.e., it is turned ON. This ensures that the SCR does not turn ON accidentally due to excessive voltage and operates within its safe operating range.
  2. Gate Control: The breakover voltage allows the SCR to be controlled with a small gate voltage. This capability means that the SCR can be turned ON and OFF by applying a small positive voltage to the gate terminal, making it ideal for applications requiring precise control.
  3. Thermal Management: The breakover voltage significantly impacts the thermal management of the SCR. If the supply voltage exceeds this threshold, the SCR can undergo thermal runaway, potentially causing damage. Therefore, considering the breakover voltage is crucial when designing and selecting SCRs for specific applications.
  4. Design and Selection: The breakover voltage is a key parameter in the design and selection of SCRs for specific applications. Commercially available SCRs have breakover voltages ranging from 50 V to 500 V, determining their suitability for various uses.
  5. Safety: The breakover voltage ensures that the SCR does not unintentionally turn ON due to excessive voltage, which could lead to damage or even a fire. This makes it a crucial safety consideration in the design and operation of SCR-based systems.

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