In a thyristor, holding current is

Question

Examoogle · Accepted Answer

less than $I_L$

Explanation: In a thyristor (SCR), the holding current $I_H$ is the minimum anode current required to maintain the device in the 'ON' state. The latching current $I_L$ is the minimum anode current required to turn the device 'ON' from the 'OFF' state; since turning the device on requires overcoming the initial depletion region, $I_L$ is always greater than $I_H$.

Working Principle: The latching current $I_L$ must be achieved immediately after the gate pulse is removed to ensure the internal regenerative feedback of the thyristor reaches a stable conducting state. If the anode current falls below the holding current $I_H$, the device loses its regenerative latching mechanism and reverts to the blocking (OFF) state. Because $I_L$ includes the initial charging of internal capacitances and the start of regeneration, $I_L > I_H$.

Key Formulas: $I_L > I_H$ — The fundamental inequality for thyristor conduction.

Key Points: Latching current $I_L$ is associated with the turn-on process.; Holding current $I_H$ is associated with the turn-off process.; The ratio $I_L / I_H$ is typically between 2 to 3 for most commercial thyristors.; If anode current $I_A

Advantages: Protects against unintentional turn-off due to minor current dips; Defines specific operating boundaries for power electronics design

Disadvantages: High latching current requires larger gate pulses for reliable turn-on; Holding current limits the minimum controllable load in certain dimming circuits

Applications: Design of SCR firing circuits; Protection circuit design for power switches; DC to DC chopper circuit analysis

Additional Info: Holding current is generally lower because the internal charge carriers are already established in the device layers after turn-on.. Option B is correct because the threshold for latching is higher than the threshold for merely maintaining existing conduction.

Answer: B is correct — Latching current is higher because it encompasses the energy required to initiate the internal regenerative action of the thyristor, whereas holding current only maintains it.

Exam Tip: In exams, always remember: Latching = 'Launch' (starting, needs more), Holding = 'Hold' (sustaining, needs less). $I_L > I_H$ is a frequent numerical problem trap.

Related Questions

Discussion (0)