In a thyristor, ratio of latching current to holding current is

Question

Examoogle · Accepted Answer

2.5

Explanation: In a thyristor (SCR), the latching current is the minimum anode current required to maintain the device in the 'ON' state immediately after it has been triggered and the gate signal is removed. The ratio of latching current ($I_L$) to holding current ($I_H$) is typically in the range of 2 to 3, with 2.5 being the standard accepted representative value.

Working Principle: The latching current is associated with the turn-on process, requiring enough carriers to sustain regenerative feedback within the internal $p-n-p-n$ structure. The holding current is associated with the turn-off process, representing the threshold current below which the regenerative action fails and the device switches to the 'OFF' state. Since sustaining the turn-on process requires more charge carriers than merely maintaining the conduction state, $I_L$ is always greater than $I_H$.

Key Formulas: $I_L > I_H$ — Fundamental condition for SCR operation; $	ext{Ratio} = \frac{I_L}{I_H} \approx 2.5$ — Standard design guideline

Key Points: Latching current is defined during the turn-on phase.; Holding current is defined during the turn-off phase.; The SCR cannot remain ON if the anode current falls below $I_H$.; If anode current fails to reach $I_L$ after triggering, the SCR turns OFF.

Advantages: Ensures stable transition to conduction state.; Provides a clear design margin for triggering circuits.

Disadvantages: Higher latching current requires more robust gate driver circuitry.; Variations in these currents occur with temperature fluctuations.

Applications: Power electronic switching converters.; Phase-controlled rectifiers.; AC voltage regulators.

Additional Info: Latching current is typically defined with the gate pulse present, then removed.. Option D (6.0) is typically considered too high for standard commercial SCRs.

Answer: C is correct — The ratio of latching current to holding current in a thyristor is typically approximately 2.5, as latching current is inherently higher to ensure stable turn-on.

Exam Tip: Always remember that L comes before H alphabetically, and Latch (turn-on) comes before Hold (turn-off) in the device cycle, aligning with $I_L > I_H$.

Related Questions

Discussion (0)