--- title: "Thyristor V-I; latching vs holding current" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:thyristor-vi-latching-vs-holding" source: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-vi-latching-vs-holding site: BCT Engineering --- # Thyristor V-I; latching vs holding current Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2081 Baishakh, 6+2 marks - Video: [Latching and holding current of a thyristor](https://www.youtube.com/watch?v=_24_ImQjXao) — •Learning Space• - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-vi-latching-vs-holding - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Explain the V-I characteristic of thyristor. Also compare latching current with holding current with suitable examples. ## Short answer The thyristor V-I characteristic has reverse blocking, forward blocking, and forward conduction modes; with gate current, breakover voltage reduces, and once ON voltage drops to about 1-2 V. Latching current is the minimum anode current needed at turn-on with gate present so the SCR stays ON; holding current is the minimum current to keep it conducting with gate open. IL is about 2-3 times IH. ## Answer A **thyristor (SCR)** is a four-layer P-N-P-N, three-junction ($J_1, J_2, J_3$) semiconductor switch with three terminals: anode (A), cathode (K) and gate (G). It can be turned ON by a small gate pulse when forward biased, but once ON the gate loses control; it turns OFF only when its anode current falls below the holding current. #### V-I characteristic ```text IA ^ | |<- ON state (VT = 1-2 V) | | IL ---|--| IH ---|--+. | `. Ig2 > Ig1 > Ig=0 | `-.___ ___ -VBR | `----' | -----+-------+---------------------+--> VAK | | forward blocking VBO | | (leakage current) | reverse blocking | (leakage current) v reverse avalanche ``` 1. **Reverse blocking mode:** cathode is positive with respect to anode. Junctions $J_1$ and $J_3$ are reverse biased and $J_2$ is forward biased, so only a small reverse leakage current flows. If the reverse voltage reaches the **reverse breakdown voltage** $V_{BR}$, avalanche breakdown occurs at $J_1$ and $J_3$; the current rises sharply and the device is usually destroyed. 2. **Forward blocking mode:** anode is positive and gate is open. $J_1$ and $J_3$ are forward biased but $J_2$ is reverse biased, so only a small forward leakage current flows. The SCR is OFF and blocks the forward voltage. 3. **Forward conduction (ON) mode:** if the forward voltage is raised to the **forward breakover voltage** $V_{BO}$ (with $I_g = 0$), junction $J_2$ breaks down by avalanche and the SCR switches suddenly to the ON state (the negative-resistance jump). The voltage across it drops to about 1 to 2 V and the current is limited only by the load. With gate current, $J_2$ breaks down at a lower voltage: the larger $I_g$, the smaller the breakover voltage ($I_{g2} > I_{g1}$). In practice the SCR is always turned ON by a gate pulse at a voltage well below $V_{BO}$. #### Latching current versus holding current - **Latching current ($I_L$):** the minimum anode current that must flow through the SCR immediately after it is triggered, while the gate pulse is still present, so that it stays ON after the gate pulse is removed. If the anode current has not reached $I_L$ when the gate pulse ends, the SCR turns OFF again. - **Holding current ($I_H$):** the minimum anode current that must keep flowing to hold an already conducting SCR in the ON state (gate open). If the anode current falls below $I_H$, the SCR turns OFF and returns to forward blocking. - $I_L$ is greater than $I_H$, usually $I_L \approx 2$ to $3\,I_H$. Example: for an SCR with $I_L = 40$ mA and $I_H = 15$ mA, the gate pulse must last until the anode current reaches 40 mA, but once ON it stays ON until the current drops below 15 mA. | Point | Latching current $I_L$ | Holding current $I_H$ | |---|---|---| | Related to | Turn-on process | Turn-off (ON-state) condition | | Gate | Gate pulse present | Gate removed | | Magnitude | Larger | Smaller ($I_L \approx 2$-3 $I_H$) | | Practical effect | Sets minimum gate pulse width | Sets minimum load current to stay ON | Example: with an inductive load, current rises slowly, so a longer gate pulse (or a pulse train) is needed for the current to reach $I_L$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In reverse blocking mode of an SCR, which junctions are reverse biased? A. J2 only B. J1 and J2 C. J1 and J3 D. J3 only Answer: C. With cathode positive, J1 and J3 are reverse biased and J2 is forward biased; only reverse leakage flows. (Key point: Reverse blocking) 2. What is latching current IL? A. Minimum current needed to keep an already ON SCR conducting with gate open B. Minimum anode current right after triggering for the SCR to stay ON once the gate pulse is removed C. Maximum current the SCR can carry D. Leakage current in forward blocking Answer: B. If the anode current has not reached IL when the gate pulse ends, the SCR turns OFF again. (Key point: Latching current) 3. If the anode current of a conducting SCR falls below the holding current IH, what happens? A. It stays ON until the gate is removed B. It returns to forward blocking (turns OFF) C. It enters reverse blocking D. It breaks down at J2 Answer: B. IH is the minimum current to remain ON; below it the SCR returns to the forward blocking state. (Key point: Holding current) 4. With IL = 40 mA and IH = 15 mA, what must the gate pulse last for? A. Until anode current reaches 15 mA B. Until anode current reaches 40 mA C. Until anode current drops to zero D. Only a few nanoseconds regardless Answer: B. IL is larger than IH (about 2 to 3 times), and it sets the minimum gate pulse width; with inductive loads a longer pulse is needed. (Key point: IL vs IH practical effect) ## Related in Power Electronics - [Thyristor V-I, holding/latching; vertical BJT](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-vertical-bjt) — Power Electronics, chapter 1 - [Thyristor V-I, latching/holding, gate signal circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-latching-gate-circuit) — Power Electronics, chapter 1 - [Latching and holding current](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Alatching-holding-current) — Power Electronics, chapter 1