--- title: "Thyristor V-I, avalanche and forward breakdown" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:thyristor-vi-breakdown" source: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-vi-breakdown site: BCT Engineering --- # Thyristor V-I, avalanche and forward breakdown Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2079 Baishakh, 4 marks - Video: [How does a Thyristor work?](https://www.youtube.com/watch?v=0AgPUikpvpM) — Sabin Civil Engineering - 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-breakdown - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Draw the V-I characteristics curve of thyristor. What is an avalanche breakdown and forward breakdown voltage of thyristor? Mention with its symbolic diagram. ## Short answer Avalanche breakdown occurs in reverse bias when voltage reaches VBR, causing destructive carrier multiplication at junctions J1 and J3. Forward breakover voltage VBO is the forward voltage at which the middle junction J2 breaks down by avalanche, turning the thyristor on without gate current. ## Answer The thyristor (SCR) is a four-layer PNPN device with anode A, cathode K and gate G. Its V-I characteristic shows anode current $I_A$ against anode-cathode voltage $V_{AK}$. ```text Symbol Structure A A | | +---+ _V_ | P | |\ +---+ J1 | \ G | N | K +---+ J2 | P |---- G +---+ J3 | N | +---+ | K ``` ```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 ``` - **Avalanche (reverse) breakdown:** in reverse bias, $J_1$ and $J_3$ block. When the reverse voltage reaches the reverse breakdown voltage $V_{BR}$, carriers gain enough energy to knock out more carriers (avalanche multiplication), and the reverse current rises sharply. This normally destroys the device, so it is operated below $V_{RRM}$. - **Forward breakover voltage $V_{BO}$:** in forward bias with gate open, $J_2$ blocks. $V_{BO}$ is the forward voltage at which $J_2$ breaks down by avalanche and the SCR switches from forward blocking to the ON state without any gate signal. With gate current, the breakover voltage becomes smaller. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What happens at the reverse breakdown voltage VBR of a thyristor? A. Carriers gain enough energy to knock out more carriers (avalanche multiplication) and reverse current rises sharply B. The SCR turns ON and conducts forward C. The gate regains control D. The SCR enters negative resistance region safely Answer: A. Avalanche multiplication makes reverse current rise sharply, which normally destroys the device, so it is operated below VRRM. (Key point: Avalanche breakdown) 2. What is the forward breakover voltage VBO? A. The reverse voltage at which J1 fails B. The forward voltage at which J2 breaks down and the SCR turns ON with the gate open C. The gate voltage needed to fire D. The voltage drop of the SCR in the ON state Answer: B. With the gate open, J2 avalanches at VBO and the SCR switches from forward blocking to ON without a gate signal. (Key point: Forward breakover voltage) 3. How does applying gate current change the forward breakover voltage? A. VBO increases B. VBO is unchanged C. VBO becomes smaller D. VBO becomes equal to VBR Answer: C. The larger the gate current, the smaller the forward breakover voltage. 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