--- title: "Thyristor di/dt and dv/dt protection" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:thyristor-didt-dvdt-protection" source: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-didt-dvdt-protection site: BCT Engineering --- # Thyristor di/dt and dv/dt protection Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2079 Bhadra, 8 marks; 2078 Bhadra, 8 marks; 2074 Chaitra, 8 marks; 2069 Chaitra, 8 marks (asked 4 times) - Video: [21. a. di/dt and dv/dt Protection for Thyristor](https://www.youtube.com/watch?v=k4uA4A0Knpg) — Electronic Waves - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-didt-dvdt-protection - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Explain the di/dt protection scheme and dv/dt protection scheme for a thyristor with necessary diagram and waveforms. ## Short answer di/dt protection uses a series inductor Ls limiting anode current rise to Vs/Ls, chosen from the di/dt rating. dv/dt protection uses an RC snubber across the SCR to slow voltage rise and limit capacitor discharge current; Rs is chosen from dv/dt and peak current limits. ## Answer A thyristor must be protected against a fast rate of rise of anode current ($di/dt$) at turn-on and a fast rate of rise of forward voltage ($dv/dt$) while it is OFF. A **series inductor** gives di/dt protection and an **RC snubber** across the device gives dv/dt protection. ```text Ls (di/dt) o----/\/\/\/----+-----------+ | | A | Rs SCR | K | Cs | | o---------------+-----------+ (RC snubber across SCR) ``` #### di/dt protection **di/dt protection (series inductor):** when an SCR is turned ON, conduction starts in a small area near the gate and then spreads over the whole cathode area at about 0.1 mm/µs. If the anode current rises faster than this spreading, the current density in the small conducting area becomes very high, causing local hot spots and damage. 1. A small inductor $L_s$ is connected in series with the SCR. At turn-on the current can rise only at the rate $$ \frac{di}{dt} = \frac{V_s}{L_s} $$ so $L_s$ is chosen as $L_s \ge \frac{V_m}{(di/dt)_{rated}}$. 2. A strong gate pulse (high gate current with a fast rise) is also used, so that a larger area of the cathode turns ON at the start. 3. The snubber resistance $R_s$ limits the capacitor discharge current at turn-on, which is the other source of high $di/dt$. ```text iA ^ without Ls (very steep) | | ____________ | | / with Ls | |/ .---------- | | / slope = Vs/Ls | | / -+----+/--------------------> t gate pulse ``` #### dv/dt protection **dv/dt protection (RC snubber):** when an SCR is in forward blocking, junction $J_2$ is reverse biased and behaves like a capacitor $C_j$. A fast-rising anode voltage drives a charging current $i = C_j\,\frac{dv}{dt}$ through the device. If this current is large enough it acts like a gate current and turns the SCR ON falsely. To prevent this, a series $R_s C_s$ snubber is connected across the SCR. 1. When a voltage step appears, the capacitor $C_s$ initially acts as a short circuit, so the voltage across the SCR cannot jump suddenly; it rises at the rate at which $C_s$ charges through the load and $R_s$. 2. With a series circuit inductance $L$, the rate is roughly $\frac{dv}{dt} \approx \frac{R_s V_s}{L}$, so $R_s$, $C_s$ (and $L$) are chosen to keep $dv/dt$ below the rated value. 3. $R_s$ limits the discharge current of $C_s$ through the SCR when it turns ON (the discharge current is about $V_s/R_s$), which protects against high $di/dt$ from the snubber itself. It also damps the $L$-$C_s$ oscillation. A diode across $R_s$ (polarised snubber) gives better dv/dt limiting while still limiting discharge current. ```text v across SCR ^ without snubber | ________________ | | with snubber | | ___.--------- | | .' | |/ slower rise --+----+--------------------> t step applied ``` **Design note:** with source voltage $V_s$, series inductance $L_s$ and snubber $R_s, C_s$, the usual relations are $$ \begin{aligned} \left(\frac{di}{dt}\right)_{max} &= \frac{V_s}{L_s}, & \left(\frac{dv}{dt}\right)_{max} &\approx \frac{R_s V_s}{L_s}, & R_s &\ge \frac{V_s}{I_{T(peak)}} \end{aligned} $$ so $L_s$ is fixed by the di/dt rating and $R_s$ then by the dv/dt rating. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why can a very high di/dt at turn-on damage an SCR? A. The gate loses control of the device B. Junction J2 behaves as a capacitor and falsely triggers C. Current crowds in the small area near the gate, causing local hot spots D. The holding current is exceeded Answer: C. Conduction starts near the gate and spreads at about 0.1 mm/us. If current rises faster, density in the small area becomes very high. (Key point: Cause of di/dt damage) 2. A series inductor Ls limits the rate of rise of current to approximately: A. Vs x Ls B. Ls/Vs C. Vs/Ls D. Rs x Vs/Ls Answer: C. At turn-on, di/dt = Vs/Ls, so Ls is chosen as Vm divided by the rated di/dt. (Key point: Series inductor for di/dt) 3. How does a fast rise of anode voltage turn on a blocking SCR falsely? A. It forward-biases J1 and J3 directly B. The charging current Cj dv/dt through J2 acts like gate current C. It reduces the holding current to zero D. It reverse biases the gate Answer: B. J2 acts like a capacitor Cj when reverse biased. The current i = Cj dv/dt can trigger the SCR as if gate current flowed. (Key point: Cause of false turn-on by dv/dt) 4. What is a role of the snubber resistor Rs? A. It makes the voltage across the SCR jump instantly B. It limits the capacitor discharge current (about Vs/Rs) when the SCR turns ON C. It increases the dv/dt across the SCR D. It supplies the gate pulse Answer: B. Rs limits Cs discharge current through the SCR at turn-on, preventing high di/dt from the snubber, and it damps L-Cs oscillation. (Key point: Role of Rs in snubber) ## Related in Power Electronics - [di/dt, dv/dt protection and gate circuit design](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Aprotection-gate-design-factors) — Power Electronics, chapter 1 - [Why high di/dt and dv/dt harm a thyristor](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Ahigh-didt-dvdt-protection) — Power Electronics, chapter 1 - [SCR di/dt, dv/dt and current ratings numerical](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Ascr-snubber-didt-dvdt-numerical) — Power Electronics, chapter 1 - [Snubber circuit and dv/dt protection](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Asnubber-dvdt-protection) — Power Electronics, chapter 1, asked 2× - [Series inductor for di/dt limit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Aseries-inductor-didt-limit) — Power Electronics, chapter 1 - [Thyristor protection and diode reverse recovery numerical](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Advdt-didt-protection-reverse-recovery) — Power Electronics, chapter 1 ## Related in other subjects - [Unit protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aunit-protection-scheme) — Switchgear and Protection, chapter 7 - [i(0+) and di/dt(0+) with sinusoidal source](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_circuit_theory%3Ainitial-values-ac-source-switch-k) — Electrical Circuit Theory, chapter 2