--- title: "Why high di/dt and dv/dt harm a thyristor" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:high-didt-dvdt-protection" source: https://www.bctengineer.com/q/semester_7/power_electronics/high-didt-dvdt-protection site: BCT Engineering --- # Why high di/dt and dv/dt harm a thyristor Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2071 Chaitra, 8 marks - Video: [Mastering High dv/dt And High di/dt Protection For SCR In Power Electronics | GATE](https://www.youtube.com/watch?v=aPNB7jIoojs) — Ekeeda GATE & ESE - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/high-didt-dvdt-protection - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question In which circumstances, a thyristor may subjected to high di/dt and dv/dt. Why these are harmful to thyristor. Explain how a thyristor can be protected against high di/dt and dv/dt. ## Short answer High di/dt occurs at turn-on with resistive or capacitive loads or snubber discharge; high dv/dt occurs while off during switching transients or reapplied voltage. di/dt causes local hot spots and junction melting; dv/dt may false-trigger the SCR. Protect with a series inductor for di/dt, an RC snubber for dv/dt, plus a strong gate pulse. ## Answer A thyristor is exposed to high **di/dt** at turn-on and to high **dv/dt** while it is OFF. Both can damage it or make it misbehave, so a series inductor and an RC snubber are used for protection. #### When high di/dt occurs - Firing the SCR when the supply voltage is high and the load is resistive or capacitive (the current can rise almost instantly). - Discharge of the snubber capacitor or a load capacitor through the SCR at turn-on. - Very low circuit inductance (short leads, capacitor banks). #### Why high di/dt is harmful At turn-on, conduction starts only in a small area near the gate and spreads over the cathode at about 0.1 mm/µs. If the current rises faster than this, it is concentrated in the small conducting area, causing local **hot spots**, junction melting and failure. #### When high dv/dt occurs - Switching transients when the supply is suddenly connected, or when other devices in the converter switch. - Forward voltage reapplied after commutation; line surges and lightning. #### Why high dv/dt is harmful In forward blocking, the reverse-biased junction $J_2$ acts like a capacitance $C_j$. A rapid voltage rise drives a charging current $i = C_j\,dv/dt$ through the device. This current acts like a gate current and can **turn the SCR ON falsely**, causing loss of control or a short circuit. #### Protection ```text Ls (di/dt) o----/\/\/\/----+-----------+ | | A | Rs SCR | K | Cs | | o---------------+-----------+ (RC snubber across SCR) ``` **di/dt protection (series inductor):** the aim is to keep the rate of rise of anode current below the rate at which the conducting area can spread. 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 $di/dt$. **dv/dt protection (RC snubber):** to stop the false turn-on described above, 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 only as fast as $C_s$ 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 (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 the discharge current. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a very high di/dt at turn-on harmful to a thyristor? A. It reverse biases junction J2 B. Current concentrates in a small area near the gate, causing hot spots and junction melting C. It lowers the holding current D. It makes the gate lose control after turn-off Answer: B. Conduction spreads from the gate at only about 0.1 mm/us, so a faster current rise overloads the small turned-on area. (Key point: Why high di/dt is harmful) 2. How can a high dv/dt turn an SCR ON falsely? A. It forward biases J1 and J3 permanently B. It increases the holding current C. It opens the gate circuit D. The charging current C_j dv/dt through junction J2 acts like gate current Answer: D. J2 behaves as a capacitance in forward blocking; i = C_j dv/dt can act as a gate current and trigger the SCR. (Key point: Why high dv/dt is harmful) 3. How does a series inductor Ls protect against di/dt? A. It limits the rate of rise of current to di/dt = Vs/Ls at turn-on B. It bypasses current around the SCR C. It slows the voltage rise across the SCR D. It lowers the gate current Answer: A. At turn-on the supply voltage appears across the inductor, so the current can rise only at Vs/Ls; Ls >= Vm/(di/dt)rated. (Key point: di/dt protection) 4. In the RC snubber across the SCR, what does Rs do besides helping limit dv/dt? A. It supplies gate current B. It increases the discharge current of Cs C. It limits the discharge current of Cs through the SCR at turn-on and damps oscillation D. It makes Cs act as a short for steady voltage Answer: C. Without Rs the capacitor would discharge through the SCR at turn-on, creating a high di/dt; Rs limits it to about Vs/Rs. 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