--- title: "Snubber circuit and dv/dt protection" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:snubber-dvdt-protection" source: https://www.bctengineer.com/q/semester_7/power_electronics/snubber-dvdt-protection site: BCT Engineering --- # Snubber circuit and dv/dt protection Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2073 Shrawan, 8 marks; 2070 Asar, 6 marks (asked 2 times) - Video: [Snubber circuit in power electronics through Animation (Thyristor Protection)](https://www.youtube.com/watch?v=XWW2VLHjcH4) — The PiSquare Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/snubber-dvdt-protection - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Draw a snubber circuit for an SCR. Explain the dv/dt protection method for a thyristor: how does the snubber provide dv/dt protection? ## Short answer An RC snubber across an SCR limits dv/dt by having the uncharged capacitor act as a short circuit, so anode voltage rises slowly as Cs charges. With series inductance, dv/dt is about Rs Vs/L, kept below the device rating. Rs also limits capacitor discharge current at turn-on and damps oscillation. ## Answer A **snubber circuit** is a series resistor-capacitor ($R_s$-$C_s$) network connected across a thyristor to limit the rate of rise of forward voltage ($dv/dt$) across it and so prevent false turn-on. #### Snubber circuit for an SCR ```text Ls (di/dt) o----/\/\/\/----+-----------+ | | A | Rs SCR | K | Cs | | o---------------+-----------+ (RC snubber across SCR) ``` A polarised version places a diode $D_s$ in parallel with $R_s$: the capacitor charges through the diode (good dv/dt limiting) and discharges through $R_s$ (limited discharge current). #### Why dv/dt protection is needed In forward blocking, junction $J_2$ is reverse biased and acts like a capacitor $C_j$. A rapidly rising anode voltage causes a charging current $$ i_j = C_j\frac{dv}{dt} $$ to flow through $J_2$. This current acts like gate current. If it exceeds the triggering level, the SCR turns ON without any gate signal, which is a false (unwanted) turn-on. #### How the snubber limits dv/dt 1. When a forward voltage step $V_s$ is applied across the OFF thyristor, the uncharged capacitor $C_s$ acts momentarily as a short circuit, so the anode voltage cannot jump instantly. 2. The capacitor charges through the load (and series inductance $L$) and $R_s$. The voltage across the SCR therefore rises at a controlled rate; with series inductance $$ \frac{dv}{dt} \approx \frac{R_s V_s}{L} $$ which is designed to be below the device's rated dv/dt. 3. When the SCR is later fired, $C_s$ discharges through it. $R_s$ limits this discharge current to about $V_s/R_s$, protecting the SCR from a high di/dt and peak current. 4. $R_s$ also damps the oscillation between the circuit inductance and $C_s$. ```text v across SCR ^ without snubber | ________________ | | with snubber | | ___.--------- | | .' | |/ slower rise --+----+--------------------> t step applied ``` The snubber values are a compromise: a larger $C_s$ gives better dv/dt protection but more loss: the power lost in $R_s$ is about $\frac{1}{2}C_sV_s^2 f$, where $f$ is the switching frequency. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is an RC snubber across an SCR used for? A. To limit di/dt at turn-on B. To limit dv/dt and prevent false turn-on C. To increase the holding current D. To supply the gate pulse Answer: B. The series Rs-Cs network limits the rate of rise of forward voltage across the thyristor, preventing false triggering. (Key point: Snubber purpose) 2. When a voltage step is applied, why can the anode voltage not jump instantly with a snubber? A. Rs blocks all current B. The uncharged Cs acts momentarily as a short circuit and charges gradually C. The SCR is already ON D. The load inductance becomes zero Answer: B. Uncharged Cs initially behaves as a short across the SCR, so voltage rises only as Cs charges through the load and Rs. (Key point: Capacitor action) 3. When the SCR is fired, Cs discharges through it. How is this discharge current limited? A. By the capacitor itself B. By the gate resistor C. By the load inductance D. By Rs, to about Vs/Rs Answer: D. Rs limits the discharge current to about Vs/Rs, protecting the SCR from high di/dt and peak current. (Key point: Discharge at turn-on) 4. What is the drawback of choosing a larger Cs? A. Worse dv/dt protection B. More loss in Rs (about 1/2 Cs Vs^2 f) C. Lower voltage rating D. False triggering increases Answer: B. A larger Cs gives better dv/dt protection but dissipates more energy in Rs each cycle, so the values are a compromise. 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