Snubber circuit and dv/dt protection

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

         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 $C_s$.

 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.