Thyristor di/dt and dv/dt protection

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.

         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 $di/dt$.

 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.

 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.