Why high di/dt and dv/dt harm a thyristor

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

         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.