Rate of rise of forward voltage, dv/dt
Rate of rise of forward voltage:
Fig: dv/dt suppression circuit – basic circuit Fig: dv/dt suppression circuit – circuit variations
- The junctions of any semiconductor exhibit some unavoidable capacitance. A changing voltage impressed on this junction capacitance results in a current, I = C dv/dt.
- If this current is sufficiently large a regenerative action may occur causing the SCR to switch to the on state. This regenerative action is similar to that which occurs when gate current is injected.
- The critical rate of rise of off-state voltage is defined as the maximum value of rate of rise of forward voltage which may cause switching from the off-state to the on-state.
- Since dv/dt turn-on is non-destructive, this phenomenon creates no problem in applications in which occasional false turn-on does not result in a harmful affect at the load. Heater application is one such case.
- However, at large currents where dv/dt turn-on is accompanied by partial turn-on of the device area a high di/dt occurs which then may be destructive. The majority of inverter applications, however, would result in circuit malfunction due to dv/dt turn-on.
- One solution to this problem is to reduce the dv/dt imposed by the circuit to a value less than the critical dv/dt of the SCR being used. This is accomplished by the use of a circuit similar to those in Figure to suppress excessive rate of rise of anode voltage. Z represents load impedance and circuit impedance.
- Variations of the basic circuit is also shown where the section of the network shown replaces the SCR and the R-C basic snubber.
- Since circuit impedances are not usually well defined for a particular application, the values of R and C are often determined by experimental optimization.
- A technique can be used to simplify snubber circuit design by the use of nomographs which enable the circuit designer to select an optimized R-C snubber for a particular set of circuit operating conditions.
- Another solution to the dv/dt turn-on problem is to use an SCR with higher dv/dt turn-on problem is to use an SCR with higher dv/dt capability. This can be done by selecting an SCR designed especially for high dv/dt applications, as indicated by the specification sheet.
- Emitter shorting is a manufacturing technique used to accomplish high dv/dt capability.