Question

How does dV/dt triggering occur in SCRs?

Answer 1

In an SCR (Silicon Controlled Rectifier), dV/dt triggering refers to the phenomenon where the device is turned on (triggered into conduction) due to a rapid rate of change of the voltage across the SCR. This can happen even without a gate signal, which is typically used for turning the SCR on.

Here's a simple breakdown of how dV/dt triggering occurs:

1. SCR Structure: An SCR is made of four layers of semiconductor material (PNPN), with three junctions. The device remains in the "off" state (blocking mode) as long as the voltage across it is below a certain threshold.
   

1. The Role of dV/dt: If the voltage across the SCR increases too quickly (i.e., a very high rate of voltage change), it can cause the internal capacitances of the SCR to charge up rapidly.
   

1. Breakdown of Junctions: The charging of the capacitances can lead to a situation where the internal voltage across the junctions exceeds the breakdown voltage of one or more of the junctions in the SCR. This results in the sudden triggering of the device.
   

1. Turn-on: When the SCR is triggered by dV/dt, it begins conducting current even if no gate signal is applied. The device stays on (in conduction mode) as long as the current through it is above a certain threshold, even after the voltage change slows down.
   

In summary, dV/dt triggering is a result of a fast voltage rise that can cause the internal capacitances of the SCR to charge up rapidly, leading to the breakdown of one of the junctions, and thus the SCR turning on. This is usually an unwanted behavior in many circuits, so circuit designers use techniques like snubber circuits to limit the rate of voltage change and prevent accidental triggering of the SCR.