Question

How does an SCR behave during forward recovery?

Answer 1

During forward recovery in an SCR (Silicon Controlled Rectifier), the device transitions from its off state (non-conducting) to its on state (conducting). Forward recovery occurs when the SCR is initially switched on after being reverse-biased or forward-biased but not conducting.

Here's how it behaves:

1. Off State (Before Switching On):
   

   The SCR is in the off state, where it behaves like an open switch. No current flows through it, and it blocks the voltage in the reverse direction.

1. Triggering the SCR:
   

   When a small gate current is applied (for a positive gate signal), the SCR starts to turn on. The SCR starts to conduct only when the anode voltage is positive and reaches a certain threshold, typically referred to as the breakover voltage.

1. Transition to Forward Conduction:
   

   During forward recovery, as the SCR is turning on, there is an initial delayed current rise. This is due to the time required for the device to charge the internal capacitances and overcome the stored charge in the device.

1. Characteristic of Forward Recovery:
   

   - Voltage Spike: At the moment of turning on, the SCR might show a brief voltage spike. This happens because the stored charge from the off state needs to dissipate. The internal junctions of the SCR momentarily behave like capacitors, leading to a short delay before it starts conducting fully.
   - Current Rise: After the voltage spike, the current through the SCR starts rising rapidly and the device enters a conducting state where it behaves like a closed switch.

1. On State:
   

   After the forward recovery period, the SCR reaches its steady-state conducting condition, where it allows current to flow easily from the anode to the cathode. It will continue to conduct as long as the anode voltage remains positive and the SCR is not turned off (which typically requires removing the gate current or the current dropping below a certain holding current).

In summary, during forward recovery, the SCR shows a brief delay with a possible voltage spike as it charges internal capacitances and begins to conduct. After this brief recovery period, it settles into the conducting state, allowing current to flow freely through it.