The forward region characteristics of a SCR

[abhi_verma812]

Can any one explain me the forward region characteristics of a SCR ( silicon controlled rectifier)...

 

[vicky]

Re: SCR Question

DEAR

An SCR can be seen as a conventional rectifier controlled by a gate signal. It is a 4-layered 3-terminal device. A p-type layer acts as anode and an n-type layer as a cathode; the p-type layer closer to the n-type(cathode) acts as a gate. k': k; "k


An SCR (left) can be thought of as two BJT transistors working together (right).

Modes of operation

In the normal 'off' state the device restricts current flow to the leakage current. When the gate to cathode voltage exceeds a certain threshold, the device turns 'on' and conducts current. The device will remain in the 'on' state even after gate current is removed so long as current through the device remains above the holding current. Once current falls below the holding current for an appropriate period of time, the device will switch off.

If the applied voltage increases rapidly enough, capacitive coupling may induce enough charge into the gate to trigger the device into the 'on' state; this is referred to as dv/dt triggering. This is usually prevented by limiting the rate of voltage rise across the device, perhaps by using a snubber. dv/dt triggering may not switch the SCR into full conduction rapidly and the partially-triggered SCR may dissipate more power than is usual, possibly harming the device.

SCRs can also be triggered by increasing the forward voltage beyond their rated breakdown voltage, but again, this does not rapidly switch the entire device into conduction and so may be harmful so this mode of operation is also usually avoided. Also, the actual breakdown voltage may be substantially higher than the rated breakdown voltage, so the exact trigger point will vary from device to device.