Understanding Silicon Controlled Rectifiers (SCRs)

Silicon Controlled Rectifiers (SCRs)

Introduction

SCRs are unidirectional devices, meaning they conduct current in only one direction, unlike TRIACs, which are bidirectional. SCRs are typically triggered by current flowing into the gate, whereas TRIACs can be triggered by either positive or negative current applied to their gate.

Construction

SCRs comprise four semiconductor layers, forming NPNP or PNPN structures with three P-N junctions (J1, J2, J3) and three terminals: anode, cathode, and gate. The anode connects to the p-type material, the cathode to the n-type layer, and the gate to the p-type material nearest the cathode.

These layers alternate between p- and n-type silicon with added dopants. Junctions are formed through diffusion or alloying. Low-power SCRs utilize planar construction with diffused junctions, while high-power SCRs use mesa construction, where J2 is diffused and the outer layers are alloyed for high current handling. These high-power SCRs are reinforced with tungsten or molybdenum plates and attached to a copper stud for heat sinking.

SCR characteristics resemble thyristors, which are a broader family of multilayer devices exhibiting bistable on/off switching behavior.

Operating Modes

Forward Blocking Mode

In this mode, the anode is positive, the cathode is negative, and the gate is at zero potential. J1 and J3 are forward-biased, while J2 is reverse-biased, resulting in a small leakage current. The SCR remains off until the voltage reaches the breakover value, causing J2 to undergo avalanche breakdown and initiate conduction.

Forward Conduction Mode

The SCR transitions from blocking to conduction either by exceeding the breakover voltage or by applying a positive gate pulse. Once conducting, the gate signal is no longer needed. Turning off the SCR requires reducing the current below the holding current or momentarily shorting the anode and cathode.

Reverse Blocking Mode

Some SCRs offer reverse blocking capability, increasing the forward voltage drop due to a long, low-doped P1 region. Reverse and forward blocking voltage ratings are typically the same. These SCRs are common in current-source inverters.

Asymmetrical SCRs (ASCRs)

SCRs lacking reverse blocking capability are called asymmetrical SCRs (ASCRs). Their reverse breakdown rating is usually in the tens of volts. ASCRs are used with a parallel reverse conducting diode (e.g., in voltage-source inverters) or where reverse voltage won't occur (e.g., switching power supplies).

Reverse Conducting Thyristors (RCTs)

ASCRs can be packaged with a reverse conducting diode, forming a Reverse Conducting Thyristor (RCT).