What Is a Silicon-Controlled Rectifier (SCR)?

An SCR is a unidirectional thyristor that has three terminals and blocks forward voltage until it is triggered. After it is triggered, an SCR can remain conductive after the gate trigger pulse ends when enough current coming in from its anode (A) establishes latching. SCRs conduct current mainly from the A to the K terminal. SCRs will turn off when the main current through an SCR drops below the holding level and remains below that point for a sufficient period of time so that the SCR may recover. In AC applications, the natural zero-crossing of current allows the SCR to turn off; in DC applications, the SCR current must be interrupted or diverted in another way.

What Is a Silicon-Controlled Rectifier?

An SCR belongs to the thyristor family of semiconductor devices and consists of four alternating layers of semiconductor material, commonly referred to as PNPN. An SCR has three physical terminals: anode (A), cathode (K), and gate (G). The Basics on the thyristor (SCR) structure and its application by STMicroelectronics illustrates how the SCR achieves such a rapid change from blocking to conducting state through the internal regenerative action created within an SCR.

The term “rectifier” describes the preferred current flow direction through an SCR, which is from anode to cathode; a gate current can initiate conduction when the SCR is forward biased; however, unlike a MOSFET, the on-state of an SCR is not maintained continuously by the gate control circuit.

Why Does the PNPN Structure Latch?

The main component of a PNPN structure is the combination of four layers of semiconductor material which creates three junctions in total. However, because of this combination, it is also possible to represent PNPN structures with an equivalent PNP transistor and NPN transistor connected with regenerative feedback. Current produced by one transistor reinforces the drive to the other, so a small gate current can trigger a rapid increase in internal conduction.

After the amount of current flowing through the SCR reaches a point where the regenerative effect becomes established, the actual current flowing through the SCR will continue to flow without the need for a continuous application of the gate current. Once the amount of current flowing through the SCR decreases below the minimum level required to maintain the conducting state and the stored charge has had time to clear, then the SCR will again enter a blocking state.

Which SCR Terminal Is A, K, or G?

The letter “A” represents the anode terminal, “K” the cathode terminal, and “G” the gate terminal. The anode and cathode form the main current path, while the gate is the control terminal referenced to the cathode. Rotating the symbol does not change these logical functions; readers who are unsure about diode polarity can first review anode and cathode on diode symbols.

The logical terminal names do not define a universal physical pin order, even when the logical terminals are the same.

While knowing the package style will not necessarily give you the terminal arrangement, related package families can have different tab connections or die connections. You should read the complete part marking, and then compare it to the package drawing, lead numbering, and metal-tab connection from the exact datasheet, to ensure that you identify every component on the board based upon the accurate part markings on each component, not based upon the way they look.

How Does an SCR Turn On?

When the anode is positive relative to the cathode, an untriggered SCR normally remains in a forward-blocking state. The application of current to the gate-to-cathode causes free charge carriers to be injected into the crystal and drives the SCR into a regenerative conduction transition. In addition to the gate voltage, the gate current and gate pulse duration are important in triggering the SCR.

STMicroelectronics’ parameter list for SCRs, Triacs, AC Switches, and Diacs separates gate trigger current, gate trigger voltage, latching current, holding current, turn-off time, and other specified ratings. This means that the circuit that supplies the gate must supply the trigger current without exceeding the gate limits of the selected device.

When a gate current is applied while the main conducting path of an SCR is reverse-biased, it will NOT result in normal forward conduction through the SCR. Since the series gate resistor is primarily used in SCR circuits to set the gate pulse current, the value of the series resistor is determined by the gate ratings and drive voltage used with the selected SCR. There is no standard value for series resistors for all applications.

How Can an SCR Be Triggered?

In most SCR circuits, a gate pulse is the intended method of control; however, other means of achieving turn-on can occur under the right conditions. Littelfuse identifies gate drive, static dv/dt, and breakover voltage as distinct turn-on mechanisms in Gating, Latching, and Holding of SCRs and Triacs.

SCR V-I Characteristics

The SCR’s behaviour is characterised by an abrupt transition between the second and third regions on the V-I curve; in normal gate-triggered operation, this abrupt transition occurs under the influence of an applied gate signal to the SCR. This is significant because the SCR’s gate drive can trigger the device into the on-state well below its breakover voltage. Once an SCR reaches the on-state it remains there because the anode current maintains the regenerative conduction state.

Why Does an SCR Stay On After the Gate Pulse Ends?

Internal positive feedback maintains the on-state of the SCR once conduction has been established through enough of the SCR structure. The removal of the gate pulse does not terminate the anode current that maintains regeneration, thereby allowing the SCR to remain in a regenerative mode of operation. The gate serves primarily as a turn-on signal rather than a general turn-off control.

STMicroelectronics treats latching current and holding current separately in Thyristors and TRIACs: latching current and Thyristors and TRIACs: holding current – an important parameter. The required current for latching is normally greater than that required to sustain conduction for a given device.

The gate pulse must remain on for as long as necessary for the load current to rise above I_L. Thus, while a pulse may be sufficient to cause I_GT to be reached, it may not be sufficient to cause sustained conduction if the load current rises too slowly to reach I_L before the gate pulse is removed.

How Does an SCR Turn Off in AC and DC Circuits?

To turn off an SCR, the main current must drop below the holding current, and turn-off conditions must remain favorable for the required recovery period. Removing the gate signal alone is not a general turn-off method for a latched SCR.

The natural way to turn off an SCR in a line-commutated AC circuit is through zero crossing of load current, occurring once per half-cycle. With reactive loads and various other factors, the actual current zero can be displaced from the voltage zero, making the timing of recovery important.

In contrast to line-commutated AC sources, DC sources do not provide a periodic natural current zero condition. Because of this, you need to either break the current path, redirect the current to a different path, use a commutation network, or alternatively, reduce the anode current below the holding current condition for the required recovery time.

Which SCR Datasheet Parameters Matter Most?

Choosing an SCR based solely on the current is not possible. STMicroelectronics’ AN4608 selection guide connects the current rating, voltage rating, triggering current, temperature, dV/dt, dI/dt, and the SCR’s turn-off behavior to the application.

SCR vs. Diode vs. TRIAC vs. MOSFET

The symbols and polarity conventions are compared in NMOS vs. PMOS symbols.

The decision process of selecting a device usually comes down to two questions before the application is even started: 1) does the load require bidirectional AC power, or 2) can you incorporate (or theoretically incorporate) a method of current zero or commutation event. A “yes” answer to (1) can point toward a TRIAC; a “no” answer to (2) eliminates an SCR as a viable solution, regardless of the technical specifications it meets.

Common SCR Applications

AC Phase Control

In a half-wave AC controller, delaying a gate firing pulse allows control over how much of the positive half-cycle is supplied to a resistive load. Triggering the SCR early in the half-cycle results in the delivery of most of the power during the half-cycle. Controlling the angle at which the SCR is triggered becomes the primary means of controlling average power. An SCR controls one half-cycle only, so it can support simple heater or lamp dimming but leaves the load unpowered and asymmetric on the other half-cycle. To control both half-cycles in an AC motor speed control or power tools type situation, two SCRs connected in an anti-parallel configuration or a full bridge configuration can be used. For a resistive AC load, there is a natural current zero at the end of each conducting cycle; there is no need to provide a separate commutation circuit.

Crowbar Overvoltage Protection

A crowbar circuit is triggered by monitoring a power supply at a specific threshold (typically the threshold is established well above normal operating voltage and below the maximum voltage rating of the downstream devices), forcing an SCR to create a low-impedance path across the rail upon a trip event. When using a crowbar circuit, the SCR is generally chosen due to the speed at which it can go from a blocking state to a latch state in microseconds with no contact bounce; this means that there is no mechanical contact wear. The SCR remains latched until the current drops below a specific hold current (I_H) and at this time the crowbar will not clear itself; it will hold the fault until either a fuse, current limiting circuit or an upstream protection stage has removed power to the circuit and at this point the SCR will return to the blocking state and the circuit will be in a state to be reset.

Controlled Rectifiers and Motor Drives

SCR bridges combine rectification with firing-angle control: the gate pulse is delayed by a controlled angle within each half-cycle, and delaying it further lowers the average DC output without dissipating the difference as heat the way a linear regulator would. Because the bridge is line-commutated, each SCR turns off naturally as line current reverses without a separate commutation circuit, making SCR bridges suitable for large DC motor drives, where firing angle influences speed and available torque, and for battery chargers and industrial rectifiers that require an adjustable DC level from a fixed AC supply.

Inrush Current Limiting and High-Power Switching

The most common method for limiting transient load current is to use either resistors or NTC thermistors to absorb the initial surge current before firing an SCR (with or without bypass) onto the load once the surge current has peaked or decayed out, thus allowing for a smooth transition into normal load current (steady state), without the limiter dissipating continuous power. STMicroelectronics documents SCR-based inrush-current limiter topologies and the related current, voltage, and thermal selection factors in AN4608 and its SCR application documentation.

How Do dV/dt, dI/dt, Surge Current, and Heat Affect an SCR?

An SCR can be electrically within its steady-state current rating and still fail if transient voltage, turn-on current rise, surge energy, or junction temperature exceeds its limits.

Many of the limitations listed above cannot be determined from a generic table. A snubber may be sized for one source impedance, but if another source is used, it may be undersized. Also, a fuse rated for a certain duration of a surge may not provide adequate protection against a shorter, higher-amplitude surge than the one to which it has been rated. A practical way to use the numbers on the datasheet is to match them to the waveforms measured or simulated in the actual application circuit, as opposed to a textbook approximation.

How to Test an SCR with a Multimeter

Handheld multimeters can help identify obvious anode-to-cathode shorts, open or abnormal gate-cathode junctions, and terminal assignments that conflict with the datasheet. However, they cannot reproduce the rated blocking voltage, load current, surge current, dV/dt, dI/dt, junction temperature, and dynamic commutation conditions of an SCR.

Begin by referencing the exact datasheet pinout, then isolate and discharge the SCR before testing. In diode-test or resistance mode, test the gate-to-cathode behavior in both polarity directions and make sure the anode-to-cathode path is not simply shorted.

Certain meters and sensitive SCRs can supply sufficient current to the gate and main path to cause a brief latch to occur, while some combinations are unable to reach the latching current (I_L) even if the SCR is functional. Using a current-limited, low-voltage supply and load setup can allow for recreation of the entire forward blocking, gate trigger, main current above I_L, latching after gate removal, and turn-off below I_H sequences.

Why Won’t an SCR Turn On or Turn Off?

If an SCR does not turn on, it may be due to reversed anode-cathode bias or incorrect pin identification. In addition, it is possible that there has not been sufficient gate current supplied to trigger the SCR, that the gate pulse ends too soon, or that the load current has not exceeded I_L. Excessive noise or poor gate return can create intermittent triggers for SCRs.

If an SCR does not turn off, it is commonly because the main current has not dropped below I_H or the low-current interval has not reached the required recovery time. In reactive circuits, the current waveform does not necessarily reach zero when the voltage reaches zero; therefore, the most useful indicators of SCR turn-off in reactive circuits will be current waveforms and the timing of commutation.