A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state current controlling device. The name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Robert N. Hall and commercialized by Frank W. "Bill" Gutzwiller in 1957.
Some sources define silicon controlled rectifiers and thyristors as synonymous, other sources define silicon controlled rectifiers as a subset of a larger family of devices with at least four layers of alternating N and P-type material, this entire family being referred to as thyristors.
SCRs are unidirectional devices (i.e. can conduct current only in one direction) as opposed to TRIACs which are bidirectional (i.e. current can flow through them in either direction). SCRs can be triggered normally only by currents going into the gate as opposed to TRIACs which can be triggered normally by either a positive or a negative current applied to its gate electrode.
Modes of operation
This device is generally used in switching applications. In the normal "off" state, the device restricts current 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 as 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 gate is pulsed and the current through the device is below the latching current, the device will remain in the "off" state.
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 (also called as break over voltage), but again, this does not rapidly switch the entire device into conduction and so may be harmful; therefore this mode of operation is also usually avoided.
SCR are available with reverse blocking capability. Reverse blocking capability adds to the forward voltage drop because of the need to have a long, low doped P1 region.[clarification needed] Usually, the reverse blocking voltage rating and forward blocking voltage rating are the same. The typical application for reverse blocking SCR is in current source inverters.
SCR incapable of blocking reverse voltage are known as asymmetrical SCR, abbreviated ASCR. They typically have a reverse breakdown rating in the 10's of volts. ASCR are used where either a reverse conducting diode is applied in parallel (for example, in voltage source inverters) or where reverse voltage would never occur (for example, in switching power supplies or DC traction choppers).
Asymmetrical SCR can be fabricated with a reverse conducting diode in the same package. These are known as RCT, for reverse conducting thyristor.
Thyristor turn on methods
- forward voltage triggering
- gate triggering
- dv/dt triggering
- temperature triggering
- light triggering
Forward voltage triggering occurs when the anode-cathode forward voltage is increased with the gate circuit opened. This is known as avalanche breakdown, during which junction j2 will breakdown. At sufficient voltages, the thyristor changes to its on state with low voltage drop and large forward current. In this case, J1 and J3 are already forward biased.
Application of SCRs
SCRs are mainly used in devices where the control of high power, possibly coupled with high voltage, is demanded. Their operation(it can switch large current on and off) makes them suitable for use in medium to high-voltage AC power control applications, such as lamp dimming, regulators and motor control.
SCRs and similar devices are used for rectification of high power AC in high-voltage direct current power transmission. They are also used in the control of welding machines, mainly MTAW (Metal Tungsten Arc Welding) and GTAW (Gas Tungsten Arc Welding) process.
Compared to SCSs
A silicon-controlled switch (SCS) behaves nearly the same way as an SCR, aside from a few distinctions. Unlike an SCR, a SCS switches off when a positive voltage/input current is applied to another anode gate lead. Unlike an SCR, a SCS can also be triggered into conduction when a negative voltage/output current is applied to that same lead.
SCSs are useful in practically all circuits that need a switch that turns on/off through two distinct control pulses. This includes power-switching circuits, logic circuits, lamp drivers, counters, etc.
Compared to Triacs
TRIACs resemble SCRs in that they both act as electrically controlled switches. Unlike SCRs, TRIACS can pass current in either direction. Thus, TRIACs are particularly useful for AC applications. TRIACs have three leads: a gate lead and two conducting leads, referred to as MT1 and MT2. If no current/voltage is applied to the gate lead, the TRIAC switches off. On the other hand, if the trigger voltage is applied to the gate lead, the TRIAC switches on.
TRIACs are suitable for light-dimming circuits, phase-control circuits, AC power-switching circuits, AC motor control circuits, etc.
- high-voltage direct current
- Gate turn-off thyristor
- Insulated-gate bipolar transistor
- Integrated gate-commutated thyristor
- Voltage regulator
- Crowbar (circuit)
- Christiansen, Donald; Alexander, Charles K. (2005); Standard Handbook of Electrical Engineering (5th ed.). McGraw-Hill, ISBN 0-07-138421-9
- International Electrotechnical Commission 60747-6 standard
- Dorf, Richard C., editor (1997), Electrical Engineering Handbook (2nd ed.). CRC Press, IEEE Press, Ron Powers Publisher, ISBN 0-8493-8574-1
- Thyristor Theory and Design Considerations; ON Semiconductor; 240 pages; 2006; HBD855/D. (Free PDF download)
- Industrial and Power Electronics; G.K Mithal.
- Power Electronics; K B Khanchandani.