- A silicon controlled rectifier, also known as a semiconductor controlled rectifier, is a solid-state current-controlling device with four layers. General Electric’s trade name for a type of thyristor is “silicon controlled rectifier.” Moll, Tanenbaum, Goldey, and Holonyak of Bell Laboratories invented four-layer p-n-p-n switching in 1956.
- Thyristors are a type of bipolar semiconductor device that has four (or more) alternating N-P-N-P layers. Thyristors are comprised of the following components: silicon controlled rectifier (SCR), TRIAC, gate turn off switch (GTO), silicon controlled switch (SCS), AC diode (DIAC), unijunction transistor (UJT), and programmable unijunction transistor (PUT). This section focuses solely on the SCR, though the GTO is mentioned.
- Some sources define silicon-controlled rectifiers and thyristors as synonymous, while others define thyristors as a proper subset of the set of silicon-controlled rectifiers; the latter being devices with at least four layers of alternating n- and p-type material. According to Bill Gutzwiller, the terms “SCR” and “controlled rectifier” were used first, and the term “thyristor” was added later as the device’s use spread internationally.
- SCRs are unidirectional devices (they can only conduct current in one direction), whereas TRIACs are bidirectional (i.e. charge carriers can flow through them in either direction). In contrast to TRIACs, which can be triggered normally by either a positive or a negative current applied to its gate electrode, SCRs can only be triggered normally by a positive current going into the gate.
- The standard electronic symbol for an SCR is depicted on the figure below. It depicts the part’s three lead pin outs, with the upper one representing the anode, the lower one representing the cathode, and the central extension representing the gate. The symbol looks like a regular rectifier diode with an extra lead from the cathode side. SCRs, like diodes, rectify alternating current in response to DC electrical triggers on their gate inputs.
Silicon Controlled Rectifier (SCR)
- The silicon controlled rectifier is a four layer diode with a gate connection similar to the one shown in Figure 1. (a). When activated, it behaves like a diode for one polarity of current. It is nonconducting if it is not turned on. The operation is described in terms of the compound connected transistor equivalent shown in Figure 1. (b). Between the gate and cathode terminals, a positive trigger signal is applied. As a result, the NPN equivalent transistor conducts. The conducting NPN transistor’s collector pulls low, causing the PNP base to move towards its collector voltage, causing the PNP to conduct.
- The conducting NPN transistor’s collector pulls low, causing the PNP base to move towards its collector voltage, causing the PNP to conduct. The conducting PNP’s collector pulls high, causing the NPN base to move in the direction of its collector. This positive feedback (regeneration) reinforces the NPN’s current state of operation. Furthermore, even in the absence of a gate signal, the NPN will now conduct. An SCR will conduct as long as there is a positive anode voltage present. This is true for the DC battery shown. SCRs, on the other hand, are most commonly used with an alternating current or pulsating DC supply.
- Conduction stops when the positive half of the sinewave at the anode expires. Furthermore, most practical SCR circuits rely on the AC cycle reaching zero to cut off or commutate the SCR.
- The doping profile of an SCR is depicted in Figure (a). It is worth noting that the cathode, which corresponds to the equivalent emitter of an NPN transistor, is heavily doped, as indicated by N+. The anode is also doped heavily (P+). It is a PNP transistor’s equivalent emitter. The two middle layers, which correspond to the equivalent transistor’s base and collector regions, are less heavily doped: N- and P. In high power SCRs, this profile may be spread across an entire semiconductor wafer of significant diameter.
- Figures above show the schematic symbols for an SCR and a GTO (b & c). The basic diode symbol indicates that, like a diode, cathode to anode conduction is unidirectional. The addition of a gate lead indicates diode conduction control. The gate turn off switch (GTO) has bidirectional arrows around the gate lead, indicating that conduction can be disabled or initiated by a negative pulse.
- Aside from the common silicon-based SCRs, experimental silicon carbide devices have been developed. Silicon carbide (SiC) operates at higher temperatures and has the highest heat conductivity of any metal, second only to diamond. This should enable either physically smaller or more powerful devices.
DC Thyristor / SCR Circuit,/h2>
- Many applications call for an SCR circuit to control the operation of a DC load. This can be used for switching DC motors, lamps, or any other load.
- The basic SCR circuit shown below can control power to a load by using a small switch to initiate power application to the load.
- With S1 closed and S2 open, no current will flow at first. Only when S2 is closed and causes gate current to flow, does the SCR circuit turn on and current flow in the load.
- Until the anode circuit is broken, current will continue to flow. S1 can be used for this. Another method is to place the switch S1 across the SCR and briefly close it, causing the voltage across the SCR to disappear and the SCR to stop conducting.
- Because of their functions in this SCR circuit, S1 and S2 may be referred to as the Off and ON switches, respectively. In this configuration, S1 must be able to carry the full load current, while S2 must only carry the gate current.
- Once the SCR is turned on, the switch can be released and remain open because the SCR’s action maintains the current flow through the device and thus the load.
- R1 connects the gate to the power supply via the switch. When S2 is closed, current flows through the resistor, enters the gate, and activates the SCR. R1 must be calculated to provide enough gate current to turn the SCR circuit.
- R2 is included to reduce the SCR’s sensitivity so that it does not fire on any noise that is detected.
Basic AC thyristor / SCR circuit
- When using a thyristor circuit with AC, a few changes must be made, as shown below.
- The reason for this is that alternating current reverses polarity throughout the cycle. This means that the SCR will become reverse-biased, effectively lowering the anode voltage to zero and causing it to turn off for one-half of each cycle. As a result, there is no need for an off switch because this is accomplished through the use of an AC supply.
- The circuit operates slightly differently than the DC SCR circuit. When the switch is turned on, the circuit must wait for sufficient anode voltage to be available as the AC waveform progresses along its path. In addition, the SCR circuit will have to wait until the voltage within the gate section of the circuit is high enough to trigger the SCR. The switch must be in the closed position for this.
- Once triggered, the SCR will remain conducting for the duration of the positive half of the cycle. As the voltage falls, the anode cathode voltage will become insufficient to support conduction. At this point, the SCR will no longer operate.
- The SCR will then not operate during the negative half of the cycle. The process will only be repeated when the next positive half of the cycle returns.
- As a result, this circuit will only operate when the gate switch is closed.
- One disadvantage of using this type of SCR circuit is that it cannot provide more than 50% power to the load because it does not conduct during the negative half of the AC cycle because the SCR is reverse biased.
Applications of Thyristor Circuit Design
- Power Switching Circuit
- Controlled Rectifier
- AC power control circuits
- Speed control of DC shunt motor
- SCR Crowbar
- Computer logic circuits
- Timing Circuits
- Inverters
- Battery Charging Regulators
- Temperature control systems