Lecture 05.pdf

BASIC ELECTRONICS LECTURE 05: THYRISTORS Solomon Nunoo Dept. of Electrical and Electronic Eng. University of Mines and Technology, Tarkwa Content ◦ Introduction ◦ The Four-Layer Diode (Shockley Diode) ◦ The Silicon-Controlled Rectifier (SCR) ◦ The Diac ◦ The Triac ◦ The S...

BASIC ELECTRONICS LECTURE 05: THYRISTORS Solomon Nunoo Dept. of Electrical and Electronic Eng. University of Mines and Technology, Tarkwa Content ◦ Introduction ◦ The Four-Layer Diode (Shockley Diode) ◦ The Silicon-Controlled Rectifier (SCR) ◦ The Diac ◦ The Triac ◦ The Silicon-Controlled Switch (SCS) Limitation of Power Semiconductor Devices Majority carrier devices, like Schottky diode, MOSFET exhibit very fast switching responses, controlled essentially by the charging of the device capacitances. – However, forward voltage drops of these devices increases quickly with increasing breakdown voltage. Minority carrier devices, like BJT, IGBT can exhibit high breakdown voltages with relatively low forward voltage drop. – But they can have longer switching times due to stored minority charges. Energy is lost during switching transitions, due to a variety of mechanisms. – The resulting average power loss, or switching loss, is equal to this energy loss multiplied by the switching frequency. So need of a mechanism to have a compensation between these issues. Thyristor ◦ Thyristor, a three terminal, four layers solid-state semiconductor device, each layer consisting of alternately N-type or P-type material, i.e., P-N-P-N, that can handle high currents and high voltages, with better switching speed and improved breakdown voltage. ◦ Name ‘thyristor’, is derived by a combination of the capital letters from THYRatron and transISTOR. ◦ Thyristor has characteristics similar to a thyratron tube which is a type of gas filled tube used as a high energy electrical switch and controlled rectifier. ◦ But from the construction view point, a thyristor (pnpn device) belongs to transistor (pnp or npn device) family. ◦ This means that thyristor is a solid-state device like a transistor and has characteristics similar to that of a thyratron tube. Thyristors ◦ Thyristors are a class of semiconductor devices characterised by 4-layers of alternating p- and n-material. Four-layer devices act as either open or closed switches; for this reason, they are most frequently used in control applications. ◦ Some thyristors and their symbols are (a) 4-layer diode (b) SCR (c) Diac (d) Triac (e) SCS Thyristors Anode ◦ The concept of 4-layer devices is usually shown as an equivalent circuit of a pnp and an npn pn junction 1 transistor. Ideally, these devices would not Q1 conduct, but when forward biased, if there is sufficient leakage current in the upper pnp device, pn junction it can act as base current to the lower npn device 2 causing it to conduct and bringing both transistors Q2 into saturation. pn junction 3 Cathode  ANODE Equivalent Circuit ANODE P Q1 N N BJT_PNP_VIRTUAL GATE P Q2 P GATE BJT_NPN_VIRTUAL N CATHODE CATHODE Thyristor Family Members ◦ SCR: Silicon-Controlled ◦ LASCS: Light Activated Silicon Rectifier Control Switch ◦ DIAC: Diode on Alternating ◦ LASCR: Light Activated Silicon Current Control Rectifier ◦ TRIAC: Triode for Alternating Current ◦ SITh : Static Induction Thyristor ◦ SCS: Silicon-Control Switch ◦ RCT: Reverse Conducting ◦ SUS: Silicon Unilateral Switch Thyristor ◦ SBS: Silicon Bidirectional ◦ GTO : Gate Turn-Off thyristor Switch ◦ MCT: MOSFET Controlled ◦ SIS: Silicon Induction Switch Thyristor ◦ ETOs: Emitter Turn ON thyristor The Four-Layer Diode ◦ The 4-layer diode (or Shockley diode) is a type of thyristor that acts something like an ordinary diode but conducts in the forward direction only after a certain anode to cathode voltage Anode (A) A called the forward-breakover voltage is reached. p 1 n ◦ The 4-layer diode has two leads, labeled the 2 anode (A) and the cathode (K). The symbol p 3 reminds you that it acts like a diode. It does not n conduct when it is reverse-biased. Cathode (K) K The Four-Layer Diode ◦ The characteristic curve for a 4-layer diode shows the forward blocking region. When the anode-to-cathode voltage exceeds VBR, conduction occurs. The switching current at this point is IS. IA Forward- Once conduction begins, it will continue conduction On region until anode current is reduced to less than IH the holding current (IH). This is the only IS Off Forward- VAK blocking way to stop conduction. 0 VBR(F) region Question 1 ◦ A certain 4-layer diode is biased in the forward-blocking region with an anode-to-cathode voltage of 20 V. Under this bias condition, the anode current is 1 μA. Determine the resistance of the diode in the forward-blocking region. ◦ If the anode current is 2 μA and VAK = 20 V, what is the 4-layer diode’s resistance in the forward-blocking region? Question 2 ◦ Determine the value of anode current in the figure below when the device is on. VBR(F) = 10 V. Assume the forward voltage drop is 0.9 V. ◦ What is the resistance in the forward-conduction region of the 4- layer diode in the figure above? 12 The Silicon-Controlled Rectifier A ◦ The SCR had its roots in the 4-layer diode. By adding a gate connection, the SCR could be triggered into conduction. This G improvement made a much more useful device than the 4- layer diode. K IA The SCR can be turned on by exceeding the forward breakover I H0 IG2 > IG1 IG1 > IG0 I = 0 G0 voltage or by gate current. Notice I H1 that the gate current controls the amount of forward breakover VR I H2 VF voltage required for turning it on. 0 VBR(F 2) VBR(F1) VBR(F0) IR The Silicon-Controlled Rectifier ◦ Like the 4-layer diode, the SCR will conduct as long as forward current exceeds IH. There are two ways to drop the SCR out of conduction: 1) anode current interruption and 2) forced +V commutation. Anode current can be interrupted by breaking the anode IA = 0 current path (shown here), providing a path around the SCR, or dropping the anode voltage to the point that IA < IH. RA Force commutation uses an external circuit to momentarily force current in the opposite direction to forward conduction. SCRs are commonly used in ac circuits, which forces the G SCR out of conduction when the ac reverses. SCR Specifications IF Three important SCR specifications are: Forward-breakover voltage, VBR(F): This is the voltage Forward- at which the SCR enters the forward-conduction region. conduction region (on) Holding current, IH: This is the value of anode current for IG = 0 below which the SCR switches from the forward- IH VBR(R) conduction region to the forward-blocking region. VR VBR(F) VF 0 Reverse- blocking Forward- Gate trigger current, IGT: This is the value of gate Reverse- region blocking avalanche region (off) current necessary to switch the SCR from the forward- region blocking region to the forward-conduction region under specified conditions. IR SCR Applications SCRs are used in a variety of power control applications. One of the most common applications is to use it in ac circuits to control a dc motor or appliance because the SCR can both rectify and control. I R1 The SCR is triggered on the positive A cycle and turns off on the negative cycle. R2 A circuit like this is useful for speed R4 R3 control for fans or power tools and other B related applications. M SCR Applications Another application for SCRs is in crowbar circuits (which get their name from the idea of putting a crowbar across a voltage source and shorting it out!) SW Fuse The purpose of a crowbar circuit is to shut down a DC power supply in case of over-voltage. Once power supply VOUT triggered, the SCR latches on. The SCR can handle a large current, which causes the fuse (or circuit D1 breaker) to open. R1 D2 VTRIG R3 R2 "Crowbar circuit" Question 3 ◦ Determine the gate trigger current and the anode current when the switch, SW1, is momentarily closed in the figure below. Assume VAK = 0.2 V, VGK = 0.7 V and IH = 0.5 A. Will the SCR turn on if VA is reduced to 12 V? Explain. 18 Question 4 ◦ Show the voltage waveform across the SCR in the figure below from anode to cathode (ground) in relation to the load current for 180°, 45°, and 90° conduction. Assume an ideal SCR. 19 Solution 4 ◦ When there is load current, the SCR is conducting and the voltage across it is ideally zero. When there is no load current, the voltage across the SCR is the same as the applied voltage. The waveforms are as shown below. 20  A1 The Diac The diac is a thyristor that acts like two back-to-back 4-layer diodes. It can conduct current in either direction. Because it is bidirectional, the terminals are equivalent and labeled A1 and A2. A2 IF The diac conducts current after the breakdown voltage is reached. At that point, the diac goes into avalanche conduction, creating a current IH VBR(R) VR VF pulse sufficient to trigger another thyristor (an 0 VBR(F) –IH SCR or triac). The diac remains in conduction as long as the current is above the holding current, IH. IR The Triac A1 The triac is essentially a bidirectional SCR but the anodes are not interchangeable. Triggering is done by applying a current pulse to the G gate; breakover triggering is not normally used. A2 IA When the voltage on the A1 terminal is positive with respect to A2, a gate current IG2 IG1 IG0 pulse will cause the left SCR to conduct. IH0 IH1 When the anode voltages are reversed, the –VA VBR(R0) VBR(R1) VBR(R2) IH2 VA VBR(F2) VBR(F1) VBR(F0) –IH2 gate current pulse will cause the right SCR to –IH1 –IG0 –IG1 –IG2 –IH0 conduct. –IA Triac Application Triacs are used for control of ac in applications like electric range heating controls, light dimmers, and small motors. Triac on Like the SCR, the triac latches after RL IL triggering and turns off when the current A1 Delay is below the IH, which happens at the Vin angle Conduction end of each alteration. G angle A2 VG The Silicon-Controlled Switch Anode (A) Anode gate The SCS is similar to an SCR but with two gates. It can be (G A) Cathode triggered on with a positive pulse on the cathode gate, and gate can be triggered off with a positive pulse on the anode gate. (GK) Cathode (K) +VCC VGK A GA In this example, the SCS is controlling a dc source. The load is in the cathode circuit, which has the VGA GK advantage of one side of the load being on circuit VL ground. K RL Summary 4-layer diode The type of 2-terminal thyristor that conducts current when the anode-to-cathode voltage reaches a specified “breakover” value. Thyristor A class of four-layer (pnpn) semiconductor devices. SCR Silicon-controlled rectifier; a type of three terminal thyristor that conducts current when triggered by a voltage at the single gate terminal and remains on until anode current falls below a specified value. Summary LASCR Light-activated silicon-controlled rectifier; a four layer semiconductor device (thyristor) that conducts current in one direction when activated by a sufficient amount of light and continues to conduct until the current falls below a specified value. Diac A two-terminal four-layer semiconductor device (thyristor) that can conduct current in either direction when properly activated. Triac A three-terminal thyristor that can conduct current in either direction when properly activated. Summary SCS Silicon-controlled switch; a type of four-terminal thyristor that has two gate terminals that are used to trigger the device on and off.