1 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS UNIT - I Power Electronics & its Roll : Power electronics is a technology that deals with the conversion and control of electrical power with high-efficiency switching mode electronic devices for a wide range of applications. Power electronics makes it possible to transport electricity over long distances with minimum losses, which is accomplished by Power Converters. Power Converters convert alternating current (AC) into high-voltage direct current (HVDC) and vice-versa Power Electronics Devices are also used many applications like speed control of AC / DC Motors, electrochemical process, heating and lighting control, electronic welding, power line volt–ampere reactive (VAR) and harmonic compensators, high-frequency (HF) heating, and motor drives. Silicon Controlled Rectifier ( SCR ) / Thyristor A silicon controlled rectifier is a four-layer solid state current-controlling device which control high Voltage and Power. A SCR is device / component have three terminals namely anode (A), cathode (K), Gate (G), four Layers P-N-P-N, and three Junctions J 1, J 2, J 3 . It allows the current in one direction from Anode to Cathode only and the Gate controls the flow of current between the Anode and Cathode. The primary function of SCR / Thyristor is to control electric power and current by acting as a switch. Figure shows the Construction & Symbol of SCR: 2 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS SCR V-I Characteristics Forward Characteristics When Anode is positive with respect to Cathode, the curve between V and I is called the forward characteristics. In figure, OMNK is the forward characteristics of SCR at IG=0. If the supply voltage is increased from zero, a point reached ( point M ) when the SCR starts conducting and the voltage across SCR suddenly drops as shown by dotted curve MN and then to any point in between N and K. Since the anode current in this mode will only be limited by the load, so based on the value of load the anode current will change and may at any point in between N and K. Thus NK represents the forward conduction of SCR and supply voltage appears across the load resistance RL . If proper gate current is made to flow, SCR can close at much smaller supply voltage. Reverse Characteristics When Anode is negative with respect to Cathode, the curve between V and I is known as reverse characteristics. In figure, MPQ is the reverse characteristics of SCR. In reverse characteristics, Initially voltage increases from O and reached up to P Point, only negligible leakage current will flow through SCR. At Point P, SCR reached at reverse breakdown voltage ( VBR ) 3 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS represents this reverse breakdown voltage in the V-I characteristics. It can be seen that, there is a sharp increase in reverse current at this voltage. This increased reverse current may result in more losses in the SCR which in turn may damage the SCR. Therefore the reverse voltage across the SCR terminals should not exceed reverse breakdown voltage during its operation. Important Terms in V-I Characteristics of SCR 1. Forward Break-Over Voltage :- Forward break over voltage of SCR is the minimum forward voltage at which SCR starts conducting. Thus, if the break-over voltage of an SCR is 200 V, it means that it can block a forward voltage (i.e. SCR remains open) as long as the supply voltage is less than 200 V. If the supply voltage is more than this value, then SCR will be turned on. In practice, the SCR is operated with supply voltage less than break-over voltage and it is then turned on by means of a small voltage applied to the gate. 2. Peak Reverse Voltage (PRV) / Reverse Breakdown Voltage: It is the maximum reverse voltage (cathode positive w.r.t. anode) applied to an SCR up-to which it remains safe i. e. without damaging in the reverse direction.” PRV is an important consideration while connecting an SCR in an a.c. circuit. During the negative half of A. C. supply, reverse voltage is applied across SCR. If PRV is exceeded, there may be avalanche breakdown and the SCR will be damaged if the external circuit does not limit the current. Commercially available SCRs have PRV ratings up-to 2.5 kV. 3. Forward Current Rating: It is the maximum anode current that an SCR is capable of passing without destruction. Every SCR has a safe value of forward current which it can conduct. If the value of current exceeds this value, the SCR may be destroyed due to intensive heating at the junction. For example, if an SCR has a forward current rating of 40 A, it means that the SCR can safely carry only 40 A. Any attempt to exceed this value will result in the destruction of the SCR. 4 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS 4. Holding Current: It is the minimum anode current at which the thyrister / SCR continue to conduct. If the anode current less than Holding current, the thyrister will be turned off. Holding current of SCR or thyrsistor is that minimum value of current below which anode current must fall to come in OFF state. This means if the value of holding current is 5 mA, then anode current of SCR must become less than 5 mA to stop conducting. 5. Latching Current: Latching current is the minimum Anode current required to maintain the thyristor in the ON-STATE after the gate pulse is removed.” Difference Between latching and holding current. Sr. No. Latching Current Holding Current It is related with turn on process of 1) It is related to turn off process. SCR or thyristor. Minimum value of anode current Minimum current above which gate 2) below which it must fall to stop losses its control. conducting in forward direction. Value of latching current is more 3) It is less than latching current. than that of holding current. Latching current is generally 2 to 3 4) – times of the holding current. 5 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS Two -Transistor analogy of SCR ww.electrical.com The two-transistor analogy of SCR is a method of representing an SCR as a combination of an N-P-N and a P-N-P transistor. SCR is a three terminal device having a P- N-P-N structure. The three terminals are the cathode, anode and the gate terminal. The two transistor equivalent circuit shows that the collector current of the NPN transistor TR 2 feeds directly into the base of the PNP transistor TR 1, while the collector current of TR 1 feeds into the base of TR 2. These two inter-connected transistors with each other for conduction as each transistor gets its base-emitter current from the other’s collector-emitter current. So until one of the transistors is given some base current nothing can happen even if an Anode-to-Cathode voltage is present. If the Anode terminal is made positive with respect to the Cathode, the two outer P- N junctions are now forward biased but the centre N-P junction is reverse biased. Therefore forward current is also blocked. If a positive current is injected into the base of the NPN transistor TR 2, the resulting collector current flows in the base of transistor TR 1. This in turn causes a collector current to flow in the PNP transistor, TR 1 which increases the base current of TR 2 and so on If the thyristors Anode terminal is negative with respect to the Cathode, the centre N- P junction is forward biased, but the two outer P-N junctions are reversed biased and it behaves very much like an ordinary diode. Therefore a thyristor blocks the flow of reverse current until at some high voltage level the breakdown voltage point of the two outer junctions is exceeded and the thyristor conducts without the application of a Gate signal. 6 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS Then we can see that a thyristor blocks current in both directions of an AC supply in its “OFF” state and can be turned “ON” and made to act like a normal rectifying diode by the application of a positive current to the base of transistor, TR 2 which for a silicon controlled rectifier is called the “Gate” terminal. SCR Triggering: Switching the SCR from forward blocking state (OFF- state) to forward conduction state (ON- state) is known as turning ON process of SCR . It is also called as SCR triggering. SCR Triggering Methods: The SCR can be made to conduct or switching into conduction mode is performed by any one of the following methods. i. Forward voltage triggering. ii. Temperature triggering. iii. dv / dt triggering. iv. Light triggering. v. Gate triggering. i. Forward Voltage Triggering : One of the commonly used SCR Turn On methods is by increasing the forward anode to cathode voltage. By doing this, the depletion layer width is also increasing at junction J 2. This also causes to increase the minority charge carriers accelerating voltage at junction J2. This further leads to an avalanche breakdown of the junction J 2 at a forward break-over voltage V BO . At this stage SCR turns into conduction mode and hence a large current flow through it with a low voltage drop across it. During the turn ON state the forward voltage drop across the SCR is in the range of 1 to 1.5 volts and this may be increased with the load current.