Thyristor PDF

THYRISTO R  Thyristor is also called as Silicon Controlled Rectifier (SCR).  Using thyristor is better than diode for rectification.  SCR is ON condition if gate current is increased and to OFF the SCR, applied voltage has to reduce below holding current.  Thyristor also can prod...

THYRISTO R  Thyristor is also called as Silicon Controlled Rectifier (SCR).  Using thyristor is better than diode for rectification.  SCR is ON condition if gate current is increased and to OFF the SCR, applied voltage has to reduce below holding current.  Thyristor also can produce half wave and full wave rectification.  Particularly useful for controlling power circuits  Simple to use and cheap to buy and often thyristor circuits are easy to build and use. WHAT IS A THYRISTOR?  consists of four layers of differently doped silicon rather than the three layers of the conventional bipolar transistors.  thyristor has a p-n-p-n structure with the outer layers with their electrodes referred to as the anode (p-type) and the cathode (n- type)  control terminal of the SCR is named the gate and it is connected to the p-type layer that adjoins the cathode layer. Construction Symbol  Thyristors are usually manufactured from silicon, although, in theory other types of semiconductor could be used.  The first reason for using silicon for thyistors is that silicon is the ideal choice because of its overall properties. It is able to handle the voltage and currents required for high power applications. Additionally it has good thermal properties.  The second major reason is that silicon technology is well established and it is widely used for a variety of semiconductor electronics components. As a result it is very cheap and easy for semiconductor manufacturers to use. HOW DOES A THYRISTOR WORK?  different to other devices  Normally no current flows across the device  However if a supply is connected across the device, and a small amount of current is injected into the gate, then the device will "fire" and conduct. It will remain in the conducting state until the supply is removed.  J2 J1 J3  Working of SCR  When Gate is Open  J2 reverse biased & J1, J3 Forward biased.  No current will through RL  V increased until breaks down & SCR ON-stroke. Gate is +ve cathode  J2 reverse biased & J3 forward biased.  ON- V small Vb/over decreases. IG fl owing & Ia increases more e at J2 – breaks up SCR ON.  Gate loses control. Remove SCR VG still ON  Switching SCR OFF  Reduce apply voltage to almost zero which the anode current is reduced below certain value called holding current.  To see how the thyristor operates, it is worth looking at a thyristor equivalent circuit. For the sake of an explanation, the thyristor circuit can be considered as two back to back transistors. The first transistor with its emitter connected to the cathode of the thyristor is an n- p-n device, whereas a second transistor with its emitter connected to the anode of the thyristor, SCR is a p-n-p variety. The gate is connected to the base of the n-p-n transistor as shown  When voltage is applied across a thyristor, no current fl ows because neither transistor is conducting. As a result there is no complete path across the device. If a small current is passed through the gate electrode, this will turn "on" the transistor TR2.  When this occurs it will cause the collector of TR2 to fall towards the voltage on the emitter, i.e. the cathode of the whole device.  When this occurs it will cause current to flow through the base of TR1 and turn this transistor "on". Again this will now try to pull the voltage on the collector of TR1 towards its emitter voltage.  This will cause current to flow in the emitter of TR2, causing its "on" state to be maintained. In this way it only requires a small trigger pulse on the gate to turn the thyristor on.  Once switched on, the thyristor can only be turned off by removing the supply voltage.  It can be seen that current will only flow in one direction through the thyristor. If a reverse voltage is applied, then no current will flow, even if some gate current is applied.  In this way for thyristor circuits used for AC, operation only occurs over one half of the AC waveform.  For the other half of the cycle the device remains inoperative and no current can flow. DIFFERENCES BETWEEN NORMAL DIODES AND SCR / THYRISTORS Normal Diode Thyristor Consists of 2 layers (P and N) Consists of 4 layers of P and N In forward bias, ac will convert to In forward bias, there are no dc output unless the gste is activated. Convert a.c to d.c Convert a.c to d.c and invert d.c to a.c One diode can have +ve or –ve One thyristor can interchange +ve output only at one time or –ve outputs depends on gate signal Connection to anode and cathode Connection to anode, cathode and gate Several applications of SCR:  Motor starter (soft starter)  Varying motor speed (VFD)  Converter circuit  Inverter circuit  Somehow, SCR cannot be used as rectifier circuit in brushless alternator because of SCR require firing control circuit which is not possible to connect through brushless type alternator. SCR IN MOTOR STARTER Some characteristics of SCR applied in soft starter: - Voltage output varies according to the desired values but frequency remains the same. - Use as voltage limiter in both positive and negative cycle of ac supply. - Used as a starting method of ac motor to reduce starting current and bypass after achieving rated speed (e.g induction motor) SCR IN VARYING MOTOR SPEED (VFD) Several characteristics of SCR in VFD:  Frequency supply varies but voltage remains the same.  Use in converting ac to dc and inverting dc to ac of supply voltage to vary frequency  Use to control speed of high power electrical motor (e.g propulsion motor) EXERCISES: 1. What is a thyristor? 2. How does thyristor works? 3. Explain the construction of a thyristor by the aid of a diagram. 4. What are the applications of a thyristor? 5. What are the differences between thyristor and a normal diode? END