Thyristor or Silicon Controlled Rectifiers - Essay Example

Chopper Control College: Chopper Control Introduction There are two forms of electrical energy supplies ly; AC (Alternating Current) and DC (Direct Current). In Ac power supplies both the voltage and the current vary in magnitude with time whereas in DC power supplies both the voltage and the current are constant i.e. they don't vary with respect to time. Most, if not all electrical machines / appliances run directly on mains supply. The electronic devices on the other hand utilize the DC power supply. However, the available mains power is the AC type. For the electronics devices to run on this supply there is the need for an interface known as the power converters. Converters can be made using different technologies which depend on; the type of semiconductor devices used and the power ratings of the load. Several semi conductor devices are used such as the diodes, Thyristors, Triacs, GTO (gate turn off) among others (Gureich, 2008). Thyristor or Silicon Controlled Rectifiers (SCRs) have been the traditional workhorses for bulk power conversion and control in industry. The Thyristor came from its gas tube equivalent, thyratron. Often it is a family name that includes SCRs, Triac, GTO, MCT and IGCT. A Thyristor is a controlled rectifier where the unidirectional current flow from anode to cathode is initiated by a small signal current from gate to cathode. Thyristors are classified as standard or slow-phase-control-type and fast switching, voltage fed inverter type (Ulrich et al, 1998). Thyristor switching characteristics Initially, when forward voltage is applied across a device, the off-state, or static dv / dt, must be limited so that it does not switch on spuriously. The dv / dt creates displacement current in the depletion layer capacitance of the middle junction, which induces emitter current in the component transistors and causes switching action (Dorf, 1997). When the device turns on; the anode current di / dt can be excessive, which can destroy the device by heavy current concentration. During conduction, the inner P-N regions remain heavily saturated with minority carriers and the middle junction remains forward biased. To recover the forward voltage capability, a reverse voltage is applied across the device to sweep out the minority carriers and the phenomena are similar to that of a diode. However, when the recovery current goes to zero, the middle junction still remains forward-biased. This junction eventually blocks with an additional delay when the minority carriers die by the recombination process. The forward voltage can then be applied successfully, but the reapplied dv / dt will be somewhat less than the static dv / dt because of the presence of minority carriers. The volt-ampere characteristics of the device indicate that at gate current IG =0, if forward voltage is applied on the device, there will be a leakage current due to blocking of the middle junction (Littelfuse Inc, 2008). If the voltage exceeds a critical limit (break over voltage) the device switches into conduction. As the magnitude of Ig increases, the forward break over voltage is reduced and eventually at Ig, the device behaves like a diode with the entire forward blocking region removed. The device will turn on successfully if the minimum current, called a latching current, is maintained. During conduction, if the gate current is zero and the anode current falls below a critical limit, called the holding current, the device reverts to the forward blocking state. With reverse voltage, the end P-N junctions of the device become reverse biased and the V-I curve becomes essentially similar to that of a diode rectifier. This indicates that the thyristor is composed of two diodes connected back to back. Modern Thyristors are available with very large voltage (several KV) and current ratings (several KA). In order to turn the thyristor off, the load current must be reduced below its holding current (IH) for sufficient time to allow all the mobile charge carriers to vacate the junction. This is achieved by "forced commutation" in DC circuits or at the end of the conducting half cycle in AC circuits. (Forced commutation is when the load circuit causes the load current to reduce to zero to allow the thyristor to turn off) At this point, the thyristor will have returned to its fully blocking state. If the load current is not maintained below IH for long enough, the thyristor will not have returned to the fully blocking state by the time the anode-to-cathode voltage rises again. It might then return to the conducting state without an externally-applied gate current. Note that IH is also specified at room temperature and like IL, it reduces at higher temperatures. The circuit must therefore allow sufficient time for the load current to fall below IH at the maximum expected operating temperature for successful commutation. To turn off (commutate) a thyristor (or Triac), the load current must be < IH for sufficient time to allow a return to the blocking state. This condition must be met at the highest expected operating temperature. In a converter circuit, a thyristor can be turned off (or commutated) by a segment of reverse AC line or load voltage (defined as line or load commutation respectively), or by an inductance-capacitance, circuit-induced transient reverse voltage (defined as forced commutation). Converter applications Applications of converters may include the following; Electromechanical processes such as electroplating, anodizing ,metal refining, and chemical gas (hydrogen, oxygen, chlorine etc) Adjustable speed DC and AC motors. High voltage DC (HVDC) systems DC and AC general purpose power supplies, including UPS (uninterruptible power supply) systems DC to AC power conversion from solar cells, fuel cells etc. with interface to the utility system. Thyristor converters and inverters employ similar switching characteristics hence some conversion operations can be modified a little bit and be used for inversion purposes. Again diodes, thyristors and any other semi conductor device used in power conversion or inversion have similar operational properties. Again, it is worth noting that the semi conductor devices build on short comings of the other. Commutation techniques can be viewed at a broader angle to be of two types: the first one is natural commutation and forced commutation. Natural commutation of thyristors occurs when dealing with AC power supplies while forced commutation applies to DC circuits and it is achieved mainly by reverse biasing the thyristors. Several methods of thyristor turn-off/ commutation are employed in converter circuits namely; I. Parallel capacitance II. Resonant turn-off circuits III. Commutation by a load carrying thyristor IV. Coupled pulse Parallel capacitance In this method of DC line commutation a capacitor (C) of sufficiently high value is connected across the load so that at fundamental frequency, the effective load has a leading power factor. The purpose of the capacitor is to have load commutation of the thyristors. In this case, the DC link inductance is very high and has near perfect filtering of the harmonic currents by the capacitor. There exist several methods of considering the capacitance ratings. The capacitor can be of a fixed capacitance or variable capacitance type. When using the variable capacitance type it becomes cumbersome since varying the capacitance is not easy and is less accurate. The only remaining option is to use a capacitor with fixed capacitance C. This means that the other parameters of the converter can be altered but the capacitor ratings will remain constant; this is easier to implement as compared to the variable capacitance type. Figure 1: parallel capacitance1 The thyristor pairs Q1Q2 and Q3Q4 are switched alternatively for angle to impress a square current wave at the output. The fundamental component of the load current leads the nearly sinusoidal load voltage wave by o. When thyristor pair Q1Q2 is switched on, outgoing pair Q3Q4 is impressed with a negative voltage segment for duration o, causing load commutation. Since = w t q, the minimum value of should be sufficient to turn off the thyristors during time tq. The thyristors can be replaced by symmetric blocking GTOs (Gate Turn off Thyristors) or IGBTs (Insulated Gate Bipolar Transistors) with series diodes (Mohan, 2002). The load current is at lagging power factor angle and can be resolved into a reactive component and an active component. The leading capacitor current I C overcomes the lagging current I q so that the effective power factor becomes leading. The effective load can be considered as parallel resonant circuit and the inverter frequency is higher than the resonance frequency (Bimal, 2001). Resonant turn-off circuits This provides a new and improved method for controlling a power converter by using and suitably controlling an auxiliary resonant commutation circuit in order to achieve soft-switching of all switching devices employed in the power converter. Resonant turn-off circuits provide a way of controlling a power converter employing an auxiliary resonant commutation circuit in order to achieve soft-switching of all of the converter's switching devices such that high quality AC line current waveforms are attainable even with a reduced filter size and at high switching frequencies. With this method, the gating and conduction times of the converter's switching devices are controlled in such manner as to generate a boost energy which is added to the resonant operation in order to ensure soft-switching of all the switching devices. In this case, the power converter includes an inverter with two main switching devices per phase, each switching device having a diode connected in anti-parallel therewith and further having a relatively large snubber capacitor coupled there across. The auxiliary resonant commutation circuit includes auxiliary switching devices coupled in series with an inductor and the snubber capacitors (Lander, 1994; ). Advantageously, the control method of the present invention controls the gating and conduction times of the inverter's switching devices. The auxiliary switching devices ensures that the output voltage attempts to overshoot, i.e. at least reaches, the positive and negative converter rail voltages during each resonant commutation cycle, thereby approaching true soft-switching of all the main devices (Erickson, 2001). The auxiliary resonant circuits In operation, the auxiliary resonant commutation circuit is triggered into conduction by a respective auxiliary switching device, thereby coupling the LC resonant circuit to a forcing potential substantially equal to one-half the DC supply voltage. With this forcing potential, the output resonating voltage should ideally have a peak-to-peak excursion equal to the DC supply voltage. In order to ensure that the resonating output voltage reaches the ideal peak-to-peak voltage excursion, the control method of the present invention involves adding a boost current to the resonant current by appropriately controlling the conduction times of the auxiliary switching devices and the main switching devices. A predetermined boost current level adds sufficient energy to the resonant operation to ensure that the output voltage attempts to overshoot the respective converter rail voltages, hence forward biasing the corresponding anti-parallel diode and clamping the output voltage to the respective rail voltage. During the clamping interval, the control commutates the corresponding switching device with substantially no switching losses. a control method is provided for determining the gating sequence and conduction times of main switching devices and auxiliary switching devices to achieve soft-switching thereof. In particular, the timing is controlled in such a manner as to provide a boost current. This boost current, when added to the resonant inductor current ensures that the resonant output voltage attempts to overshoot the DC rail voltages and during each resonant interval. As a result, the corresponding diode is forward biased, clamping the resonant voltage to the respective rail. This diode clamping interval provides a zero-voltage turn-on opportunity for the main switching devices. Furthermore, since the main switching devices turn off with a relatively large capacitance in parallel, there are substantially no main device turn-off switching losses, i.e. turn-off occurs with substantially zero voltage there across. Still further, the auxiliary switching devices exhibit substantially lossless switching. In particular, the auxiliary switching devices turn on with substantially zero current through due to the presence of a relatively large inductor coupled in series. The auxiliary switching devices turn off when the resonant current reaches zero, i.e. with substantially zero current, Hence, soft-switching of all active devices employed in the power converter is achieved by the control method of the present invention. The sequence and directions of current flow during commutation is initiated with current flowing in anti-parallel diode. In conjunction therewith, the resonant and load currents Ir and IL, respectively, and the resonant output voltage. Also, the time intervals during which the respective circuit devices are active are indicated by dashed vertical lines. The positive direction of the load current is assumed to be a constant current source during any commutation interval. With diode conducting current, the commutation process begins by turning ON auxiliary switching device. As a result, a forcing potential equal to one-half the DC supply is applied across resonant inductor (Lander, 1994 and Sze, 2002). This initiates a ramp-up commutation phase wherein the resonant current increases at a linear rate of 2Lr /Vdc. During the ramp-up Commutation phase, main switching device remains gated ON, even though it is not conducting current. As the resonant current ir increases, it displaces the load current initially flowing in diode D2. When the resonant current ir exceeds the load current level IL, the boost phase of the commutation cycle begins. In the boost phase, the current in diode D2 decreases to zero, and a boost current ib flows in main switching device S2, with the boost current ib being determined by the expression ib =ir -IL. In accordance with the present invention, the boost current ib adds sufficient energy to the resonant cycle to ensure that the resonant voltage VF attempts to overshoot the positive rail voltage Vdc. The boost current ib increases in main switching device S2 at a linear rate; hence, the duration of the boost phase can be controlled using a simple time delay. Commutation by another load carrying thyristor This method of commutation employs two inductive loads. The line inductance is neglected for simplicity. The load inductance is assumed to be very large so that the load current is ripple-free. In the positive half cycle, the thyristors Q1Q2 are forward-biased, and when these two devices are triggered into conduction at firing angle , the load current will flow through the devices as shown in the figure below. Figure 2: showing commutation by another load carrying thyristor2 Since the load is inductive, the thyristor current will continue to flow beyond angle when the voltage vs reverse its polarity. Thyristors Q3Q4 are fired symmetrically at angle in the next half-cycle. This places a reverse voltage across Q1Q2, which turns off, or commutates and the load current is taken by Q3Q4. A finite leakage inductance in the line will cause gradual current transfer from the outgoing thyristor to the incoming thyristor. When the line-line voltage is shorted and the supply volt-seconds area is absorbed by the leakage inductances in series until the current transfer is completed then commutation occurs. At this time the load voltage dwells at an intermediate level between the two-phase voltages. In this case, the commutation equation for voltage is; Va = LcdiQl/dt + v'd In this analysis, the load current is assumed to be constant. There are some losses associated with this method and they are referred to as commutation losses. On the other hand, adequate angle margin should be provided to ensure successful commutation. The thyristors require minimum turn-off time for successful commutation which correspondingly determines the fluctuation angle (Lander, 1994). Coupled pulse Another type of thyristor turn-off method is by using a coupled pulse. This is a type of forced commutation that does not require any additional power circuit components. In this method, the converter thyristors are commutated by periodically interrupting the DC linking current. Assume, for example, that commutation is desired from thyristor Q2 to Q4, when Q3 on the positive side is conducting. At point A, the line-side converter is blocked. The converter will be dragged into the inverting mode when negative Vr pumps the energy stored in Ld and the machine phases into the line and Id falls to zero. During current interruption, inverter conducting thyristors Q2 and Q3 will turn off, but then, at point B, Q3 and QA are fired and the line-side converter is enabled to re-establish current Id. Thus, conduction is successfully transferred from Q2 to Q4. The DC link bypass thyristor Q4 can help fast current interruption. When the line-side converter is blocked, QA is forced to lock the current in Ld through it. This helps faster current interruption in Q2 and Q3 because of reduced feedback energy. This mode of control is repeated every /3 interval and the machine phase current becomes slightly less than 2/3 wide per half cycle with a small gap in the middle. This cycle goes on and on such that a control switching frequency is obtained. This cycle is N times that of the AC supply frequency. These clock pulses are counted by the counter and the values are sent as address signals. The signals are sent to the DC link voltage command to produce the bypass gate pulses which force the rectifier leg to short-circuit causing commutation. Alternatively, the coupled pulse can be produced by an independently turn off circuit (Lander, 1994). This is normally a pulse voltage source. In most cases, this method is not used because it is expensive since it requires additional transformers to run the gates. For this circuit, the pulse is only directed to two thyristors unlike other methods where all gates are powered. Conclusion Thyristors are power semi conductor devices with wide applications in conversion and inversion systems. Thyristors are not fully controllable switches. They are switched on by a gate signal, but even after the gate signal is removed, the thyristor remains in the on state until a turn-off / commutation condition occurs. There are several methods of thyristor commutation namely: parallel capacitance, resonant turn off, commutation by a load carrying thyristor and coupled pulse. The recent trend in semiconductor technology has seen the emergence of GTO (Gate Turn Off thyristor). The GTO can be turned on by a gate signal and can also be turned off by a gate signal of negative polarity. References Batarseh, I. 2003. Power Electronic Circuits. New York: John Wiley. Bimal, K. 2001. Modern Power Electronics and AC Drives. New Jersey: Prentice hall publishers. Dorf, R. C. 1997. Electrical Engineering Handbook (2nd ed.). New York: CRC Press. Erickson, R. W. 2001. Fundamentals of Power Electronics. New York: Springer publishers Gureich, V. 2008. Electronic Devices on Discrete Components for Industrial and Power Engineering. New York: CRC Press. Lander, C.W. 1994. Power Electronics. New York: Mc Graw-Hill publishers. Littelfuse Inc. 2008. Fundamental Characteristics of Thyristors. Teccor brand Thyristors AN1001. [Online]. Available at: http://www.littelfuse.com/data/en/Application_Notes/AN1001.pdf . Accessed 10 October 2009. Hughes, A. 2006. Electric Motors and Drives: Fundamentals, Types and Applications. Burlington MA: Newness Publishers. Mohan, N. 2002. Power Electronics: Converters, Applications and Design. New York: John Wiley and sons. Sze, S. M. 2002. Semiconductor Devices, Physics and Technology, 2nd ed. New York: John. Wiley and sons publishers. Toomas, R.; Aleksand, V.; Galina, R.; and Mihhail, P. 2007. Comparison of turn-on Characteristics of thyristor structures based on wide bandgap materials. Department of electronics, Tallinn University of Technology. [Online]. Available at: http://www.kirj.ee/public/Engineering/2007/issue_4/eng-2007-4-16.pdf Ulrich, N.; Tobias, R.; Jrgen, P. and Josef, L. 1998. Application Manual IGBT- and MOSFET-Power Modules. Read More