Solid state devices find more and more applications in industry and, therefore, it becomes imperative to have preliminary knowledge of such devices. The main devices usually used for motor control are diode, thyristor or silicon controlled rectifier, triac, diac, power transistors etc. Every device has certain maximum voltage, current and time response (speed) capabilities. These can very rarely be achieved simultaneously in a single device. Only one of these is realized in one device.
Device # 1. Diode:
‘The term diode denotes a two-electrode device. A semiconductor diode is simply a P-N junction with connecting leads or terminals on the two sides of the P-N junction. A diode is a unidirectional device permitting the easy flow of current in one direction but restraining the flow in opposite direction.
A major application of diodes is in rectification i.e., in conversion of ac into dc. Semiconductor diode is becoming more and more popular these days due to its smaller size, cheapness, robustness and higher operating efficiency.
The symbol for a semiconductor diode is shown in Fig. 2.10. The arrow in the symbol indicates the direction of conventional current flow when the diode is forward biased i.e., from the positive terminal through the device to the negative terminal. The P-side of the diode is always the positive terminal for forward bias and is designated the anode. The N-side is called the cathode and is the negative terminal when the device is forward biased.
The semiconductor diode may be either silicon one or germanium one. The silicon junction diode is similar in appearance to the germanium diode but differs in properties. Silicon diodes have, in general, higher peak inverse voltage (PIV) and current ratings and wider temperature ranges than germanium diode.
The volt-ampere characteristics of a diode are illustrated in Fig. 2.11. When the junction is in forward biased very little current, called the forward current, flows until the forward voltage exceeds the junction barrier potential (0.3 V for Ge and 0.7 V for Si). The characteristic follows an exponential law.
With the increase in forward voltage forward current increases almost linearly and the P-N junction starts behaving as a resistor and when the applied voltage exceeds a certain value, extremely large current would flow and the P-N junction may get damaged due to overheating. A practical diode has a very low forward resistance (of the order of 75 Ω in case of Ge and 150 Ω in case of silicon) and has a voltage drop of 0.3 to 0.7 V at all current levels in forward direction.
When an ordinary P-N junction diode is reverse biased, normally only very small reverse saturation current flows. It is almost independent of the voltage applied. However, if the reverse bias is increased, a point is reached when the junction breaks down and the reverse current increases abruptly, as shown in Fig. 2.11. This current could be large enough to destroy the junction.
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If the reverse current is limited by means of a suitable series resistor, the power dissipation at the junction will not be excessive, and the device may be operated continuously in its breakdown region to its normal (reverse saturation) level. It is found that for a suitably designed diode, the breakdown voltage is very stable over a wide range of reverse currents. This quality gives the breakdown diode many useful applications as a voltage reference source.
An ideal diode presents zero impedance to current flow in one direction and infinite impedance in opposite direction. The symbol and V-I characteristic of an ideal diode are shown in Fig. 2.12. An ideal diode acts like an automatic switch. The switch is closed when the diode is forward biased and is opened when reverse biased.
The main parameters of a diode are blocking or peak inverse voltage or PIV (about 1,000 V for Si and 400 V for Ge), average forward current and maximum operating junction temperature (nearly 125°C for Si and 60° for Ge). Protection is necessarily provided against voltage surges, over-currents and excessive temperature rise.
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A “freewheeling” diode is commonly employed in motor control systems to provide an alternate path for continuity of current in the inductive circuit following the switching off of some power device between the motor and the supply source.
i. Zener Diode:
Zener diode, also sometimes called the breakdown diode, is a P-N junction diode specially designed for operation in the breakdown region in reverse bias condition. The breakdown diode may be silicon or germanium one but silicon is preferred over germanium because of higher operating temperature and current capability. The knee point is also sharper in case of a silicon diode. These diodes are analogous to gas diodes in which large current appears on reaching the breakdown potential.
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This is similar to that of a normal diode except that the line representing the cathode is bent at both ends i.e., the bar is turned into Z-shape; stands for zener.
The zener diode, though not a true power controlling device, is quite often used as a voltage control and sensing device in many motor controllers.
ii. Power Diode:
Diodes find many applications in electronics and electrical engineering circuits. Power diodes play a significant role in power electronics for conversion of electric power. A diode acts as a switch to perform various functions, such as switching in rectifiers, freewheeling in switching regulators, charge reversal of capacitor and energy transfer between components, voltage isolation, energy feedback from load to the power source, and trapped energy recovery.
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Power diodes can be assumed as ideal switches for most applications but practical diodes differ from the ideal characteristics and have certain limitations. The power diodes are similar to P-N junction signal or low-power diodes. However, the power diodes have large power-, voltage-, and current- handling capabilities than those of ordinary diodes. The frequency response (or switching speed) is low in comparison to that of ordinary diodes.
Power diodes are of three types: General purpose, high speed (or fast-recovery), and Schottky. General-purpose diodes are available up to 6000 V, 4500 A, and the rating of fast- recovery diodes can go up to 6000 V, 1100 A. The reverse recovery time varies between 0.1 and 5 |is. The fast-recovery diodes are essentially required for high-frequency switching of power converters. Schottky diodes have low on-state voltage and very small recovery time, typically nanoseconds. The leakage current increases with the voltage rating and their ratings are limited to 100 V, 300 A. A diode conducts when its anode voltage is higher than that of the cathode; and the forward voltage drop of a power diode is very small, typically 0.5 and 1.2 V.
Device # 2. Thyristor:
Thyristor is the general name given to a family of semiconductor devices having four layers with a control mechanism, although this term is most commonly applied to the SCR (silicon controlled rectifier). This term is derived from thyratron and transistor because the device combines the rectification action of thyratron and control action of transistor.
Since its inception the thyristor has come to stay as a basic building block in many industrial and power system applications. Its ability to be controlled, compactness, fast response, high reliability, better efficiency, large power handling capacity, high voltage and current ratings, good trigger sensitivity, static operation, large power gain, sturdy construction, long life, very little maintenance and low cost of fabrication—due to advancement in the field of fabrication—have given the thyristor a colourful reception in every field.
Today thyristors are finding applications in the control of dc/ac motors; for the improvement of power factor; and as switching devices. Thyristors have helped in further cost reduction and in the development of drive system by changing the emphasis from dc motors to ac motors. With cycle converters and inverters, the speed of an ac motor can also be controlled with ease and reliability.
Apart from these main applications it finds use as a switching device, particularly in the improvement of power factor of transmission lines and mains. Thyristor can be used as a power switching device with a power handling capacity ranging from a few watts to as high as 4 MW (2,500 A at 1,600 V). Some thyristors have a rating as high as 35,000 A, 10,000 V for use in hv dc transmission lines.
Thyristors, with its large number of advantages and tremendous control capabilities, have numerous applications and have completely replaced the electromagnetic control systems. Thyristor basically serves two functions viz electronic switching and electronic control.
Some of the applications of thyristors are listed below:
i. Speed control of dc and ac motors.
ii. As rectifier for conversion of ac into dc.
iii. As inverter for conversion of dc into ac.
iv. As dc chopper or dc to dc converter for converting dc at one level to dc at another level.
v. As cycloconverter for converting ac of one frequency into ac of another frequency.
vi. Control of temperature, level, position and illumination.
vii. Power switches (dc and ac circuit breakers).
viii. As static switches.
ix. Control of induction heating.
x. Relay control.
xi. Phase control.
SCRs and triacs having high voltage and current ratings are widely employed for power control applications whereas other members of thyristor family are employed for small power applications and for switching in control and digital circuits.
Device # 3. Silicon Controlled Rectifier (SCR):
The SCR is a four-layer, three-junction and a three-terminal device and is shown in Fig. 2.14 (a). The end P-region is the anode, the end N-region is the cathode and the inner P-region is the gate. The anode to cathode is connected in series with the load circuit. Essentially the device is a switch. Ideally it remains off (voltage blocking state), or appears to have an infinite impedance until both the anode and gate terminals have suitable positive voltages with respect to the cathode terminal.
The SCR then switches on and current flows and continues to conduct without further gate signals. Ideally the SCR has zero impedance in conduction state. For switching off or reverting to the blocking state, there must be no gate signal and the anode current must be reduced to zero.
Current can flow only in one direction. Now SCRs of voltage rating 10 kV and an rms current rating of 35,000 A with speed of 1 µs corresponding to a power-handling capacity of 30 MW are available. It can be switched on by a low voltage supply of about 1 A and 10 W. Obviously it has tremendous power amplification (of the order of 3 x 106). Because of its compactness, high reliability and low loss, the SCR has almost replaced the earlier power switching devices—thyratron and a magnetic amplifier.
Volt-Ampere Characteristics:
The SCR is a four layer device with three terminals, namely, the anode, the cathode and the gate. When the anode is made positive with respect to the cathode, junctions J1 and J3 are forward biased and junction J2 is reverse biased and only the leakage current will flow through the device. The SCR is then said to be in the forward blocking state or in the forward mode or off state. But when the cathode is made positive with respect to the anode, junctions J1 and J3 are reverse biased, a small reverse leakage current will flow through the SCR and the SCR is said to be in the reverse blocking state or in reverse mode.
When the anode is positive with respect to cathode i.e., when the SCR is in forward mode, the SCR does not conduct unless the forward voltage exceeds certain value, called the forward breakover voltage, VFBO. In non-conducting state, the current through the SCR is the leakage current which is very small and is negligible. If a positive gate current is supplied, the SCR can become conducting at a voltage much lesser than forward breakover voltage. The larger the gate current, lower the breakover voltage. [Fig. 2.15(c)] With sufficiently large gate current, the SCR behaves identical to PN rectifier. Once the SCR is switched on, the forward voltage drop across it is suddenly reduced to very small value, say about 1 volt. In the conducting or on-state, the current through the SCR is limited by the external impedance.
When the anode is negative with respect to cathode i.e., when the SCR is in reverse mode or in blocking state no current flows through the SCR except very small leakage current of the order of few microamperes. But if the reverse voltage is increased beyond a certain value, called the reverse breakover voltage, VRBO avalanche breakdown takes place. Forward breakover voltage VFBO is usually higher than reverse breakover voltage, VRBO.
From the foregoing discussion, it can be seen that the SCR has two stable and reversible operating states. The change over from off-state to on-state, called turn-on, can be achieved by increasing the forward voltage beyond VFBO, A more convenient and useful method of turning on the device employs the gate drive. If the forward voltage is less than the forward breakover voltage, VFBO, it can be turned on by applying a positive voltage between the gate and the cathode. This method is called the gate control. Another very important feature of the gate is that once the SCR is triggered to on-state the gate loses its control.
The switching action of gate takes place only when:
(i) SCR is forward biased i.e., anode is positive with respect to cathode, and
(ii) Suitable positive voltage is applied between the gate and the cathode.
Once the SCR has been switched on, it has no control on the amount of current flowing through it. The current through the SCR is entirely controlled by the external impedance connected in the circuit and the applied voltage. There is, however, a very small, about 1 V, potential drop across the SCR. The forward current through the SCR can be reduced by reducing the applied voltage or by increasing the circuit impedance. There is, however, a minimum forward current that must be maintained to keep the SCR in conducting state. This is called the holding current rating of SCR. If the current through the SCR is reduced below the level of holding current, the device returns to off- state or blocking state.
The SCR can be switched off by reducing the forward current below the level of holding current which may be done either by reducing the applied voltage or by increasing the circuit impedance.
Alternatively the SCR can be switched off by applying negative voltage to the anode (reverse mode), the SCR naturally will be switched off.
Here one point is noteworthy the SCR takes certain time to switch off. The time, called the turn-off time, must be allowed before forward voltage may be applied again otherwise the device will switch on with forward voltage without any gate pulse. The turn-off time is about 15 microseconds, which is immaterial when dealing with power frequency, but this becomes important in the inverter circuits, which are to operate at high frequency.
There are mainly two types of SCRs for use in motor controllers—inverter grade and converter grade. Former ones are used in inverters, cycloconverters, and brushless dc motor systems while the latters are used in choppers, phase-controlled rectifiers, ac regulators etc. These two grades vary in their time responses. Inverter grade SCRs are costlier than the converter grade.
Typical SCR Parameters:
Commercially available SCRs have a wide range of voltage and current ratings, turn-on and turn-off times and dynamic resistances in cut-off and in saturation regions. Typical values of these parameters for SCRs are given below in tabular form.
Device # 4. Triac:
The triac is another three-terminal ac switch that is triggered into conduction when a low-energy signal is applied to its gate terminal. Unlike the SCR, the triac conducts in either direction when turned on. The triac also differs from the SCR in that either a positive or negative gate signal triggers it into conduction. Thus the triac is a three terminal, four layer bidirectional semiconductor device that controls ac power whereas an SCR controls dc power or forward biased half cycles of ac in a load. Because of its bidirectional conduction property, the triac is widely used in the field of power electronics for control purposes. Triacs of 16 kW rating are readily available in the market.
“Triac” is an abbreviation for three terminal ac switch. ‘Tri’- indicates that the device has three terminals and ‘ac’ indicates that the device controls alternating current or can conduct in either direction.
Though the triac can be turned on without any gate current provided the supply voltage becomes equal to the breakover voltage of the triac but the normal way to turn on the triac is by applying a proper gate current. As in case of SCR, here too, the larger the gate current, the smaller the supply voltage at which the triac is turned on.
Triac can conduct current irrespective of the voltage polarities of terminals MT1 and MT2 with respect to each other and that of gate and terminal MT2.
Typical V-I characteristic of a triac is shown in Fig. 2.16 (b). The triac has on- and off-state characteristics similar to SCR but now the characteristic is applicable to both positive and negative voltages. This is expected because triac consists of two SCRs connected in parallel but opposite in directions.
Next to SCR, the triac is the most widely used member of the thyristor family. In fact, in many of control applications, it has replaced SCR by virtue of its bidirectional conductivity. Motor speed regulation, temperature control, illumination control, liquid level control, phase control circuits, power switches etc. are some of its main applications.
Device # 5. Diac:
A diac is an important member of the thyristor family and is usually employed for triggering triacs. A diac is a two-electrode bidirectional avalanche diode which can be switched from off-state to the on-state for either polarity of the applied voltage. This is just like a triac without gate terminal. Its equivalent circuit is a pair of inverted four layer diodes. Two schematic symbols are shown in Fig. 2.17 (a). Again the terminal designations are arbitrary since the diac, like triac, is also a bilateral device. The switching from off-state to on-state is achieved by simply exceeding the avalanche breakdown voltage in either direction. Volt-ampere characteristic is shown in Fig. 2.17 (b).
The diacs, because of their symmetrical bidirectional switching characteristics, are widely used as triggering devices in triac phase control circuits used in heat control, universal motor speed control etc.
Device # 6. Gate Turn-Off (GTO) Thyristors:
Conventional thyristors are used as ideal switches in power electronic applications. They have the capability of blocking several thousand volts in the off-state and conducting several thousand amperes in the on-state with only small on-state voltage drop. The only drawback of conventional thyristor is that when the gate turns into on-state, the gate loses the control over the device and, therefore, it cannot be turned off without the technique called commutation. The circuitry required for commutation makes the device costlier and bulky. This can be avoided by using a device of thyristor family, known as the gate turn-off thyristor (GTO).
A GTO like an SCR can be turned on by applying a positive gate signal. However, a GTO can be turned off by a negative gate signal. A GTO is a non-latching device and can be built with current and voltage ratings similar to those of an SCR. The device incorporates the character of both of a thyristor and a transistor.
The GTOs have the advantages over SCRs:
(i) Elimination of commutating components in forced commutation, resulting in reduction in cost, weight and volume
(ii) Faster turn-off permitting high switching frequencies
(iii) Improved efficiency of converters
(iv) Reduction in acoustic and electromagnetic noise due to elimination of commutation chokes
(v) High blocking voltage and large current capability.
In low-power applications, GTOs have the following advantages over BJTs:
i. Higher blocking voltage capability.
ii. High ratio of peak controllable current to average current.
iii. High ratio of peak surge current to average current, typically 10:1.
iv. High on-state gain (anode current and gate current) typically 600.
v. Pulsed gate signal of short duration.
Under surge conditions, a GTO goes into deeper saturation due to regenerative action. On the other hand, a BJT tends to come out of saturation.
However, the GTO has the following disadvantages:
i. High latching and holding currents.
ii. More on-state voltage drop and associated loss.
iii. Higher triggering gate current due to multi-cathode structure as compared to conventional thyristor.
Applications:
GTOs are mostly used in voltage-source converters in which a fast recovery antiparallel diode is required across each GTO. Thus, GTOs normally do not require reverse voltage capability. Such GTOs are known as asymmetric GTOs. This is achieved by a so-called buffer layer, a heavily doped N+ layer at the end of the N layer. Asymmetric GTOs have lower voltage drop and higher voltage and current ratings.
GTO devices are now used in:
(i) High performance drive systems, such as the field-oriented control scheme used in rolling mills, robotics and machine tools,
(ii) Traction purposes because of their lighter weight and
(iii) Adjustable-frequency inverter drives. At present, GTOs with ratings up to 5,000 V and 3,000 A are available.
Device # 7. Power Transistors:
The transistor is a two junction, three terminal, current controlled solid state device. This device is a 3-layer P-N-P (or N-P-N) structure. The transistor is capable of amplification and in most respect it is analogous to a vacuum triode. The transistor is only six decade old, yet it has replaced vacuum tubes in almost all applications.
The reasons are obviously its advantages over vacuum tubes such as compact size, light weight, rugged construction more resistive to shocks and vibrations, instantaneous operation (no heating required), low operating voltage, high operating efficiency and long life with essentially no ageing effect if operated within permissible limits of temperature and frequency.
In normal operation the emitter-base junction is forward biased and collector-base junction is reverse biased. The collector current is almost equal but slightly less than the emitter current. The collector current can be controlled by base current. Except for small motors, transistors are used in the switching (on/off) mode. The common-emitter configuration is normally employed as it has high power gain.
Power transistors are now available in ratings suitable for motor control. The common-emitter saturation voltage VCE(Sat) for typical power transistors varies from 0.3 to 0.7 V. This is sufficiently less than the on-state anode-cathode voltage drop of an SCR, and therefore, the average power loss in a transistor is less than that of an SCR of an equivalent power rating.
The switching of a power transistor is controlled through the base current. A continuous base current is required to keep the transistor in the saturated state, i.e., the on-state. In a high- current device, a base current of several amperes is required to keep the device in the on-state owing to its low current gain (about 10) and, therefore, the power loss in the base circuit may be appreciable.
Power transistors have no surge current capacity and can withstand only a low rate of change of current. The switching operations of power transistors are generally faster than those of SCRs, and the commutation problems of SCR are not present in transistors.
Power transistors are becoming more and more popular in low-to-medium-power applications, where they compete successfully with thyristors and GTOs. Power transistor applications are in the range of a few kW to several hundred kWs size in voltage-fed choppers and inverters with switching frequency up to 10-15 kHz.