Regulated Power Supply - Assignment Example

BASIC POWER SUPPLIES Introduction A power supply is an electronic circuit composed of a power transformer and a circuit that converts the voltage from AC to DC, DC to AC, or DC to DC voltages. A power supply is the heart of any electronic circuit. It provides life to appliances, amplifiers, electronic circuits. Living things have a heart, while electronic and electrical appliances have a power supply. Without a power supply, any appliance or electronic circuit won’t function. The goal of the power system is to distribute power effectively to each section of the entire product and to do it in a fashion that meets the needs of each subsection within the product. (Brown 5) The transformer transforms AC voltage and current to lower or higher voltage current. AC voltage then passes through a circuit where it is processed by means of a regulator, capacitor, diode, resistor, and other filters, to produce DC current. Rectifier diodes are used for rectification purposes. There are various types of rectifier circuits that offer half-wave and full-wave rectifications. The most common is the bridge type rectifier circuit which is composed of four diodes, arranged in a bridge formation. The capacitor filters, and added is a load resistor. Some other types have complex circuits which use various capacitors, transistors, and regulators. Figure 1. Block diagram of a simple power supply From the block diagram of a simple power supply, the different stages are emphasized. This is a simplified power supply for a very simple electronic circuit, for example an AM radio. First we have the input voltage which may be 110 or 220 volts. The AC (alternating current) voltage passes through the transformer, which is being transformed depending on the current/voltage need. If the transformer is a step-down transformer, it transforms the 110 volts to a much lower voltage, for example 6 volts. This voltage passes through a rectification process by way of diode/s or IC. The voltage is further processed and goes through a filtering stage by way of capacitors. Solid state ICs and transistors act as regulators. This is applicable in circuits that require accurate DC voltage and current. The whole process is done in micro-second of a process which maybe in full wave or half-wave depending upon the type of rectifier circuit. Full-rectification offers full-wave and the result is a filtered direct current. The input voltage, which is the output of the transformer, is 6 volts AC. When this AC current becomes DC, it becomes 6 volts DC, which is needed by our AM radio. Power transformers come in an enormous variety of secondary voltages and currents: outputs as low as 1 volt or so up to several thousand volts, current ratings from a few milliamps to hundreds of amps. Typical transformers for use in electronic instruments might have secondary voltages from 10 to 50 volts, with current ratings of 0.1 to 5 amps or so. Circuits with capacitors and inductors are more complicated than the resistive circuits, i.e. those using resistors. A voltage divider containing a capacitor or inductor will have a frequency-dependent division ration. With the coming in of the solid state semi-conductors such as the transistors, diodes and IC, electronic circuits have become complex, mostly needing milli- and micro-voltages and currents. This is the advent of the nano technology or the super small electronic circuits. ICs have dominated most electronic circuits and power supplies. Transistors are solid state which dominate inverters and converters taking places alongside inductors, capacitors, discrete semiconductors, and integrated circuit modules. A power supply can be a Converter or Inverter. Our example of power supply above is a converter type power supply, because it converts certain AC voltage to DC voltage and current. Inverter/Converter An inverter is a device, circuit, or system that inverts or delivers AC voltage when energized or supplied from a source of dc power, for example a battery. An inverter is usually applicable if ac current is needed with dc current as the given. A converter converts ac current dc current or dc to dc. More sophisticated power supplies are needed inside a television set. The electronic circuits inside a television set have different voltage and current needs, which may be low or high voltages. Many electrical and electronic equipment contain voltage regulated power supplies. The accuracy of the voltage produced by these supplies is important insofar as proper operation and maximum life of the equipment are concerned. Present day electronic equipments contain many timing, waveshaping, and other critical circuits that require a highly regulated voltage source. The magnetic amplifier can be used to obtain this regulated voltage by being used as the regulating element for the output of the power supply. The magnetic amplifier type regulated power supply is very reliable under varying load conditions. Other important parameters in a power supply include the RMS. This is the AC input current which is the measurement of the AC RMS input current at the full load worst case condition of either low line or high line voltage. This also needs efficiency. Efficiency is a measure of the power consumed in the process of power conversion. For AC/DC and AC/AC converters, the measurements process requires that careful attention be paid to the RMS measurement of both voltage and current, as well as the phase relationship between them. The total AC power is then calculated from the measured data. Instruments are available that can make this measurement directly. (Crandall, 1997, p. 273) Circuit Operation All power supplies work under the same basic principle, whether the supply is a linear or a more complicated switching supply. All power supplies have at their heart a closed negative feedback loop. This feedback loop does nothing more than hold the output voltage at a constant value. A regulated power supply is most common in many electronic circuits nowadays. One example of a regulated power supply is the linear regulator. Linear regulators are step-down regulators only; that is, the input voltage source must be higher than the desired output voltage. Linear power supplies are the simplest of the DC/DC converters. There are several factors in every application of linear supplies that are important for their reliable operation. These are thermal design, output regulation, stability considerations and its transient response, any of which could cause the system to behave badly. Linear regulators are used much more often than switching regulators. They can be found distributed throughout products POL (point of load supplies, where local circuit regulation is needed voltage bus quieting for noise sensitive circuits, and inexpensive voltage bus generation. (Brown, 2008, p. 1) There are two types of linear regulators: the shunt regulator and the series-pass regulator. The shut regulator is a voltage regulator that is placed in parallel with the load. An unregulated current source is connected to a higher voltage source, the shunt regulator draws output current to maintain a constant voltage across the load given a variable input voltage and load current. A common example of this is a Zener diode regulator. The series-pass linear regulator is more efficient than the shunt regulator and uses an active semiconductor as the series-pass unit, between the input source and the load. Linear regulators can be designed to meet a variety of cost and functional needs. The design examples that follow illustrate that linear regulator designs can range from the very elementary to the more complex. Due to the relatively large power loss of linear regulators, the thermal considerations typically represent a significant problem. The Zener shunt regulator is typically used for very local voltage regulation for less than 200 mW of a load. A series resistance is placed between a higher voltage and is used to limit the current to the load and Zener diode. The Zener diode is most common used as a regulator. The Zener diode compensates for the variation in load current. The Zener voltage will drift with the temperature. The drift characteristics are given in many Zener diode datasheets. Its load regulation is adequate for most supply specifications for integrated circuits. It also has a higher loss than the series-pass type of linear regulator, since its loss is set for the maximum load current, which for any load remains less than that value. Figure 2: A Zener shunt regulator The zener shunt regulator uses a simple formula where voltage in is greater than voltage out; the output is controlled by the zener diode which is 3 volts. Formula is the square of maximum voltage in minus voltage output multiplied by the resistance R. The Zener shunt regulator is typically used for very local voltage regulation for less than 200mW of a load. A series resistance is placed between a higher voltage and is used to limit the current to the load and Zener diode. The Zener diode compensates for the variation in load current. The Zener voltage will drift with temperature. The drift characteristics are given in many Zener diode datasheets. Its load regulation is adequate for most supply specifications for integrated circuits. It also has a higher loss than the series-pass type of liner regulator, since its loss is set for the maximum load current, which for any load remains less than that value. Figure 3: Basic Three-Terminal Positive Regulator Design using IC, MC 7805 The main characteristic of this IC is that its voltage output is 5 volts. The name MC 7805 connotes the voltage output, read the last two digits = 05. There are other ICs of the same brand but with different voltage outputs and their names also suggest their outputs, for example MC 7812 whose output is 12 voltages. This simple and cheap but valuable IC is common in electronic circuits which need lower regulated power supply, mostly up to 12 volts, but there are as high as 24 volts. This example of a 3-terminal regulator illustrates the design considerations. Many designers view only he electrical specifications of the regulators and forget the thermal derating of the part. In majority of the 3-terminal applications, the heatsink determines the regulator’s maximum output current. The following example illustrates a typical recommended design procedure. Specification Input: 12 VDC (max) 8.5 VDC (min) Output: 5.0 VDC 0.1A to 0.25A Temperature: -40 to +50°C It should be noted that diode IN4001 is required for discharging the 100uF capacitor when the system is turned off. THERMAL DESIGN Design of a Regulated Power Supply This regulated power supply uses two transistors, 2N3055 and 2N3904, the input of 12v to 25 is unregulated, and the output is +10V at the emitter of 2N3055. The description for this is that R1, which has a resistance of 1K holds Q1 on; when the output reaches 10 volts, Q2 goes into conduction (base at 5V), preventing further rise of output voltage by shunting base current from Q1’s base. The supply can be made adjustable by replacing R2 and R3 by a potentiometer. This is actually an example of negative feedback: Q2 “looks at” the output and does something about it if the output isn’t at the right voltage. Conclusion A regulated power supply answers to the needs of complex and sophisticated circuits common in appliances using ICs and nano technology. Nowadays circuits uses ICs which demand milli- and micro-voltages; the power supply has to be much complicated because small ICs or microcomputers need regulated DC voltages. We were used to linear regulated power supply. High tech and sophisticated circuits have been introduced into what we call nano technology using micro components. The present rule of the game is minutization or using minute materials such ICs that can fit even the smallest of gadgets. This technology would need a regulated power supply with the milli- or micro-volts output. References Brown, M., 2008. Power sources and supplies. Oxford, UK: Elsevier Inc. Crandall, E., 1997. Power supply testing handbook: strategic approaches in test cost reduction. United States of America: Chapman & Hall. Electronics Talk, 2010. DC/DC converter for 50W LED applications. Available from: http://www.electronicstalk.com/news/lne/lne654.html [Accessed 4 May 2010] Electronics Talk, 2010. Linear unveils 200mA PNP low-dropout regulator. Available from: http://mytalk.pro-talk.com/registration/index/ [Accessed 4 May 2010] 6. Appendices Appendix 1: Linear Technology has announced the LT3492, a 2.1MHz DC/DC converter designed to operate as a three-channel constant current LED driver. DC/DC converter for 50W LED applications SOURCE: Electronics Talk (http://www.electronicstalk.com/news/lne/lne654.html) Appendix 2: Linear unveils 200mA PNP low-dropout regulator The LT3082 200mA PNP low dropout regulator features a new architecture using a current reference and voltage follower. SOURCE: Electronics Talk (http://mytalk.pro-talk.com/registration/index/) Appendix 3: IC Datasheet Source: An IC Datasheet featuring LM324 Description The LM324 consists of four independent, high gain, internally frequency compensated operational amplifiers which were designed specifically to operate from a single power supply over a wide range of voltages. Operation from split power supplies the magnitude of the power supply voltage. Application areas include transducer amplifiers, DC gain blocks and all the conventional op amp circuits. Features •Wide range of supply voltages Low supply current drain independent of supply voltage Low input biasing current •Low input offset voltage and offset current •Input common-mode voltage range includes ground Differential input voltage range equal to the power supply voltage DC voltage gain 100 V/ mV Typ PIP - 14 SOP - 14 Package Internal Block Diagram SOURCE: www.alldatasheet.com Read More