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# Linear power supply basics tutorial

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### Linear power supply basics tutorial

1. 1. Linear Power Supply Basics Tutorial - summary or tutorial about the basics of linear power supplies, their design, operation and advantages and disadvantages. In this section • Linear power supply basics • Shunt voltage regulator • Series voltage regulator Linear power supplies are widely used because of the advantages they offer in terms of overall performance. Linear power supplies are often used in exacting situations where the regulation and removal of noise is paramount. While linear power supplies may not be as efficient as other types of power supply technology, they offer the best performance and are therefore used in many applications where noise is of great importance. Often audio amplifiers and many other items of electronic equipment use linear power supplies to obtain the best performance. Linear power supply basics Linear power supplies gain their name from the fact that they use linear, i.e. non-switching techniques to regulate the voltage output from the power supply. The term linear power supply implies that the power supply is regulated to provide the correct voltage at the output. Sometimes the sensing of the voltage may be accomplished at the output terminals, or on some occasions it may be achieved directly at the load. In terms of the overall make-up of a linear power supply, it can be split into several blocks as detailed below. Block diagram of linear power supply architecture The main elements of the linear power supply are: • Input transformer: As many power supplies take their source power from an AC mains input, it is common for linear power supplies to have a step down or occasionally a step up transformer. This also serves to isolate the power supply from the mains input for safety. • Rectifier: As the input from an AC supply is alternating, this needs to be converted to a DC format. Various forms of rectifier circuit are available. Note on Diode Rectifiers: Diode rectifiers are an essential element within many circuits including power supplies and RF detectors. They convert the AC signal into DC. Various forms are available from single diode half wave rectifiers to multiple diode full-wave rectifiers. The type chose will depend upon the application and its requirements. Click for more information on Diode Rectifiers Even for DC powered regulators, a rectifier may be placed at the input to guard against inverse connection of the supply. • Smoothing: Once rectified from an AC signal, the DC needs to be smoothed to remove the varying voltage level. Large reservoir capacitors are used for this.
4. 4. It should also be noted that for the shunt regulator circuit, the series resistance is comprised of the series resistor value, plus any source resistance. In most cases the value of the series resistor will dominate and the source resistance can be ignored, but this may not always be the case. Shunt regulator with feedback loop The basic shunt voltage regulator above does not have any feedback, i.e. it runs in an open loop manner. As imagined, the performance of this form of shunt regulator is sufficient for many applications, but much higher levels of performance can be achieved by providing feedback based on the output voltage of the shunt voltage regulator and feeding this back into the system to ensure that the required output voltage is accurately maintained. Block diagram of shunt voltage regulator with feedback Using a shunt voltage regulator with feedback as shown above, the output voltage is sensed and the voltage compared to a reference. The level of the shunt current is then altered to return the output voltage to the required level. Full shunt regulators are not widely used because shunt regulators offer a low efficiency level. Series regulators are widely available and offer higher efficiency levels, although they are not as high as switch mode supplies. By Ian Poole Series Voltage Regulator (Series Pass Regulator) - overview and theory of the series regulator or series pass regulator used in many linear power supply circuits. In this section • Linear power supply basics • Shunt voltage regulator • Series voltage regulator The series voltage regulator format or as it is sometimes called the series pass regulator is the most commonly used format for providing the final voltage regulation in a linear voltage regulator circuit. As the name suggests, the series voltage regulator or series pass voltage regulator operates by using a variable element in series with the load. In this way a series voltage regulator provides an effective form of voltage regulation within a linear power supply.
5. 5. Series voltage regulator basics The series voltage regulator or series pass voltage regulator uses a variable element placed in series with the load. By changing the resistance of the series element, the voltage dropped across it can be varied to ensure that the voltage across the load remains constant. The advantage of the series voltage regulator is that the amount of current drawn is effectively that used by the load, although some will be consumed by any circuitry associated with the regulator. Unlike the shunt regulator, the series regulator does not draw the full current even when the load does not require any current. As a result the series regulator is considerably more efficient. Concept of the series voltage regulator / series pass regulator Simple emitter follower voltage regulator One of the simplest implementations of this concept is to use a single pass transistor in the form of an emitter follower configuration, and a single Zener diode drive by a resistor from the unregulated supply. This provides a simple form of feedback system to ensure the Zener voltage is maintained at the output, albeit with a voltage reduction equal to the base emitter junction voltage - 0.6 volts for a silicon transistor It is a simple matter to design a series pass voltage regulator circuit like this. Knowing the maximum current required by the load, it is possible to calculate the maximum emitter current. This is achieved by dividing the load current, i.e. transistor emitter current by the Β or hfe of the transistor. Simple emitter follower series pass regulator The Zener diode will generally need a minimum of around 10mA for a small Zener to keep its regulated voltage. The resistor should then be calculated to provide the base drive current and the minimum Zener current from a knowledge of the unregulated voltage, Zener voltage and the current required. [ (Unregulated voltage - Zener voltage ) / current ]. A small margin should be added to the current to ensure that there is sufficient room for margin when the load, and hence the transistor base is taking the full current.
6. 6. The power dissipation capacity for the Zener diode should be calculated for the case when the load current, and hence the base current is zero. In this case the Zener diode will need to take the full current passed by the series resistor. Series pass regulator with feedback In order to provide improved levels of performance it is possible to add a more sophisticated feedback network into the regulator circuit. Using feedback within a voltage regulator enables the output to be sampled, and compared with a stable reference voltage. The error is then used to correct the output voltage. In this way, a far higher level of performance can be obtained in terms of the required output voltage as well as ripple and spikes. Series pass voltage regulator with feedback It is possible to use a simple two transistor circuit for a series pass regulator with voltage sensing and feedback. Although it is quite straightforward to use an operational amplifier, which will provide higher levels of feedback, and hence better regulation, this two transistor circuit illustrates the principles well. Simple two transistor series pass regulator In this circuit TR1 forms the series pass transistor. The second transistor, TR2 acts as the comparator, feeding the error voltage between the reference diode and the sensed output voltage which is a proportion of the output voltage as set by the potentiometer. The resistor, R1 provides the current for the collector of TR2 and the reference diode ZD1. Voltage reference Any linear voltage regulator can only be as good as the voltage reference that is used as the basis of the comparison within the system. While a battery could in theory be used, this is not satisfactory for most applications. Instead Zener diode based references are almost universally used. Integrated circuit regulators and references use sophisticated on-chip combinations of transistors and resistors to obtain temperature compensated and precise voltage reference sources.
7. 7. The voltage reference must be driven from the unregulated supply. It cannot be taken from the regulated output as there are start-up issues. At start-up there is no output and therefore the reference output will be zero and this will be maintained until the reference starts-up. Simplified reference source for series pass voltage regulator Often the output from the reference source is fed via a potential divider. Not only does this reduce the output voltage which is normally very useful, but it also allows a capacitor to be added to the output to help remove any ripple or noise that may be present. The reduced voltage is also useful because the minimum voltage output is governed by the reference voltage. Output sampling The simple emitter follower series voltage regulator circuit directly compared the output with the voltage reference. In this way the output voltage was equal to that of the reference, neglecting the base emitter voltage drop. However it is possible to sample a proportion of the output voltage and compare this to the reference. If this is done, then the output voltage becomes greater than the reference voltage as the negative feedback in the circuit fights to keep the two compared voltages the same. If for example the reference voltage is 5 volts and the sampling or potential divider provides 50% of the output voltage, then the output voltage will be maintained at 10 volts. Series pass voltage regulator with sampled output The potential division or sampling can be made variable, and in this way, the output voltage can be adjusted to the required value. Normally this method is only used for small adjustments as the minimum output level obtained by this method is an output equal to the reference voltage. It should be remembered that using a potential divider has the effect of reducing the feedback loop gain. This has the effect of reducing the loop gain and thereby reducing the regulation performance. Normally there is sufficient loop gain for this not to be a major problem except when only a very small proportion of the output is sampled. Care should also be taken not to increase the voltage of the output to a point where the regulator does not have sufficient drop across it to regulate the output voltage sufficiently. Low drop out series voltage regulators One of the considerations of any regulator is the voltage that must be placed across the series pass element. In addition to the drop across the regulator itself, there must be sufficient voltage to run the drive circuitry. In some circuits, a low drop out regulator is important - i.e. where the
8. 8. level of voltage drop available across the series regulator element is limited. This minimum drop out voltage can be important and is often a specified parameter in many integrated regulator chips. While the circuits shown here are simple transistor circuits, the same principles are used in larger circuits and also within integrated circuits. The same series pass regulator concepts as well as the reference diode circuits, sampling and other areas all use the same elements. By Ian Poole Capacitor Smoothing Circuits & Calculations - notes, details & calculations for smoothing capacitor circuits used with rectifiers with details of ripple voltage and ripple current. In this section • Diode rectifier circuits • Half wave rectifier circuit • Full wave rectifier circuit • Two diode full wave rectifier • Bridge rectifier circuit • Rectifier capacitor smoothing circuit Rectifiers are normally used in circuits that require a steady voltage to be supplied. To provide a steady DC output. The raw rectified DC requires a smoothing capacitor circuit to enable the rectified DC to be smoothed so that it can be used to power electronics circuits without large levels of voltage variation. Capacitor smoothing basics The raw DC supplied by a rectifier on its own would consist of a series of half sine waves with the voltage varying between zero and √2 times the RMS voltage (ignoring any diode and other losses). A supply of this nature would not be of any use for powering circuits because any analogue circuits would have the huge level of ripple superimposed on the output, and any digital circuits would not function because the power would be removed every half cycle. To smooth the output of the rectifier a reservoir capacitor is used - placed across the output of the reciter and in parallel with the load.. This capacitor charges up when the voltage from the rectifier rises above that of the capacitor and then as the rectifier voltage falls, the capacitor provides the required current from its stored charge. Smoothing action of a reservoir capacitor It should be remembered that the only way discharge path for the capacitor, apart from internal leakage is through the load to the rectifier / smoothing system. The diodes prevent backflow through the transformer, etc.. Smoothing capacitor value The choice of the capacitor value needs to fulfil a number of requirements. In the first case the value must be chosen so that its time constant is very much longer than the time interval between the successive peaks of the rectified waveform:
10. 10. • Capacitor charging current: On the charge cycle of the smoothing capacitor, the capacitor needs to replace all the lost charge, but it can only achieve this when the voltage from the rectifier exceeds that from the smoothing capacitor. This only occurs over a short period of the cycle. Consequently the current during this period is much higher. The large the capacitor, the better it reduces the ripple and the shorter the charge period. In view of the large currents involved, care must be taken to ensure that he ripple current does not exceed the rated values for the capacitor. By Ian Poole 1. linear power supply Presentation Transcript • A presentation of eSyst.org • LINEAR Block diagram and functions of a transformer, rectifier, POWER SUPPLY Types of rectifier, filter, voltage regulator and voltage divider. filter and regulator circuits A presentation of eSyst.org • Power For All electronic circuits need a power source to work. Supply electronic circuits made up of transistors and/or ICs, this power source must be a DC voltage of a specific A battery is a common value. DC voltage source for some types of electronic equipment especially portables like cell Most non-portable equipment uses phones and iPods. power supplies that operate from the AC power line but produce one or more DC outputs. A presentation of eSyst.org • Power The input is the 120 volt 60 Hz AC Supply Characteristics power The power supply converts the AC line. into DC and provides one or more Some modern electronic circuits DC output voltages. need two or A good example of a modern power more different voltages. supply is the one inside a PC that furnishes 12, 5, 3.3 and 1.2 volts. A presentation of eSyst.org • Main circuits in most power supplies. Components of a Power Supply A presentation of eSyst.org