Power Supply Essay

August 15, 2017 General Studies

Aimed at system interior decorators whose involvement focal points on other Fieldss. this note reviews the basic power supply design knowhow assumed in the remainder of the book. In mains-supplied electronic systems the AC input electromotive force must be converted into a DC electromotive force with the right value and grade of stabilisation. Figures 1 and 2 show the simplest rectifier circuits. In these basic constellations the peak electromotive force across the burden is equal to the peak value of the AC electromotive force supplied by the transformer’s secondary twist. For most applications the end product rippling produced by these circuits is excessively high.

However. for some applications – driving little motors or lamps. for illustration – they are satisfactory. If a filter capacitance is added after the rectifier diodes the end product electromotive force wave form is improved well. Figures 3 and 4 show two authoritative circuits normally used to obtain uninterrupted electromotive forces get downing from an alternating electromotive force. The Figure 3 circuit uses a center-tapped transformer with two rectifier rectifying tubes while the Figure 4 circuit uses a simple transformer and four rectifier rectifying tubes. Figure 1: Basic Half Wave Rectifier Circuit. Figure 3: Full Wave Rectified Output From the Transformer/rectifier Combination is filtered by C1.

Figure 4: This Circuit Performs Identically to that Shown in Figure 3.

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Figure 2: Full Wave Rectifier Wich uses a Center-tapped Transformer.

Figure 5 shows the uninterrupted electromotive force curve obtained by adding a filter capacitance to the Figure 1 circuit. The subdivision b-c is a consecutive line. During this clip it is the filter capacitance that supplies the burden current. The incline of this line increases as the current additions. conveying point degree Celsiuss lower. Consequently the rectifying tube conductivity clip ( c-d ) additions. increasing rippling. With zero burden current the DC end product electromotive force is equal to the peak value of the rectified AC electromotive force. Figure 6 shows how to obtain positive and negative end products referred to a common land. Useful design informations for this circuit is given in figures 7. 8 and 9. In peculiar. the curves shown in Figure 7 are helpful in finding the electromotive force rippling for a given burden current and filter capacitance value. The value of the electromotive force rippling obtained is straight relative to the burden current and inversely relative to the filter capacitance value.



Figure 5: End product Wave forms from the Half-wave Rectifier Filter. Figure 8: District of columbia to Peak Ratio for Half Wave rectifiers.

Figure 6: Full-wave Split Supply Rectifier.

Figure 9: District of columbia to Peak Ratio for Full-wave Rectifiers.

Figure 7: Ripple Voltage vs. Filter Capacitor Value ( full-wave Rectifier ) .

Figure 10: DC Characteristics of a 50 VA Nonregulated Supply.


Mains ( 220V ) +20 % +15 % +10 % –10 % –15 % –20 % Secondary Voltage 28. 8V 27. 6V 26. 4V 24V 21. 6V 20. 4V 19. 2V DC Output Voltage ( VO ) IO = 0 43. 2V 41. 4V 39. 6V 36. 2V 32. 4V 30. 6V 28. 8V IO = 0. 1A 42V 40. 3V 38. 5V 35V 31. 5V 29. 8V 28V IO = 1A 37. 5V 35. 8V 34. 2V 31V 27. 8V 26V 24. 3V

thermic protection maps. Figures 16. 17 and 18 show how these circuits are used. Mention to the datasheets for more information. Figure 11: Ripple Reduction Produced by a Single Section Inductance-capacitance Filter.

The public presentation of a supply normally used in consumer applications – in audio amplifiers. for illustration – is described in figure 10 and table 1. When a low rippling electromotive force is required an LC filter web may be used. The consequence on the end product electromotive force of this add-on is shown in figure 11. As figure 11 shows. the residuary rippling can be reduced by 40 dubnium. But frequently the inductance is dearly-won and bulky. Often the grade of stableness provided by the circuits described above is deficient and a stabilizer circuit is needed. Figure 12 shows the simplest solution and is satisfactory for tonss of up to about 50mA. This circuit is frequently used as a mention electromotive force to use to the base of a transistor of to the input of an op A to obtain higher end product current. The simplest illustration of a series regulator is shown in Figure 13. In this circuit the transistor is connected as a electromotive force follower and the end product electromotive force is about 600 – 700mV lower than the zener electromotive force.

The resistance R must be dimensioned so that the zener is right biased and that sufficient base current is supplied to the base of Q1. For high burden currents the basal current of Q1 is no longer negligible. To avoid that the current in the zener drops to the point where effectual ordinance is non possible a darlington may be used in topographic point of the transistor. When better public presentation is required the op A circuit shown in Figure 14 is recommended. In this circuit the end product electromotive force is equal to the mention electromotive force applied to the input of the op A. With a suited end product buffer higher currents can be obtained.

The end product electromotive force of the Figure 14 circuit can be varied by adding a variable splitter in analogue with the zener rectifying tube and with its wiper connected to the op amp’s input. The design of stabilised supplies has been simplified dramatically by the debut of electromotive force regulator ICs such as the L78xx and L79xx three-terminal series regulators which provide a really stable end product and include current clipper and

Figure 12: Basic Zener Regulator Circuit.

Figure 13: The Series Pass Zener-based Regulator Circuit can Supply Load Currents up to about 100mA.


Figure 14: The Op-amp-based Regulator can Supply 100mA with Excellent Regulation.

Figure 15: Zener Regulator Circuit Modified for Low-noise Output.

Figure 16: A Three Terminal 1A Positive Regulator Circuit is really Simple and Performs really Well.


Figure 17: A Three Terminal 1A Negative Voltage Regulator.

Figure 18: Complete ± 12V – 1A Split Supply Regulator Circuit.



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This publication supersedes and replaces all information antecedently supplied. SGS-THOMSON Microelectronicss merchandises are non authorized for usage as critical constituents in life support devices or systems without express written blessing of SGS-THOMSON Microelectronicss.

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