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61 . 50V 5A LAB POWER SUPPLY REGULATOR t 70 ‘3r I dn 40 DROPOUT POWER 4 eta. sis \ VOLTS LOAD ADJUST e QNO 7 NIP Our new switchmode supply has adjustable voltage output from 3 to 50V and a maximum 5A output at voltages below 35V. averages out the waveform from the collector of Q to give a DC voltage across RL. The magnitude of this voltage is given by the equation, V, = Vcc x where t is the period when Q is conducting and T is the total period of the waveform applied to the base of Q. For an ideal power supply, this output voltage is independent of the output current. This is another way of saying that the power supply has zero output impedance. From the above formula we can see that any factors which would tend to cause changes in the output voltage can be compensated for by adjusting the on – time (or duty cycle) of Q. To do this we need a control circuit which can modulate the pulse width of the signal applied to the base of Q while monitoring the output voltage for any changes. This is then referred to as "pulse width modulation". The reason for the high efficiency of the switchmode regulator is that the pass transistor, which is the principal source of losses, is operated at its two most efficient points. At cutoff, there is a large voltage across the transistor but little current through it. Conversely, at saturation there is little voltage across the transistor but a large current through it. Either way, very little power is dissipated in the switching transistor so efficiency is high. And heatsinks can either be small or, in some cases, dispensed with entirely. Basic PWM circuit Now refer to Fig. 2 which illustrates the basic components of a PWM circuit and how they function together. There is an RC oscillator which produces a sawtooth output, an error amplifier which monitors the output voltage and a comparator which actually produces the square wave pulse train which is fed to RT RAMP OSCILLATOR OUTPUT utptput PWM COMPARATOR OUTPUT OSCILLATOR //// PWM COMPARATOR 4 ERROR AMPLIFIER o OFF the base of switching transistor Q. OK. Now the RC oscillator produces a sawtooth waveform with its period defined by RT and CT. Not that surprising really, is it? In our power supply, the operating frequency is just beyond the limit of audibility, at around 20kHz. But the actual frequency is not important. The error amplifier performs the same function as the error amplifier in any linear regulator circuit. It compares a proportion of the power supply output voltage against a very stable reference voltage. The output of the error amplifier is then essentially a constant voltage on which is superimposed an amplified ver- [NUR_ ERROR AMPLIFIER OUTPUT O ON Fig. 2 FROM REGULATOR OUTPUT REFERENCE IVOLTAGE ot sion of the small fluctuations in the power supply output. In other words, as far as we are concerned, the output of the error amplifier is essentially a constant DC voltage. This voltage from the error amplifier is compared to the sawtooth voltage from the oscillator by the PWM comparator. When the sawtooth voltage is the higher, the PWM comparator will have a high output and Q will be turned on. When the error amplifier voltage is the higher the PWM comparator output is low and Q is turned off. By looking at the timing diagram which is part of Fig. 2, this operation can be fur – ELECTRONICS Australia, May,
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103 50 & 25 YEARS AGO "Electronics Australia" is bne of the longest running technical publications In the world. We started as "Wireless Weekly" in August 922 and became "Radio and Hobbies In Australia" in April 939. The title was changed to "Radio, Television and Hobbies" In February 955 and finally, to "Electronics Australia" in April 965. Below we feature some items from past Issues. May 933 Progress of Television: "Television in the United States," said Mr Scott (of the Scott Radio Laboratories, Chicago, USA, and who is currently visiting Sydney), "is a farce. I do not believe that any advancement in technique during the past 2 months has been made, and it does not appear that television will ever be brought to the necessary state of perfection to make ordinary radio receivers obsolete. Even if television becomes an accomplished fact, the broadcast receiver will be necessary for the sound, and the television equipment will be in the form of an accessory". ú r xy New Circuit: The usual monthly meeting of the Institution of Radio Engineers of Australia was held at Science House, Grosvenor Street, on Monday, April 24. It heard a short talk by Mr A. G. Hull, Technical Editor of "Wireless Weekly", on the subject of a novel and interesting circuit, with which he has been experimenting for the past couple of weeks. Describing his circuit, Mr Hull said. he did not wish to claim that he had invented the circuit, but merely brought the circuit before the meeting on account of its interesting nature and possibilities. Mr Hull then went on to fully explain the operation of the circuit, and mentioned its many advantages over existing audio amplifiers. (Editorial note: The circuit was that of a simple phase splitter, permitting push-pull operation without the need for a transformer. It was to become quite famous and remain popular until the end of the valve era.) Shades of things to come?: Japanese radio manufacturers are looking for foreign markets, and are going to sell sets in Singapore at from 35/- to E4 each, in competition with American sets at about 50 each, or with, shall we say, Australian sets at what price? Copyright problems: The English Postmaster -General told the English "Newspaper World" that he did not approve the connection of dictaphones to loud -speakers for taking down radio talks. But it is understood that there is no objection to listeners taking down talks in this manner, provided they make no public use of them. RADIO TELEVISION AND HOBBIES May 958 Anti -gravity Research: A new international race going on behind the scientific scenes of Britain, Russia and America may yield a substance that falls upwards. Recent discoveries in nuclear research indicate that the existence of such a substance is not as impossible as it sounds. The first nation to produce it will almost certainly win the race into space, for a material which overcomes gravity will overcome the biggest problem facing rocket designers. Since ancient times scientists have been aware of the laws of gravitation – the mutual attraction of masses toward each other. But no one has ever been able to explain why it works. But research work in nuclear physics has yielded sufficient intriguing facts to convince some scientists that they may be able not only to control gravity, but to reverse it. One line of research into the problem follows up the field theory of relativity which Einstein was working on when he died. The theory linked gravity closely with space and time. Now other scientists are examining thé theory. They hope that by transposing space-time values, they may produce new values of gravity. In this fantastic search, success would mean a material with negative weight. It could mean a spaceship which would leave the earth without so much as a child’s cracker under it. Stereo Records: Progress towards the stereophonic disc is proceeding rapidly, and reports indicate that already records of this type are available in limited quantities in the USA. Major companies are planning for releases later this year, but even now equipment is coming on the market. The stereo future seems likely to be a major event. Of special interest (is) the new stereo type record cutting head which, developed by Westrex in the USA, seems certain to be used for making the industry’s newest contribution to the art of recorded sound. It now seems quite certain that this type of head (45-45 ) will be used in preference to the vertical -lateral type, mainly because it is much easier to obtain good balance between the channels. Atomic power for NSW?: Atomic power stations would not be built in New South Wales for 0 years at least, according to Mr Conde, Electricity Commission Chairman. Adequate supplies of relatively cheap black coal were available for the commission’s thermal stations. He added: "I cannot see where we will have a nuclear station for at least 0 years. "It probably will be longer. "Tremendous advances have been made in the field of commercial power from atomic energy." 2.4 ELECTRONICS Australia, May,
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62 50V/5A laboratory power supply BASIC PRINCIPLES RT a 0.V DEAD 4 `+ TIME – CONTROL 0.6mA PIN 5 CAPACITOR CT PIN 3 FEEDBACKIPWM COMPARATOR PIN a DEAD TIME CONTROL Fig. 3 DEAD TIME COMPARATOR 3 COMPENSA TIONfPWM COMPARATOR INPUT WAVEFORM AT POINT A’ FLIP-FLOP T INPUT FLIP-FLOP O FLIPFLOP O OUTPUT 0 EMITTER OUTPUT 02 EMITTER OUTPUT CONTROL GND : PARALLEL OPERATION HIGH : ALTERNATE OPERATION ERROR AMPLIFIER ERROR AMPLIFIER ther elucidated. Note that if the error amplifier output was higher than shown in the timing diagram, the intersection times of the two waveforms would be shorter and hence the pulses from the comparator output would be correspondingly shorter (although still having the same repetition rate, ie, 20kHz). By the same process, if the error amplifier output was lower, the intersection times of the two waveforms would be longer and the comparator’s output PULSE STEERING FLIP-FLOP T CONTROL 3 O O 5V REFERENCE -7/////77Y 60 ELECTRONICS Australia, May, 983 L_ J 2 VCC VREF T GND.Cl C2 02 0E2 pulses would also be longer. Well, that essentially describes how a pulse -width modulation circuit works. Such a circuit could employ a number of discrete semiconductors at the simplest level, or in the most refined versions use a specially designed IC. Such an IC is the Fairchild µa494 switchmode regulator which is the heart of our new power supply. Fig. 3 is a full block and timing diágram for the internal circuit of the µa494. It differs from the simplified diagram in several ways. Firstly, components for an extra mode of operation have been added so that the µa494 may be used in push-pull or bridge type circuits with transformer coupled outputs. These components include an extra transistor, logic circuitry and an extra comparator (called the dead time comparator). Secondly, instead of one there are two error amplifiers provided. One error amplifier is used for output voltage control in the same manner as in the simplified diagram of Fig. 2 while the other error amplifier is usually used to provide current limiting. To do this the voltage drop across a small resistor in series with the load is measured. If the voltage drop exceeds a preset value (indicating excessive current) then the output of the error amplifier rises and reduces the output transistor on time so that the output current is kept to a safe level. Logic circuitry The logic circuitry is arranged so that the mode of operation of the output transistors can be selected by the appropriate voltage level applied to the output control, pin 3. A high level on the output control and the output transistors turn on and off alternately in sympathy with the flipflop outputs. A low level (or ground) applied to the output control causes both output transistors to operate in parallel and ignore the flipflop outputs. With the output control grounded, the state of the output transistors at any time is dependent upon the output state of the dead time and PWM comparators. Both comparators high and the output transistors are on, either or both comparators low and the output transistors are off. The inputs to the comparators differ from the simplified diagram in that there are DC offset voltages applied. To gain an understanding of the operation of the comparators assume firstly that the dead time control (pin 4) is connected to ground. The 0.V offset which is now applied to the inverting (-) terminal of the dead time comparator means that for the comparator output to go high, the voltage (ramp) across CT must be greater than 0.V. Since CT is discharged below this voltage at the beginning of each ramp, there will always be a short interval at the beginning of each cycle when the output of the comparator is low and hence the output transistors are off. This is called "dead time" and is required to preclude the possibility of both output transistors being on together when con –