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Fundamentals Of Power Supply Design Robert A. 32


Power electronics impact our lives in many ways - from new power circuits that extend battery life to voltage regulators that help manage and distribute energy more efficiently from the grid to the consumer. This five-part power fundamentals course covers an overview of power electronics, components, topologies and control theory.




Fundamentals of Power Supply Design Robert A. 32



Beware of using the nameplate. This is a legality rating and will usually give a much higher volt-ampere rating than the unit will ever draw. For example, consider a unit with a nameplate that reads 90 to 240 volts at 4 to 8 amps with a 500 W power supply. In the nameplate reading, the numbers are backward. The larger amperage goes with the lower voltage. If you assume a nominal 120 volts at 8 amps, you get 960 VA. A PF of 0.95 would yield 912 W. No power supply is that inefficient, and a power supply almost never runs at full power. Therefore, it is highly unlikely that this device will ever draw more than 500 W of power, but if you want to be really conservative, multiply by 1.1 and figure 550 W of input power.


The history of switching power supplies turns out to be pretty interesting. While most people view the power supply as a boring metal box, there's actually a lot of technological development behind it. There was, in fact, a revolution in power supplies in the late 1960s through the mid 1970s as switching power supplies took over from simple but inefficient linear power supplies, but this was a few years before the Apple II came out in 1977. The credit for this revolution should go to advances in semiconductor technology, specifically improvements in switching transistors, and then innovative ICs to control switching power supplies.[2]Some background on power suppliesIn a standard desktop computer, the power supply converts AC line voltage into DC, providing several carefully regulated low voltages at high currents. Power supplies can be built in a variety of ways, but linear and switching power supplies are the two techniques relevant to this discussion. (See the notes for more about obsolete technologies such as largemechanical motor-generator systems[3] and ferroresonant transformers[4][5].)A typical linear power supply uses a bulky power transformer to convert the 120V AC into a low AC voltage, converts this to low voltage DC with a diode bridge, and then uses a linear regulator to drop the voltage to the desired level. The linear regulator is an inexpensive easy-to-use transistor-based component that turns the excess voltage into waste heat to produce a stable output. Linear power supplies are almost trivial to design and build.[6] One big disadvantage however, is they typically waste about 50-65% of the power as heat,[7] often requiring large metal heat sinks or fans to get rid of the heat. The second disadvantage is they are large and heavy. On the plus side, the components (other than the transformer) in linear power supplies only need to handle low voltages and the output is very stable and noise-free.A switching power supply works on a very different principle: rapidly turning the power on and off, rather than turning excess power into heat. In a switching power supply, the AC line input is converted to high-voltage DC, and then the power supply switches the DC on and off thousands of times a second, carefully controlling the time of the switching so the output voltage averages out to the desired value. Theoretically, no power gets wasted, although in practice the efficiency will be 80%-90%. Switching power supplies are much more efficient, give off much less heat, and are much smaller and lighter than linear power supplies.The main disadvantage of a switching power supply is it is considerably more complex than a linear power supply and much harder to design.[8] In addition, it is much more demanding on the components, requiring transistors that can efficiently switch on and off at high speed under high power. The switches, inductors, and capacitors in a switching power supply can be arranged in several different arrangements (or topologies), with names such as Buck, Boost, Flyback, Forward, Push-Pull, Half Wave, and Full-Wave.[9]History of switching power supplies to 1977Switching power supply principles were known since the 1930s[6] and were being built out of discrete components in the 1950s.[10] In 1958, the IBM 704 computer used a primitive vacuum-tube based switching regulator.[11] The company Pioneer Magnetics started building switching power supplies in 1958[12] (and decades later made a key innovation in PC power supplies[13]). General Electric published an early switching power supply design in 1959.[14] In the 1960s the aerospace industry and NASA[15] were the main driving force behind switching power supply development, since the advantages of small size and high efficiency made up for the high cost.[16] For example, NASA used switching supplies for satellites[17][18] such as Telstar in 1962.[19]The computer industry started using switching power supplies in the late 1960s and they steadily grew in popularity.Examples include the PDP-11/20 minicomputer in 1969,[20] the Honeywell H316R in 1970,[21] and Hewlett-Packard's 2100A minicomputer in 1971.[22][23]By 1971, companies using switching regulators "read like a 'Who's Who' of the computer industry: IBM, Honeywell, Univac, DEC, Burroughs, and RCA, to name a few."[21]In 1974, HP used a switching power supply for the 21MX minicomputer,[24] Data General for the Nova 2/4,[25] Texas Instruments for the 960B,[26] and Interdata for their minicomputers.[27]In 1975, HP used an off-line switching power supply in the HP2640A display terminal,[28] Matsushita for their traffic control minicomputer,[29] and IBM for its typewriter-like Selectric Composer[29] and for the IBM 5100 portable computer.[30]By 1976, Data General was using switching supplies for half their systems, Hitachi and Ferranti were using them,[29] Hewlett-Packard's 9825A Desktop Computer[31] and 9815A Calculator[32] used them, and the decsystem 20[33] used a large switching power supply. By 1976, switching power supplies were showing up in living rooms, powering color television receivers.[34][35]Switching power supplies also became popular products for power supply manufacturers starting in the late 1960s.In 1967, RO Associates introduced the first 20Khz switching power supply product,[36] which they claim was also the first switching power supply to be commercially successful.[37] NEMIC started developing standardized switching power supplies in Japan in 1970.[38]By 1972, most power supply manufacturers were offering switching power supplies or were about to offer them.[5][39][40][41][42]HP sold a line of 300W switching power supplies in 1973,[43] and a compact 500W switching power supply[44] and a 110W fanless switching power supply[45] in 1975. By 1975, switching power supplies were 8% of the power supply market and growing rapidly, driven by improved components and the desire for smaller power supplies for products such as microcomputers.[46]Switching power supplies were featured in electronics magazines of this era, both in advertisements and articles.Electronic Design recommended switching power supplies in 1964 for better efficiency.[47]The October 1971 cover of Electronics World featured a 500W switching power supply and an article "The Switching Regulator Power Supply".A long article about power supplies in Computer Design in 1972 discussed switching power supplies in detail and the increasing use of switching power supplies in computers, although it mentions some companies were still skeptical about switching power supplies.[5]In 1973, Electronic Engineering featured a detailed article "Switching power supplies: why and how".[42]In 1976, the cover of Electronic Design[48] was titled "Suddenly it's easier to switch" describing the new switching power supply controller ICs,Electronics ran a long article on switching power supplies,[29]Powertec ran two-page ads on the advantages of their switching power supplies with the catchphrase "The big switch is to switchers",[49] and Byte magazine announced Boschert's switching power supplies for microcomputers.[50]A key developer of switching power supplies was Robert Boschert, who quit his job and started building power supplies on his kitchen table in 1970.[51] He focused on simplifying switching power supplies to make them cost-competitive with linear power supplies, and by 1974 he was producing low-cost power supplies in volume for printers,[51][52] which was followed by a low-cost 80W switching power supply in 1976.[50] By 1977 Boschert Inc had grown to a 650-person company[51] that producing power supplies for satellites and the F-14 fighter aircraft,[53] followed by power supplies for companies such as HP[54] and Sun.People often think of the present as a unique time for technology startups, but Boschert illustrates that kitchen-table startups were happening even 40 years ago.The advance of the switching power supply during the 1970s was largely driven by new components.[55]The voltage rating of switching transistors was often the limiting factor,[5] so the introduction of high voltage, high speed, high power transistors at a low cost in the late 1960s and early 1970s greatly increased the popularity of switching power supplies.[5][6][21][16]Transistor technology moved so fast that a 500W commercial power supply featured on the cover of Electronics World in 1971 couldn't have been built with the transistors of just 18 months earlier.[21]Once power transistors could handle hundreds of volts, power supplies could eliminate the heavy 60 Hz power transformer and run "off-line" directly from line voltage. Faster transistor switching speeds allowed more efficient and much smaller power supplies.The introduction of integrated circuits to control switching power supplies in 1976 is widely viewed as ushering in the age of switching power supplies by drastically simplifying them.[10][56]By the early 1970s, it was clear that a revolution was taking place. Power supply manufacturer Walt Hirschberg claimed in 1973 that "The revolution in power supply design now under way will not be complete until the 60-Hz transformer has been almost entirely replaced."[57] In 1977, an influential power supply book said that "switching regulators were viewed as in the process of revolutionizing the power supply industry".[58]The Apple II and its power supplyThe Apple II personal computer was introduced in 1977. One of its features was a compact, fanless switching power supply, which provided 38W of power at 5, 12, -5, and -12 volts. Holt's Apple II power supply uses a very simple design, with an off-line flyback converter topology.[59]Steve Jobs said that every computer now rips off Rod Holt's revolutionary design.[1] But is this design revolutionary? Was it ripped off by every other computer?As illustrated above, switching power supplies were in use by many computers by the time the Apple II was released. The design is not particularly revolutionary, as similar simple off-line flyback converters were being sold by Boschert[50][60] and other companies. In the long term, building the control circuitry out of discrete components as Apple did was a dead-end technology, since the future of switching power supplies was in PWM controller ICs.[2] It's surprising Apple continued using discrete oscillators in power supplies even through the Macintosh Classic, since IC controllers were introduced in 1975.[48] Apple did switch to IC controllers, for instance in the Performa[61] and iMacs.[62]The power supply that Rod Holt designed for Apple was innovative enough to get a patent,[63] so I examined the patent in detail to see if there were any less-obvious revolutionary features. The patent describes two mechanisms to protect the power supply against faults. The first (claim 1) is a mechanism to safely start the oscillator through an AC input. The second mechanism (claim 8) returns excess energy from the transformer to the power source (especially if there is no load) through a clamp winding on the transformer and a diode.This is the AA11040-B power supply for the Apple II Plus.[59] AC power enters, on the left, is filtered, goes through the large switching transistor to the flyback transformer in the middle, is rectified by the diodes to the right (on heatsinks), and then is filtered by the capacitors on the right. The control circuitry is along the bottom. Photo used by permission from kjfloop, Copyright 2007.The AC start mechanism was not used by the Apple II,[59] but was used by the Apple II Plus,[64] Apple III,[65] Lisa,[66] Macintosh,[67] and Mac 128K through Classic.[68] I could not find any non-Apple power supplies that use this mechanism,[69] except for a 1978 TV power supply,[70] and it became obsoleted by IC controllers, so this mechanism seems to have had no impact on computer power supply design.The second mechanism in Holt's patent, the clamp winding and diode to return power in a flyback converter, was used in a variety of power supplies until the mid-1980s and then disappeared.Some examples are the Boschert OL25 power supply (1978),[60]Apple III (1980),[65]Apple's power supply documentation (1982),[59]Tandy hard drive (1982),[71]Tandy 2000 (1983),[72][73]Apple Lisa (1983),[66]Apple Macintosh (1984),[67]Commodore Model B128 (1984),[74]Tandy 6000 (1985),[75]andMac Plus (1986) to Mac Classic (1990).[68]This flyback clamp winding seems to have been popular with Motorola in the 1980s, appearing in the MC34060 controller IC datasheet,[76] a 1983 designer's guide[77] (where the winding was described as common but optional), and a 1984 application note.[78]Is this flyback clamp winding the innovation of Holt's that other companies ripped off? I thought so, until I found a 1976 power supply book that described this winding in detail,[35] which ruined my narrative. (Also note that forward converters (as opposed to flyback converters) had used this clamp winding dating back to 1956,[79][80][81] so applying it to a flyback converter doesn't seem like a huge leap in any case.)One puzzling aspect of power supply discussion in the book Steve Jobs[1] is the statement that the Apple II's power supply is "like those used in oscilloscopes", since oscilloscopes are just one small use for switching power supplies. This statement apparently arose because Holt had previously designed a switching power supply for oscilloscopes,[82] but there's no other connection between Apple's power supply and oscilloscope power supplies.The biggest impact of the Apple II on the power supply industry was on Astec - the Hong Kong company that manufactured the power supply. Before the Apple II came out, Astec was a little-known manufacturer, selling switching DC-DC inverters. But by 1982, Astec had become the world's top switching-powers-supply manufacturer, almost entirely based on Apple's business, and kept the top spot for a number of years.[83][84] In 1999, Astec was acquired by Emerson,[85] which is currently the second-largest power supply company after Delta Electronics.[86]A little-known fact about the Apple II power supply is that it was originally assembled by middle-class California housewives as piecework.[83] As demand grew, however, power supply construction was transferred to Astec, even though it cost $7 a unit more. Astec was building 30,000 Apple power supplies monthly by 1983.[83]Power supplies post-AppleIn 1981, the IBM PC was launched, which would have lasting impact on computer power supply designs. The power supply for the original IBM 5150 PC was produced by Astec and Zenith.[83] This 63.5W power supply used a flyback design controlled by a NE5560 power supply controller IC.[87]I will compare the IBM 5150 PC power supply with the Apple II power supply in detail to show their commonalities and differences. They are both off-line flyback power supplies with multiple outputs, but that's about all they have in common. Even though the PC power supply uses an IC controller and the Apple II uses discrete components, the PC power supply uses approximately twice as many components as the Apple II power supply. While the Apple II power supply uses a variable frequency oscillator built out of transistors, the PC power supply uses a fixed-frequency PWM oscillator provided by the NE5560 controller IC. The PC uses optoisolators to provide voltage feedback to the controller, while the Apple II uses a small transformer. The Apple II drives the power transistor directly, while the PC uses a drive transformer. The PC checks all four power outputs against lower and upper voltage limits to make sure the power is good, and shuts down the controller if any voltages are out of spec. The Apple II instead uses a SCR crowbar across the 12V output if that voltage is too high. While the PC flyback transformer has a single primary winding, the Apple II uses an extra primary clamp winding to return power, as well as another primary winding for feedback. The PC provides linear regulation on the 12V and -5V supplies, while the Apple II doesn't. The PC uses a fan, while the Apple II famously doesn't. It's clear that the IBM 5150 power supply does not "rip off" the Apple II power supply design, as they have almost nothing in common. And later power supply designs became even more different.The IBM PC AT power supply became a de facto standard for computer power supplies. In 1995, Intel introduced the ATX motherboard specification,[88] and the ATX power supply (along with variants) has become the standard for desktop computer power supplies, with components and designs often targeted specifically at the ATX market.[89]Computer power systems became more complicated with the introduction of the voltage regulator module (VRM) in 1995 for the Pentium Pro, which required lower voltage at higher current than the power supply could provide directly. To supply this power, Intel introduced the VRM - a DC-DC switching regulator installed next to the processor that reduces the 12 volts from the power supply to the low voltage used by the processor.[90] (If you overclock your computer, it is the VRM that lets you raise the voltage.) In addition, graphics cards can have their own VRM to power a high-performance graphics chip. A fast processor can require 130 watts from the VRM. Comparing this to the half watt of power used by the Apple II's 6502 processor[91] shows the huge growth in power consumption by modern processors. A modern processor chip alone can use more than twice the power of the whole IBM 5150 PC or three times that of the Apple II.The amazing growth of the computer industry has caused the power consumption of computers to be a cause for environmental concern, resulting in initiatives and regulations to make power supplies more efficient.[92] In the US, Energy Star and 80 PLUS certification[93] pushes manufacturers to manufacture more efficient "green" power supplies.These power supplies squeeze out more efficiency through a variety of techniques: more efficient standby power, more efficient startup circuits, resonant circuits (also known as soft-switching and ZCT or ZVT) that reduce power losses in the switching transistors by ensuring that no power is flowing through them when they turn off, and "active clamp" circuits to replace switching diodes with more efficient transistor circuits.[94] Improvements in MOSFET transistor and high-voltage silicon rectifier technology in the past decade has also led to efficiency improvements.[92]Power supplies can use the AC line power more efficiently through the technique of power factor correction (PFC).[95] Active power factor correction adds another switching circuit in front of the main power supply circuit. A special PFC controller IC switches this at a frequency of up to 250kHZ, carefully extracting a smooth amount of power from the power s


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