Apple didn't revolutionize power supplies; new transistors did

The new biography Steve Jobs contains a remarkable claim about the power supply of the Apple II and its designer Rod Holt:[1] Instead of a conventional linear power supply, Holt built one like those used in oscilloscopes. It switched the power on and off not sixty times per second, but thousands of times; this allowed it to store the power for far less time, and thus throw off less heat. "That switching power supply was as revolutionary as the Apple II logic board was," Jobs later said. "Rod doesn't get a lot of credit for this in the history books but he should. Every computer now uses switching power supplies, and they all rip off Rod Holt's design." I found it amazing to think that computers now use power supplies based on the Apple II's design, so I did some investigation. It turns out that Apple's power supply was not revolutionary, either in the concept of using a switching power supply for computers or in the specific design of the power supply. Modern computer power supplies are totally different and do not rip off anything from Rod Holt's design. It turns out that Steve Jobs was making his customary claim that everyone is stealing Apple's revolutionary technology, entirely contrary to the facts. 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 supplies In 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 large mechanical 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 1977 Switching 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 supply The 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.