A switched-mode power supply (SMPS), also called switching-mode power supply, switch-mode power supply, switched power supply, or simply switcher, is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, a SMPS transfers power from a DC or AC source (often mains power, see AC adapter) to DC loads, such as a personal computer, while converting voltage and current characteristics. Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high-dissipation transitions, minimizing wasted energy. Voltage regulation is achieved by varying the ratio of on-to-off time (also known as duty cycle). In contrast, a linear power supply regulates the output voltage by continually dissipating power in the pass transistor. The switched-mode power supply's higher electrical efficiency is an important advantage. Switched-mode power supplies can also be substantially smaller and lighter than a linear supply because the transformer can be much smaller. This is because it operates at a high switching frequency which ranges from several hundred kHz to several MHz in contrast to the 50 or 60 Hz mains frequency used by the transformer in a linear power supply. Despite the reduced transformer size, the power supply topology and electromagnetic compatibility requirements in commercial designs result in a usually much greater component count and corresponding circuit complexity. Switching regulators are used as replacements for linear regulators when higher efficiency, smaller size or lighter weight is required. They are, however, more complicated; switching currents can cause electrical noise problems if not carefully suppressed, and simple designs may have a poor power factor.
History 1836 Induction coils use switches to generate high voltages. 1910 An inductive discharge ignition system invented by Charles F. Kettering and his company Dayton Engineering Laboratories Company (Delco) goes into production for Cadillac. The Kettering ignition system is a mechanically switched version of a flyback boost converter; the transformer is the ignition coil. Variations of this ignition system were used in all non-diesel internal combustion engines until the 1960s when it began to be replaced first by solid-state electronically switched versions, then capacitive discharge ignition systems. 1926 On 23 June, British inventor Philip Ray Coursey applies for a patent in his country and United States, for his "Electrical Condenser". The patent mentions high frequency welding and furnaces, among other uses. c. 1932 Electromechanical relays are used to stabilize the voltage output of generators. See Voltage regulator § Electromechanical regulators. c. 1936 Car radios used electromechanical vibrators to transform the 6 V battery supply to a suitable plate voltage for the vacuum tubes. 1959 Transistor oscillation and rectifying converter power supply system U.S. patent 3,040,271 is filed by Joseph E. Murphy and Francis J. Starzec, from General Motors Company. 1960s The Apollo Guidance Computer, developed in the early 1960s by the MIT Instrumentation Laboratory for NASA's Moon missions (1966–1972), incorporated early switched-mode power supplies. c. 1967 Bob Widlar of Fairchild Semiconductor designs the μA723 IC voltage regulator. One of its applications is as a switched-mode regulator. 1970 Tektronix starts using high-efficiency power supplies in its 7000-series oscilloscopes produced from about 1970 to 1995. 1970 Robert Boschert develops simpler, low-cost switched-mode power supply circuits. By 1977, Boschert Inc. had grown to a 650-person company. After a series of mergers, acquisitions, and spin offs (Computer Products, Zytec, Artesyn, Emerson Electric) the company is now part of Advanced Energy. 1972 HP-35, Hewlett-Packard's first pocket calculator, is introduced with transistor switching power supply for light-emitting diodes, clocks, timing, ROM, and registers. 1973 Xerox uses switching power supplies in the Alto minicomputer. 1976 Robert Mammano, a co-founder of Silicon General Semiconductors, develops the first integrated circuit for SMPS control, model SG1524. After a series of mergers and acquisitions (Linfinity, Symetricom, Microsemi), the company is now part of Microchip Technology. 1977 The Apple II is designed with a switched-mode power supply. 1980 The HP8662A 10 kHz–1.28 GHz synthesized signal generator was designed with a switched-mode power supply.
Explanation A linear power supply (non-SMPS) uses a linear regulator to provide the desired output voltage by dissipating power in ohmic losses (e.g., in a resistor or in the collector–emitter region of a pass transistor in its active mode). A linear regulator regulates either output voltage or current by dissipating the electric power in the form of heat, and hence its maximum power efficiency is voltage-out divided by voltage-in since the voltage difference between input and output is wasted. In contrast, a SMPS changes output voltage and current by switching ideally lossless storage elements, such as inductors and capacitors, between different electrical configurations. Ideal switching elements (approximated by transistors operated outside of their active mode) have no resistance when on and carry no current when off, so converters with ideal components would operate with 100% efficiency (i.e., all input power is delivered to the load; no power is wasted as dissipated heat). In reality, these ideal components do not exist, so a switching power supply cannot be 100% efficient, but still provides a significant improvement in efficiency over a linear regulator.
Different switching configurations are used in SMPS designs. A boost converter acts like a step-up transformer for DC signals. A buck–boost converter works in a similar manner, but yields an output voltage which is opposite in polarity to the input voltage. Other buck circuits exist to boost the average output current with a reduction of voltage.
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