Switched-mode power supply - GME switching power supply

A switched-mode power supply (also switching-mode power supply, SMPS, or simply switcher) is an electronic power supply unit (PSU) that incorporates a switching regulator in order to provide the required output voltage. An SMPS is actually a power converter that transmits power from a source (e.g., a battery or the electrical power grid) to a load (e.g., a personal computer) with ideally no losses. The function of the converter is to provide a reliable output voltage often at a different level than the input voltage.
When mechanical shafts are rotating, a simple gear train can deliver power to a shaft at one speed from a shaft at a different speed. However, fluid power can be converted from a source with one pressure–flow ratio to another pressure–flow level without rotation by using the switching action of a hydraulic ram. Similarly, when AC power is being delivered from an AC source, a simple transformer can be used to convert power at one voltage level to power at another voltage level with low losses. Likewise, the switched action of an SMPS can convert DC power with low losses.

Explanation

A linear regulator maintains the desired output voltage by dissipating excess 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 output current by dissipating power, and hence its maximum power efficiency is 50%. In contrast, a switched-mode power supply regulates output current by switching ideal storage elements, like inductors and capacitors, into and out of different configurations. Ideal switching elements (e.g., transistors operated outside of their active mode) have no resistance when "closed" and carry no current when "open", and so the converters can theoretically operate with 100% efficiency (i.e., all input power is delivered to the load; no power is wasted as dissipated heat).
For example, the DC component (i.e., the time average) at one terminal of an inductor will match the DC component at the other terminal. If a DC source, an inductor, a switch, and the corresponding electrical ground are placed in series and the switch is driven by a square wave, the voltage waveform measured across the switch will also be a square wave. Because the inductor ensures that the average value of the output waveform matches the DC source voltage, the peak amplitude of the voltage across the switch will be twice the voltage of the input. If a diode-and-capacitor combination are placed in parallel to the switch, the peak voltage can be stored in the capacitor, and the capacitor can be used as a DC source with voltage higher than the DC voltage driving the circuit. This so-called boost converter acts like a step-up transformer for DC signals.
In an SMPS, the output current flow depends on the input power signal, the storage elements and circuit topologies used, and also on the pattern used (e.g., PWM with an adjustable duty cycle) to drive the switching elements. Typically, the spectral density of these switching waveforms has energy concentrated at relatively high frequencies. As such, switching transients, like ripple, introduced onto the output waveforms can be filtered with small LC filters.

Advantages and Disadvantages

The main advantage of this method is greater efficiency because the switching transistor dissipates little power when it is outside of its active region (i.e., when the transistor acts like a switch and either has a negligible voltage drop across it or a negligible current through it). Other advantages include smaller size and lighter weight (from the elimination of low frequency transformers which have a high weight) and lower heat generation due to higher efficiency. Disadvantages include greater complexity, the generation of high-amplitude, high-frequency energy that the low-pass filter must block to avoid electromagnetic interference (EMI), and a ripple voltage at the switching frequency and the harmonic frequencies thereof.
Very low cost SMPS may couple electrical switching noise back onto the mains power line, causing interference with A/V equipment connected to the same phase. Non power-factor-corrected SMPSs also cause harmonic distortion.