Switching Mode Power Supplies

A problem of linear power supply is the size and weight of the transformer. The size is required because of the low frequency (50 to 60 Hz). For the same power output, the transformer size goes down as the frequency goes up (up to a point). The SMPS takes advantage of this by chopping the AC line’s waveform into lots of little pieces and changing them to a desired voltage level with a much smaller transformer. A key fact is that the switching element is either off or fully on (saturated). The voltage drop across the transistor is small (for either a bipolar transistor or a MOSFET), meaning little power is being wasted in it. When it’s off, no power is being dissipated. This is one of the efficiency wins of a SMPS.

The filter capacitors can also be smaller at these higher frequencies and chokes are more effective. A lower frequency limit is 25 kHz (to stay above the human hearing range) and the state-of-the-art upper limit is currently around 3 MHz. Most switching supplies use frequencies in the range of about 50 kHz to 1 MHz.

Parasitic behaviors and the skin effect in conduction become important at the higher switching frequencies, especially because the waveforms are square waves and rich in harmonics. In passive elements like capacitors and inductors, ESR (equivalent series resistance) becomes important and leads to inefficiencies. Resistors need to be non-inductive. Careful, optimized switching mode designs can realize efficiencies of 95%, but the typical SMPS is around 75% efficient, still much better than the typical linear supply. This is one reason they are universally seen in personal computers.

Another advantage with Switching Mode Power Supplies (SMPS) is that the switching can be modulated in various ways, depending on the load conditions. The power supply output is regulated with a feedback circuit which adjusts the timing (duty cycle ) with which the MOSFETS are switched on or off.

The benefits of switching mode power supplies don’t come with some costs. The higher frequencies and switching mean higher levels of electromagnetic interference (EMI), both radiated and conducted. This can put switching noise back into the power line. The control electronics also become more complicated (even more so recently because of the desire to have higher power factors).

Switching mode power supplies may struggle to produce low voltages. This is because the transistor has to switch current — i.e., the SMPS can’t operate unless enough current is flowing. Because of this, switching mode power supplies often have a minimum output voltage specification.

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[...] can be 90% efficient or better. Linear power supplies are typically much less efficient than switching mode power supplies. [...]

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