what are switch mode power supply

An issue of linear power supply often is the size and weight of this transformer. The size is required due to the low frequency (50 to 60 Hz). For the similar power output, the transformer size goes down as the frequency rises (up to a point). The SMPS takes benefit of this by chopping the AC line's waveform into lots of little pieces and changing those to a desired voltage level which has a smaller transformer. A major truth is that this switching element is either off or fully on (saturated). The voltage drop round the transistor is small (for whether bipolar transistor or a MOSFET), meaning little power has been wasted within it. When it's off, no power will be dissipated. This is one of the efficiency wins of a SMPS.

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

Parasitic behaviors as well as the skin effect in conduction become important in the higher switching frequencies, especially as the waveforms are square waves and loaded in harmonics. In passive elements like capacitors and inductors, ESR (equivalent series resistance) becomes important and leads to inefficiencies. Resistors need be non-inductive. Careful, optimized switching mode designs can realize efficiencies of 95%, though the typical SMPS is around 75% efficient, still as well as the typical linear supply. That is one reason these are universally noticed in notebooks.

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

The advantages of switching mode power supplies don??t come with some costs. The better frequencies and switching mean higher numbers of electromagnetic interference (EMI), both radiated and conducted. This will put switching noise into the power line. The control electronics also be a little more complicated (much more so recently due to the desire to have higher power factors).

Switching mode power supplies may struggle to produce low voltages. It is because the transistor needs to switch current? i.e., the SMPS can't operate unless enough current is flowing. For this reason, switching mode power supplies usually have the very least output voltage specification.