Switch-mode power supplies (SMPS), also called switching power supplies, have become more popular than linear power supplies in the past ten years because they provide a regulated voltage with more efficiency and they don't require the larger transformers and filtering devices that the linear power supplies require. For example, the linear power supplies generally have average conversion efficiencies of 30%, while the SNIPS have efficiencies up to 80%. Since the SMPS don't need the larger components, they are more usable in modern circuits where cabinet space and board space is at a premium. Designers are continually trying to reduce the size and weight of electronic controls, and one easy way has been to change to SMPS.
Fig. 1 shows an electrical block diagram of a switch-mode power supply. This diagram will aid in understanding how the SMPS converts a dc input voltage to a new value of dc voltage that is filtered and regulated. The first block of the power supply is called the rectifier and filter section and it's shown in the diagram as a diode and capacitor, indicating the ac voltage is rectified to pulsing dc and then filtered to reduce the amount of ripple. The second block in the diagram shows the symbols of a MOSFET and bipolar transistor. This section is called the high-frequency switching section and it uses either MOSFETs or bipolar transistors to convert the dc voltage to a high-frequency ac square wave. The high-frequency ac square wave can be 20-100 kHz. The incoming ac voltage is rectified to dc and then the high-frequency switching section changes it back to ac for several reasons. First, the incoming voltage is always fluctuating and it's full of transient voltages that can be damaging to solid-state components if they are allowed to reach them. The two-step conversion helps to isolate these fluctuations and transients. The second reason: higher frequencies allow for higher conversion efficiencies.