| There are many types and designs of power supplies. | | | | voltages, depending on circuit topology. |
| This article will describe what is commonly known as a | | | | In a popular AC to DC SMPS design, the incoming AC |
| switching power supply, sometimes referred to as a | | | | mains voltage is first converted to DC using a bridge |
| switched mode power supply (SMPS), or 'switcher'. | | | | rectifier. This DC voltage is then fed to a 'chopper' |
| All electronic circuits require a voltage source in order | | | | stage containing highly efficient MOSFET switching |
| to operate. In the United States, the most common | | | | transistors. The gates of the transistors are fed by an |
| voltage available to consumers from the electric utility | | | | oscillator providing a constant frequency, pulse width |
| is the 120V RMS nominal alternating current found at | | | | modulated (PWM) signal (usually in the range of 50 to |
| outlets throughout homes and businesses. In contrast, | | | | 300KHz) in order to 'chop' the DC into a high |
| most electronic circuits in use today require a stable | | | | frequency AC voltage. This AC voltage is fed into the |
| direct current source with a much lower voltage - | | | | primary winding of a high frequency transformer. The |
| usually 3 to 12 volts. | | | | transformer secondary winding voltage is then rectified |
| Virtually every electronic device that operates from | | | | to DC for use in the circuits where it is required. |
| the AC mains must include a circuit that will derive a | | | | Regulation of the output voltage is accomplished by a |
| different voltage from the 120 volts that the utility | | | | circuit that samples the output voltage, and signals the |
| provides. This is accomplished by a type of circuit | | | | PWM oscillator to vary the ratio of transistor 'on' time |
| referred to as a power supply. Power supplies can be | | | | to 'off' time to maintain a constant voltage output. |
| generally grouped into two classes of operation. | | | | There are significant advantages of this design over |
| Linear power supply designs are quite common and | | | | the linear design. The high efficiency of the switching |
| familiar to most people. They use a 'brute force' | | | | transistors (80 to 90%) means less energy is wasted |
| method of power conversion and regulation. The AC | | | | as heat, and the high frequency of operation allows for |
| mains voltage is fed directly to a transformer which | | | | a much smaller and lighter transformer to be used. |
| converts the 120V AC to the highest voltage required | | | | Should an AC input voltage not be available - such as |
| by the circuit it is designed to supply. The lower voltage | | | | in an automotive application - the designer need only |
| is then converted to DC via a bridge rectifier, and then | | | | remove the rectifier stage from the power supply |
| regulated by a transistor operating within its linear | | | | input, and use a high frequency transformer with the |
| range - thus being dubbed a 'linear' power supply. | | | | appropriate turns ratio to provide the desired output |
| The advantage of linear power supply designs is that | | | | voltage. |
| they are simple and straightforward. Disadvantages | | | | In summary, the principal advantages of the SMPS are |
| include inefficiency (50% or less), large physical size | | | | its ultra high efficiency, along with a significant reduction |
| and weight, and generation of heat. | | | | in size and weight. In today's world where efficiency |
| Switching power supply designs are typically more | | | | and portability have become more important than ever, |
| complex than their linear counterparts. As a result, the | | | | the SMPS is quickly becoming the preferred design for |
| SMPS is also more versatile. SMPS design allows for | | | | power supply circuits. |
| either AC or DC input, and can output either AC or DC | | | | |