How to use a Switching Power Supply

A switched-mode power supply (SMPS) incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a DC or AC source to DC loads, such as a personal computer, while converting voltage and current characteristics. Unlike a linear power supply, the pass transistor of a switching-mode power supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high dissipation transitions, which minimizes wasted energy. Voltage regulation in a switching power supply is achieved by varying the ratio of on-to-off time (also known as duty cycles). In contrast, a linear power supply regulates the output voltage by continually dissipating power in the pass transistor. This higher power conversion efficiency is an important advantage of a switched-mode power supply. Switched-mode power supplies may also be substantially smaller and lighter than a linear supply due to the smaller transformer size and weight.

A switching power supply takes an AC input, but rectifies and filters into DC first, is converted back into AC at some high switching frequency, steps down the voltage with a transformer, then is rectified and filtered into a DC output. 

Switching Power Supply - Advantages / Disadvantages

• Small & Light
• Efficiency around 70-95%
• Complex Design
• EMI Filtering Required
• Low Cost

A linear power supply requires larger semiconductor devices to regulate the output voltage and therefore generates more heat, resulting in lower energy efficiency. A linear power supply normally operates around 60% efficiency for 24V outputs, whereas a switch-mode power supply operates at 80% efficiency or more.

Switching power supplies are compact, lightweight, and highly efficient. Used correctly, they can improve the reliability of your electric equipment.

There are many types of switching power supplies manufactured and used worldwide, some for AC and some for DC operation. Before using a switching power supply:

• check the local voltage;
• determine whether it is AC or DC;
• know the allowable voltage range;
• and the input selection method.

Be aware that input outside of the specified range may damage the power supply. Even if the input voltage is within the specified range, distortion in the input voltage may impair the proper functioning of the power supply. Although there are some exceptions, in switching power supplies that directly rectify AC input, the most common method of rectification is the capacitor input method, in which current runs through a smoothing capacitor. In such cases, the input current is determined by the output power, the input voltage, power factor, and efficiency.

In switching power supplies, when the power is turned on, peak current flows while the input smoothing capacitor is charging. This current is called "inrush current". The inrush current varies depending on the input timing and whether or not an inrush current prevention circuit is being used, and it occurs between 10 and 20 times before the normal operating condition is reached.

If several switching power supplies are used at once, the inrush current increases. This is an important factor to consider when selecting power cables, fuses, and switches. If the input fuse blows (circuit opens), it may mean that there is a problem with the switching power supply.

In general, the power factor of switching power supplies using the capacitor input method is between 0.4 and 0.6. However, the power factor of the resonant power supply is approximately 1, due to the use of a PFC (Power Factor Collection) circuit. This means that the input current requirement is lower than for other switching power supplies. Input voltage x Power factor x Efficiency

Advantages of SMPS

High Efficiency – They generate far less heat. Lower-powered units will often not require a heat shield which means that they can be mounted directly onto printed circuit boards. 

Compact Form Factor – Because switchers operate at a higher frequency, the value, and thereby the size of their associated filtering capacitors and inductors will be smaller and the overall unit will take up less space.

Versatile Design – Switchers can be designed to step up voltage or step-down down voltage as the application requires.

You may also refer to the following Power Management video for additional overview information about power.