Complete Guide to Switching Power Supplies: How They Work and How to Choose

Comprehensively explains the basics, mechanisms, selection points, 24V/12V application examples, and noise countermeasures of switching power supplies. Packed with information that beginner to intermediate engineers want to know. Please also take a look at our UNIFIVE Products and support system.

What Is a Switching Power Supply? Easy-to-Understand Explanation of Basic Mechanism and Key Applications

For designers and engineers of electrical and electronic equipment, switching power supplies are essential for achieving high efficiency, compact size, and low cost. This article comprehensively explains the basic mechanism and operation of switching power supplies, how to choose them by application, noise countermeasures, and safety standards. The content is organized to be easy to understand, even for entry-level and mid-level engineers or those new to power design, so we encourage you to read through to the end.

History of Switching Power Supplies

The history of switching power supplies dates back to the 1800s.

It’s said that the world’s first switching power supply was developed in 1958 by IBM. At the time, the switching power supply designed by IBM was based on vacuum tube technology. Around the same time, GM also filed patents for similar oscillator technologies using transistors. These technological innovations significantly expanded the options available to device designers. The 1960s were an energetic era across many fields including aerospace, telecommunications, electronics, and computing.

Mechanism and Operating Principle of Switching Power Supplies

*Example of a basic block diagram of a switching power supply

A switching power supply is a power converter that converts AC input into DC by rectification and then rapidly turns on and off high-frequency switching elements (such as MOSFETs) to control the voltage. Compared to linear power supplies, the following advantages stand out:

• High efficiency: Only the required power is supplied through switching operation, reducing ripple and heat generation, allowing efficient operation.
• Compact size: High-frequency switching allows for smaller transformers and filter components, saving board space.
• Wide input voltage compatibility: Many products can flexibly support voltages ranging from 100V to 200V, depending on the control circuit.

The basic operation flow is: AC input → Rectifier circuit → High-frequency switching circuit (PWM control, etc.) → Isolation transformer (if required) → Rectification and smoothing → Output. Control circuits using PWM (Pulse Width Modulation) help maintain stable output voltage and current, and many include protection features such as overvoltage and overcurrent prevention.

Difference from Linear Power Supplies and Selection Tips

A common comparison to switching power supplies is the linear power supply (series regulator). Linear power supplies use transistors or regulator ICs to drop voltage and offer simple design with low high-frequency noise but have the following disadvantages:

• Low efficiency: Excess power is dissipated as heat, requiring heatsinks or other cooling solutions.
• Large and heavy: Transformers and heat sinks need to be large.

*Comparison table of switching and linear power supplies

While switching power supplies may require noise countermeasures, the ability to deliver compactness and high efficiency makes them attractive. Here are key points to check when selecting a power supply:

1. Output voltage and current range
2. Input voltage (AC/DC) range
3. Efficiency, heat generation, and cooling method
4. Noise characteristics
5. Protection features (overvoltage, overcurrent protection, etc.)
6. Size, weight, and cost

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