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What Is a Switching Power Supply? A Complete Guide to How It Works and How to Choose One
Comprehensively explains the basics and mechanisms of switching power supplies, selection points, application examples for 24V and 12V, and noise countermeasures. Packed with information that beginner to intermediate engineers want to know. Please also see the introduction to our UNIFIVE Products and support system.
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What Is a Switching Power Supply? A Clear Guide to Its Basic Mechanism and Key Application Points
For electrical and electronic equipment design engineers, switching power supplies are essential for achieving high efficiency, compact size, and cost reduction. This article comprehensively explains the basic mechanism and operating principles of switching power supplies, how to select them by application, as well as key points such as noise countermeasures and safety standards. It is organized to be easy to understand for beginner to mid-level engineers and those new to power supply design, so please read through to the end.
History of Switching Power Supplies
The history of switching power supplies dates back to the 1800s.
It is said that IBM developed the world’s first switching power supply in 1958. The switching power supply designed by IBM at that time was based on vacuum tube technology. Around the same time, GM filed a similar patent as a transistor oscillator, and this technological innovation is said to have dramatically expanded the options available to equipment designers. The 1960s was a particularly active era across fields such as space, communications, electronics, and computers.
Mechanism and Operating Principle of Switching Power Supplies
*Example of a basic block diagram of a switching power supply
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A switching power supply is a power converter that rectifies AC (alternating current) input into DC (direct current), then controls the voltage by rapidly turning high-frequency switching devices (such as MOSFETs) on and off. Compared to linear power supplies, it has the following key features:
- High efficiency: By supplying only the required power through switching operation, ripple and heat generation are suppressed, enabling efficient operation.
- Compact size: High-frequency switching allows transformers and filter components to be miniaturized, saving board space.
- Wide input voltage compatibility: Depending on the control circuit, many products flexibly support systems from 100V to 200V.
The operating principle generally follows this flow: AC input → Rectifier circuit → High-frequency switching circuit (such as PWM control) → Isolation transformer (if required) → Rectification and smoothing → Output. In the control circuit, **PWM (pulse width modulation)** maintains constant output voltage and current, and many units include protection functions to prevent overvoltage and overcurrent.
Differences from Linear Power Supplies and Selection Points
A frequently compared alternative is the linear power supply (series regulator). Linear supplies step down voltage using transistors or regulator ICs. They produce less high-frequency noise and are easier to design, but have the following disadvantages:
- Low efficiency: Excess power is dissipated as heat, requiring cooling design such as heat sinks.
- Large and heavy: Transformers and heat dissipation components must be larger.
| Item | Switching Power Supply | Linear Power Supply |
|---|---|---|
| Conversion Method | High-frequency switching | Continuous control using transistors, etc. |
| Efficiency | High (80%-95%, etc.) | Lower (50%-70%, etc.) |
| Heat Generation | Low | High |
| Size/Weight | Compact/lightweight | Large/heavy |
| Noise (high frequency) | High | Low |
*Comparison table of switching and linear power supplies
While switching power supplies may require noise countermeasures, they are attractive because they achieve compact size and high efficiency simultaneously.
When selecting a unit, check the following points:
- Output voltage and current range
- Input voltage (AC/DC) range
- Efficiency, heat generation, and cooling method
- Noise characteristics
- Protection functions (overvoltage, overcurrent, etc.)
- Size, weight, and cost
Why Switching Power Supplies Are Efficient
Switching power supplies are highly efficient because the switching device operates either fully on or fully off, with extremely short time spent in intermediate voltage states. Unlike linear supplies, continuous power loss does not occur, resulting in less heat generation and contributing to energy savings.
In addition, by controlling the switching frequency and pulse width according to load fluctuations, they can maintain efficiency even during standby or light-load conditions.
Types of Switching Power Supply Circuit Topologies
Buck-Boost Converter
This non-isolated topology allows both step-up and step-down operation. It is used to obtain an output voltage with opposite polarity to the input voltage. When the switch is on, energy flows into the inductor; when the switch is off, energy is released from the inductor through a diode. Compared to the buck type, a larger current flows through the output filter capacitor.
Buck Converter
This non-isolated topology does not use a transformer. It is used when the input voltage is higher than the output voltage. The output voltage is regulated by adjusting the duty ratio of the switch on-time relative to the input voltage. Also called a step-down chopper method, it is a common DC-DC converter topology. When the switch is on, energy is stored in the inductor coil; when off, energy is released through a diode or MOSFET. Its relatively simple configuration makes it cost-effective.
Boost Converter
Also known as a chopper method (reverse polarity type), this topology is used when a higher output voltage than the input voltage is required. When the switching device such as a MOSFET is on, energy is stored in the inductor; when off, energy is delivered to the load through a diode. The stored inductor energy is added to the input voltage, resulting in a higher output voltage.
Push-Pull Topology
This multi-switch topology connects two primary windings of equal turns in series, with the center tap connected to the positive supply and both ends connected to low-side switches. By alternately turning the switches on and off, an alternating magnetic field is generated in the transformer core. Since the secondary-side conversion frequency is twice that of the primary side, filter components can be reduced in size. It is suitable for relatively high-capacity power supplies.
Forward Topology
A single-switch on-type converter commonly used for small to medium power supplies up to several hundred watts. During the on period of the primary switching device, power is transferred to the secondary side through a transformer. Although it requires both a transformer and a choke coil, it offers high efficiency.
Flyback Topology
A single-switch on-off type converter. When the switching device is on, energy is stored in the primary winding of the transformer. When off, the stored energy is released from the secondary winding through a diode, rectified and smoothed to produce DC voltage. It does not require a separate choke coil but involves a more complex transformer design and relatively large capacitor currents.
Full-Bridge Topology
A multi-switch topology where switching devices connected in series are arranged in parallel with the DC input, and the transformer primary winding is placed between their midpoints. By alternately switching on and off, it is suitable for large-capacity power supplies and is also used for computing equipment that requires high current.
Half-Bridge Topology
This method divides the DC input voltage using two capacitors and connects two switching devices in series, with a transformer placed at their midpoint. It offers high transformer efficiency and is suitable for high input voltages.
Among these, buck, boost, and buck-boost converters are non-isolated types, while forward, flyback, push-pull, half-bridge, and full-bridge are isolated types. Other isolated methods include the Royer and Jensen topologies.
Features and Applications of 24V Switching Power Supplies
Advantages and Applications of 24V Power Supplies
24V switching power supplies are widely used in industrial equipment, factory automation systems, and machine tools. Their advantages include safety, standardization with compatible sensors and actuators, and easy integration with other voltages such as 5V and 12V.
Benefits in Industrial Equipment
In industrial applications requiring stable long-term operation under high loads, 24V switching power supplies offer easy thermal design, parallel operation for higher output current or redundancy, and flexible form factors such as board-mounted or DIN rail types.
Technologies Enhancing Reliability and Safety
In addition to isolation, overvoltage and overcurrent protection, technologies such as temperature-sensor-based fanless cooling and enhanced noise filtering improve reliability and safety. Many products now include **PFC (power factor correction)** and comply with international standards such as UL and EN.
How to Choose a 12V Switching Power Supply
Applications and Selection Criteria
12V switching power supplies are used in LED lighting, automotive devices, and communication equipment. Key selection criteria include rated output power (W), efficiency and heat generation, and compliance with safety standards such as UL and EN.
Checking Input Voltage and Output Current
Confirm the input voltage range (e.g., AC100-240V) and the maximum load current. For example, supplying 2A at 12V requires about 24W of rated capacity. Consider inrush current and load fluctuation peaks to ensure sufficient margin.
Miniaturization and Energy Savings Achieved by Switching Power Supplies
Efficiency Improvement Through Switching Technology
By increasing the speed of switching device on-off operation, losses are minimized, enabling the use of smaller transformers and achieving overall compactness and lightweight design. Advanced control circuits also improve efficiency during standby and partial load conditions.
Demand for Compact Form Factors
Compact power supplies are especially required for built-in applications such as PCs and medical devices. Board-mounted types and external AC adapters should be selected according to installation space and thermal design requirements.
Environmental Benefits of Energy Savings
High efficiency reduces CO2 emissions and enables smaller heat sinks and cooling fans, contributing to resource savings and longer product life. Energy-saving performance is also important for meeting international standards and environmental regulations.
Noise Issues and Solutions
Understanding Causes and Effects
High-speed switching operation can generate noise that interferes with surrounding circuits. Causes include electromagnetic leakage at switching frequencies and rapid current changes during rectification. Adequate countermeasures are particularly important in audio equipment.
Countermeasures Using Capacitors and Diodes
- Proper capacitor placement: Use bypass capacitors near switching devices and ICs to reduce ripple and high-frequency noise.
- Diode selection: Use fast recovery or Schottky barrier diodes to suppress excessive noise.
- Snubber circuits: Absorb surge voltage generated in transformers and switching devices.
Noise Filters and Circuit Design
Additional measures include properly designed noise filters and shielding, careful grounding design, and optimized wiring layout to prevent ground loops.
Detailed Explanation of Circuits and Rectification Methods
Role of Diodes and Transformers
Front-end stages often use bridge rectifier and PFC circuits. Transformers provide isolation and voltage conversion, while diodes are used for high-frequency rectification and protection. Proper selection significantly affects efficiency and durability.
Control Methods and Applications
- PWM control: Widely used and suitable for many applications.
- PFM control: Effective for large load fluctuations.
- Resonant type: Achieves high efficiency with reduced noise but is more complex to implement.
Advances in AC/DC Conversion
Next-generation semiconductors such as SiC (silicon carbide) and GaN (gallium nitride) enable higher switching frequencies and lower losses, allowing smaller, more efficient, and lower-noise designs.
How to Choose a Switching Power Supply Manufacturer and Our Solutions
Features of Unifive
Unifive provides power supplies for industrial, communication, and medical applications, offering quality management compliant with ISO, UL, and EN standards, extensive experience and support, a wide product lineup (5V, 12V, 24V, 48V, etc.), and technical advantages such as low-noise design and parallel operation capability.
Our Optimal Solutions
In addition to standard products, we propose optimal switching power supplies tailored to customer requirements, including product selection support, technical customization, and small-lot semi-custom production.
Conclusion
This article explained the basic mechanisms and operating principles of switching power supplies, differences from linear supplies, features of 24V and 12V models, noise countermeasures, and safety standards. Switching power supplies offer significant advantages in efficiency, miniaturization, and energy savings, making them indispensable in modern electronics. Proper knowledge and appropriate manufacturer and product selection are essential.
At Unifive, we offer a wide range of highly reliable switching power supplies and provide flexible technical consultation and customization support. If you have any questions regarding selection or implementation, please feel free to contact us using the button below.
We also accept requests for various samples at any time.