A switching power supply is a type of power supply that uses modern power electronics technology to control the on and off time ratio of switching transistors to maintain a stable output voltage. Switching power supplies are generally composed of a pulse-width modulation (PWM) control IC and MOSFETs. Compared to linear power supplies, the cost of both increases with increasing output power, but the rates of increase differ.
At a certain output power point, the cost of linear power supplies becomes higher than that of switching power supplies; this point is called the cost inversion point. With the development and innovation of power electronics technology, switching power supply technology is also constantly innovating, and this cost inversion point is increasingly shifting towards lower output power levels, providing ample room for the development of switching power supplies.
The trend towards higher frequencies is the future direction of switching power supplies. Higher frequencies enable miniaturization and allow switching power supplies to enter a wider range of applications, particularly in high-tech fields, driving the miniaturization and portability of high-tech products. Furthermore, the development and application of switching power supplies are of great significance in energy conservation, resource conservation, and environmental protection.
The power electronic devices used in switching power supplies are mainly diodes, IGBTs, and MOSFETs.
SCRs are used in small quantities in the input rectifier circuits and soft-start circuits of switching power supplies. GTRs are difficult to drive and have low switching frequencies, so they are gradually being replaced by IGBTs and MOSFETs.
Three conditions for switching power supplies
1. Switch: Power electronic devices operate in a switching state rather than a linear state.
2. High frequency: Power electronic devices operate at high frequencies rather than low frequencies close to the power frequency.
3. DC: Switching power supplies output DC, not AC.
Classification of switching power supplies
In the field of switching power supply technology, the development of related power electronic devices and switching frequency conversion technology are proceeding simultaneously. These two aspects mutually promote each other, driving the development of switching power supplies at a double-digit annual growth rate towards lighter, smaller, thinner, lower-noise, higher-reliability, and more interference-resistant designs. Switching power supplies can be broadly classified into AC/DC and DC/DC converters. DC/DC converters have now achieved modularization, and their design technology and manufacturing processes are mature and standardized both domestically and internationally, gaining user acceptance. However, the modularization of AC/DC converters, due to their inherent characteristics, encounters more complex technical and manufacturing challenges during the modularization process. The following sections will describe the structure and characteristics of both types of switching power supplies.
2.1 DC/DC Conversion
DC/DC conversion transforms a fixed DC voltage into a variable DC voltage; it is also called DC chopping. There are two operating modes for choppers: one is pulse width modulation (PWM), where Ts remains constant and ton is changed (generally applicable); the other is frequency modulation, where ton remains constant and Ts is changed (prone to interference). The specific circuits fall into the following categories:
(1) Buck circuit - step-down chopper, its average output voltage
U0 is less than the input voltage Ui, and they have the same polarity.
(2) Boost circuit - boost chopper, its average output voltage
U0 is greater than the input voltage Ui, and they have the same polarity.
(3) Buck-Boost circuit—a buck or boost chopper, its
The average output voltage U0 is greater than or less than the input voltage Ui, with opposite polarities, and is transmitted through inductance.
(4) Cuk circuit – a buck or boost chopper whose average output voltage is...
When voltage U0 is greater than or less than input voltage Ui, and the polarities are opposite, capacitance transfer occurs.
There are also Sepic and Zeta circuits.
The above are non-isolated circuits. Isolated circuits include forward converters, flyback converters, half-bridge converters, full-bridge converters, and push-pull converters.
Today's soft-switching technology has brought about a qualitative leap in DC/DC converters. VICOR Corporation in the United States designs and manufactures various ECI soft-switching DC/DC converters with maximum output powers of 300W, 600W, and 800W, corresponding to power densities of (6.2), 10, and 17) W/cm³, and efficiencies of (80-90)%. Nemic Lambda Corporation in Japan has recently launched the RM series of high-frequency switching power supply modules using soft-switching technology. These modules have a switching frequency of (200-300) kHz and a power density of 27 W/cm³. By employing a synchronous rectifier (MOS-FETs instead of Schottky diodes), the overall circuit efficiency is increased to 90%.
2.2 AC/DC Conversion
AC/DC conversion transforms alternating current (AC) into direct current (DC). The power flow can be bidirectional; power flowing from the power source to the load is called "rectification," and power flowing back from the load to the power source is called "active inversion." AC/DC converters accept 50/60Hz AC input. Because rectification and filtering are necessary, relatively large filter capacitors are essential. Furthermore, due to safety standards (such as UL and CCEE) and EMC directives (such as IEC, FCC, and CSA), EMC filtering and the use of safety-compliant components are required on the AC input side. This limits the miniaturization of AC/DC power supplies. Additionally, the high-frequency, high-voltage, and high-current switching operations increase the difficulty of addressing EMC issues, placing high demands on the internal high-density circuit design. For the same reason, high-voltage and high-current switching increases power loss, limiting the modularization of AC/DC converters. Therefore, power system optimization design methods must be employed to achieve satisfactory efficiency.
AC/DC converters can be classified according to their circuit wiring method into half-wave circuits and full-wave circuits. They can also be classified according to the number of power phases into single-phase, three-phase, and multi-phase. Furthermore, they can be classified according to the operating quadrant into one-quadrant, two-quadrant, three-quadrant, and four-quadrant circuits.
Selection of switching power supply
In terms of input interference immunity, switching power supplies, due to their inherent circuit structure (multi-stage series connection), are highly resistant to common input interference such as surge voltages. They also exhibit a significant advantage over linear power supplies in terms of output voltage stability, which can reach 0.5% to 1%. As a power electronic integrated device, the following points should be considered when selecting a switching power supply module:
3.1 Selection of Output Current
Because switching power supplies have high operating efficiency, generally exceeding 80%, the selection of their output current requires accurate measurement or calculation of the maximum current absorbed by the electrical equipment. This ensures that the selected switching power supply offers a high performance-to-price ratio. The typical output current calculation formula is as follows:
Is=KIf
Where: Is—rated output current of the switching power supply;
If—the maximum current absorbed by the electrical equipment;
K—Margin coefficient, typically taken as 1.5 to 1.8;
3.2 Grounding
Switching power supplies generate more interference than linear power supplies. For equipment sensitive to common-mode interference, grounding and shielding measures should be taken. According to EMC standards such as ICE1000, EN61000, and FCC, switching power supplies must incorporate EMC electromagnetic compatibility measures. Therefore, switching power supplies generally should have an EMC filter. For example, in Leadway Technology's HA series switching power supplies, connecting the FG terminal to ground or the user's chassis is necessary to meet the aforementioned electromagnetic compatibility requirements.
3.3 Protection Circuit
Switching power supplies must have overcurrent, overheat, and short-circuit protection functions in their design. Therefore, switching power supply modules with complete protection functions should be selected first during the design process, and the technical parameters of their protection circuits should be matched with the operating characteristics of the electrical equipment to avoid damaging the electrical equipment or the switching power supply.
Development Trends of Switching Power Supply Technology
The development trend of switching power supplies is towards high frequency, high reliability, low power consumption, low noise, interference immunity, and modularity. Since high frequency is a key technology for making switching power supplies lightweight, small, and thin, major international switching power supply manufacturers are committed to simultaneously developing new, highly intelligent components, particularly improving the losses of secondary rectifier devices, and increasing technological innovation in power ferrite (Mn-Zn) materials to achieve high magnetic performance at high frequencies and high magnetic flux densities (Bs). Capacitor miniaturization is also a key technology. The application of SMT (Surface Mount Technology) has enabled significant progress in switching power supplies, allowing components to be arranged on both sides of the circuit board to ensure lightweight, small, and thin designs. The high frequency of switching power supplies inevitably leads to innovation in traditional PWM switching technology. Achieving ZVS (Zero Switching Variables) and ZCS (Zero Switching Variables) soft-switching technologies has become the mainstream technology for switching power supplies, significantly improving their operating efficiency. Regarding high reliability, American switching power supply manufacturers have significantly improved product reliability by reducing operating current and junction temperature to reduce component stress.
Modularization is the general trend in the development of switching power supplies. Modular power supplies can be used to form distributed power systems, and N+1 redundant power supply systems can be designed to achieve capacity expansion through parallel connection. Addressing the drawback of high operating noise in switching power supplies, pursuing higher frequencies alone will inevitably increase noise. Theoretically, partial resonant converter technology can achieve both higher frequencies and lower noise. However, the practical application of partial resonant converter technology still faces technical challenges. Therefore, much work is still needed in this area to make this technology practical.
Continuous innovation in power electronics technology has given the switching power supply industry broad development prospects. To accelerate the development of my country's switching power supply industry, we must follow the path of technological innovation and forge a path of industry-academia-research collaboration with Chinese characteristics, contributing to the rapid development of my country's national economy.
Disclaimer: This article is a reprint. If it involves copyright issues, please contact us promptly for deletion (QQ: 2737591964 ) . We apologize for any inconvenience.
