The continuous rise in global energy prices has forced all industries to seriously consider energy conservation. Among various energy-saving methods, improving the efficiency of switching power supplies is an important means. The United States, after proposing the 80 PLUS program, launched the 85 PLUS and 88 PLUS programs, which were implemented in a short period. Therefore, maximizing the efficiency of switching power supplies is a constant goal pursued by the power supply industry. To improve the efficiency of switching power supplies, it is essential to understand the factors affecting them. Generally speaking, flyback converters and their derivative circuits have the lowest efficiency, and multi-stage converter circuit topologies are less efficient than single-stage converters. Therefore, these circuit topologies should be avoided as much as possible in applications. For forward converters, a larger duty cycle results in higher relative efficiency; therefore, in practical designs, the largest possible duty cycle should be selected, ideally close to 1. Switching power supplies with a large input voltage variation range are less efficient than those with a small input voltage variation range. Avoiding large input voltage variations or minimizing unnecessary input voltage variation margins is one of the simplest ways to improve the efficiency of switching power supplies. As switching frequencies increase, switching losses become a significant factor. The simplicity of the RCD snubber circuit is an important factor affecting efficiency. Therefore, using soft switching and zero-voltage switching can effectively eliminate switching losses. When using MOSFETs with voltage ratings of 400–700V, conduction losses may account for two-thirds of the total losses. Therefore, reducing the on-resistance of the MOSFET can effectively reduce MOSFET losses. Based on the above factors, we propose a method to achieve a high-efficiency switching power supply using a combination of PFC and an unregulated isolation converter. PFC control is implemented using an MC33368, and DC/DC conversion is performed using an IRS2453 self-oscillating full-bridge converter. Brief Description of Working Principle: Since PFC has a voltage regulation function, in applications where output voltage stability is not very high, the isolation converter can use a non-PWM control method with a 100% duty cycle instead of the conventional PWM control method, thus improving efficiency by 2% or more. Output voltage stability can be achieved by the voltage regulation function of the PFC stage, and its principle block diagram is shown in Figure 1. [align=center]Figure 1. Block diagram of the PFC + unregulated isolation converter combination[/align] This allows the unregulated isolation converter to be considered part of the PFC, while the PFC becomes an isolated PFC. Since the isolated PFC has isolation, voltage shutdown, and voltage regulation functions, it becomes a truly isolated switching power supply. The maximum duty cycle obtained by using a non-PWM operating mode allows the switching transistors in the full-bridge circuit to conduct at "zero voltage," achieving "zero-voltage switching." This improves the reliability of the circuit, further reduces losses, and increases efficiency. IRS2453 Function Introduction : The IRS2453 self-oscillating full-bridge converter was developed by International Rectifier Corporation based on the IR2153 self-oscillating high-voltage bridge driver. The IRS2453 self-oscillating full-bridge driver has a high operating voltage (600V) and an internal oscillator with a settable frequency, similar to a 555 timer. The full-bridge gate drive ensures precise dead time. Inside this chip, a full-bridge circuit composed of MOSFETs is included. This construction saves on external components, reducing costs, and saves space, making the overall power supply smaller. More importantly, the duty cycle of the switching transistors is fixed inside the chip. In this full-bridge circuit front-end structure, the duty cycle of each switching transistor can approach 50%, allowing the duty cycle of the switching transistors in the full-bridge circuit to reach 95% or even higher. Normally, the maximum duty cycle of the switching transistors is less than 80%. Thus, the duty cycle of the switching transistors in the full-bridge circuit increases, and conduction losses decrease. Moreover, this design is a non-adjustable pulse width form, greatly improving efficiency. The block diagram of the IRS2453 self-oscillating full-bridge driver is shown in Figure 2. [align=center] Figure 2 Block diagram of IRS2453[/align] Complete Circuit Analysis and Circuit Design Essentials The complete circuit schematic and detailed parameters of the entire power supply are shown in Figure 3. The entire circuit is composed of a PFC circuit unit using MC33368 and an unregulated isolated converter using IRS2453. [align=center]Figure 3 Circuit schematic of PFC + unregulated isolated converter combination[/align] The reason for choosing IRS2453 as the control chip for the unregulated isolated converter is mainly because the unregulated isolated converter does not require PWM function, but it needs a clock to obtain the required output pulse. This is why IRS2453 has its own oscillator. IRS2453 itself has full-bridge drive capability. Its two-bridge converter requires high and low side drive, while the full-bridge requires two high-side drive circuits. Since IRS2453 has the above functions, it is the best choice as the drive control chip for the unregulated isolated converter. Due to the selection of a large duty cycle operating state, the efficiency of the wound transformer under normal conditions reaches its maximum at this time. Test Results As can be seen from the circuit diagram, in this design, the mains power, after passing through the power factor correction circuit, obtains a stable DC output voltage. This eliminates the need for voltage regulation, thus improving the power supply efficiency to a certain extent. Furthermore, the output duty cycle of the IRS2453 self-oscillating full-bridge converter is constant, eliminating the need for pulse width modulation, further enhancing the power supply efficiency. Detailed product parameters are as follows: ● Dimensions: 100mm × 60mm × 40mm ● Switching transistor drain/source voltage test (see Figure 4) [align=center] Figure 4 Source-drain voltage waveform of the switching transistor[/align] ● Output ripple voltage test under full load (see Figure 5) [align=center] Figure 5 Output ripple voltage under medium load[/align] ● Input current and harmonic analysis (see Figure 6) [align=center] Figure 6 Input current and harmonic analysis[/align] The overall efficiency test results are shown in Table 1. As can be seen from the above analysis, the technical solution presented in this paper can easily improve the efficiency of the switching power supply, and the power factor and higher harmonic parameters of the power supply fully meet the requirements. Conclusion This design is a simple high-efficiency power converter implemented using the IRS2453 chip, which is low in cost and small in size. Testing and calculations show that the overall efficiency of this device is 89.5%. Its output voltage is 24V DC, making it suitable for a wide range of applications. With careful refinement, the efficiency can exceed 90% under 25-100% load conditions, making it a truly high-efficiency switching power supply.