Comprehensive analysis of the four major trends in switching power supply technology

At present, China's switching power supplies account for 80% of the world market, but we have almost no share in the high-end market. my country is currently short of energy, and the power supply industry is an industry closely related to energy consumption, so the government and academic groups should provide guidance on the development direction of power supplies in several aspects.

　　1. Rapid development of non-isolated DC/DC technology

　　In recent years, non-isolated DC/DC technology has developed rapidly. At present, a set of electronic equipment or electronic systems requires the power supply system to provide multiple voltage levels due to different loads. For example, a desktop PC requires four voltages of +12V, +5V, +3.3V, and -12V, as well as a standby +5V voltage, and the motherboard requires 2.5V, 1.8V, 1.5V, or even 1V. It is impossible to give so many voltage outputs in an AC/DC, and most low-voltage power supply currents are very large, so many non-isolated DC/DCs have been developed, which can basically be divided into two categories. One type contains power switching elements inside, called DC/DC converters. The other type does not contain power switches and requires external power MOSFETs, called DC/DC controllers. According to the circuit function, there are step-down step-down, boost step-up, buck-boost or SEPIC step-down, and positive voltage to negative voltage INVERTOR. Among them, the most varieties and the fastest development are step-down step-down. According to the output current, it is divided into single-phase, two-phase and multi-phase. The control method is mainly PWM, and a small part is PFM.

　　In non-isolated DC/DC conversion technology, TI's pre-detection gate drive technology uses digital technology to control synchronous BUCK. The DC/DC conversion efficiency using this technology can reach up to 97%, among which TPS40071 is its representative product. In the BOOST boost mode, there are also synchronous BOOST products that use MOSFET instead of diode. In the low-voltage field, the increase in efficiency is very large, and efforts are being made to further eliminate the conduction and reverse recovery problems of the MOSFET body diode.

　　2. Switching power supply blows the horn of digitalization

　　At present, in the entire electronic analog circuit system, television, audio equipment, photo processing, communication, network, etc. have gradually realized digitalization, and the last bastion without digitalization is the power supply field. In recent years, the research momentum of digital power supply has not diminished, and more and more results have been achieved. The companies that are ahead in the digitalization of power supply are TI and Microchip. TI not only has advantages in DSP, but also merged with UNITRODE, a professional manufacturer of PWMIC. The company has used TMS320C28F10 to make a 48V output high-power power supply module for communication, in which the PFC and PWM parts are completely digitally controlled. Now, TI has developed a number of digital PWM control chips. At present, they are mainly UCD7000 series, UCD8000 series and UCD9000 series, which will become the pathfinders of the next generation of digital power supply. They generally include both hardware and software programming. The hardware part includes the logic part of PWM, clock, analog-to-digital conversion of amplifier loop, digital-to-analog conversion, digital processing, drive, detection and processing of synchronous rectification, etc.

　　Currently, the competition in the power supply field is mainly a competition of performance and price, so digital power supply still has a long way to go. However, the clarion call for digitalization in the power supply field has already been sounded.

　　3. Primary PWM control IC is continuously optimized

　　Active clamping technology has been popular for more than ten years. Since the expiration of VICOR's patent in 2002, new active clamping control ICs developed by various companies have sprung up, providing users with ample choices.

　　TI, which controls the early active clamp control technology, not only maintains the original UCC3580 series, but also newly develops the more superior UCC2891-94, which adopts current-type control, combines the high-side clamp and low-side clamp control schemes, and provides a new control technique. OnSemi first launched the low-voltage (100V) active clamp NCP1560 control chip, and then launched the high-voltage application control chip NCP1280, which not only solves the requirements of LCDTV plasma TV power supply, but also directly targets the next generation of fanless PC power supply.

　　The 5000 series of NS company in the United States has a special active clamp control IC of LM5025. Even the little-known Semtech company has also provided an active clamp control chip, model SC4910, which shows that there are huge market opportunities behind it. Until recently, TI company launched the active clamp control ICUCC2897, which has made the PWM control of active clamp perfect. And the Taiwanese company Fairchild has provided the cheapest active clamp control IC, namely SD7558 and SD7559.

　　In the field of high power, full-bridge phase-shifted ZVS soft switching technology has made great contributions to solving the efficiency of switching power supplies. From TI's UC3875 to UCC3895, and from Linear's LTC1922 to LTC3722, adaptive detection technology has been added, making full-bridge phase-shifting technology reach its peak. However, with the widespread application of synchronous rectification technology today, it cannot achieve the best ZVS synchronous rectification. Because the full-bridge phase-shifting circuit is essentially asymmetrical, it cannot achieve complete ZVS synchronous rectification. Since half of its opening and closing processes are always hard switches, its efficiency is not as good as the synchronous rectification of the ZVS mode of the symmetrical circuit topology. The latest scientific and technological achievement should be the PWM symmetrical full-bridge ZVS control IC-ISL6752 launched by INTERSIL. It can control the four MOS switches on the primary side to the ZVS working state, and can accurately give the driving signal to control the synchronous rectification on the secondary side to the ZVS working state. The 400W DC/DC made with this IC and the advanced power MOSFET can achieve a conversion efficiency of 95%.

　　For low-power switching power supplies, the flyback converter PWM control IC is still used, but it must be able to solve the control method of synchronous rectification on the secondary side. OnSemi's NCP1207 and NCP1377 are leaders in the field of high-voltage AC/DC. If they are equipped with TI's flyback converter synchronous rectification control IC-UCC27226, they can become almost flawless high-efficiency power supplies. Among the flyback converter control ICs in the low-voltage DC/DC field, Linear's LTC3806 is a superior product. LTC3806 can not only control PWM well, but also provide accurate secondary-side synchronous rectification drive signals. It is a masterpiece of low-voltage and low-power power supply control ICs.

　　In summary, the best control method and the best circuit topology can be selected when designing a switching power supply. High power should be full-bridge ZVS plus secondary-side ZVS synchronous rectification, and the typical control IC is ISL6752; medium power to low power should be active clamp forward conversion ZVS soft switch with synchronous rectification of pre-detection gate drive technology on the secondary side; and low power should be flyback conversion with synchronous rectification. Of course, there is no absolute boundary here, but there should be corresponding optimal choices under different conditions.

　　4. Synchronous rectification technology achieves high efficiency

　　Since the birth of synchronous rectification technology in the late 1990s, switching power supply technology has made great progress. The synchronous rectification scheme using IC control technology has been generally accepted by R&D engineers. The current synchronous rectification technology is striving to achieve ZVS and ZCS synchronous rectification.

　　Since Galaxy Corporation of the United States published the ZVS synchronous rectification technology in 2002, it has been widely used. This type of synchronous rectification cleverly adjusts the pulse signal of the secondary side driving the synchronous rectification to be ahead of the rising edge of the primary side PWM pulse signal and behind the falling edge to achieve the ZVS mode operation of the synchronous rectification MOS. Almost all the latest dual-output PWM control ICs have added control terminals for ZVS synchronous rectification on the secondary side in the control logic. For example: Linear's LTC3722, LTC3723, INTERSIL's ISL6752, etc. These ICs not only strive to solve the soft switching of the primary side power MOSFET, but also focus on solving the ZVS synchronous rectification of the secondary side, and the conversion efficiency can reach more than 94%.

　　In the asymmetric switching power supply circuit topology, especially for the forward circuit or forward active clamp circuit with good performance, in the synchronous rectification on the secondary side, in order to realize the synchronous rectification of ZVS mode and eliminate the conduction loss of MOSFET body diode and the loss caused by reverse recovery time, TI's patented technology "pre-detection gate drive technology" adds a lot of digital control technology to the control chip. The birth of the control chip UCC27228 for synchronous rectification of the forward circuit has made the efficiency of the forward circuit reach an unprecedented high efficiency.

　　Combined with the active clamping technology on the primary side, this latest circuit mode can achieve soft switching ZVS operation on the primary side, solve the core reset and energy feedback, reduce the voltage stress of the power MOSFET, and achieve synchronous rectification in the optimal ZVS state on the secondary side. The small and medium-power DC/DC converters that use these two technologies have an efficiency of more than 94% and a power density of more than 200W/in.

　　5. Expert opinion: Energy shortage urgently requires the introduction of energy-saving policies

　　At present, China's switching power supplies account for 80% of the world market, but we have almost no share in the high-end market. my country is currently short of energy, and the power supply industry is an industry closely related to energy consumption, so the government and academic groups should provide guidance on the development direction of power supplies in several aspects.

　　First, the no-load power consumption of the color TV power supply. In cities, many families use remote control to shut down the TV after watching TV at night, which wastes electricity. At this time, the no-load power consumption of the color TV is mostly above 3.5W, while the European standard is less than 1W and the Japanese standard is less than 0.6W.

　　Second, domestic home appliance manufacturers do not have high requirements for power efficiency, but only price. For example, when DVD manufacturers use external power adapters, they would rather choose an adapter with a conversion efficiency of less than 80% and a no-load loss of 1.5W, which costs 49 yuan, but are unwilling to choose an adapter with a conversion efficiency of more than 90% and a no-load loss of less than 0.6W, which costs 59 yuan.

　　At present, our country imports more than 50% of its oil, and is still a country with a shortage of oil. If there are more private cars, where can we get oil? Should we use laws and policies to encourage companies and engineers to develop and produce more efficient power sources?