Overview of the development of high-frequency switching power supply technology for communications

1 Development of high-frequency switching power supply technology for communications

The development of high-frequency switching power supply technology for communications can basically be reflected in several aspects: converter topology, modeling and simulation, digital control and magnetic integration.

1.1 Converter topology

Soft switching technology, power factor correction technology and multi-level technology are hot topics in converter topology in recent years. The use of soft switching technology can effectively reduce switching loss and switching stress, which helps to improve converter efficiency; the use of PFC technology can improve the input power factor of AC/DC converters and reduce harmonic pollution to the power grid; and multi-level technology is mainly used in three-phase input converters for communication power supplies, which can effectively reduce the voltage stress of the switch tube. At the same time, due to the high input voltage, the use of appropriate soft switching technology to reduce switching losses is an important research direction for multi-level technology in the future.

In order to reduce the volume of the converter, it is necessary to increase the switching frequency to achieve high power density, and smaller magnetic materials and passive components must be used. However, increasing the frequency will greatly increase the switching loss and drive loss of the MOSFET, and the application of soft switching technology can reduce the switching loss. The most widely used in current communication power engineering is the active clamping ZVS technology, the ZVS phase-shifted full-bridge technology born in the early 1990s, and the synchronous rectification technology proposed in the late 1990s.

1.1.1 ZVS Active Clamp

Active clamping technology has gone through three generations, and all of them have been patented. The first generation is the active clamping ZVS technology of VICOR in the United States, which increases the operating frequency of DC/DC to 1MHZ and the power density is close to 200W/in3, but its conversion efficiency does not exceed 90%. In order to reduce the cost of the first generation of active clamping technology, IPD applied for a patent for the second generation of active clamping technology, which uses P-channel MOSFET and is used for active clamping of forward circuit topology on the secondary side of the transformer, which greatly reduces the product cost. However, the zero voltage switching (ZVS) boundary conditions of the MOSFET formed by this method are narrow, and the operating frequency of PMOS is not ideal. In order to prevent the magnetic energy from being consumed in vain when the magnetic core is reset, a Chinese-American engineer applied for a patent for the third generation of active clamping technology in 2001. Its feature is that the energy released when the magnetic core is reset is transferred to the load on the basis of the second generation of active clamping, so a higher conversion efficiency is achieved. It has three circuit schemes: one of the schemes can use N-channel MOSFET, so the operating frequency can be higher. This technology can combine ZVS soft switching and synchronous rectification technology together, so it can achieve an efficiency of up to 92% and a power density of more than 250W/in3.

1.1.2 ZVS Phase-Shifted Full Bridge

Since the mid-1990s, ZVS phase-shifted full-bridge soft switching technology has been widely used in the field of medium and high power power supplies. This technology plays a great role in improving the efficiency of the converter when the switching speed of the MOSFET is not ideal, but it also has many disadvantages. The first disadvantage is the addition of a resonant inductor, which leads to a certain volume and loss, and the electrical parameters of the resonant inductor need to be kept consistent, which is difficult to control during the manufacturing process; the second disadvantage is the loss of the effective duty cycle. In addition, since synchronous rectification is more convenient to improve the efficiency of the converter, the control effect of the phase-shifted full-bridge on the secondary-side synchronous rectification is not ideal. The original PWMZVS phase-shifted full-bridge controllers, UC3875/9 and UCC3895, only control the primary side, and need to add logic circuits to provide accurate secondary synchronous rectification control signals; now the latest phase-shifted full-bridge PWM controllers such as LTC1922/1 and LTC3722-1/-2, although they have added secondary synchronous rectification control signals, still cannot effectively achieve ZVS/ZCS synchronous rectification on the secondary side, but this is one of the most effective measures to improve converter efficiency. Another major improvement of LTC3722-1/-2 is that it can reduce the inductance of the resonant inductor, which not only reduces the volume and loss of the resonant inductor, but also improves the loss of duty cycle.

1.1.3 Synchronous Rectification

Synchronous rectification includes self-drive and external drive. The self-drive synchronous rectification method is simple and easy, but the secondary voltage waveform is easily affected by many factors such as transformer leakage inductance, resulting in low reliability in mass production and seldom used in actual products. For the conversion of output voltage from 12V to around 20V, a dedicated external driver IC is often used, which can achieve better electrical performance and higher reliability.

TI has proposed the chip UCC27221/2 with predictive drive strategy, which dynamically adjusts the dead time to reduce the conduction loss of the body diode. ST has also designed a similar chip STSR2/3, which is not only used for flyback but also for forward, and improves the performance of continuous and discontinuous conduction modes. The Center for Power Electronics Systems (CPES) of the United States has studied various resonant drive topologies to reduce drive losses, and in 1997 proposed a new type of synchronous rectification circuit, called quasi-square wave synchronous rectification, which can greatly reduce the conduction loss and reverse recovery loss of the synchronous rectifier body diode, and easily realize the soft switching of the primary main switch tube. The synchronous rectification control chips LTC3900 and LTC3901 launched by Linear Technology can be better applied to forward, push-pull and full-bridge topologies.

ZVS and ZCS synchronous rectification technologies have also begun to be applied, such as the synchronous rectification driver for active clamp forward circuits (NCP1560), and the synchronous rectification driver chips for dual transistor forward circuits LTC1681 and LTC1698, but none of them have achieved the excellent effect of symmetrical circuit topology ZVS/ZCS synchronous rectification.

1.2 Modeling and Simulation

There are two main modeling methods for switching converters: small signal analysis and large signal analysis.

Small signal analysis method: mainly the state space averaging method, proposed by RDMiddlebrook of California Institute of Technology in 1976. It can be said that this is the first real breakthrough in modeling and analysis in the field of power electronics. Later, there appeared current injection equivalent circuit method, equivalent controlled source method (this method was proposed by Chinese scholar Zhang Xingzhu in 1986), three-terminal switching device method, etc., all of which belong to the category of circuit averaging method. The disadvantages of the averaging method are obvious. The signal is averaged and ripple analysis cannot be performed effectively; stability analysis cannot be performed accurately; it may not be suitable for resonant converters; the key point is that the model obtained by the averaging method is independent of the switching frequency, and the applicable condition is that the natural frequency generated by the inductor and capacitor in the circuit must be much lower than the switching frequency, so that the accuracy will be higher.

Large signal analysis methods: analytical method, phase plane method, large signal equivalent circuit model method, switch signal flow method, nth harmonic three-port model method, KBM method and general average method. Another one is the equivalent small parameter signal analysis method proposed by Professor Qiu Shuisheng of South my country University of Technology in 1994, which is not only applicable to PWM converters but also to resonant converters, and can perform output ripple analysis.

The purpose of modeling is to simulate and then conduct stability analysis. In 1978, R. Keller first used RDMiddlebrook's state space average theory to perform SPICE simulation of switching power supplies. In the past 30 years, many scholars have established various model theories in the modeling of the average SPICE model of switching power supplies, thus forming various SPICE models. These models have their own strengths, and the more representative ones are: Dr. Sam Ben Yaakov's switching inductor model; Dr. Ray Ridley's model; the average PSpice model of the switching power supply based on Dr. Vatche Vorperian's Orcad9.1; the average ISspice model of the switching power supply based on Steven Sandler's ICAP4; the average model of the switching power supply based on Dr. Vincent G. Bello's Cadence, etc. On the basis of using these models, the macro model is constructed in combination with the main parameters of the converter, and the DC/DC converter constructed by the constructed model is used to perform DC analysis, small signal analysis and closed-loop large signal transient analysis on the professional circuit simulation software (Matlab, PSpice, etc.) platform.

As converter topologies are changing with each passing day and developing at a very fast speed, the requirements for converter modeling are becoming more and more stringent. It can be said that converter modeling must keep up with the development of converter topologies in order to be more accurately applied to engineering practice.

1.3 Digital Control

The simple application of digitalization is mainly to protect and monitor circuits, as well as to communicate with the system. It has been widely used in communication power supply systems. It can replace many analog circuits, complete the start-up of the power supply, input and output over-voltage and under-voltage protection, output over-current and short-circuit protection, and overheating protection, etc. Through a specific interface circuit, it can also complete communication and display with the system.

More advanced applications of digitization include not only realizing perfect protection and monitoring functions, but also outputting PWM waves, controlling power switch devices through drive circuits, and realizing closed-loop control functions. At present, TI, ST, and Motorola have all launched dedicated motor and motion control DSP chips. At present, the digitization of communication power supplies mainly adopts the combination of analog and digital. The PWM part still uses a dedicated analog chip, while the DSP chip mainly participates in duty cycle control, frequency setting, output voltage adjustment, protection and monitoring functions.

In order to achieve faster dynamic response, many advanced control methods have been gradually proposed. For example, ON Semiconductor proposed improved V2 control, Intersilicon proposed Active-droop control, Semtech proposed charge control, Fairchild proposed Valley current control, IR proposed multi-phase control, and many universities in the United States have also proposed a variety of other control ideas [7, 8, 9]. Digital control can improve the flexibility of the system, provide better communication interface, fault diagnosis capability, and anti-interference ability. However, in precision communication power supply, control accuracy, parameter drift, current detection and current sharing, and control delay will be practical problems that need to be solved urgently.

1.4 Magnetic Integration

As the switching frequency increases, the size of the switching converter decreases and the power density is greatly improved, but the switching loss will increase and more magnetic devices will be used, thus occupying more space.

Foreign research on magnetic component integration technology is relatively mature, and some manufacturers have applied this technology to actual communication power supplies. In fact, magnetic integration is not a new concept. As early as the late 1970s, Cuk proposed the idea of ​​magnetic integration when he proposed the Cuk converter. Since 1995, the Center for Power Electronic Systems (CPES) in the United States has done a lot of research on the integration of magnetic devices. It has used the concept of coupled inductors to conduct in-depth research on multi-phase BUCK inductor integration and applied it to various types of converters. In 2002, Yim-Shu Lee and others from the University of Hong Kong also proposed a series of discussions and designs on magnetic integration technology.

Conventional magnetic component design methods are extremely cumbersome and need to be considered from different angles, such as the selection of the size of the core, the determination of the material and winding, and the estimation of iron loss and copper loss. However, in addition to this, magnetic integration technology must also consider the problem of flux imbalance, because the equivalent total flux of the magnetic flux distribution in each part of the core is different, and some parts may saturate in advance. Therefore, the analysis and research of magnetic device integration will be more complex and difficult. However, the advantage of high power density it brings will definitely be a major development trend of communication power supply in the future.

1.5 Manufacturing process

The manufacturing process of high-frequency switching power supply for communication is quite complicated and directly affects the electrical function, electromagnetic compatibility and reliability of the power supply system. Reliability is the primary indicator of communication power supply.

The adoption of complete testing methods, complete process monitoring points and anti-static measures in the manufacturing process has largely continued the best design performance of the product, and the widespread use of SMD patch components will greatly improve the reliability of welding. European and American countries will require lead-free processes for electronic products from 2006, which will put forward higher and stricter requirements on the selection of components in communication power supplies and the control of the manufacturing process.

The more attractive technology at present is the concept of integrated power electronics module (IPEM) proposed by the US Power Electronics Systems Center (CPEC) in recent years, commonly known as "building blocks". It uses advanced packaging technology to reduce parasitic factors to improve voltage ringing and efficiency in the circuit, and integrates the drive circuit with the power device to increase the drive speed and thus reduce switching losses. Power electronics integration technology can not only improve transient voltage regulation, but also improve power density and system efficiency. However, such integrated modules currently face many challenges, mainly the integration of passive and active devices, and it is difficult to achieve the best thermal design. CPEC has conducted many years of research on power electronics integration technology and proposed many useful methods, structures and models.

2 Conclusion

The development of high-frequency switching power supplies for communications towards integration and miniaturization will be the main trend in the future. The power density will become increasingly larger, and the requirements for technology will become increasingly higher. Before there are new breakthroughs in semiconductor devices and magnetic materials, major technological progress may be difficult to achieve, and the focus of technological innovation will be on how to improve efficiency and reduce weight. Therefore, process technology will also occupy an increasingly high position in power supply manufacturing. In addition, the application of digital control integrated circuits is also a direction for the development of switching power supplies in the future, which will depend on the further improvement of DSP operating speed and anti-interference technology.