The integration and miniaturization of high-frequency switching power supply for communication is the main trend of development in the future. The power density will be larger and larger, and the requirements for technology will be higher and higher. At present, the development of high-frequency switching power supply technology for communication mainly includes the following aspects: converter topology, modeling and simulation, digital control and magnetic integration.
1. Converter topology
In recent years, soft switching technology, power factor correction technology and multilevel technology are hot spots in converter topology. Using soft switching technology can effectively reduce the loss and switching stress of high-frequency switching power supply for communication, which is helpful to improve the efficiency of the converter; PFC technology can improve the input power factor of AC / DC converter and reduce the harmonic pollution to power grid; Multilevel technology is mainly used in three-phase input converter of communication power supply, which can effectively reduce the voltage stress of switch tube. At the same time, due to the high input voltage, using appropriate soft switching technology to reduce switching loss is an important research direction of multilevel 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 use smaller magnetic materials and passive components. However, increasing the frequency will greatly increase the switching loss and driving loss of MOSFET, and the application of soft switching technology can reduce the switching loss. At present, the most widely used communication power engineering are active clamp ZVS technology, ZVS phase-shifting full bridge technology and synchronous rectification technology.
However, the efficiency of the first generation of DC clamping technology is close to 200W / in3% after the first generation of DC clamping technology. The second generation of active clamping technology adopts p-channel MOSFET and is used for active clamping of forward circuit topology on the secondary side of transformer, which greatly reduces the product cost. However, the zero voltage switching (ZVS) boundary condition of MOSFET formed by this method is narrow, and the working frequency of PMOS is not ideal. The third generation active clamp technology transfers the energy released during core reset to the load on the basis of the second generation active clamp, so it realizes higher conversion efficiency. It has three circuit schemes: one scheme can adopt n-channel MOSFET, so the working frequency can be higher. Using this technology, ZVS soft switching and synchronous rectification technology can be combined together, so it can achieve an efficiency of 92% and a power density of more than 250W / in3.
ZVS phase shifted full bridge soft switching technology plays a great role in improving the efficiency of the converter when the switching speed of MOSFET is not ideal, but it also has many disadvantages. The first disadvantage is to add a resonant inductor, which leads to a certain volume and loss, and the electrical parameters of the resonant inductor need to be consistent, which is difficult to control in the manufacturing process; The second disadvantage is the loss of an effective duty cycle. In addition, because synchronous rectification is more convenient to improve the efficiency of the converter, the control effect of phase-shifting full bridge on the secondary side synchronous rectification is not ideal. The original pwmzvs phase-shifting full bridge controller, UC3875 / 9 and ucc3895 only control the primary stage, and additional logic circuit is needed to provide accurate secondary pole synchronous rectification control signal. Although the current phase-shifting full bridge PWM controllers such as LTC1922 / 1 and ltc3722-1 / - 2 have added the secondary side synchronous rectification control signal, they still can not effectively achieve the secondary side ZVS / ZCS synchronous rectification, but can effectively improve the efficiency of the converter. Ltc3722-1 / - 2 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.
Synchronous rectification includes self drive and external drive. The self driven 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 less application in practical products. For the conversion of output voltage above 12V to about 20V, special external drive IC is mostly used, which can achieve better electrical performance and higher reliability.
2. Modeling and simulation
There are two main modeling methods of switching converter: small signal and large signal analysis.
Small signal analysis method: mainly state space average method, which can be said to be the first significant breakthrough in modeling and analysis in the field of power electronics. Later, such as current injection equivalent circuit method, equivalent controlled source method and three terminal switching device method belong to the category of circuit average method. The disadvantage of the average method is obvious. The signal is averaged, but the ripple analysis can not be carried out effectively; The stability analysis cannot be carried out accurately; It may not be suitable for resonant converters; The key point is that the model obtained by the average method has nothing to do with the switching frequency, and the applicable condition is that the natural frequency generated by the inductance and capacitance in the circuit must be much lower than the switching frequency for high accuracy.
Large signal analysis methods: analytical method, phase plane method, large signal equivalent circuit model method, switching signal flow method, Nth harmonic three port model method, KBM method, general average method and equivalent small parameter signal analysis method.
The purpose of modeling is to simulate and then analyze the stability. In the past 30 years, in the modeling of the average SPICE model of switching power supply, many scholars have established a variety of model theories, thus forming a variety of SPICE models. These models have their own advantages. The representative ones are: Dr. samben Yaakov's switching inductance model; Ridley's model; Based on Dr. vatchevorperian's orcad9 1 Average PSPICE model of switching power supply; The average ispice model of switching power supply based on Steven Sandler's icap4; Based on Dr. Vincent G Bello's cadence's switching power supply average model and so on. On the basis of using these models, combined with the main parameters of the converter, the macro model is constructed, and the DC / DC converter composed of the built model is used for DC analysis, small signal analysis and closed-loop large signal transient analysis on the platform of professional circuit simulation software (Matlab, PSpice, etc.).
3. Digital control
The simple application of digitization is mainly the protection and monitoring circuit, as well as the communication with the system. At present, it has been widely used in high-frequency switching power supply for communication. It can replace many analog circuits to complete the power start, input and output overvoltage and undervoltage protection, output overcurrent and short circuit protection, overheating protection, etc. through a specific interface circuit, it can also complete the communication and display with the system.
The more advanced application of digitization includes not only realizing the perfect protection and monitoring function, but also outputting PWM wave, controlling the power switching device through the driving circuit, and realizing the closed-loop control function. At present, the digitization of communication power supply mainly adopts the combination of analog and digital. The PWM part still adopts special analog chip, while the DSP chip is mainly involved in duty cycle control, frequency setting, output voltage regulation, protection and monitoring.
Digital control can improve the flexibility of the system and provide better communication interface, fault diagnosis ability and anti-interference ability. However, in the precision communication power supply, factors such as control accuracy, parameter drift, current detection and current sharing, and control delay will be practical problems that need to be solved urgently.
4. Magnetic set
With the increase of switching frequency, the volume of switching converter will be reduced and the power density will be greatly improved, but the switching loss will increase, and more magnetic devices will be used, so it will occupy more space.
Since 1995, the American Center for power electronics systems integration (CPES) has done a lot of research work on magnetic device integration. Using the concept of coupled inductor, multiphase buck inductor integration has been deeply studied and applied to various types of converters.
The conventional design method of magnetic components is extremely cumbersome and needs to be considered from different angles, such as the size selection of magnetic core, the determination of 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 magnetic flux imbalance, because the magnetic flux is distributed in each part of the core, its equivalent total magnetic flux is different, and some parts may be saturated in advance. Therefore, the analysis and research of magnetic device integration will be more complex and difficult. However, its advantage of high power density must be a major development trend of high-frequency switching power supply for communication in the future.
5. Manufacturing process
The manufacturing process of high-frequency switching power supply for communication is quite complex, and directly affects the electrical function, electromagnetic compatibility and reliability of the power supply system, and reliability is the primary index of communication power supply. The adoption of complete detection means, complete process monitoring points and anti-static measures in the production and manufacturing process continues the best design performance of the product to a great extent, and the wide use of SMD chip devices will greatly improve the reliability of welding. European and American countries have required lead-free technology for electronic products since 2006, which puts forward higher and stricter requirements for the selection of devices in communication power supply and the control of production and manufacturing process.
At present, the most attractive technology is the concept of power electronic integration module (IPEM), commonly known as "building block", proposed by the American power electronic system center (CPEC) in recent years. Advanced packaging technology is adopted to reduce parasitic factors to improve the voltage ringing and efficiency in the circuit. The driving circuit and power devices are integrated to improve the driving speed and reduce the switching loss. Power electronics integration technology can not only improve the regulation of transient voltage, but also improve power density and system efficiency. However, there are many challenges in such integration modules, mainly the integration of passive and active devices, and it is difficult to achieve the best thermal design.