Brief introduction of the development of high-frequency switching power supply technology for communication

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

The development of high-frequency switching power supply technology for communication 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 multilevel technology have been 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 the efficiency of the converter; the use of PFC technology can improve the input power factor of the AC / DC converter and reduce the harmonic pollution to the power grid; and multi-level The technology is mainly used in the three-phase input converter of the communication power supply, 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 future research direction of multilevel technology.

In order to reduce the size of the converter, it is necessary to increase the switching frequency to achieve high power density. It is necessary to use smaller-sized magnetic materials and passive components. However, increasing the frequency will greatly increase the switching loss and driving loss of the MOSFET. Soft switching technology Application can reduce switching losses. At present, the most widely used communication power engineering projects are active clamping ZVS technology, ZVS phase-shifted full-bridge technology born in the early 1990s and synchronous rectification technology proposed in the late 1990s.

1.1.1 ZVS active clamp

Active clamping technology has gone through three generations, and all have applied for patents. The first generation is the active clamping ZVS technology of the American VICOR company, 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 Company declared a patent for the second generation of active clamping technology, which uses a P-channel MOSFET and is used for active clamping of the forward circuit topology on the secondary side of the transformer , Which makes the product cost much lower. However, the zero-voltage switching (ZVS) boundary condition of the MOSFET formed by this method is narrow, and the PMOS operating frequency 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 the third-generation active clamping technology patent in 2001. Its feature is that the magnetic energy is based on the second-generation active clamping. The energy released when the core is reset is transferred to the load, so a higher conversion efficiency is achieved. It has three circuit schemes: one scheme can use N-channel MOSFET, so the operating frequency can be higher. Using this technology can combine ZVS soft switching and synchronous rectification technology, so it achieves up to 92% efficiency and Power density above 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 there are also many disadvantages. The first disadvantage is to add a resonant inductor, which results in a certain volume and loss, and the electrical parameters of the resonant inductor need to maintain consistency, which is more difficult to control during the manufacturing process; the second disadvantage is that the effective share is lost Air ratio. In addition, because the synchronous rectification is more convenient to improve the efficiency of the converter, the phase-shifted full bridge is not ideal for controlling the secondary side synchronous rectification. The original PWMZVS phase-shifted full-bridge controller, UC3875 / 9 and UCC3895 only control the primary, need to add logic circuit to provide accurate secondary pole synchronous rectification control signal; now the latest phase-shifted full-bridge PWM controller such as LTC1922 / 1 LTC3722-1 / -2, although the secondary side synchronous rectification control signal has been added, it still cannot effectively achieve the ZVS / ZCS synchronous rectification of the secondary side, but this is one of the most effective measures to improve the efficiency of the converter. Another major improvement of LTC3722-1 / -2 is to 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 the duty cycle.

1.1.3 Synchronous rectification

Synchronous rectification includes self-driving and external driving. 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 actual products. For the conversion of output voltage above 12V to around 20V, special external driver ICs are mostly used, so that better electrical performance and higher reliability can be achieved.

TI has proposed a chip UCC27221 / 2 that predicts the driving strategy, and dynamically adjusts the dead time to reduce the conduction loss of the body diode. ST Company has also designed a similar chip STSR2 / 3, which is not only used for flyback but also for forward excitation, while improving the performance of continuous and discontinuous conduction mode. The United States Power Electronics System Center (CPES) 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 body diode of the synchronous rectifier Conduction loss and reverse recovery loss, and it is easy to realize the soft switching of the primary main switch tube. Linear Technology's synchronous rectification control chips LTC3900 and LTC3901 can be better applied in forward, push-pull and full-bridge topologies.

ZVS and ZCS synchronous rectification technologies have also been applied, such as the synchronous rectification drive of active clamp forward circuits (NCP1560), the synchronous rectification drive chips LTC1681 and LTC1698 of two-transistor forward circuits, but they have not achieved symmetric circuit extension The excellent effect of Park ZVS / ZCS synchronous rectification.

1.2 Modeling and simulation

Switching converters mainly have two modeling methods for small signal and large signal analysis.

Small signal analysis method: mainly the state space averaging method, proposed by the RDMiddlebrook of the California Institute of Technology in 1976. It can be said that this is the first truly significant breakthrough in modeling and analysis in the field of power electronics. Subsequent emergence such as 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 belong to the category of circuit average method. The shortcomings of the averaging method are obvious. The signal is averaged and the ripple analysis cannot be effectively performed; the stability analysis cannot be accurately performed; the resonant converter may not be suitable; the key point is that the averaging method yields The model of 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 the accuracy to be higher.

Large signal analysis method: there are analytical method, phase plane method, large signal equivalent circuit model method, switching signal flow method, n-th harmonic three-port model method, KBM method and general average method. There is also an equivalent small parameter signal analysis method proposed by Mr. Qiu Shuisheng, a professor at South China University of Technology in 1994, which is not only applicable to PWM converters but also resonant converters, and can perform output ripple analysis.

The purpose of modeling is to simulate and then perform stability analysis. In 1978, R. Keller first used RDMiddlebrook's state space averaging theory for SPICE simulation of switching power supplies. In the past 30 years, in the modeling of the average SPICE model of the switching power supply, many scholars have established various model theories, thus forming various SPICE models. These models have their own strengths and are more representative: Dr. Sam Ben Yaakov's switching inductance model; Dr. Ray Ridley's model; Dr. VatcheVorperian's Orcad9.1 switching power supply average Pspice model; StevenSandler's ICAP4 switching power supply Average Isspice model; average model of switching power supply based on Dr.VincentG.Bello's Cadence and so on. 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 composed of the built model is used to perform DC analysis on the professional circuit simulation software (Matlab, Pspice, etc.) platform. Small signal analysis and closed-loop large signal transient analysis.

Since the topology of the converter is changing with each passing day and the development speed is extremely fast, accordingly, the requirements for modeling the converter are becoming more and more strict. It can be said that the modeling of converters must catch 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 digitization is mainly protection and monitoring circuits, as well as communication with the system, which has been widely used in communication power systems. It can replace many analog circuits, complete power supply start-up, input and output over-voltage, under-voltage protection, output over-current and short-circuit protection, and overheat protection, etc., through a specific interface circuit, can also complete the communication with the system and display.

The more advanced digital applications include not only the realization of perfect protection and monitoring functions, but also the output of PWM waves, the control of power switching devices through the drive circuit, and the realization of closed-loop control functions. At present, TI, ST and Motorola have launched dedicated motor and motion control DSP chips. At present, the digitalization of communication power supply mainly adopts the combination of analog and digital. The PWM part still uses a special analog chip, while the DSP chip mainly participates in duty cycle control, frequency setting, output voltage adjustment, and 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, Intersil Corporation proposed Active-droop control, Semtech Corporation proposed charge control, Fairchild Corporation proposed Valley current control, IR Corporation proposed multi-phase control, and many universities in the United States also Various other control ideas have been proposed [7, 8, 9]. Digital control can improve the flexibility of the system, provide better communication interface, fault diagnosis ability, and anti-interference ability. However, in precision communication power supplies, factors such as control accuracy, parameter drift, current detection and current sharing, and control delay will be practical problems that need to be resolved urgently.

1.4 Magnetic integration

As the switching frequency increases, the volume 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.

Research on magnetic component integration technology is relatively mature abroad, 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 had proposed the idea of ​​magnetic integration when it proposed the Cuk converter. Since 1995, the Center for Power Electronics Systems (CPES) of the United States has done a lot of research on the integration of magnetic devices. Using the concept of coupled inductors, it has conducted in-depth research on the integration of multi-phase BUCK inductors and applied it to various types of transformation器 中. In 2002, Hong Kong University Yim-ShuLee and others 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 perspectives, such as the choice of core size, the determination of materials and windings, and the evaluation of iron and copper losses. But in addition to magnetic integration technology, the problem of magnetic flux imbalance must also be considered, because the magnetic flux is distributed in each part of the core and 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 complicated and difficult. However, the advantage of high power density brought by it must 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, and reliability is the primary indicator of communication power supply

. The complete testing methods in the manufacturing process, the use of complete process monitoring points and anti-static measures have largely continued the best design performance of the product, and the widespread use of SMD chip devices will greatly improve the reliability of welding Sex. European and American countries will require lead-free technology for electronic products from 2006, which will place higher and stricter requirements on the selection of components in the communication power supply and the control of the manufacturing process.

At present, the more attractive technology is the concept of Power Electronics Integrated Module (IPEM) proposed by the United States Power Electronics System Center (CPEC) in recent years, commonly known as "building blocks". Using advanced packaging technology to reduce parasitic factors to improve the voltage ringing and efficiency in the circuit, the drive circuit and power devices are integrated together to increase the driving 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, there are many challenges for such integrated modules, 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 has proposed many useful methods, structures and models.

2 conclusion

The development of high-frequency switching power supplies for communication towards integration and miniaturization will be the main trend in the future, and the power density will be greater and greater, and the requirements for the process will be higher and higher. Before new breakthroughs are made in semiconductor devices and magnetic materials, major technological advancements may be difficult to achieve. The focus of technological innovation will be on how to improve efficiency and reduce weight. Therefore, process technology will also occupy a higher and higher 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. (Organization of China Educational Equipment Purchasing Network)

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