Several aspects of the future development of switching power supply technology
The rapid development of the communication industry has greatly promoted the development of communication power. Switching power supply is at the core of the communication system and has become the mainstream of modern communication power supply systems. In the field of communication, a high frequency rectifier is generally referred to as a primary power source, and a direct current-direct current (DC/DC) converter is referred to as a secondary power source. With the development of large-scale integrated circuits, power modules are required to be miniaturized, and thus it is necessary to continuously increase the switching frequency and adopt a new circuit topology, which puts higher requirements on the high-frequency switching power supply technology.
1 Development of high frequency switching power supply technology for communication
The development of high-frequency switching power supply technology for communication can be basically 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 soft switching technology can effectively reduce the switching loss and switching stress, which helps to improve the efficiency of the converter. The PFC technology can improve the input power factor of the AC/DC converter and reduce the harmonic pollution to the power grid. The technology is mainly applied in the three-phase input converter of 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 research direction for multi-level technology in the future.
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 a smaller size of magnetic material and passive components, but increasing the frequency will greatly increase the switching loss and driving loss of the MOSFET, and soft switching technology. Applications can reduce switching losses. The most widely used communication power engineering applications are active clamp ZVS technology, ZVS phase-shift full-bridge technology born in the early 1990s, and synchronous rectification technology proposed in the late 1990s.
1.1.1 ZVS Active Clamp
Active clamp technology has gone through three generations and has been patented. The first generation is VICOR's active clamp ZVS technology, which increases the DC/DC operating frequency to 1 MHZ and the power density is close to 200 W/in3. However, its conversion efficiency does not exceed 90%. To reduce the cost of the first generation of active clamping technology, IPD has filed a patent for the second generation of active clamping technology that uses P-channel MOSFETs and is used on the secondary side of the transformer for active clamps in the forward circuit topology. This makes the cost of the product much lower. However, the zero voltage switching (ZVS) boundary conditions of the MOSFET formed by this method are narrow, and the PMOS operating frequency is also not ideal. In order to make the magnetic energy not be consumed in the reset of the magnetic core, a Chinese-American engineer applied for the third-generation active clamp technology patent in 2001, which is characterized by magnetic on the basis of the second generation of active clamps. The energy released during core reset is transferred to the load, thus achieving higher conversion efficiency. It has three circuit schemes: one of which can use N-channel MOSFETs, so the operating frequency can be higher. This technology can combine ZVS soft switching and synchronous rectification technology, thus achieving up to 92% efficiency and Power density above 250 W/in3.
1.1.2 ZVS phase shift full bridge
From the mid-1990s, ZVS phase-shifted full-bridge soft-switching technology has been widely used in medium and high-power power supplies. This technology plays a big role in improving the efficiency of the converter when the switching speed of the MOSFET is not ideal, but its disadvantages are also many. The first drawback is the addition of a resonant inductor, which results in a certain volume and loss, and the electrical parameters of the resonant inductor need to be consistent, which is more difficult to control during the manufacturing process; the second drawback is the loss of effective Air ratio . In addition, since synchronous rectification is more convenient to improve the efficiency of the converter, the effect of the phase shift full bridge on the secondary side synchronous rectification is not ideal. The original PWM ZVS phase-shifted full-bridge controller, UC3875/9 and UCC3895 only control the primary, additional logic is required to provide accurate sub-polar synchronous rectification control signals; today's latest phase-shifted full-bridge PWM controllers such as the LTC1922/1 Although the LTC3722-1/-2 has increased the secondary side synchronous rectification control signal, it still cannot effectively achieve the secondary side ZVS/ZCS synchronous rectification, but this is one of the most effective measures to improve the converter efficiency. Another major improvement of the LTC3722-1/-2 is that it reduces the inductance of the resonant inductor, which not only reduces the volume of the resonant inductor and its losses, but also improves the loss of 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 leakage inductance of the transformer, resulting in low reliability in mass production and less application in actual products. For the conversion of the output voltage from 12 V to 20 V, a special external driver IC is used, which can achieve better electrical performance and higher reliability.
TI has proposed a chip-driven UCC27221/2 for predictive driving strategy, dynamically adjusting the dead time to reduce the conduction loss of the body diode. ST also designed a similar chip, STSR2/3, which is used not only for flyback but also for forward excitation, while improving the performance of continuous and discontinuous conduction modes. The US Center for Power Electronics Systems (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 synchronous rectification. The conduction loss and reverse recovery loss of the body diode are easy to implement, and the soft switching of the primary main switch tube is easy . Linear Technology's synchronous rectification control ICs, the LTC3900 and LTC3901, are better suited for use in forward, push-pull and full-bridge topologies.
ZVS and ZCS synchronous rectification technologies have also been applied, such as synchronous rectification drive of active clamp forward circuit (NCP1560), synchronous rectification driver chip LTC1681 and LTC1698 of two-transistor forward circuit, but they have not achieved symmetrical circuit extension. The excellent effect of Park ZVS/ZCS synchronous rectification.
1.2 Modeling and Simulation
Switching converters mainly have two modeling methods: small signal and large signal analysis.
Small-signal analysis: mainly state space averaging method , proposed by RDMiddlebrook of the California Institute of Technology in 1976, it can be said that this is the first real significance of modeling and analysis in the field of power electronics. breakthrough. Later, such as the current injection equivalent circuit method, the equivalent controlled source method (which was proposed by Chinese scholar Zhang Xingzhu in 1986), and the 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 cannot be effectively analyzed for ripple; the stability analysis cannot be performed accurately; it may not be suitable for the resonant converter; the key point is that the average method yields The model 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, and the accuracy will be higher.
Large signal analysis method: 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. Another is the equivalent small-parameter signal analysis method proposed by Professor Qiu Shuisheng of South China University of Technology in China in 1994 , which is not only applicable to PWM converters but also to resonant converters, and can perform ripple analysis of output. .
The purpose of modeling is to simulate and then perform stability analysis. In 1978, R.Keller first used R.D. Middlebrook's state space averaging 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, such as: Switching inductance model of Dr.SamBenYaakov; Dr. RayRidley model; Switching power supply average Pspice model based on Dr.VatcheVorperian Orcad9.1; Switch of ICAP4 based on Steven Sandler The average power Isspice model; based on Dr. Vincent G. Bello's Cadence switching power supply average model and so on. On the basis of using these models, the macro-model is constructed by combining the main parameters of the converter, and the DC/DC converter constructed by the model is used for DC analysis on the professional circuit simulation software (Matlab, Pspice, etc.) platform. Small signal analysis and closed loop large signal transient analysis.
Due to the ever-changing topology of the converter, the development speed is extremely fast, and accordingly, the requirements for modeling the converter are becoming more and more strict. It can be said that the modeling of the converter must catch up with the development of the converter topology in order to be more accurately applied to engineering practice.
1.3 Digital Control
The simple application of digitization is mainly the protection and monitoring circuit, and the communication with the system, and has been widely used in communication power supply systems. It can replace many analog circuits, complete the power supply start, input and output overvoltage, undervoltage protection, output overcurrent and short circuit protection, and overheat protection. Through specific interface circuits, communication with the system can also be completed. display.
More advanced digital applications include not only perfect protection and monitoring functions, but also PWM waves, control of power switching devices through drive circuits, and closed-loop control. At present, TI, ST and Motorola have introduced dedicated motor and motion control DSP chips. At present, the digitization of communication power supply mainly adopts the combination of analog and digital. The PWM part still uses a special analog chip, and the DSP chip mainly participates in the duty cycle control, frequency setting, output voltage regulation, protection and monitoring functions.
In order to achieve a faster dynamic response, many advanced control methods have been gradually proposed. For example, ON Semiconductor proposes improved V2 control, Intel Corporation proposes Active-droop control, Semtech proposes charge control, Fairchild proposes Valley current control, IR proposes multiphase control, and many US universities also A variety of other control ideas have been proposed [7, 8, 9]. Digital control increases system flexibility, provides better communication interfaces, troubleshooting capabilities, and immunity to interference. 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 solved urgently.
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