Method for improving the reliability of switching power supply

    Two qualities that are indispensable for the quality of electronic products - technical and reliability. As a successful electronic product, the comprehensive level of the two aspects affects product quality. As an important component in an electronic system, the reliability of the power system determines the safety performance of the whole system. Switching power supplies are widely used in various fields due to their small size and high efficiency. However, how to improve the reliability of switching power supplies is power electronic technology. An important turning point in striding.

    1. Electromagnetic compatibility (EMC) design technology

    The switching power supply mostly adopts pulse width modulation (PWM) technology. The pulse waveform is rectangular, and its rising and falling edges contain a large number of harmonic components. In addition, the reverse recovery of the output rectifier also generates electromagnetic interference (EMI), which is the influence. Unfavorable factors of reliability make electromagnetic compatibility of the system an important issue. There are three necessary conditions for generating electromagnetic interference: interference source, transmission medium, sensitive receiving unit, and EMC design is to destroy one of these three conditions.

    For the switching power supply, the interference source is mainly suppressed, and the interference source is concentrated in the switching circuit and the output rectifier circuit. The technologies used include filtering technology, layout and wiring technology, shielding technology, grounding technology, and sealing technology.

    2, power equipment reliability thermal design technology

    Experts pointed out that in addition to electrical stress, temperature is one of the most important factors affecting equipment reliability. Statistics show that for every 2 °C increase in temperature of electronic components, the reliability decreases by 10; when the temperature rises by 50 °C, the lifetime rises only 25 1/6 at °C. Due to the influence of temperature, it is necessary to take technical measures to limit the temperature rise of the chassis and components - thermal design. The principle of thermal design is to reduce the heat generation, that is, to select better control methods and technologies, such as phase shift control technology, synchronous rectification technology, etc., in addition, to select low-power devices, reduce the number of heat-generating devices, and increase The width of the rough line increases the efficiency of the power supply. The second is to enhance heat dissipation, that is, the use of conduction, radiation, convection technology to transfer heat, including radiator design, air cooling (natural convection and forced air cooling) design, liquid cooling (water, oil) design, thermoelectric cooling design, heat pipe Design and so on. Forced air cooling heat dissipation is more than ten times larger than natural cooling, but it is necessary to increase the fan, fan power supply, interlocking device, etc. In the design, the heat dissipation method should be selected according to the actual situation.

    3. Switching power supply electrical reliability engineering design technology

    For the power factor correction technology, it is specifically that the harmonic current of the switching power supply pollutes the power grid and interferes with other common network equipment, which may cause the neutral current of the three-phase four-wire system to be too large, causing an accident. The general solution is to adopt Switching power supply with power factor correction technology.

    In terms of protection circuits, in order to make the power supply work reliably under various harsh environments, various protection circuits such as surge protection, overvoltage and undervoltage, overload, short circuit, and overheating should be added during the design.

    For the choice of control strategy, it is traced back to the medium- and small-power power supply. Current-mode PWM control is a widely used method. In the DC-DC converter, the output ripple can be controlled at 10mV, which is superior to the conventional power supply for voltage-type control. Hard switching technology is limited by switching loss, the switching frequency is generally below 350kHz; soft switching technology is to make the switching device switch at zero voltage or zero current state, realize switching loss is zero, so that the switching frequency can be raised to the level of megahertz. This technology is mainly used in high-power systems, which are less common in low-power systems.

    For the power supply mode, it is generally divided into a centralized power supply system and distributed power supply. Modern power electronic systems generally use distributed power supply systems to meet the requirements of high reliability equipment.

    Since the components directly determine the reliability of the power supply, the selection of components is particularly important. Component failures are mainly concentrated in the following four points: manufacturing quality issues, device reliability issues, design issues, and loss issues. This should be given sufficient attention in use.

    For the circuit topology, the switching power supply generally adopts eight kinds of topologies such as single-ended forward type, single-ended flyback type, double-tube forward type, double single-ended forward type, double forward type, push-pull type, half bridge, and full bridge. Among them, the switching pressure of the double-tube forward-excited, double-excited and half-bridge circuits is only the input power supply voltage, and it is relatively easy to select the 600V switching tube when the 60 derating, and there is no problem of unidirectional polarization saturation. In general, these three topologies are widely used in high voltage input circuits.