What does the reliability design of switching power supply include?

Date：2022-08-03 18:03:55Views：724

Reliability design is an important part of the overall engineering design of the system. It is a series of analysis and design techniques to ensure the reliability of the system. As an important component in an electronic system, the reliability of power supply determines the reliability of the whole system. Switching power supply is widely used in various fields because of its small size and high efficiency. How to improve its reliability is an important aspect of power electronics technology.

Engineering design technology for electrical reliability of switching power supply

1.1 selection of power supply mode

Power supply modes are generally divided into centralized power supply system and distributed power supply. Modern power electronic system generally adopts distributed power supply system to meet the requirements of high reliability equipment.

1.2 selection of circuit topology

Switching power supply generally adopts eight topologies, including single end forward, single end flyback, double transistor forward, double single end forward, double forward, push-pull, half bridge, full bridge, etc. Among them, the switch tubes of double switch forward, double forward and half bridge circuits bear pressure only for the input power supply voltage. It is easier to choose 600V switch tubes when 60% derating, and there will be no problem of unidirectional magnetic saturation. These three topologies are widely used in high-voltage input circuits.

1.3 power factor correction technology

The harmonic current of switching power supply pollutes the power grid, interferes with other common network equipment, and may also make the neutral line current of three-phase four wire system too large, causing accidents. One of the solutions is to adopt switching power supply with power factor correction technology.

1.4 selection of control strategy

Among the power supplies with medium and small power, current mode PWM control is widely used. The output ripple in DC-DC converter can be controlled at 10mV, which is better than the conventional power supply with voltage mode control.

Due to the limitation of switching loss, the switching frequency of hard switching technology is generally below 350 kHz; Soft switching technology enables switching devices to switch at zero voltage or zero current to achieve zero switching loss, so that the switching frequency can be increased to the megahertz level. This technology is mainly used in high-power systems, and it is rare in low-power systems.

1.5 selection of components

Because components directly determine the reliability of power supply, the selection of components is very important. The failure of components mainly focuses on the following four points: manufacturing quality problems, component reliability problems, design problems, loss problems. This should be paid enough attention in use.

1.6 protection circuit

In order to make the power supply work reliably in various harsh environments, a variety of protection circuits should be added in the design, such as anti surge impact, over and under voltage, overload, short circuit, overheating and other protection circuits.

Electromagnetic compatibility (EMC) design technology

Switching power supply mostly adopts pulse width modulation (PWM) technology. The pulse waveform is rectangular, and its rising edge and falling edge contain a large number of harmonic components. In addition, the reverse recovery of the output rectifier will also produce electromagnetic interference (EMI), which is an adverse factor affecting reliability, which makes the electromagnetic compatibility of the system become an important problem.

There are three necessary conditions for electromagnetic interference: interference source, transmission medium and sensitive receiving unit. EMC design is one of these conditions.

For switching power supply, it is mainly to suppress interference sources, which are concentrated in switching circuit and output rectifier circuit. The adopted technologies include filtering technology, layout and wiring technology, shielding technology, grounding technology, sealing technology, etc.

Reliability Thermal Design Technology of power supply equipment

Statistics show that the reliability of electronic components decreases by 10% when the temperature rises by 2 ℃; The service life when the temperature rises by 50 ℃ is only 1/6 of that when the temperature rises by 25 ℃. In addition to electrical stress, temperature is the most important factor affecting the reliability of equipment. This requires technical measures to limit the temperature rise of chassis and components, which is thermal design. The principle of thermal design is to reduce the calorific value, that is, to select better control methods and technologies, such as phase-shifting control technology, synchronous rectification technology and other technologies, and to select low-power devices, reduce the number of heating devices, increase the width of thick printed wiring, and improve the efficiency of power supply. The second is to strengthen 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, etc.

The heat dissipation of forced air cooling is more than ten times greater than that of natural cooling, but fans, fan power supply, interlocking devices, etc. should be increased, and the heat dissipation mode should be selected according to the actual situation in the design.

Safety design technology

For power supply, safety has always been identified as the most important performance. Unsafe products not only cannot complete the specified functions, but also may cause serious accidents, and even cause huge losses of machine destruction and human death. In order to ensure high safety of products, safety design must be carried out. The safety design of power products includes the prevention of electrical hazards and overheating hazards.

For the commercial equipment market, the representative safety standards include UL, CSA, VDE, etc. the content varies according to the purpose. The allowable leakage current is between 0.5 ~ 5mA, and the leakage current specified in the Chinese military standard gjb1412 is less than 5 mA. The leakage current of power supply equipment to ground depends on the capacity of Y capacitor of EMI filter, as shown in Figure 2. From the perspective of EMI filter, the larger the capacity of Y capacitor is, the better, but from the perspective of safety, the smaller the capacity of Y capacitor is, the better. The capacity of Y capacitor is determined according to the safety standard. According to GJB151A, 50 Hz equipment is less than 0.1 μ F. 400Hz equipment is less than 0.02 μ F。 If the safety performance of X capacitor is poor, it may be broken down when the transient peak of power grid occurs. Its breakdown does not endanger personal safety, but it will make the filter lose its filtering function.

Three prevention design technology

Three prevention design refers to moisture-proof design, salt spray proof design and mold proof design. The three prevention design should be carried out for all applications in the south of the Yangtze River, coastal areas and military power supply.

The surface of electronic equipment will adsorb a thin wet water layer, namely water film, in the humid marine atmosphere, but when the water film reaches 20 ~ 30 molecular layer thickness, it forms the electrolyte film necessary for chemical corrosion. This salt rich electrolyte has strong corrosive activity to the bare metal surface. In addition, the sudden change of temperature will produce dew point in the air, which will reduce the insulation resistance between printed lines, mold the components, produce copper green, corrosion and fracture of pins, etc.

The humid and hot environment provides favorable conditions for the breeding of mold. Mold takes organic matter in electronic equipment as nourishment, adsorbs water and secretes organic acids, destroys insulation, causes short circuit and accelerates metal corrosion.

In engineering, corrosion-resistant materials can be selected, and then through plating, coating or chemical treatment, that is, the performance of electronic equipment and parts can be covered with a room of metal or non-metal protective film to isolate it from the surrounding media, so as to achieve the purpose of protection. The structure adopts sealed or semi sealed form to isolate the external adverse environment. Coating special three proofing varnish on printed boards and components can effectively avoid corona and breakdown between conductors and improve the reliability of power supply. The transformer shall be painted and sealed at the end to prevent short circuit accidents caused by moisture entering.

Three prevention design and electromagnetic shielding are often contradictory. If the three prevention design is excellent, it will have good electrical insulation, while the electrically insulated shell will not have good shielding effect. These two aspects need to be considered comprehensively. In the design of the whole machine, the requirements of shielding and grounding should be fully considered, and a reasonable process should be adopted to ensure the long-term continuity of the surface with electrical contact.

Anti vibration design technology

Vibration is also an important cause of power failure. In the vibration test, the leads of tantalum capacitors and aluminum electrolytic capacitors are often broken by vibration, which requires reinforcement design. Generally, tantalum capacitors can be fixed with silica gel, aluminum electrolytic capacitors with a height of more than 25cm and a diameter of more than 12cm can be equipped with fixing clips, and printed boards can be equipped with ribs.

The above suggestions are only applicable to industrial and military power supplies, and different choices can be made for commercial products in some aspects. In short, the reliability of power supply equipment is not only related to electrical design, but also to assembly, process, structural design, processing quality and other aspects. Reliability is based on design. In practical engineering applications, feedback data should be obtained through various tests to improve the design and further improve the reliability of power supply.