The quality of electronic products is a combination of both technical and reliability. As an important part of an electronic system, the reliability of the power system determines the reliability of the whole system. Switching power supply is widely used in various fields due to its small size and high efficiency. How to improve its reliability is an important part of power electronics technology. aspect.
1 Switching Power Supply Electrical Reliability Engineering Design Technology
1.1 Choice of power supply mode
Power supply methods are generally divided into: 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.
1.2 Circuit topology selection
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. The switching tube of the double-tube forward-excited, double-excited and half-bridge circuits is only the input power supply voltage. It is easier to select the 600V switch tube when 60% derating, and there is no problem of unidirectional polarization saturation. These three topologies are widely used in high voltage input circuits.
1.3 Power Factor Correction Technology
The harmonic current of the switching power supply pollutes the power grid, interferes with other common network equipment, and may cause the neutral current of the three-phase four-wire system to be too large, causing an accident. One of the solutions is to use a switching power supply with power factor correction technology.
1.4 Choice of control strategy
In the medium and small 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 350 kHz; 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 megahertz level This technology is mainly used in high-power systems, which are less common in low-power systems.
1.5 Selection of components
Since the components directly determine the reliability of the power supply, the selection of components is very 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.
1.6 Protection circuit
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 overheat protection circuits should be added during design.
2 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: the interference source, the transmission medium, and the sensitive receiving unit. The EMC design destroys 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.
3 power equipment reliability thermal design technology
Statistics show that for every 2 °C increase in temperature of electronic components, the reliability is reduced by 10%; when the temperature rises by 50 °C, the lifetime is only 1/6 of that at 25 °C. In addition to electrical stress, temperature is the most important factor affecting equipment reliability. This requires technical measures to limit the temperature rise of the chassis and components, which is the 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.
4 Safety design technology
For power supplies, safety has historically been identified as the most important performance. Unsafe products can not only perform the specified functions, but also can cause serious accidents and even cause huge losses. To ensure a high level of safety, a safety design is required. The safety product design of the power supply includes protection against electrical hazards and overheating.
For the commercial equipment market, representative safety standards include UL, CSA, VDE, etc. The content varies depending on the application, and the leakage current is allowed to be between 0.5 and 5 mA. The leakage current specified by China's military standard GJB1412 is less than 5 mA. The magnitude of the leakage current of the power supply device to ground depends on the capacity of the Y capacitor of the EMI filter, as shown in Figure 2. From the perspective of the EMI filter, the larger the capacity of the Y capacitor, the better, but the smaller the capacity of the Y capacitor from the safety point of view, the better. The capacity of the Y capacitor is determined according to safety standards. According to the GJB151A, the 50 Hz device is less than 0.1 μF and the 400 Hz device is less than 0.02 μF. If the safety performance of the X capacitor is poor, the transient spike of the power grid may be broken down, and its breakdown does not endanger personal safety, but it will cause the filter to lose its filtering function.
5 three-proof design technology
The three-proof design refers to moisture-proof design, salt spray prevention design and anti-fungal design. All applications should be carried out in the south of the Yangtze River, coastal areas and military power sources.
The surface of the electronic device will adsorb a very thin layer of wet water in the humid ocean atmosphere, that is, the water film, but when the water film reaches 20 to 30 layers thick, the electrolyte membrane necessary for chemical corrosion is formed. The salt-containing electrolyte has a strong corrosive activity on bare metal surfaces. In addition, the temperature is abrupt, and the dew point is generated in the air, which causes the insulation resistance between the printed wires to decrease, the components are moldy, the patina is generated, and the pins are corroded and broken.
The hot and humid environment provides favorable conditions for the growth of mold. Molds use organic matter in electronic equipment as a nutrient to absorb moisture and secrete organic acids, destroy insulation, cause short circuits, and accelerate metal corrosion.
In engineering, corrosion-resistant materials can be selected, and then coated, coated or chemically treated to cover the metal or non-metal protective film of the electronic equipment and parts to isolate it from the surrounding medium, thereby achieving the purpose of protection. . The structure is sealed or semi-sealed to isolate the external adverse environment. The application of special three-proof varnish to the printed board and components can effectively avoid corona and breakdown between the wires and improve the reliability of the power supply. The transformer should be immersed in paint and sealed at the end to prevent moisture from entering the short circuit.
Three-proof design and electromagnetic shielding are often contradictory. If the three-proof design is excellent, it has good electrical insulation, and the electrically insulated outer casing has no good shielding effect. These two aspects need to be considered comprehensively. In the design of the whole machine, the shielding and grounding requirements should be fully considered, and a reasonable process should be adopted to ensure long-term conduction of the surface with electrical contact.
6 Anti-vibration design technology
Vibration is also an important cause of power failure. In the vibration test, tantalum capacitors and aluminum electrolytic capacitor leads are often broken, which requires reinforcement design. Generally, a tantalum capacitor can be fixed by using a silicone rubber, and a fixing clip is attached to an aluminum electrolytic capacitor having a height of more than 25 cm and a diameter of more than 12 cm, and a rib is attached to the printed board.
7 Summary
The above recommendations are only applicable to industrial products and military power supplies, and can be made in different aspects for commercial grade products. In short, the reliability of power supply equipment is not only related to electrical design, but also related 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 the power supply.
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