The quality of electronic products is a combination of both technical and reliability aspects. As an important part of an electronic system, the reliability of the power supply determines the reliability of the whole system. The 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 factor in power electronics technology. aspect.
Electrical Reliability Engineering Technology of Switching Power Supply
1.1 power supply options
Power supply is generally divided into: centralized power supply system and distributed power supply. Modern power electronic systems generally use a distributed power supply system to meet the requirements of high reliability equipment.
1.2 circuit topology selection
Switching power supply generally single-ended forward, single-ended flyback, double forward, dual single-ended forward, double forward, push-pull, half bridge, full bridge eight topologies. One double-forward, dual forward and half-bridge circuit switch voltage is only the input power supply voltage, 60% derating 600V switch is easier to use, and does not appear unidirectional biasing saturation problem , These three topologies in the high-voltage input circuit is widely used.
1.3 power factor correction technology
The harmonic current of the switching power supply pollutes the power grid and disturbs the other common network equipment. It may also cause the midline current of three-phase four-wire system to be too large to cause an accident. One way to solve this problem is to adopt the switching power supply with power factor correction technique.
1.4 Selection of control strategy
In the small and medium-sized power supply, current-mode PWM control is a large number of methods used in DC-DC converter output ripple can be controlled at 10mV, better than the conventional power supply voltage control.
Hard switching technology due to switching losses, the switching frequency is generally below 350 kHz; soft-switching technology is to make switching devices in the zero-voltage or zero current state switch to achieve zero switching loss, which can switch the frequency to the level of megahertz , This technology is mainly used in high-power systems, low-power systems are rare.
1.5 components of the selection
Because the components directly determine the reliability of the power supply, so the selection of components is very important. Component failure is mainly concentrated in the following four points: manufacturing quality problems, device reliability issues, design problems, loss problems. In use should be given sufficient attention.
1.6 Protection circuit
To make the power supply in a variety of harsh environments to work reliably, should be added in the design of a variety of protection circuits, such as anti-surge, over voltage, overload, short circuit, overheating protection circuit.
Electromagnetic Compatibility (EMC) Design Technology
Switching power supply using pulse width modulation (PWM) technology, the pulse waveform is rectangular, the rising edge and falling edge contains a large number of harmonic components, the other output rectifier reverse recovery will produce electromagnetic interference (EMI), which is the impact Reliability of the negative factors, which makes the system has become an important issue of electromagnetic compatibility.
There are three necessary conditions for the generation of electromagnetic interference: interference, transmission media, sensitive receiving unit, EMC design is to destroy one of these three conditions.
For switching power supply, the main source is to suppress interference, interference sources in the switching circuit and the output rectifier circuit. The technology used includes filtering technology, layout and wiring technology, shielding technology, grounding technology, sealing technology and other technologies.
3 thermal reliability of power supply equipment design technology
Statistics show that the temperature of electronic components increased by 2 ℃, reliability decreased by 10%; temperature rise at 50 ℃ when the temperature is only 25 ℃ when the temperature of 1 / In addition to electrical stress, the temperature is the most important factor affecting the reliability of the equipment. This requires technical measures to limit the temperature rise of the chassis and components, which is thermal design. Thermal design principles, one is to reduce the heat, that is to choose better control methods and technologies, such as phase-shifting control technology, synchronous rectification technology, the other is the choice of low-power devices to reduce the number of heating devices, increase The width of the rough print line increases the efficiency of the power supply. The second is to strengthen the heat, that is the use of conduction, radiation, convection heat transfer technology, which includes radiator design, air cooling (natural convection and forced air cooling) design, liquid cooling (water, oil) design, Design and so on.
Forced air cooling heat than natural cooling more than ten times more, but to increase the fan, fan power, interlock devices, etc., in the design to select the cooling mode according to the actual situation.
4 security design technology
For power, the security has always been identified as the most important performance, unsafe products not only can not complete the required function, but also may have serious accidents, or even cause a huge loss of machine crash. In order to ensure the product has a very high security, must be designed for safety. Power supply product safety design includes the prevention of electrical hazards, overheating hazards.
For the commercial equipment market, the representative safety standards are UL, CSA, VDE, etc., the content varies with the use, allowing leakage current between 0.5 ~ 5mA, China's military standards GJB1412 leakage current less than 5 mA. The leakage current of the power supply depends on the Y capacitor of the EMI filter, as shown in Figure 2. From the point of view of the EMI filter Y capacitor capacity as possible, but from the safety point of view Y capacitance of the smaller the better, Y capacitor capacity based on safety standards to decide. 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 peak of the power grid may appear to be broken down, and its breakdown will not endanger personal safety, but will make the filter loss of filtering.
5 three anti-design technology
Three-proof design refers to moisture-proof design, anti-salt spray design and anti-mildew design. Where the application of China's south of the Yangtze River, coastal areas and military power supply should be three anti-design.
The surface of electronic equipment in the humid marine atmosphere will adsorb a thin layer of wet water, the water film, but the water film to 20 to 30 molecular layer thickness, the formation of chemical corrosion necessary for the electrolyte membrane, the rich Salt-containing electrolyte on the exposed metal surface has a strong corrosive activity. Another temperature mutation, the dew point in the air, the printed circuit will reduce the insulation resistance between the components moldy, resulting in copper green, the pin is corrosion fracture and so on.
Hot and humid environment for the breeding of mold provides favorable conditions. Mold to electronic equipment in the organic matter as nutrients, adsorption of water and secretion of organic acids, destruction of insulation, lead to short circuit, accelerate the metal corrosion.
In engineering, you can use corrosion-resistant materials, and then through the plating, coating or chemical treatment that is through the performance of electronic equipment and parts of a housing to cover a metal or non-metallic protective film, so with the surrounding medium isolation, so as to achieve the purpose of protection . In the structure of the use of sealed or semi-sealed form to isolate external adverse environment. The performance of the printed circuit board and component coating three special varnish can effectively avoid the corona between the wires, breakdown, improve the reliability of the power supply. Transformer should be dipped paint, seal, to prevent moisture into the cause short-circuit accident.
Three anti-design and electromagnetic shielding is often contradictory. If the three anti-designed to have good electrical insulation, and electrical insulation of the shell is no good shielding effect, these two aspects to be taken into account. In the whole design, should take full account of shielding and grounding requirements, to take reasonable process to ensure that there is electrical contact surface long-term conduction.
6 anti-vibration design technology
Vibration is also a cause of power failure is an important reason. In the vibration test often occurs tantalum capacitor and aluminum electrolytic capacitor lead is broken, which requires reinforcement design. Generally can be fixed with silica gel tantalum capacitors, to a high degree of more than 25cm and diameter of more than 12cm of aluminum electrolytic capacitors to install a fixed clip to the printed circuit board to install ribs.
7 Summary
The above recommendations apply only to industrial products and military power, for commercial-grade products in some ways to make different choices. In short the level of reliability of power supply equipment, not only with the electrical design, but also with the assembly, process, structural design, processing quality and other aspects. Reliability is based on design, in practical engineering applications, but also through a variety of tests to obtain feedback data to improve the design, to further improve the reliability of the power supply.
