Do you understand military switching power supply reliability technology?

Do you know the reliability technology of military switching power supply? This article makes a more detailed analysis and comparison of the factors affecting the reliability of military PWM switching power supply, and puts forward some suggestions to improve the reliability of switching power supply based on engineering reality.

The design of electronic products, especially military regulated power supplies, is a systematic project. Not only the parameter design of the power supply itself must be considered, but also electrical design, electromagnetic compatibility design, thermal design, safety design, three-proof design, etc. Because even the smallest negligence in any aspect may lead to the collapse of the entire power supply, we should fully realize the importance of reliability design of power supply products.

2. Electrical reliability design of switching power supply

2.1 Selection of power supply method

The deviation between the outputs of the centralized power supply system and the voltage difference caused by the difference in transmission distance reduce the power supply quality. Moreover, when a single power supply is used for power supply, the system may be paralyzed when the power supply fails. Because the power supply unit is close to the load, the distributed power supply system has improved dynamic response characteristics, good power supply quality, small transmission loss, high efficiency, energy saving, and high reliability. It is easy to form an N + 1 redundant power supply system, and it is relatively easy to expand power. . Therefore, the use of distributed power supply systems can meet the requirements of high reliability equipment.

2.2 Selection of circuit topology

Switching power supplies generally use eight topologies, including single-ended forward, single-ended flyback, dual-tube forward, dual single-ended forward, dual forward, push-pull, half-bridge, and full-bridge. The pressure of single-ended forward, single-ended flyback, dual single-ended forward, and push-pull switching tubes is more than twice the input voltage. If used with a 60% derating, it will be difficult to select the switching tube. In push-pull and full-bridge topologies, unidirectional bias magnetic saturation may occur, causing damage to the switch tube. However, the half-bridge circuit will not have this problem because it has the ability to automatically resist imbalance. The pressure-bearing pressure of the two-tube forward and half-bridge circuit switch tubes is only the maximum input voltage of the power supply. Even if it is used with a 60% derating, it is relatively easy to select a switch tube. These two types of circuit topologies are generally used in high-reliability projects.

2.3 Selection of control strategy

In small and medium-power power supplies, current-type PWM control is a widely used method. It has the following advantages over the voltage-controlled type: cycle-by-cycle current limitation is faster than voltage-type control, and the switching tube will not be damaged due to overcurrent. Reduce overload and short-circuit protection; excellent grid voltage regulation rate; fast transient response; stable loop, easy to compensate; ripple is much smaller than the voltage control type. Production practice shows that the output ripple of the current-controlled 50W switching power supply is around 25mV, which is much better than the voltage-controlled type.

Hard switching technology is limited by switching losses, and the switching frequency is generally below 350kHz. Soft switching technology applies the resonance principle to make the switching device switch on and off at zero voltage or zero current, achieving zero switching loss, thereby increasing the switching frequency. To the megahertz level, this converter using soft switching technology combines the advantages of both PWM converters and resonant converters, and has close to ideal characteristics, such as low switching loss, constant frequency control, appropriate energy storage component size, and relatively small size. Wide control range and load range, but this technology is mainly used in high-power power supplies, and PWM technology is still dominant in small and medium-power power supplies.

2.4 Selection of components

Because components directly determine the reliability of the power supply, the selection of components is very important. The failure of components is mainly concentrated in the following four aspects:

(1) Manufacturing quality issues

Failures caused by quality problems have nothing to do with working stress. Those with unqualified quality can be eliminated through strict inspection. Mature products from designated manufacturers should be selected for engineering applications. Uncertified products are not allowed to be used.

(2) Component reliability issues

The reliability problem of components is the problem of basic failure rate. This is a random failure. The difference from quality problems is that the failure rate of components depends on the working stress level. Under a certain stress level, the failure rate of components will be greatly reduced. In order to eliminate components that do not meet the requirements for use, including unqualified electrical parameters, unqualified sealing performance, unqualified appearance, poor stability, early failure, etc., a screening test should be conducted, which is a non-destructive test. Through screening, the component failure rate can be reduced by 1 to 2 orders of magnitude. Of course, the cost of screening tests (time and expense) is very high, but comprehensive maintenance, logistics support, and joint testing of the entire aircraft are still cost-effective, and the development cycle will not be extended. General requirements for screening tests of main components of power supply equipment:

① The resistance is 100% tested according to technical conditions at room temperature, and unqualified products are eliminated.

② Ordinary capacitors are 100% tested according to technical conditions at room temperature, and unqualified products are eliminated.

③ Various parameters of connectors are sampled and tested according to technical conditions.

④ Semiconductor devices are screened according to the following procedures:

Visual inspection → Preliminary test → High temperature storage → High and low temperature shock → Electric power aging → High temperature test → Low temperature test → Normal temperature test

After screening, the rejection rate Q should be calculated

Q=(n/N)×100%

In the formula: N——The total number of samples tested; n——The number of samples rejected; If Q exceeds the upper limit specified by the standard, all components in this batch will not be allowed to be put on the machine and will be processed according to relevant regulations.

When the standards are met, the qualified components will be marked with paint spots and then sent to a special warehouse for installation.

(3) Design issues

The first is to properly select the right components:

① Try to use silicon semiconductor devices and use less or no germanium semiconductor devices.

② Use more integrated circuits to reduce the number of discrete devices.

③ Using MOSFET for the switch tube can simplify the drive circuit and reduce losses.

④ Try to use diodes with soft recovery characteristics for the output rectifier.

⑤ Devices with metal packaging, ceramic packaging, or glass packaging should be selected. It is prohibited to use plastic packaged devices.

⑥ Integrated circuits must be Class I products or military products with quality levels above MIL-M-38510 and MIL-S-19500 standards B-1.

⑦ Use as few relays as possible during design. If necessary, use sealed relays with good contact.

⑧ In principle, potentiometers should not be used. Those that must be retained should be sealed.

⑨ The distance between the absorption capacitor, the switching tube and the output rectifier tube should be very close. Because high-frequency current flows, it is easy to heat up. Therefore, these capacitors are required to have high-frequency, low-loss and high-temperature resistance characteristics.

In humid and salt spray environments, aluminum electrolytic capacitors will suffer from shell corrosion, capacity drift, increased leakage current, etc. Therefore, it is best not to use aluminum electrolytic capacitors on ships and humid environments. Since the electrolyte will decompose when bombarded by space particles, aluminum electrolytic capacitors are not suitable for power supplies of aerospace electronic equipment.

Tantalum electrolytic capacitors have good temperature and frequency characteristics, are resistant to high and low temperatures, have long storage times, and have stable and reliable performance. However, tantalum electrolytic capacitors are heavy, have low volume ratio, are not resistant to back pressure, have few high-voltage varieties (>125V), and are expensive.

Regarding derating design:

The basic failure rate of electronic components depends on working stress (including electricity, temperature, vibration, impact, frequency, speed, collision, etc.). Except for a few components that fail under low stress, the higher the working stress, the higher the failure rate. In order to reduce the failure rate of components, derating design must be carried out during circuit design. In addition to reliability, the degree of derating also needs to consider factors such as volume, weight, and cost. Different components have different derating standards. Practice shows that the basic failure rate of most electronic components depends on electrical stress and temperature, so derating is mainly to control these two stresses. The following is the derating of commonly used components in switching power supplies. Amount coefficient:

① The power derating coefficient of the resistor is between 0.1 and 0.5.

② The power derating coefficient of the diode is below 0.4, and the reverse withstand voltage is below 0.5.

③ The voltage derating coefficient of the light-emitting diode is below 0.6, and the power derating coefficient is below 0.6.

④ The voltage derating coefficient of the power switch tube is below 0.6, and the current derating coefficient is below 0.5.

⑤ The voltage derating coefficient of ordinary aluminum electrolytic capacitors and non-polar capacitors is between 0.3 and 0.7.

⑥ The voltage derating coefficient of tantalum capacitors is below 0.3.

⑦ The current derating coefficient of the inductor and transformer is below 0.6.

(4) Loss problem

Component failure caused by loss depends on the length of working time and has nothing to do with working stress. When aluminum electrolytic capacitors work at high frequency for a long time, the electrolyte will gradually lose, and the capacity will also decrease simultaneously. When the electrolyte loss is 40%, the capacity will drop by 20%; when the electrolyte loss is 0%, the capacity will drop by 40%. At this time, the capacitor The core has basically dried up and can no longer be used. In order to prevent malfunctions, the replacement time of aluminum electrolytic capacitors should generally be indicated on the drawing, and replacement must be forced upon expiration.

2.5 Protection circuit settings

In order for the power supply to work reliably in various harsh environments, a variety of protection circuits should be installed, such as protection circuits against surge impact, overvoltage, undervoltage, overload, short circuit, overheating, etc.

3. Electromagnetic compatibility (EMC) design

Because the switching power supply uses pulse width modulation (PWM) technology, its pulse waveform is rectangular, and both the rising and falling edges contain a large number of harmonic components. In addition, the reverse recovery of the output rectifier will also produce electromagnetic interference (EMI). This is Unfavorable factors that affect reliability, thus making electromagnetic compatibility an important issue for the system.

As shown in Figure 1, there are three necessary conditions for the generation of electromagnetic interference: interference source, transmission medium, and sensitive receiving unit. EMC design is to destroy one of these three conditions.

Figure 1 Three conditions for the formation of electromagnetic interference:

For switching power supplies, the main purpose is to suppress interference sources, which are concentrated in the switching circuit and the output rectifier circuit. The technologies used include filtering technology, layout and wiring technology, shielding technology, grounding technology, sealing technology, etc. EMI is divided into conducted interference and radiated interference according to the propagation path. The frequency range of conducted noise is very wide, from 10kHz to 30MHz. Although we know the cause of interference, in terms of efficiency, it may not be a good way to solve it by controlling the rise and fall time of the pulse waveform. One of the solutions is to add Install the power EMI filter, output filter and absorption circuit, see Figure 2.

The power supply EMI filter is actually a low-pass filter. It transmits 50Hz or 400Hz AC power to electronic equipment without attenuation, but greatly attenuates the incoming interference signal. At the same time, it can suppress the interference signal generated by the equipment itself. Prevent it from entering the power grid and harming other equipment on the public network. Selecting an EMI filter is based on the size of the insertion loss, selecting the filter network structure and component parameters, and selecting parameters such as rated voltage, rated current, leakage current, insulation resistance, temperature conditions, etc. based on actual requirements. The power EMI filter is best installed near the socket where the power cord enters the chassis. Countermeasures to suppress output noise are basically solved in three frequency bands: 10kHz to 150kHz, 150kHz to 10MHz, and above 10MHz. The range of 10kHz to 150kHz is mainly normal noise, which is generally solved by using a general-purpose LC filter. The noise in the range of 150kHz ~ 10MHz is mainly common mode component, which is usually solved by using common mode suppression filter. The common mode choke should use ferrite magnetic material with high magnetic permeability and good frequency characteristics. The inductance is (1 ~ 2) mH and the capacitance is between 3300pF ~ 4700pF. If the noise in the low frequency band is controlled, it can be appropriately Increase the value of LC. The countermeasure in the frequency range above 10MHz is to improve the shape of the filter. The reverse recovery of the output rectifier diode will also cause electromagnetic interference. In this case, an RC absorption circuit can be used to suppress the rising rate of the current. Usually R is between (2 ~ 20)Ω, C is between 1000pF ~ 10nF, and C should be Use high-frequency ceramic capacitors.

Good layout and wiring techniques are also an important means of controlling noise. In order to reduce the occurrence of noise and prevent malfunctions caused by noise, the following points should be noted:

① Minimize the area surrounded by high-frequency pulse current.

② The buffer circuit should be as close as possible to the switching tube and output rectifier diode.

③ Keep the area where the pulse current flows away from the input and output terminals to separate the noise source from the outlet.

④ The control circuit and the power circuit should be separated and single-point grounding should be used. Large-area grounding can easily cause antenna effects, so it is recommended not to use large-area grounding.

⑤ If necessary, the output filter inductor can be placed on the ground return line.

⑥ Use multiple low ESR (equivalent series resistance) capacitors in parallel for filtering.

⑦ Use copper foil for low-inductance and low-resistance wiring.

⑧ There should not be too long parallel lines between adjacent printed lines. Try to avoid parallel lines and use vertical intersections. Do not change the line width or make sudden corners. Ring routing is prohibited.

⑨ The input and output lines of the filter must be separated. It is prohibited to bundle the input and output wires of the switching power supply together.

For radiation interference, sealing and shielding technology is mainly used to implement electromagnetic sealing on the structure, which requires good electromagnetic contact between various parts of the shell to ensure electromagnetic continuity. At present, in order to reduce weight, aluminum alloy casings are mostly used, but aluminum alloys have poor magnetic permeability, so the casing needs to be plated with a layer of nickel or sprayed with conductive paint, and the inner wall is covered with a shielding material with high magnetic permeability. The permanent connection of the shell should be firmly bonded with conductive adhesive or a continuous weld structure should be used. If it needs to be disassembled, it can be pressed with conductive rubber strips to ensure electromagnetic continuity. Conductive materials require high conductivity, elasticity, and a minimum width-to-thickness ratio.

4. Reliability thermal design of power supply equipment

Besides electrical stress, temperature is the most important factor affecting equipment reliability. The temperature rise inside the power supply equipment will cause component failure. When the temperature exceeds a certain value, the failure rate will increase exponentially. When the temperature exceeds the limit value, component failure will occur. Foreign statistics show that for every 2°C increase in temperature of electronic components, the reliability drops by 10%; the lifespan when the temperature rises to 50°C is only 1/6 of that when the temperature rises to 25°C. Technical measures need to be taken to limit the temperature rise of the chassis and components. This is thermal design. The principles of thermal design are: first, to reduce heat generation, that is, to use better control methods and technologies, such as phase-shift control technology, synchronous rectification technology, etc.; the other is to use low-power devices, reduce the number of heating devices, and increase heating. Thick printed line width improves power supply efficiency. The second is to enhance heat dissipation, that is, to use conduction, radiation, and convection technologies to transfer heat. This includes the use of radiators, air cooling (natural convection and forced air cooling), liquid cooling (water, oil), thermoelectric cooling, heat pipes and other methods.

The heat dissipation of forced air cooling is more than ten times greater than that of natural cooling, but it requires additional fans, fan power supplies, interlocking devices, etc. This not only increases the cost and complexity of the equipment, but also reduces the reliability of the system. In addition, it also increases Noise and vibration, so under normal circumstances, natural cooling should be used as much as possible instead of cooling methods such as air cooling and liquid cooling. When laying out components, the heating components should be placed downwind or on the upper part of the printed board. The radiator should be treated with an oxidation and blackening process to increase the radiation rate. It is not allowed to be coated with black paint. Spraying conformal paint will affect the heat dissipation effect, so it is necessary to increase the margin appropriately. The surface of the radiator installation device must be smooth and flat, and silicone grease is generally applied to the contact surface to improve thermal conductivity. Transformers and inductors should use thicker wires to suppress temperature rise.

5. Safety design

For power supplies, safety has always been determined to be one of the most important properties. Unsafe products not only fail to complete the specified functions, but may also cause serious accidents, causing huge losses such as machine crashes and fatalities. In order to ensure that the product has a high level of safety, safety design must be carried out. The safety design of power supply products mainly focuses on preventing electric shock and burns.

For the commercial equipment market, representative safety standards include UL, CSA, VDE, etc. The contents vary depending on the use. The allowable leakage current is between 05mA and 5mA. The leakage current specified by my country's military standard GJB1412 is less than 5mA. The size of the leakage current of the power supply equipment to the ground depends on the capacity of the EMI filter capacitor Cy, as shown in Figure 2. From the perspective of EMI filter, the larger the capacity of capacitor Cy, the better, but from the perspective of safety, the smaller the capacity of capacitor Cy, the better. The capacity of capacitor Cy is determined according to safety standards. If the safety performance of the capacitor Cx is poor, it may be broken down when a transient spike occurs in the power grid. Although its breakdown does not endanger personal safety, it will cause the filter to lose its filtering function. In order to prevent accidental electric shock, in principle, the product end (non-power end) of the plug socket is a pin, and the grid end (power end) is a hole; the input end of the power supply device is a pin, and the output end is a hole.

In order to prevent burns, for exposed parts (radiators, chassis, etc.) that may come into contact with the human body, when the ambient temperature is 25°C, the maximum temperature should not exceed 60°C, and the maximum temperature of the panel and manually adjusted parts should not exceed 50°C. .

6. Three-proof design

Three-proof design refers to moisture-proof design, salt-spray-proof design and mold-proof design.

During design, sealing measures should be taken for components that require sealing; epoxy resin potting can be used for irreparable combined devices; the moisture absorption of components and raw materials used should be small, and materials containing cotton, linen, and silk should not be used. and other mold-prone products; protective nets should be installed on sealed chassis and cabinets to prevent insects and rodents from entering; the outer top of devices directly exposed to the atmosphere should not use a concave structure to avoid corrosion caused by water accumulation; corrosion-resistant materials can be selected. Then, the surface of the electronic equipment and its parts is covered with a metal or non-metal protective film through plating, coating or chemical treatment to isolate the surrounding medium; the structure is sealed or semi-sealed to isolate the external adverse environment; the printed board is Special three-proof varnish coating on the surface of components can effectively avoid corona and breakdown between wires and improve the reliability of the power supply; inductors and transformers should be dipped in paint and end-sealed to prevent moisture from entering and causing short-circuit accidents.

7. Conclusion

The above recommendations only apply to military power supplies. Different choices may be made in some aspects for commercial and industrial products. In short, the reliability of power supply equipment is not only related to electrical design, but also to components, structure, assembly, technology, processing quality, etc. Reliability is based on design. In actual engineering applications, feedback data should be obtained through various tests to improve the design and further improve the reliability of the power supply. The above is the reliability technology of military switching power supply. I hope it can help you.