Definition of switching power supply

Switching powersupply is to use electronic switching devices (such as transistors, fieldeffect tubes, thyristors, etc.) to make the electronic switching devicesconstantly "on" and "off" through the control circuit, sothat the electronic switching devices pulse modulate the input voltage, so asto realize DC / AC, DC / DC voltage conversion, and adjustable and automaticvoltage stabilization of the output voltage.

What is the bestswitching power supply?

Switching power supply is an indispensable part of various electronicequipment and the heart of electronic equipment. Its performance is directlyrelated to the technical indicators of electronic equipment and whether it canwork safely and reliably. In the early design stage of each electronicequipment, the characteristics of switching power supply may not be consideredfirst, such as size, weight, heat dissipation, etc. it is often considered toselect an appropriate switching power supply after the design of its mainelectronic equipment is completed. This also brings great challenges to theselection or design of switching power supply. In terms of customers’ actualuse, it is necessary to comprehensively consider various factors such asproduct performance, practical application, quality, cost, delivery time andsoon to select products that meet their own needs. Competitive price and high-qualityproducts that meet or exceed their own needs are the best switching powersupply products. There is no best switching power supply, only theswitching power supply that is most suitable for our required applications is thebest.

Switching power supply is a collection of hundreds of components. Thefield of switching power supply involves many technical fields, such as powerelectronics, semiconductor devices, frequency conversion technology,electromagnetic technology, computer (microprocessor) technology, integratedautomatic control and so on. Generally speaking, a good switching power supplycan be found from the following aspects.

A. Appearance
A good switchingpower supply is generally designed to be very beautiful, like a work of art. Itis a baby cultivated by engineers through team cooperation and hard work. It isthe crystallization of wisdom and will bring people a feeling of beauty. A goodswitching power supply will comprehensively consider the best air duct / heatdissipation design, wiring, processing technology, reliability and otherfactors. When a switching power supply is turned on, we can get the mostintuitive aesthetic feeling from its structure and PCB layout. A bad power supplycan be seen on its PCB board, which is messy, asymmetric and has no sense ofbeauty. Of course, this is the simplest and intuitive judgment, and a switchingpower supply with beautiful appearance is not necessarily a good switchingpower supply. The core of switching power supply is to ensure its basic workingcharacteristics, stable output and not easy to damage. Therefore, reliabilityis very important.

B. Reliability

As an importantpart of electronic system, the reliability of switching power supply directlyaffects the reliability of the whole system. We can understand the reliabilityof switching power supply from the following aspects.

1. Engineering design technology of electricalreliability of switching power supply

1.1 Selection of power supply mode

The power supply mode is generally divided into centralized power supplysystem and distributed power supply. Modern power electronic system generally adoptsdistributed power supply system to meet the requirements of high reliabilityequipment.

1.2 Selection of circuit topology

Switching power supply generally adopts eight topologies: single endforward, single end flyback, double transistor forward, double single endforward, double forward, push-pull, half bridge and full bridge. Among them,the switch tubes of double transistor forward, double forward and half bridgecircuits bear pressure only for the input power supply voltage. It is easier toselect 600V switch tubes when 60% derating, and there will be no problem ofunidirectional magnetic biassaturation. These three topologies are widely usedin high-voltage input circuits.

1.3 Power factor correction technology

The harmonic current of switching power supply pollutes the power grid andinterferes with other common network equipment. It may also cause excessiveneutral current of three-phase four wire system and cause accidents. One of thesolutions is to adopt switching power supply with power factor correctiontechnology.

1.4 Selection of control strategy

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

Due to the limitation of switching loss, the switching frequency of hard switchingtechnology is generally below 350 kHz; Soft switching technology enablesswitching devices to switch under zero voltage or zero current state to achievezero switching loss, so as to increase the switching frequency to MHz level.This technology is mainly used in high-power systems, which is rare in low-powersystems.

1.5 Selection of components

Because the components directly determine the reliability of the powersupply, the selection of components is very important. The failure ofcomponents mainly focuses on the following four points: manufacturing quality,component reliability, design and loss. Enough attention should be paid to thisin use.

1.6 Protection circuit

In order to make the power supply work reliably in various harshenvironments, a variety of protection circuits should be added in the design,such as anti-surge impact, overvoltage and under voltage, overload, shortcircuit, overheating and other protection circuits.

2. Electromagnetic compatibility (EMC) design technology

Switching power supplies mostly use pulse width modulation (PWM)technology. The pulse waveform is rectangular, and its rising and falling edgescontain a large number of harmonic components. In addition, the reverserecovery of the output rectifier will also produce electromagneticinterference(EMI), which is an adverse factor affecting the reliability, whichmakes the electromagnetic compatibility of the system become an importantproblem.

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

For switching power supply, it is mainly to suppress interference sources,which are concentrated in switching circuit and output rectifier circuit. Theadopted technologies include filtering technology, layout and wiringtechnology, shielding technology, grounding technology, sealing technology andso on.

3. Reliability Thermal Design Technology of power supplyequipment

Statistics show that the reliability of electronic components decreases by10% when the temperature increases by 2 ℃; The service lifewhen the temperature rises by 50 ℃ is only 1 /6 of that when the temperature rises by 25 ℃. In addition toelectrical stress, temperature is an important factor affecting equipmentreliability. This requires technical measures to limit the temperature rise ofchassis and components, which is thermal design. The principle of thermal designis to reduce the calorific value, that is, select better control methods and technologies,such as phase-shifting control technology and synchronous rectificationtechnology. In addition, select low-power devices, reduce the number of heatingdevices, increase the width of thick printed lines and improve the efficiencyof power supply. The second is to strengthen heat dissipation, that is, totransfer heat by using conduction, radiation and convection technologies,including radiator design, air cooling (natural convection and forced aircooling) design, liquid cooling (water and oil) design, thermoelectric coolingdesign, heat pipe design, etc.

The heat dissipation of forced air cooling is more than ten times greaterthan that of natural cooling, but the fan, fan power supply, interlockingdevice, etc. shall be added, and the heat dissipation mode shall be selectedaccording to the actual situation in the design.

4. Safety design technology

For power supply, safety has always been identified as an importantperformance. Unsafe products not only can not complete the specified functions,but also may have serious accidents and even cause huge losses of machinedestruction and human death. In order to ensure high safety of products, safetydesign must be carried out. The safety design of power products includes theprevention of electrical hazard and overheating hazard.

For the commercial equipment market, representative safety standardsinclude UL, CSA, VDE, etc. the contents vary according to the purpose, and theallowable leakage current is between 0.5 ~ 5mA. The leakage current of powersupply equipment to ground depends on the capacity of Y capacitor of EMIfilter. From the perspective of EMI filter, the larger the capacity of Ycapacitor is, the better, but from the perspective of safety, the smaller thecapacity of Y capacitor is, the better. The capacity of Y capacitor isdetermined according to the safety standard. If the safety performance of X capacitoris poor, it may be broken down when the transient peak of power grid occurs.Its breakdown does not endanger personal safety, but will make the filter loseits filtering function.

5. Coating design technology

Coating design refers to moisture-proof design, salts pray preventiondesign and mold prevention design. Coating design shall be carried out for allapplications in the damp condition, coastal areas and special environment.

The surface of electronic equipment will adsorb a thin wet water layer,namely water film, in the humid marine atmosphere, but when the water filmreaches 20 ~ 30 molecular layer thickness, the electrolyte film necessary forchemical corrosion will be formed. This salt rich electrolyte has strongcorrosive activity to the bare metal surface. In addition, the sudden change oftemperature will produce dew point in the air, which will reduce the insulationresistance between printed lines, moldy components, produce copper green,corrosion and fracture of pins, etc.

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

In engineering, corrosion-resistant materials can be selected, and thenplated, coated or chemically treated, that is, the performance of electronicequipment and parts can be covered with a room of metal or non-metal protectivefilm to isolate it from the surrounding media, so as to achieve the purpose ofprotection. The structure adopts sealed or semi sealed form to isolate theexternal adverse environment. Coating special three proofing varnish on printedboards and components can effectively avoid corona and breakdown betweenconductors and improve the reliability of power supply. The transformer shallbe painted and end sealed to prevent short circuit accident caused by moisture.

Coating design and electromagnetic shielding are often contradictory. Ifthe coating design is excellent, it has good electrical insulation, while theelectrically insulated shell has no good shielding effect. These two aspectsneed to be considered comprehensively. In the design of the whole machine, therequirements of shielding and grounding shall be fully considered, andreasonable process shall be adopted to ensure the long-term conduction of thesurface with electrical contact.

6. Ant vibration design technology

Vibration is also an important cause of power failure. The lead oftantalum capacitor and aluminum electrolytic capacitor is often broken invibration test, which requires reinforcement design. Generally, tantalumcapacitors can be fixed with silica gel, aluminum electrolytic capacitors with aheight of more than 25cm and a diameter of more than 12cm can be equipped withfixing clips, and printed boards can be equipped with ribs.

At last, a good switching power supply, especially a complex switching power supply, includes comprehensive consideration and balance in all aspects. From the development trend of switching power supply, miniaturization, high frequency, high efficiency and digital control intelligence are the sustainable development trend. Only when it meets the actual application needs of customers, exceeds the most severe environmental application conditions and works stably without failure is a good power supply that meets the requirements. For this topic, we will continue to update and hope to get your feedback to discuss how to design and make a better, more stable and more powerful switching power supply. Welcome your valuable comments.