From the switch power supply design and production process began to describe it, first talk about the design of printed board. Switching power supply works in high frequency, high pulse state, which belongs to a special kind of analog circuit. The principle of high frequency circuit routing shall be followed when the board is laid.
1. Layout: The pulse voltage connection line is as short as possible, in which the input switching tube is connected to the transformer and the output transformer is connected to the rectifier tube. The pulse current loop is as small as possible as the input filter capacitance is positive to the transformer and the return capacitance is negative. The X capacitor in the transformer circuit should be as close to the input end of the switching power supply as possible. The input line should avoid being parallel with other circuits. The Y capacitance shall be placed at the housing ground terminal or FG connection end. Keep a distance between the common touch induction and the transformer to avoid magnetic coupling. If it is not easy to deal with, a shield can be added between the common inductance and the transformer.
The output capacitors are generally two, one near the rectifier tube and the other near the output terminal, which can affect the output ripple index of the power supply. The parallel effect of two small capacity capacitors should be better than that of one large capacity capacitor. Heating device and electrolytic capacitor to maintain a certain distance, in order to extend the whole machine life, electrolytic capacitor is the switch power supply life bottle strength, such as transformer, power tube, high-power resistor and electrolytic to maintain a distance, electrolytic should also be set aside heat dissipation space, conditions allow it can be placed in the air inlet.
Note the control part: Weak signal high impedance circuit connection to short feedback loop such as sampling, should try to avoid its interference when processing circuit, current sampling signal, especially in the current control circuit, deal with easy to appear some unexpected accidents, some of the skills, to now 3843 circuit for example shown in figure (1) a better figure a in figure 2, figure 2 on the full load current waveform oscilloscope for observed obviously superimposed spike, due to the interference current limit point lower than design value, without this kind of phenomenon, and as a switch tube driving signal circuit, switch pipe driving resistance close to switch, switch tube can be improved working reliability, This is related to the high DC impedance voltage driving characteristics of power MOSFET.
Some principles of PCB wiring are discussed below.
Line spacing: With the continuous improvement and improvement of the manufacturing process of printed circuit board, there is no problem that the outgoing line spacing in general processing plants is equal to or less than 0.1mm, which can fully meet most applications. Considering the components and production process used for switching power supply, the minimum line spacing for general double panel is set as 0.3mm, the minimum line spacing for single panel as 0.5mm, and the minimum line spacing for bonding pad to bonding pad, bonding pad to bonding hole, or bonding pad to bonding hole is set as 0.5mm, which can avoid "bridging" phenomenon during welding operation. , so that most of the plate manufacturers can easily meet the production requirements, and the yield control can be very high, but also achieve a reasonable wiring density and a more economical cost.
The minimum line spacing is only suitable for signal control circuits and low-voltage circuits with voltage less than 63V. When the voltage between lines is greater than this value, the line spacing can be generally taken according to the empirical value of 500V/1mm.
In view of some relevant standards that have clear provisions on line spacing, they should be strictly followed, such as ac inlet to fuse end connection. Some power sources have high volume requirements, such as module power. It is proved to be feasible in practice that the input sideline spacing is 1mm. For ac input and (isolated) DC output power products, the safety spacing shall be greater than or equal to 6mm, which is of course determined by related standards and execution methods. The general safety distance can be taken as a reference by the distance on both sides of the feedback optocoupler, which is greater than or equal to this distance in principle. Also can be in the photocoupler below the printed board grooves, so that creepage distance increased to meet the insulation requirements. Generally, the distance between components on the ac input side of switching power supply and components on the board and the uninsulated shell and radiator should be greater than 5mm, while the distance between components on the output side and the shell and radiator should be greater than 2mm, or it should be strictly implemented in accordance with the safety specifications.
Common methods: The circuit board grooving method mentioned above is suitable for some occasions with insufficient spacing. By the way, this method is also commonly used to protect the discharge gap and is commonly seen in the TV picture tube tail plate and ac input of power supply. This method has been widely used in module power supply and can get good results under the condition of sealing.
Method 2: Cushion insulating paper can be made of green paper, polyester film, teflon directional film and other insulating materials. General general power supply with blue paper or polyester film pad in the circuit board between the metal shell, this material has high mechanical strength, has a certain moisture resistance. Ptfe directional membrane has been widely used in module power supply due to its high temperature resistance. An insulating film can also be placed between the element and the surrounding conductor to improve the electrical resistance of the insulation.
Note: Some device insulation coverings cannot be used as an insulating medium to reduce the safe spacing, such as the outer skin of the electrolytic capacitor, which may contract under high temperatures. Large electrolytic explosion-proof tank front to set aside space to ensure that the electrolytic capacitor in extraordinary circumstances can be unimpeded pressure.
To talk about some matters concerning the wiring of printed copper sheet:
Current density: Most electronic circuits are now made of copper insulated panels. Common circuit board copper skin thickness is 35 m, the wiring can be based on 1A/mm empirical value to take the current density value, specific calculation can see the textbook. To ensure the mechanical strength of wiring, the line width should be greater than or equal to 0.3mm(other non-power circuit board may have a smaller minimum line width). Circuit boards of 70 m copper thickness are also commonly used in switching power supplies, so current densities can be higher.
In addition, the commonly used circuit board design tools and software generally have design specifications, such as line width, line spacing, dry plate through hole size and other parameters can be set. When designing circuit board, the design software can automatically execute according to the specification, which can save a lot of time, reduce part of the workload, and reduce the error rate.
Generally, double panel can be used for high reliability requirements or wiring line density. It is characterized by moderate cost, high reliability and can meet most applications.
Some products of module power supply line also adopt multilayer board, which is mainly convenient to integrate transformer inductance and other power components, optimize wiring, power pipe heat dissipation, etc. It has the advantages of good artistic consistency and good heat dissipation of transformer, but its disadvantages are high cost and poor flexibility, and it is only suitable for industrial mass production.
Single panel and market circulation general switching power supply almost adopt single side circuit board, it has the advantage of low cost, in the design and production process to take some measures to ensure its performance.
Talk about some experience of single-side printed board design, because the single panel has the characteristics of low cost, easy to manufacture, widely used in switching power supply circuit, because it has only one side to bind copper, the electrical connection of the device, mechanical fixed to rely on the layer of copper skin, must be careful when dealing with.
In order to ensure good welding mechanical structure performance, the single panel welding plate should be a little larger to ensure good adhesion of copper and substrate, and not be shaken off copper. Generally, the width of welding ring should be greater than 0.3mm. The diameter of the hole should be slightly larger than the pin diameter of the device, but should not be too large, to ensure the shortest distance between the pin and the solder. The size of the hole should not interfere with the normal inspection, and the diameter of the hole is generally greater than the diameter of the pin 0.1-0.2mm. Multi-pin devices can also be larger to ensure a smooth check.
The electrical connection shall be as wide as possible, and the principle width shall be larger than the diameter of the welding plate. In special cases, the line shall be widened when the connection line meets the welding plate (commonly known as tear drop) to avoid the fracture of the line and the welding plate in some conditions. Principle Minimum line width should be greater than 0.5mm.
Components on a single panel shall be close to the circuit board. For devices requiring overhead heat dissipation, sleeve should be added to the pin between the device and the circuit board to support the device and increase insulation, and to minimize or avoid the impact of external impact on the connection between the pad and the pin, so as to enhance the soundness of welding. The heavier components on the circuit board can increase the support connection points and strengthen the connection strength with the circuit board, such as transformers and power device radiators.
Under the condition of not affecting the distance between the single panel welding surface and the shell, the pin can be left longer. Its advantage is that the strength of the welding part can be increased, the welding area can be increased, and the virtual welding phenomenon can be found immediately. When the pin is long, the welding force is small. In Taiwan and Japan, the device pin is often bent on the welding surface to a 45-degree Angle with the circuit board, and then the welding process, the same as above. Today to talk about some items in the design of double panel, in some of the requirements are higher, or wire density is larger in the application environment with double-sided printed board, its performance and various aspects of the index is much better than a single panel.
Because the holes have been metallized with high strength, the welding ring is smaller than that of the single panel, and the hole diameter of the welding plate can be slightly larger than the diameter of the pin, because in the welding process, it is beneficial for the solder solution to penetrate into the top plate through the holes, so as to increase the welding reliability. However, there is a disadvantage, if the hole is too large, part of the device may float up under the impact of jet tin during wave soldering, resulting in some defects.
For the processing of high-current line running, the line width can be processed according to the front post. If the width is not enough, tin plating on the line can be used to increase the thickness to solve the problem. There are many methods:
1. Set the running wire as the property of the welding plate, so that the running wire will not be covered by flux during the manufacturing of the circuit board, and the hot air will be coated with tin at ordinary times.
2. Place the welding plate at the wiring location and set the welding plate to the shape of wire running. Pay attention to setting the hole of the welding plate to zero.
3. Line placement in the solder mask is the most flexible method. However, not all PCB manufacturers will understand your intention. Solder will not be applied where the solder mask is placed.
Several methods of tin plating line as mentioned above, it should be noted that, if the wide line is all coated with tin, after welding, will adhere to a large number of solder, and the distribution is very uneven, affecting the appearance. Generally, thin strips with tin plating width of 1~1.5mm and length can be determined according to the circuit. The tin plating interval of 0.5~1mm double-sided circuit board provides great selectivity for layout and wiring, which can make wiring more reasonable. About grounding, power and signal must be separated, the two can converge on the filter capacitor, to avoid a large pulse current through the signal ground connection and the accident factors lead to instability, signal control circuit by one-point earthing method as far as possible, there is a skill, as far as possible put the grounding line is placed on the same wiring layer, the last shop in another layer of earth. The output line generally passes through the filter capacitance first and then to the load, and the input line must pass through the capacitance first and then to the transformer. The theoretical basis is to let the ripple current pass through the brigade filter capacitance.
Voltage feedback sampling. In order to avoid the influence of large current flowing through the wire, the feedback voltage sampling point must be placed at the end of the power supply output to improve the load effect index of the whole machine.
Go line from a wiring layer change to another wiring commonly used hole connected, unfavorable by device pin pad, because in the instrumentation devices that may destroy the connection relations, and in every 1 a current through, at least 2 hole, hole diameter is greater than 0.5 mm, usually 0.8 mm to ensure processing reliability.
Device heat dissipation, in some small power power supply, circuit board wiring can also heat dissipation function, its characteristics is to wire as wide as possible, in order to increase heat dissipation area, not coated with flux, conditions can be evenly placed through the hole, enhance thermal conductivity.
The application of aluminum substrate in switching power supply and the application of multilayer printed board in switching power supply circuit are discussed.
Aluminum substrate is constructed by itself and has the following characteristics: excellent thermal conductivity, single side copper binding, devices can only be placed on the copper binding surface, and electrical connection holes cannot be opened, so jumper wire cannot be placed as per single panel.
The SMT device, switch tube and output rectifier tube are generally placed on the aluminum substrate to conduct the heat out through the substrate, with low thermal resistance and high reliability. The transformer adopts planar SMT structure, which can also dissipate heat through the substrate, and its temperature rise is lower than the conventional one. The transformer of the same specification adopts aluminum substrate structure, which can obtain larger output power. Aluminum substrate jumpers can be treated in a bypass manner. The power supply of aluminum substrate is generally composed of two printed boards, and another board is placed to control the circuit. The two boards are integrated by physical connection.
Due to excellent thermal conductivity aluminum plate, difficult in small amount of manual welding, solder cooling too fast, easy problems existing a simple and practical method, will be a common electric iron used ironing temperature adjustment function is), turn it over, ironing, oriented fixed well, the temperature to 150 ¡æ or so, put the aluminum plate on top of the iron, heating time, and then affix components and welding according to the conventional method, the iron temperature with device is easy to weld advisable, high device damage when possible, copper or aluminum plate, low temperature welding effect is bad, be flexible.
In recent years, with the multilayer circuit board used in switching power supply circuit, making it possible to print line transformer, because of the sandwich plate, layer spacing is small, also can make full use of the transformer window section, can add one or two on the main circuit board piece composed of multilayer printed coil to use window, reduce the line current density as a result of printed coil, reduce manual intervention, transformer good consistency and flat structure, the leakage inductance is low, good coupling. Open magnetic core, good heat dissipation conditions. Because it has many advantages, it is beneficial to mass production, so it is widely used. However, it is not suitable for small-scale production due to the large investment in the initial stage of development.
Switching power supply can be divided into two forms: isolated and non-isolated. The topological form of isolating switching power supply is mainly discussed here. The isolated power supply can be divided into two types according to its structure: positive excitation type and flyback type. Flyback type refers to the cut-off of the secondary side when the original side of the transformer is switched on. The transformer stores energy. When the original edge is cut off, the secondary edge is switched on and the energy is released to the working state of the load. In general, conventional flyback power supply has more single tubes than double tubes. Positive excitation refers to the induction of the corresponding voltage output to the load on the primary side of the transformer while the secondary side, and the energy is directly transferred through the transformer. According to the specifications can be divided into conventional positive excitation, including single tube positive excitation, double tube positive excitation. Half - bridge and bridge circuit belong to positive excitation circuit.
Both forward and flyback circuits have their own characteristics, which can be used flexibly in the process of circuit design to achieve the optimal cost performance. Generally, flyback type can be used in low power situations. Single tube positive circuit can be used for slightly larger, double tube positive circuit or half bridge circuit can be used for medium power, push pull circuit can be used for low voltage, and the same working state as half bridge. High power output, generally using bridge circuit, low voltage can also use push-pull circuit.
Flyback power supply is widely used in small and medium power supply because of its simple structure and the inductance which is similar to the volume of transformer is eliminated. In some introductions, flyback power supply can only achieve tens of watts, output power more than 100 watts has no advantage, it is difficult to achieve. I think the general situation is like this, but also can not be generalized, PI company's TOP chip can achieve 300 watts, there are articles about flyback power can achieve kilowatts, but I have never seen the real thing. The output power is related to the output voltage.
Flyback power supply is the leakage inductance of the transformer is a very key parameter, with the need of flyback power supply transformer store energy, to make full use of the transformer core, typically KaiQi gap in the magnetic circuit, the purpose is to change the core of hysteresis loop of the slope, the transformer to withstand large pulse current shock, into a state without iron core saturation nonlinear, high magnetic circuit gas gap in the magnetic resistance
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