Talking about the design experience of switch power supply for many years, from the design of switch power PCB, PCB wiring, PCB copper wiring, aluminum substrate and multilayer PCB in the application of switch power, to the duty ratio of flyback power, absolute practice essence!
First, the switch power supply PCB design
First from the switch power supply design and production process to describe it, talk about the PCB design.Switching power supply works in high frequency, high pulse state, which belongs to a special kind of analog circuit.High frequency circuit wiring should be followed when laying the board.
Layout: the pulse voltage connection should be as short as possible, in which the input switch tube is connected to the transformer, and the output transformer is connected to the rectifier tube.The pulsed current loop should be as small as possible as the input filter capacitance is positive to the transformer and back capacitance is negative to the switch tube.The output part of the transformer from the output end to the rectifier tube to the output electricity feel the output capacitance return transformer circuit X capacitor should be as close as possible to the switch power supply input end, the input line should avoid parallel with other circuits, should avoid.
The Y capacitor should be placed at the chassis ground terminal or FG connection terminal.The common touch induction is kept at a certain distance from the transformer to avoid magnetic coupling.If it is not easy to deal with, a shield can be added between the common inductor and the transformer. The above items have a great impact on the EMC performance of the switching power source.
Generally speaking, the output capacitor can be used with two small capacitance capacitors close to the rectifier tube and the other one close to the output terminal, which can affect the output ripple index of the power supply. The parallel connection effect of two small capacitance capacitors should be better than that of one large capacity capacitor.Heating devices and electrolytic capacitor to maintain a certain distance, to extend the life of the whole machine, electrolytic capacitor is the life of the switch power bottle, such as transformer, power tube, high power resistance and electrolysis to keep the distance, between electrolysis must leave space for heat dissipation, conditions allow it to be placed in the air inlet.
Control part should pay attention to: high impedance weak signal circuit wiring should be as short as possible, such as sampling feedback loop, in the processing should try to avoid its interference, current sampling signal circuit, especially the current control circuit, processing is not easy to appear some unexpected accidents.
Ii. Some principles of PCB wiring
Line spacing: with the continuous improvement and improvement of printed circuit board manufacturing process, the general processing plant to produce line spacing equal to or even less than 0.1mm has no problems, can fully meet most applications.Considering the components and production process of switching power supply.
General double panel set minimum line spacing of 0.3 mm, single panel set minimum line spacing of 0.5 mm, welding plate and welding plate, welding plate with a hole or a hole with a hole, a minimum distance of 0.5 mm can be avoided in the process of welding operation "bridging" phenomenon, so most of the system board factory can easily meet the production requirement, and can control the yield is very high, also can achieve reasonable wiring density and has a more economic cost.
The minimum line spacing is only suitable for signal control circuit and low-voltage circuit whose voltage is lower than 63V. When the voltage between lines is higher than this value, the line spacing can be obtained according to the empirical value of 500V/1mm.
In view of some relevant standards on the line spacing has a clear provision, it should be strictly in accordance with the standard, such as ac inlet end to fuse end wire.Some power sources require a lot of volume, such as module power.The practice shows that the input sideline spacing is 1mm.For ac input and (isolated) dc output power products, it is strictly stipulated that the safe spacing should be greater than or equal to 6mm, which is determined by relevant standards and implementation methods.
In general, the safe distance can be referred to by the distance between the two sides of the feedback optical coupling, and the principle is greater than or equal to this distance.Also can be in the optocoupler under the printed board grooves, so that creepage distance to meet the insulation requirements.In general, ac input side wiring or board components should be more than 5mm away from non-insulated housing and radiator, and output side wiring or devices should be more than 2mm away from housing or radiator, or strictly in accordance with safety specifications.
Common methods: the above mentioned circuit board slotting method is suitable for some space is not enough, incidentally, this method is also used to protect the discharge gap, common in the television picture tube tail plate and the power supply ac input.This method has been widely used in module power supply and can get good results under the condition of pouring and sealing.
Method 2: pad insulation paper, can use green shell paper, polyester film, teflon directional film and other insulation materials.General general power supply with green shell paper or polyester film pad in the circuit board between the metal casing, this material has high mechanical strength, has a certain moisture resistance ability.Polytetrafluoroethylene (ptfe) directional film is widely used in module power supply because of its high temperature resistance.An insulating film may also be placed between the element and the surrounding conductor to improve the electrical resistance of the insulation.
Note: insulation covers for some devices cannot be used as insulation to reduce the safe distance, such as the skin of an electrolytic capacitor, which may shrink at high temperatures.The front end of the large electrolytic explosion-proof groove should be set aside to ensure that the electrolytic capacitor can discharge pressure without hindrance in exceptional circumstances.
Three, PCB copper sheet wiring matters needing attention
Wire current density: most electronic circuits are now insulated with copper.The copper skin thickness of common circuit board is 35 microns, and the current density can be calculated according to 1A/mm empirical value.In order to ensure the principle of mechanical strength, the wire width should be greater than or equal to 0.3mm (other non-power circuit board may have a smaller minimum wire width).Circuit boards with a thickness of 70 microns are also common in switching power supplies, so the current density can be higher.
Add a point, is 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.In the design of circuit board, the design software can be implemented in accordance with the specification automatically, which can save a lot of time, reduce part of the workload and reduce the error rate.
Generally high reliability requirements of the line or wiring line density can be used double panel.Its features are moderate cost, high reliability, can meet most applications.
Module power supply line also has some products using multilayer board, mainly to facilitate the integration of transformer inductance and other power components, optimize wiring, power tube heat dissipation and so on.It has the advantages of good process appearance and consistency and good transformer heat dissipation, but its disadvantage is high cost and poor flexibility, which is only suitable for large-scale industrial production.
Single panel, market circulation general switching power supply almost all USES single side circuit board, which has the advantage of low cost, in the design, and production process to take some measures to ensure its performance.
Four, how to design one side printing
Because the single panel has the low cost, easy to manufacture the characteristic, obtains the widespread application in the switch power supply circuit, because it only has one side to tie the copper, the electric appliance connection of the device, the mechanical fixation must rely on that layer copper skin, must be careful when handling.
In order to ensure good mechanical performance of welding, the single panel welding pad should be slightly larger to ensure the good binding force between the copper skin and the substrate, so as not to peel and break the copper skin when subjected to vibration.Generally, the width of welding ring should be greater than 0.3mm.The diameter of the welding disc hole should be slightly larger than the pin diameter of the device, but should not be too large. The distance between the pin and the welding disc should be ensured to be the shortest by soldering.Multi-pin devices can also be larger to ensure smooth checking.
The electric wire should be as wide as possible, the principle width should be larger than the diameter of the welding pad, in special cases, the line should be widened when connecting with the welding pad (commonly known as the formation of tear drops), to avoid breaking the line and the welding pad in some conditions.The 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, a casing should be added to the pin between the device and the circuit board, which can play a dual role of supporting the device and increasing insulation. The impact of external force on the connection between the welding pad and the pin should be minimized or avoided to enhance the firmness of welding.Support connection point can be added for components with large weight on circuit board, and connection strength between circuit board can be strengthened, such as transformer, power device radiator.
The pins on the welding surface of single panel can be left longer without affecting the distance from the shell, which has the advantage of increasing the strength of the welding position, increasing the welding area, and discovering the phenomenon of virtual welding immediately.When the long shear leg of the pin, the stress on the welding site is small.In Taiwan, Japan often use the device pin in the welding surface into a 45-degree Angle with the circuit board, and then welding process, the same reason.Today we are going to talk about some of the issues in the design of double panel, in some of the higher requirements, or high line density of the application environment, the use of double-sided printed board, its performance and all aspects of the index is much better than the single panel.
Due to the high strength of the hole metallized, the welding ring is smaller than the single panel, and the diameter of the hole is slightly larger than the diameter of the pin, because it is conducive to the penetration of solder solution through the hole to the top welding pad in the welding process, so as to increase the welding reliability.However, there is a disadvantage, if the hole is too large, part of the device may float under the impact of the efflux tin during wave soldering, resulting in some defects.
For the handling of high-current wiring, the width of the wires can be handled in accordance with the preceding treatment. If the width is not enough, generally, tin plating on the wiring can be used to increase the thickness. There are many methods:
1, set the wiring to the properties of the pad so that the circuit board in the manufacture of the wiring will not be covered by flux resistance, hot air will be coated with tin.
2. Place the pad at the wiring place and set the pad to the shape that needs to be wired. Pay attention to setting the hole of the pad to zero.
3, place the wire in the solder mask, this method is the most flexible, but not all circuit board manufacturers will understand your intention, you need to text. Where the wire is placed on the solder mask, no flux will be applied.
As mentioned above, it should be noted that, if the very wide wiring 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 strip tin plating width in 1~1.5mm, length can be determined according to the circuit, tin plating partial interval of 0.5~1mm double-sided circuit board for layout, wiring provides a great choice, can make the 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 usually passes through the filter capacitor first, and then to the load. The input line must also pass through the capacitor first, and then to the transformer. The theoretical basis is to let the ripple current pass through the filter capacitor.
Voltage feedback sampling, in order to avoid the influence of large current through the wiring, the sampling point of feedback voltage must be placed at the end of the power 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 holes, hole diameter is greater than 0.5 mm, usually 0.8 mm to ensure processing reliability.
Device heat dissipation, in some small power supply, circuit board wiring can also heat dissipation function, its characteristic is the wiring as wide as possible, to increase heat dissipation area, not coated with flux, conditions can be evenly placed through the hole, enhance thermal conductivity.
Contact Person: Mr. Steven Luo
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