Switching power supplies operate at high frequencies and high pulse states and are a special type of analog circuit. The layout of the high frequency circuit must be followed when laying the board.
1. Layout: The pulse voltage connection is as short as possible. The input switch is connected to the transformer and the output transformer is connected to the rectifier. The pulse current loop is as small as possible as the input filter capacitor is positive to the return capacitance of the transformer to the switch. Output part of the transformer output to the rectifier to the output inductor to the output capacitor return transformer circuit X capacitor should be as close as possible to the input end of the switching power supply, the input line should avoid parallel with other circuits, should be avoided. The Y capacitor should be placed at the chassis ground terminal or FG connector. The total inductance is kept at a certain distance from the transformer to avoid magnetic coupling. If it is not handled well, a shield can be added between the common inductor and the transformer. The above items have a great influence on the EMC performance of the switching power supply.
The output capacitor can generally be used with two ones close to the rectifier and the other should be close to 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 using a large-capacity capacitor. The heating device should be kept at a certain distance from the electrolytic capacitor to extend the life of the whole machine. The electrolytic capacitor is the bottle strength of the switching power supply life. For example, the transformer, the power tube, the high-power resistor should be kept away from the electrolysis, and the heat dissipation space must be left between the electrolysis. , conditions allow it to be placed at the air inlet.
The control part should pay attention to: the high-impedance weak signal circuit connection should be as short as possible, such as the sampling feedback loop. In the processing, try to avoid the interference, current sampling signal circuit, especially the current control type circuit, the processing is not easy, some unexpected The accident, as well as the switch tube drive signal circuit, the switch tube drive resistor should be close to the switch tube, which can improve the reliability of the switch tube, which is related to the high DC impedance voltage drive characteristics of the power MOSFET.
Let's talk about some of the principles of printed circuit board wiring.
Line spacing: With the continuous improvement and improvement of the printed circuit board manufacturing process, there is no problem in the general processing factory to make the line spacing equal to or less than 0.1mm, which can fully satisfy most applications. Considering the components and production process used in the switching power supply, the minimum line spacing of the dual-panel is set to 0.3mm, the minimum line spacing of the single-panel is set to 0.5mm, and the pads and pads, pads and vias or vias are used. Holes with a minimum spacing of 0.5mm avoid "bridging" during soldering operations. Therefore, most of the board manufacturers can easily meet the production requirements, and can control the yield very high, and can achieve reasonable wiring density and a more economical cost.
The minimum line spacing is only suitable for the signal control circuit and the low voltage circuit with a voltage lower than 63V. When the line voltage is greater than this value, the line spacing can generally be taken according to the empirical value of 500V/1mm.
In view of the fact that there are some related standards that have clear provisions on the line spacing, it must be strictly in accordance with the standard, such as the AC inlet end to the fuse end connection. Some power supplies are very demanding, such as module power. The general transformer input side line spacing of 1mm has proven to be feasible. For AC input, (isolated) DC output power products, the stricter rule is that the safety spacing should be greater than or equal to 6mm, which is determined by the relevant standards and implementation methods. The general safety spacing can be taken as the reference by the distance between the two sides of the feedback optocoupler. The principle is greater than or equal to this distance. It is also possible to slot the printed circuit board under the optocoupler to increase the creepage distance to meet the insulation requirements. Generally, the spacing of the AC input side of the switching power supply or the components on the board is not more than 5mm from the non-insulated housing, and the distance between the output side wiring or the device from the housing or the heat sink is greater than 2mm, or strictly in accordance with safety regulations.
Common methods: The above mentioned method of slotting the circuit board is suitable for some occasions where the spacing is not enough. By the way, this method is also commonly used as a protection discharge gap, which is common in the TV picture tube tail plate and the power AC input. . This method has been widely used in module power supply and can achieve good results under potting conditions.
Method 2: pad insulation paper, which can be insulated materials such as green paper, polyester film, and PTFE oriented film. Generally, the general-purpose power supply uses a green paper or a polyester film pad between the circuit board and the metal casing. This material has high mechanical strength and has a certain resistance to moisture. PTFE oriented film is widely used in module power supplies due to its high temperature resistance. An insulating film may also be interposed between the component and the surrounding conductor to improve the insulation resistance.
Note: Some device insulation covers cannot be used as an insulating medium to reduce the safety spacing, such as the outer skin of electrolytic capacitors, which may be subject to heat shrinkage under high temperature conditions. Space should be left at the front end of the large electrolytic explosion-proof tank to ensure that the electrolytic capacitor can be unimpeded in the extreme case.
Let's talk about some things about the copper plating of printed boards:
Trace current density: Most electronic circuits are now made of insulating plates. The copper thickness of the common circuit board is 35μm, and the current density value can be taken according to the empirical value of 1A/mm. For the specific calculation, please refer to the textbook. In order to ensure the mechanical strength of the routing, the line width should be greater than or equal to 0.3mm (other non-power circuit boards may have a smaller minimum line width). The copper thickness is 70μm. The circuit board is also commonly used in switching power supplies, so the current density can be higher.
In addition, the commonly used circuit board design tool software generally has design specifications, such as line width, line spacing, dry disk via size and other parameters can be set. When designing a circuit board, the design software can be automatically executed according to specifications, saving a lot of time, reducing part of the workload and reducing the error rate.
Generally, a double panel is used for a line or wiring line having a relatively high reliability requirement. It is characterized by moderate cost and high reliability, which can satisfy most applications.
Some products in the module power supply line also use multi-layer boards, which are mainly for integrating power devices such as transformer inductors, optimizing wiring and power tube heat dissipation. The utility model has the advantages of good process consistency and good heat dissipation of the transformer, but the disadvantage is that the cost is high and the flexibility is poor, and it is only suitable for industrial large-scale production.
Single-panel, market-distributed general-purpose switching power supplies almost all use single-sided circuit boards, which have the advantage of low cost, and some measures in design and production process can also ensure their performance. #p#分页头#e#
Today, I talk about some experience of single-sided printed board design. Because of the low cost and easy manufacturing, single-panel is widely used in switching power supply lines. Because it only has one side of copper, the electrical connection of the device and the mechanical fixing must be Rely on that layer of copper, you must be careful when handling.
In order to ensure good soldering mechanical structure performance, the single-panel pad should be slightly larger to ensure good bonding force between the copper foil and the substrate, without being peeled off or broken when subjected to vibration. Generally, the width of the welding ring should be greater than 0.3mm. The diameter of the pad hole should be slightly larger than the diameter of the device pin, but it should not be too large, so that the soldering distance between the pin and the pad is the shortest. The size of the hole does not hinder the normal inspection. The diameter of the pad hole is generally larger than the pin. The diameter is 0.1-0.2mm. Multi-pin devices are also larger to ensure a smooth check.
The electrical connection should be as wide as possible. The width of the principle should be larger than the diameter of the pad. In special cases, the line must be widened at the intersection with the pad (commonly known as teardrop formation) to avoid breakage in certain condition lines and pads. The principle minimum line width should be greater than 0.5mm.
The components on the single panel should be close to the circuit board. For devices that require overhead heat dissipation, it is necessary to add a sleeve to the pin between the device and the circuit board to support the device and increase the insulation. To minimize or avoid external force impact on the pad and pin connection. The effect is enhanced to enhance the firmness of the weld. The heavier components on the board can increase the support connection point and strengthen the connection strength with the circuit board, such as transformers, power device heat sinks.
The single-panel soldering surface pin can be left longer without affecting the distance between the outer casing and the outer casing. The advantage is that the strength of the welded portion can be increased, the welding area can be increased, and the phenomenon of virtual welding can be found immediately. When the pin is cut long, the welded part is less stressed. In Taiwan and Japan, the process of bending the device pins on the soldering surface at a 45-degree angle to the board and then soldering is the same as above. Today, I will talk about some of the issues in the dual-panel design. In some application environments where the requirements are relatively high, or the density of the traces is relatively large, the performance of the double-sided printed board is much better than that of the single panel.
The double-pad pad has a higher metallization strength than the hole, and the solder ring can be smaller than the single-layer. The hole diameter of the pad hole is slightly larger than the diameter of the pin, because the solder solution penetrates into the top layer through the solder hole during the soldering process. Pads to increase soldering reliability. However, there is a drawback. If the hole is too large, some devices may float under the impact of the jet tin during wave soldering, causing some defects.
For the processing of large current traces, the line width can be processed according to the previous post. If the width is not enough, it can generally be solved by adding tin to the trace on the trace to solve the problem.
1. Set the trace to the pad property so that the trace is not covered by the solder resist when the board is manufactured, and the hot air is plated with tin.
2. Place the pad at the wiring and set the pad to the shape that requires the trace. Be careful to set the pad hole to zero.
3, placing 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 use text description. No solder resist is applied to the place where the solder mask is placed
Several methods of tinning the line are as above. It should be noted that if the wide traces are all plated with tin, after soldering, a large amount of solder will be bonded, and the distribution is uneven, which affects the appearance. Generally, the width of tin plating can be 1~1.5mm, the length can be determined according to the line, and the tin plating part is 0.5~1mm. The double-sided circuit board provides great selectivity for layout and routing, which can make wiring more. It tends to be reasonable. Regarding the grounding, the power ground and the signal ground must be separated, and the two grounds can be merged at the filter capacitor to avoid the occurrence of unstable accidental factors caused by the large pulse current passing through the signal ground. The signal control loop uses a grounding method as much as possible. There is a trick to place ungrounded traces on the same wiring layer and finally on the other layer. The output line generally passes through the filter capacitor and then to the load. The input line must also pass through the capacitor and then to the transformer. The theoretical basis is to let the ripple current pass through the travel filter capacitor.
Voltage feedback sampling, in order to avoid the influence of large current through the trace, the sampling point of the feedback voltage must be placed at the end of the power output to improve the load effect of the whole machine.
The change of the trace from one wiring layer to the other is generally via vias, which should not be achieved through the device pin pads, because it is possible to break the connection when the device is inserted, and when every 1A current is passed. There should be at least 2 vias, the via hole principle should be greater than 0.5mm, and generally 0.8mm to ensure processing reliability.
The device dissipates heat. In some low-power power supplies, the circuit board traces can also function as a heat sink. The feature is that the traces are as wide as possible to increase the heat dissipation area, and no solder resist is applied. The vias can be uniformly placed to enhance the thermal conductivity. .
Then talk about the application of aluminum substrate in switching power supply and the application of multilayer printed board in switching power supply circuit.
The aluminum substrate is constructed by itself, and has the following characteristics: the thermal conductivity is very good, the copper is single-sided, the device can only be placed on the copper surface, and the electrical connection hole cannot be opened, so the jumper cannot be placed in the same manner as the single panel.
A chip device is generally placed on the aluminum substrate, and the switch tube and the output rectifier tube conduct heat through the substrate, and the heat resistance is low, and high reliability can be achieved. The transformer adopts a planar patch structure, and can also dissipate heat through the substrate, and its temperature rise is lower than conventional. The same specification transformer adopts an aluminum substrate structure to obtain a large output power. Aluminum substrate jumpers can be handled by bridge. The aluminum substrate power supply generally consists of two printed boards, and the other board is placed with a control circuit, and the two boards are integrated by physical connection.
Due to the excellent thermal conductivity of the aluminum substrate, it is difficult to solder in a small amount, the solder is cooled too fast, and the problem is easy to occur. There is a simple and practical method for ironing a normal electric iron (preferably with a temperature adjustment function). Turn over, iron the face up, fix it, adjust the temperature to about 150 °C, put the aluminum substrate on the iron, warm it for a while, then paste the components and solder according to the conventional method, the temperature of the iron is easy to solder. If it is too high, the device may be damaged. Even the copper substrate of the aluminum substrate is peeled off. The temperature is too low and the welding effect is not good. It is necessary to be flexible.
In recent years, with the application of multi-layer circuit boards in switching power supply circuits, printed circuit transformers have become possible. Due to the small layer spacing of multilayer boards, it is also possible to make full use of the transformer window section, which can be used on the main circuit board. Add one or two printed coils composed of multi-layer boards to achieve the purpose of using the window to reduce the current density of the line. Due to the use of printed coils, manual intervention is reduced, the transformer has good consistency, the planar structure, the leakage inductance is low, and the coupling is good. . Open core, good heat dissipation. Because of its many advantages, it is advantageous for mass production, so it is widely used. However, the initial investment in research and development is large, and it is not suitable for small-scale students.
Switching power supply is divided into two forms: isolated and non-isolated. Here we mainly talk about the topological form of isolated switching power supply. In the following, unless otherwise specified, it refers to isolated power supply. Isolated power supplies can be divided into two broad categories according to their structural form: forward and flyback. The flyback means that the secondary side is cut off when the primary side of the transformer is turned on, and the transformer stores energy. When the primary side is turned off, the secondary side is turned on, and the energy is released to the working state of the load. Generally, the conventional flyback power supply has a single tube and the double tube is not common. The forward type refers to the conduction of the primary side of the transformer while the secondary side induces the corresponding voltage output to the load, and the energy is directly transmitted through the transformer. According to the specifications, it can be divided into regular positive, including single tube forward and double tube forward. Half-bridge and bridge circuits are all forward circuits.
Both forward and flyback circuits have their own characteristics, and they can be used flexibly in order to achieve optimal cost performance in the process of designing circuits. Flyback is usually available in low power applications. A slightly larger one can use a single-tube forward circuit, a medium power can use a double-tube forward circuit or a half-bridge circuit, and a low voltage voltage push-pull circuit is used, which is the same as the half-bridge operation state. High-power output, generally adopts bridge circuit, and low-voltage can also adopt push-pull circuit.
Because of its simple structure, the flyback power supply eliminates an inductor similar in size to the transformer, and is widely used in small and medium power supplies. In some introductions, the flyback power supply can only achieve tens of watts, and the output power exceeds 100 watts, so there is no advantage, and it is difficult to implement. I think this is the case in general, but it cannot be generalized. PI's TOP chip can achieve 300 watts. There is an article about the flyback power supply that can achieve kilowatts, but I have never seen anything in kind. The output power is related to the output voltage level.
The leakage inductance of the flyback power transformer is a very critical parameter. Since the flyback power supply requires the transformer to store energy, in order to make full use of the transformer core, it is generally necessary to open the air gap in the magnetic circuit, the purpose of which is to change the core hysteresis. The slope of the return line enables the transformer to withstand large pulse current surges, so that the core enters a saturated non-linear state, the air gap in the magnetic circuit is in a high reluctance state, and the leakage flux in the magnetic circuit is much larger than the fully closed magnetic circuit. .
The coupling between the primary poles of the transformer is also a key factor in determining the leakage inductance. To make the primary pole coil close as much as possible, a sandwich winding method can be used, but this will increase the distributed capacitance of the transformer. Use the iron core as much as possible to use the longer magnetic core of the window to reduce the leakage inductance. For example, the core effect of EE, EF, EER and PQ is better than that of the EI type.
Regarding the duty cycle of the flyback power supply, in principle, the maximum duty cycle of the flyback power supply should be less than 0.5, otherwise the loop is not easy to compensate and may be unstable, but there are some exceptions, such as the TOP series chip introduced by the US PI company. Can work under conditions where the duty cycle is greater than 0.5. The duty ratio is determined by the turns ratio of the primary and secondary sides of the transformer. My opinion on the flyback is to first determine the reflected voltage (the output voltage is reflected to the primary side by the transformer coupling), and the reflected voltage is increased within a certain voltage range. The working duty ratio is increased and the switching tube loss is reduced. When the reflected voltage is lowered, the duty ratio of the operation is reduced, and the loss of the switching tube is increased. Of course, this also has a precondition. When the duty ratio is increased, it means that the output diode conduction time is shortened, in order to keep the output stable, more often it will be guaranteed by the output capacitor discharge current, and the output capacitor will withstand higher frequency. The ripple current is flushed and the heat is exacerbated, which is not allowed under many conditions.
When the duty ratio is increased and the transformer turns ratio is changed, the leakage inductance of the transformer is increased, and the overall performance is changed. When the leakage inductance energy is large to a certain extent, the low loss caused by the large duty of the switch tube can be fully offset. When there is no longer increasing the meaning of the duty cycle, it may even break the switch tube because the leakage inductance anti-peak voltage is too high. Due to the large leakage inductance, the output ripple and other electromagnetic indicators may be deteriorated. When the duty ratio is small, the effective value of the current through the switch tube is high, and the effective value of the primary current of the transformer is large, which reduces the efficiency of the converter, but can improve the working condition of the output capacitor and reduce heat generation.
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