Reasonable design of switching power supply - Solutions - Huaqiang Electronic Network

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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, which has no advantage and 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 pulse voltage connection is as short as possible, in which the input switch tube is connected to the transformer and the output transformer is connected to the rectifier connection line. 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.

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.

Some principles of PCB layout When designing a printed board, the influence of interference on the system should be taken into consideration. The analog part of the circuit and the circuit of the digital part should be strictly separated. The core circuit should be protected, the system ground should be surrounded, and the wiring should be as thick as possible. The power supply adds a filter circuit, adopts DC-DC isolation, the signal is optically isolated, and the isolated power supply is designed to analyze parts that are prone to interference (such as clock circuits, communication circuits, etc.) and parts that are easily interfered (such as analog sampling circuits, etc.). Take measures for each of these two types of circuits. Inhibition measures are taken for the interference components, isolation and protection measures are taken for the sensitive components, and they are spatially and electrically separated. When designing at the board level, it is also important to note that the components are placed away from the edges of the printed board, which is advantageous for shielding against air discharge.

With the continuous improvement and improvement of the printed circuit board manufacturing process, there is no problem in the general processing factory to produce a line spacing equal to or less than 0.1 mm, 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.

Method 1: The method of slotting the circuit board mentioned above 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 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.

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.

Some matters concerning the copper trace of the printed circuit board. Current density: Most electronic circuits are now made of insulating copper. 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 trace, the line width should be greater than or equal to 0.3mm. The copper thickness is 70μm. The circuit board is also commonly used in switching power supplies, so the current density can be higher.

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.

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 soldering 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. The 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 diameter of the pin by 0.1-0.2mm. The multi-pin device can also be larger to ensure smooth inspection.

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.

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 production.

There is also a certain factor in the reflected voltage of the flyback power supply. The reflected voltage of the military switching power supply is also related to one parameter, that is, the output voltage. The lower the output voltage, the larger the transformer turns ratio, the greater the leakage inductance of the transformer, and the higher the voltage withstand by the switch tube. High, it is possible to break through the switch tube, and the absorption circuit consumes more power, which may cause the absorption loop power device to permanently fail. In the optimization process of designing low-voltage output low-power flyback power supply, it must be handled carefully. There are several ways to deal with it:

a Use a large power class core to reduce the leakage inductance, which can improve the conversion efficiency of the low-voltage flyback power supply, reduce the loss, reduce the output ripple, and improve the cross-adjustment rate of the multi-output power supply. Such as CD player, DVB set-top box and so on.

b If the condition does not allow the core to be enlarged, only reduce the reflected voltage and reduce the duty cycle. Reducing the reflected voltage can reduce the leakage inductance but it is possible to reduce the power conversion efficiency. The two are contradictory. There must be an alternative process to find a suitable point. During the transformer replacement experiment, the primary side of the transformer can be detected. The anti-peak voltage, as far as possible to reduce the width and amplitude of the anti-peak voltage pulse, can increase the working safety margin of the converter. Generally, the reflected voltage is suitable at 110V.

c Enhance coupling, reduce loss, adopt new technology, and winding technology. In order to meet safety regulations, transformers will take insulation measures between primary and secondary sides, such as insulating tape and insulating end-space tape. These will affect the leakage inductance performance of the transformer. In actual production, the winding of the primary winding around the secondary can be used. Or the secondary is wound with a triple insulated wire, the insulation between the primary and secondary is eliminated, the coupling can be enhanced, and even a wide copper sheath can be used.

The flyback power transformer core is operating in a unidirectional magnetization state, so the magnetic circuit needs to open an air gap, similar to a pulsating DC inductor. Part of the magnetic circuit is coupled through the air gap. The principle of why the air gap is opened is understood by me: Since the power ferrite also has an operating characteristic curve (hysteresis loop) similar to a rectangle, the Y-axis represents the magnetic induction intensity (B) on the operating characteristic curve, and the current production process is generally The saturation point is above 400mT. Generally, this value should be suitable in the design of 200-300mT, and the X-axis represents the magnetic field strength (H). This value is proportional to the magnetizing current intensity. The open air gap of the magnetic circuit is equivalent to tilting the hysteresis loop of the magnet to the X-axis. Under the same magnetic induction, it can withstand a larger magnetizing current, which is equivalent to storing more energy in the core. This energy is cut off at the switch. When the transformer is discharged to the load circuit through the secondary of the transformer, the flyback power core has two functions. #p#分页头#e#

The transformer of the flyback power supply operates in a unidirectional magnetization state, and not only transmits energy through magnetic coupling, but also acts as a multiple function of voltage conversion input and output isolation. Therefore, the handling of the air gap needs to be very careful. The air gap is too large to increase the leakage inductance, the hysteresis loss increases, the iron loss and the copper loss increase, which affects the performance of the power supply. Too small an air gap may saturate the transformer core and cause power damage.

The continuous and discontinuous mode of the so-called flyback power supply refers to the working state of the transformer. In the full load state, the transformer operates in a mode in which the energy is completely transmitted or not completely transmitted. Generally, it should be designed according to the working environment. The conventional flyback power supply should work in continuous mode, so the loss of the switch tube and the line is relatively small, and the working stress of the input and output capacitors can be alleviated, but there are some exceptions. Due to the characteristics of the manufacturing process, the high back pressure diode has a long reverse recovery time and a low speed. In the continuous current state, the diode recovers when there is a forward bias, and the energy loss in the reverse recovery is very large, which is not conducive to the converter performance. The improvement is lighter, the conversion efficiency is lowered, the rectifier tube is severely heated, and even the rectifier tube is burned. Since the diode is reverse biased at zero bias in discontinuous mode, the loss can be reduced to a lower level.

The flyback switching power supply transformer should work in continuous mode, which requires a relatively large winding inductance. Of course, there is a certain degree of continuity. It is unrealistic to pursue absolute continuity excessively. It may require a large magnetic core, and many The number of turns in the coil, along with a large leakage inductance and distributed capacitance, may not be worth the loss. So how to determine this parameter, through many practices, and analysis of peer design, I believe that in the nominal voltage input, the output reaches 50% ~ 60% transformer from intermittent, transition to continuous state is more appropriate.

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.

The processing line width of the large current trace can be processed according to the previous post. If the width is not enough, the tin plating can be used to increase the thickness on the trace to solve the problem. There are many methods.

a Set the trace to the pad properties so that the trace is not covered by the solder mask when the board is manufactured, and the hot air is plated with tin.

b 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.

c Place the wire in the solder mask. This method is the most flexible, but not all circuit board manufacturers will understand your intentions. Several methods of tinning the solder resist line at the place where the solder resist layer is placed are as above. Generally, the width of tin plating can be 1~1.5mm, the length can be determined according to the line, and the tinned 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.

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.

Aluminum substrate and multi-layer printed board for use in switching power supply Aluminum substrate (metal-based heat sink (including aluminum substrate, copper substrate, iron substrate)) is a unique metal-based copper clad plate with good thermal conductivity. Electrical insulation properties and machinability. The aluminum-based copper clad laminate is a metal circuit board material composed of copper foil, thermal conductive insulating layer and metal substrate. Its structure is divided into three layers: Cireuitl.Layer circuit layer: copper clad board equivalent to ordinary PCB, line copper foil thickness loz To 10oz. DielcctricLayer insulation: The insulation is a low thermal resistance thermal insulation material. The thickness is 0.003" to 0.006" inches, which is the core technology of aluminum-based copper clad laminates and has obtained UL certification.

The BaseLayer base layer is a metal substrate, typically aluminum or a choice of copper.

Aluminum-based copper clad laminates and conventional epoxy glass cloth laminates, etc., currently on the market is the Fosslet aluminum substrate. The circuit layer (ie, copper foil) is usually etched to form a printed circuit, so that the various components of the assembly are connected to each other. In general, the circuit layer is required to have a large current carrying capacity, so that a thick copper foil should be used, and the thickness is generally 35 μm. 280μm; thermal conductive insulation layer is the core technology of aluminum substrate. It is generally composed of special polymer filled with special ceramics. It has small thermal resistance, excellent viscoelasticity, anti-heat aging ability, and can withstand mechanical and thermal stress. The thermal insulation layer of the high-performance aluminum substrate produced by the company uses this technology to make it have excellent thermal conductivity and high-insulation electrical insulation properties; the metal base layer is a supporting member of the aluminum substrate and requires high thermal conductivity. Generally, it is an aluminum plate, and a copper plate (where a copper plate can provide better thermal conductivity) can be used, which is suitable for conventional machining such as drilling, punching and cutting.

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.