How to complete the reasonable design of switching power supply from conception to practice

　　Switching power supplies have become the protagonist of our circuit design, and can even be said to have become an inseparable part of the industry's development. Compared with traditional linear power supplies, the cost of linear power supplies is higher than that of switching power supplies at a certain output power point. Common switching power supplies can be divided into two types: isolated and non-isolated.

　　In this article, we will mainly discuss the topology of isolated switching power supplies. Therefore, in the following article, unless otherwise specified, the power supplies mentioned in the article will all refer to isolated power supplies. Isolated power supplies can be divided into two categories according to their structural forms: forward and flyback. Flyback refers to a working state in which 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 cut off, the secondary side is turned on, and the energy is released to the load. Generally, conventional flyback power supplies have more single tubes, and double tubes are uncommon. Forward refers to a working state in which the secondary side senses the corresponding voltage and outputs it to the load when the primary side of the transformer is turned on, and the energy is directly transmitted through the transformer. According to the specifications, it can be divided into conventional forward, including single-tube forward and double-tube forward. Half-bridge and bridge circuits are both forward circuits.

　　The forward and flyback circuits each have their own characteristics. In the process of designing the circuit, they can be used flexibly to achieve the best cost-effectiveness. Generally, the flyback type can be selected for low power occasions. A single-tube forward circuit can be used for slightly larger power, a double-tube forward circuit or a half-bridge circuit can be used for medium power, and a push-pull circuit is used for low voltage, which is the same as the working state of the half-bridge. For high power output, a bridge circuit is generally used, and a push-pull circuit can also be used for low voltage.

　　Flyback power supply is widely used in small and medium power supply because of its simple structure and the elimination of an inductor which is about the same size as the transformer. Some introductions say that the power of flyback power supply can only reach tens of watts, and it has no advantage if the output power exceeds 100 watts, and it is difficult to achieve. I think this is generally true, but it cannot be generalized. PI's TOP chip can reach 300 watts, and there are articles that introduce flyback power supply that can reach thousands of watts, but I have never seen the real thing. The output power is related to the output voltage.

　　The leakage inductance of the flyback power supply transformer is a very critical parameter. Since the flyback power supply needs the transformer to store energy, in order to make full use of the transformer core, an air gap is generally required in the magnetic circuit. The purpose is to change the slope of the core hysteresis loop so that the transformer can withstand large pulse current shocks without causing the core to enter a saturated nonlinear state. The air gap in the magnetic circuit is in a high magnetic resistance state, and the leakage magnetic flux generated in the magnetic circuit is much greater than that of a completely closed magnetic circuit.

　　The pulse voltage connection line should be as short as possible, including the connection line from the input switch tube to the transformer, and the connection line from the output transformer to the rectifier tube. The pulse current loop should be as small as possible, such as the positive input filter capacitor to the transformer to the switch tube return capacitor negative. The output part of the transformer output to the rectifier tube to the output inductor to the output capacitor return transformer circuit should be as close to the input end of the switching power supply as possible, and the input line should avoid being parallel to other circuits and should be avoided. The Y capacitor should be placed at the chassis ground terminal or FG connection terminal. The common mode inductor should be kept at a certain distance from the transformer to avoid magnetic coupling.

　　Generally, two output capacitors can be used, one close to the rectifier tube and the other close to the output terminal, which can affect the output ripple index of the power supply. The effect of two small-capacity capacitors in parallel should be better than using a large-capacity capacitor. Heat-generating devices should be kept at a certain distance from electrolytic capacitors to extend the life of the whole machine. Electrolytic capacitors are the life of switching power supplies. For example, transformers, power tubes, and high-power resistors should be kept at a distance from electrolytic capacitors. Heat dissipation space must also be left between electrolytic capacitors. If conditions permit, they can be placed at the air inlet.

　　Some principles of PCB wiring When designing a printed circuit board, the impact of interference on the system should be taken into consideration. The analog and digital parts of the circuit should be strictly separated. The core circuit should be protected. The system ground wire should be surrounded and the wiring should be as thick as possible. The power supply should add a filter circuit, DC-DC isolation should be used, and the signal should use photoelectric isolation. An isolated power supply should be designed. The parts that are prone to interference (such as clock circuits, communication circuits, etc.) and the parts that are easily interfered with (such as analog sampling circuits, etc.) should be analyzed, and measures should be taken for these two types of circuits. Suppression measures should be taken for interfering components, isolation and protection measures should be taken for sensitive components, and they should be spaced and electrically separated. When designing at the board level, it should also be noted that the components should be placed away from the edge of the printed circuit board, which is beneficial for protecting against air discharge.

　　With the continuous improvement and improvement of the manufacturing process of printed circuit boards, it is no longer a problem for general processing plants to produce line spacing equal to or even less than 0.1mm, which can fully meet most applications. Considering the components and production processes used in switching power supplies, the minimum line spacing of double-sided boards is generally set to 0.3mm, and the minimum line spacing of single-sided boards is set to 0.5mm. The minimum spacing between pads, pads and vias, or vias and vias is set to 0.5mm to avoid the "bridging" phenomenon during the welding operation. In this way, most board manufacturers can easily meet production requirements, control the yield rate to a very high level, and 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 voltages below 63V. When the line voltage is greater than this value, the line spacing can generally be selected according to the empirical value of 500V/1mm.

　　Method 1: The circuit board slotting method mentioned above is suitable for some occasions where the spacing is not enough. By the way, this method is also often used as a protective discharge gap, which is common in the tail plate of the TV picture tube and the AC input of the power supply. This method has been widely used in module power supplies and can achieve good results under potting conditions.

　　Method 2: Use insulating paper, such as green paper, polyester film, polytetrafluoroethylene oriented film, etc. Insulating materials such as green paper, polyester film, and polytetrafluoroethylene oriented film can be used. Generally, general power supplies use green paper or polyester film to pad between the circuit board and the metal casing. This material has high mechanical strength and a certain degree of moisture resistance. Polytetrafluoroethylene oriented film is widely used in module power supplies due to its high temperature resistance. Insulating film can also be used between components and surrounding conductors to improve insulation and electrical resistance.

　　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 interlayer spacing of multi-layer boards, the window section of the transformer can be fully utilized. One or two printed coils composed of multi-layer boards can be added to the main circuit board to achieve the purpose of utilizing the window and reducing the line current density. Due to the use of printed coils, manual intervention is reduced, the transformer has good consistency, flat structure, low leakage inductance, good coupling. Open core, good heat dissipation conditions. Because it has many advantages, it is conducive to mass production, so it has been widely used. However, the initial investment in research and development is large, and it is not suitable for small-scale production.

　　There is another determining factor for the reflected voltage of the flyback power supply. The reflected voltage of the military switching power supply is also related to a parameter, that is, the output voltage. The lower the output voltage, the greater the transformer turns ratio, the greater the transformer leakage inductance, the higher the voltage the switch tube withstands, and the switch tube may be broken down. The greater the power consumption of the absorption circuit, the more likely it is that the power device of the absorption circuit will fail permanently. Care must be taken in the optimization process of designing a low-voltage output low-power flyback power supply. There are several ways to deal with it:

　　A. Use a magnetic core with a larger power level to reduce leakage inductance. This can improve the conversion efficiency of the low-voltage flyback power supply, reduce losses, reduce output ripple, and improve the cross-regulation rate of multi-channel output power supplies. It is generally common in switching power supplies for household appliances, such as optical disc players, DVB set-top boxes, etc.

　　B. If the conditions do not allow the magnetic core to be enlarged, the only option is to reduce the reflected voltage and duty cycle. Reducing the reflected voltage can reduce leakage inductance but may reduce the power conversion efficiency. The two are contradictory. A replacement process is necessary to find a suitable point. During the transformer replacement experiment, the reverse peak voltage of the primary side of the transformer can be detected. The width and amplitude of the reverse peak voltage pulse can be reduced as much as possible to increase the working safety margin of the converter. Generally, the reflected voltage is more suitable at 110V.

　　C. Enhance coupling, reduce loss, adopt new technology and winding process. To meet safety regulations, transformers will take insulation measures between the primary and secondary sides, such as padding with insulating tape and adding insulating end tape. These will affect the leakage inductance performance of the transformer. In actual production, the primary winding can be wrapped around the secondary. Or the secondary can be wound with triple insulated wire and the insulation between the primary and secondary can be eliminated to enhance coupling. Even wide copper foil can be used for winding.

　　The flyback power transformer core is working in a unidirectional magnetization state, so the magnetic circuit needs to have an air gap, similar to a pulsating DC inductor. Part of the magnetic circuit is coupled through an air gap. I understand the principle of opening the air gap as follows: Since the power ferrite also has a working characteristic curve (hysteresis loop) that is approximately rectangular, the Y axis on the working characteristic curve represents the magnetic induction intensity (B). The current production process generally has a saturation point above 400mT. Generally, this value should be taken in the design at 200-300mT. The X axis represents the magnetic field intensity (H), which is proportional to the magnetization current intensity. Opening an air gap in the magnetic circuit is equivalent to tilting the magnet hysteresis loop toward the X axis. Under the same magnetic induction intensity, it can withstand a larger magnetization current, which is equivalent to storing more energy in the magnetic core. This energy is discharged to the load circuit through the secondary of the transformer when the switch tube is turned off. The air gap in the flyback power core has two functions.

　　The transformer of the flyback power supply works in a unidirectional magnetization state. It not only transfers energy through magnetic coupling, but also bears the multiple functions of voltage conversion input and output isolation. Therefore, the air gap needs to be handled very carefully. If the air gap is too large, the leakage inductance will increase, the hysteresis loss will increase, and the iron loss and copper loss will increase, affecting the overall performance of the power supply. If the air gap is too small, the transformer core may be saturated, resulting in damage to the power supply.

　　The so-called continuous and discontinuous modes of the flyback power supply refer to the working state of the transformer. Under full load, the transformer works in a working mode of complete or incomplete energy transfer. Generally, it should be designed according to the working environment. Conventional flyback power supplies should work in continuous mode, so that the losses of the switch tube and the line are relatively small, and the working stress of the input and output capacitors can be reduced, but there are some exceptions. Due to the characteristics of the manufacturing process, high reverse voltage diodes have a long reverse recovery time and low speed. In the continuous current state, the diode recovers when there is a forward bias. The energy loss during reverse recovery is very large, which is not conducive to improving the performance of the converter. At the least, the conversion efficiency is reduced, the rectifier tube is seriously heated, and at worst, the rectifier tube is even burned. Since in the discontinuous mode, the diode is reverse biased under zero bias, the loss can be reduced to a relatively low level.

　　The flyback switching power supply transformer should work in continuous mode, which requires a relatively large winding inductance. Of course, continuity also has a certain degree. It is unrealistic to pursue absolute continuity excessively. It may require a large magnetic core and a lot of coil turns, accompanied by large leakage inductance and distributed capacitance, which may not be worth the cost. So how to determine this parameter? Through many practices and analysis of the designs of peers, I believe that when the nominal voltage is input, the output reaches 50%~60% and the transformer transitions from intermittent to continuous state is more appropriate.

　　This article introduces how to reasonably design a switching power supply from structure to circuit board design. It also provides solutions to some problems. It can be said that it covers the entire process of a switching power supply design, and has a detailed introduction from design to actual production. It is a rare and valuable knowledge for novices. I hope everyone can learn how to reasonably design a switching power supply from this article and use it flexibly.