From PCB to flyback power supply switching power supply design

　　Talking about the design experience of switching power supplies over the years, from the design of switching power supply printed circuit boards, printed circuit board wiring, printed circuit board copper routing, the application of aluminum substrates and multi-layer printed circuit boards in switching power supplies, to the duty cycle of flyback power supplies, it is an absolute practical essence!

　　Design of Switching Power Supply Printed Circuit Board

　　Let's start with the design and production process of the switching power supply, and first talk about the design of the printed circuit board. The switching power supply works at a high frequency and high pulse state, and is a special type of analog circuit. When laying out the board, you must follow the high-frequency circuit wiring principles.

　　1. Layout: The pulse voltage connection 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 X capacitor in the output part 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. The input line should avoid being parallel to other circuits and should be avoided. The Y capacitor should be placed on the chassis grounding terminal or FG connection terminal. The common mode inductor should be kept at a certain distance from the transformer to avoid magnetic coupling. If it is difficult to handle, a shield can be added between the common mode inductor and the transformer. The above items have a greater impact on the EMC performance of the switching power supply.

　　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.

　　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, and it should be avoided as much as possible during processing. The current sampling signal circuit, especially the current control circuit, is prone to unexpected accidents if not handled properly. There are some tricks. Take the 3843 circuit as an example. See Figure (1). The effect of Figure 1 is better than that of Figure 2. When Figure 2 is fully loaded, the current waveform is observed with an oscilloscope and obviously superimposed with spikes. This is because the interference current limit point is lower than the design value. Figure 1 does not have this phenomenon. There is also a switch tube drive signal circuit. The switch tube drive resistor should be close to the switch tube to improve the reliability of the switch tube. This is related to the high DC impedance voltage drive characteristics of the power MOSFET.

　　Switching power supply printed circuit board wiring principles

　　Let's talk about some principles of printed circuit board wiring.

　　Line spacing: With the continuous improvement and improvement of printed circuit board manufacturing technology, 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.

　　In view of the fact that some relevant standards have clear regulations on line spacing, they must be strictly implemented in accordance with the standards, such as the connection from the AC input end to the fuse end. Some power supplies have high volume requirements, such as module power supplies. Generally, the line spacing on the input side of the transformer is 1mm, which has been proven to be feasible in practice. For power products with AC input and (isolated) DC output, the more stringent regulations require that the safety spacing must be greater than or equal to 6mm, of course, this is determined by relevant standards and implementation methods. The general safety spacing can be used as a reference for the distance on both sides of the feedback optocoupler, which is greater than or equal to this distance in principle. 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 between the AC input side wiring or components on the board of the switching power supply and the non-insulated housing or heat sink should be greater than 5mm, and the spacing between the output side wiring or components and the housing or heat sink should be greater than 2mm, or strictly follow the safety specifications.

　　Common methods: 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 resistance.

　　Note: The insulation sheath of some devices cannot be used as an insulating medium to reduce the safety distance, such as the outer skin of the electrolytic capacitor. Under high temperature conditions, the outer skin may shrink due to heat. Space should be left at the front of the large electrolytic explosion-proof tank to ensure that the electrolytic capacitor can release pressure without hindrance in an emergency.

　　Precautions for copper wiring on printed circuit boards

　　Current density of wiring: Most electronic circuits are now made of copper bonded to an insulating board. The copper thickness of a common circuit board is 35μm. The current density value of the wiring can be taken according to the empirical value of 1A/mm. For specific calculations, please refer to the textbook. In principle, the line width should be greater than or equal to 0.3mm to ensure the mechanical strength of the wiring (other non-power circuit boards may have a smaller minimum line width). The copper thickness of 70μm is also common in switching power supplies, so the current density can be higher.

　　In addition, the commonly used circuit board design tool software generally has design specification items, such as line width, line spacing, dry plate via size and other parameters can be set. When designing a circuit board, the design software can automatically execute according to the specifications, which can save a lot of time, reduce some workload, and reduce the error rate.

　　Generally, double-sided boards can be used for circuits with high reliability requirements or high wiring density. Its characteristics are moderate cost, high reliability, and can meet most application occasions.

　　Some products in the module power supply line also use multi-layer boards, which are mainly convenient for integrating power devices such as transformer inductors , optimizing wiring, power tube heat dissipation, etc. It has the advantages of good consistency in process appearance and good heat dissipation of transformers, but its disadvantages are high cost and poor flexibility, and it is only suitable for industrial large-scale production.

　　Single-sided board: Almost all general switching power supplies on the market use single-sided circuit boards, which have the advantage of low cost. Some measures in design and production process can also ensure its performance.

　　Today I will talk about some experiences in single-sided printed circuit board design. Because single-sided boards are low-cost and easy to manufacture, they are widely used in switching power supply circuits. Since they only have one side of copper binding, the electrical connection and mechanical fixation of the device all rely on that layer of copper, so you must be careful when handling it.

　　To ensure good mechanical structure performance of welding, the pad of single-sided board should be slightly larger to ensure good bonding between copper and substrate, so as to avoid peeling and breaking of copper when vibrated. Generally, the width of solder ring should be greater than 0.3mm. The pad hole diameter should be slightly larger than the device pin diameter, but not too large to ensure the shortest distance between the pin and pad by solder connection. The size of the pad hole should not hinder normal inspection. The pad hole diameter is generally 0.1-0.2mm larger than the pin diameter. Multi-pin devices can also be larger to ensure smooth inspection.

　　The electrical connection should be as wide as possible. In principle, the width should be greater than the pad diameter. In special cases, the line must be widened (commonly known as generating teardrops) when the line intersects with the pad to avoid the line and pad breaking under certain conditions. In principle, the minimum line width should be greater than 0.5mm.

　　Components on a single-sided board should be close to the circuit board. For devices that require overhead heat dissipation, sleeves should be added to the pins 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 pad and the pin should be minimized or avoided to enhance the firmness of welding. For heavier components on the circuit board, support connection points can be added to strengthen the connection strength between the circuit board, such as transformers and power device heat sinks.

　　The pins on the welding side of a single-sided board can be left longer without affecting the spacing with the shell. The advantage is that it can increase the strength of the welding part, increase the welding area, and detect cold welding immediately. When the pins are cut, the welding part is less stressed. In Taiwan and Japan, the process of bending the device pins on the welding surface to a 45-degree angle with the circuit board and then welding is often used. The reason is the same as above. Today, let's talk about some matters in the design of double-sided boards. In some application environments with higher requirements or higher wiring density, double-sided printed boards are used, and their performance and various indicators are much better than single-sided boards.

　　The double-sided board pad has a higher strength because the hole has been metallized. The solder ring can be smaller than the single-sided board, and the pad hole diameter can be slightly larger than the pin diameter, because it is conducive to the solder solution to penetrate through the solder hole to the top pad during the soldering process to increase the soldering reliability. However, there is a disadvantage. If the hole is too large, some components may float up under the impact of the jet tin during wave soldering, resulting in some defects.

　　For high current routing, the line width can be processed according to the previous post. If the width is not enough, it can generally be solved by tinning the line to increase the thickness. There are many ways to solve this problem.

　　1. Set the trace to pad attribute, so that the trace will not be covered by solder mask during circuit board manufacturing and will be tinned during hot air leveling.

　　2. Place a pad at the wiring location and set the pad to the shape where the wiring needs to be routed. Be sure to set the pad hole to zero.

　　3. Place the wire on the solder mask layer. This method is the most flexible, but not all PCB manufacturers will understand your intentions, so you need to explain it in words. No solder mask will be applied to the area where the wire is placed on the solder mask layer.

　　Several methods of tinning the circuit are as above. It should be noted that if a very wide trace is tinned, a large amount of solder will be adhered after welding, and the distribution is very uneven, affecting the appearance. Generally, a thin strip of tinning can be used with a width of 1~1.5mm, and the length can be determined according to the circuit. The interval of the tinned part is 0.5~1mm. The double-sided circuit board provides a lot of options for layout and routing, which can make the wiring more reasonable. Regarding grounding, the power ground and the signal ground must be separated. The two grounds can be combined at the filter capacitor to avoid large pulse currents passing through the signal ground connection and causing unexpected unstable factors. The signal control loop should try to use a single-point grounding method. There is a trick to try to place the non-grounded traces on the same wiring layer, and finally lay the ground wire on another layer. The output line generally 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 passing through the wiring, the sampling point of the feedback voltage must be placed at the very end of the power supply output to improve the load effect index of the whole machine.

　　When the wiring changes from one wiring layer to another, vias are generally used for connection. It is not advisable to achieve this through the device pin pads, because this connection relationship may be destroyed when the device is inserted. In addition, there should be at least 2 vias for every 1A current passing through. The via aperture should be greater than 0.5mm in principle. Generally, 0.8mm can ensure processing reliability.

　　Device heat dissipation. In some low-power power supplies, the circuit board routing can also serve as a heat dissipation function. Its characteristics are that the routing is as wide as possible to increase the heat dissipation area, and no solder resist is applied. If conditions permit, vias can be evenly placed to enhance thermal conductivity.

　　Application of aluminum substrate and multilayer printed circuit board in switching power supply

　　Next, let's talk about the application of aluminum substrates in switching power supplies and the application of multilayer printed circuit boards in switching power supply circuits.

　　The aluminum substrate has the following characteristics due to its own structure: excellent thermal conductivity, single-sided copper bonding, devices can only be placed on the copper-bonded surface, and electrical connection holes cannot be opened, so jumpers cannot be placed like single-sided boards.

　　SMD devices, switch tubes, and output rectifier tubes are generally placed on aluminum substrates to conduct heat away through the substrate. The thermal resistance is very low, and high reliability can be achieved. The transformer adopts a planar SMD structure, and heat can also be dissipated through the substrate. Its temperature rise is lower than the conventional one. The same specification transformer adopts an aluminum substrate structure to obtain a larger output power. Aluminum substrate jumpers can be handled in a bridging manner. Aluminum substrate power supplies are generally composed of two printed circuit boards, and the other board is placed on the control circuit. The two boards are physically connected to form a whole.

　　Due to the excellent thermal conductivity of the aluminum substrate, it is difficult to manually solder a small amount of soldering. The solder cools too quickly and problems are likely to occur. Here is a simple and practical method: turn an ordinary electric iron (preferably with a temperature control function) for ironing clothes over, with the ironing side facing up, fix it, and adjust the temperature to about 150°C. Place the aluminum substrate on the iron, heat it for a while, and then stick and solder the components according to the conventional method. The temperature of the iron should be suitable for the device to be easy to solder. If it is too high, the device may be damaged or even the copper skin of the aluminum substrate may peel off. If the temperature is too low, the welding effect is not good, so it must be controlled flexibly.

　　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.

　　Switching power supplies are divided into two types: isolated and non-isolated. Here we mainly talk about the topology of isolated switching power supplies. In the following text, unless otherwise specified, all refer to isolated power supplies. Isolated power supplies can be divided into two categories according to their different 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 situation in which the primary side of the transformer is turned on and the secondary side senses the corresponding voltage and outputs it to the load, and the energy is directly transferred 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 coupling between the primary poles of the transformer is also a key factor in determining the leakage inductance. The primary pole coils should be as close as possible. Sandwich winding can be used, but this will increase the distributed capacitance of the transformer. When selecting an iron core, try to use a magnetic core with a relatively long window to reduce the leakage inductance. For example, the effect of EE, EF, EER, and PQ type magnetic cores is better than that of 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 chips launched by the American PI company, which can work under the condition of a duty cycle greater than 0.5. The duty cycle is determined by the ratio of the primary and secondary turns of the transformer. My opinion on flyback is to first determine the reflected voltage (the voltage value reflected to the primary side by the output voltage through the transformer coupling). Within a certain voltage range, the reflected voltage increases, the working duty cycle increases, and the switch tube loss decreases. When the reflected voltage decreases, the working duty cycle decreases, and the switch tube loss increases. Of course, this also has a prerequisite. When the duty cycle increases, it means that the conduction time of the output diode is shortened. In order to maintain output stability, it will be guaranteed by the discharge current of the output capacitor more often. The output capacitor will be subjected to a larger high-frequency ripple current, which will increase its heating, which is not allowed under many conditions. Increasing the duty cycle and changing the transformer turns ratio will increase the transformer leakage inductance and change its overall performance. When the leakage inductance energy reaches a certain level, it can fully offset the low loss caused by the large duty cycle of the switch tube. At this time, there is no point in increasing the duty cycle. The switch tube may even be broken down due to the excessive reverse peak voltage of the leakage inductance. Due to the large leakage inductance, the output ripple and some other electromagnetic indicators may deteriorate. When the duty cycle is small, the effective value of the current passing 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 conditions of the output capacitor and reduce heat generation. How to determine the transformer reflected voltage (i.e., duty cycle)

　　Some netizens mentioned the parameter setting of the feedback loop of the switching power supply and the working state analysis. Because I was not good at advanced mathematics in school, I almost had to take a make-up exam for "Principles of Automatic Control". I still feel scared about this subject. I still can't write out the closed-loop system transfer function completely. I feel vague about the concepts of zeros and poles of the system. I can only roughly tell whether it is divergent or convergent by looking at the Bode plot. So I dare not say anything nonsense about feedback compensation, but I have some suggestions. If you have some math skills and some study time, you can find the university textbook "Principles of Automatic Control" and digest it carefully, and combine it with the actual switching power supply circuit and analyze it according to the working state.

　　Duty cycle of flyback power supply

　　Finally, let's talk about the duty cycle of the flyback power supply (I am concerned about the reflected voltage, which is consistent with the duty cycle). The duty cycle is also related to the withstand voltage of the switch tube. Some early flyback power supplies used relatively low withstand voltage switch tubes, such as 600V or 650V as the switch tube for the AC 220V input power supply. Perhaps it was related to the production process at the time. High withstand voltage tubes were not easy to manufacture, or low withstand voltage tubes had more reasonable conduction losses and switching characteristics. The reflected voltage of this line cannot be too high, otherwise, in order to make the switch tube work within a safe range, the power absorbed by the circuit loss is also considerable. Practice has proved that the reflected voltage of a 600V tube should not be greater than 100V, and the reflected voltage of a 650V tube should not be greater than 120V. When the leakage inductance peak voltage value is clamped at 50V, the tube still has a 50V working margin. Now, due to the improvement of the manufacturing process level of MOS tubes, general flyback power supplies use 700V or 750V or even 800-900V switch tubes. For a circuit like this, the overvoltage resistance is stronger and the reflected voltage of the switch transformer can be made higher. The maximum reflected voltage is more suitable at 150V, which can achieve better overall performance. PI's TOP chip is recommended to be 135V with transient voltage suppression diode clamping. However, the reflected voltage of his evaluation board is generally lower than this value, about 110V. These two types have their own advantages and disadvantages:

　　Category 1: Disadvantages: Weak overvoltage resistance, small duty cycle, and large transformer primary pulse current. Advantages: Small transformer leakage inductance, low electromagnetic radiation, high ripple index, small switch tube loss, and conversion efficiency is not necessarily lower than that of the second category.

　　Category 2: Disadvantages: switch tube loss is larger, transformer leakage inductance is larger, and ripple is worse. Advantages: stronger overvoltage resistance, large duty cycle, lower transformer loss, and higher efficiency.

　　There is another determining factor for the reflected voltage of the flyback power supply

　　The reflected voltage of the flyback 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 (especially the circuit using transient voltage suppression diodes). 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:

　　1. Use a magnetic core with a larger power level to reduce leakage inductance, which 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.

　　2. 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.

　　3. 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, and even wide copper foil can be used for winding.

　　The low voltage output in this article refers to the output less than or equal to 5V. For this kind of small power supply, my experience is that the forward type can be used for power output greater than 20W to obtain the best cost performance. Of course, this is not absolute. It is related to personal habits and application environment. Next time, let's talk about the magnetic core used in flyback power supply and some understanding of air gap in magnetic circuit. I hope that experts can give me some advice.

　　The flyback power transformer core is working in a unidirectional magnetization state, so the magnetic circuit needs to be gapped, 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 similar to a rectangle, 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). This value 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. One is to transfer more energy, and the other is to prevent the magnetic core from entering a saturation state.

　　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. It should be pointed out here that due to the characteristics of the flyback power supply, it is also more suitable to be designed as a high-voltage power supply, and the high-voltage power supply transformer generally works in discontinuous mode. I understand that because the high-voltage power supply output requires a high-voltage rectifier diode. Due to the characteristics of the manufacturing process, the high reverse voltage diode has a long reverse recovery time and low speed. In the continuous current state, the diode is recovered 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 the worst, the rectifier tube is even burned. Because in the discontinuous mode, the diode is reverse biased under zero bias, the loss can be reduced to a relatively low level. Therefore, the high-voltage power supply works in discontinuous mode, and the operating frequency cannot be too high. There is another type of flyback power supply that works in a critical state. Generally, this type of power supply works in frequency modulation mode, or frequency modulation and width modulation dual mode. Some low-cost self-excited power supplies (RCC) often use this form. To ensure output stability, the transformer operating frequency changes with the output current or input voltage. When approaching full load, the transformer always remains between continuous and intermittent. This power supply is only suitable for low-power output, otherwise the handling of electromagnetic compatibility characteristics will be a headache.

　　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 think that when the nominal voltage is input, the output reaches 50%~60%, and the transformer transitions from intermittent to continuous state is more appropriate. Or at the highest input voltage state, when the output is full load, the transformer can transition to the continuous state.