Talking about the design experience of switching power supply for many years, from the design of switching power supply printed board, printed board wiring, printed board copper wiring, the application of aluminum substrate and multi-layer printed board in switching power supply, to the duty cycle of flyback power supply, absolute practice essence!
Design of Printed Circuit Board for Switching Power Supply
First, let's describe the design and production process of switch mode power supplies. Let's first talk about the design of printed circuit boards. Switching power supplies operate at high frequencies and high pulse states, and are a relatively special type of analog circuit. When laying out boards, the principle of high-frequency circuit wiring must be followed.
Layout: The pulse voltage connection should be as short as possible, with input switch tube to transformer connection and output transformer to rectifier tube connection. The pulse current loop should be as small as possible, such as the input filtering capacitor being positive and the return capacitor being negative from the transformer to the switching tube. The output part of the transformer should be from the output end to the rectifier tube to the output inductor to the output capacitor. The X capacitor in the transformer circuit should be as close as possible to the input end of the switching power supply, and the input line should avoid being parallel to other circuits. The Y capacitor should be placed at the casing grounding terminal or FG connection terminal. Maintain a certain distance between the common touch electric induction and the transformer to avoid magnetic coupling. If it is not easy to handle, a shield can be added between the common touch inductor and the transformer. The above factors have a significant impact on the EMC performance of the switching power supply.

The output capacitor can generally be two, 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 parallel effect of two small capacity capacitors should be better than using one large capacity capacitor. Heating devices should be kept at a certain distance from electrolytic capacitors to extend the lifespan of the entire machine. Electrolytic capacitors are the key to the lifespan of switching power supplies, such as transformers, power tubes, high-power resistors, and electrolysis. Cooling space should also be left between electrolysis, and if conditions permit, they can be placed in the air inlet.
Attention should be paid to the control part: the wiring of high impedance weak signal circuits should be as short as possible, such as sampling feedback loops. During processing, it is important to avoid interference and current sampling signal circuits, especially current control circuits, which may cause unexpected accidents if not handled well.
Let's talk about some principles of printed circuit board wiring.
Line spacing: With the continuous improvement and enhancement of the manufacturing process of printed circuit boards, there is no problem for general processing plants to produce a line spacing equal to or even less than 0.1mm, which can fully meet most application scenarios. Considering the components and production process used in switch mode power supplies, the minimum line spacing for double-sided panels is generally set to 0.3mm, the minimum line spacing for single panel is set to 0.5mm, and the minimum spacing between pads and pads, pads and vias, or vias and vias is set to 0.5mm. This can avoid the phenomenon of "bridging" during welding operations, making it easy for most board factories to meet production requirements and control the yield very high, It can also achieve reasonable wiring density and have a relatively economical cost.
The minimum line spacing is only suitable for signal control circuits and low-voltage circuits with voltage below 63V. When the line to line voltage is greater than this value, the line spacing can generally be taken based on the empirical value of 500V/1mm.
Given that some relevant standards have clear regulations on line spacing, it is necessary to strictly follow the standards, such as connecting the AC inlet end to the fuse end. Some power supplies have high volume requirements, such as module power supplies. It has been proven feasible to have a line spacing of 1mm on the input side of a typical transformer. For power products with AC input and (isolated) DC output, the strict requirement is that the safety distance should be greater than or equal to 6mm, which is determined by relevant standards and implementation methods. The general safety distance can be referred to by the distance on both sides of the feedback optocoupler, and should be greater than or equal to this distance. Slots can also be made on the printed circuit board below the optocoupler to increase the creepage distance and meet insulation requirements. The distance between the AC input side wiring or components on the board of a general switching power supply and the non insulated casing or heat sink should be greater than 5mm, and the distance between the output side wiring or components and the casing or heat sink should be greater than 2mm, or strictly follow safety regulations.
Common methods: The method of slotting circuit boards mentioned earlier is suitable for situations where the spacing is not enough. By the way, this method is also commonly used as a protective discharge gap and is commonly used at the tail board of TV graphics tubes and the AC input of power supplies. This method has been widely used in modular power supplies and can achieve good results under sealing conditions.

Method 2: Use insulating paper, which can be made of insulating materials such as green shell paper, polyester film, and polytetrafluoroethylene directional film. Generally, general-purpose power supplies are padded between the circuit board and the metal casing with green shell paper or polyester film. This material has high mechanical strength and certain moisture resistance. Polytetrafluoroethylene directional membranes are widely used in modular power supplies due to their high temperature resistance. Insulating films can also be placed between components and surrounding conductors to improve insulation and electrical resistance.
Attention: Some device insulation covers cannot be used as insulation media to reduce safety distance, such as the outer skin of electrolytic capacitors, which may shrink due to heat under high temperature conditions. A space should be left at the front end of the large electrolytic explosion-proof tank to ensure that the electrolytic capacitor can discharge voltage unobstructed in emergency situations.
Precautions for copper wire routing on printed circuit boards
Line current density: Nowadays, most electronic circuits are composed of insulated boards bound with copper. The commonly used copper sheet thickness for circuit boards is 35 μ m. The current density value can be taken based on the empirical value of 1A/mm for wiring, and specific calculations can be found in textbooks. To ensure the mechanical strength of the wiring, 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 thickness of the copper sheet is 70 μ M circuit boards are also commonly used in switching power supplies, so the current density can be higher.
To add, commonly used circuit board design tool software generally has design specification items, such as line width, line spacing, dry tray through-hole size, and other parameters that can be set. When designing circuit boards, the design software can automatically execute according to specifications, saving a lot of time, reducing some workload, and reducing error rates.
Double sided boards can be used for circuits or wiring with high reliability requirements or high density. Its characteristics are moderate cost, high reliability, and can meet most application scenarios.
Some products in the modular power supply industry also use multi-layer boards, which are mainly convenient for integrating power devices such as transformers and inductors, optimizing wiring, and cooling power tubes. It has the advantages of good consistency in process aesthetics and good heat dissipation of transformers, but its disadvantages are high cost, poor flexibility, and is only suitable for industrial large-scale production.
Single panel, commonly used switch mode power supplies in the market almost all use single-sided circuit boards, which have the advantage of low cost. Some measures in design and production process can also ensure its performance.
Some Experiences on Single sided Printed Circuit Board Design
Due to its low cost and ease of manufacturing, single panels are widely used in switch mode power supply circuits. As they only have one side bound with copper, the electrical connections and mechanical fixation of devices rely on that layer of copper sheet, and care must be taken when handling them.
To ensure good welding mechanical structural performance, the single panel solder pads should be slightly larger to ensure good adhesion between the copper sheet and the substrate, and not to peel or break off the copper sheet when subjected to vibration. The general welding ring width should be greater than 0.3mm. The diameter of the solder pad hole should be slightly larger than the diameter of the device pins, but should not be too large to ensure that the distance between the pins and the solder pad is the shortest. The size of the solder pad hole should not hinder normal inspection. The diameter of the solder pad hole is generally 0.1-0.2mm larger than the diameter of the pins. Multi pin devices can also be larger to ensure smooth component inspection.
Electrical connections should be as wide as possible, with a principle that the width should be greater than the diameter of the solder pad. In special cases, the line must be widened (commonly known as generating tears) when the connection intersects with the solder pad to avoid breaking under certain conditions. The minimum line width should be greater than 0.5mm.
The components on a single panel should be tightly attached to the circuit board. For devices that require overhead heat dissipation, a sleeve should be added to the pins between the device and the circuit board to support the device and increase insulation. It is necessary to minimize or avoid the impact of external forces on the connection between the solder pad and the pins, and enhance the firmness of the welding. The heavier components on the circuit board can have additional support connection points, which can enhance the connection strength with the circuit board, such as transformers and power device heat sinks.
The pins on the single panel welding surface can be left longer without affecting the spacing with the shell. Its advantage is that it can increase the strength of the welding area, increase the welding area, and detect virtual welding phenomena in real time. When the pins are long and the legs are cut, the welding area is subjected to less force. In Taiwan and Japan, the process of bending device pins at a 45 degree angle to the circuit board on the soldering surface and then soldering is commonly used, and the same principle applies. Today, let's talk about some issues in dual panel design. In some application environments with high requirements or high wiring density, using double-sided printed boards has much better performance and various indicators than single panels.
Due to the high strength of the metalized holes in the double-sided solder pads, the welding ring can be smaller than that of a single panel, and the hole diameter of the solder pads can be slightly larger than the diameter of the pins. This is because during the welding process, it is beneficial for the solder solution to penetrate through the solder holes to the top layer of the solder pads, thereby increasing welding reliability. But there is a drawback, if the hole is too large, some components may float under the impact of jet tin during wave soldering, resulting in some defects.
For the treatment of high current wiring, the wire width can be treated according to the previous label. If the width is not enough, tin plating can generally be used to increase the thickness of the wiring. There are many methods to solve this problem:
1. Set the wiring to solder pad properties, so that the wiring will not be covered by solder mask during circuit board manufacturing, and the hot air leveling will be tin plated.
2. Place a solder pad at the wiring location, set the solder pad to the shape that requires wiring, and be sure to set the solder pad hole to zero.
3. Placing wires on the solder mask layer is the most flexible method, but not all circuit board manufacturers will understand your intention and require written explanation. The area where the wire is placed in the solder mask layer will not be coated with solder mask.There are several methods for tinning circuits as mentioned above. It should be noted that if a wide wire is fully tinned, a large amount of solder will be adhered after welding, and the distribution will be uneven, affecting the appearance. Generally, thin and long strips with a tin plating width of 1-1.5mm can be used, and the length can be determined according to the circuit. The double-sided circuit board with a tin plating interval of 0.5-1mm provides great selectivity for layout and wiring, which can make the wiring more reasonable. Regarding grounding, the power ground and signal ground must be separated, and the two grounds can be combined at the filtering capacitor to avoid unstable accidental factors caused by large pulse currents passing through the signal ground connection. The signal control circuit should adopt a single point grounding method as much as possible. There is a technique to place non grounded wiring on the same wiring layer as much as possible, and finally lay a ground wire on another layer. The output line generally first passes through the filtering capacitor before reaching the load, and the input line must also pass through the capacitor before reaching the transformer. The theoretical basis is to let the ripple current pass through the filtering capacitor.
Voltage feedback sampling: To avoid the influence of large current passing through the wiring, the sampling point of feedback voltage must be placed at the end of the power output to improve the overall load effect index of the machine.
The routing from one wiring layer to another is generally connected through via holes and should not be achieved through device pin pads, as this connection relationship may be disrupted during device insertion. Additionally, there should be at least 2 via holes every 1A of current passing through, and the via hole diameter should be larger than 0.5mm. Generally, 0.8mm can ensure machining reliability.
Device heat dissipation: In some low-power power supplies, the wiring of the circuit board can also serve as a heat dissipation function. Its characteristic is to have the wiring as wide as possible to increase the heat dissipation area, without applying solder mask, and can be evenly placed through holes to enhance thermal conductivity if conditions permit.

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 multi-layer printed boards in switching power supply circuits.
The aluminum substrate is constructed by itself and has the following characteristics: excellent thermal conductivity, single-sided copper binding, devices can only be placed on the copper binding surface, and electrical wiring holes cannot be opened, so jumper wires cannot be placed like single panels.
Surface mount devices, switching tubes, and output rectifier tubes are generally placed on aluminum substrates to conduct heat through the substrate, resulting in low thermal resistance and high reliability. The transformer adopts a flat surface mount structure, which can also dissipate heat through the substrate. Its temperature rise is lower than that of conventional transformers. Similarly, using an aluminum substrate structure for transformers of the same specifications can achieve larger output power. Aluminum substrate jumpers can be treated by bridging. Aluminum substrate power supply is generally composed of two printed boards, with the other board containing a control circuit. The two boards are physically connected to form a whole.
Due to the excellent thermal conductivity of aluminum substrates, it is difficult to manually weld them in small quantities, and the solder cools too quickly, which can easily lead to problems. There is a simple and practical method to do this: turn over a regular electric iron for ironing clothes (preferably with a temperature adjustment function), turn it over, iron the surface upwards, fix it, and adjust the temperature to around 150 ℃. Place the aluminum substrate on top of the iron, heat it up for a period of time, and then stick and weld the components according to the conventional method, The temperature of the iron should be suitable for easy soldering of the device. If it is too high, the device may be damaged, and even the aluminum substrate copper skin may peel off. If the temperature is too low, the soldering effect is not good, and it should be flexibly controlled.
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 transformer window section can also be fully utilized. One to 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. The use of printed coils reduces manual intervention, The transformer has good consistency, planar structure, low leakage inductance, and good coupling. Open magnetic core with good heat dissipation conditions. Due to its many advantages, it is conducive to large-scale production and has been widely used. But the initial investment in research and development is relatively large, which is not suitable for small-scale production.
Switching power supplies are divided into two forms: isolated and non isolated. Here, we will mainly discuss the topology of isolated switching power supplies. In the following text, unless otherwise specified, they all refer to isolated power supplies. Isolation power supplies can be divided into two categories according to their structural forms: forward and flyback. Flyback type refers to the secondary side being cut off when the primary side of the transformer conducts, and the transformer stores energy. When the primary side is cut off, the secondary side conducts and energy is released to the working state of the load. Generally, conventional flyback power supplies have more single switches, while double switches are not common. Forward excitation refers to the process where the primary side of a transformer conducts while the secondary side induces the corresponding voltage output to the load, and energy is directly transmitted through the transformer. According to specifications, it can be divided into conventional forward excitation, including single tube forward excitation and double tube forward excitation. Half bridge and bridge circuits belong to forward circuits.
Both forward and flyback circuits have their own characteristics, and can be flexibly applied in the design process to achieve optimal cost-effectiveness. Generally, flyback type can be used in low-power situations. A slightly larger one can use a single transistor forward circuit, a medium power can use a dual transistor forward circuit or a half bridge circuit, and a push-pull circuit is used at low voltage, which is the same as the half bridge working state. High power output is generally achieved through bridge circuits, and push-pull circuits can also be used for low voltage.
Flyback power supplies are widely used in small and medium-sized power supplies due to their simple structure, which eliminates an inductor that is similar in size to a transformer. In some introductions, it is mentioned that the power of the flyback power supply can only reach a few tens of watts, and there is no advantage when the output power exceeds 100 watts, making it difficult to implement. I think this is generally the case, but it cannot be generalized. PI Company's TOP chip can achieve 300 watts, and there is an article introducing that flyback power can reach up to kilowatts, but I have not seen a physical product before. The output power is related to the output voltage.
The leakage inductance of the flyback power transformer is a very critical parameter. As the flyback power requires the transformer to store energy, in order to fully utilize the transformer core, an air gap is usually opened in the magnetic circuit. The purpose is to change the slope of the hysteresis loop of the core, so that the transformer can withstand large pulse current shocks without the core entering a saturated non-linear state, and the air gap in the magnetic circuit is in a high magnetic resistance state, The leakage in the magnetic circuit is much greater than that in a completely closed magnetic circuit.
The coupling between the initial poles of a transformer is also a key factor in determining leakage inductance. To make the initial pole coil as close as possible, a sandwich winding method can be used, but this will increase the distributed capacitance of the transformer. When selecting iron cores, try to use magnetic cores with longer windows to reduce leakage inductance. For example, using EE, EF, EER, and PQ type magnetic cores will result in better performance than 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 for and may be unstable. However, there are some exceptions, such as the TOP series chips launched by PI Company in the United States, which can operate under conditions where the duty cycle is greater than 0.5. The duty cycle is determined by the ratio of turns on the primary and secondary sides of the transformer. My opinion on doing flyback is to first determine the reflected voltage (the voltage value reflected on the primary side through transformer coupling). Within a certain voltage range, as the reflected voltage increases, the working duty cycle increases and the switching losses decrease. If the reflected voltage decreases, the working duty cycle decreases and the switching 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 stable output, more of the time it will be ensured by the discharge current of the output capacitor. The output capacitor will withstand larger high-frequency ripple currents, which will intensify its heating. This is not allowed under many conditions. Increasing the duty cycle and changing the turn ratio of the transformer will increase the leakage inductance of the transformer, improving 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. Therefore, there is no significance in increasing the duty cycle again, and it may even cause the switch tube to break down due to the high reverse peak voltage of the leakage inductance. Due to high leakage inductance, it may cause output ripple and other electromagnetic indicators to 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. However, it can improve the working conditions of the output capacitor and reduce heating. How to determine the reflected voltage (i.e. duty cycle) of a transformer
Some netizens mentioned the parameter settings and working state analysis of the feedback loop of the switching power supply. Due to my poor performance in advanced mathematics when I was in school, I almost took the make-up exam for "Principles of Automatic Control". I still feel scared about this subject and cannot fully write the closed-loop system transfer function. I feel very vague about the concepts of system zeros and poles. Looking at the Bode diagram, I can only roughly see whether it is divergent or convergent. Therefore, I dare not speak nonsense about feedback compensation, but there are some suggestions. If you have some mathematical skills and some study time, you can carefully digest the university textbook "Principles of Automatic Control" and combine it with actual switch power supply circuits to analyze according to the working state.

The duty cycle of the flyback power supply
Finally, let's talk about the duty cycle of the flyback power supply (I pay attention to the reflection voltage, which is consistent with the duty cycle). The duty cycle is also related to the selection of the withstand voltage of the switching tube. Some early flyback power supplies used relatively low withstand voltage switching tubes, such as 600V or 650V as the switching tube for AC 220V input power supply, which may be related to the production process at that time. High withstand voltage tubes are not easy to manufacture, Alternatively, low voltage withstand tubes may have more reasonable conduction losses and switching characteristics. For circuits like this, the reflected voltage should not be too high. Otherwise, in order to keep the switching tubes operating within a safe range, the power absorbed by circuit losses is also considerable. Practice has proven that the reflection voltage of 600V pipes should not exceed 100V, and the reflection voltage of 650V pipes should not exceed 120V. When the leakage sensing peak voltage value is clamped at 50V, the pipes still have a working margin of 50V. Nowadays, due to the improvement of MOSFET manufacturing technology, most flyback power supplies use switch transistors with a voltage of 700V, 750V, or even 800-900V. This type of circuit has a stronger ability to withstand overvoltage, and the reflection voltage of the switch transformer can also be relatively high. The maximum reflection voltage is suitable at 150V, which can achieve good comprehensive performance. The top chip recommended by PI company is 135V, which uses transient voltage suppression diode clamping. But his evaluation board generally reflects a voltage lower than this value, around 110V. Both types have their advantages and disadvantages:
      Type 1: Disadvantages include weak overvoltage resistance, low duty cycle, and high primary pulse current in transformers. Advantages: Transformers have low leakage inductance, low electromagnetic radiation, high ripple index, low switching losses, and may not necessarily have lower conversion efficiency than the second type.
Type 2: Disadvantages include higher losses in switching tubes, higher leakage inductance in transformers, and poorer ripple. Advantages: Strong overvoltage resistance, high duty cycle, lower transformer losses, 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, which is the output voltage. The lower the output voltage, the larger the turn ratio of the transformer, the greater the leakage inductance of the transformer, and the higher the voltage borne by the switch tube. It is possible to breakdown the switch tube and consume more power in the absorption circuit, which may cause permanent failure of the absorption circuit power device (especially in circuits using transient voltage suppression diodes). In the optimization process of designing low-voltage output low-power flyback power supplies, careful handling is necessary. There are several methods to handle this:
1. By using a magnetic core with a higher power level to reduce leakage inductance, the conversion efficiency of low-voltage flyback power supplies can be improved, losses can be reduced, output ripple can be reduced, and the cross adjustment rate of multiple output power supplies can be improved. This is generally seen in switch mode power supplies for household appliances, such as CD players, DVB set-top boxes, etc.
2. If conditions do not allow for increasing the magnetic core, the reflection voltage can only be reduced to reduce the duty cycle. Reducing the reflection voltage can reduce leakage inductance, but it may also lower the power conversion efficiency. This is a contradiction, and a replacement process is necessary to find a suitable point. In the transformer replacement experiment, the back peak voltage of the transformer's primary side can be detected, and the width and amplitude of the back peak voltage pulse can be reduced as much as possible, which can increase the working safety margin of the transformer. Generally, a reflection voltage of 110V is more suitable.
3. Enhancing coupling, reducing losses, adopting new technologies and winding processes, transformers will take insulation measures between the primary and secondary sides to meet safety standards, such as padding insulation tape and adding insulation end air tape. These will affect the leakage inductance performance of transformers, and in practical production, the winding method of wrapping the primary winding with the secondary winding can be used. Alternatively, the secondary can be wound with triple insulated wire, eliminating the insulation material between the primary stages to enhance coupling, and even wide copper foil winding can be used.
The low-voltage output in the article refers to an output less than or equal to 5V. In my experience, for low-power power supplies like this, if the power output is greater than 20W, a forward converter can be used to achieve the best cost-effectiveness. However, this is not a definitive decision, as it depends on personal habits and the application environment. Next time, I will talk about some understanding of using magnetic cores and opening air gaps in the magnetic circuit for flyback power supplies. I hope that experts can provide guidance.
The magnetic core of the flyback power transformer operates 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 air gaps. Why is the principle of opening an air gap understood by myself as follows: due to the fact that 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). Currently, the saturation point of the production process is generally above 400mT, and this value should be suitable in the design range of 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 hysteresis loop of the magnet towards 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 into the load circuit through the transformer secondary when the switching tube is turned off. The opening of the air gap in the flyback power supply magnetic core has two functions. One is to transfer more energy, and the other is to prevent the magnetic core from entering a saturated state.The transformer of the flyback power supply operates in a unidirectional magnetization state, which not only transfers energy through magnetic coupling, but also plays multiple roles in voltage conversion input output isolation. So the handling of air gaps needs to be very careful. If the air gap is too large, it can increase leakage inductance, magnetic hysteresis loss, iron loss, copper loss, and affect the overall performance of the power supply. A small air gap may cause saturation of the transformer core, leading to power damage.
The continuous and intermittent modes of the flyback power supply refer to the working state of the transformer, in which the transformer operates in a mode of complete or incomplete energy transfer at full load. Generally, the design should be based on the working environment. Conventional flyback power supplies should operate in continuous mode, so that the losses of switching tubes and circuits are relatively small, and the working stress of input and output capacitors can be reduced. However, there are some exceptions to this.
It should be pointed out here that due to the characteristics of the flyback power supply, it is also suitable to be designed as a high-voltage power supply, and high-voltage power transformers generally work in intermittent mode. I understand that the high-voltage power supply output requires the use of high voltage rectifier diodes. Due to the manufacturing process characteristics, high reverse voltage diodes have a long reverse recovery time and low speed. In a continuous current state, the diode recovers when there is a forward bias, and the energy loss during reverse recovery is very large, which is not conducive to improving the performance of the converter. In some cases, the conversion efficiency is reduced, and the rectifier tube heats up severely. In severe cases, the rectifier tube may even burn out. Due to the fact that in intermittent mode, the diode is reverse biased under zero bias, the loss can be reduced to a relatively low level. So the high-voltage power supply operates in intermittent mode and the operating frequency cannot be too high.
There is also a type of flyback power supply that operates in a critical state. Generally, this type of power supply operates in frequency modulation mode or frequency width modulation dual mode. Some low-cost self-excited power supplies (RCC) often use this form. To ensure output stability, the operating frequency of the transformer changes with the output current or input voltage. When approaching full load, the transformer always remains between continuous and intermittent. This type of power supply is only suitable for low-power output, Otherwise, the handling of electromagnetic compatibility characteristics can be very headache inducing.
The flyback switching power supply transformer should operate in continuous mode, which requires a relatively large winding inductance. Of course, continuity also has a certain degree. Overly pursuing absolute continuity is not realistic. It may require a large magnetic core, a large number of coil turns, and accompanied by large leakage inductance and distributed capacitance, which may not be worth the loss. So how to determine this parameter? Through multiple practices and analysis of peer designs, I believe that it is more appropriate for the transformer to transition from intermittent to continuous state when the output reaches 50% to 60% of the nominal voltage input. Alternatively, at the highest input voltage state and full load output, the transformer can transition to a continuous state.