Let's start with the description of the design and production process of the switching power supply, and let's talk about the design of the printed board. Switching power supply works in high frequency and high pulse state, which belongs to a special type in analog circuits. The layout of the board must follow the principle of high-frequency circuit wiring.

1. Layout: The pulse voltage connection is as short as possible, in which the input switch is connected to the transformer, and the output transformer is connected to the rectifier. The pulse current loop is as small as possible, such as the input filter capacitor is positive to the transformer to the switch tube and the return capacitor is negative. The output terminal of the transformer from the rectifier tube to the output inductor to the output capacitor returns to the transformer circuit. The X capacitor should be as close as possible to the input terminal of the switching power supply. The input line should be avoided parallel to other circuits and should be avoided. The Y capacitor should be placed at the chassis ground terminal or the FG connection. The common-mode inductance is kept 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 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. Heating devices should keep a certain distance from electrolytic capacitors to prolong the life of the whole machine. Electrolytic capacitors are the lifespan of switching power supplies. For example, transformers, power tubes, and high-power resistors should keep a distance from electrolysis, and space for heat dissipation should also be left between electrolysis. , it can be placed in the air inlet if conditions permit.

Attention should be paid to the control part: the connection of the high-impedance weak signal circuit should be as short as possible, such as the sampling feedback loop, and it should be avoided as far as possible during processing. The current sampling signal circuit, especially the current-controlled circuit, is prone to some unexpected things if it is not handled well. There are some tricks in it. Now take the 3843 circuit as an example as shown in Figure (1). The effect of Figure 1 is better than that of Figure 2. In Figure 2, when the oscilloscope is used to observe the current waveform at full load, spikes are obviously superimposed on the current waveform. On the low side, there is no such phenomenon in Figure 1, and there is a switching tube driving signal circuit. The switching tube driving resistance should be close to the switching tube, which can improve the working reliability of the switching tube, which is related to the high DC impedance voltage driving characteristics of the power MOSFET.

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

Line spacing: With the continuous improvement and improvement of the manufacturing process of printed circuit boards, there is no problem in the general processing factory manufacturing line spacing equal to or even less than 0.1mm, which can fully meet most applications. Considering the components and production process used in the switching power supply, the minimum line spacing of the double-sided panel is generally 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 and vias. Holes with a minimum spacing of 0.5mm to avoid "bridging" during welding operations. , so that most board factories can easily meet the production requirements, and can control the yield to a very high level, and can also 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 a voltage lower than 63V. When the line-to-line voltage is greater than this value, the line spacing can generally be taken according to the empirical value of 500V/1mm.

In view of the fact that some relevant standards have clear regulations on the line spacing, it must be strictly implemented in accordance with the standards, such as the connection between the AC inlet end and the fuse end. Some power supplies have high volume requirements, such as modular power supplies. The general transformer input side line spacing of 1mm has been proved to be feasible. For AC input, (isolated) DC output power supply products, the stricter regulation is that the safety distance should be greater than or equal to 6mm. Of course, this is determined by the relevant standards and implementation methods. Generally, the safety distance can be used as a reference for the distance on both sides of the feedback optocoupler, and the principle is greater than or equal to this distance. Slots can also be made on the printed board under the optocoupler to increase the creepage distance to meet the insulation requirements. Generally, the distance between the AC input side wiring or on-board components of the switching power supply should be greater than 5mm from the non-insulated casing and radiator, and the distance between the output side wiring or device and the casing or radiator should be greater than 2mm, or strictly follow safety regulations.

Common method: 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 often used as a protective discharge gap, which is commonly used in the tail plate of TV kinescopes and the AC input of power supply. . This method has been widely used in module power supply, and good results can be obtained under the condition of potting.

Method 2: Pad insulating paper, which can be made of green shell paper, polyester film, PTFE oriented film and other insulating materials. Generally, green shell paper or polyester film is used for general power supply to pad between the circuit board and the metal casing. This material has high mechanical strength and has a certain ability to resist moisture. PTFE oriented film is widely used in module power supply due to its high temperature resistance. An insulating film can also be placed between the element and the surrounding conductors to improve the insulation resistance.

Note: The insulating coating of some devices cannot be used as an insulating medium to reduce the safety distance, such as the outer skin of an electrolytic capacitor. Under high temperature conditions, the outer skin may shrink due to heat. Space should be reserved at the front end of the large electrolytic explosion-proof tank to ensure that the electrolytic capacitor can discharge pressure unhindered in an emergency.

Finally, let’s talk about the duty cycle of the flyback power supply (I pay attention to the reflected voltage, which is consistent with the duty cycle). The duty cycle is also related to the withstand voltage of the selection switch. Some early flyback power supplies use lower withstand voltage switches. Tube, such as 600V or 650V as the switch tube of AC 220V input power supply, may be related to the production process at that time, high withstand voltage tubes are not easy to manufacture, or low withstand voltage tubes have more reasonable conduction loss and switching characteristics, such as this circuit The reflected voltage should not be too high, otherwise, in order to make the switch tube work within a safe range, the power absorbed by the circuit will be considerable. Practice has proved that the reflected voltage of the 600V tube should not be greater than 100V, and the reflected voltage of the 650V tube should not be greater than 120V. When the leakage inductance peak voltage value is clamped at 50V, the tube still has a working margin of 50V. Now, due to the improvement of the MOS tube manufacturing process level, the general flyback power supply uses 700V or 750V or even 800-900V switch tubes. Like this kind of circuit, the ability to resist overvoltage is strong, and the reflected voltage of some switching transformers can also be made higher. The maximum reflected voltage is 150V, which is suitable to obtain better overall performance. PI's TOP chip recommends using a transient voltage suppression diode clamp for 135V. However, his evaluation board generally has a reflected voltage lower than this value at about 110V. Both types have advantages and disadvantages:

The first category: the disadvantage is that the anti-overvoltage capability is weak, the duty cycle is small, and the primary pulse current of the transformer is large. Advantages: transformer leakage inductance is small, electromagnetic radiation is low, ripple index is high, switching tube loss is small, and the conversion efficiency is not necessarily lower than the second type.

The second category: the disadvantage is that the loss of the switch tube is larger, the leakage inductance of the transformer is larger, and the ripple is worse. Advantages: stronger anti-overvoltage capability, larger duty cycle, lower transformer loss, and higher efficiency.

There is also a 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 more power a circuit consumes, the potential for permanent failure of the snubber circuit power device (especially a circuit using a transient voltage suppression diode). In the optimization process of designing low-voltage output and low-power flyback power supplies, care must be taken. There are several processing methods:

1. Use a magnetic core with a larger power level 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-regulation rate of the multi-channel output power supply, which is generally used in home appliances switches. Power supplies, such as CD players, DVB set-top boxes, etc.

2. If the conditions do not allow to increase the magnetic core, only the reflected voltage can be reduced and the duty cycle can be reduced. Reducing the reflected voltage can reduce the leakage inductance, but it may reduce the power conversion efficiency. The two are contradictory. There must be a replacement process to find a suitable point. During the transformer replacement experiment, the primary side of the transformer can be detected. Inverse peak voltage, try to reduce the width and amplitude of the inverse peak voltage pulse, which can increase the working safety margin of the converter. Generally, the reflected voltage is more suitable at 110V.

3. To enhance coupling, reduce loss, adopt new technology and winding process, in order to meet safety regulations, the transformer will take insulating measures between the primary side and the secondary side, such as insulating tape and insulating tape. These will affect the leakage inductance performance of the transformer. In actual production, the primary winding can be used to wrap the secondary winding. Or the secondary is wound with triple insulated wire, eliminating the insulation between the primary and secondary, which can enhance the coupling, and can even be wound with a wide copper sheet.

The low-voltage output in the text refers to the output less than or equal to 5V. Like this type of low-power power supply, my experience is that the output of the power output is greater than 20W, and the forward type can be used to obtain the best cost performance. Of course, this is not absolute, and Personal habits are related to the application environment. Next time, I will talk about some understanding of the magnetic core for flyback power supply and the air gap in the magnetic circuit. I hope you can give me some advice.

The magnetic core of the flyback power transformer is working 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 an air gap. The principle of why the air gap is opened is as follows: since the power ferrite also has a working characteristic curve (magnetic hysteresis loop) that is similar to a rectangle, the Y-axis on the working characteristic curve represents the magnetic induction intensity (B), and the current production process is generally The saturation point is above 400mT. Generally, the value of this value should be 200-300mT in the design. The X-axis represents the magnetic field strength (H). This value is proportional to the magnetizing current strength. Opening the air gap in the magnetic circuit is equivalent to tilting the hysteresis loop of the magnet to the X axis. Under the same magnetic induction intensity, it can withstand a larger magnetizing current, which is equivalent to storing more energy in the magnetic core. This energy is turned off when the switch tube is turned off. When it is discharged to the load circuit through the transformer secondary, the air gap of the flyback power supply magnetic core has two functions. One is to transfer more energy, and the other is to prevent the core from going into saturation.

The transformer of the flyback power supply works in a one-way magnetization state, not only to transfer energy through magnetic coupling, but also to perform multiple functions of voltage conversion input and output isolation. Therefore, the handling of the air gap needs to be very careful. 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, which will affect the overall performance of the power supply. Too small an air gap may saturate the transformer core and cause damage to the power supply

The so-called continuous and discontinuous mode of the flyback power supply refers to the working state of the transformer. In the fully loaded state, the transformer works in the working mode of complete energy transfer or incomplete transfer. Generally, it should be designed according to the working environment. The conventional flyback power supply should work in the 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 needs to be specially pointed out here: due to the characteristics of the flyback power supply, it is more suitable to be designed as a high-voltage power supply, and the high-voltage power supply transformer generally works in the discontinuous mode. Due to the characteristics of the manufacturing process, the high reverse voltage diode has long reverse recovery time and low speed. In the continuous current state, the diode recovers when there is forward bias voltage, and the energy loss during reverse recovery is very large, which is not conducive to the performance of the converter. The improvement of the conversion efficiency will reduce the conversion efficiency, the rectifier tube will be seriously heated, and the rectifier tube will even be burned in severe cases. Since in discontinuous mode the diode is reverse biased at zero bias, losses 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 also a 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. In order to ensure stable output, the transformer The working frequency changes with the output current or input voltage. When the transformer is close to full load, it is always kept between continuous and intermittent. This kind of power supply is only suitable for small power output, otherwise the processing of electromagnetic compatibility characteristics will be very troublesome.

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 too much. It may require a large magnetic core. The number of turns of the coil, along with the large leakage inductance and distributed capacitance, may outweigh the gain. So how to determine this parameter, through many practices and analysis of the design of the same industry, I think that when the nominal voltage input, the output reaches 50%~60% of the transformer, it is more appropriate to transition from intermittent to continuous state. Or at the highest input voltage state, when the output is fully loaded, the transformer can transition to the continuous state.

Let's talk about some matters about the copper routing of the printed board:

Trace current density: Now most electronic circuits are composed of insulating plates bound with copper. The copper thickness of the commonly used circuit board is 35μm, and the current density value of the wiring can be taken according to the empirical value of 1A/mm. The specific calculation can be found in the textbook. In order 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). Circuit boards with a copper thickness of 70 μm are also common in switching power supplies, so the current density can be higher.

It is added that 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 circuit boards, the design software can automatically execute according to the specifications, which can save a lot of time, reduce part of the workload and reduce the error rate.

Generally, double-sided boards can be used for lines with high reliability requirements or high wiring line density. It is characterized by moderate cost and high reliability, which can meet most applications.

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

Single-sided, almost all general-purpose switching power supplies in the market use single-sided circuit boards, which have the advantage of low cost. Taking some measures in design and production technology can also ensure their performance.

Today, I will talk about some experience in the design of single-sided printed boards. Because single-sided printed boards are characterized by low cost and easy manufacture, they are widely used in switching power supply circuits. Relying on that layer of copper, care must be taken when handling.

In order to ensure good welding mechanical structure performance, the single-panel pad should be slightly larger to ensure a good binding force between the copper skin and the substrate, so that the copper skin will not peel off and break off when subjected to vibration. Generally, the width of the welding ring should be greater than 0.3mm. The diameter of the pad hole should be slightly larger than the diameter of the device pin, but it should not be too large to ensure the shortest connection distance between the pin and the pad by soldering. Diameter 0.1-0.2mm. 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 larger than the diameter of the pad. In special cases, the wire must be widened when the connection meets the pad (commonly known as teardrop generation) to avoid breakage between the wire and the pad under certain conditions. In principle, the minimum line width should be greater than 0.5mm.

The components on the single panel should be close to the circuit board. Devices that require overhead heat dissipation,