Reasonable design technology of switching power supply

With the continuous improvement and improvement of the manufacturing process of printed circuit boards, it is no longer a problem for general processing plants to produce line spacing equal to or even less than 0.1mm, which can fully meet most applications. Considering the components and production processes used in switching power supplies, the minimum line spacing of double-sided boards is generally set to 0.3mm, and the minimum line spacing of single-sided boards is set to 0.5mm. The minimum spacing between pads, pads and vias, or vias and vias is set to 0.5mm to avoid the "bridging" phenomenon during the welding operation. In this way, most board manufacturers can easily meet production requirements, control the yield rate to a very high level, and achieve a reasonable wiring density and a more economical cost.

The minimum line spacing is only suitable for signal control circuits and low-voltage circuits with voltages below 63V. When the line voltage is greater than this value, the line spacing can generally be selected according to the empirical value of 500V/1mm.

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

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

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 by bridging. 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.

3 Some matters concerning copper wiring of 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. The copper thickness of 70μm is also common in switching power supplies, so the current density can be higher.

Some products in the module power supply line also use multi-layer boards, which are mainly 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.

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.

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 double-sided board pad has a higher strength because the holes have been metallized. The solder ring can be smaller than that of the single-sided board, and the pad hole diameter can be slightly larger than the pin diameter. This is because it is beneficial for the solder solution to penetrate through the solder hole to the top pad during the soldering process, thereby increasing the soldering reliability.

4. Handling of 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 trace to increase the thickness. There are many ways to do this.

a Set the trace to pad properties so that the trace will not be covered by solder resist during circuit board manufacturing and will be tinned during hot air leveling.

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

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

Several methods of circuit tinning are as follows. 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. Double-sided circuit boards provide 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.

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.

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

Aluminum substrate (metal-based heat sink (including aluminum substrate, copper substrate, iron substrate)) is a unique metal-based copper-clad laminate, which has good thermal conductivity, electrical insulation and mechanical processing properties. Aluminum-based copper-clad laminate is a metal circuit board material, composed of copper foil, thermal insulation layer and metal substrate. Its structure is divided into three layers:

Circuit layer: equivalent to the copper clad board of ordinary PCB, the circuit copper foil thickness is from 10oz to 10oz.

DielcctricLayer Insulation Layer: The insulation layer is a layer of low thermal resistance thermally conductive insulation material. The thickness is: 0.003" to 0.006" inches. It is the core technology of aluminum-based copper clad laminate and has obtained UL certification.

The BaseLayer layer is a metal substrate, generally aluminum or optionally copper.

Aluminum-based copper-clad laminates and traditional epoxy glass cloth laminates, etc., the mainstream on the market is Foslite aluminum substrate. The circuit layer (i.e. copper foil) is usually etched to form a printed circuit to connect the various components of the assembly. In general, the circuit layer requires a large current-carrying capacity, so a thicker copper foil should be used, with a thickness of generally 35μm~280μm; the thermal conductive insulation layer is the core technology of the aluminum substrate, which is generally composed of a special polymer filled with special ceramics, with low thermal resistance, excellent viscoelastic properties, resistance to thermal aging, and the ability to withstand mechanical and thermal stress. The company's high-performance aluminum substrate thermal conductive insulation layer uses this technology, which makes it have extremely excellent thermal conductivity and high-strength electrical insulation performance; the metal base is the supporting component of the aluminum substrate, which requires high thermal conductivity, generally aluminum plate, and copper plate (copper plate can provide better thermal conductivity), which is suitable for conventional mechanical processing such as drilling, punching and cutting.

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 by bridging. 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.

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

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

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

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

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

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

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

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

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