Switching power supply design

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In any switching power supply design, the physical design of the PCB board is the last link. If the design method is not appropriate, the PCB may radiate too much electromagnetic interference, causing unstable power supply operation. As a designer, you must understand the physical working principle of the circuit and design a high-quality PCB . The switching power supply contains high-frequency signals . Any trace on the PCB can act as an antenna. The length and width of the trace will affect its impedance and inductance, thereby affecting the frequency response. Even traces that pass DC signals will couple to RF signals from adjacent traces and cause circuit problems ( even radiate interference signals again ) . Therefore, all traces that pass AC current should be designed to be as short and wide as possible, which means that all components connected to the traces and other power lines must be placed very close. The length of the trace is proportional to the inductance and impedance it exhibits, while the width is inversely proportional to the inductance and impedance of the trace. The length reflects the wavelength of the trace response. The longer the length, the lower the frequency of electromagnetic waves that the trace can send and receive, and it can radiate more RF energy. Choosing the right MOSFET for the power switch or synchronous rectification function can also help reduce EMI. When the MOSFET device is powered off, low C ▼ oss ▼ ( like FDS

6690A) can reduce the interference of spike pulses. The main current loops of the three main switching power supply structures, pay attention to their differences. Each switching power supply has four current loops, which are relatively independent of each other. In a well-layout PCB , the order of importance is as follows: 　　Power switch AC loop 　　Output rectifier 　　AC loop Input signal source current loop 　　Output load current loop 　　The input signal source and output load current loops usually do not have problems. The current waveforms in these loops are the superposition of large DC current and small AC current. Special filters are usually required in these two loops to prevent AC noise from leaking into the surrounding environment. The input and output current loops should be connected to the power supply only from the terminals of the filter capacitor. The input loop charges the input capacitor through a current that is approximately DC, but cannot provide the high-frequency current pulses required by the switching power supply. The filter capacitor mainly plays a broadband energy storage role; similarly, the output filter capacitor is also used to store high-frequency energy from the output rectifier and eliminate the DC energy of the output load loop. Therefore, the terminals of the input and output filter capacitors are very important. If the connection between the input / output loop and the power switch / rectifier loop cannot be directly connected to the terminals of the capacitor, the AC energy will " flow through " the input or output filter capacitor and radiate to the environment. The current waveforms of the two basic PWM operating modes produce harmonic current waveforms that are much higher than the switching frequency. The AC loops of the power switch and rectifier contain high-amplitude trapezoidal current waveforms. The harmonic components in these waveforms are very high, and their frequencies are much higher than the switching fundamental frequency. The peak amplitude of these AC currents can be as high as 5 times the continuous input / output DC current amplitude , and the transition time is usually about 50ns . These two loops are most likely to generate electromagnetic interference. Designers must lay out these AC loops before other printed wiring in the power supply. The three main components of each loop ( filter capacitor, power switch or rectifier, inductor or transformer ) should be placed adjacent to each other, and the components should be positioned so that the current path between them is as short as possible to ensure the shortening of the current path length. The printed wiring in these loops also has the greatest impact on the converter's measured efficiency. Selecting a package such as DPAK or SO-8 allows for simultaneous heat dissipation and signal transmission. Products from Fairchild and other suppliers combine the functions of heat dissipation and signal transmission. Grounding is important. Grounding is the bottom branch of the current loop discussed earlier, but it plays an important role as a common reference point for the circuit. Therefore, the placement of ground wires should be carefully considered in the layout. Mixing various grounds will cause unstable power supply operation. There are three main grounding schemes for switching power supply structures. When designing, make sure that an additional " control ground " has been considered . This is the ground point connected to the control IC and all related passive components. It is extremely sensitive, so it should only be placed after the other AC loops are laid out. The point where the control ground is connected to other grounds is very special. Usually, the connection point is located at the common end of all components of the control IC that sense small voltages. These connection points include the common end of the current sensing resistor in the current mode switching converter and the bottom end of the output resistor divider. Its function is to establish a low-noise Kelvin connection between the sensitive component and the input sensitive to voltage errors or current amplification. If the control ground is connected to any other point, the noise generated in those additional loops will be superimposed on the control signal, which will affect the operation of the control integrated circuit. Designers should ensure that each high-current ground terminal uses a printed line as short and wide as possible. Usually, the common terminal of the filter capacitor should be the only connection point for other ground points to couple to the high-current AC ground. High-voltage AC node There is a node in each switching power supply that has the highest AC voltage compared to other nodes. This node is the AC node that appears at the drain ( or collector ) of the power switch tube. In non-isolated DC/DC converters, this node can also be connected to the inductor and connected to ( or output to ) the rectifier; in the structure of the isolation transformer, this node is separated from the coil of the transformer. It still behaves as a common node in electrical performance, but it is only reflected through the transformer, and each one should be designed separately. This node will have different problems. Its AC voltage can be coupled to the printed lines of different metal layers nearby through capacitors and radiate electromagnetic interference. However, the printed lines usually have to dissipate heat for the power switch tube and rectifier, especially for surface-mounted power supplies. From an electrical point of view, the printed lines should be as small as possible, but from a heat dissipation point of view, they should be larger. In surface mount designs, there is a good compromise method to make the top PCB board the same as the bottom PCB board and connect them together through many holes ( or vias ) . A good way to enhance the heat dissipation capacity of the PCB board and reduce the capacitive coupling of other traces. This technology greatly reduces the capacitive coupling to other traces, but it doubles the heat dissipation and surface area. With an SO8 packaged N- channel power MOSFET ( such as FDS























6670A) as an example, in the upper layer there is only325 mm▲ 2 ▲ The copper-clad area has a thermal resistance of 50 (C/W in contact with the air . When another identical board is added to the bottom layer of the PCB and connected together through 8 vias, the thermal resistance drops to 39 (C/W . Because there is no metal wire carrying different signals on the other side of the board, the capacitance will drop by more than an order of magnitude. In via applications, other signals and grounding must be kept away from AC printed wires with high voltages and parts used for heat dissipation. In offline converters, the ground wire may couple energy from this node and lead it out of the product through the AC plug, which generates excessive conducted electromagnetic interference. Parallel filter capacitors are often used in parallel to reduce the parallel equivalent series resistance (ESR) of the filter capacitors. This approach also allows each capacitor to shunt a portion of the ripple current so that each capacitor can work normally within the specification of its ripple current. Only when the trace impedance between the capacitors and each ripple current source is the same, the ripple current will be " averaged and shunted " , which requires that the traces between the capacitors between the rectifier or power switch must be of equal length and width. The correct placement of parallel capacitors is one of the keys to the design of switching power supplies. Placing capacitors in columns and connecting them in sequence is very beautiful, but this layout will cause the capacitor closest to the power switch or rectifier to bear more ripple current than other capacitors, thereby shortening the service life of the capacitor.

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What parameters should be paid attention to when choosing a switching power supply? How is it different from a linear power supply?

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Note: The input signal source and output load current loops usually do not have problems. The current waveforms in these loops are the superposition of large DC current and small AC current. Special filters are usually required in these two loops to prevent AC noise from leaking into the surrounding environment. The input and output current loops should be connected to the power supply only from the terminals of the filter capacitor. The input loop charges the input capacitor through a current that is approximately DC, but cannot provide the high-frequency current pulses required by the switching power supply. The filter capacitor mainly plays a broadband energy storage role; similarly, the output filter capacitor is also used to store high-frequency energy from the output rectifier and eliminate the DC energy of the output load loop. Therefore, the terminals of the input and output filter capacitors are very important. If the connection between the input/output loop and the power switch/rectifier loop cannot be directly connected to the capacitor terminals, the AC energy will "flow through" the input or output filter capacitor and radiate into the environment. Grounding is very important. Grounding is the bottom branch of the current loop discussed above, but it plays an important role as a common reference point for the circuit. Therefore, the placement of the ground wire should be carefully considered in the layout. Mixing various grounds will cause unstable power supply operation. Capacitors are often used in parallel to reduce the parallel equivalent series resistance (ESR) of the filter capacitor. This practice also allows each capacitor to shunt a portion of the ripple current so that each capacitor can work normally within the specification of its ripple current. The ripple current will only be "averaged" when the trace impedance between the capacitors and each ripple current source is the same, which requires that the traces between the capacitors between the rectifier or power switch must be of equal length and width. The correct placement of parallel capacitors is one of the keys to the design of switching power supplies. Placing capacitors in columns and connecting them in sequence is very beautiful, but this layout will cause the capacitor closest to the power switch or rectifier to bear more ripple current than other capacitors, thereby shortening the service life of the capacitor. Difference: The switching power supply contains high-frequency signals. Choosing the right MOSFET for the design of the power switch or synchronous rectification function can also help reduce electromagnetic interference. When the MOSFET device is powered off, low C▼oss▼ (like FDS6690A) can reduce the interference of spike pulses.