How to design a suitable power supply

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How to design a suitable power supply [Copy link]

Nowadays, for an electronic system, the design of the power supply part is becoming more and more important. By discussing the experience of power supply design, we aim to make in-depth and progress in power supply design. Q1: How to evaluate the power supply requirements of a system Answer: For an actual electronic system, its power supply requirements should be carefully analyzed. Not only should we care about the input voltage, output voltage and current, but we should also carefully consider the total power consumption, the efficiency achieved by the power supply, the transient response capability of the power supply part to load changes, the tolerance range of key components to power supply fluctuations and the corresponding allowable power supply ripple, as well as heat dissipation issues, etc. Power consumption and efficiency are closely related. If the efficiency is high, the total power consumption will be less when the load power consumption is the same, which is very beneficial to the power budget of the entire system. Compared with LDO and switching power supply, the efficiency of switching power supply is higher. At the same time, evaluating efficiency is not just about looking at the efficiency of the power supply circuit when it is fully loaded, but also about the efficiency level when it is lightly loaded. As for the load transient response capability, there will be strict requirements for some high-performance CPU applications, because when the CPU suddenly starts to run heavy tasks, the required startup current is very large. If the power supply circuit does not respond quickly enough, the instantaneous voltage drops too much or too low, causing the CPU to run incorrectly. Generally speaking, the actual value of the required power supply is mostly ±5% of the nominal value, so the allowable power supply ripple can be calculated based on this, and of course a margin must be reserved. Heat dissipation is more important for those high-current power supplies and LDOs, and calculations can also be used to evaluate whether they are suitable. Q2: How to choose a suitable power supply implementation circuit Answer: According to the specific technical indicators obtained by analyzing the system requirements, you can choose a suitable power supply implementation circuit. Generally, for the weak current part, it includes LDO (linear power converter), switching power supply capacitor buck converter and switching power supply inductor capacitor converter. In comparison, LDO design is the easiest to implement, with small output ripple, but the disadvantages are that the efficiency may not be high, the heat generation is large, and the current that can be provided is not large compared to the switching power supply, etc. The switching power supply circuit design is flexible and efficient, but the ripple is large, the implementation is relatively complex, and the debugging is relatively cumbersome, etc. Q3: How to choose suitable components and parameters for the switching power supply circuit Answer: Many engineers who have not used switching power supply design will have a certain fear of it, such as worrying about the interference problem of the switching power supply, PCB layout problems, and the parameters and type selection of components. In fact, as long as you understand it, it is very convenient to use a switching power supply design. A switching power supply generally includes a switching power supply controller and an output part. Some controllers will integrate MOSFET into the chip, which makes it easier to use and simplifies the PCB design, but the design flexibility is reduced. The switch controller is basically a closed-loop feedback control system, so there is generally a sampling circuit for feedback output voltage and a control circuit for the feedback loop. Therefore, the design of this part is to ensure accurate sampling circuits and control the feedback depth, because if the feedback loop responds too slowly, it will have a lot of impact on transient response capabilities. The output part design includes output capacitors, output inductors, MOSFETs, etc. The selection of these is basically to meet a balance between performance and cost. For example, a high switching frequency can use a small inductor value (which means a small package and cheap cost), but a high switching frequency will increase interference and switching losses on the MOSFET, thereby reducing efficiency. The result of using a low switching frequency is the opposite. The selection of the ESR of the output capacitor and the Rds_on parameter of the MOSFET is also very critical. A small ESR can reduce the output ripple, but the capacitor cost will increase, and a good capacitor will be expensive. The driving capability of the switching power supply controller should also be noted. Too many MOSFETs cannot be driven well. Generally speaking, the supplier of the switching power supply controller will provide specific calculation formulas and usage solutions for engineers to refer to. Q4: How to debug the switching power supply circuit Answer: There are some experiences that can be shared with everyone 1: The output of the power supply circuit is connected to the board through a low-resistance high-power resistor. In this way, the power supply circuit can be debugged first without soldering resistors to avoid the influence of the subsequent circuits. 2: Generally speaking, the switching controller is a closed-loop system. If the output deterioration exceeds the range that the closed loop can control, the switching power supply will not work properly. So in this case, it is necessary to carefully check the feedback and sampling circuits. Especially if a large ESR output capacitor is used, a lot of power ripple will be generated, which will also affect the operation of the switching power supply.