How to design a suitable system power supply (Part 1)

        For an electronic system now, the design of the power supply part is becoming more and more important. I would like to discuss some of my own experience in power supply design with you to start a discussion so that we can all have a deeper understanding 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. With high efficiency, the total power consumption is 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 only about the efficiency of the power supply circuit at full load, but also about the efficiency level at light load. 


        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. 

        The heat dissipation problem 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 is flexible in design and high in efficiency, but the ripple is large, the implementation is more complicated, and the debugging is more cumbersome, etc. 

        Q3: How to choose the right 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 of the switching power supply, PCB layout problems, parameters and type selection of components, etc. 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 two parts: a switching power supply controller and an output. 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 switching 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 an accurate sampling circuit and to control the feedback depth, because if the feedback loop responds too slowly, it will have a lot of impact on the transient response capability. 

        The output design includes output capacitors, output inductors, and 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 paid attention to. Too many MOSFETs cannot be driven well.

        Generally speaking, suppliers of switching power supply controllers will provide specific calculation formulas and usage plans for engineers to refer to.