Q1: how to select appropriate components and parameters for switching power supply circuit?
Answer: many engineers who have not used the design of switching power supply will be afraid of it, such as the interference of switching power supply, PCB layout, parameter and type selection of components. In fact, it is very convenient to use the switching power supply design as long as you understand it.
A switching power supply generally includes switching power supply controller and output. Some controllers will integrate MOSFET into the chip, which makes it easier to use and simplifies PCB design, but reduces the flexibility of design.
Switch controller is basically a closed-loop feedback control system, so there is usually a sampling circuit for feedback output voltage and a control circuit for feedback loop. Therefore, the design of this part is to ensure the accurate sampling circuit and control the feedback depth, because if the response of the feedback loop is too slow, it will have a great impact on the transient response capability.
The design of the output part includes output capacitance, output inductance, MOSFET, etc. the selection of these devices is basically to meet the balance between performance and cost. For example, a small inductance value can be used for a high switching frequency (meaning small packaging and cheap cost), but a high switching frequency will increase interference and switching loss to MOSFET, thus reducing efficiency. The result of low switching frequency is the opposite.
ESR for output capacitance and RDS for MOSFET_ On parameter selection is also very critical. A small ESR can reduce the output ripple, but the cost of capacitance will increase. A good capacitance will be expensive. It should also be noted that too many MOSFETs cannot be well driven.
Generally speaking, the supplier of switching power supply controller will provide specific calculation formula and use scheme for engineers' reference.
Q2: how to debug the switching power supply circuit?
Answer: there are some experiences to share with you: (1) the output of the power circuit is connected to the board through a low resistance high-power resistor. In this way, the power circuit can be debugged first without welding the resistor to avoid the influence of later circuits. (2) Generally speaking, the switch 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 normally. Therefore, in this case, it is necessary to carefully check the feedback and sampling circuits. Especially if the output capacitor with large ESR value is used, a lot of power supply ripple will be generated, which will also affect the operation of switching power supply.
Q3: why grounding?
Answer: the introduction of grounding technology was originally a protective measure taken to prevent power or electronic equipment from being struck by lightning. The purpose is to introduce the lightning current generated by lightning to the earth through lightning rods, so as to protect buildings. At the same time, grounding is also an effective means to protect personal safety. When the phase line (such as poor wire insulation, line aging, etc.) caused by some reason touches the equipment shell, the equipment shell will have dangerous voltage, and the resulting fault current will flow through the PE line to the earth, thus playing a protective role. With the development of electronic communication and other digital fields, only considering lightning protection and safety in grounding system is far from meeting the requirements. For example, in the communication system, the interconnection of signals between a large number of devices requires that each device should have a reference 'ground' as the reference ground of the signal. Moreover, with the complexity of electronic equipment, the signal frequency is getting higher and higher. Therefore, in the grounding design, electromagnetic compatibility problems such as mutual interference between signals must be paid special attention. Otherwise, improper grounding will seriously affect the reliability and stability of system operation. Recently, the concept of "ground" has also been introduced into the signal return technology of high-speed signals.
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