When designing the switching power supply PCB, what needs to be paid attention to is: the introduction point of the feedback line and who the freewheeling diode is for. As can be seen from Figure 3, when U1 is turned on, the current I2 enters the inductor L1. The characteristic of the inductor is that the current cannot suddenly occur or disappear when it flows through the inductor. There is a time process for the change of the current in the inductor. Under the action of the pulse current I2 flowing through the inductor, part of the electrical energy is converted into magnetic energy, and the current gradually increases. At a certain point, the control circuit U1 turns off I2. Due to the characteristics of the inductor, the current cannot disappear suddenly. At this time, the diode works. It takes over the current I2, so it is called a freewheeling diode. It can be seen that the freewheeling diode is used for the inductor. The freewheeling current I3 starts from the negative terminal of C3, flows through D1 and L1 and then flows into the positive terminal of C3. This is equivalent to a water pump, using the energy of the inductor to increase the voltage of capacitor C3. There is also the problem of the introduction point of the voltage detection feedback line. It should be fed back after filtering, otherwise the output voltage ripple will be larger. These two points are often overlooked by many of our PCB designers. They think that the same network is not the same where it is connected. In fact, the connection is in different places, and the performance impact is huge. Figure 4 is the PCB diagram of the LM2575 switching power supply. Let’s see what is wrong with the wrong picture.