Summary of key points of switching power supply PCB layout, worth collecting!

Before the article begins, let’s talk about the 8 key points of switching power supply PCB layout:

1. The capacitance of the bypass ceramic capacitor should not be too large, and its parasitic series inductance should be as small as possible. Multiple capacitors connected in parallel can improve the impedance characteristics of the capacitor;

2. The parasitic parallel capacitance of the inductor should be as small as possible, and the farther the distance between the inductor pin pads, the better;

3. Avoid placing any power or signal traces on the ground layer;

4. The area of ​​the high-frequency loop should be reduced as much as possible;

5. The placement of via holes should not destroy the path of high-frequency current on the ground layer;

6. Different circuits on the system board require different ground layers, and the ground layers of different circuits are connected to the power ground layer through a single point;

7. The drive circuit loop from the control chip to the upper and lower field effect transistors should be as short as possible;

8. The switching power supply power circuit and control signal circuit components need to be connected to different ground layers. These two ground layers are generally connected through a single point.

This is a schematic diagram of a step-down switching power supply used in consumer electronics. The designer should be able to distinguish components in the power circuit from components in the control signal circuit on this circuit diagram. If the designer treats all components in the power supply as components in a digital circuit, the problem will be quite serious. Usually, you first need to know the path of high-frequency current in the power supply, and distinguish between small-signal control circuits and power circuit components and their wiring.

Generally speaking, the power circuit of the power supply mainly includes input filter capacitors, output filter capacitors, filter inductors, and upper and lower power field effect transistors. The control circuit mainly includes PWM control chip, bypass capacitor, bootstrap circuit, feedback voltage dividing resistor, and feedback compensation circuit.

[Power supply control circuit (thin wire), power circuit (thick wire)]

Power supply power circuit PCB layout

The correct placement and routing of power components on the PCB will determine whether the entire power supply is working properly. Designers must first have a certain understanding of the voltage and current waveforms on switching power supply power devices.

The following shows the current and voltage waveforms on a step-down switching power supply power circuit component.

Since the current flowing through the input filter capacitor (Cin), the upper field effect transistor (S1) and the lower field effect transistor (S2) is an alternating current with high frequency and high peak value, it is represented by Cin-S1-S2. The area of ​​the loop formed should be minimized. At the same time, the loop area composed of S2, L and the output filter capacitor (Cout) should also be minimized.