Electronic Design - Portable System Switching Power Supply PCB Design

1. The correct switching power supply PCB design technology is an important step in the development of portable devices

In the current switching power supply development, designers mostly choose AC/DC adapters that are easy to purchase in the PCB market, and install multiple sets of DC power supplies directly on the circuit board of the system. However, since the switching power supply will generate electromagnetic waves and affect the normal operation of its electronic products, the correct power supply PCB design technology becomes very important.

In many cases, a power supply that is perfectly designed on paper may not work properly during the initial commissioning. The reason is that there are many problems in the PCB design of the power supply. For example, for a schematic diagram of a step-down switching power supply on a consumer electronic device, the designer should be able to distinguish the components in the power circuit from the components in the control signal circuit on this circuit diagram. If the designer treats all the components in this power supply as components in a digital circuit, the problem will be quite serious. The switching power supply PCB design is completely different from the digital circuit PCB design. In digital circuit layout, many digital chips can be automatically arranged by PCB software and the connecting lines between the chips can be automatically connected by PCB software. The switching power supply typeset by automatic typesetting will definitely not work properly. Therefore, designers need to master and understand the correct switching power supply PCB design technical rules, and of course, they also need to have a clearer understanding of the technical status of the switching power supply at all levels.

Second, the technical rules of switching power supply PCB design

2.1 The capacitance of the bypass ceramic capacitor should not be too large, and its parasitic series inductance should be minimized. Multiple capacitors in parallel can improve the high-frequency impedance characteristics of the capacitor. why is it like this? This is because of the high frequency filtering characteristics of capacitors.

This formula shows: reducing the distance between the capacitor pins (d) and increasing the cross-sectional area (A) will increase the capacitance of the capacitor itself

This formula shows: reducing the distance between the capacitor pins (d) and increasing the cross-sectional area (A) will increase the capacitance of the capacitor itself.

Capacitors usually have two parasitic parameters: equivalent series resistance (ESR) and equivalent series inductance (ESL).

The resonant frequency (fo) of a capacitor can be obtained from its own capacitance (c) and equivalent series inductance (LESL):

The resonant frequency (fo) of a capacitor can be obtained from its own capacitance (c) and equivalent series inductance (LESL)

When the working frequency of a capacitor is below fo, the capacitance impedance Zc decreases with the increase of frequency; when the working frequency of the capacitor is above fo, the capacitance impedance Zc will become like an inductor impedance and increase with the increase of frequency; when the capacitor works When the frequency is close to fo, the impedance of the capacitor is equal to its equivalent series resistance (RESR).

Electrolytic capacitors generally have a large capacitance and a large equivalent series inductance. Because of its low resonant frequency, it can only be used for low-frequency filtering. Tantalum capacitors generally have larger capacitance and smaller equivalent series inductance, so their resonance frequency will be higher than that of electrolytic capacitors, and can be used in medium and high frequency filtering. The capacitance and equivalent series inductance of ceramic capacitors are generally very small, so its resonance frequency is much higher than that of electrolytic capacitors and tantalum capacitors, so it can be used in high-frequency filtering and bypass circuits. Since the resonant frequency of a small-capacity ceramic capacitor will be higher than that of a large-capacity ceramic capacitor, ceramic capacitors with too high capacitance cannot be selected when choosing a bypass capacitor. In order to improve the high frequency characteristics of the capacitor, multiple capacitors with different characteristics can be used in parallel.

When the operating frequency of an inductor is below fo, the impedance of the inductor increases with the increase of frequency; when the operating frequency of the inductor is above fo, the impedance of the inductor decreases with the increase of frequency; when the operating frequency of the inductor is close to fo, the inductor impedance is equal to Its equivalent parallel resistance (REPR).

In the application of switching power supply, the equivalent parallel capacitance (CP) of the inductor should be controlled as small as possible. At the same time, it must be noted that the inductance of the same inductance will produce different equivalent parallel capacitance values (CP) due to different coil structures.

It can be seen that the equivalent parallel capacitance values of the two inductors with the same inductance are actually ten times different. In high-frequency filtering, if the equivalent parallel capacitance of an inductor is too large, high-frequency noise will easily flow directly to the load through its parallel capacitance. Such an inductance also loses its high-frequency filtering function.

2.3 Avoid placing any power or signal traces on the ground.

The concept of mirror surface in electromagnetic theory will be of great help for designers to master the PCB design concept of switching power supply. Scenario when direct current flows over a ground plane. At this time, the return direct current on the ground is very evenly distributed across the ground. Scenario when high-frequency alternating current flows above the same formation. At this time, the return AC current on the ground plane can only flow in the middle of the ground plane and there is no current at all on both sides of the ground plane.

2.4 The area of the high-frequency AC loop should be minimized.

There are many high frequency AC loops composed of power devices in the switching power supply PCB. If these loops are not handled well, it will have a great impact on the normal operation of the power supply. In order to reduce the electromagnetic noise generated by the high-frequency AC loop, the area of the loop should be controlled very small. If the area Ac of the high-frequency alternating current loop is large, a large electromagnetic interference will be generated inside and outside the loop. If the same high-frequency AC current, when the loop area is designed to be very small, the internal and external electromagnetic fields of the loop will cancel each other out, and the entire circuit will become very quiet.