Introduction to Switching Power Supply PCB Layout Technical Rules and Applications
Nowadays, due to the electromagnetic waves generated by switching power supplies, which affect the normal operation of their electronic products, the correct PCB layout technology for power supplies has become very important.
In many cases, a power supply designed perfectly on paper may not function properly during initial debugging due to various issues with its PCB layout. For example, for a step-down switching power supply schematic on a consumer electronic device, the designer should be able to distinguish between components in the power circuit and components in the control signal circuit on this circuit diagram. However, if the designer treats all components in this power supply as components in the digital circuit, the problem will be quite serious. The layout of switch power supply PCB is completely different from that of digital circuit PCB. In digital circuit layout, many digital chips can be automatically arranged through PCB software, and the connection lines between chips can be automatically connected through PCB software. The switch power supply produced by automatic typesetting will definitely not work properly. So, designers need to master and understand the correct technical rules for PCB layout of switch power supplies.
Technical rules for PCB layout of switch power supply
The capacitance of bypass ceramic capacitors should not be too large, and their parasitic series inductance should be minimized as much as possible. Parallel connection of multiple capacitors can improve the high-frequency impedance characteristics of capacitors
When the operating frequency of a capacitor is below fo, the capacitance impedance Zc decreases with the increase of frequency; When the operating frequency of the capacitor is above fo, the capacitance impedance Zc will increase like the inductance impedance with the increase of frequency; When the operating frequency of a capacitor approaches fo, the capacitance impedance is equal to its equivalent series resistance (RESR).
Electrolytic capacitors generally have a large capacitance and a large equivalent series inductance. Due to its low resonant frequency, it can only be used for low-frequency filtering. Tantalum capacitors generally have a large capacitance and a small equivalent series inductance, so their resonant frequency is higher than that of electrolytic capacitors and can be used in mid to high frequency filtering. Ceramic capacitors generally have small capacitance and equivalent series inductance, so their resonant frequency is much higher than that of electrolytic capacitors and tantalum capacitors, making them suitable for high-frequency filtering and bypass circuits. Due to the fact that the resonant frequency of small capacitance ceramic capacitors is higher than that of large capacitance ceramic capacitors, therefore
When selecting bypass capacitors, it is not advisable to only choose ceramic capacitors with excessively high capacitance values. In order to improve the high-frequency characteristics of capacitors, multiple capacitors with different characteristics can be used in parallel. Figure 1 (a) shows the improved impedance effect after multiple capacitors with different characteristics are connected in parallel. It is not difficult to understand the importance of this layout rule through analysis. Figure 1 (b) shows different wiring methods for inputting power (VIN) to load (RL) on a PCB. In order to reduce the ESL of the filtering capacitor (C), the lead length of the capacitor pin should be minimized as much as possible: and the wiring from VIN positive to RL and VIN negative to RL should be as close as possible.
