Technical Rules and Applications of PCB Layout for Switching Power Supplies
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 perfectly designed power supply on paper may not work properly during initial debugging due to many issues with the PCB layout of the power supply. For example, in the schematic diagram of a step-down switching power supply on a consumer electronic device, the designer should be able to distinguish between components in the power circuit and those in the control signal circuit on this circuit diagram. However, if the designer treats all components in this power supply as if they were components in the digital circuit, the problem can 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 connecting lines between chips can be automatically connected through PCB software. The switch power supply produced by automatic typesetting will definitely not work properly. Therefore, designers need to master and understand the correct PCB layout technical rules for switching power supplies.
Technical rules for PCB layout of switching power supply
The capacitance of the bypass ceramic capacitor should not be too large, and its 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 become like the inductance impedance and increase with the increase of frequency; When the operating frequency of a capacitor approaches 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. Due to 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 resonant frequency is higher than that of electrolytic capacitors and can be used in medium to high frequency filtering. The capacitance and equivalent series inductance of ceramic capacitors are generally very small, so their resonant frequency is much higher than that of electrolytic capacitors and tantalum capacitors, so they can be used in 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
When selecting bypass capacitors, it is not advisable to simply use ceramic capacitors with high capacitance values. In order to improve the high-frequency characteristics of capacitors, multiple capacitors with different characteristics can be connected in parallel for use. 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 the different wiring methods from input power (VIN) to load (RL) on a PCB. In order to reduce the ESL of the filter capacitor (C), the lead length of the capacitor pin should be minimized as much as possible, while the routing from VIN positive to RL and from VIN negative to RL should be as close as possible.
