An in-depth explanation of power supply voltage drop in PCB design

In PCB design, power supply is a topic that cannot be ignored, especially now that the power supply voltage of many products is getting lower and lower, and the current is getting higher and higher, often hundreds of amps, so now everyone is paying more and more attention to the integrity of the power supply. This article focuses on some issues with power supply voltage drop.
Many confident engineers will probably think, isn’t it just about the current carrying capacity? I have calculated the number of vias and copper width according to empirical formulas. The power supply must be adequate and there can be no problems. I have passed it and will Casually being a donkey’s age.

In fact, does the voltage drop only depend on the voltage at the consumer end? No, the power supply voltage drop is a system that affects the whole system. Modifying any parameter in the system will affect the final result. To understand this system, you need to know where the power flows.
As shown in the figure, there is a power plane above. The marked route is the part with the highest current density. The green part is the shortest path from the power supply to the return ground plane. As can be seen from the above, the closer the path, the more current passes through. The current is just like us, we also like to take shortcuts. We all want to choose a path with a smaller resistance to pass through, so as to save power to the power end.

This characteristic will cause the current density in some areas to be relatively high and the current passing through some vias to be relatively large. Therefore, instead of adding a corresponding number of vias according to the empirical formula, the current will pass through the vias in an evenly distributed manner. This causes the current passing through some vias to exceed the capacity range. After the board is used for a period of time, the middle of the vias may break, affecting the life of the vias and the life cycle of the board. Therefore, for some high-current power supplies, neatly adding vias may affect the size of the via current. There is a trick to adding vias at this time, and the current of the vias close to the power supply output will be larger. In this case, It is recommended to use simulation to guide the addition of via arrays.

The same is true for current density. On the nearest path between the power output end and the power consumption end, the current density will be relatively large. If the nearest path happens to be a bottleneck area, the power path needs to be modified.

There is another factor that also affects the voltage drop of the power supply, which is temperature and wind speed. Temperature mainly affects the resistivity of the conductor. As the temperature increases, the resistivity will also increase, and the DC resistance in the conductor will also increase. Therefore, in the case of high power consumption, the power supply design also needs to consider the issue of heat dissipation.

To summarize, when we design a power supply, in addition to meeting the current-carrying copper width and number of vias, we also need to pay attention to the current size of each via, the current density on the power path, the working environment of the board, temperature rise and other factors .

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