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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, the current is getting larger and larger, and the current is hundreds of amperes, so now everyone is paying more and more attention to power integrity. This article China PCBA manufacturer – SysPCB will focus on some problems of power supply voltage drop.

Theoretically, calculating the voltage drop should be junior high school physics knowledge. The power supply voltage drop <v*tolerance=i*r (DC resistance) can be said to be very simple. The method of reducing the voltage drop is also well known, as long as the DC impedance on the power path and the return path is reduced.

Many confident engineers might think, isn’t it the current-carrying capacity? I have calculated the number of vias and the width of the copper sheet according to the empirical formula. The power supply must be proper and there is no problem.

In fact, the voltage drop only depends on the voltage of the power terminal? No, the power voltage drop is a system that affects the whole body. Modifying any parameter in the system will affect the final result. To understand this system, one must know the direction of power flow.

As shown in the figure above, it is a power plane, the marked route is the part with the highest current density, and the green part is the closest route from the power supply to the return ground plane. It can be seen from the above that the closer the path, the more current will pass. The current is like human, also prefer to take shortcuts. We all want to choose a path with a smaller resistance to pass through to save power to the power consumption end.

This feature will cause the current density in some areas to be too large and the current through some vias to be relatively large. So instead of adding the corresponding number of vias according to the empirical formula, the current will pass through the vias in an evenly distributed manner. This causes some vias to pass the current exceeding the capacity range. After the board is used for a period of time, the middle of the via will break, which affects the life of the via and also affects the life of the board. Therefore, for some high-current power supplies, the neat addition of vias may affect the size of the via current. At this time, there are tricks to add vias, and the current of the vias close to the output of the power supply 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. The current density will be larger on the closest path between the power supply output terminal and the power consumption terminal. If the closest path happens to be the bottleneck area, the power supply path needs to be modified.

Another factor also affects the voltage drop of the power supply, that is, temperature and wind speed. Temperature mainly affects the resistivity of the conductor. As the temperature rises, the resistivity will increase, and the DC resistance in the conductor will also increase. Therefore, in the case of high power consumption, heat dissipation needs to be considered in power supply design.

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