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Integrity of power in PCB circuit

January 13, 2020

PCB Image:Integrity of power in PCB circuit

Power integrity and signal integrity are closely related to each other. Under many circumstances, the main cause for distortion of signal is from power system. For example, rebounding noise from the ground is too large; the design of decoupling capacitance is not appropriate; returning impact is serious; division of multi-power or multi-surface is not good; unreasonable design of the floor; uneven current distribution, etc..

1)、Power distribution system

Power integrity is very complicated and the key lies in how to control the resistance between the power systems (between the power and the surface). In theory, the lower the resistance between the power systems is; the better it is. That’s because when the resistance becomes smaller, nose will also less intense and voltage loss will also be less. In practice, we can get the desired resistance by regulating the highest voltage and the range of power supply, to achieve the desired resistance in the power system by adjusting the relevant factors in the circuits.

2)、Rebound from the ground

When the peripheral rates of high-speed components are less than 0.5n, data exchanges from the bus with large volume data are very fast. Instability in the power will occur when electric waves are generated in the power layer to affect the signal. When current changes occur in the return circuits coupled with shortening of the rising edge, current changing rates will increase and voltage rebounding will be enhanced as voltage will be generated in the returning inductance. At this time, the ideal electrical level at the ground is not zero, nor is the power the ideal direct current potential. When gate circuits of the on-off switch increase, rebounding from the ground will be more serious. For the bus with 128 bits, there may be 50 to 100 pieces of I/O wire which can be shifted at the same clock edge. At this time, both power which returns and is shifted to I/O drive and the inductance in the return circuits should be as low as possible, or it will be static when connected to the same ground and electric brush may occur. Rebounding on the ground can occur anywhere, such as, in chips, enclosing, connectors, circuit board, etc., which may affect power integrity.

3)、Decoupling capacitance

As we all know, noise from the system can be reduced by adding capacitance between the power and the ground. But how much capacitance should be added, after all? What’s the value of every capacitance to be added? How to place the capacitance properly? Actually, we haven’t thought too much about questions like this but do it only by experience. Sometimes, some people may think the less the better. In designing high speed, we must consider the stray parameter of the capacitance to calculate the number of decoupling capacitance and the value as well as their position, to make sure that the resistance of the system is under control. One basic rule is that the decoupling capacitance should neither be too much nor too less.