How to prevent voltage fluctuations caused by power lines? ADI teaches you a solution to easily solve it
When using a buck regulator or linear regulator power supply, the voltage is generally regulated to a set value to power the load. In some applications (for example, laboratory power supplies or electronic systems that require long cables to connect various components), it is not possible to ensure that the accurate regulated voltage is always provided at the required location due to various voltage drops on the interconnection lines.
The control accuracy depends on many parameters, one is the accuracy of the DC voltage when the load requires a continuous constant current , and the other is the accuracy of the AC voltage generated , which depends on how the generated voltage changes with load transients. Factors affecting the DC voltage accuracy include the required reference voltage (which may be a resistor divider), the behavior of the error amplifier, and some other factors affecting the power supply. Key factors affecting the AC voltage accuracy include the selected power level, backup capacitors, and the architecture and design of the control loop.
However, in addition to all of these factors that affect the accuracy of the generated supply voltage, there are other effects that must be considered. If the power supply is spatially separated from the load it needs to power, there will be a voltage drop between the regulated voltage and where the power is needed. This voltage drop depends on the resistance between the regulator and the load. It could be a cable with plug contacts or a long trace on a circuit board.
Figure 1 shows that there is a resistance between the power supply and the load. The voltage loss across this resistance can be compensated by slightly increasing the voltage generated by the power supply. Unfortunately, the voltage drop across the line resistance depends on the load current, i.e. the current flowing through the line. A high current results in a higher voltage drop than a low current. As a result, the load is powered by a rather inaccurately regulated voltage that depends on the line resistance and the corresponding current.
Figure 1. Physical distance between a regulator and associated load.
A solution to this problem has long been to add an extra pair of connections in parallel to the actual wiring and measure the voltage on the electronic load side using Kelvin sense leads. In Figure 1, these extra lines are shown in red. These measurements are then integrated into the supply voltage control on the power supply side. This works well, but has the disadvantage of requiring additional sense leads. Since they do not have to carry high currents, the diameter of these leads is usually very small. However, having measuring leads in the connection cable for higher currents results in additional effort and higher costs.
The voltage drop on the connection line between the power supply and the load can also be compensated without an additional pair of sense leads. This is particularly meaningful for applications where the cabling is complex and expensive and the generated EMC interference can easily couple to the voltage test leads. The second solution is to use a dedicated line voltage drop compensation IC such as the LT6110. This IC is inserted on the voltage generation side and measures the current before entering the connection line. The output voltage of the power supply is then adjusted according to the measured current, making it possible to adjust the load side voltage very accurately regardless of the load current.
Figure 2. Using the LT6110 to regulate the power supply output voltage to compensate for voltage drops on connecting wires.
With components such as the LT6110, the supply voltage can be adjusted to the corresponding load current; however, this adjustment requires information about the line resistance. This information is available in most applications. If the connection wires are replaced with longer or shorter ones during the life of the device, the voltage compensation implemented with the LT6110 must also be adjusted accordingly.
If the line resistance may change during device operation, a component such as the LT4180 can be used to virtually predict the connection line resistance through an AC signal when there is input capacitance on the load side, thereby providing a high-precision voltage at the load end.
Figure 3 shows an application using the LT4180 where the resistance of the transmission line is unknown. The line input voltage is regulated based on the corresponding line resistance. Using the LT4180, voltage regulation is achieved by simply changing the line current in steps and measuring the corresponding voltage change without the need for a Kelvin sense line. The measurement results are used to determine the voltage loss in the unknown line. The voltage loss information enables optimal regulation of the DC/DC converter output voltage.
Figure 3. Virtual remote measurement of a line using the LT4180.
This measurement works well as long as the nodes on the load side have low AC impedance. This is useful in many applications, because the load after a long connection line requires a certain amount of energy storage. Due to the low impedance, the output current of the DC/DC converter can be regulated and the line resistance can be determined by measuring the voltage on the front side of the connection line. Whether a stable supply voltage can be obtained depends not only on the voltage converter itself, but also on the supply line to the load.
The required DC accuracy can be improved by adding additional Kelvin sense lines. Alternatively, integrated circuits can be used to compensate for voltage drops on the lines without the need for Kelvin sense lines. This is useful if the cost of Kelvin sense lines is too high or if existing lines must be used and additional sense lines are not available. Using these design techniques, higher voltage accuracy can be easily achieved.
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