Ways to Prevent Voltage Fluctuations Caused by Power Lines

Control accuracy depends on many parameters. One is the DC voltage accuracy when the load requires continuous constant current. Another is the AC accuracy of the generated voltage, which depends on how the generated voltage changes with load transients. Factors that affect DC voltage accuracy include the required reference voltage (perhaps a resistive divider), the behavior of the error amplifier, and some other influence of the power supply. Key factors that affect AC voltage accuracy include the selected power level, backup capacitance, and control loop architecture and design.

However, in addition to all these factors that affect the accuracy of the generated supply voltage, there are other effects that must be considered. If the power source is physically 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 header contacts or a longer trace on a circuit board.

Figure 1. Physical distance between voltage regulator and associated load.

Figure 1 shows the resistance between the power supply and the load. The voltage loss across this resistor can be compensated by slightly increasing the voltage generated by the power supply. Unfortunately, the voltage drop developed across the line resistance depends on the load current, which is the current flowing through the line. High current results in a higher voltage drop compared to low current. The load is therefore powered by a regulated voltage with rather low accuracy, which depends on the line resistance and the corresponding current.

There has long been a solution to this problem. Can be connected in parallel with the actual connection to add an extra pair of connections. Use a Kelvin detection line to measure the voltage on the electronic load side. In Figure 1, these additional lines are shown in red. These measurements are then integrated into the supply voltage control on the supply side. This method is effective, but has the disadvantage of requiring additional detection leads. Since there is no need to carry high currents, the diameter of these leads is usually very small. However, locating the measurement wires in the connecting cable for higher currents brings additional work and higher costs.

Voltage drops in the connection wires between the source and load can also be compensated for without the need for an additional pair of sense leads. This is particularly relevant in applications where cabling is complex and costly and the resulting EMC interference can easily couple into the voltage test leads. The second option is to use a dedicated line voltage drop compensation IC such as the LT6110. Plug this IC into the voltage generating side and measure the current before entering the connecting wire. The power supply's output voltage is then adjusted based on the measured current, allowing the load-side voltage to be adjusted very precisely regardless of the load current.

Figure 2. Use the LT6110 to regulate the power supply output voltage to compensate for voltage drops on the connecting lines.

Using components such as the LT6110, it is possible to adjust the supply voltage based on the corresponding load current; however, this adjustment requires knowledge of the line resistance. Most apps provide this information. If the connection wires are changed to longer or shorter ones during the lifetime of the device, the voltage compensation implemented with the LT6110 must also be adjusted accordingly.

If the line resistance is likely to change during device operation, components such as the LT4180 can be used to provide a high-precision voltage to the load side by virtually predicting the connection line resistance from the AC signal when the load side has input capacitance.

Figure 3. Virtual remote measurement of lines using the LT4180.

Figure 3 shows an application using the LT4180 where the resistance of the transmission line is unknown. Line input voltage is regulated based on the corresponding line resistance. With the LT4180, there is no need for a Kelvin sensing line; voltage regulation can be achieved by simply gradually changing the line current and measuring the corresponding voltage changes. Use the measurement results to determine the voltage loss in the unknown line. Optimal regulation of the DC/DC converter output voltage is achieved based on voltage loss information.

This method of measurement works well as long as the node on the load side has low AC impedance. This is effective in many applications where the load behind 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 adjusted and the line resistance determined by measuring the voltage on the front side of the connecting line.