Control accuracy depends on many parameters, one is the dc voltage accuracy when the load requires a continuous constant current, and the other is the ac accuracy of the generated voltage, which depends on how the generated voltage varies with load transients. Factors that affect dc voltage accuracy include the required reference voltage (possibly a resistor divider), the behavior of the error amplifier, and some other influence of the power supply. Key factors affecting AC voltage accuracy include the selected power class, backup capacitance, and control loop architecture and design.
However, in addition to all of these factors that affect the accuracy of the generated supply voltage, other influences must also be considered. If the power source 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 header contacts or a long trace on a circuit board.
Figure 1 shows that there is a resistance between the source and the load. The voltage loss across this resistor can be compensated by slightly increasing the voltage generated by the power supply. Unfortunately, the resulting voltage drop across the line resistance depends on the load current, which is the current flowing through the line. High currents result in higher voltage drops than low currents. Thus, the load is powered by a rather inaccurate regulated voltage that depends on the line resistance and the corresponding current.
Figure 1. Physical distance between the regulator and the associated load.
There has been a solution to this problem for a long time. It can be connected in parallel with the actual wiring, add an additional pair of connections, and use Kelvin detection lines to measure the voltage on the electronic load side. In Figure 1, these extra lines are shown in red. These measured values are then integrated into the supply voltage control on the supply side. This works well, but has the disadvantage of requiring an additional sense lead. These leads are usually very small in diameter since they do not need to carry high currents. However, setting the measuring wires in the connecting cable for higher currents involves additional work and higher costs.
Compensation for voltage drops on the connection line between the source and the load is also possible without an additional pair of sense leads. This is especially relevant for some applications where cabling is complex and costly and the resulting EMC interference can easily couple to the voltage test leads. The second option is to use a dedicated line drop compensation IC such as the LT6110. Insert this IC into the voltage generation side, and measure the current before entering the connecting wire. The output voltage of the power supply is then regulated based on the measured current, enabling very precise regulation of the load-side voltage regardless of the load current.
Figure 2. Using the LT6110 to regulate the output voltage of a power supply to compensate for voltage drops on connecting lines.
With components such as the LT6110, the supply voltage can be adjusted for 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 may vary during device operation, a component such as the LT4180 can be used to provide a highly accurate voltage at the load by virtually predicting the resistance of the connection line from an AC signal when there is input capacitance on the load side.
Figure 3 shows an application using the LT4180 where the resistance of the transmission line is unknown. The line input voltage is adjusted according to the corresponding line resistance. With the LT4180, voltage regulation is achieved by simply changing the line current step by step and measuring the corresponding voltage change without Kelvin sensing the line. Use the measurements to determine voltage losses in unknown lines. Optimum regulation of the output voltage of the DC/DC converter is achieved based on the voltage loss information.
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. Effective in many applications where the load behind the long connecting 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 connecting line. Whether a stable power supply voltage can be obtained is not only related to the voltage converter itself, but also related to the power supply line of the load.
The required DC accuracy can be increased by configuring additional Kelvin sense lines. Alternatively, integrated circuits can be used to compensate for voltage drops on the lines, eliminating the need for Kelvin sense lines. This can be useful if the cost of Kelvin sense wires is too high, or if existing wires must be used without additional sense wires. Higher voltage accuracy can be easily achieved using these design tips.
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