Most engineers think that using a power supply is a pretty simple task. But as currents go up, keeping the correct voltage on the device under test (DUT) can become a challenge.
As current flows through the load leads (the wires between the power supply and the DUT), a voltage drop appears across the resistance in the load leads. This voltage drop means that the voltage at the DUT is lower than the voltage at the output terminals of the power supply. When the DUT demands low current, this voltage drop is probably insignificant. At higher currents, the voltage drop can mean there is insufficient voltage at the DUT for the DUT to operate properly (Fig. 1).
When your DUT is a Power-over-Ethernet device running on a 44-V nominal supply voltage, it can tolerate a few hundred millivolts of drop in each load lead and operate happily at 43 V. But when your DUT is an FPGA running at 1.1 V or a cell phone running on a 3.6-V battery, a few hundred millivolts of drop in each load lead means there is not enough voltage to properly operate. As nominal supply voltages drop, currents rise, causing the voltage drop through the load leads to be even higher.
What should you do when faced with too little voltage for your DUT to operate? One solution is to use thicker wire, which will have lower resistance and a smaller voltage drop as current flows through it. There are limits to this approach. If you have long load leads, you may not be able to keep the total resistance low enough to keep the voltage drop low enough. However, as the wire grows thicker, it can become difficult to manage, especially if you need to connect thick wires to small pins on a DUT.
Another approach would be to turn up the voltage on the supply. But when the current drops to a lower level, the voltage drop through the wires goes down, and then you have too much voltage at the DUT. As the currents go up and down, you might try to use a computer to create a regulation loop by setting the voltage on the power supply, measuring the voltage at the DUT, and adjusting the voltage up or down on the power supply to account for the actual voltage drop in the load leads at that moment.
But using the computer as part of the feedback loop complicates your test setup. More importantly, this feedback loop can be slow, as it could take up to 100 ms for each execution of the loop to measure and reset the power supply. The response time of this loop is most likely not fast enough to keep up with the changing current flowing through the DUT.
The Remote Sensing Solution
Thankfully, power-supply designers have included a feature to solve this problem. Known as remote sensing, it is found on higher-current power supplies where users are more likely run into voltage regulation problems due to voltage drops in load leads at higher currents. Using remote sensing, the power supply itself can perform this feedback loop and adjust its output to compensate for the voltage drop in the load leads, even as the current changes.
For remote sensing to work, the power supply has four output terminals. The “+” and “–” terminals are the standard power-supply output terminals to which the load leads are attached and through which high currents will flow into the DUT. Sometimes these are called the force terminals.
The other two terminals are the sense terminals, typically labeled “+S” and “–S.” The wires connected to them are called the sense leads (Fig. 2). The sense terminals are measurement terminals and have high input impedance so no current will flow into the sense terminals. Since no current flows into the sense terminals, no current will flow through the sense leads. If no current is flowing, then regardless of the resistance or the gauge of the sense wires, there will be no voltage drop in the sense leads. Thin wire, such as 18 or 22 gauge, can be used.
Voltage at the DUT will then appear at the sense terminals, allowing the power supply to sense the actual voltage at the DUT. The power supply will regulate the voltage at the output terminals to keep the voltage at the sense terminals exactly at the programmed value even as the voltage drop through the load leads changes during current changes.
There are some limitations and drawbacks to remote sensing:
• Your test setup is more complex, as two sets of leads (load leads and sense leads) need to be run between the power supply and the DUT.
• The power supply will have to be larger to account for the losses in the load leads. If there are large voltage drops in the load leads, due to high resistance wire or high currents, the power supply needs to be capable of generating higher voltages to compensate for the voltage drops in the load leads. For example, if you want 12 V at the DUT and there is 1 V of drop in each lead, then the power supply needs to go to 14 V.
• The load leads are a loop of wire that is an unwanted inductance in the load lead path. The same is true for the sense leads. To avoid the addition of unwanted inductances created by long loops of wire, the two load leads should be twisted together. Also, separate from the load leads, the two sense leads should be twisted together. When twisted together, there is no longer any loop area between the leads and therefore unwanted inductance is reduced.
As DUT currents go up, there will be voltage drops in the load leads, resulting in reduced voltage at the DUT that could cause the DUT to stop operating. Using thicker wires can only help to a degree. Remote sensing allows a power supply to adjust its output voltage to compensate for the voltage drop in the load leads and hold the voltage at the DUT to the programmed voltage. Although adding remote sense leads increases test setup complexity, remote sensing can give you more stable voltage regulation even when your DUT current changes from low to high current.