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Power tip: Compensating for cable drop

Posted: 21 Oct 2013 ?? ?Print Version ?Bookmark and Share

Keywords:power supply? remote sensing? USB charger? differential amplifier? transconductance?

In this circuit, the output current is 2.5 amps, and the resulting current-sense resistor voltage is 125 mV. The offset voltage of the differential amp is 3 mV, which is amplified to 10 mV for about a 2 per cent set-on error. You can get better set-on error with a better and more expensive amplifier. This circuit provides about 625 mV of cable drop compensation with 125 mV current sense voltage and a gain of five in the amplifier.

Figure 3 shows the measured performance of this circuit.

There are three curves: the uncorrected Vo with 0.25 Ohms of resistance of the cable, the power supply output with cable drop compensation, and the load voltage with cable drop compensation. The uncomp Vo curve shows that, without cable drop compensation, the output voltage would fall outside a 5 per cent window. The comp PS Vo curve shows the output of the power supply with cable compensation ranges about 600 mV over a 2.5-amp load current range.

Figure 3: Increasing output voltage compensates cable drop.

The no-load voltage regulation point is 4.92V compared to a desired 5.00V, for an error of about 1.6 per cent. This is less than the potential worst-case error. The significant error terms are reference accuracy (0.7 per cent), divider resistors R4 and R7 (1.6 per cent with 1 per cent resistors), and U4A offset voltage (0.3 per cent). The comp cable Vo curve shows the voltage at the end of the cable connected to the load with cable compensation. The goal of 5 per cent accuracy is easily achieved with this approach.

To summarise, it is possible to put a positive load line on a power supply with a handful of inexpensive parts. In many cases, the added complexity of this circuit is small compared to remote sensing. Furthermore, it is safer due to concerns over fault conditions with the remote sense connections. The impact on the loop compensation is minor as long as the current sense connection is on the downstream side of the output capacitor.

About the author
Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.

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