Back to Basics: Using Remote Sensing to Ensure Accurate Point of Load Voltage Control

April 30, 2014

Today’s ASICs and processors typically operate on a low voltage and therefore require high current. This means that if they are placed at any distance from the power supply or DC-DC converter that feeds them, transmission losses can become an issue. Because of resistance and therefore voltage drop in the power path between the converter and load, the voltage across the load will be lower than that across the converter’s output terminals. The difference is given by I x R, where I is the current drawn by the load and R is the resistance of the power path.

On a PCB, the power path is usually provided by the PCB’s power plane, so R is relatively small. However, as mentioned, I can be large so the voltage drop between the converter and load can be significant. This is even more important in a low voltage/high current environment, because a loss of say 0.5 V is a much higher proportion of a 3 V target voltage than it is of 48 V.

Remote sense, a popular and effective way of overcoming these issues, is used by many power product manufacturers.  Most Vicor products have provision for this; the only exceptions are the VI Chip® family, which uses Adaptive Loop Control and the Micro Brick family. Given their power levels, the Micro Brick (Quarter Brick) products simply do not require remote sense for most applications. For the rare cases where it is required, an Application Note (see pages 9 and 10 of this Applications Manual) describing implementation of external remote sensing for these products is available.

Fig.1 shows the remote sense concept. The S+ and S- Sense wires are connected across the load at the points where the correct voltage should be applied.  Note that this is a ‘remote sense’ configuration; as we shall see, ‘local sense’ arrangements also have their role to play.

Fig.1: Four-wire remote voltage sensing on a DC-DC converter

Fig.1: Four-wire remote voltage sensing on a DC-DC converter

S+ and S- are high-impedance inputs, so negligible current is drawn through the Sense lines; voltage drop is therefore negligible. This means that the Sense pins provide the converter with an accurate measurement of the voltage across the load resistance RL. By contrast, until modified by remote sense control,  the voltage at Vout+ and Vout- will be lower than the voltage across the load by an amount I x RP where I is the current drawn by the load and RP is the resistance of the power path. The converter can use this difference between the S and Vout voltage levels to accurately compensate for the voltage drop.

Some applications may not require the remote sense feature. If so, the remote sense lines should not simply be left open, as the power supply could drive upwards to an unwanted and possibly destructive level. Instead, the power supply should be configured in ‘local sense’ mode, with a low-value resistor connected between the S+ and Vout+ terminals, and another between S-and Vout-.

Westcor products make this particularly easy and reliable to set up, as they have 5 Ω resistors pre-installed in parallel with the remote sense circuits. They also have Westcor’s Autosense capability, which allows them to use the remote sense lines if connected, or automatically switch to the 5 Ω resistors if not. This feature protects the Westcor products against open loop mode failure as well as simplifying configurations with no remote sense connections. Similar provision can be made for other Vicor products by using externally-connected low-value resistors.

It should be remembered that remote sense wires are part of the power supply’s control loop, so any radiated noise they pick up can introduce instability into the power system.  This can be mitigated by using light-gauge wiring – which is viable as the sense lines draw minimal current – and twisting the S+ and S- wires together in addition to shielding them.

Although remote sense allows loads to receive correct voltage levels, it should not be considered as a substitute for good power layout practice. If insufficient care is used in minimising the power path voltage drop, this drop could exceed the compensation range of the power source and make full compensation impossible. In any case, excessive power path voltage drops are to be avoided; for example a drop of 0.5 V in a path carrying 100 A current equates to 50 W losses with associated implications for efficiency and cooling. These issues mean that investment in adequate gauge cabling and power planes and good-quality connectors is easily justified.

In using remote sense, care must be taken to ensure that the power supply is not driven up into an over-voltage protection (OVP) region; this would cause the power supply to shut down, denying power to the load. It is also important that the Vout+ or – lines are not opened while power is applied. This would cause current to flow in the Sense lines. With good design, however, using remote sense is an invaluable technique that ensures the voltage at the load is exactly that value that was intended.

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