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Compensate for wiring losses with remote sensing

-November 18, 2010

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Compensate for wiring losses with remote sensing figure 1 Wires and connectors have resistance. This simple, unavoidable truth dictates that a power source’s remote load voltage is less than the source’s output voltage (Figure 1). You could maintain the intended load voltage by raising the regulator output. Unfortunately, line resistance and load variations introduce uncertainties, thus limiting the correction accuracy. You could add a locally positioned regulator, but this approach is inefficient due to regulator losses (Figure 2). A classic approach uses “four-wire” remote sensing to eliminate line-drop effects (Figure 3). In this case, load-referred sense wires feed the power supply’s sense inputs. The sense inputs’ high impedance negates the sense-line-resistance effects. This scheme works well but requires dedicated sense wires, a significant disadvantage in many applications.

Compensate for wiring losses with remote sensing figure 2

“Virtual” remote sensing

Compensate for wiring losses with remote sensing figure 3You can eliminate the sense loads and still retain the advantages of classic four-wire remote sensing (Figure 4). In this case, a Linear Technology VRS (Virtual Remote Sense) IC alternates output current between 95 and 105% of the nominal required output current (Reference 1). The IC forces the power supply to provide a dc current plus a small square-wave current with peak-to-peak amplitude equal to 10% of the dc current. Typical systems generally require decoupling capacitor CLOAD for low impedance under transient conditions. In this case, it takes on an additional role by filtThe power supply can be a linear or switching regulator, a module, or any other power source capable of variable output. You can also synchronize the power supply to the sense IC’s operating frequency, which is adjustable over three decades. The sense IC has optional spread-spectrum operation to improve EMI (electromagnetic interference). The IC’s 3 to 50V input range allows you to use it in many designs.ering out the VRS square-wave excursions.

Compensate for wiring losses with remote sensing figure 4You size CLOAD to produce an ac short at the square-wave frequency. Thus, VOUTAC, the square-wave voltage at the power supply, is equal to 0.1×IDC×RW p-p, where IDC is the square-wave current and RW is the wire’s resistance. Thus, the square-wave voltage at the power supply has a peak-to-peak amplitude equal to one-tenth the dc wiring drop. This figure represents a direct measurement—not an estimate—of wiring drop and is accurate over all load currents. The IC provides signal processing that produces a dc voltage from this ac signal. The IC introduces it into the power-supply feedback loop to provide accurate load regulation (see sidebar “A primer on VRS operation”).

Compensate for wiring losses with remote sensing figure 5The power supply can be a linear or switching regulator, a module, or any other power source capable of variable output. You can also synchronize the power supply to the sense IC’s operating frequency, which is adjustable over three decades. The sense IC has optional spread-spectrum operation to improve EMI (electromagnetic interference). The IC’s 3 to 50V input range allows you to use it in many designs.

This technique employs an estimate—not a direct measurement—of load voltage, so the resultant correction is only an approximation—but a good one (Figure 5). In this example, load current increases from 0A until it produces a 2.5V wiring drop. Load voltage drops only 73 mV at maximum current. A voltage drop equivalent to 50% of load voltage results in only a 1.5% shift in load-voltage value. Smaller wiring drops produce even better results.

Linear regulators

You can use VRS with a linear regulator. The IC senses current through a 0.2Ω shunt resistor (Figure 6). Feedback controls Q1 with Q2, completing a control loop. You design Q2 in cascode to the IC’s open-drain output to control a high voltage at Q1’s gate. Components at the IC’s compensation pin furnish loop stability and provide good transient response. The design shows good response to load-step waveforms (Figure 7). Loop compensation, load capacitance, and the remote-sense IC’s sampling rate determine the transient response (Figure 8).

Compensate for wiring losses with remote sensing figure 6


Compensate for wiring losses with remote sensing figure 7You can also apply this technique to a monolithic regulator (Figure 8a). This approach allows you to add current limiting and simplifies the loop compensation. The transient response is similar to that of the circuit in Figure 6. As before, the sense IC’s low-voltage drain pin requires you to place a transistor in cascode to control the high voltage at the regulator’s set pin.

Switching regulators

You can also design VRS into switching regulators. A flyback voltage-boost configuration has similar architecture to that of the linear examples, although the output voltage is higher than the input voltage (Figure 8b). In this case, the sense IC’s open-drain output is directly compatible with the boost regulator’s low-voltage VC pin, so no cascode stage is necessary.

You can also design VRS into switching step-down, or buck, regulators (Figure 8c). As before, you can control the VC pin of the regulator directly from the sense IC’s open-drain output. You design a single-pole roll-off to stabilize the loop. The design maintains a 12V, 1.5A output from a 22 to 36V input despite a 0 to 2.5Ω wiring-drop loss.

Isolated supplies

Compensate for wiring losses with remote sensing figure 8You can adapt the VRS approach to isolated output supplies (Figure 8d). You use an approach similar to that in the previous examples to supply a fully isolated 24V output. The VRS feature accommodates a 10Ω wire resistance. The flyback-regulator IC and T1 form a transformer-coupled power stage. You use optocoupled feedback to maintain output isolation.

You can design step-down isolated converters that incorporate remote virtual sensing (Figure 8e). This 48V input to the 3.3V, 3A output has a fully isolated output. The regulator IC drives T1 through Q1. T1’s rectified and filtered secondary supplies output power, which the remote-sense IC corrects for line drops. You maintain isolation by transmitting the feedback signal with an optoisolator. The optoisolator’s output collector ties back to the regulator IC’s VC pin, closing the control loop.

Compensate for wiring losses with remote sensing figure 9

Compensate for wiring 
losses with remote sensing figure 10
Compensate for wiring losses with remote sensing figure 11You can also add VRS to a brick or half-brick isolated input module (Figure 9). You don’t use the module’s sense terminals. Instead, you introduce the sense IC’s wiring-drop correction at the module’s trim pin. The power-brick-module trim pin’s transient response defines the available control bandwidth (Figure 10a). The trim pin’s dynamics dictate your expectation for the loop response of this module (Figure 10b). The load-step response is less than 40 msec with this Vicor module. The module’s trim-pin dynamics limit the clean and well-controlled response envelope. Turn-on dynamics into a 2.5A load are equally well-behaved (Figure 11). The sense IC’s operation arrests the initial abrupt rise at the third vertical division (Figure 12). The sense IC controls the output ascent’s conclusion to the regulation point in a damped fashion. You can barely discern the sense IC’s sampling square wave in the output waveform’s settled portion.

You can also apply VRS to offline power supplies (Figure 13). A typical VRS-aided offline isolated output supply has a 5V output with 2A capacity. The schematic appears complex, but inspection reveals it to be an ac-line-powered variant of an isolated approach. The sense IC provides remote sensing and closes an isolated feedback loop with optical transmission.

Compensate for wiring losses with remote sensing figure 12Lamp circuit

You can also use a VRS IC to stabilize the drive to a halogen lamp (Figure 14). The circuit is a buck-boost SEPIC (single-ended primary-inductance converter, Reference 2). This 12V, 30W automotive-lamp output remains constant despite a 9 to 15V input-voltage variation along with any line resistance or connection uncertainties (Figure 15). Additional benefits include a constant color output and an extended lamp life due to greatly reduced lamp turn-on current (figures 16 and 17). The regulator reduces inrush current to 7A, one-third of the unregulated value.



References
  1. LT4180 Virtual Remote Sense Controller,” Linear Technology Corp, 2010.
  2. Ridley, Ray, PhD, “Analyzing the Sepic Converter,” Power Systems Design Europe, November 2006.

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