Compensate for wiring losses with remote sensing
By multiplexing a small ac signal on the power wires, you can infer wiring losses.
Jim Williams, Jesus Rosales, Kurk Mathews, and Tom Hack, Linear Technology -- EDN, November 18, 2010
At A Glance
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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.
You 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.
You 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”).
The 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).
You 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.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.
You 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 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.
Lamp circuitYou 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.
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References |
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A primer on VRS operation
Voltage drops in wiring can produce considerable load-regulation errors in electrical systems (Figure A). As load current, IL, increases, voltage drop in the wiring increases, and the load voltage decreases. The traditional approach to solving this problem, remote sensing, regulates the voltage at the load, increasing the power-supply voltage, VOUT, to compensate for voltage drops in the wiring. Although remote sensing works well, it requires an additional pair of wires to measure at the load, which can sometimes be impractical.
The LT4180 eliminates the need for a pair of remote-sense wires by creating VRS (virtual remote sensing). The IC achieves VRS by measuring the incremental change in voltage that occurs with an incremental change in current in the wiring (Figure B). You can use this measurement to infer the total dc voltage drop in the wiring, which you can then compensate for. The VRS IC takes over control of the power supply through its feedback pin, VFB, maintaining tight regulation of load voltage VL.
Figure C shows that a new cycle begins when the power supply and VRS close the loop around the supply voltage. Both the supply voltage and the supply current slew and settle to a new value, and the VRS stores these values. The device opens a new supply-voltage feedback loop and sets up a new feedback loop that commands the power supply to deliver 90% of the previously measured current (0.9A). The supply voltage drops to a new value as the power supply reaches a new steady state, and the VRS IC also stores this information. At this point, the device has measured and stored the change in the output voltage for a −10% change in output current. The next VRS cycle uses this voltage to compensate for voltage drops due to wiring resistance.
Voltage drops in wiring can produce considerable load-regulation errors in electrical systems (Figure A). As load current, IL, increases, voltage drop in the wiring increases, and the load voltage decreases. The traditional approach to solving this problem, remote sensing, regulates the voltage at the load, increasing the power-supply voltage, VOUT, to compensate for voltage drops in the wiring. Although remote sensing works well, it requires an additional pair of wires to measure at the load, which can sometimes be impractical.
The LT4180 eliminates the need for a pair of remote-sense wires by creating VRS (virtual remote sensing). The IC achieves VRS by measuring the incremental change in voltage that occurs with an incremental change in current in the wiring (Figure B). You can use this measurement to infer the total dc voltage drop in the wiring, which you can then compensate for. The VRS IC takes over control of the power supply through its feedback pin, VFB, maintaining tight regulation of load voltage VL.
Figure C shows that a new cycle begins when the power supply and VRS close the loop around the supply voltage. Both the supply voltage and the supply current slew and settle to a new value, and the VRS stores these values. The device opens a new supply-voltage feedback loop and sets up a new feedback loop that commands the power supply to deliver 90% of the previously measured current (0.9A). The supply voltage drops to a new value as the power supply reaches a new steady state, and the VRS IC also stores this information. At this point, the device has measured and stored the change in the output voltage for a −10% change in output current. The next VRS cycle uses this voltage to compensate for voltage drops due to wiring resistance.
Talkback
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It's a good IC implementation. I had a wise old engineer working for me in the early 90's who just added a touch of positive feedback to his regulator circuit to null the DC output resistance - since our application (a plug in power supply) could not support direct remote sensing. That was a good idea then too.
Steve Hageman - 2011-21-6 15:17:24 PDT -
I read about this idea several months ago, but the description left quite a few questions. It certainly is an interesting idea, and actually a totally new concept, it appears. It will be quite interesting to see how widely it is used, since the more standard remote sensing scheme is so very common presently.
William Ketel - 2010-19-11 18:23:19 PST
