Design Ideas: January 4, 1996

A power supply usually needs to provide a regulated voltage across a load, but there is always some voltage drop across the conductors that carry the current to the load. Two common techniques help the supply to achieve the proper voltage across the load. One is to set the voltage at the power-supply output terminals high enough to compensate for the IR drop of the conductors. This voltage may be 2% high on a 5V power supply, which is 5.1V. The other method is to offer remote sensing. Remote sensing merely regulates the voltage at the load instead of at the power supply’s output terminals.

A third and alternative approach, called automatic local sensing, is to offer both features on the same power supply. In other words, have the power supply use traditional remote sensing for demanding applications, but when remote sensing is unnecessary (and, possibly, undesirable because of the additional cost of cabling), have the voltage at the power-supply output terminals increase a precise amount, say 2%.

On power supplies that have remote sensing, you have to do one or more things to accomplish local sensing. Jumpers may be necessary on the power supply. Unfortunately, these jumpers set the voltage at the power-supply output terminals to the same voltage that would have appeared across the load had you used remote sensing. Other power supplies operate with the sense leads floating, but the output voltage is seldom correct and often not well-regulated. So, in either case, the power-supply output voltage generally requires readjustment. Automatic local sensing can eliminate the need for readjustment and provide excellent regulation.

The circuit in Figure 1a shows a typical, simplified voltage-control loop for an auxiliary output (the so-called magamp output) of a power supply that has remote sensing. A main output might use a similar control. IC₁ could drive an opto-coupler instead of Q₁, which resets the magnetic core on a supply’s magamp circuit. On power supplies that do not have remote sensing, R₃ and R₆ would be 0 Ohms, and you would adjust R₄ to set the output high enough to compensate for the IR drop that R₇ and R₈ (conductor resistance) cause. This circuit does not generate a well-defined output voltage when you don’t use remote sensing (that is, when you don’t connect +S and -S), because the anode current, I_ANODE, from IC₁ varies as the supply regulates. This variability causes the voltage drop across R₆ to be larger than the voltage drop across R₃, and this voltage varies as regulation occurs.

The circuit in Figure 1b implements automatic local sensing by simply adding a differential amplifier to the power-supply control loop. This modified circuit keeps the voltage drop across R₃ and R₆ constant and precise when you use local and not remote sensing. The differential amplifier allows the anode current of IC₁ to return directly to V-. A simple way to state the role of the unity gain, differential amplifier is that it is "re-references" the voltage at the junction of R₅ and R₆ to V-. When using local sensing, the values of R₁₀ through R₁₃ are high enough at 20 kV to cause a minimal influence on the current flowing through R₃ and R₆. This current is approximately 500 µA, which produces a 50-mV drop across each of these resistors.

Many op amps, such as all LM324 types, work well for IC₂. The criteria for choosing IC₂ is that the bandwidth of the circuit must be greater than the bandwidth of the switching power-supply’s control loop. This loop’s bandwidth is typically 20 kHz. Also, if you use a rail-to-rail op amp for IC₂, then R₁ is unnecessary. You can add a margining feature by making a connection to the junction of R₄ and R₅ and making that connection accessible. You or another user can then pull up or down the nominal power-supply voltage setting by placing a resistance between this margining pin and -S or +S, respectively. (DI#1807)