Sunday, March 31, 2013

Remote sensing can affect load regulation performance

Back in September of 2011, I posted about what load effect was (also known as load regulation) and how it affected testing (see The voltage load effect specification tells you the maximum amount you can expect the output voltage to change when you change the load current. In addition to the voltage load effect specification, some power supplies have an additional statement in the remote sensing capabilities section about changes to the voltage load effect spec when using remote sensing. These changes are sometimes referred to as load regulation degradation.

For example, the Agilent 6642A power supply (20 V, 10 A, 200 W) has a voltage load regulation specification of 2 mV. This means that for any load current change between 0 A and 10 A, the output voltage will change by no more than 2 mV. The 6642A also has a remote sensing capability spec (really, a “supplemental characteristic”). It says that each load lead is allowed to drop up to half the rated output voltage. The rated output voltage for the 6642A is 20 V, so half is 10 V meaning when remote sensing, you can drop up to 10 V on each load lead. Also included in the 6642A remote sensing capability spec is a statement about load regulation. It says that for each 1 volt change in the + output lead, you must add 3 mV to the load regulation spec. For example, if you were remote sensing and you had 0.1 ohms of resistance in your + output load lead (this could be due to the total resistance of the wire, connectors, and any relays you may have in series with the + output terminal) and you were running 10 A through the 0.1 ohms, you would have a voltage drop of 10 A x 0.1 ohms = 1 V on the + output lead. This would add 3 mV to the load regulation spec of 2 mV for a total of 5 mV.

There are other ways in which this effect can be shown in specifications. For example, when remote sensing, the Agilent 667xA Series of power supplies expresses the load regulation degradation as a formula that includes the voltage drop in the load leads, the resistance in the sense leads, and the voltage rating of the power supply. Output voltage regulation is affected by these parameters because the sense leads are part of the power supply’s feedback circuit, and these formulas describe that effect:

One more example of a way in which this effect can be shown in specifications is illustrated by the Agilent N6752A. Its load effect specification is 2 mV and goes on to say “Applies for any output load change, with a maximum load-lead drop of 1 V/lead”. So the effect of load-lead drop is already included in the load effect spec. Then, the remote sense capability section simply says that the outputs can maintain specifications with up to a 1 V drop per load lead.

When you are choosing a power supply, if you want the output voltage to be very well regulated at your load, be sure to consider all of the specifications that will affect the voltage. Be aware that as your load current  changes, the voltage can change as described by the load effect spec. Additionally, if you use remote sensing, the load effect could be more pronounced as described in the remote sensing capability section (or elsewhere). Be sure to choose a power supply that is fully specified so you are not surprised by these effects when they occur.

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