With rack space at a premium and constant pressures to drive the cost of systems down, having the ability to produce a wide range of voltages and currents with one system power supply instead of several is very beneficial.

During test, a single dc power supply with an autoranging output characteristic can be used to cover the range of several supplies with rectangular output characteristics, or cover the range of a single, much larger supply.
A programmable dc power supply output characteristic graph shows the borders of an area containing all valid voltage and current combinations for that particular output. Dc power supply output characteristics primarily fall into one of two categories: rectangular or autoranging. With the more typical rectangular output characteristic, maximum power is produced at a single point coincident with the maximum voltage and maximum current values. With an autoranging output characteristic, maximum power is produced throughout a range of voltage and current values, thereby aautomatically' extending the range of valid operating voltages and currents without requiring an increase in the maximum power.
Autoranging power supplies are appropriate to use as the source of dc power in a variety of test situations. Their use makes sense when:
* The device under test (DUT) requires a wide range of input voltages and currents, all at roughly the same power level. For example, when testing a dc/dc converter with a 24V nominal input voltage, the input voltage can range from 14V to 40V.
* There are a variety of different DUTs of similar power consumption, but different voltage and current requirements. Again, different dc/dc converters in the same power family can have nominal input voltages of 12V, 24V, or 48V, resulting in input voltages as low as 9V, and as high as 72V.
* A known change is coming for the dc input requirements without a corresponding change in input power. For example, the input voltage on automotive accessories could be changing from 12V nominal to 42V nominal, but the input power requirements will not necessarily change.
* Extra margin on input voltage and current is needed, especially if future test changes are anticipated, but the details are not presently known.

In general, the maximum power available from a particular power supply will determine its size and cost, where, of course, more power corresponds to larger size and higher price. Depending upon the application, autoranging power supplies can be used in place of several supplies of a similar power rating, or one power supply with a higher power rating. In either case, the autoranging supply will occupy less space and cost less.
For example, consider the input power requirements for testing a dc/dc converter with specifications as shown in the table below.
As part of a complete set of tests, the dc/dc converter should be tested at full power out (42W) across the entire input voltage range -- in this case, 14V to 40V in. Selection of an input power source to provide the dc input power to the converter is driven by considering the minimum and maximum converter input voltages, and the corresponding input currents at full power out of the converter. With a maximum of 42W out and 90percent efficiency, the input current can easily be calculated at the minimum, nominal, and maximum input voltages:

Pin = Pout/efficiency = 42W/0.9 = 46.67W

Iin = Pin / Vin

At 14V in, Iin = 46.67W / 14V = 3.33A.
At 24V in, Iin = 46.67W / 24V = 1.94A.
At 40V in, Iin = 46.67W / 40V = 1.17A.

These values are summarised in the following table:
Therefore, at full output power, this dc/dc converter draws input currents from a little more than 1A to nearly 3.5A depending on the input voltage. Note that this is at a typical efficiency of 90percent. If the efficiency is lower, the input current will be higher for a given input voltage.
Since there is a 40V maximum input voltage requirement, and at least a 3.33A maximum input current requirement, a single input power source with a rectangular output characteristic would require at least 133W (40V*3.33A) of power to cover the full range of needed input voltages and currents. A typically available 50V, 4A, 200W supply would cover this range as shown below.
But the 40V and 3.33A are not needed simultaneously, so using a 200W rectangular output power supply is overkill in power (more than 4 times!) since only 46.67W are needed. Another alternative is to use several power supplies each with lower power to cover the required dc/dc converter input voltage and current requirements. Again, this solution provides the necessary voltages and currents, as shown below, but will take up more space and cost more than one autoranging output power supply.
A more suitable alternative is to use a single autoranging power supply, such as the recently released Agilent Technologies N6751A, rated for 50V, 5A, 50W maximum. Using the autoranging solution, one suitably powered output covers the entire range of required voltage and current combinations, eliminating the need for a larger product of unnecessarily high power, or the need for multiple outputs that occupy more space than required as shown in below.
The N6751A power supply system is part of a new platform of dc power supplies that offers a variety of autorangers at 50W and 100W per output available in a very flexible mainframe system containing up to four outputs per mainframe, all in 1U high packaging. The selection of outputs is user configurable with the user choosing from autoranging and rectangular outputs as well as standard and precision accuracies.
Having the ability to configure up to four outputs per mainframe can further simplify testing dc/dc converters by using the additional outputs on converter inputs such as output on/off, trim up, and trim down. They have a full feature set including built-in dc voltage and current measurement, and optional built-in voltage and current digitisation capability. An integrated LAN, USB, and GPIB interface completes the package.
There are a variety of test applications in which an autoranging power supply is a more sensible choice compared to a rectangular output power supply. In these applications, the DUT (or collection of DUTs) is subjected to a wide variety of voltages and currents, frequently at a roughly constant maximum power.
One of these applications provides input power to dc/dc converters during test. While the test points could be covered by a single rectangular output power supply of higher power, or multiple rectangular output supplies, the autoranging supply is a better choice.

Gary Raposa is with Agilent Technologies, Palo Alto, Ca, USA. www.agilent.com