Most often DC power supplies have a rectangular output
characteristic, as depicted in figure 1. With an increasing load they output a
fixed output voltage up to the current limit, at which point the voltage drops
in order to maintain the current fixed at its limit.
Figure 1: DC power supply rectangular output
characteristic.
There is however DC power supplies that offer multiple
output ranges. One example of a multiple (dual in this case) output range DC
power supply is our N678xA series DC source measure modules. Their output characteristics
are depicted in Figure 2.
Figure 2: Agilent N678xA series source measure modules
output characteristics
Unlike the output characteristic of a single output range
DC power supply, you cannot get both
the maximum current and maximum voltage of a multiple output range DC power
supply at the same time.
What is the purpose of having multiple output ranges on a
DC power supply?
There are times, especially when having to test a variety
of devices, the need for greater current or voltage, but not necessarily
needing both maximum voltage and current at the same time. In these situations many times these test
power needs are better served by a DC power supply having multiple output
ranges. The advantages of a multiple output range DC power supply are smaller
size, less power dissipation, and less input power required, in comparison to a
single output range DC power supply of comparable voltage and current
capability. If the N678xA series DC source measure modules had a single output
range they would need to have a 60 watt output to cover the span of voltage
they now provide with 20 watts of output power. An even more extreme example is our B2900
series source measure units. They output up to 31.8 watts continuously, but can
provide up to 210 volts and up to 3.03 amps over three output ranges.
The downside of having multiple output ranges is somewhat
greater complexity. Figure 3 depicts a conceptual design for a dual output
range DC power supply.
Figure 3: Conceptual dual output range DC power supply
Because the transformer efficiently converts AC power by
square of its turn ratio there is very little impact on its size to accommodate
secondary windings with multiple taps or multiple secondary windings that can
be alternately connected in series or parallel, in order to accommodate multiple
output power ranges. Similarly, the linear series pass element dissipates about
the same maximum power whether it is operating at a higher voltage with lower
current, or at a lower voltage with a higher current.
