Power Supplies and Output Power rating versus Voltage

As a rule, power supplies are rated for maximum power output only at their maximum output voltage. That means that as the programmed output voltage is decreased, the power output capability decreases proportionally. For example, a 10kW rated power supply with a 0 to 100V voltage range can deliver 100A at 100V or 100 x 100 = 10kW but only 50 x 100 = 5kW at 50V output as the current remains limited at 100A max.  This is true for both DC power supplies (DC output) as well as AC power sources (AC output). Such power supplies/sources are called ‘point rated’ as they provide the maximum rated output power only at the maximum voltage setting or one set point.

More recently, manufacturers of T&M power supplies and sources have started offering models that are not point rated but rather offer maximum output power over a portion of their output voltage range.  Such power supplies or sources are referred to as Constant Power (CP) voltage range models. For the case of AC power sources, let’s take a closer look at what this means.

Constant Power Voltage Range AC Source

A constant power voltage range source allows operation at maximum output power VA and Watt over a portion of its voltage range. This means the maximum available current at each programmed voltage set point increases as the voltage setting decreasing, thus maintaining a fixed V x I = P power profile. What this means is that the maximum power output set point is no longer restricted to the highest voltage setting.

Constant Power Voltage Range Comparisons

Although AC or DC power source models from different vendors may all claim Constant Power mode voltage range capability, that does not mean they are all created equal. This is best illustrated by comparing the technical specifications for similar output power rated AC source models side by side using the same 300Vac voltage range as is done in the table below.

Power Rating
Brand A
Brand B
Pacific Power
AC Voltage Range 0 – 300Vac 0 – 300Vac 0 – 300Vac
Max. Current 62.5A rms 75A rms 125A rms
CP Mode Range 80% - 100% 67% - 100% 40% - 100%
Max. Power @ 230Vac/Phs 14,375 VA 15,000 VA 15,000 VA
Max. Power @ 115Vac/Phs 7,187 VA 8,625 VA 15,000 VA

Table 1: Constant Power Voltage Range Comparison for different brand power sources

Note that all models reviewed have constant power voltage mode over some of their output voltage range but the range of it is rather limited for some.  The AFX Series is the notable exception, supporting constant power operation all the way from 100% to 40% of its 300Vac voltage range or down to 120Vac.  The same applies for its DC mode were full power is available from 100% to 40% of its 425Vdc range (425Vdc - 170Vdc).

The two graphs below (Figures 1 & 2) illustrate the significantly larger usable operating range of the Pacific Power AFX Series AC source compared to the other brands.  Figure 1 shows available max. load current as a function of programmed output voltage.

AFX-vs-Competition-Current-vs-Voltage.pngFigure 1: Available Current per phase for 45kVA AC power sources from different vendors

A similar comparison can be made by plotting available power as a function of programmed output voltage as shown in Figure 2. Again, the usable range of the Pacific AFX Series is considerably larger.

AFX-vs-Competition-Power-vs-Voltage.pngFigure 2: Available Power per phase for 45kVA AC power sources from different vendors

Why Does it Matter?

An increasing number of products support a wide input voltage range, also referred to as universal input range. To develop and test these products, they must be evaluated under both low voltage and high voltage input conditions, including under and over voltage stress testing, often beyond published specification.  This requires a power source with a wide voltage and current profile over which it can deliver full power.  It also means switching between a high and low voltage range is not acceptable as this invariably requires the output to the unit under test to be interrupted for some period of time during range change.

Have a wide constant power voltage range as found on the AFX Series eliminates the need to oversize the programmable power source used.  The AFX Series from Pacific Power meets these requirements better than its competition.


While product features like constant power mode voltage range capability may seem similar between equivalent power rated power sources from various vendors, it often pays to evaluate in greater detail the detailed technical specifications before making a selection. Such differences in specs can have significant impact on the usability of the power source for a range of situations.  The extent of the available constant power voltage range is one such example.