The Benefits of Autoranging vs Conventional DC Power Supplies

How Autoranging Saves Capital Costs and Rack Space (and How True Autoranging Saves Even More!)

You need a high-capacity power supply. Should it be a conventional power supply with a rectangular output characteristic or a power supply with an autoranging, constant power output characteristic? That depends on the load the power supply must support.

If the load is a constant power output device such as a motor or a DC-DC power supply, then an autoranging power supply can be very advantageous. The autoranging supply can meet the power delivery requirements and save you on cost and precious test rack space.

In addition, not every autoranging programmable power supply functions the same. Some units actually can perform the autoranging function over a limited voltage range, so it’s not able to provide the full benefit of an autoranging device.

Let’s define what we mean by autoranging and illustrate these differences with an example.

What is an autoranging programmable power supply?

With an autoranging output, the programmable DC power supply generates a wide output range to ensure it can power a load, especially a constant power load over the load’s full power operating range. The power supply seamlessly delivers increased current when the load operates at a lower voltage to allow the load to maintain full power output.

Figure 1 shows the voltage-current output curve for an autoranging power supply. This curve illustrates the characteristic output of an autoranging 5 kW power supply. In this case, the power supply is an EA Elektro-Automatik 5 kW programmable power supply.

A conventional rectangular output characteristic of a 5 kW power supply with the same maximum voltage would compare with the output of as autoranging supply, as shown in Figure 2. The autoranging output of a comparable wattage power supply provides significantly more range. Current output is three times greater for the autoranging supply. The rectangular output supply can generate 10 A while the autoranging supply can produce 30 A.

True autoranging programmable DC power supplies

What is unique to EA programmable DC power supplies is they have “true” autoranging.

True autoranging allows full power output from maximum voltage all the way down to 1/3 of the maximum output voltage. Thus, an EA power supply can deliver full power over the load’s complete voltage operating range. In most cases, the EA supply can deliver full power beyond the operating range of the load. In contrast, a power supply with a rectangular output supplies maximum output power only at one point, at the point of maximum voltage and maximum current.

Figure 1. Autoranging power supply output characteristic of a 5 kW supply with 500 V and 30 A output ratings

Figure 2. Conventional 5 kW, 500 V, 10 A rectangular output power supply characteristic (blue shade) overlayed on the 5 kW output characteristic

Example: powering a DC server source

Now let’s use an example to see how an autoranging programmable power supply saves on costs and on rack space. Manufacturers are using DC energy to power server farms. Consider a DC server power source that supplies 3000 W to a set of servers. The server source is designed to deliver full power when sourced from 190 VDC to 400 VDC. That is quite a wide voltage range.

The power supply testing the server during the manufacturing process or powering the server when it is in operation must be capable of delivering 3000 W over the range from 190 VDC to 400 V VDC. Programmable DC power supplies capable of meeting the capacity requirement must have a voltage of at least 400V. In general, the closest voltage that power supply manufacturers offer is 500 VDC.

Using the formula, Power (P) = V (voltage) x I (current), you can determine the current required at the voltage extremes to provide the necessary full power to the server power source.

• At the maximum voltage, 400 V, I = 3000 W/400 V = 7.5 A.
• At the minimum voltage, 190 V, I = 3000 W/190 V = 15.8 A.

So, the 5000 W rectangular output power supply, whose characteristic output is shown in Figure 2, can meet the requirements of the server source at 400 V, but it cannot meet the current requirement at 190 V. A rectangular output supply that would meet the power requirement for both voltages would need: 500 V x 15.8 A = 7900 W.

Manufacturers do not make 8000 W power supplies. The next capacity is 10,000 W. For a rectangular output supply to meet the power requirements of the 3000 W server source, the rectangular output supply would have to be a 10,000 W, 500 V power supply. See Figure 3.

Figure 3. A 10 kW power supply with a rectangular output is needed to cover the total operating range of the server source while a only 5 kW autoranging power supply is needed.

In contrast, Figure 3 shows that an EA autoranging power supply with 5000 W capacity can supply both required current levels at 400 V and 190 V. The autoranging programmable power supply can meet the requirements with ½ the power of the rectangular output power supply. Here is where the cost savings come in.

• Since the price of a power supply is directly related to its total wattage, a 5000 W autoranging power supply can yield significant capital cost savings by potentially costing ½ as much as a conventional 10,000 W power supply.
• Having to dissipate less power allows for lower capital costs for cooling infrastructure.
• The smaller wattage power supply saves on annual utility costs.

As well as providing lower instrument and infrastructure costs, an EA autoranging power supply, such as the 5000 W PS 10500-30 DC programmable power supply, has high-power density and is only 3U high, much smaller than a 10 kW conventional power supply. Thus, the EA supplies save rack space and potentially requires fewer power racks in a total system. Fewer power racks can reduce facility overhead costs.

Autoranging Benefits

Along with the benefits of lower costs and rack space savings, EA programmable DC power supplies offer other valuable features. EA supplies can have a built-in arbitrary waveform generator that allows the addition of a custom waveform to the DC signal and simulation of power sources such as solar cells, batteries, and fuel cells. Bidirectional DC power supplies source or sink power; and can be regenerative, returning absorbed power to the AC power lines. With numerous standard and optional interfaces, the power supplies can interface with PCs and programmable logic controllers (PLCs).

For more information on how EA supplies can meet your needs and assistance with selection of a programmable power supplies, contact us.