When Selecting a DC Power Supply, Understand its Output Characteristic

­An important but overlooked aspect of power supply selection is the type of output characteristic that the power supply has. A typical power supply has a rectangular output in which maximum power is delivered only at the supply’s maximum rated voltage and maximum rated current. Another option is a power supply with an autoranging characteristic. This supply type has a constant power output characteristic that can deliver full power output across a wider range of output values. The autoranging supply can output a higher voltage and a higher current than an equivalently powered supply with a rectangular output characteristic.

In many applications, a test engineer can select an autoranging DC power supply with a lower power rating than what a rectangular output supply would require. Thus, the autoranging supply can save cost and potentially instrument size, and cooling costs. The wide voltage and current range of the autoranging supplies relative to the conventional rectangular output supplies can also reduce the number of different power supplies needed for test systems, allowing the facility to manage fewer power supply models. Autoranging DC power supplies provide flexibility for future test systems should a new product have a higher voltage range than current products. Thus, an autoranging supply can address an existing application and offer the flexibility for use in future applications, meaning test managers can purchase fewer power supplies given the extra flexibility autoranging features provide. A DC power supply with a rectangular output cannot match what an autoranging DC power supply can offer.

This article addresses the differences between the autoranging power supply and the rectangular output supply. The paper also presents applications that show the value of the autoranging output supplies.

Figure 1 shows a power supply's autoranging output compared to a supply with a rectangular output. The autoranging output is essentially a constant power output. The autoranging output allows higher voltage output at lower load currents. This type of supply also allows higher load currents at lower voltages. Full power output can be realized all along the curve of the autoranging output. Numerous combinations of voltage and current deliver full power.

The rectangular output shown in Figure 1 can only supply maximum power at one point, at maximum voltage. At any voltage lower than the maximum rated voltage, the supply delivers less than the maximum power.

The limitations from ideal of active circuit components and circuit configurations prevents an autoranging power supply from providing constant power between the extremes of maximum voltage at no current to maximum current at no voltage. Unlike other autoranging power supplies, EA Elektro-Automatik units achieve full power all the way down to 33% of the maximum rated voltage. Other typical autoranging power supplies begin to lose power before dropping to just 50% of their rated output voltages.

As a result, with the autoranging design provided by EA Elektro-Automatik, there’s no need to oversize the power supply purchased, and it will be more likely to support future tests that are required.

Testing Servers

Consider the need to power a server farm where each server requires 3,150 W. Server designs operate over a wide voltage range of 192 to 400 V. At both of those voltages and any voltage in between, the server requires 3150 W. The required load currents at the minimum and maximum voltages are:

                   3,150 W/400 V = 7.9 A

                   3,150 W/192 V = 16.4 A

If a test engineer chooses a power supply with a conventional rectangular output characteristic, the required supply would have to have a voltage over 400 V and have a current rating of 16.4 A. Manufacturers typically offer supplies with a 500 V output. To accommodate the full voltage and current range needed, then the required supply would have to have a capacity of:

                  500 V x 16.4 A = 8,200 W.

With 5 kV, 7.5 kV, 10 kV power supplies available, a test system engineer would need to select a 10 kW power supply.

Alternatively, EA Elektro-Automatik can offer an autoranging supply with the same maximum voltage, 500 V, and with a 30 A maximum output; however, the power output is only 5,000 W. The required EA Elektro-Automatik power supply would be a model PSI 10500-30.

                  5,000 W/192 V = 26.0 A

                  5,000 W/400 V = 12.5 A

The EA Elektro-Automatik power supply can supply half the power and have extra current capacity to meet the test requirements. 

If there was a need to test at higher than 400 V, then an alternative selection for a power supply could be a model PSI 10750-20, also a 5,000 W power supply. At 192 V, the supply can still source 20 A. Now the test engineer has a supply with a much greater range than the 10,000 W conventional output supply even though the power is twice as high. The 10,000 W supply with 750 V output would only provide 13.3 A which is less than the required current of the low, 192 V state. The higher voltage, 10,000 W rectangular output supply cannot meet the load requirements of the server under the low voltage condition. With 750 V output, the rectangular output supply would have to be a 15,000 W supply to support the 16 A load. That is 3 times the power of the autoranging power supplies.

Enabling the use of a lower power-capacity supply certainly saves substantially on cost as cost is directly rated to the number of watts of power. There are also reduced cooling requirements to support the lower wattage supply. In addition, the selection of the 750 V supply allows for testing new servers that operate at higher voltages.

Testing server farms is one example of how autoranging supplies can maintain a load over a wide voltage range using a lower power instrument than a conventional rectangular output supply. Testing DC-DC converters with a wide input voltage rating is a similar application. Like servers, DC-DC converters must maintain their load over a specified input voltage range which can be quite large.

Testing Photovoltaic inverters

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Figure 2: Solar cell I-V output characteristic 

Solar arrays operate over a wide dynamic range as shown by the characteristic curve for a solar array in Figure 2. The output of the solar arrays connects to a photovoltaic inverter which supplies the power line grid with AC power. To maximize efficiency, the inverter is designed to maintain the solar array’s output at its maximum power point, MPP. Due to weather conditions and the time of day, changes in the light intensity on the solar array causes the I-V curve and its maximum power point to shift. The inverter is designed to maintain the solar array at its maximum power point. As a result, the inverter must have a wide input voltage operating range. Table 1 lists some parameters of a photovoltaic inverter. The red boxes highlight the maximum input voltage of the inverter and the maximum current. Below the maximum current, the table indicates that the start-up voltage for the inverter is 200 V. The inverter must operate over a range of 200 – 520 V. Selecting a rectangular power supply that can meet the necessary voltage and current ratings would require a 30 kW supply which would output 600 V and 50 A. An EA Elektro-Automatik 15 kW autoranging power supply, the model PSI 10750-60, can satisfy the requirements with a maximum voltage of 750 V and a maximum current of 60 A. The PSI 10750-60 provides both more voltage and more current headroom at ½ the power.

Again, the autoranging supply yields a significant cost savings, a savings in rack space, and less cooling infrastructure. The higher voltage supply allows for testing future inverters that may run on voltages higher than 520 V.

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Table 1: Input power parameters of a photovoltaic inverter

Testing on-board chargers

Electric vehicles (EVs) have on-board battery chargers that operate on 400 V and 800 V. The chargers can draw 22 kW. To test either output on-board charger, a power supply would need to supply 22,000 W/400 V = 55 A and 22,000 W/800 V = 27.5 A. A rectangular output power supply would need to cover an 800 V and 55 A envelope or 44 kW. The selected supply would have to be a 50 KW supply which is what most power supply manufacturers make at power levels above 40 kW. With its autoranging power supplies, EA Elektro-Automatik can provide a 30 kW, model PSI-11000-80, with a 1000 V maximum voltage and a 80 A maximum current. The autoranging supply saves the test engineer 20 kW, resulting in capital cost savings and, most likely, rack space and cooling power savings. Furthermore, the autoranging supply provides an extra 200 V for use in a future test system.

Conclusion: The value of a true autoranging output for a DC power supply

A power supply's output characteristic can impact your test system's cost, size, and cooling requirements. An autoranging output power supply provides higher voltage, more load current, and more combinations of voltage and current for maximum power output compared with an equivalently powered supply with a rectangular output characteristic. As shown by the examples above, a test engineer can use a lower power, autoranging supply than would be required to meet the application with a rectangular output supply. The wide voltage range of an EA Elektro-Automatik autoranging supply can reduce the number of power supply models required in a test facility. The test facility can purchase and stock fewer types of models. Fewer models decrease the number of different power supplies that a test department must learn to service and reduce the number of spare parts that have to be inventoried. Finally, the wider range of an autoranging power supply gives the test engineer flexibility to install the same power supply in a future test system if a new device-under-test has a higher voltage rating. Investigate the output characteristic for future supply needs. The autoranging supplies will provide the most value.

EA Elektro-Automatik