Essentials of DC Power Supplies and Loads

Power consumption and conservation are vital functions for many systems and testing these functions help determine the efficiency and productivity of an electronic design. Power measurements rely on two workhorse units: DC power supplies and loads. They are vital for controlling energy on the workbench and often serve as starting points for new product development. DC loads and power supplies work together to define the behavior of a device under test (DUT) in response to applied energy, not just in performance but in reliability and compatibility with other electronic devices.

The simplest starting point when searching for a DC power supply is the amount of energy it supplies: does it provide enough power to energize a DUT of interest? That amount of power is available in terms of the power supply’s voltage and current ranges, how well they are regulated or controlled, how accurately a DC power supply reports the power it is taking from an AC source and providing to a DC load, the amounts of variations and noise in the DC power, and how efficiently it is converting power from AC form to DC form. DC power supplies are available as linear or switched-mode devices, each with benefits. Switched-mode supplies feature high efficiency, small size, and low cost compared to linear power supplies, but linear supplies typically provide higher output accuracy and resolution with less noise than switched-mode power supplies.

Power supplies are haunted by two forms of noise: normal-mode output noise and common-mode output noise. Normal-mode noise is generated by a power supply’s internal circuitry while common-mode noise is earth-referenced noise originating from the power line or source. Linear power supplies have lower normal-mode noise levels than switched mode power supplies, but switched-mode power supplies are capable of lower power-conversion efficiency than linear power supplies. Switched-mode power supplies can generate higher output levels than linear power supplies for the same size enclosure. When testing DUTs sensitive to normal-mode noise, linear power supplies provide a more sensible choice.

Setting Voltages

Power supplies can supply low and high voltages, although usually in different units. For example, the Tektronix PWS2185 linear power supply is not characterized in terms of output power but is rated for output voltages of 0 to 18 VDC and output current of 0 to 5 A when fed with a 110- or 220-VAC source at 47 to 63 Hz. The voltage can be set with 10-mV resolution and the current with 10-mA resolution. The amount of line regulation is within 0.1% + 5 mV of the voltage setting and 0.1% + 2 mA of the current setting while the accuracy of the voltage and current settings are within 0.02% + 5 mV and 0.10% + 2 mA, respectively. Peak-to-peak noise and voltage variations or ripple are controlled to better than 3 mV. Model PWS2185 measures 8.55 × 3.57 × 14.24 in. (217 × 91 × 362 mm) without additional carrying boots and handle. It has a numeric keypad for data entry and an easy-to-read vacuum fluorescent display and has built-in memory for 20 operating setups.

In contrast, the Sorenson XG20-84 programmable DC power supply is rated for more than 1500 W output power from a compact 1U rack-mount enclosure. It has an output voltage range of 0 to 20 VDC voltage and current range of 0 to 84 A. It is powered by 100 to 240 VAC at 47 to 63 Hz, providing 82% efficiency from a 110-VAC source and 85% efficiency from a 240-VAC source. For computer programmability, several interfaces are provided, including USB, GPIB, and RS-232/RS-485 ports. The power supply has constant-current and constant-voltage modes and built-in sequencing capability to simplify powering routines. For simple setting, users can tune an analog control range from 0 to 10 VDC.

When higher voltages are required, the Chroma 62150H-600S DC power supply provides as much as 15 kW output power from a 3U rack-mount enclosure. It features an open-circuit voltage range of 0 to 600 VDC and short-circuit current range of 0 to 25 A. The power supply is well equipped with measurement circuitry and a general assortment of interfaces for connection to external computers, including USB, RS-232, and RS-485 ports as well as GPIB and Ethernet ports as options. In addition to providing better than 0.1% full-scale (FS) measurement accuracy, the built-in high-speed-digital (HSD) measurement circuitry can produce current-voltage (I-V) plots for display of output power behavior and responses to a DUT.

Easing the Loads

While DC power supplies are vital for energizing DUTs, DC loads help evaluate power conversion devices and systems, including power supplies and rechargeable batteries for electric vehicles (EVs). For flexibility, DC loads work in different operating modes, such as constant current (CI), constant voltage (CV), constant power (CP), and constant resistance (CR) modes. Electronic loads that can operate in master/slave configurations, with or without an external computer, provide the means of reaching a high-power/current level for a wide range of measurements.

The Chroma 63206A-600-420 high-power DC electronic load handles power levels to 6 kW along with 600 VDC maximum voltage and 420 A maximum current. It integrates a 200-MHz microprocessor for self-contained automation and programmability via front-panel controls that include a soft panel. It also includes GPIB, USB, and Ethernet interfaces for remote control using an external computer. This DC load provides low-, medium-, and high-power operating ranges so that current limits can be set at 42, 210, and 420 A, respectively, while working in static-mode voltage ranges of 1.4 to 600.0 VDC, 7.0 to 600.0 VDC, and 14.0 to 600.0 VDC, respectively. In addition to CI, CV, CP, and CR modes, it offers an “auto mode” that automatically switches among the four constant parameter modes to measure DUTs with dynamic behavior, such as lithium-ion batteries during charging mode.

The more conservative BK Precision 8510 DC electronic load is rated for 600 W output power, with output voltage range of 0 to 120 VDC and output current range of 0 to 120 A. It offers 1-mV resolution for regulated voltage output to 18 VDC in CV mode and 1 mA resolution for regulated current as high as 24 A in CC mode, with 10-mA resolution to full rated output current. It measures 17.30 × 3.46 × 18.07 in. (439.6 × 88 × 459 mm) and weighs 31 lbs. (14 kg). An AC line switch in the rear selects between 110- and 220-VAC operation.

As a way of boosting efficiency to 94%, the EA Elektro-Automatik ELR10080-1000 4U programmable electronic DC load employs an energy-recovery function to feed energy back into the local grid. It features a wide input range 342 to 528 VAC (and 208-VAC models available for the U. S.) for use on 380-, 400-, 480-VAC grids and delivers as much as 30-kW continuous power, with output voltage range of 0 to 80 VDC and current range of 0 to 1000 A. It also protects local grids using a novel water-cooling architecture to dissipate heat internally. The robust DC load integrates a 5-in. TFT color touch panel for local control as well as multiple USB ports and control software for Windows PCs for remote operation.

This is a small sampling of modern DC loads and power supplies for test applications. For more information on these and many other DC loads and power supplies, please visit the Axiom Test Equipment website at www.axiomtest.com.