You need some DC power supplies for your new program. There must be 1,000 or more models available from more than two dozen suppliers. How do you choose the best one for your application?
In the broad view, three factors are involved in selecting a DC power supply. The first is generic. You have commercial input power, and you need certain DC output voltages at specific currents, regulated to meet your requirements even with line and load changes. The second factor considers physical definitions such as size, package configuration, displays, and compatibility with the ambient conditions in your laboratory or test area.
In many cases, these two factors are all that matter. Hundreds of models are available at reasonable prices, and each performs the basic function of converting commercial AC power into well-regulated DC outputs.
The third selection factor personalizes the power supplies to your requirements. They should be application-friendly, meeting your need for:
Multiple voltages to power the units under test in the system.
Compatibility with surrounding equipment in the system so they do not interfere with proper operation.
Communications with the system controller/monitor, typically your computer, in both directions.
Supplying power to devices now and later.
Responding to and providing status and other information for operators and maintenance personnel.
“Customers continue to demand solid value in their test equipment,” said Herman van Eijkelenburg, product marketing manager at California Instruments. “This does not necessarily mean more features but rather the right set of features to solve application-specific problems.”
The Friendly Features
The right set of friendly features is unique for each application. But some common ones are multiple outputs in one unit, compatibility with surrounding equipment, a close relationship with the system controller, the capability to handle new types of circuitry, and ease of operation with simple controls and meaningful diagnostics.
Multiple Outputs
The common DC power supply is a general-purpose device offering a single, regulated output. But often, you must purchase two or three units to supply the different voltages required to operate a system.
Because separate supplies occupy much needed rack space, some manufacturers offer a multiple-output supply. Protek, for example, has the 3015B and 3032B Power Supplies, each with two independent ±30-V outputs and a front-panel readout of voltage and current. These cater to the requirements of analog circuitry and products that need both positive and negative voltage. The 3033B adds a +5-V output for cases where the application requires high current for conventional digital circuit development and test.
With multiple outputs, several companies offer a master-slave arrangement so two outputs can be used in parallel to double the available current. In some cases, two outputs can be used in series to double the available voltage.
Surrounding Equipment
The DC power supply must coexist with surrounding equipment and personnel. This involves compatibility with electromagnetic radiation restrictions as well as conformance to safety standards. Manufacturers marketing their power products in Europe are keenly aware of the need for such compatibility, better known as the CE Marking.
Today, many U.S. products are designed to meet the specifications for the CE Marking. Hewlett-Packard’s Jane Wu, product manager for power products, explained, “It isn’t too soon to make sure that the products you design today meet 2001 CE regulatory requirements for standardized use of public AC mains systems. If they don’t, they’ll have to be redesigned to comply in the very near future.”
The System Controller
In many cases, a programmable power supply is an integral part of a computer-controlled system. Operating voltages must be changed frequently and automatically as part of a test sequence, and the voltage and current must be monitored and recorded at certain points in the test sequence.
Control via the RS-232-C interface is one approach. A faster, more popular method uses the IEEE 488 bus and its command protocol, the Standard Commands for Programmable Instruments (SCPI). This arrangement provides simultaneous commanding and monitoring, allowing the system computer to step through a test sequence quickly and accurately.
New Devices
Since +5 VDC has been the conventional supply for digital circuits, many power supply manufacturers offer models with +5-VDC output and a very narrow range of adjustment, such as 4.75 V to 5.25 V. But as Jerry Price, vice president of Power Ten, noted, “With new semiconductor architectures allowing devices to run at lower and lower voltages, many customers need the precise, programmable voltage capability of only 3 VDC. With the new P63C-31800, we offer up to 1,800 A at 3 VDC.”
Operation and Diagnostics
Incorporating a microprocessor in the power supply gives you a wide range of operational and diagnostic parameters for display. It also enhances your ability to get the most from the supply and intercept operational problems before they become acute.
The Sorensen/Elgar DHP Series of power supplies, for example, displays
output power, voltage, and current; all set points and limits; current and maximum AC input voltage; diagnostics current and highest air-inlet temperature; and fault report to cover microprocessor status, maximum air-inlet temperature, maximum AC input voltage, and the integrity of each internal module.
The availability of this status information is especially useful when you have an interactive system controller. In a high-volume, high-speed production application, you can develop software to monitor these parameters at computer speeds, making maximum use of the power supply’s self-diagnostic capabilities.
Another feature in some supplies is an audible alarm that is accessible to the system controller and visible to the operator. This is especially useful in a high-speed automated test system where most routine transactions are handled without operator intervention.
Some microprocessor-controlled DC power supplies store several setups for quick recall which can speed the production-line response to a change in test sequences. A subset of this capability restores the previous setup at the start of the day or after a power outage.
One example of operator-friendliness is the Hewlett-Packard 66309D Mobile Communications DC Source which supplies all the tools needed for production testing of mobile phones in one unit. Two DC outputs at 0 to 15 V and 0 to 12 V support testing of the main battery power and the battery-charger circuits. A built-in auxiliary digital voltmeter gives you the capability to measure voltages within the phone, switching voltages, or other parameters. The equipment also detects open sense wire connections automatically.
Other examples are the Keithley 2300 and 2400 High-Speed Precision Power Supplies with programmable output impedance to simulate various battery types in different discharge states. The 2306 also simulates a battery charger. The 2303 and 2304A handle heavy pulse loading, and the 2430 is a DC source and meter with the capability to generate pulses of varying widths and up to 1,000 W power.
The Source of New Designs
Looking again at the selection of an application-friendly DC supply, we see that power supply designers have been doing their homework. The equipment is being revised frequently to offer more interesting options, as evidenced by the sampling of DC power supplies found in Table 1.
You can be a part of the design team for the next family of power supplies. Power supply design engineers want to stay abreast of industry needs, and this is possible only if they hear from you. Next year’s DC power supplies will be based on this year’s phone calls, letters, and e-mail messages.
AC Power Sources
For applications where AC power sources are required, such as in testing DC power supplies, some of the most recent models and their features are provided in Table 2.
Copyright 1999 Nelson Publishing Inc.
September 1999
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