Selecting the right power supply for a circuit-evaluation project or a new production test setup seems to be so straightforward that almost no one gives it the attention it deserves–until the supply proves to be the wrong choice. To prevent future frustration, we are presenting a refresher on DC power supply performance parameters, including their definitions and potential effects on UUT or system performance.
The Obvious Choices
Voltage and Current Range–These parameters are most easily established since they are purely a function of the circuits to be powered. If the supply is to provide only a fixed DC voltage needed to power the UUT, consider whether you will want to test the UUT at high- and low-supply voltage extremes.
Most fixed output supplies enable you to change the output voltage by +10%. If this is not adequate, a fixed output supply providing greater variations or a variable-output supply is needed.
If the supply is to be used for a variety of test setups, predict the maximum current it must provide for 75% to 90% of your applications. For the remaining needs, you can parallel two or more supplies. Supplies used to power semipermanent test stands should have a current capacity of at least 125% of anticipated requirements to accommodate limited expansion.
Operating Mode–Almost all DC supplies normally operate in the constant-voltage mode; that is, they provide a settable constant voltage output regardless of the amount of current drawn–until a predetermined limit is exceeded. Many also can operate in a constant-current mode, again within limitations.
Limits are not only imposed by maximum allowable voltage or current handling capabilities but also by the maximum power the supply can deliver. In an autoranging mode, the supply will track the V*I product and automatically set voltage or current limits.
A few supplies can also function as electronic loads. In this mode, they can test other voltage- or current-producing sources.
Response Time–If the supply is to be remotely controlled or to be part of an automated test system, the time lag between issuing a command (for example, turn-on or change voltage) and obtaining the desired output may become a significant selection factor. Response time usually ranges from a few milliseconds to fractions of a second. A few supplies feature extremely fast slew rates (>1V/µs) and can be used as programmable high-power arbitrary waveform sources.
Resolution–The smallest voltage or current increment to which the output is settable is determined by this specification parameter. Some supplies provide the same resolution over the entire output range; others feature resolutions that vary as a function of the selected output range.
Accuracy, Convenience, Expansion and Safety
Remote Sensing–To enable very accurate control of voltages or currents at the load, a supply with remote sensing capability provides a feedback path to its regulator circuit via separate sense lines. This feature compensates for any voltage drop that may occur between the supply and the UUT.
Programmability–Some supplies feature analog programmability of voltage or current via a variable DC voltage or resistance value. For applications involving several predetermined test scenarios or sequencing of output values, control via IEEE 488 or RS-232 is appropriate. A few supplies also include a multiple instrument setup storage capability, a feature that speeds bus-programmed or bench operation.
Parallel or Series Operation–When one supply cannot provide the full voltage or current levels required, two or more supplies (or multiple sections of some multiple output supplies) can be connected in parallel or in series, provided they are designed for such operation. In this case, continuous communication between the regulator and the control circuits of the interconnected supplies is maintained, with one supply acting as master and the others as slaves.
Overload Protection–Supplies often provide power to a variety of circuits, the current consumption of which may be unknown. To avoid damage, a range of protection circuits is in use.
Almost all supplies limit the output or turn themselves off when maximum allowable values are exceeded. Many have programmable limits and some include an automatic voltage-current regulation crossover feature. This feature changes the manner of regulation from constant voltage to constant current, and vice versa, when settable limits of either are exceeded.
Added protection diodes can prevent damage due to inadvertent connection of the supply to external sources with reversed polarity. Heat sensors are also included in some units to avoid damage to the supply due to prolonged overload or cooling failures.
Inherent Potential Detriments
Ripple and Noise–Ideally, DC supplies furnish pure DC. However, some undesirable remains of the rectified line power (referred to as ripple) plus high-frequency repetitive transients, in the case of switch mode power supplies (SMPS), will always be present. These two components plus any other noise spikes generated by the supply are usually referred to as periodic and random deviation (PARD).
The amount of PARD that can be tolerated is very much a function of the noise sensitivity of the UUT. If high-gain amplifiers are part of the circuitry to be tested, it is likely that any PARD frequency components falling into the bandwidth of the circuit will be amplified. If the PARD is of significant magnitude and no special filtering is provided for the first amplifier stage, the resulting interference can create havoc with test results.
For linear supplies, the ripple voltage is usually the most prevalent component of PARD and is specified as an rms voltage or as a Vrms/Vout percentage. For switching supplies, Vpeak-to-peak provides a more meaningful measure or purity; however, manufacturers of linear as well as switching supplies may provide only rms or only peak-to-peak or both values.
Regulation–As line voltage or load current fluctuates, so will the output voltage of a DC supply. The extent of this fluctuation depends on the characteristics of the regulator circuit, the energy stored in filter capacitors and energy consumption rates.
If the input to the power supply comes from a relatively constant source, minimal line regulation is required. If the circuit to be powered is relatively insensitive to voltage changes or represents a nearly constant load, minimal load regulation will suffice. Regulation is usually specified as a percentage change of output when stepping from no load to full load or as line voltage changes between defined limits.
Internal Impedance–A relatively high internal supply impedance, as seen by the load when looking back into the supply terminals, can be detrimental in two ways. First, it will contribute to poor load regulation. But more importantly, any fluctuations in the current drawn by the load will produce voltage fluctuations superimposed on the DC output. These fluctuations have the same effect as ripple and noise emanating from the supply and can be amplified by the circuit under test and cause erroneous results.
Load Transient Response or Recovery–Values associated with these terms indicate the degree to which the supply’s regulator circuit can cope with sudden changes in output loading. There are two elements to consider when transients are induced by sudden load changes: the output deviation and the response time required for the output to recover and return to its original value.
These parameters are usually specified as a peak-voltage deviation in millivolts and recovery time in milliseconds, both for a 10% change in load. Some suppliers, however, specify recovery time for greater load current changes, such as a 50% to 100% rated output current change.
Ancillary Issues
Power supply technology is still evolving. “The factors spurring this evolutionary phenomena include continuing advancements in switch mode technology plus customers’ increasing demands for more complex programming capabilities, higher quality standards and competitive challenges,” remarked Chrystal Shaw at Xantrex.
“In the past, many customers refused to consider SMPS for many applications because of perceived or actual performance limitations,” Ms. Shaw continued. “But their numbers are decreasing as SMPS have become more sophisticated and approach linear noise and response levels.”
Several techniques are applied concurrently to bring about these performance improvements. For instance, “Low ripple and noise are being achieved in our Series 6680 DC power supplies through controlled slope switching, extensive filtering and careful component layout,” explained Terri Lynch, Product Marketing Engineer at Hewlett-Packard.
But there are still applications for which linear supplies are preferred, cautioned Ms. Shaw. These may include powering or testing A to D and D to A conversion circuitry, lab- grade analog measuring equipment and sensitive RF applications.
As to demands for better programmability, the power supply industry is responding by providing suitable interfaces and creating application software. “This involves supporting emerging and de-facto virtual instrument environments as well as providing compatibility with standards such as IEEE 488.2 and the SCI instrumentation protocol,” said Norm Precourt, Vice President of Engineering at Elgar.
A case in point is the Hameg HM8142. “It is available with programmability-oriented options such as IEEE 488 or RS-232 interfaces as well as drivers for LabWindows, LabVIEW and PowerLab,” said Fred Katz, Chief Engineer of Hameg.
But power supplies should not only facilitate sophisticated remote-control operation, but also allow the engineer to perform setups and conduct experiments from the front panel, emphasized David Pereles, Applications Engineer at the Diagnostics Tool Division of Fluke. They should also be easily programmable and recover gracefully from overloads.
Not everyone requires remote control, and individual demands vary widely. A representative sampling of currently available DC power supplies with their key specifications and features is provided in Table 1.
Most of today’s DC power supplies are off-the-shelf units. But rather surprising is the increasing demand for custom or modified power supplies.
“Some companies have needs that can only be addressed with custom units,” said Bob Kral, Product Manager of B+K Precision, one of the companies that specialize in custom power supplies in addition to its extensive catalog offerings. “Our requests range from blank front panels to analog programmability.”
For applications where AC sources are required, such as for testing DC power supplies, a sampling of recently introduced models and their features is provided in Table 2. If the DC supplies you select will be used outside the United States, they must comply with various regulations, including those imposing limitations on power factor, harmonic content and inrush current. AC power sources are essential to verify compliance with these specifications.1
References.
1. Jacob, G., “Electronic AC Sources,” Evaluation Engineering, September 1994, pp. 45-53.
Instrumentation
Copyright 1995 Nelson Publishing Inc.
September 1995