You can't physically bend a benchtop DC power supply or AC source, but in all other senses of the word, they are flexible with a capital F. Sure, you still can buy a simple, dumb, single-output supply. But other than to save a few dollars, why would you? Even a $300 DC supply offers remote programmability; on-board memory to store V-I settings; constant power output characteristics; and overvoltage, overcurrent, and overtemperature output protection.
Generally, these capabilities and advanced features such as digital closed-case calibration and fast pulse outputs have been developed to address real customer requirements. Of course, if one company's power supplies or sources are gaining market share, their features soon will be copied. Nevertheless, there is an underlying demand for most of the newer functions that is dictated by specific applications.
For example, semiconductor testing needs short power pulses to avoid self-heating that would alter the device characteristics being measured. And, there is continuing emphasis on battery life in portable equipment. To minimize power consumption, many small design changes are made, and at each step, it's important to accurately measure the load voltage and current.
In addition, power supplies and sources have benefited from newer semiconductor devices and control techniques that have made possible higher efficiency, faster transient response, and greater peak power in a smaller or lighter-weight package. At first glance, these characteristics and a steadily increasing list of standard features such as programmability, a universal input voltage range, and accurate regulation may appear to be only evolutionary. Some things are, but among the specifications are some truly innovative advances.
Noteworthy Innovations
Constant Power
When you specify a power supply, you have to address the present requirements as well as how they may change in the future. The move from 5-V digital ICs to 3.3 V is a good example. If you bought a conventional 0-V to 5-V, 10-A, 50-W supply to handle the 5-V ICs, you still are limited to 10 A for the 3.3-V devices. Newer supplies with a constant power mode support the maximum output current for a 50-W rating or 15.15 A in this example.
There are limits, so don't expect 100 A at 0.5 V from the same 50-W supply. But within the manufacturer's specifications, you can trade volts for amps, keeping the maximum power constant. Kepco's Series KLP features Hyperbolic Power™ technology, a descriptive name for a constant power mode. For Model KLP 75-33-1.2K, the supply's 1,200-W maximum power can support any combination of voltage and current between 75 V @ 16 A and 36 V @ 33.3 A. The -1.2K suffix indicates a built-in LAN port, a standard feature of the LXI-compatible KLP Series.
To View the DC Power Sources Comparison Chart click here.
To View the AC Power Sources Comparison Chart click here.
Chroma ATE's 62000P Series has a constant-power operating envelope also reflected in the model number. The 1200W/80V/60A can provide 1,200 W output power at any combination of voltage and current bounded by 80 V @ 15 A and 20 V @ 60 A. And, B&K Precision's 100-W Model 9110 offers a wide operating area from 60 V @1.66 A to 20 V @ 5 A.
The Agilent Technologies N675XA and N676XA DC Supplies use the term autoranging to describe the constant power mode. For all constant power supplies, the locus of allowed V-I combinations is a hyperbola.
The DC Power Supply Comparison Chart that accompanies this article includes the maximum voltage available from the lowest to highest voltage model in a series. Similarly, the current corresponds to the maximum available from the models in that range. For supplies with a constant power mode, the power is listed as 100 W max, for example. The maximum qualification is not used with conventional supplies.
Universal Input
DC supplies and AC sources often can accept more than one range of input voltage. Typically, a choice must be made between 115-V or 230-V nominal ranges although this may be an automatic selection made by the instrument when power is applied. Larger supplies and sources generally operate from three-phase power. Sometimes, as with the Pacific Power Source AC sources, many input voltages are available—100, 110, 120, 200, 208, 220, 230, and 240 to list just the single-phase voltages.
In the AC Sources Comparison Chart, several series of products have been listed. In one case, the entry for a series was subdivided into groups of models. This approach allowed a distinction to be made between lower-power units that can operate from either 115-V or 230-V and higher-power units that must have a 230-V input. In other cases, the products in a series are treated as a group, but because two output voltage ranges were available, specifications were listed for both conditions.
Agilent's N67XX DC Supplies feature a universal input rated from 86 VAC to 264 VAC at 50 Hz to 400 Hz. There are no switches to set or fuses to change when the supplies are used in different countries or in test systems running from different AC voltages. Lambda America's compact ZUP Series operates with inputs from 85 V to 265 VAC. Equipment with a truly universal input range works well at either extreme and anywhere in between.
Aside from the convenience of not having to manually configure a supply for a particular input voltage, a universal input range has an important benefit. If the power supply or source is used with a 230-V input, it will continue to operate correctly down to about 40% of the nominal value. No selectable input configuration offers that degree of immunity to AC supply dips or sags.
Power Factor Correction
IEC 61000-3-2 sets limits on harmonic currents for products that draw less than 16 A per phase but more than 75 W. Professional equipment that consumes more than 1 kW is excluded. However, many DC supplies and AC sources that use switch-mode technology to reduce size, weight, and cost are covered by the standard.
Typically, switching supplies create a raw DC bus by rectifying the input AC voltage. The usual capacitor-input configuration draws current pulses near the peaks of the voltage sine wave, producing harmonic distortion. To reduce the distortion, power factor correction (PFC) circuitry attempts to make the input current resemble a sine wave. Active PFC schemes can be very effective, producing power factors near unity. Passive PFC may be less expensive on lower-power products but also is less effective and not suitable for high-power applications.
Data Acquisition
DC supplies and AC sources with a readback feature are capable of responding to a controller or PC with a measured value of output voltage and current. To do this, the equipment must have a built-in measurement system with a digital output. A readback capability is not new, but the responsiveness of today's faster measurement systems is.
Agilent's recently introduced Model N67XX DC Power Analyzer takes advantage of the 50-kS/s sampling rate of the measurement ADC in the N67XX supplies. Up to four N67XX power supplies can be housed in the analyzer mainframe and controlled from a front panel with an integral color display. Individual outputs can be programmed to turn on/off in a particular sequence with specified timing and levels.
In addition to fast measurements, the N67XX supplies also have sufficient output bandwidth to function as arbitrary waveform generators (Arbs), directly generating user-defined disturbances. Supplemental characteristics now specify a 3-dB frequency bandwidth at five output levels for many of the supplies in the series. For example, the 35-V Model N6774A 300-W Supply achieves 125-Hz bandwidth at 0.35-V or 0.7-V pk-pk but only 40 Hz at 3.5 V and 20 Hz at 35 V.
As useful as the power analyzer is, its performance depends entirely on the power supplies it uses. Agilent promotes the instrument as combining a DMM, oscilloscope, Arb, and datalogger, but these capabilities are only as good as those of the individual power modules. You do not get a 10-MHz bandwidth general-purpose scope although the 10-kHz bandwidth corresponding to a 50-kS/s sampling rate is helpful.
If you're only concerned about the changes occurring to the power driving your DUT, the power analyzer can do the whole job. It's likely that you would use it to control the power sequencing and deliberate disturbances needed to test a DUT at the corners of its input power specification. However, you will need a conventional scope to monitor points of interest in the DUT circuitry to determine unusual behavior such as high-frequency oscillation. Only sampled supply output voltage and current can be viewed on the analyzer display.
Fast Transient Response
Fast transient response is important in many applications, portable device development being a very popular one at the moment. Many of these products can transition almost instantaneously from a near-zero-power sleep mode to a full-power-on state. The trade-off encountered when designing a wide-bandwidth power supply output stage is stability for a range of loads. Digital control is attractive because it can tune the output characteristics to the load, reducing transient voltage droop and the time to recover to large load changes.
Agilent's Kevin Cavell, product manager for power products, explained some of the techniques used to achieve the required performance: “Our most recent products use digital control to protect the end customer's load through digital-based monitoring of key facets of the power supply. We also use digital control in the most complex way, which includes digital regulation of the output voltage and current through both linear and digital feedback systems. And, digital control is used for power management for redundancy and budgeting in limited power environments.”
Specifications
Accuracy
Although many data sheets for DC supplies and AC sources are well written and comprehensive, some are not. A common failing is to assume that the reader will understand the meaning of 0.1% accuracy. If every one meant 0.1% of the actual value, that would be fine. Unfortunately, it's very convenient to create overall specifications for a series of power supplies or sources based on percentage of rated output or range or full scale—the same things.
It's typical for a higher voltage output to have greater ripple and noise than a much lower-level output. Specifying ripple, noise, or accuracy based on a full-scale output covers the whole series of products in a simple one-line statement. A problem occurs when it's unclear if the percentage relates to full scale or the actual output value, and it can make a big difference.
Periodic and Random Deviations
On another topic, consider the periodic and random deviations (PARD) Vrms and Vpk-pk values listed for several DC supplies. PARD generally is measured in a 20-MHz bandwidth.
Immediately obvious is the distinction between bulk DC supplies and more closely regulated precision supplies. Chroma Systems Solutions' Series 62000B quotes up to 200-mV pk-pk PARD. The 62000B Series of 1.5-kW modules can be configured to provide up to 120 kW. In contrast, precision supplies may have only a few millivolts of PARD whether measured as rms or pk-pk.
It's interesting to note the wide range of ratios between listed rms and pk-pk PARD values. A general rule of thumb for relating rms and pk-pk is to use a factor of about 6:1. Mathematically, there is no direct relationship, because if the PARD truly is random, eventually the maximum pk-pk value could be very large. Practically, 6:1 is a reasonable value. Nevertheless, for companies that list both rms and pk-pk PARD, the factor varies greatly.
Chroma's 62000P Series quotes 15-mV rms and 100-mV pk-pk, close to 6:1. In contrast, Kepco's KLP Series has 10-mV rms and 125-mV pk-pk, a 12:1 ratio. Protek's Model 6006S has 5-mV rms vs. 100-mV pk-pk, a ratio of 20:1. Perhaps the larger ratios indicate that more of the noise is related to the switching frequency and consequently not random. Very narrow noise pulses add little to the rms value but greatly influence the pk-pk measurement.
Output Flexibility
There's nothing special about the polarity of a DC supply unless you need the other one. Many supplies specify ??Vout, and they mean it. The outputs are floating and can be configured for either polarity of operation. In contrast, some multi-output supplies may connect all the low output terminals to the same ground reference, eliminating the flexibility you have with separate supplies. It's a small point that may not even be highlighted in the data sheet other than to list the output rating with a ?? prefix.
A similarly easy-to-overlook specification relates to supply output parallel operation. Especially given the trend to higher-current, lower-voltage ICs, paralleling N power supply outputs often is done. Active current sharing ensures that each supply provides 1/N of the total. Many supplies can be operated in parallel, but only a careful reading of the data sheet will reveal how the output currents will be distributed.
Standard vs. Optional Features
Confusion between what is and isn't claimed and under what conditions is especially prevalent in AC source data sheets. Some of these products can provide virtually any waveform you may need at a wide range of power levels. The problem is that you may require a special controller or software to access all the capabilities.
Understandably, a manufacturer wants to highlight the functionality, accuracy, flexibility, programmability, and applicability of its product. And, many times, new features are provided in software or through a controller firmware upgrade. But all the great new capabilities often displace basic specifications, making the data sheet less informative than it should be. Make certain that the unit fundamentally meets your requirements, and then determine if the extra capabilities might be useful in your application.
Summary
Table 1 summarizes a long-term trend toward smaller, lighter DC supplies and AC sources. Advances in switch-mode power control are largely responsible, together with microprocessors, better semiconductor processes, and improved high-frequency transformer and choke materials. Although this table shows basic changes, it doesn't indicate the greatly increased capabilities of these products that were gradually added over the years.
According to Lambda Americas' Product Manager for High Power John Breickner who developed the table, “Users increasingly are using digital control via serial, GPIB, or LXI interfaces. They usually write their own system software but turn to the power supply manufacturer for drivers as the building blocks peculiar to a specific supply. It's most convenient for users to have a series of supplies or sources with identical drivers and the same command structures.”
Certainly, it can be advantageous to use the most efficient, smallest, and lowest-cost product that will do the job. On the other hand, surprises may be lurking in a test application just waiting to catch the unwary engineer.
Mitchel Orr, sales manager at Pacific Power Source, cautioned, “The linear amplifier type of AC source still has its place, outperforming many switch-mode designs with its lower output impedance and faster load transient response. The uninformed test engineer frequently selects a product that's too large or small for his application and may not understand the important differences between switch-mode and linear technology.”
Table 2 gives a direct comparison of the company's AMX linear and ASX switch-mode AC sources and emphasizes the points Mr. Orr made. To be fair, the transient response figures are only typical values, but the AMX Series boasts a 50-kHz small-signal bandwidth compared to 5 kHz for the ASX Series. The AMX also can drive a more diverse range of load impedances with its high 6:1 peak-to-rms current capacity. The ASX is a fine series of products and appropriate for many applications. Nevertheless, there are differences in the characteristics that linear and switch-mode technologies support, and they could be important in your application.