“Power supplies come in an extremely wide range of ratings, physical sizes, and form factors. While typically optimized for size, efficiency, and cost (especially in power-constrained applications such as wearables), in some applications the power supply unit (PSU) needs to be its parameters are adjusted. This is especially true for PSUs in benchtop or automated test applications and environments.
Power supplies come in an extremely wide range of ratings, physical sizes, and form factors. While typically optimized for size, efficiency, and cost (especially in power-constrained applications such as wearables), in some applications the power supply unit (PSU) needs to be its parameters are adjusted. This is especially true for PSUs in benchtop or automated test applications and environments.
As a result, more and more power supplies are beginning to offer varying degrees of field flexibility, ranging from over-the-air firmware updates for efficiency optimization to always-on remote monitoring and control to ensure accuracy, scalability, redundancy and efficient Power array load balancing. Programmable features can speed product design and evaluation, enhance system functionality and provide the flexibility needed. However, among the ever-growing list of programmable options available, a few stand out in particular.
This article will explore the role, functions and features of the latest generation of advanced PSUs in self-contained enclosures that go far beyond typical self-contained, accurate and responsive power supplies. We then highlight the features, capabilities and subsequent benefits of the latest generation of fully networked, highly programmable PSUs, using products from XP Power as an example.
PSU vs. Open Frame Power Supplies
In many designs, the AC/DC power supply is built or squeezed onto the main PC board or a separate circuit board tucked into a corner. But in other products, a unique, independent and isolated power supply unit is required. Sometimes referred to as “rack mount” or “open frame” power supplies, these power supplies are self-contained and meet the necessary packaging, performance, and regulatory requirements. Many of these power supplies are available from multiple suppliers as secondary or alternative sources of form, fit and function.
These power supplies are represented by units such as XP Power’s UCH600PS36, which is a 36-volt, 4.16-amp (A), 600-watt (W) open frame power supply with no user interface, as it is not required (Figure 1). Instead, they are embedded into the final product, and once in use, users cannot adjust it. They have very few input/output connections: AC in, DC out, and maybe a remote sense lead.
In Contrast, engineering projects require a power supply with a flexible, easy-to-use interface implemented through a combination of switches, knobs, soft buttons, gauges, indicators, and even alphanumeric readout displays. These fully adjustable PSUs are designed for easy parameter adjustment, including output voltage, maximum current, and voltage/current limits, among other factors. They are used to meet the needs of engineering teams during the design, prototype evaluation and commissioning stages and are often referred to as “benchtop” or “lab” power supplies. For convenience and neatness, they can also be rack-mounted in a fixed semi-permanent configuration as part of automatic test equipment (ATE) or other long-term installations (Figure 2).
Today’s PSUs have the same basic functions as PSUs of a few decades ago, but the demands they need to meet are much more complex. In addition to basic voltage and current readings and manual adjustment of output voltage values, the PSU must enable other manually controlled functions and provide remote access.
PSUs like XP Power’s PLS600 Series Programmable DC Power Supplies do just that, adjusting operation with convenient, organized front-panel controls and a variety of rear-panel connectivity options, including USB, Ethernet, and analog parameters (Figure 3). In addition, the PSU must monitor its own condition and load condition and report the condition directly remotely as needed and in the event of an exception to maintain control over the device itself and the larger system.
The functions of the front panel (1 to 7 in Figure 3) are described in more detail in the user manual, in ascending order: power on/off; current setting; voltage setting; output on/off; Display; And the power output lead socket.
Full Programmability Brings Additional Benefits
Claiming that a PSU is “programmable” is one thing, but it’s important to figure out what that means for modern PSUs. First, the PSU must have a user-settable output voltage, not a fixed output voltage; in many cases, the PSU can also function as a user-settable current source. For convenience, the values of these main parameters can be easily adjusted from the front panel as needed. When paired with a digital readout, the rotary controls remain the most comfortable way to quickly set, adjust or “fine-tune” target value settings.
Other parameters that the user can set include the important Over Voltage Protection (OVP), Over Current Protection (OCP), and even Over Power Protection (OPP) values. The OPP value is useful for applications where the “worry” is not the 600 watt power limit of the PLS600 PSU, but the maximum amount of power (voltage x current) that the load is allowed to draw from the power supply to prevent damage to the power supply.
Often, after making various adjustments to voltage, current, power, or other setpoints under time pressure and debug and test stress, users may neglect to record the values they actually set for these factors. For this reason and others, the PLS600 PSU allows quick Display of parameter values. In addition, parameter values are stored internally, so there is no need to re-enter them at power-up.
This basic programmability is only the first aspect of a truly versatile PSU. For many test and evaluation situations, it is necessary to have the power supply execute predefined real-time “scripts” independent of network connectivity. To this end, the PLS600 series offers advanced integrated scripting capabilities that allow users to write custom programs to generate user-defined output profiles to meet a variety of unique requirements and upload them to the power supply for execution on command.
This allows the power supply to play an advanced role in a larger system and thus be an effective element in product performance sequences or advanced life cycle testing such as Highly Accelerated Life Testing (HALT), etc., and may aid in discovery and final product Minor exceptions related to power subsystem characteristics.
Connectivity and control capabilities from simple manual operation to advanced networking settings
While a desktop PSU should have front panels for basic and instant access, user-friendly hand-operated controls, these alone are not enough for efficient system-level power. In addition to the convenient rotary voltage and current adjustment controls, the PLS600 series supports remote control via USB, Ethernet and analog control inputs.
Analog control may seem outdated, but it allows for direct and easy setup of basic remote control scenarios, which may be required in some traditional situations. Note that benchtop instruments typically have a longer lifespan and there are IEEE-488 General Purpose Interface Bus (GPIB) units still in use. Analog control is also convenient when the power supply is used in a closed-loop feedback configuration, where the supply voltage must be adjusted in real-time based on some sensed or derived voltage.
In addition to basic analog control, all PLS600 PSUs are LAN Instrumentation Extensions (LXI) certified, thus meeting the interoperability standard for LAN-based instruments. Standard LabVIEW and Interchangeable Virtual Instrument (IVI) drivers work with all standard software. These units support Standard Commands for Programmable Instruments (SCPI), as well as user-developed SCPI-based software. The USB and Ethernet inputs are SCPI compliant and LabVIEW drivers are available on the National Instruments website. To ensure confidence in setup and readback values, the PSU includes embedded 12-bit digital-to-analog and analog-to-digital converters for accurate measurement and reporting of voltage and current.
Combining remote networked settings with the ability to change values manually or under program control and reporting on power status and alarm conditions is a huge plus. It reduces the need for engineers to “watch” the DUT test and find and correlate exceptions as they occur. When combined with instruments such as data loggers or digital oscilloscopes with deep memory and suitable triggers, it is suitable to perform long-term tests and then download the results for a more comprehensive analysis.
Solve Remote Inspection and Calibration Problems
All current-carrying leads and power rails are affected by the current resistance (IR) voltage (V) drop. Basic calculations based on Ohm’s law (V = IR) show the severity of the problem. As a result, the voltage delivered on the load can easily be lower than its nominal value on the power supply, ranging from a few millivolts to tens or even hundreds of millivolts.
One way to account for this drop is to compensate by increasing the nominal voltage at the PSU by an amount equal to the drop, but this is considered a bad practice because the IR drop is a function of the current drawn, Hence there are fluctuations. Thus, sometimes the voltage at the load may actually be too high when the current and resulting IR drop is small.
For this reason, a common solution is to use remote sensing by adding two extra wires to the Kelvin sensing configuration. In this configuration, the actual voltage on the load is sensed and fed back to the power supply to dynamically adjust the output so that the voltage on the load always has the desired value. This widely used solution has become accepted standard practice and generally works well, but it does have some drawbacks.
First, two extra leads are required, which may seem trivial but adds to the clutter of the bench. Second, adding two additional low-resistance contacts at the load is not always easy, especially if the design of the load contacts cannot accommodate these contacts. Anyone trying to connect a 24 AWG sense lead to a screw or other terminal designed for 14/12/10 AWG current-carrying power rails has had difficulty.
Finally, the two extra sense leads may appear to be passive wires, but they are not. Electrically, they form a feedback loop for an amplifier, which happens to be a power supply. Anytime such a feedback loop exists, there is the potential for noise pickup or even oscillation due to unconstrained and often poorly defined loops. Therefore, while the remote sensing technique can solve the problem of IR drop, it can also lead to a larger hidden danger of power supply output oscillation. More filtering of the correct type may be required, but such filtering may also alter the dynamic transient response of the power supply and degrade the corresponding performance.
Remote Detection – No IR Drop Sense Leads
To avoid the mechanical, electrical and even aesthetic issues associated with remote sensing, the PLS600 series offers an alternative method of digitally compensating for these resistors using proprietary techniques without the need for any additional wires. Briefly, the user activates remote detection mode from the front panel, shorts the load wires at the load, and sets the PSU current to be at least as large as the load is expected to draw (Figure 4).
The PSU measures the output current and total voltage drop in the load wires, and then calculates the resistance of the load wires. Next, the PSU can adjust the output voltage on its power terminals in real time to correct for voltage drops in the load cables. Thus, a separate sense lead is no longer required in actual installation.
Premium PSU also provides calibration flexibility
While PSUs like the PLS600 series do not normally require calibration, in some cases verification of the unit’s output voltage performance is required, as well as some calibration adjustments. To calibrate the output voltage and current and the displayed voltage and current, the PLS600 Series requires a calibrated voltmeter and a calibrated shunt.
Set the PSU to calibration mode and leave its output open only if the voltmeter is connected. Simply put, the displayed value of the PSU matches the value of the voltmeter, and pressing the PSU panel button registers these values. Next, connect the shunt across the output and connect a voltmeter to the shunt. Then adjust the PSU output until the external voltmeter reads exactly the current shown on the power supply display (Figure 5). Note that, according to Ohm’s law, the voltage displayed on the gauge also depends on the value of the shunt used.
How to get more voltage or current
While the PSUs in the PLS600 series offer a combination of voltage and current ratings, there are undoubtedly situations where more use of one or both of these parameters is required. An obvious solution is to use a larger power supply, but this has the disadvantage of increasing cost. Since this need may only exist for a short period of time, it is difficult to justify it. Another alternative is to consider connecting two or more PLS600 PSUs in series for more voltage, or in parallel for more current.
However, getting more voltage or current is not just a matter of connecting or paralleling two power supplies in series. When two PSUs are combined in this way, one of three things can happen:
This configuration does not provide the desired output, control is lost, and the power supply may be damaged
The configuration works to some extent, but it does not achieve the desired performance, accuracy, consistency, or confidence
This configuration works due to luck (usually not good engineering strategy) or due to careful design
Results 1 and 2 are neither desirable nor acceptable, but there are ways to compensate for their shortcomings to some extent with some carefully selected external components such as shunt resistors or blocking Diodes (Figure 6) . A similar scheme is also used for voltage pairing. Even if the configuration is feasible, the overall performance will be limited by the specification of the smaller of the two power supplies and the mismatch between the added components and will cause performance degradation due to these components.
So the general idea is that using a single power supply in an application that is rated to match the application’s requirements causes far fewer problems than using two or more power supplies in parallel or in series. But if these power supplies are designed for series or parallel operation, the “normal operation” target result 3 can occur, as is the case with the PLS600 series PSUs.
To place PLS600 PSUs in parallel or in series, one of the power supplies must be set as the master and the rest as slaves. Up to two power supplies can be connected in series (and they must be the same) to boost voltage, and up to four identical units can be paralleled to increase current. The setup and assignment of master and slave devices is done through the front panel controls, but it must be understood that there are some maximum limitations for safety and performance reasons.
Improve Convenience, Compliance, and Efficiency with Racks and Stacking
In terms of the visual appearance of the engineer’s workbench, some are pretty neat, others are incredibly cluttered. The status quo is that many workbenches start out neat and tidy, but tend to gradually become “cluttered,” and single or multiple PSUs and their leads add to this level of clutter. In other cases, the PSU is provided as part of a rack-mounted instrument package for one of several reasons:
It is part of an independent ATE or long-term assessment program
Provides system integrity and enhances reliability by ensuring that all components are placed where expected and all cables are routed and stress relieved
Requires shipping and eventual reinstallation
For these reasons, XP Power offers the PLS600 Rack Mount Kit for the PLS600 PSU (Figure 7).
Since all members of the PLS600 series have the same housing size, this kit is suitable for all members. Installing a PSU with this kit is a quick and easy task, and the kit allows two PSUs to be installed side by side.
Desktop power supply units are distinct in form and function from embedded power supplies and provide multiple user controls or adjustments. Benchtop or “lab” PSUs are a must-have tool for prototyping, debugging and testing, and fixed-position test benches. Well-designed and feature-rich laboratory PSUs, such as XP Power’s PLS600 series PSUs, not only provide outstanding performance, but also have other features required for efficient and flexible use of PSUs, including convenient front panel controls, networked access and script-driven programmability, etc.
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