Conquer design challenges: Skills for power supplies

In the dynamic world of engineering design, the escalating requirements placed on power systems frequently give rise to intricate design challenges. The evolving landscape of DC power systems introduces complexities that can manifest stumbling blocks in the design process. Fundamental skills related to power supplies play a crucial role in mitigating these challenges.

Today’s advanced DC power systems are not immune to design problems, and a solid foundation in power supply knowledge can prove instrumental in navigating and overcoming hurdles. Whether it’s discerning the intricacies of device under test (DUT) voltage or current, addressing unforeseen temperature fluctuations, or managing noise sensitivity; a fundamental understanding of power supplies empowers designers to identify and tackle the nuanced issues embedded within a power system.

Understanding constant voltage and constant current

One of the most important concepts for anyone using power supplies is understanding constant voltage (CV) and constant current (CC). For engineers getting started with power supplies they must know some of the basic rules when it comes to CV and CC. The output of a power supply can operate in either CV or CC mode depending on the voltage setting, current limit setting, and load resistance.

In scenarios where the load current remains low and the drawn current falls below the preset current limit, the power supply seamlessly transitions into CV mode. This mode is characterized by the power supply regulating the output voltage to maintain a constant value. In essence, the voltage becomes the focal point of control, ensuring stability, while the current dynamically adjusts based on the load requirements. This operational behavior is particularly advantageous when dealing with varying loads, as it allows the power supply to cater to diverse current demands while steadfastly maintaining a consistent voltage output.

In instances where the load current surges to higher levels, surpassing the predefined current setting, the power supply shifts into CC mode. This response involves the power supply imposing a cap on the current, restricting it to the pre-set value. Consequently, the power supply functions as a guardian, preventing the load from drawing excessive current.

In CC mode, the primary focus of regulation shifts to the current, ensuring it remains consistent and in line with the predetermined setting. Meanwhile, the voltage dynamically adjusts in response to the load’s requirements. This operational behavior is particularly crucial in scenarios where the load’s demands fluctuate, as it ensures a stable and controlled current output, preventing potential damage to both the power supply and the connected components. Understanding this interplay between voltage and current dynamics is essential for engineers and users to harness the full potential of power supplies, especially in applications with varying load conditions.

Most power supplies are designed in such a way that it is optimized for CV operation. This means that the power supply will look at the voltage setting first and adjust all other secondary variables to achieve the programmed voltage. For a visual representation, see Figure 1 on the operating locus of a CC/CV power supply.

Figure 1 The operating locus of a CC/CV power supply. Source: Keysight

Boosting voltage or current

In instances where the demands of an application exceed the capabilities of a single power supply, a practical solution is to combine two or more power supplies strategically. This can be particularly useful when users need more voltage or current than a single power supply unit can deliver.

For scenarios demanding higher voltage, the method involves connecting the outputs of the power supplies in series. This arrangement effectively adds the individual voltage outputs, resulting in an aggregate voltage that meets the specified requirements. On the other hand, requiring a higher current, connecting the power supply outputs in parallel proves advantageous. This configuration combines the current outputs, providing a cumulative current output that satisfies the application’s demands.

To achieve optimal results, it is crucial to set each power supply output independently. This ensures that the voltages or currents align harmoniously, summing up to the total desired value. By following these simple yet effective steps, users can harness the collective power of multiple power supplies, tailoring their outputs to meet the specific voltage and current requirements of the application.

For higher voltage, first set each output to the maximum desired current limit the load can safely handle. Then, equally distribute the total desired voltage to each power supply output. For example, if engineers are using three outputs, set each to one third the total desired voltage:

Never exceed the floating voltage rating (output terminal isolation) of any of the outputs.
Never subject any of the power supply outputs to a reverse voltage.
Only connect outputs that have identical voltage and current ratings in series.

For higher current, equally distribute the total desired current limit to each power supply:

One output must operate in constant voltage (CV) mode and the other(s) in constant current (CC) mode.
The output load must draw enough current to keep the CC output(s) in CC mode.
Only connect outputs that have identical voltage and current ratings in parallel.

See Figure 2 for a visual representation of a series connection with remote sense to a load.

Figure 2 Series connection to a load with remote sense. Source: Keysight

In the parallel setup, the CV output determines the voltage at the load and across the CC outputs (Figure 3). The CV unit will only supply enough current to fulfill the total load demand.

Figure 3 Parallel connection to the load with remote sense; the CV output determines the voltage at the load and across the CC outputs. Source: Keysight

Dealing with unexpected temperature effects

Temperature fluctuations not only impact the behavior of DUTs but also exert a significant influence on the precision of measurement instruments. For example, during a chilly winter day, an examination of Lithium-ion batteries at room temperature yielded unexpected results. Contrary the user’s anticipation of a decrease, the voltage of the cells drifted upward over time.

This phenomenon was attributed to the nighttime drop in room temperature, which paradoxically led to an increase in cell voltage. This effect proved more pronounced than the anticipated decrease resulting from cell self-discharge during the day. It’s worth noting that the power supplies responsible for delivering power to these cells are also susceptible to temperature variations.

To accurately characterize the output voltage down to microvolts, it becomes imperative to account for temperature coefficients in the application of power. This adjustment ensures a more precise understanding of the voltage dynamics, accounting for the impact of temperature on both the DUTs and the measurement instruments.

The following is an example using a power supply precision module that features a low-voltage range. The test instrumentation specification table documents the valid temperature range at 23°C ±5°C after a 30-minute warm-up.

To apply a temperature coefficient, engineers must treat it like an error term. Let’s assume that the operating temperature is 33°C, or 10°C above the calibration temperature of 23°C and a voltage output of 5.0000 V.

Voltage programming temperature coefficient = ± (40 ppm + 70 μV) per °C

To correct for the 10°C temperature difference from calibration temperature, engineers will need to account for the difference in the operating temperature and voltage range specification. The low voltage range spec is valid at 23°C ±5°C or up to 28°C. Engineers will need to apply the temperature coefficient for the (5°C) difference in the operating temperature (33°C) and low voltage range spec (28°C):

± (40 ppm * 5 V + 70 μV) * 5°C = 40ppm * 5 V * 5 °C + 70 μV * 5 °C = 1.35 mV

The temperature-induced error must be added to the programming error for the low-voltage range provided in the N6761A specification table:

± (0.016 % * 5 V + 1.5 mV) = 2.3 mV

Therefore, the total error, programming plus temperature, will be:

± (1.35 mV + 2.3 mV) = ±3.65 mV

That means user output voltage will be somewhere between 4.99635 V and 5.00365 V when attempting to set the voltage to 5.0000 V in an ambient temperature of 33°C. Since the 1.35 mV part of this error is temperature-induced, as the temperature changes, this component of the error will change, and the output of the power supply will drift. The measurement drift with temperature can be calculated using the supply’s temperature coefficient.

Dealing with noise sensitive DUTs

If the DUT is sensitive to noise, engineers will want to do everything they can to minimize noise on the DC power input. The easiest thing users can do is use a low-noise power supply. But if one is not available, there are a couple of other things engineers can do.

The links between the power supply and the DUT are vulnerable to interference, particularly noise stemming from inductive or capacitive coupling. Numerous methods exist to mitigate this interference, but employing shielded two-wire cables for both load and sense connections stands out as the most effective solution. It is essential, however, to pay careful attention to the connection details.

For optimal noise reduction, connect the shield of these cables to earth ground at only one end, as illustrated in Figure 4.

Figure 4 To reduce noise connect the shield to earth ground only on one end of the cable. Source: Keysight

Avoid the temptation to ground the shield at both ends, as this practice can lead to the formation of ground loops, as depicted in Figure 5. These loops result from the disparity in potential between the supply ground and DUT ground.

Figure 5 Diagram where the shield is connected to ground at both ends resulting in a ground loop. Source: Keysight

The presence of a ground loop current can induce voltage on the cabling, manifesting as unwanted noise for your DUT. By adhering to the recommended practice of grounding the shield at a single end, you effectively minimize the risk of ground loops and ensure a cleaner, more interference-resistant connection between your power supply and the DUT.

Also, common-mode noise is generated when common-mode current flows from inside a power supply to earth ground and produces voltage on impedances to ground, including cable impedance. To minimize the effect of common-mode current, equalize the impedance to ground from the plus and minus output terminals on the power supply. Engineers should also equalize the impedance from the DUT plus and minus input terminals to ground. Use a common-mode choke in series with the output leads and a shunt capacitor from each lead to ground to accomplish this task.

Choosing the right power supply

Navigating the selection process for a power supply demands careful consideration of the specific requirements. Whether in need of a basic power supply or one with more advanced features tailored for specific applications, the ramifications of choosing a power supply with excessive power capacity can result in numerous challenges.

Common issues associated with opting for a power supply with too much power include increased output noise, difficulties in setting accurate current limits, and a compromise in meter accuracy. These challenges can be particularly daunting, but developing basic skills related to power supplies can significantly aid in overcoming these design obstacles.

By cultivating a foundational understanding of power supply principles, such as the nuances of CV and CC modes, engineers can effectively address issues related to noise, current limits, and meter accuracy. This underscores the importance of not only selecting an appropriate power supply but also ensuring that users possess the essential skills to troubleshoot and optimize the performance of the chosen power supply in their specific applications. Striking a balance between power capacity and application needs, while honing basic skills, is key to achieving a harmonious and effective power supply setup.

Andrew Herrera is an experienced product marketer in radio frequency and Internet of Things solutions. Andrew is the product marketing manager for RF test software at Keysight Technologies, leading Keysight’s PathWave 89600 vector signal analyzer, signal generation, and X-Series signal analyzer measurement applications. Andrew also leads the automation test solutions such as Keysight PathWave Measurements and PathWave Instrument Robotic Process Automation (RPA) software.

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Conquer design challenges: Skills for power supplies

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