Understanding and Addressing 5 Key Power Supply Issues
This article will take a deeper look at five key power supply problems, how to know when they arise, and the best ways to address or mitigate them.
When it comes to working with power supplies (Figure 1), there are many considerations design engineers must take into account to avoid common issues such as input under and overvoltages. This article will take a deeper look at five key power supply problems, how to know when they arise, and the best ways to address or mitigate them.
Figure 1. Power supplies are often overlooked when they work well, but as soon as the quality of their performance degrades, it becomes almost impossible to ignore them. Image provided courtesy of CUI Inc
1. Input Overvoltage and Undervoltage
Both input undervoltage and overvoltage can be extremely problematic for power supplies, and they are among the most common issues encountered.
One of the most notorious sources of both over and undervoltage is incorrectly setting the 120/240 V switch on non-universal inputs. This switch enables or disables a voltage-doubler inside the power supply, so the internal circuits operate at 240 V in either case. If the switch is set incorrectly, the internal voltage will either be too high or too low, which can lead to failure. Power supplies with universal inputs are designed to operate across the full input voltage range without the use of a switch and don’t have this problem.
Electromagnetic disturbances, such as voltage surges, sags, EFTs (Electric Fast Transient), and ESD (electrostatic discharge), can cause fast but severe over and undervoltage conditions.
Common sources include the powering up and down of machinery, lightning strikes, and the touching of the power supply by a charged object. Most power supplies will contain some amount of protection from these events. Depending on the application, more or less immunity to these disturbances may be needed.
Figure 2. EMI can be a source of overvoltage.
While power supplies are designed to withstand short transient events, such as EFTs resulting from electromagnetic interference (EMI) shown in Figure 2, sustained input overvoltages can lead to damage. Undervoltage is also problematic. It can cause problems with output regulation and control and results in increased input current and power dissipation, which can lead to failure. Some power supplies contain UVP (undervoltage protection). UVP halts power conversion if the input voltage falls below the UVP threshold.
2. Output Overpower and Overcurrent
Output current limits are an extremely important specification when selecting a power supply and are often underspecified during the design process. However, output overcurrent or overpower can have serious repercussions for the power supply and the powered device, including failure of the power supply and melted circuitry and cables. Exceeding the current ratings also leads to increased power dissipation and heat and a significant efficiency decrease.
OCP, or overcurrent protection, protects the power supply when the output current limit is exceeded. Overpower protection (OPP) protects the power supply if the output power limit is exceeded. OPP is typically applied to the primary side of a power supply and monitors the transformer current to ensure that the maximum power is not exceeded. For a single output power supply, OCP and OPP are closely related. However, for multiple output power supplies, overpower protection cannot determine the current of individual output, so it is possible to exceed the current on one output while staying below the total power limit, which can lead to failure. Overcurrent protection monitors the current directly and may be applied to each output individually to avoid this issue.
To avoid larger, more expensive power supplies, users sometimes underspecify the power or current rating. If not sized properly, transient currents can lead to repeated shutdowns. Power supplies should be sized such that transient output currents fall within the rated current limit to avoid accidentally tripping OCP or OPP.
3. Reversed Polarity
Reversed polarity refers to incorrectly connecting the positive and negative inputs or outputs of a power supply. Many components, such as electrolytic capacitors, cannot tolerate reverse polarity and will fail if subjected to it. Suppose the polarity is reversed on the power supply’s output connections. In that case, the load may be damaged, potentially leading to power supply failure, with worst-case scenarios being ruined circuits or electrical fires.
A diode may be used to protect against reversed polarity in low power applications. It may be placed in series or anti-parallel with the input or output. When in series, the diode will not conduct and keep the circuit open if the voltage is reversed. The downside to this method is that the diode will dissipate power proportional to the input current. When placed anti-parallel to the input, the diode will normally be off and only conduct if the polarity is reversed. The diode will short and trigger an overcurrent protection device, such as a fuse or the power supplies OCP when it conducts.
4. Temperature Issues
Falling either under or over the recommended temperature range for a power supply is another common issue that can cause problems for a power supply. Thermal limits ensure that the power supply safely operates within a range where you can account for its performance.
Figure 3. Operating a power supply outside of a safe operating temperature as shown in the temperature derating curve can lead to unpredictable behavior.
Outside of the minimum and maximum operating temperature, reliability, regulation, EMI, and efficiency can quickly become problems. Many components inside the power supply, such as the transistors, operate at temperatures close to their thermal limits. Operating the power supply beyond its rated temperature can cause these devices to fail.
Some power supplies allow for an extended temperature range if the output power is decreased or derated. The datasheets for these power supplies will contain derating curves, such as the one shown in Figure 3, that show the maximum temperature for specific load conditions. Derating may be required at lower temperatures as well. At very low temperatures, the values of some components, especially electrolytic capacitors, can be significantly different than under normal conditions. This can result in increased ripple voltages and start-up issues.
5. Missing External Components
Power supply performance can be seriously impacted by missing external components or connections. The performance listed in datasheets is sometimes characterized using a specific circuit, often to mimic a typical application. If a similar circuit is not applied during testing, the performance may vary from what was specified. For example, some power supplies require external capacitance at the input and/or output for performance and stability. Not including recommended or required external components can negatively affect performance or even lead to failure.
Another oft-missing set of components are pull-up and pull-down resistors, which may be needed to ensure a known voltage for on/off. A pull-up resistor, for example, creates an additional loop around critical components and makes sure that a well-defined voltage remains even when a switch is open; a pull-down resistor, on the other hand, will hold the voltage near zero when a switch is open.
EMI can be problematic for some switch mode power supplies because of the amount of noise they generate. With internal power supplies, minimal filtering may be included internally in order to reduce the size and increase design flexibility. In these cases, external filtering may be required to meet certain requirements. For example, a power supply may state that it is designed to meet Class B emissions, but without external filtering, will only meet class A. This means Class B applications will require additional external filtering. In these cases, the datasheet will often provide recommended circuits for meeting class B.
Figure 4. Output connections when not using voltage sensing (Left) and when using sense connections (Right)
Some power supplies include voltage sense connections which essentially bring the feedback loop outside of the power supply, as shown in Figure 4. This allows the power supply to compensate for the impedance of the cabling and connectors and results in tighter regulation at the load. If these sense connections aren’t used, they usually need to be shorted to the output. If left open, the feedback loop may not be closed, and the output will be uncontrolled.
CUI Power Supply Solutions
The five common power supply problems include voltage and current issues at the input and output, reversed polarity, temperature issues, and missing external components. Fortunately, careful consideration of design specifications and a few additional calculations can allow you to avoid, protect against or mitigate these issues.
CUI Inc. focuses on providing reliable power products and tools. Their experts can aid in avoiding the common power supply problems discussed in this article by assisting with developing critical specifications and eventually choosing the right power supply for any application.
Images used courtesy of CUI Inc
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