Decreased system reliability due to overloaded power supplies is a common engineering challenge. In this Q&A style article submitted by Allied Electronic & Automation, David Norton, technical marketing expert at TDK-Lambda, answers questions about the best ways to configure parallel power supplies.
What are parallel power supplies, what are they used for and why are they important?
Parallel power supply topologies have two or more power supplies configured to provide output power to the same system. These configurations can provide systems with more power (higher total output current) than a single power supply, provide redundant power to improve system reliability, and mitigate excessive heat generation to ease thermal management requirements and improve further system reliability.
For example, if the system load changes and requires more current than the existing power supply can provide, the system engineer can either install another power supply that can provide more current, or install two or more of the existing ones in parallel. However, a power supply designed for higher power output is likely to be larger than the original and could potentially be taller or deeper than the cabinet can easily accommodate. Thus, the parallel power solution offers key benefits in form and fit. Another benefit of parallel PSUs is that they will all have the same part number, which will be the same as the existing BOM and as such can help with inventory management.
Like all electronic components, power supplies have a defined operational life, after which they are likely to operate outside of their rated parameters or even fail. In a critical manufacturing process, a power outage could shut down an entire production line, leading to unplanned downtime and lost revenue. To avoid this possibility near end-of-life, system engineers may instead install two power supplies connected in parallel for redundancy or equal load sharing.
This configuration will allow the production line to continue to operate even if one of the two power supplies fails, allowing the manufacturer to select an appropriate time to service the machine rather than forcing them to react to a failure of the system. With many automated installations operating 24 hours a day, redundant parallel power supplies greatly reduce the risk that a service technician will not be available to replace a faulty unit before hours of manufacturing revenue are lost.
The temperature at which power supplies operate also has a direct effect on their rated life, and higher power output results in higher heat generation. Thus, by sharing the system load equally, the redundant parallel power supplies each operate at a lower power output and generate less heat, which can ease thermal management requirements by requiring fewer thermal control components. such as heat sinks and fans, eliminate these additional systems. costs and potential points of failure, and further improve system reliability.
When we talk about parallel power supply configurations, it is important to note that the power supplies must be able to share the load current equally. Unbalanced power supply configurations, where one provides the majority of the load current and the other very little, can cause the power supply to fail prematurely with the highest load and negate any benefits of power supply designs. parallel power supply.
How to create a redundant parallel power supply? What are the key features customers should look for?
To create a redundant parallel power supply, system engineers place a redundancy module in series between the positive output of the power supplies and the load. The redundancy module acts as a one-way valve, ensuring that current only flows to the load, and is used to decouple the power supplies in the event of one of them failing, ensuring that current does not flow into the load. faulty unit, as likely to disrupt the operation of the machine or final equipment.
The most important characteristic of power supplies and redundancy modules is long-term reliability, especially when installed in industrial factory automation equipment with high availability and strict safety requirements.
For example, the life of a power supply is primarily determined by the electrolytic capacitors used for energy storage and filtering. The colder the temperatures at which these components are used, the longer they will last. A standard rule of thumb for these components is that every 10°C increase in temperature halves their lifespan, but product data sheets and test data should specify this. Thus, product quality, accurate technical data and manufacturer transparency are also important when selecting power supplies and redundancy modules.
How can customers ensure that parallel power supplies have the same current output?
There are two types of current sharing circuits: active and passive. Active current sharing relies on a return signal wire, interconnect wire, or bus to connect all power supplies. Once connected, each unit can adjust its output voltage to balance the charging current. However, if the signal has a low potential, excessive electrical noise and spikes can cause interference and upset the balance. To solve this problem, just shorten and protect the length of the wire.
Passive sharing via output droop is very common in DIN rail mount power supplies. In this current-sharing configuration, an internal circuit reduces the output voltage as more current is drawn from the supply. This action automatically shares the load between the parallel supplies, but the load regulation is slightly degraded compared to the active current sharing configuration. However, this is not an issue for most factory automation equipment.
Some redundancy modules, such as the TDK-Lambda DRM40, have a current balance indicator option that allows users to individually adjust the outputs of the two parallel supplies until an equal output current is confirmed by an LED indicator.
What are the most common problems or concerns faced by users of parallel power supplies? Where can I find more information or get design help?
One of the most common problems people seem to have with parallel power supplies stems from the unrecognized lack of a working parallel power supply, either because they installed the wrong type of power supply or because they have not installed a parallel switch, which is usually revealed when there has been a loss of production. The best way to avoid this problem is to involve a trusted power supply manufacturer early in the machine design cycle. Experienced power supply manufacturers usually have teams of experts dedicated to helping system engineers achieve cost-effective and highly reliable power supply systems.
In addition to lost production and revenue resulting from nonfunctional parallel power supplies, there are also potential safety issues. For example, if PSUs that are not capable of sharing the load equally are used to achieve a parallel configuration, sometimes referred to as brute force mode, it is quite possible that one of the PSUs is operating at overcurrent. condition, continuously supplying 130% or more of its rated current, which can cause it to run very hot and fail prematurely.
Additionally, since the available current of the system increases when multiple power supplies are paralleled, a redundancy module capable of acting as a blocking diode should be used to improve system safety. If one of the many parallel power supplies were to short or fail, the redundancy module would prevent all system current from flowing through the failed power supply, which could create a fire hazard.
This article was submitted by Allied Electronics & Automation, a distributor of power supplies, industrial cabling and connectors. Allied Electronics & Automation is an authorized distributor of TDK-Lambda, a supplier of industrial wiring and connectors.
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