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Nowadays, the data sheet for a product series has to include everything from voltage combinations to mechanical drawings – often for dozens of different models. If you don’t know what you’re looking for, reading one can be a daunting task.
Rather than invest in a number of separate devices to meet the requirements of different applications, customers are looking to invest in a single power source. This need for interchangeability boils down to simple economics: customers want to do more with fewer devices, which puts pressure on power supply manufacturers to design products that can fit a variety of potential applications.
To be more versatile, power supplies have to include more voltage combinations. At Gresham, a product series that used to have only 15 models may now have well over 50 to account for the new voltage options. Further, manufacturers now have to offer power supplies with variable output voltages in addition to offering fixed-voltage devices. Having a variable voltage lets users adjust the power output to their preferred level, allowing the device to be used across more applications.
Keep in mind also that not only do data sheets have to list each new electrical specification, but sometimes the information has to be represented as a mechanical drawing or graph. For example, it’s no longer enough to simply list a unit’s output voltage, customers now want to see a graph that illustrates the relationship between a device’s output voltage and its temperature range.
In addition to being interchangeable, power supplies, at the same time, have to meet many specialised requirements. This includes undergoing rigorous, application-specific electrical testing. Consider EN 50155, which outlines the specifications of electronic equipment used in railway applications. This standard requires that power supplies have wider input voltages, including a range of 43 to 160 Vdc, as well as pass various tests related to electrical insulation, power surges, ESD, voltage transients and more.
The medical industry also has its fair share of tests, most of which are related to EMI, leakage current, immunity and voltage isolation. Tests like the HI pot (high potential) test, which verifies a device’s electrical insulation, are intended to protect patients coming into direct contact with medical equipment. While many of these power supplies were already manufactured to industry standards, manufacturers are now required to list the various tests and results on their data sheets.
Many applications demand both wider input ranges, higher power in smaller packages and installation in environments with both high ambient temperatures with limited cooling. In most power supply designs, the wide input can reduce the overall efficiency of the power supply, and it is this efficiency that determines the amount of waste heat produced. In simple terms, the less waste heat, the closer together components can be mounted without exceeding their specifications which in turn reduces the overall power supply size.
Current limiting of the output, heat sinking and thermal management are common ways engineers avoid overheating in power supplies. Deciding which one to use often boils down to operating environment.
Current limiting is a feature that can be built into the circuitry of a power supply, whereby control circuitry limits the amount of current emitted by the power supply. This protects the power supply from overheating and is common in units.
Heat sinking or conduction cooling eliminates heat from inside the power supply by dispersing heat externally. Most power supplies can benefit from some conduction cooling, in particular, Gresham Power offers a number of units that are packaged on U Channels or Baseplates that have heat generating components connected to the case to provide a thermal path to effectively remove heat. This method radiates heat so it doesn’t get trapped inside the power supply causing the power supply to overheat. While heat sinking also relies on free air convection, there are special cases where the power supply can be made with a base plate that functions as the heat sink. This allows the unit to be used in enclosed environments with limited or no airflow, including oil refineries or other applications that involve toxic material or explosive gas.
Most power supplies specify a convection rating based on a maximum operating temperature, and usually a dated convection rating at higher temperatures. Typically manufacturers establish a rating by running the power supply at maximum load, lowest input voltage in a thermal chamber with no moving air. The power supply’s components such as capacitors, ICs, transformers are continuously monitored for temperature to determine the convection rating.
Depending on the design of the power supply, many are designed to provide more power when forced air cooled. The forced air rating is usually reflected on the data sheet, and allows the equipment manufacturer to determine at what point controlled fans need to activate to keep the power supply within its safe operating limits.
Data sheets need to reflect all the thermal specifications needed to properly design a power supply. For one, power supply manufacturers often have to test for different thermal environments. Each new thermal management solution must also be listed alongside a mechanical drawing. Bear in mind too that each solution can be delivered a number of ways. For example, manufacturers can provide heat sinks with clamps or without, or sometimes heat sinks can be built into the bottom of the package. Listing all of these variations, along with the drawings and test results, takes up a significant amount of real estate on the data sheet.
Originally, power supplies were mounted one of two ways: PCB mounted with pins or chassis mount with screw terminal connections. But now customers want smaller power supply packages to fit ever-shrinking end products. Manufacturers have responded by providing more mounting and remote placement options.
Here’s a rundown of some of these options:
• In a PCB mount, the power supply is soldered directly onto the printed circuit board using pins. This was the standard for producing larger circuits contained in even larger multi-board designs.
• Chassis mounts allow you to remotely mount the power supply in a variety of ways: close to the load, on a frame inside the enclosure or outside the enclosure. What you ultimately choose depends on available real estate, the dimensions of your power supply and the operating environment.
• DIN rails let you mount power supplies and other industrial control equipment within enclosures or equipment racks. This mounting option utilises space efficiently, as devices can be mounted next to each other in a variety of ways to meet system requirements.
• Surface mounts are the future of the PCB mount. Nowadays, many products are getting smaller. Surface mount technology favours this trend: it utilises solder paste rather than larger pins, taking up less space.
• Wall plug-ins protect equipment from the heat and noise of the switching power supply, thereby avoiding potential damage or interference. This option also lets you easily specify adapters based on different input voltage requirements and outlets.
More options means more drawings. But your options don’t end there. Depending on the direction of your connection, you may require a right angle, straight on or vertical mount. You may also require mounting types that integrate fans, heat sinks, input and output cables, connectors and safety tabs.
Many certifications listed on data sheets require rigorous testing by the manufacturer. Data sheets are required to indicate all new certifications, testing procedures, special model numbers and designations, which adds to their length but provides plenty of useful information for end-users.
The medical industry has many standards related to leakage current, EMI resistance and voltage isolation – all of which must be avoided in machines and devices that come into direct contact with patients. In addition, the close proximity of equipment in hospitals increases the risk of noise interference which could cause medical devices to work incorrectly at a critical time. Safety specifications, particularly ones related to EMI, therefore ensure power supplies pose no risk of interference.
Power supplies used in railway applications also have their fair share of standards. Consider EN 45545-2, which specifies how materials used on trains must be fire-tested, or EN 50155, which outlines the specifications of electronic equipment used on carriages. EN 50155 also requires power supplies to pass various tests related to electrical insulation, power surges, ESD and voltage transients – all of which are intended to protect the passengers on-board. Additionally, the railway industry requires its own special input voltage range of 43 to 160 V, which doesn’t exist for other applications.
Because of the increase in safety specifications and, in many cases, the addition of new products that comply with these standards, data sheets have become longer. While on the one hand, this trend has complicated data sheets, the addition of these details makes data sheets much more useful for customers who require power supplies for medical and railway applications.
A data sheet may just be the beginning of selecting an appropriate power supply for a new product design, replacement for an existing unit or one that has gone out of production. There are always going to be questions and details that require further explanation and commercial and supply-chain implications. Such issues can be simply resolved by contacting an established power supply source such as Gresham Power Electronics.
For more information visit www.greshampower.com
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