What factors should be considered when selecting internal AC-DC power supply
Time:2022-08-02
Views:1912
A very common demand is to choose an internal AC-DC power supply for the final product, with a power range of several watts to hundreds of watts. At the low end, the alternative may be an external power supply or "adapter", but it is higher than about 100W. Usually, the built-in product is selected as the built-in module, which may be installed on the chassis or PCB. Brave people may even consider internal design, especially when the required performance is non-standard.
Why internal?
Security, EMC, and environmental compliance are most important
The size of the inlet fuse of the internal power supply shall be carefully determined
Why internal?
Let‘s first review why we choose internal power supply. For commercial products, consumers do not like bulky in-line "adapters", but at low power, "wall wart" is not a big problem. With the progress of technology, more power is squeezed out of smaller packages, so that the adapter is almost no larger than the wall plug itself. The use of external power supply also makes the product designer happy - the dangerous voltage is isolated outside, making the safety certification of the final product easier.
The disadvantage is that the length of the cable from the adapter to the product will reduce the voltage, which may require the use of additional voltage regulators in the product, and there is usually no opportunity to "smart" control the power supply, such as turning off to "sleep" mode or dynamically adjusting the output voltage. Another problem is that the overall EMI standard compliance is still the responsibility of the end product manufacturer, so adapters with production variability and uncertain cable operation must be included in EMC tests, and may produce inconsistent results. For this reason, it is not known to install the in-line adapter internally into the final product for easier security and EMI compliance.
At higher power, or when control and function are important, internal or "device" power supplies are preferred. Decisions should be made early in the product development process, as system power engineers often complain (and usually reasonably) that they must purchase built-in power supplies suitable for "any remaining space". This may lead to cost and performance compromises, and the worst case is the need for customized solutions, with associated delays and risks.
Security, EMC, and environmental compliance are most important
The built-in power supply must provide the required voltage and current, but there are many other considerations. Perhaps the most important is security, EMC and environmental compliance - the end use of products is the guide here; Different standards apply to a range of applications: such as industry, home, testing and measurement, medical and building automation. If the products are used in professional fields such as railway or military, the standards are different. Even in the field of application, there are differences - such as the patient or operator environment in medical treatment.
One trend is that the new safety standards are "risk-based", which forces manufacturers to consider how their products are abused; Choosing an internal power supply can at least ensure that inappropriate adapters will not be replaced. Choosing the right certification is very important and complex, but without experienced internal compliance engineers, reputable power suppliers can usually help.
One trend is that the new safety standards are "risk-based", which forces manufacturers to consider how their products are abused; Choosing an internal power supply can at least ensure that inappropriate adapters will not be replaced. Choosing the right certification is very important and complex, but without experienced internal compliance engineers, reputable power suppliers can usually help.
Mechanics may be the next consideration, not just shape and size, but also connectors and cooling devices. "Open" power supplies are popular and low-cost, usually with optional covers, which are necessary if technicians can access the product internally when the product is powered on. Another option is the DIN rail format common in the terminal block (Figure 1).
Internal power products usually have screw terminals or plug-in connectors for AC input and output, usually "Molex" ™” Type. In this case, cables, terminals, fuses, switches, and any chassis connectors must be properly rated and certified for the application. The AC input cable outside the power supply but inside the product is likely to receive interference, so EMI testing may indicate the need to install another certified chassis mounted filter near the power inlet.
Grounding requires special attention; If the power module connector is unplugged inside the product, it must still be grounded separately from the equipment chassis at the entrance to prevent the fire wire from loosening. Generally, all grounding connections should not be "pluggable", unless disconnecting the connector will completely remove the live connection from the product at the same time. If this is not the case, it must be grounded by "permanent" fixing, which can only be loosened by tools and includes lock washers or other anti vibration techniques. Of course, the color coding and wiring specifications must be observed according to the applied safety standards, and cable stress relief devices must be used if necessary.
The size of the inlet fuse of the internal power supply shall be carefully determined
The internal power supply connected to the chassis connector through wired AC must include a suitable single fuse or double fuse at the entrance (as the case may be). Remember that the final product AC fuse protects the upstream cables and connections, not the internal power supply, to prevent short circuits and overloads. Of course, it must pass the normal operating current and leave a certain surge margin, but it should also be rated before the fuse is disconnected after the short circuit to ground between the chassis connector and the power supply, so that the external AC cable connected to the final product will not be overloaded. Even if the rated current of the external cable is very high, the breaking value of the fuse should be lower than that of any upstream fuse or circuit breaker to avoid the fault that leads to the disconnection of multiple circuits, that is, the correct fuse "coordination" - a key problem in a professional environment (Figure 2).
Cooling considerations are important; The internal power supply can be fan, natural convection or baseboard cooling, depending on the final product and its application. In some environments (such as medical environments), fans may be excluded for noise reasons or in applications that are difficult to replace, but fan cooling power supplies are usually smaller than other types of power supplies. If the fan cooling supply is selected, the air inlet and exhaust paths must be carefully determined to avoid air "dead corners", especially if there are other system fans running. Cui and other manufacturers provide the recommended airflow direction and fan size in the data sheet of their products; These factors should be considered when designing the power supply into the system (Figure 3).
Recommendations for fan size, direction and distance should be considered to avoid air "dead corners". (source Cui)
Convective cooling power supply is sensitive to direction, so other heating components should be considered when placing it to avoid mutual overheating. The manufacturer cannot predict the final product arrangement, so the power supply will be rated according to the "working" ambient temperature. This is the "local" environment in the final product shell, which may be much higher than the external temperature, and can only be accurately known through simulation and / or measurement in the complete system under the defined load conditions.
The backplane cooling power supply can also be used to seal the housing and eliminate the uncertainty of the heat flow path. However, a flat "cold wall" is really needed, and the power supply has multiple fixtures. It may be necessary to use a thermal conductive compound at the interface with the silicone heat transfer sheet, which is another possibility.
When determining the size of the power supply and its cooling requirements, it is necessary to check the required continuous power and peak power. Sometimes, if you have a higher surge rating and the load is intermittent, you can use a smaller, lower cost power supply.
Experts can guide your choice
There are a variety of internal power supplies to choose from, which is best depends on many considerations. In order to minimize costs and risks, the type of power supply should be determined as early as possible, with due consideration to standard compliance, application, cooling environment, and ease of integration into the final product wiring and mechanical devices.
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