Key factors to consider when selecting a built-in power supply

Article By : Ron Stull

A very common need is to select an internal AC-DC power supply for an end-product in the range of a few to several hundred watts.

A very common need is to select an internal AC-DC power supply for an end-product in the range of a few to several hundred watts. At the low end, an alternative could be an external supply or ‘adapter’ but above around 100W it is normal to opt for a built-in product as a bought-in module, perhaps chassis- or PCB-mounted. The brave might even consider an in-house design, especially if the performance required is non-standard.

Why internal?

Let’s first review why an internal power supply might be chosen. For commercial products, consumers don’t like bulky in-line ‘adapters’ but then at low power, a ‘wall wart’ is no great imposition and as technology advances, more power is squeezed out of smaller packages, to the extent that the adapter is scarcely bigger than the wall plug itself. Using an external supply also pleases the product designer – dangerous voltages are kept outside, making end-product safety certification much easier.

The downsides are that the cable length from an adapter to the product drops voltage, perhaps necessitating extra regulators in the product and there is usually no opportunity for ‘smart’ control of the power supply, such as shutdown to ‘sleep’ mode or dynamic adjustment of the output voltage. Another problem is that overall EMI standards compliance is still the responsibility of the end-product manufacturer so an adapter with its production variability and indeterminate cable run must be included in EMC testing and can produce inconsistent results. For this reason, it is not unknown for in-line adapters to be fitted internally to end-products for easier safety and EMI compliance.

At higher powers, or when control and functionality are important, an internal or ‘equipment’ power supply is preferred. The decision should be made as early as possible in the product development process as it’s a common (and often justified) complaint by system power engineers that they have to source a built-in power supply that fits ‘in whatever space is left’. This can lead to cost and performance compromises and at worst the need for a custom solution with associated delays and risk.

Safety, EMC, and environmental compliance are foremost

A built-in power supply must supply the volts and amps needed, but there are many other considerations. Perhaps the most important are safety, EMC, and environmental compliance – the product end-use is the guide here; different standards apply for a range of applications: industrial, household, test and measurement, medical, and building automation for example. If the product is to be used in specialized areas such as rail or military, the standards are different again. Even within application areas, there are variations – patient or operator environments in medical for example.

A trend is for new safety standards to be ‘hazard-based’ which obliges a manufacturer to additionally consider how their product could be misused; selecting an internal power supply at least ensures that an inappropriate adapter is not swapped in. The selection of the right certifications is extremely important and complex, but reputable power supply vendors can often assist if an experienced in-house compliance engineer is not available.

Mechanics are perhaps the next consideration, not just shape and size, but also connectors and cooling arrangements. ‘Open-frame’ power supplies are popular and low-cost, often with optional covers which are necessary if a technician could be expected to have internal access to the product while it is powered. Another alternative is the DIN-rail format which is common in wiring panels (Figure 1).

Figure 1: Typical internal AC-DC power supply options. (Source CUI)

Internal power supply products usually have screw terminals or plug-in connectors for both AC input and outputs, typically the ‘Molex™’ style. In this case, cables, terminals, fuses, switches, and any chassis connector must be suitably rated and certified for the application. Interference may well be picked up on AC input cables external to the power supply but internal to the product, so EMI tests may show that a further chassis-mounted certified filter is needed close to the power inlet.

Grounding needs particular attention; if the power supply module connector is unplugged inside the product, there must still be a separate ground to the equipment chassis at the inlet in case a live wire comes loose. Generally, all ground connections must not be ‘pluggable’ unless un-mating the connector removes live connections simultaneously and completely from the product. If this is not the case, ground must be by a ‘permanent’ fixing which can only be loosened with a tool and include a lock-washer or other anti-vibration technique. Color-coding and gauge of wiring must of course be observed according to the safety standard applied, with cable strain relief where necessary.

Inlet fusing for internal power supplies should be sized carefully

Internal power supplies with wired AC connections to a chassis connector must include a suitable single or double fuse at the inlet as appropriate. Remember, an end-product AC fuse is protecting upstream cables and connections, not the internal power supply, from shorts and overloads. Certainly, it must pass the normal running current with some margin for inrush, but it should also be rated so the external AC cabling to the end-product is not overloaded before the fuse opens after a short to ground between the chassis connector and the power supply. Even if the external cable is rated for very high currents, the fuse should be lower in breaking value than any upstream fuse or breaker, to avoid a fault causing multiple circuits to be disconnected, that is, correct fuse ‘coordination’ – a critical concern in professional environments (Figure 2).

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Figure 2: Fuse values in a system should be ‘coordinated’: value Fuse 1 > Fuse 2 > Fuse 3. (Source CUI)

When taken together, the AC connection considerations discussed influence the choice of internal power supply, with the lowest cost open frame type not necessarily yielding the cheapest overall system cost if it only has marginal EMC compliance and extra filtering is required.

The cooling method dictates the power supply type

Cooling considerations are important; internal power supplies may be fan, natural convection, or baseplate-cooled with the choice depending on the end-product and its application. Fans may be ruled out in some environments such as medical, for noise reasons, or in applications where replacement would be difficult, but fan-cooled power supplies will generally be smaller than other types. If a fan-cooled supply is chosen, inlet and exhaust air paths have to be carefully identified to avoid air ‘dead spots’, especially if there are other system fans in operation. Manufacturers like CUI provide recommended airflow directions and fan sizes within the datasheets for their products; these should be considered when designing the power supply into a system (Figure 3).

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Figure 3: Recommendations for fan size, direction, and distance should be considered to avoid air ‘dead spots’. (Source CUI)

Convection-cooled power supplies are sensitive to orientation and should be located with consideration to other heat-generating components to avoid mutual overheating. Manufacturers cannot predict end-product arrangements, so will rate power supplies for an ‘operating’ ambient temperature. This is the ‘local’ ambient within the end-product enclosure which may be considerably hotter than the external temperature and will only be known accurately through simulation and/or measurement in a complete system under defined loading conditions.

Baseplate-cooled supplies are also available for sealed enclosures and take the uncertainty out of the heat flow path. A flat ‘cold wall’ does need to be available though, with multiple fixings to the power supply. A thermal compound may need to be used at the interface with silicone heat transfer sheeting another possibility.

When sizing power supplies and their cooling requirements, it is worth checking what continuous and peak power is required. Sometimes a smaller, lower-cost power supply can be used if it has a high surge power rating and the load is intermittent.

Experts can guide your choice

There is a wide choice of internal power supplies available and which is best is dependent on many considerations. For minimum cost and risk, the power supply type should be identified as early as possible with due regard to standards compliance, application, cooling environment, and ease of integration into the end-product cabling and mechanics.

Ron Stull is a Power Systems Engineer at CUI Inc. Ron has gathered a range of knowledge and experience in the areas of analog and digital power as well as ac-dc and dc-dc power conversion since joining CUI in 2009. He has played a key role on CUI’s Engineering team with responsibilities including application support, test and validation, and design. Outside of power engineering Ron can be found playing guitar, running, and touring the outdoors with his wife, where their goal is to visit all of the U.S. National Parks.

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