Why Power Supply Requirements Don't Need to Be a Design Restriction

Ron Stull, Power Systems Engineer at CUI Inc.


There are many clichés in engineering, one of which is that power is always the last thing to be considered. As with most clichés, there is an element of truth in this….

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Figure 1: There are many choices when it comes to selecting the right plug for your external power adapter (Source: CUI)

New product features tend to have a direct impact on the total power needed, which may mean increasing the capacity of dc-dc converters or increasing the number of regulators distributed throughout the design. Ultimately, feature-creep will also put greater demand on the main source of power. 

Engineers responsible for a design's power sub-systems need to ensure that the main source of power (which unless the device is only ever powered by primary batteries is likely to start with an ac outlet) can handle the demand. This, in turn, implies that an ac-dc converter is going to be needed, and then the question becomes, should that converter be internal to the design, or external? 

There are advantages to each option. An internal converter will form part of the overall design, which may alleviate the safety requirements and possibly, but not necessarily, lower the total cost. An external converter, on the other hand, offers greater freedom in terms of the total power required. If an external power adapter is specified, any subsequent increase to the output of that adapter is not going to have any design implications upon the rest of the product. For example, increasing the power output of the adapter by 20% could have a commensurate impact on the size of an internal adapter, as well as triggering a reassessment of the thermal management figures. An external adapter, however, could be changed without impacting the size, shape of other design parameters of the main product. 

In addition, if the main product uses secondary batteries, then an external adapter is almost certainly going to be the desired option, as it allows the product packaging to be designed without any consideration for the size, weight and cost of the components required for ac-dc conversion. With modern electronics, the space required for the adapter can be considerable in comparison to the highly integrated components used; we wouldn't expect to see an ac-dc adapter in a tablet, for example. Even most laptops now use an external power adapter as opposed to supporting an ac supply. 

The demands that an ac-dc adapter can impose on a design should never be ignored, but they needn't become a restriction, either, if you choose to use an external adapter. Assuming that an external adapter is the right option for your next product, you will still need to make a number of design decisions based upon various considerations, as explained here. 

Safety First

In the majority of cases, an external adapter will supply a single dc voltage to the device being powered, allowing that main supply voltage to be further stepped down and distributed around the product's sub-systems. This main dc voltage must provide enough power in a single interface to supply the entire product, so some consideration needs to be given to the type of connector used. 

There are a number of connector types widely used in external adapters. In low power portable devices (such as smart phones and tablets) the USB connector has become ubiquitous. The advent of USB 3.x with a Type C connector (now simply known as USB-C) will see this reach extend further, as it is now possible to deliver up to 100W at 20V dc if the Power Delivery part of the specification is implemented. Beyond this, the venerable barrel plug remains popular, as well as the D-shaped multipin connectors often seen on power adapters for electronic devices that require higher power levels. 

Commensurate with the type of plug used is the choice of cabling. Again, the power requirements will dictate this to some extent, in terms of wire gauge, but there are also safety aspects to consider that may apply to the cable length and other features. If the user is expected to interoperate with the connector – to change its size or style through optional adapters, for example – this may also have an impact on safety certification. 

Because ac power is potentially hazardous, adapters naturally need to be certified and marked to show compliance, at least they do if they are to be put into service in most regions around the world. While there is some motivation to make these requirements universal, many regions still retain their own. In this respect some features are common between regions, but all of the differences also need to be observed. Importantly, some applications have their own, additional requirements, such as medical appliances. It is likely that the manufacturer will know before design commences if the adapter must comply with these specifications, based on the type of application and the country/region into which it will be placed. If there is any confusion it is best to consult an expert in this area, such as CUI. 

Another related design requirement is electromagnetic interference and compatibility. Meeting EMI and EMC standards is also a largely regional requirement, and often, testing is the only way to really determine if those requirements are being met. In many cases it may be necessary to apply some form of filtering to lower emissions at certain frequencies; ferrite beads are perhaps the most commonly applied solution here. Again, experts like CUI can help manufacturers meet these necessary requirements. 

Regional Requirements 

Many aspects of power are now subject to legislation, which includes adapters, whether they are internal or external. Of course, if it is external then the figures will only apply to the adapter itself, while if it is internal the figures will probably apply to the entire product. This may seem pedantic, but in fact it is relevant, because an adapter that offers high levels of efficiency should not be subject to the inefficiencies that may be present in other parts of the product. 

As an example, the no-load power consumption of many products must comply with efficiency standards now commonly adopted by a number of countries and states. In the U.S. it is necessary to comply with the Department of Energy's Level VI standard (historically based on the California Energy Commission's first energy standard, released in 2004). This standard, now applicable across the United States, stipulates how efficient an external power adapter must be and how much standby power it can consume. As an example, an external power adapter that supplies a single dc voltage at power levels over 250W must be at least 87% efficient and consume less than 0.5W under no-load conditions. CUI offers insights into many of the world's energy standards and how they apply to external power adapters. Suffice it to say, if the power adapter doesn't comply it cannot be put into service. If the power supply complies but it resides inside a product that doesn't, then redesigning the power supply is unlikely to help. On the other hand, having an external power adapter design that does comply would allow a manufacturer to use that adapter design with a wide range of its own products, or even offer it for sale as an after-market option. 

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Figure 2: Efficiency standards and how they have evolved over time (Source: CUI) 

Other design aspects to consider include the type of plug used to connect to the ac outlet. Clearly there are regional variations here, but it is increasingly common to design external adapters that are certified to work across multiple regions. In fact, one of the main advantages of choosing an external adapter is that it also makes the main product practically universal. 

In this respect it's important to consider whether the adapter will plug directly into the outlet or use an ac input cord. This decision is likely to be dependent on two things; the product aesthetics and the power requirements. Adapters required to provide more power than, say, the 100W available from USB-C may opt for the ac cord version and, again, this presents the opportunity for a removable lead to handle the regional adaptations. 

The overall size of the adapter may also be a consideration. If it is, it may be a good idea to consider using the latest semiconductor technologies, like Gallium Nitride (GaN), which delivers higher switching efficiency. This switching efficiency leads to better thermal performance and permits an increase in switching frequency and, as a result, supports increased power density. Put simply, higher efficiency means a smaller overall package size. This can be important for an external power adapter that is needed for a portable device or slimily an improvement for the end user who must deal with a clunky large adapter under a desk, behind an appliance or to blend into their surroundings. 

Making It Your Own

As mentioned earlier, it is entirely feasible to supply external power adapters into the marketplace as an alternative to the manufacturer's own internal power supply. It also makes great sense to have the design and manufacturing of your own external power supply outsourced to a specialist like CUI. They have a wide range of options for customization of an external power adapter to match the overall design of your product. In this case, being able to have the adapter branded as if it were manufactured in your own facilities is a definite advantage. This is just one of the additional services CUI offers. 

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Figure 3: Branding an external adapter makes it part of your value proposition (Source: CUI)

We shouldn't forget that, ultimately, the adapter needs to be capable of supplying the right voltage and current levels to the device it is powering, which circles us back to the premise that the power supply is always the last thing to be designed. It is common for a design team not to know, with absolute certainty, how much power a product will need. If the ac-dc adapter is going to be internal this can present a design challenge, but if the decision to make the adapter external is taken as early as possible it frees up the engineers to focus on the feature and functionality of the product itself. This is where CUI can really help, by taking on the design and manufacturing of the external power adapter, designed to deliver the required voltage and current levels, with full international compliance to safety and efficiency requirements, branded in the customer's own style and fitted with their choice of connectors.