Power supplies: make or buy?

4 min read

Designers have wrestled for years with the dilemma of whether to design and make their own ac/dc power supply or to buy one. In recent years, the criteria by which this decision is made – the available tools and components and the demands on the supply – have changed, but the importance of the decision has not.

A typical unit operates from a nominal 120/240V, 50/60Hz supply and delivers one or several dc rails, usually between a few volts and 48V, at less than 1kW. As an example, the ac/dc supply shown in the picture provides 375W at between 12V and 56V (depending on model). Today's supplies must do more than just deliver power; they have to meet stringent safety regulations, emi/rfi standards, efficiency mandates and power factor correction (PFC) objectives. In some applications, such as medical instruments, they also have to keep leakage below a threshold and assure that component failures will not cause life threatening conditions. Today's chips make it easier to do your own design. Many embed control and algorithms for PFC, enhancing efficiency, transient response, load/line performance, while minimising emi. They feature advanced topologies and operational modes that would be difficult to design yourself. Some chips support digitally controlled supplies, where the system can monitor many internal parameters and adjust these dynamically. Some vendors offer reference designs and development tools that can, at first glance, make the design trivial. These fall into two categories. In the first, you get a detailed reference design for a specific supply (such as 375W, 48V dc) that includes a schematic, pcb layout, and bill of materials (BoM). In the second, vendor tools help to define what you need, then select the appropriate device(s) and passives, while providing a schematic, a layout and performance curves. A common reason for an in house design is an unusual form factor, common in consumer products. In addition, the higher volumes of these consumer products may be a strong justification for custom design. If you are looking to manufacture more than 1000 units/month, you'll be amortising the design/qualification process and careful BoM analysis may show you can achieve higher profit margins. Another reason to do your own design is that either your requirements fall outside of what's available or there are few vendors who can meet enough of your requirements. Examples would be supplies where the dc voltage may be high, although there may be suppliers who come close enough to what you need, or can modify what they offer. Power supply design is a matter of balancing a number of constraints, including nominal performance, efficiency, thermal issues, cost, complexity and reliability. Still, there are applications in which one parameter is so critical that only a custom supply will suffice, since no commercially available unit is prioritised for that parameter. Another reason to do it yourself is if requirements are looser than the commonly available supplies and you can 'get away' with less. A supply for a basic indicator may have loose nominal specs for output accuracy – say ±5% and few or no transient load issues – so a low cost design may be all that's needed. At the other end, you may have specs that are far better than what's available. A final reason to do it yourself is the availablity of in house expertise. If your company has been designing supplies for years and is familiar with meeting the technical and regulatory requirements and testing to them, then you are ahead of many OEMs whose expertise is in digital design. Straightforward or not? While basic supply design may be straightforward, a fully qualified design which meets all performance and regulatory specifications is not – and that's not taking into account the cost and sourcing of the components in the complete design. Start with the design itself. While chips can implement a complex topology, every ac/dc supply needs many other components. Defining and sourcing these can be a headache, especially when their secondary characteristics play a role. For example, a capacitor is defined primarily by its capacitance and working voltage, but its equivalent series resistance affects its operation, especially at higher frequencies. Even with the right part, you face supply issues. A nominally identical substitute for an inductor, for example, might result in field problems months later. You also need to decide the minimum and maximum operating ranges for your design: will it be for a restricted line voltage, such as nominal 240V ac ±10%, or full range (120/240V ac)? The former is easier to design and less costly, but it also means you need a second design if you are planning to serve worldwide markets. You will also need a test plan: how will you assure your supply works in 'corner cases', such as high/low line plus maximum ambient temperature plus line/load transients all happening at once? Then there's cooling. Are you planning to use convection cooling? Do you have the tools to model your supply and its operating environment to be certain the available airflow will be sufficient? How will the supply be mounted? – that makes a big difference to cooling – and, if you need a fan, how will you size it? When designing your own supply, you're most likely to use a chip or chipset from a vendor who also provides a reference design. Has this been built or is it a schematic supported by simulation? You may find the actual performance is not what you expected, as physical layout, routing and size of ground plane, power and control traces and connectors mean even the best simulation is only a rough approximation of the actual circuit's performance. Even if the reference design includes a pcb layout, you have to be careful if you change the layout or the BoM because an apparently trivial change can affect performance – a power supply is a closed loop amplifier that can oscillate, have transient response issues and both source and be sensitive to emi/rfi. Even a well designed and tested supply faces regulations and standards, which get more challenging as tighter standards are phased in. Amongst the issues are: • basic safety, including nsulation, isolation technique, layout spacing and design topology • emi, determined by the supply's operating frequency, internal waveforms, switching characteristics and layout • efficiency, assessed by the relationship between ac line power and dc output power • power factor correction, which defines how resistive the supply 'looks' to the ac mains as a load. If the supply is not resistive looking, the design must employ techniques to yield a power factor close to unity (IEC61000-3-2). Adding to the challenge is the worldwide nature of regulation, which means you'll be dealing with many authorities and their particular ways of testing. While today's chips, reference designs and tools make it easier than ever to design your own supply, the combination of specifications in the N2Power unit referenced above would be hard for a non expert to achieve, especially when all regulatory and manufacturing issues are factored in. Don Knowles is vp of engineering with N2Power