Should you buy a pre-made AC-DC converter solution or design your own? We tell you what factors you need to consider when choosing to design or buy your AC-DC Power Supply solution.
While most electrical components require a DC voltage, nearly everything needs to plug into the wall. Depending on where you live the wall could supply 120VAC at 60Hz, 220VAC at 50Hz, or something in between, but it will always be a large AC voltage that needs to be converted.
When to Buy
Standard power supplies are a commoditized product at this point, so it is unlikely that the average consumer will be able to create a cheaper stand-alone supply than they could simply purchase. When a transformer itself is a few dollars and an enclosed, UL-certified 5V supply with a cable is less than $10, it becomes impossible to justify the engineering time to create your own.
Even more expensive or specialized supplies are typically worth purchasing in finished form. Medical supplies can be several hundred dollars, but have already passed all necessary certifications that can cost a company tens of thousands in agency fees and were designed by engineers with experience in the rigorous standards for medical supplies.
When to Build
Power supply solutions are not one-size-fits-all. Sometimes the efficiency loss incurred by dropping from the standard 5V out of an adapter to the required 3V is simple unacceptable in your application. Sometimes an external adapter is completely off the table because you need your product to plug directly into the wall, so you have to do the AC/DC conversion on-board.
Tips for designing AC to DC power supplies
Most switch-mode power supplies (SMPS)* are centered on an ac to dc regulator or controller. Typically very little power actually flows through the controller; it is there to sense the characteristics of the input wave and control send signals to the peripheral components (like MOSFETs) that actually do the heavy lifting. Your largest expenses will likely be the transformer, large capacitors, and inductors that are found in most AC/DC designs and dictate the characteristics of the DC output. Changing the values of these components can change the output voltage and current, as well as refine the quality of the output signal by controlling voltage tolerance and current ripple.
The controller you choose will likely have an example application circuit in the datasheet and any math necessary to help you choose the values of any peripheral components. Every application is different, so you will probably need to make edits to the supplied circuit. Even if the input and outputs happen to already fit your application, these examples rarely include things like circuit protection that do need to be in your final design.
If you want to take the fast track, check out our huge collection of reference designs for single and multiple output AC to DC power supplies. You can search by application or keyword to find a design that is close to your needs, or search by a controller family to see what other people have done. Results based on the popular VIPer family from STMicroelectronics include a 6W constant-current LED driver, a 300W 3-phase industrial motor driver, and hundreds of other designs. Most of these designs are also available as physical development boards, so you can try the parts in your own environment before ever even laying out a board.
When you decide to create your own AC to DC solution, every detail counts. Several suppliers have tools to help you optimize efficiency and board space, but do remember to give appropriate thermal consideration to any current-carrying components and to always, always point those big electrolytic capacitors away from you the first time you power up a board.
Dig deeper into power supply design with our series of articles, including our breakdown of the types of switching DC to DC converters and our overview of how a switch mode power supply works. Does your design require an uninterruptible power supply? Find out.
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