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PowerLab Notes: How to Simplify an AC/DC Adaptor Design
If you have been shopping for low power AC/DC controllers on the TI website lately, you may have noticed the UCC28910. This little part is kind of a big deal! It is the first foray into high-voltage integrated FETs that like many of our latest flyback controllers, includes the advantages of primary side regulation (PSR). By combining a controller with an integrated FET and PSR, you can significantly simplify the design of an AC/DC converter. When you are trying to design anything, power supplies included, keeping it simple almost always provides the best answer. Reducing complexity generally improves reliability, reduces size and reduces cost.
Figure 1 shows just how simple a design can be using the UCC28910. This is the schematic from PMP9171, which generates an isolated 15V at 250mA from a universal AC input. Firstly, the primary side of the schematic is greatly simplified by pulling the FET inside the package with the controller. An additional benefit of doing this is that now, the start-up power can be drawn from the FET and its connection to the input voltage, through the transformer. Secondly, PSR eliminates an optocoupler, a TL431, and a handful of resistors and capacitors for compensation. PSR accomplishes this by regulating the output voltage while monitoring the voltage on the bias winding of the power transformer.
Figure 1. The PMP9171 reference design is a good example of how the UCC28910 provides a simple solution with minimal external components.
Figure 2 shows pictures of the PMP9171 circuit board, highlighting how simplifying a design translates into a compact design. The absence of the external FET, optocoupler and TL431 saves lots of board space and makes layout much easier.
Figure 2. Photographs of the PMP9171 assembly.
Of course, there will still be a need for controllers with external FETs. Consider that a controller with an external FET gives you the flexibility to pick a FET that is best suited for your design. You get to choose the voltage rating and resistance. With an integrated solution, your choices are limited. If you are trying to achieve the highest performance with your design (e.g. very high efficiency) you should probably use a controller with an external FET. However if you are concerned about small size and cost for your low power AC/DC supply, you may want to consider simplifying the design by using the UCC28910. We already have several UCC28910 designs in PowerLab, with many more to come. Here is a sampling of some the existing designs…
PS. We'll be demonstrating the UCC28910 in TI’s booth #805 at the Applied Power Electronics Conference (APEC) in Fort Worth, Texas, next week, March 17-19. If you are attending, swing by the booth and let us show you a demonstration. If you can't attend, email subscribe to the Power House blog or follow #TIdoesAPEC on Twitter to see a video demonstration of the UCC28910 and participate it the #TIdoesAPEC sweepstakes.
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