Which controller is better for AC/DC power supplies: discrete or combined?

By VA Viswanathan

If you have ever heard hallway conversations at a power supply design company, they are likely to be arguing over whether the design of a >75W power supply should use the architecture shown in Figure 1 or Figure 2. In reality, the power supply components are exactly the same for both architectures, with the only difference being the controller.

Figure 1: AC/DC design based on discrete controller IC

Figure 2: AC/DC design based on a combined controller IC

TI has supporters of both architectures and has products based on both.

Although TI has long had a rich portfolio of combined controllers and the ability to implement more functions in the same solution, I still believe that in the long run, standalone power factor correction (PFC) controllers and standalone DC/DC converters provide engineers with unparalleled advantages in designing a variety of applications, especially in the current environment where consumers are cost-conscious.

layout

Most power supply designers will tell you that printed circuit board (PCB) layout is their most frustrating power supply design technique. Poor layout often leads the design process into a dead end. Power supplies come in all shapes and sizes, requiring very different layout considerations depending on the shape requirements.

Figure 3 shows a 150W compact LED TV power supply reference design that fits the form factor. Note the considerable clearance between components. Additionally, the “small form factor” requirement in TVs keeps the layout very low, making large components look obtrusive on the board. The PFC stage and the inductor-inductor-capacitor (LLC) stage are far apart, making it very difficult to achieve an optimal layout with a combined controller. Discrete solutions do not have this problem because the controllers are in close proximity to their respective power stage components.

Figure 4 shows a 350W high-efficiency AC/DC power reference design in a form factor for industrial power. This is a space-constrained design that looks more like the dense and compact downtown of Dallas than a low-cost two-layer PCB layout. Using a single combined controller to control the signal lines is like using a large parking lot to park all of Manhattan with only one one-way street, making it very difficult to get in and out.

            Figure 3: TV power supply Figure 4: Industrial power supply

There is a fundamental conflict between the uniqueness of power supply requirements and the one-size-fits-all nature of modular controllers. Some supplies are constant current, some are constant voltage, some require high total harmonic distortion (THD), some require low standby, some are fixed load – but few require all of these at the same time.

As long as power diversity persists, there will be logical reasons why a 2-IC solution may be a better fit for your application.