What are the power-conversion trends affecting set-top-box design?

The theory that says, "if a unit can be powered off a wall outlet, power-supply efficiency and topology are not a concern" is slowly becoming passe. As high-end digital video recorders (DVRs) migrate in the direction of a compact home PC, they will eventually need to have the same 250W fan-cooled silver box. So, will all set top boxes (STBs) contain a fan at some point?

By taking advantage of the latest switch-mode power-supply ICs, designers will quickly realise that there's much room for improvement in existing designs before resorting to complicated power-supply schemes. With the number of STBs rising, it's imperative we implement efficiency-conscious power designs. Standby power can be reduced through burst-mode circuitry (see Figure), but numerous other small tradeoffs also can aid a power supply scheme.

In this article, we'll discuss ac-dc front-end IC trends, dc-dc topology considerations, and some voltage-rail-management possibilities when dealing with low-voltage signalling. We'll also delve into some of the market pressure justification surrounding STB designs.

AC-DC trends

A low-end STB can simply use a "wall brick" to convert the ac input into a single dc level that's sent into the box. This is a fair solution for low-power units that target a worldwide market. The key advantage is the ability to swap in power bricks based on the vernacular ac voltage levels inherent to the region. Unfortunately, these bricks are some of the most inefficient power-supply schemes available, often consisting of a linear transformer solution with no pulse-width modulation to vary with load requirements.

Higher-end STBs perform this initial conversion function inside the case. While the topology isn't as simple as the brick solution, it's already fairly efficiency-conscious. The major specification focus lacking from most designs revolves around standby-power monitoring. At the instant an STB is plugged into a wall outlet, it's consuming power that's awaiting a response from the remote control or corresponding with a cable network interface. Not long ago, standard STB active power levels ranged from 12 to 25W, while the standby power still averaged between 10 and 22W. This standby-power level should be regulated to within 3 to 15W, depending on device complexity.