Power Boxing

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.

Solutions like Fairchild Semiconductor's Power Switch provide a platform suited to the design of cost-effective flyback converters while maintaining minimal standby-power consumption. This is done through a burst-mode feature within the integrated PWM and MOSFET. As the load decreases, the feedback voltage decreases. The device automatically enters burst mode when the feedback voltage drops below a set level. Switching still continues, but the current limit is set to a fixed limit internally to minimise flux density in the transformer.

In fact, switching continues until the feedback voltage drops below the low-side threshold. At this point, switching stops and the output voltages start to drop at a rate that depends on the standby current load, which causes the feedback voltage to rise. Once it passes the initial high-end level, switching resumes. The feedback voltage then falls and the process repeats. Burst-mode operation alternately enables and disables switching of the power sense FET.

DC-DC trends

Ideally, the ac input power is rectified to the highest power-consumption voltage levels in the STB. That is, if the core ASIC or hard-disk-drive assembly operates off a 5V rail, the initial ac-dc step down should result in a 5V output. If more than one rail is required, it's becoming common for transformer manufacturers to provide such solutions via off-the-shelf (OTS) transformers.

It's necessary to further regulate and control dc levels throughout the board. A convenient solution for a 2.8V supply required by a low power ASIC or display driver, at the point of load (POL), is a quick linear regulator or low drop-out regulator (LDO). Even at minimal current requirements, say 500mA, the power absorbed in an LDO is equivalent to the voltage drop, and can be rather alarming. If a 5V rail is the primary available level, it may be more efficient to evaluate an integrated dc-dc switcher.

Devices similar to the FAN2011 are a good fit for POL needs. This device is a high-efficiency, low-noise, synchronous PWM current-mode dc-dc converter designed for low-voltage applications. It provides up to 1.5A continuous load current from the 4.5 to 5.5V input. The output voltage is adjustable over a wide range of 0.8V to VIN by means of an external voltage divider. Often times, a portion of the STB is idle and can be powered down. The FAN2011 is always on, while the FAN2012 has an "Enable Input." In this case, the device can be put in the shutdown mode, whereby ground current falls below 1µA.

In the past, size and voltage-level ranges of integrated dc-dc solutions have been difficult to fit within STB specifications. With new devices of this type being introduced by various semiconductor vendors on what seems like a daily basis, it's becoming easier to find a perfect fit for a given application. These days, a well-designed STB may contain two to four integrated dc-dc switchers. The trend is to have less LDOs and more switch-mode power supplies (SMPS)—but not without some caveats.

Cost and level of complexity are higher for SMPS solutions when compared to a simple LDO, but the benefits far outweigh these increases for most designs. A SMPS solution, such as the example mentioned above, can exceed 90% efficiency, while some LDO solutions struggle to meet half that level of efficiency.

Market pressure

As digital TVs become the standard, an STB will be required to decipher even terrestrial signalling. Also, most homes with an STB crave the next feature-rich solution, similar to the home PC frenzy of several years back. It's obvious that STB designers are on a swift pace to meet consumer needs, occasionally overlooking details that appear minor at first glance (e.g., power conversion).

With the number of set-top-box manufacturers also on the increase, a slip of a month or two in a program's cycle time could result in a competitor receiving the spoils. Consequently, the evaluation time of a newly integrated SMPS has got to be on a tight schedule, and the risk must be minimised. To do this, it's important to take full advantage of the semiconductor vendor's collateral. Often times with new devices, there's a recommended reference design that only requires minor tweaking.

Ed Suckow is a technical marketing engineer with Fairchild Semiconductor.

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