Advanced Energy

<p>The basic scenario of co-existing rich and real-time OS implies first off that the hypervisor must be real-time capable, meaning that it has bounded and short interrupt latencies.</p>

Eighty-five percent of all embedded developers use C or C++. Any other language is a non-starter,” said David Kleidermacher, chief technology officer of Green Hills Software. I don't have much hope a new parallel language will get a foothold,” he added.

There's nothing quite like a power supply datasheet for providing specifications that are open to interpretation, vague or downright misleading. Two apparently similar units can deliver widely differing performance and reliability. As in most things we buy, cheaper power supplies are usually cheaper for a reason – but maybe it's a reason that's buried deep in the detailed specification. This article looks at a typical datasheet for a mid-power AC/DC switcher and, from input to output, looks at how specifications are quoted and what questions need to be asked to make their meaning unambiguous. It highlights the issues that are most susceptible to 'specmanship' and suggests questions that need to be asked to ensure that a chosen power supply lives up to expectations. Most of the issues explored are equally applicable to DC/DC converters.


Starting at the input

Most AC/DC power supplies are now designed for 'universal' AC input with an input voltage range of 90-264VAC quoted. Particularly if the power supply is to be used near its full rating, it's important to check that full power is still available at the lower end of this voltage range. At lower input voltages the input current rises and, particularly in power supplies with active power factor correction (PFC), the switching current in the boost converter becomes relatively high, so overall efficiency falls. The available output power may have to be reduced by up to 30% at 90VAC compared with the quoted nominal power output. If the datasheet does not state 'full power from 90-264VAC', or something similar, look deeper into the specification for a de-rating specification or graph.

Input current is stated to enable correctly rated switches and relays to be specified – it's a function of output power, efficiency and power factor and is unambiguous. Inrush current figures are more open to interpretation and 'specmanship'. This is often specified as 'cold-start' inrush current. Here, a thermistor is used to control the inrush current, its resistance falling as it warms up. However, switching the power supply off and on after it has been running for a while can produce an inrush current many times the cold-start figure. While this is not an issue that affects the power supply itself, the higher current needs to be taken into account when selecting fuses and switching components. In many higher power units the thermistor dissipates significant power, so it normally gets switched out of circuit after the unit warms up. The device then cools and works again when it is switched back in before start-up. So, from a system design point of view, it's important to know whether inrush current figures are based on a cold start or for all conditions.


Earth leakage current is normally not an issue except in medical applications, where low figures are required to meet various international specifications that have been established to protect patients. Remember to check the input voltage at which leakage current is given and that it doesn't vary too much with temperature. These factors will affect leakage current in the final application.

The only other input specifications provided for most power supplies are fuse ratings and whether both line and neutral are fused. Fuse ratings are for fuses that are not usually user-serviceable; the fuses are over-rated and are there to protect against catastrophic failure of the power supply. In-built overload protection will operate faster in all other instances. Dual-fused power supplies are usually only required for medical applications.



Output voltage is specified at the connector of the power supply so it's important to consider the expected volt drop between power supply and the point of load, especially in low voltage, high current applications. In some instances it may be possible to adjust the power supply voltage upwards manually, typically by up to 10%, to compensate for voltage drops in the system. In others, particularly applications where the load is variable, the system designer may have to choose a power supply with a 'remote sense' function. This provides automatic compensation by detecting the voltage at the point of load and adjusting the power supply's output level to even out variations. Designers also need to be aware that varying one output rail on a multi-output power supply can result in the same percentage change in all the others. If this is a problem, independently regulated outputs will be needed, adding to the cost and size of the power supply.

Indeed, DTC estimates there were more than 12 million IPTV subscribers at year-end 2007, nearly double the number at the close of 2006. With several systems having achieved the million-subscriber mark, IPTV is now firmly established. DTC expects continued growth from IPTV, with subscribers forecast to reach more than 65 million by 2013.

In the larger context of the multichannel pay TV universe, however, IPTV remains a bit player. STB shipments for IPTV totaled just 8 million units in 2007, compared with nearly 40 million direct-to-home (DTH) satellite and nearly 37 million digital cable shipments. And the ratio is not expected to change dramatically over the next five years.

China and India alone number more than 500 million analog cable subscribers between them. The digitization of those markets will be a force in keeping digital cable STB shipments above 40 million units annually after 2009. DTH satellite, with its ability to blanket huge areas with one digital signal, will also show sustained growth worldwide, maintaining nearly double the annual shipment levels tallied for IPTV.

All STBs are not created equal. Set-tops may offer merely an MPEG-2 decoder and conditional access technology, or they might include high-end features such as digital video recording and high-definition capability. Much of the demand in the cable and DTH segments–including demand in high-growth markets such as China and India–is for lower-end, inexpensive, plain-vanilla digital STBs. DTC estimates that about 85 percent of all digital cable STBs shipped in 2007 were of the standard digital variety; for DTH, about 80 percent of all units shipped were in that category.

Though they have logged fewer overall shipments, IPTV STBs have trended to the more-sophisticated end of the spectrum. Part of the reason is that many greenfield” deployments have opted to future-proof their service offerings by beginning with the newest technology in the first place.

This article appeared in September 2009 issue of Embedded Systems Design (Europe) – ESD – to an electronic version of the whole issue is available for download.

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