power supplies 15
opposite end of the scale, a rack-mounting mainframe may accommodate more than 20 plug-in modules of differing output voltages and currents that deliver in total more than 2kW from a single-phase ac input. For some applications forced-air cooling will be essential, which introduces restrictions such as minimum clearances between the supply’s chassis and other hardware for air entry and exhaust paths. By contrast, the desktop supplies that most often power IT & medical equipment are enclosed and self-cooling, which normally restricts their power capabilities to about 150W. Most such units are single output, but multiple output versions are available.
Always check the converter’s efficiency curves for the best fit for your application. Energy savings apart, it’s worth remembering that as power supply efficiencies approach 90 per cent, a 1 per cent increase in efficiency reduces dissipation by as much as 10 per cent, with implications in not only cooling the power supply but in the heat generated within the end equipment. Recently legislation has been introduced stipulating
the minimum efficiency and no load power consumption of external power supplies. These include EISA (Energy Independence & Security Act 2007), CEC (Californian Energy Commission) and Energy Star. These pieces of legislation are specifically aimed at desktop and plug top power supplies due to the volume in use. It is thought that there are currently 1.5 billion externals in use in the US alone, which account for 6 per cent of the national electricity bill. In Europe it is thought that reducing the no load power consumption of external supplies will save up to 5TWh per year by 2010. When selecting your external power supply, it is essential to ensure that it meets these directives otherwise you could find the market for your end equipment being severely restricted. Other power supply selection criteria may include methods of fault protection, remote sensing as well as ways to control output rails such as soft-start and, using multiple supplies, sequencing.
Virtually all supplies include some form of current limiting. Two popular techniques are foldback, when the supply progressively reduces its power output as an overcurrent event takes hold, and ‘hiccup’ mode when the supply successively removes and re-applies power until the fault condition clears. Power supplies will typically have some form of over voltage protection where the unit is shut down by an independent control loop. Internally, many supplies also monitor their own operating temperatures at critical points and shut down if they detect a dangerous situation.
Remote sensing is used in high-current applications where the lead resistance from the supply to the load introduces unacceptable voltage drops. A 4-wire connection measures the voltage at the load and overcomes cable losses to regulate the output voltage at the load.
The simplest alarm/status indication is the power- good signal, which is basically the output of a voltage
Fig. 2. The EMA212 single output 212 Watt ac/ dc supply achieves 90 per cent efficiency with a power density of 10.6Watts per cubic inch in a standard 3x5 inch format.
comparator that reports the supply’s output level is within a determined threshold. Others include input power fail, over-temperature warnings and fan fail. More sophisticated supplies may include a digital interface that permits a range of programmable control and status reporting facilities. The industry-standard PMBus (power management bus) protocol that many supplies implement consists of a defined language together with SMBus (system management bus) hardware that is very similar to the two-line I2C bus. These allow the host system to continuously monitor the supply and to control many of its operating characteristics.
These are the issues most users need to consider, but others may apply. For instance, in a high-reliability scenario, it’s normal to include additional supplies that can take over if a main supply fails. The level of this redundancy technique is cost and system dependent. When backing up each supply with another (100 per cent redundancy) is too expensive, designers employ ‘n + m’ redundancy to improve reliability, with maybe one spare supply per four operational units theoretically providing a 20 times availability increase. m
Stephen Dodson is Applications Engineering Manager at XP Power in Reading, UK.
www.xppower.com
www.engineerlive.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36