TechArchive

Original article date: March 1996

Omron’s John Hickles considers the essential ac-dc power supply technologies and examines some of the features that modern designs can offer.

The power supply may not seem the most glamorous aspect of a system design but needless to say most systems would be in serious trouble without one. Feed commercial ac power directly into ICs and other sensitive electronic components in automated office and factory equipment and the results are spectacularly destructive.

Although the power requirement for any panel or system is generally fixed at the earliest stages of design this does not make selection of the power supply itself a trivial matter. There is a wide range of strategies to adopt and an even larger number of devices to buy off the shelf. Get it right and you’ll have a compact supply that will add functionality to your system.

Throw it in as an afterthought and chances are you’ll wind up with a huge transformer or control box bolted to a corner of your panel over specified and almost certainly taking up more power than it needs to.

No matter what area of product and system design you’re working in the space you have to play with is decreasing while the level of functionality is going up. So power supplies are getting smaller and current ratings are getting lower.

There are a number of power supply designs available to provide regulated dc from a commercial ac source of which switching linear and CVT (constant voltage transformer) are among the most prominent. Of these switching and linear power supplies are by far the most common. In terms of market share some 90% of today’s power supply requirements are met by switching devices. CVT designs though reliable and limited in the number of internal parts are generally large and heavy and so typically outside the scope of product and panel design.

Switching power supplies rectify the full waves of commercial ac power into dc and then step down the voltage with a high frequency (40 to 200kHz) transformer. Switching semiconductors control the switching pulse width and frequency to stabilise the output. The designs are highly efficient compact and light. They provide wide input voltage ranges and can offer features such as maintaining an output for a certain period after the input power is turned off. Some switching noise is generated but this can be minimised and is rarely a problem.

Linear power supplies convert commercial ac into dc power via a step-down transformer as a first stage followed by a rectifier and linear control circuit which steps the output down further to the required DC level and stabilises it. The output can be very stable indeed and no noise is generated but the transformer is typically very heavy and a lot of heat can be generated by the power transistor. Efficiency too can be a drawback typically in the order of 40% compared with better than 75% in switching power supplies.

Design considerations

In selecting the right power supply for the job there are a number of factors that need to be considered. Input voltage and output capacity are clearly the most important but other points include relevant safety standards and regulations that are met and how the device will be mounted.

You only have to look around at the commercial voltage standards of various countries to see why input voltage is important. Around the world you will see mains supplies of 100V 120 220 230 and 240V with frequencies of either 50 or 60Hz. If you’re designing a product to be universally applicable around the world it has to start with a power supply that will accept a universal ac input.

Safety is a key issue for all equipment and strict rules and regulations have been established such as the USA’s UL standards Canada’s CSA standards and Germany’s VDE standards. Designed to protect against electric shocks fire and other hazards these standards stipulate the construction and capabilities of individual products.

In addition to these we now have EMC regulations and CE marking to take into consideration. Power supplies are classed as components and although in the UK there is currently no legal requirement for manufacturers to ensure that their components meet the requirements of the EMC Directive this is not the case in other countries.

Omron’s policy is to build all its components to meet the EMC Directive believing that users of power supplies for example have a right to expect the manufacturers to provide inherent EMC conformity. Any final assembly or system will have to conform and the route to conformity can be that much easier if power supplies and other components offer compliance with the Directive.

Added features

Along with meeting the general power requirements of a system modern design also allows a number of added features to be built into power supplies enhancing the overall functionality of a panel or product. Such features might include visual indication (and alarm output) to warn that excess power is being drained so that there is warning of a fault before a critical system fails. A typical implementation might provide a red indicator light which is lit if the output drops to between 90% and 75% of the rated voltage. In addition to the indicator light an alarm output could be provided which could be used to trigger a warning message on an operator terminal.

Protection against current overload and overvoltage are other features that can be incorporated into modern power supply designs. When the current rises above 105% of the rated output for example a protection system could be triggered which decreases the output voltage. This overload protection could be automatically cleared when the output current falls back within the rated range. Power supplies can also offer overvoltage protection shutting off the output voltage if it should rise above typically 120% of its rated value.

Because the load wires connected to a power supply can themselves cause a voltage drop a remote sensing function can be added to detect this drop and compensate for it. Remote voltage adjustment can also be provided typically by inserting a variable resistor between the remote sensing (V+ and S+) terminals. Although many power supplies provide in-built adjustment the physical location of the power supply can make access to this difficult. By the same token a remote control function can make it possible to turn the output voltage on and off with an external signal while leaving the input voltage turned on.

• Omron

March 1996