When specifying power supplies, today's customers invariably want four things - small size, reliability, ease of control and, of course, value for money.
The size
Fifteen years ago, a 1kW SCR phase controlled programmable power supply was typically a 2U product weighing around 30kg. Today, a similarly rated supply is more likely to be a 1U package weighing just 8kg.
With larger supplies, the reductions are even more impressive. 10kW units, for example, have evolved from 7U monsters weighing 150kg or more into relatively compact 3U devices weighing in at around 50kg and delivering up to 1,000A.
Further space is saved because the designs are true 'zero stacking' so no extra space is required above or below the power supplies in the rack.
How have they done it?
Clearly, power supply manufacturers have been taking heed of customers' requests for smaller products, but just how have these size and weight reductions been achieved?
Part of the answer is the adoption of new switching topologies, complemented by the use of extensive circuit modelling during the design phase of the power supply. Modular construction and the increased use of surface mount devices are also helping to increase power density.
Further important gains have been made possible by improving the power factor and efficiency of the supplies. Better power factor means lower input currents, which in turn means smaller conductors can be used. Better efficiency means less heat has to be removed from the power supply, so size and cost can be further reduced.
It is worth noting these size reductions have been achieved without compromising the versatility of the products in any way. The latest compact designs are frequently even more versatile than their predecessors. For example, many accept wide range input voltage ranges, allowing them to operate from any input from 85V to 265Vac, without the need to change settings or make adjustments.
This feature is invaluable for companies that use the power supplies in equipment bound for export markets, as it means that the same model of power supply can be fitted irrespective of the final destination of the equipment. Not only is this convenient, it also helps to reduce inventory costs.
Reliability
Reliability is fast becoming the most important selection criterion for programmable power supplies. The keys to achieving reliability are to rate the components used in the supply as conservatively as possible, and to use highly accelerated life testing (HALT) to refine the design.
Unlike conventional testing, HALT testing is intended to stress the prototype power supply until it fails. This allows the most vulnerable components to be identified and upgraded.
If the new version of the supply is again HALT tested, the next most vulnerable components can be identified and upgraded. This process can be repeated as many times as necessary to achieve the level of reliability required in the finished design.
Reliability is also enhanced by the cooler operation of modern efficient designs, and there are important gains to be made with certain specific components which have in the past been notably failure prone. For example, if the speed of the cooling fans is controlled in proportion to the load on the power supply, the life of the fans can be greatly extended.
Modern designs also eliminate the notoriously unreliable front-panel potentiometers used traditionally for settings, replacing them with dependable optical encoders working in conjunction with digital control circuitry. The best designs go even further and also eliminate internal calibration potentiometers in favour of software-based calibration.
Ease of control
It is fair to say that ease of control has, until recently, been more of a wish than a reality.
There are a myriad of control options in use today - local front panel, remote analogue, serial and GPIB digital, and, most recently, LAN Ethernet. Unfortunately, the way in which families of power supplies evolved in the past meant little effort was usually made to provide uniform control options across the whole family.
As a result it was by no means unusual for, say, a 600W supply and a 2kW supply from the same manufacturer to have completely different front panel layouts and control facilities. This is both irritating and confusing for users.
To address this issue leading manufacturers are making it a design objective, not only to keep control interfaces identical between products of different ratings, but also to allow a seamless transition from front-panel to rear analogue or digital control, with all of the functions available in each case.
Control functions
When choosing programmable power supplies, it is always worth looking at the control functions in detail, taking into account both current and future needs. While many of the basic functions are likely to be similar between manufacturers, some manufacturers offer additional features that can bring major benefits.
A good example is the availability of safe-restart and auto-restart options, which control the way the power supply behaves after a shutdown caused, for example, by a power interruption. With safe-restart selected, the power supply restores the settings in use before the shutdown occurred but comes back up with its output off. User intervention is needed to turn the output back on.
With auto-restart selected, if the power supply output was on when the supply interruption occurred, it will come back on automatically when the supply returns - again with all of its settings restored. While this might be seen as a less safe mode of operation, there are many cases where it is invaluable, including those where the supply is used to power equipment in an unmanned location.
For applications requiring very high currents and/or maximum flexibility of power system design, advanced parallel current summing is another highly desirable feature. This allows a number of power supplies - typically up to four - to be connected together in a master-slave configuration so they behave, for all practical purposes, as a single supply. With an arrangement of this type, the master supply reports current monitoring signals corresponding to the total current sum of the group.
Control interfaces
Control interfaces also merit attention. GPIB interfaces remain popular in many applications, but even with these there can be important differences between suppliers, some of whom now offer a cost-saving GPIB multi-drop option. This allows a single GPIB-equipped master power supply to control several slave supplies via simple low-cost RS485 links, thereby avoiding the cost of having to equip every slave with a GPIB board and cable.
Ethernet is a relative newcomer to the power supply control arena but its importance is growing rapidly. The best advice here is to look for products that are LXI certified.
The LXI standard provides a basis for long lifecycle system implementations which are not limited by bandwidth constraints, software issues or architectures that depend upon specific hardware combination and configurations.
How much is it?
So much for technical matters, now let us look at real-world applications where cost considerations are never far from the prime focus. It is, however, important for users not just to look at the headline prices of products, but also to take into account the cost of options that may be needed, and to consider overall lifetime costs.
To give examples, some power supplies have integral RS232/RS485 interfaces, whereas with others they are an extra cost option. Also, the availability of products with wide range inputs can help to minimise inventory costs for users who sell their equipment in world markets.
Turning to lifetime costs, products with good power factor and high efficiency will consume less power and impose less load on cooling systems. Over the lifetime of the power supplies, the resulting savings in energy costs will repay many times over the small premium that may be involved in the initial purchase of a more efficient unit.
In addition to operating costs, it is important to consider how the manufacturer supports the product after the sale. Many leading manufacturers offer a five year warranty for programmable power supplies with service offered at company facilities or authorised service centres.
The manufacturers of programmable high current power supplies have been working hard to satisfy the ever-increasing expectations of customers. Nevertheless, currently available products are by no means all created equal. Those who do specify power supplies will find it worthwhile to survey the market carefully, paying particular attention to these points, before making the final decision on which supplies to use.
John Breickner is product manager of Lambda Americas High Power Division