Simplifying PSU selection from the top down
With such a variety of parameters to consider, system designers can find the task of selecting a power supply increasingly daunting. Regulatory issues and environmental considerations add to the complexity. Planning is the key to finding the solution that best suits your application, and it is the application you are designing that drives this top-down process. The first step is to fully understand the application’s power requirements as well as understanding how you want to integrate the power solution.
Inputs and outputs
Considering the input and output criteria is a good starting point. The majority of offline AC/DC convertors provide a universal input voltage capability spanning some 90 to 264 VAC at 47 to 63 Hz typically. This is an advantageous feature allowing configuration-free use anywhere in the world. System designers don’t need to worry that end-users might set up their equipment incorrectly. However, remember that the input fuse needs to be sized for the lowest operating voltage as the optimal input-protection fuse ratings are seldom the same for 110V and 230V operation since these voltages mean a 2:1 spread in steady-state input current. It is also worth noting that some power supplies in the market derate significantly at low line input, below 110V, so a check of the derating curve on the datasheet is recommended.
There are some applications that require the AC/DC convertor to drive load currents directly, instead of through further DC/DC conversion stages, so power supply manufacturers offer an extensive choice of current options and output voltages, including the standard 5V and 12V that traditionally power systems made up of discrete logic and analogue circuitry. In systems that use complex digital logic ICs there has been a tendency towards continually lower voltage operations in recent years, but despite this, there are many industrial designers who prefer these levels as they still offer better noise margins in electrically hostile environments. With output voltages from 3.3 to 48VDC, AC/DC supplies are easily available in single and multiple output combinations. For Distributed Power Architectures (DPA) and Intermediate Bus Architectures (IBA) single output supplies are often used, and Point of Load (POL) convertors are employed to provide the various required voltages at the local level.
It can be quite tempting for designers to over-specify the power supply’s output current capability to ensure safe operation within ratings under any and all conditions, but this can be a costly mistake, not just in terms of initial purchase cost, running costs could be adversely affected too. It is important to note here that the majority of convertors only operate at their maximum efficiency levels at the higher end of their operational range, typically 80 to 85%, so are nowhere near as efficient below 50% of their full load. In addition, some multiple output power supplies also need a minimum load to maintain regulation.
Of course, the physical dimensions of power supply units are greatly influenced by the number of output rails as well as the output power required. In typical terms both open-frame and encapsulated board-mount AC/DC convertors have between 1 and 3 outputs, starting with power levels as low as 5W. An example at the smaller end of the scale is XP Power’s ECE05 encapsulated pcb mount AC/DC power supply, measuring just 25.4 x 25.4 x 15.24 mm. An example at the other extreme might be a rack-mounted mainframe accommodating 20 or more plug-in modules, each with different currents and output voltages which combined could deliver more than 2 kW from a single-phase AC input.
Another consideration around the physical form is cooling. Forced-air cooling will be essential for some applications and with it this brings restrictions, like the need for minimum clearances between the chassis and other hardware for exhaust paths and air entry. Contrastingly, other applications need to be completely enclosed and self-cooling like desktop power supplies designed to power IT and medical equipment, but this in itself tends to restrict power capabilities to around 250W. The majority of these units are single output; however, multiple output options are available too, if required. In terms of DIN rail formats, a conventional PSU will provide just one rail with power levels from some 5W up to around 1kW. But usually, space is not a critical factor in DIN rail environments so if another output voltage is required, you can simply add a DIN rail mount DC/DC converter.
It’s crucial to look at the convertor’s efficiency curves in order to select the most appropriate fit for your application. Quite apart from the potential energy savings, it’s important to note that as the efficiency of power supplies move above 90%, an increase in efficiency of just 1% can reduce dissipation by a massive 10% with obvious knock-on implications for both cooling the power supply and in the amount of heat generated within end equipment.
There have been a number of developments in legislative requirements regarding energy consumption for power supplies over recent years. This is because as recently as 2003 there were no mandatory regulations in place, but because power supplies were not designed to shut down when the device they were powering was inactive, this led to a huge waste of energy on a global scale. Concerns about this ‘vampire’ power drain directly from live main sockets was the inspiration for the main focus of the original directives.
Many areas around the world have introduced legislation for no load power consumption and active mode efficiency for external power supplies. In the USA there are a number of bodies legislating on energy efficiency; the California Energy Commission (CEC), US Congress with its Energy Independence and Security Act (EISA) and recently the United States Department of Energy (DoE). There is also Energy Star which sets limits for electrical and electronic equipment. In Europe there is the Energy related Products (ErP) Directive formerly known as the Energy Using Products (EuP) directive, which is mandatory. There is also the EU Code of Conduct for external power supplies which is voluntary. In Canada there is Natural Resources Canada (NRCan) and in Australia the Minimum Energy Performance Standard (MEPS).
These are a just a few examples of mandatory requirements written into legislation. In recent times both the US DoE and the EU CoC have set new, more demanding, standards for both energy efficiency and no load power consumption. The EU CoC has also introduced a 10% load efficiency requirement reflecting applications which spend a large proportion of time using minimal power from the external power supply and has two tiers to drive future development. The DoE and EU CoC were both introduced in early 2014 with both the DoE and EU CoC tier 2 requirements coming into force in 2016. These new requirements mean both increased active mode efficiency and reduced no load power consumption.
In the United States an estimated 6% of the national electricity bill is taken up with powering some 1.5 billion plug top and desktop power supplies each year. When the Level V standard was implemented in Europe a number of years ago, it is thought to have saved in the region of 5TWh each year too. It is essential to make sure your power supply meets these directives or you risk restricting the potential market for your end equipment.
Most AC/DC power supplies will be designed to operate from 0°C to +70°C, with some operating from -20°C and up to +80°C. Critically, there will generally be a derating for full load operation, this can be from as low as 30°C. In practise, derating at temperatures this low could be impractical due to the temperature build up in the end customers equipment. Specifying a power supply that will operate at full power up to +50°C should suit most industrial and medical applications, whilst allowing for a little headroom.
Status reports and control
Generally speaking, all power supply units have some form of current limiting built in. Popular methods include ‘hiccup’ mode where the supply removes and reapplies power successively until the fault condition clears; and ‘foldback’ mode where the power supply reduces its power output progressively as an overcurrent event takes hold.
Power supplies generally employ over voltage protection in some form, effectively shutting the unit down through an independent control loop. Many power supplies will have the functionality to monitor internal operating temperature at critical points, allowing it to shut down if the temperature reaches a dangerous level.
For some end-user applications, it’s useful for the power supply to provide a power management interface and provide the host system with information in order to avoid data loss. Facilities at the system level might include status reporting, digital interfaces, and fault detection alarms.
Power-good is the simplest level of status indication and is simply the output from a voltage comparator reporting that the power supply’s output level is within the required threshold. Other status indication options could include over-temperature warnings, fan fail warning and input power fail warning indicators.
More sophisticated supplies are also available that implement the industry-standard PMBus (Power Management Bus) protocol which provides a fully defined command language that is used together with SMBus (System Management Bus) hardware allowing a digital interface with a range of programmable control and status reporting functions.
These are the major considerations to take into account when selecting a power supply unit for your end product, but of course, depending on your intended application, many others might apply. Always check regional legislative requirements if your device is intended for a particular area or vendor. And remember, any power supplies intended for medical or military applications will require appropriate certification.