Considering referencing power supply data sheets

Most data sheets for AC-DC or DC-DC power supplies are just 2 or 3 pages.  The headline specifications in these ‘shortform’ data sheets lean towards best-case figures with respect to power supply performance, in everything from power ratings to efficiency figures. In some instances, you may need to dig a little deeper to reveal the likely performance of a power supply in your application. This article considers five questions that shortform data may leave unanswered, in which case ‘full’ data sheets running to 20 pages or more will often be available and should be requested. A few of the questions only apply to the AC input of AC-DC converters, but most are equally applicable to DC-DC units.


Does the power rating apply at all specified input voltages?

Some universal input (90-264VAC) AC-DC power supplies need to be de-rated by 25% or more at the bottom end of the input voltage range. Others are specified as having full output across the range. A similar issue may apply to DC-DC modules - particularly those that have the ability to handle a wide range of input voltages.


Are there any other limitations that rate to the claimed power rating?

Power ratings are given at a defined ambient temperature and under specified cooling conditions. Convection cooling means just free air circulating around the unit. Forced-air cooling will often increase the power rating significantly, sometimes by as much as 50%, but the trade-off is the need to use a fan. This adds cost, complexity and a maintenance requirement (cleaning air filters), while reducing system reliability and efficiency. The final choice will often be determined by whether or not space is available for a physically larger power supply that doesn’t need a fan.

You also need to watch out for reduced power ratings at elevated ambient temperatures. Most power supplies are specified as delivering full rated output at up to 50 degrees C ambient but de-rating applies above this figure. Once again, the optimum power supply will be dependent upon the application.

 Figure 1: The power rating of these UNO series AC-DC power supplies from Phoenix Contact needs to be de-rated by 2.5% per degree C above 55 degrees C ambient temperature (up to 70 degrees C maximum)


Is the inrush current quoted as ‘cold start’ or after warm up?

Thermistors used to limit inrush current are usually switched out of circuit after warm up. Switching a ‘warm’ power supply off and back on again can result in a much higher inrush current. You may need to know how much higher so that you can choose suitably rated switches and fuses.


Under what conditions is earth leakage current specified?

Of most concern in medical applications, earth leakage current will vary with input voltage and temperature. It shouldn’t be of much concern if power supplies are medically certified but this in itself is a complex area where defined requirements vary by the type of medical application. If in doubt take specialist advice.

Figure 2: This external 100W power supply from TDK Lambda has CE, UL60601-1, EN60601-1 and IEC60601-1 certification for medical applications and typical leakage current of just 300µA at 246VAC 60Hz input



Are there any minimum load requirements?

Minimum load requirements most frequently apply in the case of multi-output power supplies – most single-output units no longer need a minimum load to maintain stability. It can add some complexity to the design of a power system if you have to ensure that all outputs experience load conditions, and it impacts efficiency too.


Figure 3: This compact, 100W, TML series AC-DC power supply from TRACO POWER has no minimum load requirement but ripple and noise will be higher if the load is not at least 3% of the maximum rating for a given output.


In summary, it’s important to remember that power supply manufacturers write data sheets without knowing anything about your application. This applies equally to power supplies from leading brands such as TRACOPOWER, TDK-Lambda, Emerson Network Power, and Phoenix Contact as it does to own brand embedded SMPS units from RS Components. Specifications relate to a specific set of conditions, and not all data sheets state performance under the same set of conditions. More detailed information is usually available in full data sheets or from power supply specialists at RS Components.


What’s the ripple and noise performance above 20MHz?

Most data sheets quote ripple and noise at 20MHz bandwidth but for some systems, the levels above this frequency can also be of concern. It pays to ask if this applies to your application.


Under what conditions are regulation figures quotes?

Line and load regulation may be quoted over a limited input voltage range or over the total range. In particular, it’s not uncommon for the figures to be based on a minimum load of 50% and variations can be much greater beneath this figure.


What happens to efficiency as the load decreases?

Most power supplies are at their most efficient when operating at, or just below, maximum load. They can become significantly less efficient below this level, generating more heat that needs to be removed through effective thermal management. The efficiency of a power supply in a given application may therefore not be anything like the figure quoted in the data sheet.