By Steve Roberts, Recom International Power

With most electronic components, the cost/performance ratio is of critical concern. An AC/DC module costs many times more than the equivalent AC line transformer, diode bridge, smoothing capacitor and linear regulator. It even costs more than a discrete AC/DC switching regulator made up from individual components. So where is the performance advantage? This is perhaps best explained with a concrete example.

Take a typical industrial application: an AC-powered PID temperature controller. The specification might be: universal input voltage (110/230 Vac), relay output, LED display, remote monitoring output and standard DIN panel mounting case. Of course, it should meet the usual safety and EMI standards. A quick guesstimate for the power supply loads gives 1W for the display, 1W for the signal conditioning electronics and relay, and 1W for the 4-20mA current loop driver. We want to keep costs down and reliability high, so we will use a standard 7107 LED display driver and analog circuitry rather than a microcontroller (see Figure 1).

So far, so good! But now we have to fit all of this into a 72 mm x 36 mm DIN standard case. After laying out the board, we find that we have about 50 mm x 30 mm real estate free on the board and around 20 mm of height available for the AC power supply (see Figure 2). A standard flat-style transformer with 3.2 VA rating already fills most of this space up, and we still haven't considered the space requirements for the rectification, regulation, input voltage range switch or input filter components. Furthermore, the transformer is the heaviest component in the design and will need additional screw fixings to avoid excessive stress to the circuit board. We could design our own discrete converter if we were familiar with switch mode power supplies, but would you like to be on the production line doing the calibration and final QA check with exposed live components and voltages exceeding 325 Vdc on the board?

This is where the advantages of an AC/DC module suddenly become obvious. For example, the Recom RAC05-05SB is a 2" x 1" module with only 15 mm height but packs a fully regulated 5W converter with universal input voltage (90~265 Vac) into this tiny space and yet weighs only 30 grams. The module is completely potted and sealed, so no dangerous internal voltages are present to shock the careless or unwary. The 3kVAC isolation and built-in filters mean that the module will meet CE, TÜV, EN60950 safety and EN55022 EMC standards with no external components, and the short circuit protection feature means that it is safe to allow the 5 VDC supply to be led out as an auxiliary supply to the connectors at the back.

The auxiliary power supply connection has a double function. With the AC/DC converter powered down, an external 5V supply can be connected to these terminals without damaging the converter. This means that all of the function tests and adjustments can be made to the temperature controller on the production line using a safe DC power supply. In fact, in this case, there is no need to test the controller with live AC power at all as every Recom AC/DC module is 100% tested and guaranteed.

There will always be applications where the low cost of a standard linear AC/DC power supply is more important than its bulky size and heavy transformer, and there will always be large volume commercial applications where the costs savings due to a discrete AC/DC converter solution outweigh the disadvantages of hazardous voltages on the PCB and the extra costs for certification, but for many other applications, the compact size, safety and ease of use of an AC/DC module will make it the preferred solution.

Steve Roberts is a Technical Support Engineer at Recom International Power. He can be reached at roberts@recom-development.at.