Engineers specifying a power supply for a given application may naturally assume that ensuring a generous output-power margin will deliver benefits such as reduced stress and a longer lifetime. In fact, the opposite is true. For optimum performance, engineers should assess the power requirements of the system accurately, and size the power supply accordingly.

Historically AC-DC PSU’s of a linear topology in applications would incorporate some deration by the user. This was mainly down to the Linear PSU having a resistive load nature within the series pass element and transformer, which were subject to I2R losses which rise quadratically with load.

Switch mode technology provides a vast increase in efficiency over a linear topology. However most SMPSUs perform optimally when operating at least 70% to 100% of their rated power. Housekeeping circuitry and critical components such as the switching MOSFET, magnetic components and regulator IC are all dimensioned to carry the maximum rated current, and at light loads the losses in these components are high relative to the output power delivered. At light loads the ringing and overshoot of the Mosfet during its off cycle, due to leakage inductance of the primary transformer contribute to wasted energy.

Although modern multi-resonant architectures with soft switching can successfully reduce this ringing, the cost of the PSU is higher. For highly cost-sensitive applications where a commodity PSU is desirable, the chosen unit should be sized to operate close to its rated output. If the PSU includes an RCD snubber circuit to protect the MOSFET, the energy absorbed in the snubber is dissipated as heat. This adds to the loss incurred in the PSU at light loads.

If the system is subsequently packaged in a small enclosure – for example to allow installation in a small space within a piece of industrial equipment – this can also place excessive thermal stresses on the PSU and surrounding components. Ultimately, the result of increased component stress caused by over-specifying the PSU can be manifested in poor reliability and a shorter MTBF.

Experience from the field shows a tendency for some engineers to specify a PSU of double the rating actually required, in the interest of reliability. Case histories, however, have shown that this can shorten the PSU lifetime by as much as 80 percent.

Specifying the right size PSU for a given application, to ensure operation close to its maximum rating, can prevent the poor efficiency and reliability experienced with over-specified unit. Valuable savings in cost, size and weight can also be achieved. The Astec LPQ family, for example, spans the power range 110W to 350W, with the 110W LPQ110 weighing 0.57kg compared to 1.8kg for the LPQ350. There is a reduction in physical size with the LPQ110 measuring 178mm x 102mm x 45.7mm, compared to the 228mm x 127mm x 63.5mm outline of the LPQ350.