What distinguishes a well designed power supply? Most might say reliability. But equally important is the behaviour of the power supply. If the specs of a power supply do not meet the specs of the equipment, then the end result would be a complete, unreliable product. qqqRien Giltay reports.

One of the factors that most stress and test a power supply is a sudden change in load conditions (caused by switching). In a machine that has both control and switching circuits any surges or dips in supply voltage may disrupt the control circuits and compromise the performance of the machine.
A high-quality power supply must react in the shortest time possible with the minimal voltage disturbance so as to promote a continuous constant output voltage. Fig.1 illustrates the dynamic response of three 6kW power supplies tested using the same dynamic load conditions. The superior dynamic response is achieved by a relatively high switching frequency when compared to the output power and an optimised control circuit of the power supply.
A good power supply requires optimum steady-state characteristics with low ripple and noise at the output. Requirements that are in conflict with optimum dynamic response characteristics. Using a special soft-switching design and carefully devised output filters a designer can achieve a superb balance between the dynamic and steady-state characteristics of a power supply, as illustrated in Fig.2.
Best quality components guarantee low noise and output ripple as well as long term reliability and stability. They also assure perfect operability of the power supply over a bigger range of operating temperatures.

Minimising losses

Converting power does not necessarily mean disrupting the ac supply. Badly designed power supplies would cause harmonics on the supply line that are difficult and expensive to filter and may create negative effects on other equipment.
With an active, three phase power factor correction (as opposed to a single phase PFC after the bridge rectifier), a power supply can not only achieve a next to unity power factor, but also would apull' a continuous power through out most of the ac cycle. Fig.3 describes the input current of the three power supplies tested. The true three phase PFC offers a next to sinusoidal current input minimising harmonics and I_R losses. When was it last that you've checked your utility bills?

Handling reactive loads

Match your power supply to your load. Sounds obvious, but did you study your load conditions? Are you switching non-resistive loads that cause voltage surges? How can a power supply react to a back surge coming from the load?
Most power supplies have an over voltage shut down, but this does not help in absorbing the power coming back from the load, hence additional circuitry is required to promote a constant output voltage also under these conditions.
A dynamic load, synchronised with the switching of the power supply would drain the power and therefore ensure that the output voltage remains clean and constant with minimal noise.
Reliability of any equipment depends on the matching of the power supply to the load.

Rien Giltay is with Delta Elektronika BV, Zierikzee, Netherlands. www.DeltaPowerSupplies.com