Ferroresonant Power Supply

This type of power supply is very stable and involves the use of the non-linear transformer, which passively provides a regulated voltage output through an electromagnetic phenomenon called ferroresonance. Ferroresonant power supplies are widely used in applications requiring constant output voltages and especially when the input voltage is likely to vary due to instabilities and other power line disturbances.

The power supply is similar to an unregulated power supply except for the inclusion of the ferroresonant transformer, specially designed to maintain the voltage constant. This stability is achieved by use of iron cores operating near a point of magnetic saturation, wherein the core is so strongly magnetized that further variation in the input voltage results in little or no increase in magnetic flux.

Among the many advantages to this design, the power supply output is isolated from surges and harmonic noise. They have excellent voltage regulation so that an input voltage change of as much as 40% will result in a mere 2% output voltage change or less. They are also more durable and have a longer service life.

Efficiency is not one of the strengths of the Ferroresonant Power Supply. At full load and ideal conditions, its efficiency can reach up to only 93% and substantially drops at light load. It is common that the constant voltage transformer be oversized by 2 to 3 times the rated load current, and this oversizing further reduces efficiency.

Another disadvantage to this design is high sensitivity to line frequency changes. They also produce a lot of heat while operating and they are heavy. Finally, although very stable the ferroresonant power supply is not as precisely regulated as the switched mode or series regulated power supplies, yet the output voltage is constant enough for many applications.