One significant drawback of a linear DC power supply is
its efficiency for most applications. You can generally design a linear DC
power supply with reasonable efficiency when both the output and input voltage
values are fixed. However, when either or both of these vary over a wide range,
after assuring the DC power supply will properly regulate at low input voltage
and/or high output voltage, it then has to dissipate considerable power the
other extremes.
For DC power supplies running off an AC line, having to
accommodate a fairly wide range of AC input voltage is a given. A 35% increase in
line voltage from the minimum to the maximum value is not uncommon. Today’s
high frequency switching based power supplies have resolved the issue of
efficiency as a function of input line voltage variance. However, prior to
widespread adaptation of high frequency switching DC power supplies, variety of
different types of low-frequency pre-regulators were developed for linear DC
power supplies
What is a pre-regulator? A pre-regulator is a circuit
that provides a regulated voltage to the linear output stage from an
unregulated voltage derived from the AC line voltage, with little loss of power.
Although not nearly as commonly used as other pre-regulator schemes, on rare
occasion ferroresonant transformers were used as an effective and efficient pre-regulator
in DC power supplies.
What is a ferroresonant transformer? It is similar to a
regular transformer in that it transforms AC voltage through primary and
secondary windings. Unlike a regular transformer however, once it reaches a
certain AC input voltage level it starts regulating its AC output voltage at a
fixed level even as the AC input voltage continues to rise, as depicted in
Figure 1. Ferroresonant transformers are also commonly called constant voltage
transformers, or CVTs.
Figure 1: Ferroresonant transformer input-output transfer
characteristic
The ferroresonant transformer employs a rather unique
magnetic structure that places a magnetic shunt leakage path between the
primary and secondary windings. This structure is illustrated in Figure 2. This
way only part of the transformer structure saturates at a higher fixed peak
voltage level during each AC half cycle. When part of the core magnetically
saturates, the primary and secondary windings are effectively decoupled. The AC
capacitor on the secondary side resonates with existing inductance. This
provides the carry-over energy to the load during this magnetically saturated
phase, holding up the voltage level. The resulting waveform is a clipped sine
wave with a fairly high level of harmonic distortion as a result. Some more modern
designs include additional filtering that can bring the harmonic distortion
down to just a few percent however.
Figure 2: Ferroresonant transformer structure
A ferroresonant transformer has some very appealing
characteristics in addition to output voltage regulation:
- Provides isolation from line spikes and noise that is normally coupled through on conventional transformers
- Provides protection from AC line voltage surges
- Provides carry over during momentary AC line drop outs that are of a fraction of a line cycle
- Limits its output current if short-circuited
- Extremely robust and reliable
Because of a number of other tradeoffs it is unlikely
that you will find them in a DC power supply today. High frequency switching
designs pretty much totally dominate in performance and cost. Ferroresonant
transformer design tradeoffs include:
- Large physical size
- Relatively expensive and specialized
- Limited to a specific line frequency as it resonates at that frequency
So, even though you are very unlikely to encounter a
ferroresonant transformer in a DC power supply today, it’s interesting to see
there still appears to be a healthy demand for ferroresonant transformers as AC
line conditioners in a wide range of sizes, up to AC line power utility
sizes. Their inherent simplicity and
robustness is hard to beat when long term, maintenance-free, reliable service is
paramount, and AC line regulation in many regions around the world cannot be
counted on to be well controlled.
