Electrical Source Conversion
One of the most basic principles of power supplies is that the user expects an ideal source. Designers of electrical equipment and electronic circuits generally assume that this type of energy supply is available. Most of us are accustomed to this idea. The electric utility grid is carefully operated to provide a sinusoidal voltage source of good quality. Student lab sources often use expensive DC supplies to ensure a fixed voltage source for projects. The idea of a device that supplies a constant voltage over a wide range of power levels is familiar.
Commercial voltage sources come quite close to ideal behavior. The typical U.S. household electrical outlet provides about 120 V AC from no load right up to the circuit breaker or fuse current limits. The 12 V DC source in a modern automobile is expected to hold firm for load currents ranging from a few milliamps to well over 100 A. Power supplies can be designed to meet very tight requirements for voltage consistency. Current sources are less familiar, but just as useful in principle. A device which supplies a fixed current over some wide voltage range is certainly a reasonable energy source.
A user of power conversion equipment would most likely expect to supply if from some source – whether the 50 Hz or 60 Hz AC mains, a battery, or some other energy source. To a power electronics designer, this expectation represents the concept of source conversion. According to this concept, any power converter would normally be designed so that its output resembles an ideal source. Not only does a user expect a converter to behave as an ideal source, but a typical power electronics engineer often intends the input energy source to show near-ideal behavior. A power converter usually is expected to manipulate energy flow between an ideal input and an ideal output.
Source conversion is the concept that describes the operation of most power converters: They control energy flow between near-ideal electrical sources.
Our typical user seeks an ideal source. A reminder of the defining characteristics of ideal sources might be helpful at this point.
A current source supplies a given current waveform into any load and at any voltage. No external circuit or device can force a current source to change.
A voltage source supplies a given voltage waveform into any load and at any current. No external device or circuit can force a voltage source to change.
The source conversion concept creates a dilemma because of Kirchhoff’s Voltage Law (KVL) and Kirchhoff’s Current Law (KCL) restrictions. Since unlike voltage sources can not be interconnected, and unlike current sources likewise can not be interconnected, we can sat that ‘voltage is not converted to voltage’ and that ‘current is not converted to current.’ No switch matrix can control energy flow between two unequal voltage sources or unequal current sources. However, any voltage can be imposed on a current source and any current can be imposed on a voltage source. This means that voltage sources can be interconnected with current sources without trouble. The concept shows us that ‘voltage converts to current’ and ‘current converts to voltage.’
The source conversion concept is a fundamental of power electronics. In a well designed power converter, both the input and output ought to have the characteristics of an ideal source. If the input is a voltage source, then the output should resemble a current source. If the input is a current source, then the output should have the properties of a voltage source. Although voltage sources are the most familiar form for energy, current sources are quite common in power electronics. In face, current and voltage sources are roughly equally prevalent. Most often, a current source represents the behavior of an inductor.
In some conversion applications, conversion of voltage to current or current to voltage is a problem. Perhaps the input and output are both near-ideal voltage source. One alternative in such cases is to introduce an intermediate source, and perform a voltage-current-voltage conversion, for instance. An internal source used in this manner is called a transfer source.
A transfer source is some electrical source element, intended to consume no energy, placed in the path between an input source and an output source. It serves as a temporary location as energy is transferred between input and output.
When a transfer source is used, it is embedded in the converter switch structure. Recall the definition of an indirect switching converter, in which elements are positioned within the switch matrix. A transfer source is one of the most common types of such elements/
Source conversion concepts play a major role in determining the topology of switching power converters. There are converters with current input and output, voltage input and current output, and indeed all combinations, for DC-DC, DC-AC, AC-DC, and all other types of conversion functions.