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DC CONVERTER AND TRANSFORMER TERMINOLOGY

Posted by Ernest Herz / April 30, 2012 / NEWS

The distinction between “power converters” and “power transformers” begins with an understanding of the two basic forms of electricity transmission. They are commonly referred to as AC and DC systems. AC is the abbreviation for “alternating current”. AC operating at 60 Hz. describes an electrical current shifting direction, to and fro, in a circuit 60 times per second. Alternating current is the electrical format typically available in most buildings from electrical utility sockets on the power grid. (120 Volts AC in North America)

What is DC power? DC is the abbreviation for “direct current”, current constantly flowing in the same direction. One of the most commonly recognizable sources of DC power is the battery.

There are some very strong practical and economic reasons, which have decided where each of these forms of power are used today:

When power companies generate electricity at macro power generation sites such as dams, their ultimate objective is to distribute energy to home and industry. In order to achieve this, power must be delivered through major power lines. Minimizing the energy lost in this process requires that these lines be operated at very high voltages. Otherwise, the physics of the situation would dictate that the size of the conductors that would be needed to carry this energy would become extremely large, heavy, expensive and physically unmanageable. High voltage lines minimize energy losses along thousands of miles distance. The intention behind the design of such power transmission lines is, that at various points along its path, power can be branched and tapped at distribution stations. There the voltage level of the main power line is adapted (transformed) to suit the consumption levels of local customers. An analogy would be a large water pipeline of huge diameter designed to deliver millions of gallons to a region. Along the way, consumers of this water tap the pipeline with much smaller diameter pipes suitable to accommodate the level of their respective consumption. This tapping becomes analogous when it comes to electricity.

This is where the electrical transformer plays a role. A transformer is an electric component used to change the value of a voltage from one level to another. It is a simple unitized electrical element, whose construction involves an iron or ferrite core and groups of copper conductors, referred to as windings, which are wound around this core. A transformer may be referred to as a “step up” or “step down” type depending on its use i.e. whether to increase or decrease voltage. The laws of physics dictate that a transformer behaves in this capacity only with respect to AC voltage. This is why it is more completely referred to as an AC transformer. The AC transformer is the fundamental component, known to engineers, for voltage changing. And when a transformer’s function is referred to in an electrical sense as transforming, what is exclusively meant is, that it changes AC voltage to AC voltage. Because this fact is taken for granted in the electrical trades, often the prefix AC is dropped and it is simply referred to as a transformer. It is the only electrical component capable of efficiently performing voltage changing on a solo basis, without the necessity of including additional electrical components. This is the major reason why the AC method was selected for power generation and transmission early in power grid evolution.

What can be the derivation of the term “DC-DC transformer”? DC to DC transformer represents a definition of a functional requirement rather than an accurate technical description. Due to the fact that DC voltage cannot be fed into a transformer constructed as above described, the term DC-DC transformer is a misnomer. Similarly, “DC transformer” also a technically incorrect term. Use of the term “DC-DC transformer” suggests a desired invocation of the function of DC-DC converters. “DC-DC converter” is an electrically accurate term to describe devices used to change DC from one level to another. Another proper term is DC DC power supply.

Today there are many good reasons for converting DC to DC. Industrial computers and electronics are used to control a myriad of processes involved in industry and daily living. The energy for operating electronic devices is invariably in DC form. The nature of a given electronic circuit determines the voltage level needed for its operation. When an environment where such a circuit is intended for installation does not make available the matching voltage, DC-DC conversion becomes necessary.

Example 1: A process control computer used to spray paint and functions at 24V. It is needed for installation in a truck with a 12V system. The installation of a DC-DC step up converter (from 12V to 24V) would enable the 24V device to be used on such a truck.

Example 2: A mobile radio transceiver designed to operate with a 12V input. It is installed on a forklift which has a 36V system. A step down DC-DC converter (from 36V to 12V) would be an appropriate interface.

What is the circuit topology of DC-DC conversion? Typically a converter DC DC, uses the DC voltage available for input and chops it into AC. This AC is then applied to the converter’s internal transformer which changes the AC into a different level. This new AC level is then converted back to a new DC level. The circuit which accomplishes the entire conversion typically consists of several circuit elements, only one of which is an AC transformer. Among the characteristics that specify DC-DC converters are input/output voltages, ability to regulate voltage, physical size and power handling capability. Another important property of DC-DC converters is isolation. An isolated DC converter has its input and output voltages disconnected from one another. In some applications dealing with high voltages, this property is crucial.

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