Introduction to Power Converter Concepts

Electrical Source Conversion

Throughout the following posts, basic concepts of the electrical energy conversion process, together with indicators of conversion quality, are introduced. We need to establish a specific set of objectives for a converter. It is important to identify performance measures to determine ether objectives are being met. How can user expectations for a power supply be translated into design goals? How do we talk about performance of quality? What should a converter try to accomplish? It is easy to demand a perfect power converter. Often, a power electronics engineer must work with a user toward reasonable performance and cost objectives. Below, an example of the interior circuits of a commercial power supply is shown. A perfect converter is impossible and a near-perfect one might involve considerable expense. How can the user’s need be met with a reliable, efficient, cost-effective circuit?

commercial power supply circuit

A commercial power supply circuit which cascades a rectifier and DC-DC converter.

Any converter design is constrained by Kirchhoff’s Voltage Law and Kirchhoff’s Current Law. The concept of source conversion is developed to link the circuit laws wo basic converter organization. As we shall see, current sources are just as common in power electronics as voltage sources.

Distortion must be considered in any converter or power supply. How much ripple or other unwanted effects will be present at a rectifier output? How pure is the output signal of a DC-DC converter or an inverter? Distortion is an inevitable result of the nonlinear switching behavior of power electronic circuits. We must consider how to reduce unwanted effects since they cannot be eliminated outright. Performance indicators such as total harmonic distortion are introduced to help characterize distortion performance. Power factor helps to address system efficiency rather than just converter efficiency. Converter efficiency is critical, but so is the energy flow taken from the input source. Power factor measures the fraction of useable flow from the input that actually contributes to output power.

Regulation defines how closely a power supply output approaches an ideal source. A real converter shows voltage drops or other changes when the load power increases or the input voltage decreases. Commercial products usually are designed to approximate a voltage source closely, and we will examine a sample specification to gain understanding of the concept.

Many converters construct pre-defined waveforms as part of their action. The full-wave rectified sinusoid is one example. When a waveform is fully determined, it can be treated as an equivalent source. It will be shown that this idea is a powerful tool for analyzing distortion waveforms and other operating characteristics of converters. Equivalent sources are especially helpful in the design of filters for converters. This brings out the interface problem. Over the next few posts, the underlying issues in the development of interfaces will be considered.

Electrical Source Conversion

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