Introduction to Switches

Kirchhoff's Voltage and Current Laws

We have seen that circuits involving switches and energy storage can be operated to give energy conversion functions. Switches come in a huge variety including mechanical and semiconductor styles. It is important to develop a systematic way to identify appropriate power electronic circuits, and to find out how they can be controlled to give the desired performance. Consider, for example, that circuits exist to:

  • Provide a very tightly fixed DC output from an uncertain input voltage source. A power supply of this type can be used anywhere in the world without adjustments. It must adapt itself to the available energy source while maintaining precise output.
  • Provide polarity reversal. The is extremely helpful in bridging digital and analog electronics.
  • Step a DC voltage up or down. The step up switching circuit we discussed previously an a kind of DC transformer.
  • Rectify, invert, and otherwise facilitate energy flow between AC and DC circuits, or between AC circuits acting at different frequencies.

The most common way to achieve these conversions is to store energy in a circuit element, then deliver it to a load to alter the voltage or current. Our previously analyzed simple switching circuits from earlier posts offer a case study list for power conversion. In these circuits, the use of switches and energy storage has been rather ad hoc. A few devices were interconnected and a power electronic function was asserted for the whole. We need a more general approach to construct and operate circuits for energy conversion. The polarity reverser and the step-up circuits both operate by switching periodically with a 50% on time. How will operation change as the on time moves from this 50% value?

One way to identify possible conversion circuits is to just write down all possible combinations of two or three switches, along with a few storage elements. This is a real brute force approach and does not give insight into how we might make the best choise for a given function. But, there is a better way! We will be concerned with basic tools for organizing and operating switch circuits. Once the switches have been organized, we will consider som of the device issues that determine the behavior of converters. Diode circuits, for instance, can be analysed by taking the device behavior into account. A key element is that switch action is periodic in time for almost all power converters. We will take advantage of this to develop tools for converter analysis.

Kirchhoff's Voltage and Current Laws

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