THE NEED FOR SWITCHING IN POWER ELECTRONIC CIRCUITS
The heart of any power electronic circuit is its semiconductor-switching network. The question arises here as to whether we have to use switches to perform electrical power conversion from the source to the load. The answer, of course, is no, as there are many circuits that can perform energy conversion without switches, including linear regulators and power amplifiers. However, the need to use semiconductor devices to perform conversion functions is very much related to converter efficiency. In power electronic circuits, the semiconductor devices are generally operated as switches, that is, either in the on-state or the off-state. This is unlike the case for power amplifiers and linear regulators where semiconductor devices operate in the linear mode. As a result, a very large amount of energy is lost within the power circuit before the processed energy reaches the output. Semiconductor switching devices are used in power electronic circuits because of their ability to control and manipulate very large amounts of power from the input to the output with relatively very low power dissipation in the switching device. Their use helps to create a very highly efficient power electronic system.
Efficiency is considered an important figure of merit and has significant implications for overall system performance. Low-efficiency power systems, large amounts of power are dissipated in the form of heat, which results in one or more of the following implications:
1. Cost of energy increases due to increased consumption.
2. Additional design complications might be imposed, especially regarding the design of device heat sinks.
3. Additional components such as heat sinks increase the cost, size, and weight of the system, resulting in low power density.
4. High-power dissipation forces the switch to operate at a low switching frequency, resulting in limited bandwidth, slow response, and most importantly, the size and weight of magnetic components (inductors and transformers) and capacitors remain large. Therefore, it is always desired to operate switches at very high frequencies. However, we will show later that as the switching frequency increases, the average switching power dissipation increases. Hence, a trade-off must be made between reduced size, weight, and cost of components versus reduced switching power dissipation, which means inexpensive low switching frequency devices.
5. Reduced component and device reliability.
For more than 30 years, it has been shown that switching (mechanical or electrical) is the best possible way to achieve high efficiency. However, unlike mechanical switches, electronic switches are far superior because of their speed and power handling capabilities as well as reliability.
The advantages of using switches do not come without a cost. Because of the nature of switch currents and voltages (square waveforms), high-order harmonics are normally generated in the system. To reduce these harmonics, additional input and output filters are normally added to the system. Moreover, depending on the device type and power electronic circuit topology used, driver circuit control and circuit protection can significantly increase both the complexity of the system and its cost.
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