The Principles of Electronic and Electromechanic Power Conversion by Ferreira Braham van der Merwe Wim & Wim van der Merwe

Author:Ferreira, Braham, van der Merwe, Wim & Wim van der Merwe
Language: eng
Format: epub
Publisher: Wiley
Published: 2013-12-25T16:00:00+00:00

Notation Note 6.1

The periodic signals observed in power electronic systems are not, as a rule, sinusoidal. For the voltage waveforms (the same assumptions also apply to current waveforms), the peak value is denoted by and the average value by . There are cases where the signal is relatively constant and . In this case, we can refer to the signal as where refers to the small variations in the signal.

Figure 6.5 Switched approach waveform.

The question now becomes: how do we generate the control signal for the switch? From the requirements thus far, we know that we want the switch to switch with a high switching frequency (so that the switching behaviour is not noticeable), the duty cycle must be continuously variable from 0 to 1 and the process must be repeatable and easy to implement. It is actually not that difficult to create the control signal of Fig. 6.5. Let us create a sawtooth signal, which we we will call the carrier. We make the frequency of this carrier signal equal to our desired switching frequency, and we can express it mathematically as

6.3

where is the peak value of the carrier signal. Let us now compare this signal with a reference value, which we will call . A circuit showing this comparison is shown in Fig. 6.6. We can now express the output, which becomes our control signal, as

6.4

Figure 6.6 Control signal generation circuit.

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