chap09 - 9 Controller Design 9.1 INTRODUCTION In all...

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9 Controller Design 9.1 INTRODUCTION In all switching converters, the output voltage v ( t ) is a function of the input line voltage the duty cycle d ( t ), and the load current as well as the converter circuit element values. In a dc–dc con- verter application, it is desired to obtain a constant output voltage v ( t ) = V , inspite of disturbances in and and in spite of variations in the converter circuit element values. The sources of these distur- bances and variations are many, and a typical situation is illustrated in Fig. 9.1. The input voltage of an off-line power supply may typically contain periodic variations at the second harmonic of the ac power system frequency (100 Hz or 120 Hz), produced by a rectifier circuit. The magnitude of may also vary when neighboring power system loads are switched on or off. The load current may con- tain variations of significant amplitude, and a typical power supply specification is that the output voltage must remain within a specified range (for example, 3.3 V ± 0.05 V) when the load current takes a step change from, for example, full rated load current to 50% of the rated current, and vice versa. The values of the circuit elements are constructed to a certain tolerance, and so in high-volume manufacturing of a converter, converters are constructed whose output voltages lie in some distribution. It is desired that essentially all of this distribution fall within the specified range; however, this is not practical to achieve without the use of negative feedback. Similar considerations apply to inverter applications, except that the output voltage is ac. So we cannot expect to simply set the dc dc converter duty cycle to a single value, and obtain a given constant output voltage under all conditions. The idea behind the use of negative feedback is to build a circuit that automatically adjusts the duty cycle as necessary, to obtain the desired output voltage with high accuracy, regardless of disturbances in or or variations in component values. This is
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332 Controller Design a useful thing to do whenever there are variations and unknowns that otherwise prevent the system from attaining the desired performance. A block diagram of a feedback system is shown in Fig. 9.2. The output voltage v ( t ) is measured, using a “sensor” with gain H ( s ). In a dc voltage regulator or dc ac inverter, the sensor circuit is usually a voltage divider, comprised of precision resistors. The sensor output signal H ( s ) v ( s ) is compared with a reference input voltage The objective is to make H ( s ) v ( s ) equal to so that accurately follows regardless of disturbances or component variations in the compensator, pulse-width modu- lator, gate driver, or converter power stage. The difference between the reference input and the sensor output is called the error signal If the feedback system works perfectly, then and hence the error sig- nal is zero. In practice, the error signal is usually nonzero but nonetheless small. Obtaining a small error is one of the objectives in adding a compensator network as shown in Fig. 9.2. Note that the output
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This note was uploaded on 01/17/2010 for the course EL 5673 taught by Professor Dariuszczarkowski during the Spring '09 term at NYU Poly.

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chap09 - 9 Controller Design 9.1 INTRODUCTION In all...

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