Controller Design

Abstract

In all switching converters, the output voltage v(t) is a function of the input line voltage v_g(t), the duty cycle d(t) and the load current i _load (t) as well as the converter circuit element values. In a dc-dc converter application, it is desired to obtain a constant output voltage v(t) = V, in spite of disturbances in v _g (t) and i _load (t and in spite of variations in the converter circuit element values. The sources of these disturbances and variations are many, and a typical situation is illustrated in Fig. 9.1. The input voltage v _g (t) 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 v_g(t) may also vary when neighboring power system loads are switched on or off. The load current i _load (t) may contain 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.