Pulse Ratio Modulation: An Interesting Technique to Implement DC/DC Conversion

Abstract

DC/DC converters may be considered as pulse modulators; the modulation laws usually implemented are those related to pulse frequency (PFM) and pulse width modulation (PWM) techniques. The pulse ratio modulation (PRM) technique, providing several advantages over conventional pulse modulation methods, has been developed for applications in a space vehicle attitude control system. Capable of operation with a high degree of precision and overcoming certain bandwidth problems associated with other modulation techniques, the modulator appears to hold excellent possibilities for applications in a broad range of control systems, such as DC and AC drives.In order to investigate the dynamical properties presented by a feedback control system employing this modulation technique, in the paper a mathematical model is presented, characterizing the PRM in terms of input output relationships. By using the state variables approach, the system is described through a vector recurrence equation and . linear bounds on the state characterizing the adopted modulation law. On the basis of this model both the steady state behaviour and the transient response are evaluated. As far as stability properties are concerned, the linearized model is developed in order to determine the local behaviour in correspondence with the steady state solutions, and a comparison is made with an equivalent PWM control system. Moreover a procedure, based on the use of projection operators in vector spaces, is presented to analyse the asymptotic stability in the large.