DSPを用いたDC-DCコンバータのロバスト制御に関する研究

摘要

The switching frequency of DC-DC conversion becomes higher as demands for downsizing andfast-response increase. At present, the switching frequency of an isolated DC-DC converter isabout 300kHz to 500kHz. PWM controllers are realized by using analog control ICs and someanalog parts. In recent years, the demands for more functions and capabilities are widelydiversified and sophisticated. To satisfy these demands, digitization of controllers has beenconsidered in academic and industrial worlds. In the practical use of DC-DC converters,characteristics of startup transient responses and dynamic load responses as well as output voltageresponses when the input voltage changing are of importance. Furthermore, in many applicationsof DC-DC converters, the loads and the input voltages cannot be specified in advance. Therefore,robust characteristics against loads and input voltage changes are necessary. Generally, the PIDcontrol is used for the digitization of the controller. The PID control is designed by trial and error,hence it is difficult to realize the robust characteristics. On the other hand, the H-infinity robustcontroller is of high-order, and its algorithm becomes too complex to implement. The robustcontroller with a simple algorithm is necessary for DC-DC converters.In this paper, an approximate 2-degree-of-freedom (2DOF) system is considered for controllers ofDC-DC converters. This system is obtained by constituting a model matching system and anapproximated inverse system and a first-order filter. The startup transient response is determinedby assigning dominant poles of model matching systems. The transfer characteristics from thedisturbance to the output are determined by one coefficient of first-order filter. Thus, the 2DOFsystem is approximately realized. This system has robust characteristics, which can be realized bya simple algorithm. When applying this approximate 2DOF controller to DC-DC converters, it isnecessary to improve the degree of approximation for stronger robustness. In this study, thecontrolled object is a forward converter whose switching frequency is 300kHz. We propose adesign method for improving the degree of approximation of both first-order and second-orderapproximate models. The approximate 2DOF digital controller is actually implemented on a DSPand is connected to the DC-DC converters. We make some simulations and experiments forexamining the startup transient responses and dynamic load responses as well as the output voltage responses when the input voltage changing. A good approximate 2DOF controller isobtained by placing the poles of the approximate 2DOF system so as not to approach the dominantpoles of model matching systems. It turns out that the characteristics are almost the same againstload changes and input voltage changes, and also that the experimental results are almost the sameas the simulation ones. The specifications are satisfied with the experiments and simulations. Thecontrol algorithm of the first order model is simpler than that of the second-order model. On theother hand, the second-order approximate model may improve the degree of approximation more.Therefore, the second-order approximate model may be used for the severer specifications.A higher switching frequency of DC-DC conversion is necessary so as to take fast-response andbetter robust characteristics. The resolution of digital PWM generators becomes lower as theswitching frequency becomes higher. Here exists an important problem of digitization ofcontrollers. Digital dither and delta-sigma modulation methods are proposed to improve thisproblem. These methods improve the resolution by averaging some switching periods. Therefore,low frequency ripples that are determined by a dither-pattern are superposed on the output voltage.The output voltage of the DC-DC converter must exist in a range of several percentage pointsincluding ripple noises, dynamic load responses, etc. Therefore, lower frequency ripples bydigitization should be as small as possible.In this study, the digital PWM resolution is improved by controlling the delay time of the PWMsignal. A delay time control system is realized by pulse-composite-circuits with two digital PWMgenerators. A method for determining control signals of two PWM generators is presented. It turnsout that the algorithm for designing both control signals are simple. The amplitude ofpulse-composite-circuits has a limit that is determined by the maximum duty cycle and switchingfrequency. In the case where the maximum duty cycle is 0.6 and the switching frequency is400kHz, the amplitude is obtained as 1/32. The relationship between the PWM resolution and theA-D converter resolution is examined. If the PWM resolution is lower than the A-D converterresolution, the output voltage oscillates in the shape of a limit cycle, and big noises occur. Whenthe pulse composition technique is applied to the DPWM generator built-in DSP, an experimentalmodel is designed and manufactured. It turns out that the PWM resolution is improved up to 1/32when the max duty cycle is 0.6 and the switching frequency is 400kHz, and that the limit cyclenoises disappear because the PWM resolution is higher than that of A-D resolution (10 bit).