Markus Krug

Model-based dual-mode controller for low-power buck converters

This paper describes the design of a digital, model-based dual-mode controller for a low-power, synchronous buck converter for battery-powered applications. In feedforward mode, the controller drives to new operating points as fast as possible without exceeding the maximum allowed battery current. The desired output voltage can be determined digitally and can be changed during operation time for dynamic voltage scaling (DVS) capability. In feedback mode, a significant reduction of the analog to digital converter (ADC) sampling rate is achieved by using a model-based approach. The model also estimates the load current, thus no current sensing is required. Furthermore, the model preserves the output voltage from limit cycles, although a low-resolution pulse width modulation (PWM) is used. A gain-scheduling scheme of the PI controller optimizes the control performance of the nonlinear behavior of the converter and keeps the adjustment energy after a load step in a range the battery can accomplish. Known load changes of the application are additionally taken into account to further improve voltage stability. The control algorithm has been implemented in a field programmable gate array (FPGA) and is tested together with a buck converter. ADC sampling rate is chosen 10x smaller than switching frequency of the converter and a battery with a maximum current drain of 40mA is used. The transient load response shows a maximum voltage drop of 10mV for known and 80mV for unknown load steps with a recovery time of 70μs. Efficiency of low-power switching regulators is improved over conventional digital designs by using slow ADCs and a low-resolution PWM.

Konzept und Realisierung eines optischen Mikrosystems zur Wiederherstellung der Akkommodation

Zusammenfassung Akkommodation, die Fähigkeit von der Nähe bis in die Ferne scharf zu sehen, ist mit ca. 50 Jahren so stark eingeschränkt, so dass wir eine Sehhilfe, z. B. eine Brille, verwenden müssen. Ein neuer Ansatz zwischen unendlich und Lesedistanz eine stufenlose Akkommodation wiederherzustellen, ist die Implantation eines optischen Mikrosystems anstelle einer Intraokularlinse. Es werden Konzept und Realisierung des Systems als Funktionsmodell im Maßstab 2:1 beschrieben.

Variable frequency digital PWM controller for low-power buck converters

This Paper describes a novel digital pulse width modulation (DPWM) controller for low-power buck converters that can adapt its switching frequency according to the actual load current during runtime. This new strategy overcomes the drawback of high output voltage ripples at light loads in pulse frequency modulation (PFM) mode and nevertheless reduces switching losses. The variable frequency DPWM in combination with the model-based controller guarantees excellent voltage regulation and high efficiency. The variable frequency DPWM, based on an enhanced counter-comparator PWM and a delay-line, provides a resolution greater than 10 bits. The resolution of the DPWM increases at small load currents, which is beneficial due to the converter nonlinearity in discontinuous current mode (DCM). The control rules for changing the switching frequency ensure that a comparable performance to a constant frequency DPWM is achieved with regard to output voltage regulation and load step response. The performance of the novel DPWM is measured by simulation and it is integrated in a FPGA-based experimental environment. Moreover, attention is turned to a resource saving implementation of the DPWM.