Matti Karppanen

Dynamical Characterization of Peak-Current-Mode-Controlled Buck Converter With Output-Current Feedforward

The paper investigates the effect of unity-gain output-current-feedforward in a peak-current-mode-controlled (PCMC) buck converter. A consistent theoretical basis is provided showing that the unity-gain feedforward can improve significantly the load invariance and transient performance of a PCMC buck converter. The nonidealities associated to the scheme would, however, deteriorate the obtainable level of invariance. The nonidealities can be maintained at acceptable level, and therefore, the scheme would provide a viable method to reduce significantly the load interactions as well as improve the load-transient response. The theoretical predictions are supported with comprehensive experimental evidence both at frequency and time domain as well as comparisons between three different buck converters

Dynamical Characterization of Input-Voltage-Feedforward-Controlled Buck Converter

The use of input-voltage-feedforward (IVFF) control has become a popular method to implement a low-cost bus converter in the telecommunications distributed power architectures because the IVFF control can provide adequate output-voltage accuracy in respect to the variations in the input voltage even at open loop, when applied to a buck converter. The method in itself is well known from the 1980s, but the dynamical features that the method would provide have not been fully investigated and reported. The results of the investigations made in this paper show that the IVFF control would provide input-noise attenuation comparable to the peak-current-mode control, voltage-loop gain largely independent of input voltage, and highly reduced input-filter interactions as compared to the voltage-mode control. The main disadvantage is the reduced output-voltage-load-transient response due to the input-voltage-dependent maximum duty ratio. The observations are supported with comprehensive experimental measurements

Dynamical Modeling and Characterization of Peak-Current-Controlled Superbuck Converter

The fourth-order converter known as superbuck is often used to interface the solar arrays into the rest of the power system due to the continuous input current it provides. Peak-current-mode (PCM) control is applied. The small-signal models of such a converter do not exist in public domain literature and its dynamical features are not known. The paper introduces the dynamical modeling based on consistent methods. The characterization shows that the PCM-controlled superbuck converter may have similar features characteristic to the conventional buck converter such as high input-voltage-noise attenuation, effectively first-order output dynamics, and high insensitivity to source interactions, when the inductor-current feedback is properly compensated. The converter is, however, more susceptible to the input-filter instability due the shape of the ideal input impedance than the conventional buck converter. Experimental evidence is provided based on a 440-kHz converter.

Source-Reflected Load Interactions in a Regulated Converter

The use of distributed power supply (DPS) systems is continuously growing in powering different electronic systems and equipment. The systems compose of regulated switched-mode converters, which typically operate in a cascaded configuration constituting a dynamically demanding constellation prone to instability and performance degradation. It is well known that the origin of the interactions is the different impedances distributed within the system. The mechanism and characterizing parameters causing the source-reflected interactions is investigated in this paper both theoretically and experimentally using a buck converter under voltage-mode (VM), peak-current-mode (PCM) and input-voltage-feedforward (IVFF) control. The reflected interactions would be eliminated if the forward-voltage transfer function can be made zero but in practice such a condition cannot be fully achieved. The investigations show that the VM-controlled convert is very sensitive to the source-reflected interactions

Dynamical Characterization of Voltage-Mode Controlled Buck Converter Operating in CCM and DCM

The paper investigates the dynamical differences imposed by the continuous and discontinuous operation modes under direct-duty-ratio or voltage-mode control. The safe load and source profiles for CCM and DCM converters are derived. It is shown e.g. that the DCM converter is more prone to instability caused by the load, while the CCM converter is sensitive to capacitive loads. The safe source profiles are shown to be almost the same for both converters but the source interactions caused e.g. by EMI-filter are shown to be smaller in the DCM converter at the vicinity of the converter output filter resonant frequency. Even if a larger magnitude variation as function of input voltage is observed in the control-to-output transfer function in DCM, the variation of magnitude in the loop gain can be made equal for both conduction modes by a proper controller design.

PCM-controlled superbuck converter with super performance and surprises

The paper investigates the dynamics associated to the fourth-order converter known as two-inductor buck, current-sourced buck or superbuck converter under peak-current-mode control. In late 1970s, the optimality of a converter was considered based on the continuity of the input current and the output current supplying the output capacitor. As a consequence, the converter named as Cuk converter according to its inventor was qualified as an optimal converter. The continuity of input and output currents does not suffice anymore but the properties such as input and output invariance, nonexistence of control anomalies, and the order of control dynamics have to be considered. The paper investigates the dynamic properties of the superbuck converter under peak-current-mode control. The investigations show that the superbuck converter contains all the desired features but it may also incorporate resonant right-half-plane (RHP) zeros and poles, which can destroy the optimality. The appearance of the RHP zeros and poles can be controlled by careful power-stage design. Practical evidence is provided based on two high-frequency converters.

EMI-Filter Interactions in a Buck Converter

The paper investigates the effects of EMI filter in the dynamics of a buck converter. It is shown theoretically that the EMI filter may increase significantly the load sensitivity of the voltage-mode-controlled converter but the peak-current-mode-controlled converter is quite insensitive to the EMI filter interactions. Experimental validations are carried out using a buck converter under three different control modes - voltage-mode, peak-current-mode and output-current-feedforward peak-current-mode control. The investigations show that the phenomenon causing the instability under peak-current-mode control is the negative-resistor¿oscillation (NRO) phenomenon, and confirm also the excess EMI-filter sensitivity of the voltage-mode controlled converter.

Load and supply invariance in a regulated converter

This paper investigates methods to ease the supply and load interactions commonly encountered in the interconnected systems. The origin of the interactions is the different impedances within the system, which are difficult to predict accurately enough for assessing the stability and performance of the system. In theory, a converter having zero output impedance and absolute attenuation of input noise would provide high load and source invariance. A peak-current-mode controlled (PCMC) buck converter with output-current feedforward (OCF) would provide near to optimal platform for designing highly load and supply invariant converter. Experimental frequency and time-domain measurements prove the theoretical predictions.

Load and supply interactions in VMC-buck converter operating in CCM and DCM

The paper investigates the dynamical differences of load and supply interactions in direct-duty-ratio or voltage-mode-controlled buck converters in continuous and discontinuous conduction modes. The origin of the interactions is the system impedances such as EMI-filters, cabling inductances, additional capacitors and input and output impedances of regulated converters. The dynamical parameters i.e. phase margin and control bandwidth of the CCM and DCM converters are designed to be the same. It is shown that the sensitivity for interactions can be concluded from the measured frequency responses. The investigations show that a converter operating in CCM is more sensitive to capacitive loads than a converter in DCM, while the DCM converter is more prone to instability caused by the load. The supply interactions caused e.g. by an EMI filter are shown to be smaller in CCM converter except near the converter output filter resonant frequency, where the forward-voltage transfer function has an amplifying effect to supply interactions.

Multi-channel semi-regulated bus converter

The supply-voltage requirements of the modern integrated circuits necessitate the use of distributed power systems. There are concerns about the use of single intermediate bus voltage to power multiple point-of-load (POL) converters such as signal integrity due to the high frequency circulating currents and narrow duty ratios of POL converters. The paper investigates the possibility to implement the bus converter as multi-output semi-regulated converter to overcome the aforementioned concerns with insignificant additional costs. The concept was implemented by means of active-clamp forward converter with synchronous rectification and input-voltage-feedforward control. The main concerns have been the input-to-output noise attenuation and cross-regulation between the outputs. Frequency and time domain results are provided to support theoretical findings.