Mikko Hankaniemi

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

Characterization of Regulated Converters to Ensure Stability and Performance in Distributed Power Supply Systems

The characterization of regulated converters is investigated in order to establish a set of dynamical parameters defining the interactions arising in the interconnected systems such as telecom distributed power supply (DPS) systems. The commercially available converters are usually vaguely specified in respect to those interactions. Provided information do not suffice for predicting the stability and performance. It is noticed that there are certain double reflections, which are not previously recognized but may increase the load sensitivity if not properly considered. The defined parameter set can be also used to design the converters to be more insensitive to different interactions

Dynamical modelling of peak-current-mode-controlled converter in continuous conduction mode

Abstract Peak-current-mode (PCM) control has been a popular control method of switched-mode converters since its publication in late 1970s due to the inherent features it provides especially in buck derived converters such as high input-noise attenuation, first-order control dynamics as well as cycle-by-cycle current limiting. Its main disadvantages are considered to be the limited duty ratio, increased output impedance, and noise sensitivity due to the fast feedback-current loop. The observed peculiar dynamical behaviour associate to the PCM control has attracted the researchers for tens of years yielding multitude of dynamical modelling approaches. Application of sampling effect as the basis for the modelling has been considered to producing most accurate dynamical models as well as explanations for the observed phenomena. A consistent and easy-to-apply modelling approach is presented in this paper, which explains the observed dynamical phenomena, provides accurate dynamical power-stage models as well as comply with the other methods proposing the existence of an infinite small-signal duty-ratio gain leading to the observed behaviour. Experimental evidence is provided based on second and fourth-order buck converters.

Load-Impedance Based Interactions in Regulated Converters

The effect of load impedance on the dynamics and performance of a regulated converter is investigated. Theoretical formulation is derived utilizing two-port modelling technique. It is definitively shown that the load interactions are reflected into the converter dynamics via the internal open-loop output impedance. At the frequencies, where the loop gain is much higher than unity, the internal closed-loop output impedances acts as a boundary for the control-bandwidth reduction. The loop gain is always affected, whenever the internal open-loop output impedance is equal or greater than the load impedance. The converters are sensitive especially to the capacitive and resonant-type loads. The sensitivity is dependent on control mode, and cannot be much reduced by means of basic controller design

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

Small-Signal Models for Constant-Current Regulated Converters

The telecom DC uninterruptible power supply (UPS) systems include a storage battery to ensure the uninterrupted operation of the system during the power outages in the commercial grid system. The storage battery behaves as an ideal DC voltage source with very low internal impedance but needs to be recharged after discharge of its energy. Therefore, the telecom rectifiers taking care of the recharging have to be provided with output overload limiting for protecting them from damage. Typical overload-protection schemes are modified-constant-power and constant-current methods. It has been observed that the crossover frequency of the output-current loop tends to increase even one decade, when the storage battery is connected as a load. In this paper, the dynamical properties of the current-output converter are investigated, and the set of transfers representing the dynamics is developed based both on the state-space averaging method and on the two-port model of the corresponding voltage-output converter. According to the investigations, the observed tendency to increasing crossover frequency can be addressed to the internal dynamics of the converter, which the storage battery invokes into effect. This means that the origin of the problem is an erroneous control design

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.

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.

Dynamical profile of a switched-mode converter - reality or imagination

An electrical device such as a switched-mode converter has unique internal dynamical profile, which mainly dictates, how the device as a subsystem in an interconnected system would behave and how it will affect the other subsystems within the constellation. The paper defines the vital representative parameters by means of which the stability and dynamics of the individual electrical device as well as the overall interconnected system can be analyzed and predicted. The very existence of such a dynamical priflle is proved based on extensive experimental evidence.

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.