Relative Stability of the Inner-Current Loop of Peak Current-Mode Controlled PWM DC-DC Converters in CCM

Current-mode control is a commonly adopted method of regulation for pulse-width modulated (PWM) dc-dc power converters in industry, but is not well understood. The advantages of current-mode control over the voltage-mode control include inherent overload and short circuit protection, faster response, line-noise rejection, and multiple converter paralleling. Current-mode controlled system consists of (1) an inner-current loop and (2) an outer-voltage loop, which sets the reference voltage to the inner loop. To ensure stable operation of the multi-loop converter, all the sequential loops in the circuit should be stable with sufficient degree of stability. The research in this dissertation is focused on the relative stability of the inner-current loop in peak current-mode (PCM) controlled PWM dc-dc converters operating in CCM. The operating principle of peak current-mode control is presented. The inner-current loop dynamics of a peak current-mode controlled dc-dc converter is investigated using perturbation theory. Considering its mixed-signal (analog and digital) behavior, the current loop is modeled using sample-and-hold theory. Taking the discrete nature of the inner-current loop into account, a closed-loop transfer function for the current loop is derived in z-domain and an equivalent-hold approximation is used to derive an approximate closed-loop transfer function in the continuous s-domain using modified Pad´e approximation. A general expression for the loop gain of the inner-loop, independent of the converter topology, is derived. Using the loop gain, a measure of relative stability of the inner loop is developed. Expressions for amount of slope compensation required at maximum duty cycle, for the inner loop to be marginally stable and to achieve a specified margin of stability, are derived. Also, expressions for maximum duty cycle at a given amount of slope compensation, for the inner loop to be marginally stable and to obtain a specified margin of stability, are derived. The control current expressions for the inner loop of peak current-mode controlled converters without and with slope compensation are derived. A procedure to design the inner-current loop is developed. Saber Sketch simulation and experimental results are presented to validate the presented theory. The dynamic behavior of the inner-current loop of peak current-mode controlled PWM dc-dc buck converter operating in CCM is analyzed. The critical path power stage transfer functions, the relevant inner-current loop transfer functions, and the control-to-output transfer function of peak current-mode controlled PWM dc-dc buck converter operating in CCM are derived. The presented model is validated using experimental Bode plots

Comparative stability analysis of droop control approaches in voltage-source-converter-based DC microgrids

Droop control has been widely applied in DC microgrids (MGs) due to its inherent modularity and ease of implementation. Among the different droop control methods that can be adopted in DC MGs, two options have been considered in this paper; I-V and V-I droop. I-V droop controls the DC current depending on the DC voltage whilst V-I droop regulates the DC voltage based on the output current. The paper proposes a comparative study of V-I/I-V droop control approaches in DC MGs focusing on steady-state power sharing performance and stability. The paper presents the control scheme for current-mode (I-V droop) and voltage-mode (V-I droop) systems, derives the corresponding output impedance of the source subsystem including converters dynamics and analyzes the stability of the power system when supplying constant power loads. The paper investigates first the impact on stability of the key parameters including droop gains, local control loop dynamics and number of sources and then performs a comparison between current-mode and voltage-mode systems in terms of stability. In addition, a generalized analytical impedance model of a multi-source, multi-load power system is presented to investigate stability in a more realistic scenario. For this purpose, the paper proposes the concept of “global droop gain” as an important factor to determine the stability behaviour of a parallel sources based DC system. The theoretical analysis has been validated with experimental results from a laboratory-scale DC MG

Trends of innovation development in Ukraine

Digital Switched mode power supplies are nowadays popular enough to be the obvious choice in many applications. Among all set-up and control techniques, the current mode DC-DC converter is often considered when performance and stability are of interest. This has also motivated all the “on chip” and ASIC implementations seen on the market, where current mode control technique is used. However, the development of FPGAs has created an important alternative to ASICs and DSPs. The flexibility and integration possibility is two important advantages among others. In this thesis report, an FPGA-based current mode buck/boost DC-DC converter is built in a stepwise manner, starting from the mathematical model. The goal is a simulation model which creates a basis for discussion about the advantages and disadvantages of current mode DC-DC converters, implemented in FPGAs

Design of robust controllers for telecom power supplies

A Telecom power supply is studied and analyzed from control system viewpoint. It consists of three stages: AC/DC rectifier, a backup battery, and a Telecom load. The AC/DC rectifier stage can be composed of paralleled DC/DC converters preceded by paralleled AC/DC converters. However, paralleled DC/DC converters are only considered in this thesis because they constitute the main dynamics in practice. A system of paralleled DC/DC converters operating in continuous inductor current mode with either voltage mode control or peak current mode control are modeled and analyzed using state-space representation. The H∞ control design is used in order to guarantee the robust stability and robust performance of the system in spite of different uncertainties. Also the H∞ loop-shaping design is used to design robust controllers in the presence of uncertainties. μ-analysis is used to evaluate the robustness of the system. Simulation results are presented to demonstrate the control design procedure and to compare between the two approaches presented. A Telecom power system can be composed of voltage-loop and current-loop subsystems. The multi-input-multi-output proportional-integral-derivative (PID) controller is first designed achieving robust stability and robust performance of the voltage-loop. Then, the multi-input-multi-output proportional-integral (PI) controller for current-loop is designed to achieve robust stability and robust performance of the overall system. μ-analysis is used to evaluate the robustness of PID and PI controllers. Simulation results are also presented to demonstrate and validate the control design. The required output characteristic of a Telecom power system contains three modes of operation: constant-voltage, modified constant-power, and constant-current modes. This nonlinear operation can be achieved by using the fuzzy-logic approach. A fuzzy PID-like controller is implemented to achieve the robust output voltage in spite of load disturbances. A fuzzy PI-like controller is implemented to ensure the overload protection reaching the optimal output characteristic of a Telecom power system. Also the internal-model control (IMC) method is applied to basic DC/DC converters: buck, boost, and buck-boost converters. IMC scheme is used to improve the dynamic performance of basic converters by achieving a robust output voltage against line and load disturbances. Simulations show good dynamic performance of the IMC controller.reviewe

Improved DC-Link Voltage Regulation Strategy for Grid-Connected Converters

In this article, an improved dc-link voltage regulation strategy is proposed for grid-connected converters applied in dc microgrids. For the inner loop of the grid-connected converter, a voltage modulated direct power control is employed to obtain two second-order linear time-invariant systems, which guarantees that the closed-loop system is globally exponentially stable. For the outer loop, a sliding mode control strategy with a load current sensor is employed to maintain a constant dc-link voltage even in the presence of constant power loads at the dc-side, which adversely affect the system stability. Furthermore, an observer for the dc-link current is designed to remove the dc current sensor at the same time improving the reliability and decreasing the cost. From both simulation and experimental results obtained from a 15-kVA prototype setup, the proposed method is demonstrated to improve the transient performance of the system and has robustness properties to handle parameter mismatches compared with the input-output linearization method

Control Loop Interactions and Their Mitigation Schemes in VSC-HVDC Systems

In line with two goals of the United Nations, i.e., providing affordable and clean energy as well as combating climate change, various converter-interfaced renewable energy sources (RESs) are being integrated into the power systems. The transfer of renewable power generated by the RESs such as offshore wind farms to remote load centers may require the use of direct current (DC) lines, which are connected to the alternating current (AC) grid via AC-DC converters. In addition to facilitating the reliable connection of RESs to the power grid, high-voltage DC (HVDC) lines may be used for the transcontinental exchange of power to transfer power over long distances. One of the major challenges in the evolution of AC systems to hybrid AC-DC systems is the control of converters. Each converter station owns various control loops that require proper tuning in their stand-alone mode of operation. Furthermore, control loops of adjacent converters may also impact one another, and as a result, there must be coordination among the control design of converters to guarantee stability and appropriate dynamic response of the entire grid. The control loop interactions among the converters worsen with increasing the size of the system and the number of converters, especially when one converter station is already in operation and re-tuning the converter's controllers is not an option. Another important aspect of future AC-DC power grids is the employment of converters built by multiple vendors, who will take part in the development of converter controllers with unique designs and know-how. These independently designed controllers will form a part of the grid control system. In this scenario, the stability of the entire system is of great importance and needs to be verified due to control loop interactions. This thesis studies both internal and external control loop interactions in voltage-sourced converters (VSCs) embedded in AC-HVDC systems. This thesis, first, studies the internal control loop interactions, where the control loops within one single converter interact with one another, and develops a method to design the individual control loops within a VSC such that the converter stability is ensured. A metric is proposed to measure interaction levels, and the impact of interactions on set-point tracking capability is also investigated. This thesis, next, considers the connections among various converters either from the AC side or the DC side and studies the external control loop interactions among the adjacent converters. Regarding the external control loop interactions caused by DC side connections, suitable system models are introduced to enable individual control design for the converters in a multi-terminal DC (MTDC)-HVDC grid. As for the AC side external control loop interactions, two scenarios are considered: 1) the converters are in the grid-following (GFL) mode of operation, and 2) the converters are in the grid-forming (GFM) mode of operation. Regarding the GFL mode of operation, the impact of control modes on the interactions is studied, and the control modes causing the highest interaction levels are identified. A novel control design framework is designed to relate the control design of each converter to the interconnected system stability. The multi-vendor issue then is considered, and the interactions are mitigated by designing individual robust controllers or by employing interaction filters. The interaction analyses are then extended to the parallel connection of GFM converters and hybrid connections of GFL and GFM converters. Stability and coupling analyses are performed among GFL and GFR converters. small-signal stability of parallel GFM converters is proved, and real-time simulations and hardware-in-the-loop-test are performed for validating the studies

Reducing Computational Time Delay in Digital Current-Mode Controllers for Dc-dc Converters

A new method to improve the performance of digital current-mode controllers used in dc-dc power conversion is introduced. The proposed scheme is based on a simple prediction method which offers more time for DSP calculations than its conventional counterparts. Therefore, there will be less DSP computational time delay, which results in faster dynamic response and more accuracy and stability in power electronic converters. Principles of operation of the proposed prediction method as well as its application to several digital control techniques are presented

Digital Control of Variable Frequency Interleaved DC-DC Converter

This paper represents a design and implementation of a digital control of variable frequency interleaved DC-DC converter using a digital signal processor (DSP). The digital PWM generation, current and voltage sensing, user interface and the new period and pulse width value calculation with DSP STM32F407VGT6 are considered. Typically, the multiphase interleaved DC - DC converters require a current control loop in each phase to avoid imbalanced current between phases. This increases system costs and control complexity. In this paper the converter which operates in discontinuous conduction mode is designed in order to reduce costs and remove the current control loop in each phase. High current ripples associated with this mode operation are then alleviated by interleaving. Pulse width modulation (PWM) is one of the most conventional modulation techniques for switching DC - DC converters. It compares the error signal with the sawtooth wave to generate the control pulse. This paper shows how six PWM signals phase-shifted by 60 degrees can be generated from calculated values. To ensure that the measured values do not contain disturbances and in order to improve the system stability the digital signal is filtered. The analog to digital converter's (ADC) sampling time must not coincide with the power transistor's switching time, therefore the sampling time must be calculated correctly as well. Digital control of the DC-DC converter makes it easy and quickly to configure. It is possible for this device to communicate with other devices in a simple way, to realize data input by using buttons and keyboard, and to display information on LED, LCD displays, etc

Dynamic analysis and QFT-based robust control design of switched-mode power converters

The use of switched-mode power converters is continuously growing both in power electronics products and systems, e.g. in Telecom applications, commercial grid systems etc. The switching converters are required to provide robust behavior and to operate without instability under a variety of operation conditions. Hence the converter system may be subject to disturbances due to load, input voltage, and system parameter variations. In the thesis a robust control design procedure based on the QFT method (Quantitative Feedback Theory) is applied successfully for switching-mode DC-DC converters in order to achieve robust output in spite of different uncertainties. Simulation results are presented to demonstrate and validate the control design, showing good dynamic performance of the QFT controller. When designing large-scale systems it is often impractical to analyze and design the system as a whole. Instead, it is desirable to divide the system into manageable subsystems which can then be designed independently. The subsystems may then be connected together to form a complete integrated system. One of the major difficulties in integrated subsystems is the stability performance degradation due to the interaction between the subsystems. A formalism to analyze the interaction between subsystems using the unterminated two-port small-signal representation is derived. Two-port models are first defined as unterminated models, where the effect of load is excluded but may be easily included using the developed reflection rules. The use of the impedance ratio as a minor loop gain, which can be used to check system stability, is outlined. Recently, there has been increasing interest in the parallel operation of DC-DC converters for reasons of increasing system reliability, facilitating system maintenance, allowing for future expansion, and reducing system design cost. However, paralleled DC-DC converters require a systematic modeling methodology and a categorical current-sharing mechanism to improve a performance of the overall system. In order to achieve desirable characteristics when operating converter modules in parallel, a unified systematic approached for modeling of parallel DC-DC converter with current-sharing control, is proposed, developed, and analyzed

Control And Topology Improvements In Half-bridge Dc-dc Converters

Efficiency and transient response are two key requirements for DC-DC converters. Topology and control are two key topics in this dissertation. A variety of techniques for DC-DC converter performance improvement are presented in this work. Focusing on the efficiency issue, a variety of clamping techniques including both active and passive methods are presented after the ringing issues in DC-DC converters are investigated. By presenting the clamping techniques, a big variety of energy management concepts are introduced. The active bridge-capacitor tank clamping and FET-diode-capacitor tank clamping are close ideas, which transfer the leakage inductor energy to clamping capacitor to prevent oscillation between leakage inductor and junction capacitor of MOSFETs. The two-FET-clamping tank employs two MOSFETs to freewheeling the leakage current when the main MOSFETs of the half-bridge are both off. Driving voltage variation on the secondary side Synchronous Rectifier (SR) MOSFETs in self-driven circuit due to input voltage variation in bus converter applications is also investigated. One solution with a variety of derivations is proposed using zerner-capacitor combination to clamping the voltage while maintaining reasonable power losses. Another efficiency improvement idea comes from phase-shift concept in DC-DC converters. By employing phase-shift scheme, the primary side and the secondary side two MOSFETs have complementary driving signals respectively, which allow the MOSFET to be turned on with Zero Voltage Switching (ZVS). Simulation verified the feasibility of the proposed phase-shifted DC-DC converter. From the control scheme point of view, a novel peak current mode control concept for half-bridge topologies is presented. Aiming at compensating the imbalanced voltage due to peak current mode control in symmetric half-bridge topologies, an additional voltage compensation loop is used to bring the half-bridge capacitor voltage back to balance. In the proposed solutions, one scheme is applied on symmetric half-bridge topology and the other one is applied on Duty-cycle-shifted (DCS) half-bridge topology. Both schemes employ simple circuitry and are suitable for integration. Loop stability issues are also investigated in this work. Modeling work shows the uncompensated half-bridge topology cannot be stabilized under all conditions and the additional compensation loop helps to prevent the voltage imbalance effectively