Amir M. Rahimi

Active Damping in DC/DC Power Electronic Converters: A Novel Method to Overcome the Problems of Constant Power Loads

Multi-converter power electronic systems exist in land, sea, air, and space vehicles. In these systems, load converters exhibit constant power load (CPL) behavior for the feeder converters and tend to destabilize the system. In this paper, the implementation of novel active-damping techniques on dc/dc converters has been shown. Moreover, the proposed active-damping method is used to overcome the negative impedance instability problem caused by the CPLs. The effectiveness of the new proposed approach has been verified by PSpice simulations and experimental results.

Loop-Cancellation Technique: A Novel Nonlinear Feedback to Overcome the Destabilizing Effect of Constant-Power Loads

Tightly regulated closed-loop converters are problematic when used as a load since they tend to draw constant power and exhibit negative incremental resistance. This negative resistance causes stability problems for the feeder system, whether it is an input filter or another converter. In multiconverter power electronic systems, which exist in different land, sea, air, and space vehicles, including electric, hybrid, plug-in hybrid, and fuel-cell vehicles, there are many converters loaded by other converters. Therefore, the destabilizing effect of the load converters, which are called constant-power loads, is a major issue. In this paper, a novel nonlinear feedback, which is called loop cancellation, is introduced. This technique is used to cancel the destabilizing effect of the constant-power loads. Theoretically, any amount of constant-power load can be compensated by this technique, and it can identically be implemented on different types of converters. The effectiveness of the proposed technique has been verified by PSpice simulations and experimental results.

Modified Pulse-Adjustment Technique to Control DC/DC Converters Driving Variable Constant-Power Loads

Multiconverter-distributed DC architectures have been utilized for power distribution in many applications such as telecommunication systems, sea and undersea vehicles, an international space station, aircraft, electric vehicles, hybrid-electric vehicles, and fuel-cell vehicles, where reliability is of prime concern. The number of power-electronic converters (AC/DC, DC/DC, DC/AC, and AC/AC) in these multiconverter electrical power systems varies from a few converters in a conventional land vehicle, to tens of converters in an advanced aircraft, and to hundreds of converters in the international space station. In these advanced applications, power-electronic converters might need to have a tight output-voltage regulation. From the output perspective, this property is highly desirable. However, since power-electronic converters are efficient, tight regulation of the output makes the converter appear as a constant-power load (CPL) at its input side. Dynamic behavior of CPLs is equivalent to negative impedance and, therefore, can result in instability of the interconnected power system. In order to mitigate the instability of the power converters loaded by CPLs, this paper presents the pulse-adjustment digital control technique. It is simple and easy to implement in application-specific integrated circuits, digital-signal processors, or field-programmable gate arrays. Moreover, its dynamic response is fast and robust. Line and load regulations are simply achievable using this technique. Analytical, as well as simulation and experimental results of applying the proposed method to a DC/DC buck-boost converter confirm the validity of the presented technique.

Digital Control of an Isolated Active Hybrid Fuel Cell/Li-Ion Battery Power Supply

One of the important issues in modern electronic equipment is providing higher peak power while preserving high energy density. Hybrid power sources composed of fuel cells and batteries combine the high-energy capabilities of fuel cells with the high-power capabilities of batteries. DC/DC power converters can appropriately control the power flow shared between the fuel cell and battery system. In this paper, we propose a new implementation of hybrid fuel cell/battery systems. A 50-W isolated power electronic interface system for a hybrid solid-oxide fuel cell (SOFC)/Li-ion battery portable power supply is designed and implemented. The control strategy presented in this paper is able to regulate the output current of the fuel cell, the charging current of the battery, and the output voltage of the power supply. The control strategy is implemented in a DSP and tested by simulation and experiments. Experimental results present the flexibility and generality of the control strategy.

Negative Impedance Stabilizing Pulse Adjustment Control Technique for DC/DC Converters Operating in Discontinuous Conduction Mode and Driving Constant Power Loads

Multiconverter power-electronics-based power systems are being increasingly used in advanced vehicles. Stability of these power electronics-intensive systems is a significant design consideration because of the potential for negative impedance instability. In this paper, in order to mitigate the problem of negative impedance instability, pulse adjustment, which is a novel fixed frequency digital control technique for converters operating with constant power loads (CPLs), is presented. This novel digital control approach treats the converter as a digital system and achieves output voltage regulation by choosing high- and low-power pulses instead of using conventional pulsewidth modulation scheme. A comparator compares the actual output voltage with the reference voltage and then switches between the appropriate states. Therefore, the digital control task is to deliver the right amount of energy to the converter by right numbers of state operations so that the average power delivery matches the required power. It needs few logic gates and comparators to implement this digital control, thus making it extremely simple and easy to develop using low-cost application specific integrated circuits. It is simple, cost effective, and dynamically fast. In this paper, a model to analyze the dc/dc buck-boost converter that is controlled by the pulse adjustment approach in the discontinuous conduction mode is derived. Stability of the buck-boost converter driving CPLs is analyzed using the introduced model. Furthermore, comprehensive sensitivity analyses of applying the pulse adjustment method to control dc/dc converters that are loaded by CPLs are presented. In addition, this paper gives design considerations to assess performance and stability robustness of the pulse adjustment method to control buck-boost converters that are loaded by CPLs. Analytical, simulation, and experimental results are presented to describe and verify the proposed technique.

Discontinuous-Conduction Mode DC/DC Converters Feeding Constant-Power Loads

One of the major issues in multiconverter power-electronic systems, which exist in different land, sea, air, and space vehicles, is the stability problem imposed by constant-power loads (CPLs). The research work done in this field has been focused mainly on continuous-conduction mode of operation. In this paper, we study the case when the loaded converter operates in discontinuous-conduction mode (DCM). We prove that the open-loop DC/DC converters operating in DCM are stable when they are loaded by CPLs. Furthermore, we present that the problem of feedback design for a converter operating in DCM and loaded by a CPL can be translated into a conventional feedback-design task for the same converter with a resistive load. Simulation and experimental results verify the presented discussion.

A lithium-ion battery charger for charging up to eight cells

Benefits of using lithium-ion batteries have made them the best choice for mobile applications like cameras, laptops, electric-vehicles, etc. On the other hand, charging and using these batteries demand observing some precautions, which would otherwise lead to battery damages. These are of more importance when batteries of multiple cells are involved. In this paper after reviewing the charging methods of multiple cells, an implemented battery charger, which can charge batteries comprised of up to eight cells, is introduced.

Design and Implementation of an Analog Constant Power Load for Studying Cascaded Converters

One of the major issues in multi-converter power electronic systems is the constant power load (CPL) behavior of the tightly regulated converters. In order to have better system performance, output of the converters should be tightly regulated. DC/DC converters when tightly regulated act as constant power loads. The current through a constant power load increases/decreases when the voltage across it decreases/increases, respectively. As a result, CPLs have negative impedance characteristics. This is a destabilizing effect known as negative impedance instability. In order to analyze cascaded DC/DC converters in multi-converter systems, an analog constant power load (A-CPL) that has a fast time response is developed and presented in this paper. It can be used as an ideal CPL for studying the mentioned issue in the laboratory. Since it is analog, the proposed A-CPL separates the problem of sub-harmonic production from the problem of negative impedance instability for research purposes. By changing the position of a manual switch, the implemented A-CPL can also operate as an adjustable power resistor or a current source for the laboratory experiments. The experimental results showing the performance of the A-CPL are also presented

Sub-Harmonic Problem in Multi-Converter Vehicular Power Systems

Power electronics is an inevitable part of vehicular multi-converter power systems. In a multi-converter power system, various buses and loads are connected through different power electronic converters and inverters. These systems consist of a few to tens of converters in electric and hybrid electric vehicles to hundreds of converters in more electric aircraft, sea vehicles, and international space station. A phenomenon that can occur in cascaded converters and multi-converter systems is sub-harmonic production. The problem with sub-harmonics is that they could not be easily cancelled out by filters. In this paper, this issue is addressed. Furthermore, some precautions to prevent the problem are introduced. Simulation and experimental results confirm the addressed concept

Analysis and Stabilization of a Buck-Boost DC-DC Converter Feeding Constant Power Loads in Parallel with Conventional Loads in Vehicular Systems

The insertion of power electronics in vehicular technologies is becoming widespread. The increasing use of power electronic controlled motor drives in land vehicles, marine vessels and aircraft, for lots of applications such as propulsion, braking, steering and actuation is creating particular problems when the power distribution system is in the form of multi-converter level DC electrical systems. This is because of many power electronic loads and associated controllers which have a constant power characteristic resulting in negative incremental input impedance. The effect of such loads in a DC system is to cause instability. This paper aims to devise a novel digital technique, pulse adjustment, for damping the instabilities and ensuring orderly operation of the system. In spite of its simplicity pulse adjustment offers a very fast and smooth response. Simulation results as well as experimental outputs are presented to verify the proposed technique