Hamid Daneshpajooh

Bidirectional DC - DC Converters for Energy Storage Systems

Bidirectional dc-dc converters (BDC) have recently received a lot of attention due to the increasing need to systems with the capability of bidirectional energy transfer between two dc buses. Apart from traditional application in dc motor drives, new applications of BDC include energy storage in renewable energy systems, fuel cell energy systems, hybrid electric vehicles (HEV) and uninterruptible power supplies (UPS). The fluctuation nature of most renewable energy resources, like wind and solar, makes them unsuitable for standalone operation as the sole source of power. A common solution to overcome this problem is to use an energy storage device besides the renewable energy resource to compensate for these fluctuations and maintain a smooth and continuous power flow to the load. As the most common and economical energy storage devices in medium-power range are batteries and super-capacitors, a dc-dc converter is always required to allow energy exchange between storage device and the rest of system. Such a converter must have bidirectional power flow capability with flexible control in all operating modes. In HEV applications, BDCs are required to link different dc voltage buses and transfer energy between them. For example, a BDC is used to exchange energy between main batteries (200-300V) and the drive motor with 500V dc link. High efficiency, lightweight, compact size and high reliability are some important requirements for the BDC used in such an application. BDCs also have applications in line-interactive UPS which do not use double conversion technology and thus can achieve higher efficiency. In a line-interactive UPS, the UPS output terminals are connected to the grid and therefore energy can be fed back to the inverter dc bus and charge the batteries via a BDC during normal mode. In backup mode, the battery feeds the inverter dc bus again via BDC but in reverse power flow direction. BDCs can be classified into non-isolated and isolated types. Non-isolated BDCs (NBDC) are simpler than isolated BDCs (IBDC) and can achieve better efficiency. However, galvanic isolation is required in many applications and mandated by different standards. The

Basic families of medium-power soft-switched isolated bidirectional dc-dc converters

This paper is concerned with the investigation of common medium-power isolated bidirectional dc-dc converters (IBDC) which are increasingly being used in many applications such as interfacing renewable energy resources to utility grid, hybrid electric vehicles and UPS systems. Although different varieties of IBDCs have been proposed by researchers, they can be conceptually classified into a few families. This paper provides an insight into the basic operation of each family by investigating the working principles of a representative member of each family. This helps in comparing different characteristics of each family and understanding their advantages and disadvantages for a certain application.

An efficient soft switched DC-DC converter for electric vehicles

This paper presents a new technique to improve the efficiency of the ZVS full-bridge dc-dc converter used to process the power between the high voltage traction battery and the 12V utility battery in a Plug-in Hybrid Electric Vehicle (PHEV). Efficient operation of the converter is crucial in order to maintain the energy of traction battery for a longer time and for increasing driving distance. Light load efficiency of the dc-dc converter is especially important because this converter is lightly loaded most of the time while the car is being driven. The passive asymmetrical auxiliary circuit used to extend the soft switching range, produces extra circulating currents that increases conduction losses. A new technique for controlling circulating currents in the auxiliary circuit is introduced that with a small increase in controller complexity, reduces conduction losses and improves the converter efficiency especially at light load. By proper duty cycle control of the full bridge switches, auxiliary circulating currents are reduced to the minimum possible values required for ZVS. While phase shift angle mainly serves as the output regulation control parameter, duty cycle is varied to keep converter in the soft switching region with minimum conduction losses. Theoretical analysis and operating principles as well as soft switching operation are discussed. Experimental results for a 2KW converter are presented that validate the significant improvement in efficiency and considerable saving of valuable energy storage.

Modified dual active bridge bidirectional DC-DC converter with optimal efficiency

In this paper a modified dual active bridge (DAB) topology with a new modulation technique is proposed for bidirectional dc-dc conversion that not only improves the soft switching range of the converter but also highly reduces the large current ripples at low voltage side. Phase shift and duty cycles of active bridges on two sides, (d1, d2, θ), are used to control the converter in order to extend the soft switching range against wide range of operating voltages on both ports and also to maximize the efficiency. The converter operation is analyzed and the soft switching conditions are extracted. For an aero space application it is shown that with a proper converter design the soft switching zones in control space can cover full power range against the wide voltage range required by standards. The extraction of optimal trajectories of control parameter values (d1opt, d2opt, θopt), for maximum efficiency are also discusses and an implementation method for the controller is proposed. Precise simulated model of the converter is developed to verify the analytic results and fine tune the design. A 2KW, 250 KHz prototype is also implemented to verify the results in practice.

Optimizing dual half bridge converter for full range soft switching and high efficiency

This paper is concerned with finding the optimum operating points of a soft-switched dual half bridge bidirectional dc-dc converter. Phase shift modulation (θ) with a fixed duty cycle is used to control the converter [1] but the soft switching range is limited. In this paper the soft switching range and efficiency of the converter are highly improved by recruiting duty cycle (d) as the second control parameter. It is shown that for a wide voltage range on both dc buses, soft switching can be achieved for any power level including zero to full load in both directions. The regions of soft switching on the (θ,d) plane are analyzed and the trajectories of the optimal (θ,d) values for highest efficiencies are extracted. A precise simulated model of the converter is developed to verify and fine tune the analytic results. Finally a 250 KHz, 1.5Kw prototype is designed and implemented to verify the results in practice.

A load/line adaptive zero voltage switching DC/DC converter used in electric vehicles

This paper presents a load/line adaptive Zero Voltage Switching (ZVS) full-bridge converter, which is able to optimize the amount of reactive current required to guarantee ZVS of the power MOSFETs. The proposed DC/DC converter is used as a battery charger for an electric vehicle. Since this application demands a wide range of load/line variations, the converter should be able to sustain ZVS under different conditions. The converter employs coupled inductors to provide the reactive current for the full-bridge semiconductor switches, which ensures ZVS at turn-on times. The coupled inductor along with the specific control system is able to generate the optimum value of the reactive current injected by the auxiliary circuit in order to minimize extra conduction losses in the power MOSFETs, as well as the losses in the coupled inductors. In the proposed approach, the peak value of the reactive current is controlled by the phase-shift between the leading leg and lagging leg of the full-bridge converter to optimize the load impact. Also, the reactive current is controlled by the switching frequency in order to compensate for the input voltage variations. Experimental results for a 2kW DC/DC converter are presented. The results show an improvement in efficiency and better performance of the converter particularly for heavy loads.

A multi-variable control technique for ZVS phase-shift full-bridge DC/DC converter

In this paper a multivariable control system is proposed for an efficient ZVS full-bridge dc-dc converter used in a Plug-in Hybrid Electric Vehicle (PHEV). This converter processes the power between the high voltage traction battery and low voltage (12V) battery. Generally, Phase-shift between the two legs of the full-bridge converter is the main control parameter to regulate the output power. However, the zero voltage switching cannot be guaranteed by merely controlling the phase-shift particularly for light load conditions. In order to extend the soft switching operation of the converter for light loads, asymmetrical passive auxiliary circuits are used to provide reactive current. However, the auxiliary circuits increase extra current burden on the power MOSFETs, leading to lower efficiency. In this paper, the duty cycle of bridge legs (as another control parameter) is also controlled to minimize the conduction losses of the converter. Basically, the multivariable controller has to adjust the control parameters in such a way that the circulating currents are kept at their minimum level for soft switching while the output power is regulated. The system operating principle, soft switching and mathematical model are discussed. Experimental results are also presented that validate the effectiveness of the control method for a 2KW prototype.

A Predictive Control Strategy For Adaptive energy storage in ZVS phase-shift-modulated full-bridge converter topologies

This paper presents a predictive control strategy for a family of zero-voltage switching phase-shift-modulated dc/dc converters initially reported for fuel cell application. The proposed control method guarantees zero-voltage switching (ZVS) operation with minimal current overshoot in auxiliary branches for all the line and load transients. The suggested method is compared with switch voltage feedback control to validate the analysis. It is shown that the proposed approach offers close to three times faster response, and reduces current overshoot by 75%. The latter allows lower safety margins for switches and auxiliary branch components, which simplifies converter design, and reduces size, weight, and cost.

Design of dc-dc converter with phase shift and duty cycle control for full range soft switching

In this paper design of a dual half bridge (DHB) converter as a soft switched isolated bidirectional dc-dc converter is discusses. The operation principles and steady state analysis of the converter with phase shift (θ) plus duty cycle (d) control are explained and soft switching conditions are extracted. The design and selection of main components are discussed for fully soft switching operation against wide range of dc bus voltage variations. Soft switching regions of the designed converter in the control space are extracted. Results show that with the designed converter soft switching can be achieved for full power range in both directions against wide voltage variations on the dc ports. Finally a 1.5KW, 250KHz prototype is implemented to verify the results and some practical issues in implementation of converter to improve its performance are explained.