Original articleDBD lamp converter design using an electrical model of the load
Introduction
Dielectric barrier discharge (DBD) excimer or exciplex lamps are used to produce UV radiation, with multiple applications such as bacteriologic sterilization or dermatology. The operating principle consists of establishing an electrical discharge in a gas mixture, which includes a rare gas and sometimes additional elements (e.g. Xe or Xe + Cl2). The discharge induces atoms to an excited state, which subsequently, associate to form excited molecules and return later to their ground state, liberating some energy in form of UV radiation [3], [4]. The structure of the lamp is presented in Fig. 1. Two coaxial silica barriers confine the gas mixture. Metallic electrodes do not have direct contact with the gas (an interesting property for a long lifetime bulb) and are arranged inside the inner wall (massive electrode) and covering the outside wall (thin cable winding, which does not screen the majority of the UV rays).
The aim of this paper is to propose a design process for the power supply of the lamp, based on the model of the latter.
Section snippets
Modelling of the lamp
An electrical model of the lamp allows the simulation of the entire system, describing the interactions between the power converter and the lamp. This model is built according to the analysis of the geometry of the lamp and its structure is presented in Fig. 2.
In this model, Cdiel represents the series equivalent capacitance of external and internal dielectric barriers; Cgas stands for the capacitive nature of gas while breakdown is not present; finally, the conductance block characterizes the
Power converter
In this section the design of the current-mode converter is described. A bidirectional current source is mandatory to avoid the indefinitely rise of the lamp voltage (in absolute value), due to the capacitive characteristic of a DBD. To fulfil this condition, configurations derived from a current source in series with a full-bridge are derived from the classical boost and buck-boost topologies; those are simplified, giving the two final structures shown in Fig. 7.
Dimensioning the converter
The algorithms used to compute the values of the different components of the converters are outlined in the following subsections. We consider only the case of the buck-boost derived topology. The control of the operating point of the lamp is the main objective of this process. The required input values are the lamp power (Plamp), the lamp operating frequency (flamp) and discharge time of the lamp (tdis).
The converter has two charge and two discharge sequences in a period, as illustrated in
Results
The best transformer, up to now, is assembled in a long cylindrical core, and three additional ferrites in “I” shape to close the squared magnetic circuit, as shown in Fig. 13. This arrangement enables to build a single layer in the secondary, minimizing the parasitic capacitance. Additionally it allows enough turns to create a magnetizing inductance that is much greater than the inductance L. For the primary windings, each one has been organized in a single layer covering the secondary, in
Conclusion
The present paper presents the design of a current-mode power supply using an electrical model of the lamp. This model permits the simulation of the entire system with consistent results.
The dimensioning method, used to build the power supply, assures the correct converter operation and automatically selects the materials and values to build the transformer and the inductance, minimizing the parasitic effects.
The electrical model of the lamp also helps to retrieve the non-measurable variables,
Acknowledgment
The authors thank Quantel for providing their patented lamp for the experiments.
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