Modelling discharge lamps for electronic circuit designers: a review of the existing methods

Summary form only given. The purpose of this paper is that to present a concise review of the methods used up to now for the description of electrical discharge lamps a part of an electronic circuit. Several attempts to describe the lamp by an equivalent electric circuit have been made with more or less accurate results. Some attempts to integrate "physical" discharge models based in the electrical conductivity of the plasma have been also achieved. Nowadays electronic power supplies for discharge lamps becomes more and more reliable and robust and occupies thereafter an important part of the light sources market. Furthermore, according new European Union commitments classic ferromagnetic ballasts are convicted to disappear in a few years from now. The study of a complex system consisting on an electronic power supply and a highly non-linear element (the lamp) necessitates a competence combination from several disciplines. However, up to now the discharge lamp scientific and engineering community remained relatively disconnected form the power electronics community. Looking in the existing literature we noticed that very complex physical models have been developed for almost any type of discharge lamp (HID, fluorescence, MHL, HPS...). Those models are able to describe in fair degree of approximation the lamp operation under DC, AC (or other) "ideal" conditions. To the other side, power electronics engineers are able to describe complicate electronic circuits by means of specialised software (Spice and its family, SUCESS, Simplorer...). However, in many cases the lamp models developed by the specialists are incompatible with the software used by the other community. Thus, electrical engineers tried to create some simple lamp models based essentially in experimental V-I characteristic of the lamp. The simplest approach consisted on the use of a fixed "negative" resistance describing the negative slope of this V-I curve. The problem with this approach is that the lamp- resistance changes as function of the current. More complicated electrical equivalents for the lamp have been presented, but in any case their domain of validity is still limited. Another, more accurate but less frequently used, approach consist on the description of the lamp conductance G(t) by means of a single differential equation. This type of models implemented in general purpose solver software can describe the lamp behaviour including the V-I hysteresis cycle. Finally, is possible to integrate a full physical discharge model based on a collisional-radiative scheme for a non-LTE system (eg fluorescent lamp) or on fluid equations for LTE devices (eg HID). This has been done by using both PSpice and SUCCESS software. The advantage of this rather complicated approach is that such type of models has a large domain of validity and it is completely free from any experimental data and/or adjustable parameters. However, the realizations of those models assumes a good knowledge of the discharge plasma and of the circuit simulator.