November 6, 1997

Spice simulates a fluorescent lamp

Christophe Basso, Pibrac, France

A hot-cathode fluorescent lamp (HCFL) contains a gaseous mixture that flows between two tungsten electrodes, or filaments. In domestic applications, the mixture comprises mercury vapor and a small quantity of inert gas (krypton or argon). The inert gas vaporizes the mercury during turn-on. To lengthen the filament's lifetime, a preheating period brings the electrodes to a high-enough temperature before avalanche occurs. The warm-up is easy to effect by supplying the filaments with ac or dc during the first few hundred milliseconds. During this emissive period, the filaments increase the electron population in the tube and consequently reduce the avalanche potential, thereby resulting in a lower striking voltage for the lamp. Once struck, the lamp maintains a quasiconstant voltage, or "arc voltage," across its terminals. A practical value for the cold-striking voltage for a 5-ft lamp is approximately 1 kV, with a corresponding arc voltage of approximately 110V rms. A Spice technique offers an easy way to model an HCFL (Listing 1). Click here to download the file from DI-SIG, #2108.

You can operate an HCFL at low or high frequency. At low frequency--for example, in a 50- or 60-Hz ballast application--the conducting gas reacts faster than the ac line. Every time the line polarity changes, the lamp current cancels, and the tube halts its conduction. The lamp then must restrike with the opposite polarity but at a voltage lower than its cold value because of the lamp's temperature. At this low operating rate, the voltage across the tube decreases as the current increases because of the negative-impedance characteristic of the conducting gas. At frequencies above a few kilohertz, these effects smooth out, and you can represent the tube using a resistive and weak capacitive filter.

You can model a fluorescent tube in several ways (Reference 1). Fitting the current-voltage characteristic represents an elegant option, but the parameter extraction from the manufacturer's data-sheet curves is complicated. Instead, the Spice macro-modeling technique offers a simplified and efficient method for generating the model. The technique uses Spice primitives to describe the lamp's complex electronic function.

The generalized model of the lamp is easy to understand (Figure 1). If the voltage on the tube is lower than the cold-strike voltage, no current flows except in the leakage elements. If the voltage increases and attains the striking voltage, the voltage-controlled switch closes, and the back-to-back zener network connects across the tube's terminals. The voltage then collapses to the arc value, and a current flows in the tube. The tube stays conductive until the current falls below the sustaining value. At this point, the switch opens, and the tube needs restriking. The netlist applies to IntuSoft's (San Pedro, CA) IsSpice4 simulator and uses standard Spice 3 elements with one of IntuSoft's Spice extensions, an if-then-else behavioral element.

Preheating the filaments reduces the striking voltage. The model accounts for this behavior and monitors the current flowing through the filaments before the first cold strike. To take advantage of this modeling feature, you should include the UIC (use-initial-conditions) key word in the transient Spice statement. In ac applications in which the period of operation is short, the thermal effects ensure a restrike voltage close to the arc value. This close conformance is especially true for high-frequency systems, such as electronic ballasts. The Spice BDIF element simply models this effect. Figure 2 shows the current-voltage characteristic of the IsSpice4 model; you can clearly see the negative-resistance effects.

Figure 3a shows a low-frequency test circuit using the tube model in a classic 50-Hz application; Figure 3b shows the simulation results. To operate the tube model at higher frequencies, simply remove the * before the RSTK element in the netlist and place one before the RNEG, DSTK1, and DSTK2 elements. Figure 4a shows a typical application for a self-oscillating ballast. You can calculate RSTK from the lamp's rms operating characteristics: RSTK=V_ARC/I_NOM. For a 36W tube, RSTK=103/0.43=240 ohms.

In Figure 4a, a saturable current transformer drives two MOSFETs. The transformer uses IntuSoft's nonlinear magnetic-core model. R₁₇ and C₁₂ drive the start-up, forcing Q₂₁ to start conducting a few milliseconds after applying the mains. Square waves then appear on the undamped L₁-C₁₃ network, and a high voltage appears across the tube ends. C₁₃ also ensures preheating of the filaments. Once the tube strikes, the resulting load changes the operating frequency of the ballast. Figure 4b shows the IsSpice4 simulation results. The model runs fast and converges without difficulties in low- and high-frequency applications. Thanks to its macro-modeling structure, the model's operating parameters easily adapt to various lamp types. Reference 2 gives further insight into self-oscillating-ballast techniques. (DI #2108)

References

1. Vladimirescu, Andrei, The Spice Book, John Wiley and Sons, ISBN 0-471-60926-9.

2. "Energy-efficient semiconductors for lighting," Application Note BR480/D, Motorola Semiconductor, Austin, TX.

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