High voltage ic-driven half-bridge gas discharge ballast

The present invention relates to a ballast circuit for a gas discharge lamp which employs a high voltage integrated circuit (HVIC) for driving a pair of serially connected switches that supply a.c. current to the lamp, and, more particularly to such a ballast circuit that applies a feedback signal to the HVIC for selecting a suitable frequency of operation during lamp starting.

One type of ballast circuit for a gas discharge lamp employs a pair of serially connected switches supplying a.c. current to the lamp, which is located in a resonant load circuit. The switches are configured in a half-bridge, Class D inverter configuration. Recently, a variety of high voltage integrated circuits (HVICs) have become available for driving such a half-bridge configuration in an alternating manner, i.e., first turning on one switch, turning it off, then turning on the second switch, turning it off, and so on. Beneficially, such HVICs could replace a variety of discrete circuit component at low cost and with reduction of ballast size. However, the HVICs are designed to provide a fixed frequency of switching of the pair of switches. While fixed frequency operation is typically suitable for steady state operation of gas discharge lamps, it is not suitable for operation during lamp ignition when it is desired that the frequency of the resonant load circuit approach its natural resonance frequency so as to result in a very high voltage spike necessary to cause lamp ignition.

It would, therefore, be desirable to overcome the foregoing deficiency of a mentioned HVIC so that, during lamp ignition, it will cause the resonant load circuit to approach its natural resonance frequency, to allow the generation of a high voltage spike to ignite the lamp.

It is, accordingly, is an object of the invention to provide a gas discharge ballast circuit incorporating a pair of serially connected switches for supplying a.c. current to a resonant load circuit, which circuit utilizes a HVIC for driving the pair of switches but which is configured to result in a frequency shift during lamp ignition towards the natural frequency of resonance of the load circuit.

In accordance with a preferred form of the invention, there is provided a ballast circuit for a gas discharge lamp. The ballast circuit includes a resonant load circuit incorporating a gas discharge lamp and including first and second resonant impedances whose values determine the operating frequency of the resonant load circuit. Further included is a d.c.-to-a.c. converter circuit coupled to the resonant load circuit so as to induce an a.c. current in the resonant load circuit. The converter includes first and second switches serially connected between a bus conductor at a d.c. voltage and ground, and has a common node through which the a.c. load current flows. A feedback circuit provides a feedback signal indicating the level of current in the resonant load circuit. A high voltage IC drives the first and second switches at a frequency determined by a timing signal which predominantly comprises the feedback signal during lamp ignition, whereby during lamp ignition the feedback signal causes the high voltage IC to drive the first and second switches towards a switching frequency which promotes resonant operation of the resonant load circuit.

The foregoing objects and further advantages and features of the invention will become apparent from the following description when taken in conjunction with the drawing, in which like reference numerals refer to like parts, and in which:

Fig. 1 is a schematic diagram, partly in block form, of a ballast circuit for a gas discharge lamp.

Fig. 2 is a voltage-versus-time graph of a typical timing signal applied to a timing input of a high voltage integrated circuit of Fig. 1.

Fig. 3 is a simplified lamp voltage-versus-angular frequency graph illustrating operating points for lamp ignition and for steady state modes of operation.

Fig. 4 is a plot of a timing voltage and related voltages versus time for steady state lamp operation.

Fig. 5 is similar to Fig. 4 but illustrates voltages during lamp ignition.

Fig. 6 is a detail, schematic diagram, partially in block form, an alternative circuit for applying a feedback voltage to a timing input of a high voltage integrated circuit.

Fig. 1 shows a ballast circuit 10 for powering a gas discharge (e.g. fluorescent) lamp, which is designated R_LAMP, because it may exhibit resistive impedance during operation. Ballast circuit 10 includes a pair of serially connected switches S₁ and S₂, such as power MOSFETs, which are connected to receive a d.c. bus voltage V_BUS between a bus conductor 12 and a ground 14. Control of switches S₁ and S₂ is provided by a high voltage integrated circuit (HVIC) 16, whose details are discussed below. By the alternate switching of S₁ and S₂, node 18 is alternately connected to bus voltage V_BUS and to ground 14. A resonant load circuit 20, connected to node 18, includes a resonant inductor L_R, a resonant capacitor C_R, and the lamp R_LAMP. A capacitor 21 provides d.c. blocking for load circuit 20. A feedback resistor R_F is further included for purposes to be discussed below. Due to its connection to node 18, a.c. current is induced in resonant load circuit 20.

HVIC 16 may comprise a half-bridge driver with oscillator, such as sold by SGS-Thompson under its product designation L6569, entitled "High Voltage Half Bridge Driver with Oscillator; or, such as sold by International Rectifier Company of El Segundo, California under its product designation IR2151, and entitled "Self-Oscillating Half-Bridge Driver." Respective high and low voltage outputs 21A and 21B from HVIC 16 drive switches S₁ and S₂. A timing resistor R_T and timing capacitor C_T are shown connected to HVIC 16. Timing resistor R_T is shown connected between a capacitor timing input 22 and a resistor timing input 24, as in conventional. Meanwhile, a timing capacitor C_T is shown connected at one end to capacitor timing input 22, as is conventional; however, the connections for the other end of timing capacitor C_T are not conventional, and, indeed, such connections relate to the inventive use of HVIC 16 in ballast circuit 10 so as to provide for the automatic generation of a very high voltage spike (e.g., 1,000-1,200 volts) across the lump R_LAMP during lamp ignition. Thus, a feedback signal, e.g., voltage V_F is applied to the lower-shown end of timing capacitor C_T by wire 26, which leads from the upper-shown end of feedback resistor R_F. In contrast, it would be conventional to connect the lower end of timing capacitor C_T directly to ground, without any feedback voltage V_F reaching timing input 22 of HVIC 16.

Both of the above-mentioned HVICs employ a timing input 22, which receives a timing signal V₂₂, with the resulting frequency of switching of switches S₁ and S₂ being determined by the respective times of transition of timing signal V₂₂ from one threshold voltage to another threshold voltage, and vice-versa. Thus, referring to Fig. 2, a possible timing signal V₂₂ is shown transitioning between a pair of voltage thresholds, which, as shown, may be 1/3 of a supply voltage V_S, which supplies HVIC of Fig. 1, and 2/3 of supply voltage V_S. Typically, when timing signal V₂₂ increases from the lower threshold and reaches the upper threshold, the upper end of timing resistor R_T becomes connected to ground 26 so that timing signal V₂₂ discharges through the timing resistor. Similarly, when timing signal V₂₂ then decays to the lower threshold, the upper end of timing resistor R_T is then connected to supply voltage V_S, causing timing signal V₂₂ to increase towards the upper threshold. At the transition points, e.g., at times t₁, t₂, t₃, and t₄, in Fig. 2, alternate switching of switches S₁ and S₂ is caused.

Prior to lamp ignition, the lamp R_LAMP appears as an extremely high resistance. During this time, the so-called "Q" or quality factor of resonant load circuit 20 is very high, because the lamp does not add a significant (i.e., low) resistive load to the circuit. During this time, it is advantageous to control switches S₁ and S₂ so that the frequency of operation of resonant load circuit 20 approaches its natural resonance point. When this occurs, the voltage placed across the lamp achieves the very high spike necessary to cause lamp ignition.

Fig. 3 shows a simplified lamp voltage-versus-angular frequency graph to explain operation of the lamp as between ignition and steady state modes. Lamp voltage is measured in decibels, and angular frequency ω is measured in radians, i.e. 2; times frequency. At angular frequency ω₂, a steady state operating point is shown at 30, at a steady state voltage V_SS. By decreasing the angular frequency to ω₁, however, the lamp voltage rises sharply to V_IGNITION, which is sufficient to cause the lamp to ignite. After ignition, the lamp exhibits a much lower resistance, and adds to the lossiness of resonant load circuit 20, decreasing its Q factor, and, hence, resulting in the lower, steady state voltage V_SS.

By applying feedback signal V_F to timing input 22 of HVIC 16, a desired shift in angular frequency will occur during lamp ignition to attain the very high voltage spike for igniting the lamp. Fig. 2 shows a timing signal V₂₂ with substantially symmetrical upward and downward exponential transitions having the same time constant such as would occur if timing input 22 of HVIC 16 were connected in the conventional manner described above. This results in a fixed frequency of operation of the lamp, which would be suitable for steady state lamp operation. Timing voltage V₂₂ on timing input 22 of HVIC 16 constitutes the sum of voltage contributions from timing capacitor C_T as it is charged or discharged, as well as a voltage contribution from feedback voltage V_F. During steady state lamp operation, feedback voltage V_F is typically quite small in relation to the contribution due to the charging or discharging of timing capacitor C_T. Thus, during steady state lamp operation, timing voltage V₂₂ is predominantly determined by the charging or discharging of timing capacitor C_T. (Other embodiments, however, might have the timing voltage predominantly controlled by a feedback voltage during steady state operation.) Fig. 4 illustrates the summation of voltages to produce timing voltage V₂₂.

In Fig. 4, the solid curve shows timing voltage V₂₂. The longer dashed-line curve 32 shows the contribution due to charging of timing capacitor C_T. Meanwhile, the shorter dashed-line curve V_F indicates a very small feedback signal. Thus, timing voltage V₂₂ is predominantly determined by the charging of capacitor C_T during steady state operation. Now, referring to Fig. 5, these same voltages during lamp ignition are illustrated.

Referring to Fig. 5, the invention takes advantage of the much higher voltages (and currents) present in resonant load circuit 20 during lamp ignition, when such circuit is essentially unloaded by the lamp (i.e., the lamp does not have a low resistance during this time). During lamp ignition, therefore, feedback signal V_F will be very much higher than during steady state lamp operation. While curve 32 showing the contribution from charging of timing capacitor C_T appears similar to as shown for the steady state case of Fig. 4, timing voltage V₂₂ in Fig. 5 does not increase as quickly. The reason is that, at timing input 22 of HVIC 16, the voltage contribution from timing capacitor C_T is summed with the inverse value of feedback voltage V_F. For illustration, however, feedback voltage V_F is shown, rather than its inverse value. Adding the inverse value of feedback voltage V_F to curve 32 results in the significant lowering of timing voltage V₂₂ noted above. As a consequence, the transition time of timing voltage V₂₂ from one threshold to another, as discussed above in connection with Fig. 2, is increased. As can be appreciated from Fig. 2, the frequency of operation of HVIC 16 is reduced. Such reduction in frequency is from a steady state operating frequency ω₂ shown in Fig. 3, towards the natural resonant frequency of resonant load circuit 20 shown at ω₁. This results in the very high lamp voltage spike necessary for lamp ignition. However, once lamp ignition is achieved, feedback voltage V_F and other voltage levels in the resonant load circuit sharply decrease, whereby such feedback voltage then has a negligible effect on timing voltage V₂₂ as described above in connection with Fig. 4.

For a 20-watt lamp, typical values for the components of ballast circuit 10 of Fig. 1 for a bus voltage V_BUS of 170 volts are as follows: resonant inductor L_R, 800 micro henries; resonant capacitor C_R. 5.6 nanofarads; feedback resistor R_F, 3.3 ohms; d.c. blocking capacitor 21, 0.22 micro farads; timing resistor R_T, 10.5 K ohms, and timing capacitor C_T, 0.001 microfarads.

Fig. 6 shows an alternative circuit for applying feedback voltage V_F to timing input 22. As shown, feedback voltage V_F is applied via a feedback timing capacitor C₁₂ to timing input 22 of HVIC 16. The lower end of timing capacitor C_T is connected directly to ground 14. In this embodiment, feedback voltage is capacitively coupled to timing input 22, and is summed at that input with the voltage across timing capacitor C_T. The behavior of the resulting ballast circuit is similar to that described above in connection with Fig. 1. In particular, the feedback voltage V_F predominates during lamp ignition in establishing timing voltage V₂₂, while the voltage across timing capacitor C_T predominates during steady state lamp operation.

As mentioned above, embodiments of the invention call be made in which the feedback voltage V_F predominates in establishing timing voltage V₂₂ both during lamp ignition and during steady state operation. For instance, the resistance of feedback resistor R_F could be increased to increase the feedback voltage V_F across it. Then, as opposed to the negligible contribution made by feedback voltage V_F according to Fig. 4, the feedback voltage V_F during steady state operation could be so large as to predominate over the contribution made by timing capacitor C_T. However, due to the increased resistive losses that would result in the feedback resistor R_F, the foregoing embodiment is not the preferred embodiment.