The present invention relates to a starting device for a discharge lamp such as a fluorescent lamp. More particularly, the invention relates to a discharge lamp starting device utilizing semiconductor switching elements.
Various starters utilizing semiconductor switching elements have been heretofore proposed, of which one example utilizing a nonlinear dielectric element and a thyristor is shown in FIG. 1. In this figure, reference numeral 1 designates a discharge lamp including filaments 101a and 101b at opposite ends of the lamp; reference numeral 2 designates an inductive stabilizer; reference numeral 3 a semiconductor switch composed of a reverse blocking triode thyristor 301, a trigger element 302 such as an SBS (Silicon Bidirectional Switch) or Diac, voltage dividing gate circuit resistors 303a and 303b, and a smoothing capacitor 304; reference numeral 4 designates a nonlinear dielectric element; reference numeral 5 a noise eliminating capacitor; and reference characters U and V power source terminals.
In the above-described starter, when an AC voltage e.sub.UV, having a waveform shown by a dotted line in FIG. 2A, is applied across the power source terminals U and V, at the beginning of the lamp starting period, the thyristor 301 turns on at a phase angle .theta..sub.1 in a positive half cycle of the power source voltage. At that time, a current flows through a path including the stabilizer 2, filament 101a, thyristor 301, and filament 101b, thereby preheating the filaments. This current will lag the source voltage due to the inductive effect of the stabilizer 2. When the current flowing through the thyristor falls below the holding current of the device, the thyristor 301 turns off at a phase angle .theta..sub.2, which occurs during the negative half cycle of the power source voltage. At the instant that the thyristor 301 is turned off, the voltage applied across the element 4 is zero. Since the power source voltage e.sub.UV is then in the proximity of the negative peak of the waveform, the element 4 is subsequently charged to the indicated polarity through the stabilizer 2.
The element 4 has a Q-V (stored charge vs. voltage) as shown in FIG. 3, wherein the stored charge becomes saturated at an applied saturation voltage E.sub.s. By appropriately selecting the Q-V characteristic of the element 4, the nonlinear region (where the charged voltage is less than the saturation voltage E.sub.s) is reached at a voltage less than the peak voltage of the power source. In this case, the charging current flowing into the element 4 is abruptly reduced at the instant the power source voltage exceeds the saturation voltage. Due to the inductive property of the stabilizer 2, the charged voltage of the element 4 then increases abruptly in the form of a pulse voltage V.sub.21, as shown in FIG. 2A which is substantially higher than the peak voltage of the power source. The pulse voltage V.sub.21 is applied across the discharge lamp 1. After the occurrence of this pulse voltage, the power source voltage e.sub.UV is applied across the lamp 1 until the thyristor 301 again turns off in the next cycle.
The above-described operation continues until the lamp 1 is started. That is, while the filaments 101a and 101b of the lamp 1 are being heated by the preheating current, the discharge of the lamp 1 is initiated by one of the positive pulse voltage V11 and the negative pulse voltage V21.
Once the lamp 1 has started, the lamp voltage is reduced below the power source voltage, thus keeping the thyristor 301 turned off. More specifically, although the lamp voltage instantaneously rises above the power source voltage, as shown by V.sub.12 and V.sub.22 in FIG. 2A, due to the charging effect of the element 4, the thyristor 301 cannot be turned on by voltages of the magnitude of V.sub.12 because of the smoothing effect of the capacitor 304.
While the above-described starting device utilizing a nonlinear dielectric element and a thyristor is advantageous in its starting performance, simplicity of circuit construction, and low cost, nevertheless, the construction shown in FIG. 1 has following difficulties:
(1) The power consumption of the circuit is higher than in a starter in which the starter is disconnected from the lamp circuit after the lamp has been started.
(2) Charging and discharging currents flowing in and out of the element 4 create annoying vibration and noise due to piezoelectric effects.
More specifically, as shown in FIG. 2B, after the lamp 1 has been started (at phase angles .theta..sub.7 and .theta..sub.8, for instance), a discharge current i.sub.11 flows through the element 4 and the lamp 1. Due to the presence of the discharge current i.sub.11, the power consumption of the lamp 1 is considerably high in comparison with starter arrangements in which the starter is fully disconnected from the lamp circuit after the lamp has been started.
FIG. 4 illustrates a second example of a conventional starting device which is intended to overcome the above-described difficulties. In this figure, reference numeral 30 designates a bidirectional semiconductor switch, reference numeral 7a designates a diode connected in parallel with the element 4, and reference numerals 6 and 7b designate a series-connected resistor and diode connected in parallel with the semiconductor switch 3 to provide a discharge circuit for the switch 30. The semiconductor switch 30 is composed of a bidirectional triode thyristor 305, a trigger element 302 such as an SBS, Diac or the like, resistors 303a and 303b, and a capacitor 304.
The operation of the conventional device will now be described with reference to FIG. 5A which shows a voltage waveform across the lamp 1.
In the beginning of the starting operation, the thyristor 305 is turned on at a phase angle .theta..sub.1 in a positive half cycle of the power source voltage e.sub.UV, at which time a preheating current flows through a path including the stabilizer 2, filament 101a, diode 7a, thyristor 305, and filament 101b. The thyristor 305 is turned off at a phase angle .theta..sub.2 in the following negative half cycle of the power source voltage e.sub.UV, at which time the preheating current is reduced to zero. The thyristor 305 is again turned on thereafter at a phase angle .theta..sub.3 by operation of the trigger element 302, thereby to create a charging current through the thyristor 305 and the element 4.
Because the element 4 has a nonlinear characteristic as in the case of the first example of the conventional device, when saturation is reached and the charging current of the element 4 drops at a phase angle .theta..sub.4 to a value lower than that required for maintaining the thyristor 305 in the conductive state, the thyristor 305 is again turned off and the power source voltage is continuously applied across the lamp 1 until a phase angle .theta..sub.6 in the positive half cycle, at which time the thyristor 305 is again turned on to pass the preheating current.
The purpose of the discharge resistor 6 and the diode 7b is as follows. When saturation is reached at the phase angle .theta..sub.3 causing an abrupt increase of the voltage across the element 4 to the maximum voltage V.sub.21, the current through the element 4 then flows through the resistor 6 and the diode 7b, thereby causing the voltage across the element 4 to substantially follow the lamp voltage. Since the voltage applied to the thyristor 305 is substantially equal to the difference between the charged voltage V.sub.21 of the element 4 and the power source voltage e.sub.UV, if there were no such discharge circuit, an extremely high voltage withstanding property would be required for the thyristor 305. Also, the diode 7b prevents charging of the element 4 during the time interval between phase angles .theta..sub.2 and .theta..sub.3, thus ensuring the generation of the high voltage pulse V.sub.21 by abruptly charging the element 4 starting from a zero potential.
Once the discharge lamp 1 has started, the lamp voltage is reduced below the power source voltage e.sub.UV, thus preventing turning on of the thyristor 305 and maintaining stable operation of the lamp 1. Furthermore, since most of the power source voltage is applied across the thyristor 305, the voltage applied to the element 4 is reduced to approximately zero. Hence the drawbacks hereinbefore described with respect to the first example of the conventional device that are caused by charging and discharging currents of the element 4 after the lamp has been started are eliminated.
Unfortunately, however, as illustrated by FIG. 5B, the connection of the diode 7a in parallel with the element 4 prevents positive voltages from being applied across the element 4. This causes the dielectric polarization of the element 4 to be shifted in one direction only, and hence the hysteresis loop of rectangular shape as shown in FIG. 3 is deformed to such an extent that the desired nonlinear characteristic of the element 4 is substantially lost. In other words, the amplitude of the pulse generated at the phase angle .theta..sub.4 is reduced to such an extent that the starting of the discharge lamp is difficult.
An object of the present invention is thus to overcome the above-described drawbacks of the conventional discharge lamp starting devices.
More specifically, it is an object of the present invention to eliminate:
(1) the high power consumption due to charging and discharging currents of the element 4 and the accompanying generation of vibration from the element 4; and
(2) the reduction of negative-side pulse voltages from the element 4.