Electronic operating circuits for fluorescent lamps of the type to which the present invention relates, usually include a rectifier circuit to which alternating power from network lines is applied. The rectified voltage is chopped to provide an output at a frequency which is substantially higher than the frequency of the power network voltage. A current limiting circuit receives the chopped output and applies it to one terminal, each, of the lamp electrodes. The other terminals of the lamp electrodes are connected to an ignition or starting circuit. This arrangement is well known and FIG. 1--to be described in greater detail hereafter--shows such a prior art arrangement.
Fluorescent lamps, in dependence on structure and gas fill, have lamp operating voltages between 30 and 150 V.sub.eff (effective voltage). The peak-to-peak voltage of the highfrequency alternating voltage U.sub.L, applied to the lamp, may be higher than the lamp voltage by a factor of about 3--in dependence on the wave shape. Thus, the high-frequency voltage may have values of between 90 to 450 volts peak-to-peak, herein referred to as V.sub.SS. The circuits usually required at the network voltage for the lamp circuit is higher than the maximum lamp operating voltage.
Operation of lamps at power network voltages customary in the United States and Canada, for example, in the order of, effectively, 100 to 120 V, results in problems. Filtered direct current voltage can be derived from the rectifiers at levels of only about 130 to 160 V. Fluorescent lamps which only have low operating voltage, that is, less than 100 V.sub.SS, can be operated after chopping with a current limiting choke without using special circuit arrangements. Only if the network voltage is slightly higher than lamp operating voltage is it possible to consistently and reliably operate such fluorescent lamps.
Fluorescent lamps have a negative impedance characteristic. This negative impedance characteristic obtains already if the lamp operating voltage U.sub.LSS, that is, the peak-to-peak voltage during operation, is only slightly below the chopped direct current voltage U.sub.SS. Due to the negative impedance characteristic, operation will be unstable. Only current limiting chokes with very small inductivity can be used. Lamps with lamp operating voltages which are higher than the network voltages, that is, lamps having operating voltages of between for example 120 to 450 V.sub.SS, cannot be operated at all utilizing current limiting merely by means of a choke.
It is possible to step up the d-c voltage by a voltage doubler rectifier circuit, for example of the type known as a Villard or a Delon circuit--see, for example, "Bauelemente der Elektronik und ihre Grundschaltungen" ("Components in Electronics and Basic Circuits Therefor"), by Buser, Kahler, Weigt, 7th edition, page 220. To obtain such high voltages, however, it is necessary to use high capacity values of electrolytic capacitors, or a hum voltage of 60 or 120 Hz will obtain with a 60-cycle supply. The light flux modulation is increased.
It is, of course, possible to transform the chopped essentially square-wave voltage by a transformer; the referenced literature "Elektronischaltungen" ("Electronic Circuits") by W. Hirschmann, page 144 shows such an arrangement. This, however, has a disadvantage in that a wound wire component must be used. This increases costs, and causes additional losses, so that the overall efficiency of the electronic circuit - lamp combination is decreased.
U.S. Pat. No. 4,544,863 describes an auxiliary circuit for fluorescent lamps in which a power network voltage of low frequency, for example 60 Hz, is transformed into a high-frequency voltage, suitable for fluorescent lamp operation. This high-frequency voltage is applied to the fluorescent lamp via an inductance and a capacitor. The inductance and capacitor are connected in series. This circuit operates with a separately controlled oscillator, requiring precise dimensioning of the components of the auxiliary circuit. The result is an arrangement which is specific to certain lamps; and the circuit is not readily adaptable, flexibly, to various lamps.