Discharge Lamp Ballast Circuit

The present invention relates to discharge lamp operating circuits, and more particularly concerns direct current ballast circuits for such lamps.

It is a general object of the invention to provide an improved direct current operating circuit for loads, especially gaseous discharge lamps.

It is a particular object of the invention to provide an improved direct current ballast circuit for gaseous discharge lamps which avoids lamp de-energization due to de-ionization of the lamp during operation, and reduces lamp re-ignition voltage.

Another object of the invention is to provide a direct current circuit of the above type which produces relatively high voltage for starting the discharge lamp.

Still another object of the invention is to provide a direct current operating circuit having relatively low current ripple characteristics.

Other objects and advantages will become apparent from the following description and the appended claims.

With the above objects in view, the present invention in one of its aspects relates to an electrical operating circuit comprising, in combination, a source of alternating current, current limiting reactance means comprising a first induction coil connected to the source of alternating current, an auxiliary induction coil inductively coupled to the first induction coil, first rectifier means connected to the output of the first induction coil, second rectifier means connected to the output of the auxiliary induction coil, and load means connected to the first and second rectifier means.

In a typical embodiment of the invention, the load means is a high intensity gaseous discharge lamp, the first induction coil serves as a ballast reactance for the lamp, and each of the rectifier means is a full wave bridge rectifier.

The invention will be better understood from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a circuit diagram of a direct current operating circuit for a gaseous discharge lamp in accordance with an embodiment of the invention;

FIG. 2 is a circuit diagram of a modification of the FIG. 1 circuit;

FIG. 3 is a circuit diagram similar to the FIG. 1 circuit incorporating a voltage doubler circuit;

FIG. 4 is a circuit diagram similar to the FIG. 3 circuit incorporating a voltage quadrupler circuit;

FIG. 5 is a diagram of the lamp operating current wave form characterizing the output of the circuits of FIGS. 1-4; and

FIG. 6 shows a portion of the FIG. 1 circuit depicting a modification thereof.

Referring now to the drawings, and particularly to FIG. 1, there is shown a circuit diagram of a typical embodiment of the invention comprising terminals 1 of a source of alternating current, and induction coil L.sub.1 connected at one side to one of the source terminals and at the other side to an input terminal of full wave bridge rectifier 2, which comprises diodes D1, D2, D3 and D4 arranged in conventional manner as shown, the other input terminal of bridge rectifier 2 being connected to the other source terminal 1. Auxiliary induction coil L.sub.2 is inductively coupled to main induction coil L.sub.1, such as by arrangement of the two coils on a common magnetic core on opposite sides of a magnetic shunt. Such an arrangement of inductively coupled coils is shown, for example, in the patent to Willis, U.S. Pat. No. 3,873,910, assigned to the same assignee as the present invention, and the disclosure thereof is accordingly incorporated herein by reference. Auxiliary induction coil L.sub.2 is connected at opposite sides respectively to the input terminals of another full wave bridge rectifier 3 similar to bridge rectifier 2 and comprising diodes D5, D6, D7 and D8. The output terminals of bridge rectifier 3 are connected to the output terminals of bridge rectifier 2 which in turn are connected across lamp load 4. Lamp 4 is typically a high intensity gaseous discharge lamp, such as a mercury vapor lamp, metal halide lamp or a high pressure sodium vapor lamp. Due to this arrangement, the direct current supplied to lamp 4 by main induction coil L.sub.1 via bridge rectifier 2 is substantially out of phase with the direct current supplied to the lamp by auxiliary coil L.sub.2 via bridge rectifier 3. As a result, the average current through the lamp and the voltage across the lamp is substantially increased over the average magnitude of current and voltage which would be applied in the absence of auxiliary coil L.sub.2 and its associated rectifier circuit, and therefore the tendency of the lamp to drop out because of de-ionization at current zero is largely prevented, and at the same time the lamp re-ignition voltage is substantially reduced so that starting of the lamp is facilitated. As will be understood, the phase shift provided by the described circuit is achieved by the approximate 180.degree. phase shift occurring in coil L.sub.2 with respect to coil L.sub.1 due to the described inductive coupling, combined with the approximate 90.degree. lead in current of the output of coil L.sub.2 due to the voltage across L.sub.1 leading the current through L.sub.1. The resultant phase shift caused by the operation of both coil circuits thus is approximately 90.degree. at lamp starting. After the lamp has started, the degree of phase shift is somewhat reduced from 90.degree..

In a typical case, the turns ratio of coils L.sub.1 and L.sub.2 is about 1:1, but this ratio may be about 1:4 or 1:5 to provide for increased line power factor where desired.

FIG. 5 illustrates a typical current wave form characteristic of the lamp current during lamp operation. As will be seen, the minimum points of the wave form are raised substantially above current zero, thereby avoiding lamp de-ionization during the operation of lamp with the unfavorable results therefrom as previously described.

FIG. 2 shows a modification of the FIG. 1 circuit wherein diodes D6 and D8 of bridge rectifier 3 are omitted and the lead from coil L.sub.2 is connected to the supply line as shown. Diodes D5 and D7 remain connected in the circuit such that they conduct on alternate half-cycles to provide uniform polarity with respect to the polarity of the output terminals of bridge rectifier 2 to which they are respectively connected. In this version, diodes D2 and D4 of bridge rectifier 2 co-act also with diodes D5 and D7 to provide full wave rectification of the current from auxiliary coil L.sub.2.

Where necessary or desirable the described circuits may be modified to provide for substantially increased lamp starting voltage. FIG. 3 illustrates a circuit similar to the FIG. 1 circuit in which voltage doubler means comprising capacitors C1 and C2 are connected in series across bridge rectifier 2. As shown, induction coil L.sub.1 is connected to the junction of capacitors C1 and C2 as well as to the junction of diodes D1 and D3. As a result, the starting voltage applied to lamp 4 is approximately double that applied in the absence of capacitors C1 and C2. In addition to producing increased starting voltage, capacitors C1 and C2 serve as a filter network for making the current ripple shown in FIG. 5 less pronounced, especially when capacitors of relatively large magnitude are selected.

FIG. 4 shows a further modification of the described circuit incorporating a voltage quadrupler network for use where even higher lamp starting voltages are necessary. In addition to capacitors C1 and C2 arranged as previously described, diode D9 is connected between the positive output terminal of bridge rectifier 2 and lamp 4, diode D10 is connected between the negative output terminal of bridge rectifier 2 and lamp 4, and capacitors C3 and C4 are connected in series across the lamp with the supply line connected to the junction thereof. Diodes D9 and D10 are oppositely poled as shown, so that diode D9 charges capacitor C3 when the output of bridge 2 is positive, while diode D10 conducts and charges capacitor C4. As a result, an approximately four-fold increase in lamp starting voltage is obtained, as compared to that produced by the circuits of FIGS. 1 and 2.

FIG. 6 shows a modification of the described circuits in which capacitor C5 is connected between auxiliary induction coil L.sub.2 and the input terminal of bridge rectifier 3. Capacitor C5 may be selected in conjunction with the leakage reactance existing between induction coils L.sub.1 and L.sub.2 to saturate the magnetic core associated therewith, so as to provide a high degree of lamp watt regulation for a wide range of input voltage and also to provide a power factor of over 90%.

While the invention has been described primarily in its application to high intensity discharge lamps, it may also be found useful as a direct current power supply for other loads where substantial filtering of the current is necessary or desirable.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.