Electronic Ballasts

This application is a continuation of PCT application PCT/AU91/00402 filed Aug. 29, 1991 titled IMPROVEMENTS IN ELECTRONIC BALLASTS which is a continuation of Australia patent application PK2084 filed Aug. 30, 1990 and titled IMPROVEMENTS IN ELECTRONIC BALLASTS.

This invention relates to electrical apparatus and more particularly although not exclusively to compact ballast circuits for fluorescent lamps of the type used as replacements for incandescent bulbs.

Because of their small size and cost constraints it is not feasible (as with larger fluorescent lamp circuits) to install a low-pass filter at the input of these circuits. As a result the compact ballast circuits presently available have a very high current harmonic distortion and a very low power factor of between 0.45 and 0.6. A typical circuit together with voltage 1 and current 2 waveforms for such existing ballasts is shown in FIGS. 1 and 2. Because of these unfavourable characteristics power authorities in many countries such as Australia will not approve these circuits for sale as separate items. Consequently the regulations can only be complied with if the ballast circuit and fluorescent lamp are sold as a complete one piece unit. This however greatly adds to their cost and reduces consumer acceptance of the product.

It is therefore an object of this invention to ameliorate the aforementioned disadvantages and accordingly an improved circuit is disclosed in which additional diode means is provided to ensure that the lamp draws current from the rectifier bridge over substantially the whole of the supply voltage waveform including periods when this voltage is less than that across smoothing capacitor C4.

One preferred form of this invention will now be described with reference to the attached illustrations in which:

FIG. 1 shows a typical prior art ballast circuit;

FIG. 2 shows voltage and current waveform;

FIG. 3 shows a schematic diagram of a circuit according to the currently preferred form of this invention; and

FIG. 4 shows the current waveform produced by the circuit of FIG. 3.

Referring first to FIG. 3 there is shown a ballast circuit which at least in terms of power factor and size has substantial advantages over that of FIG. 1 and is suitable for use with compact fluorescent lamps of 9 to 18 watts such as those sold under part numbers YDN13-2H, YDN13-2U, PLC13/82. DULUX D13W and LYNX D13W. Typical non limiting values for the various components shown may be as follows:

In this case it will be noted that a diode D2 (preferably of a high speed switching type such as FR104) is inserted into the circuit just after the full wave rectifier bridge D1 and before the smoothing capacitor C5 and C6. Further, the single smoothing capacitor in FIG. 1 has been replaced by two capacitors C5 and C6 to reduce the physical size of the unit. The lamp drive capacitor C4 however remains directly connected to the rectifier bridge output.

The purpose of this diode is to ensure that the current used by the fluorescent lamp is not entirely supplied by capacitors C5 and C6 during portions of the half cycle when the supply voltage 3 is lower than the DC Voltage on C5, C6. During this period D2 blocks the supply from C5, C6 and allows the lamp to draw current directly from the rectifier bridge. Hence, the current waveform 4 becomes more even as shown in FIG. 4. Although this curve is still not sinusoidal, it does improve the power factor to better than 0.9, and reduce the current harmonic distortion to within the limits set for even large electronic ballasts as set out for example in IEC 555/2 and AS3168).

The value of mylar capacitor C3 has to be modified to match the diode. With this particular embodiment when designed for use with a 13 watt fluorescent lamp it may be 0.047 .mu.F 400 volts.

It will thus be appreciated that this invention at least in the form of the example disclosed provides a novel and useful improvement in ballast circuits for compact fluorescent lamps. Clearly however the embodiment described is only the currently preferred form of this invention and a wide variety of modifications may be made which would be apparent to a person skilled in the art. For example the values of the various components may be changed to adapt the circuit for use with larger fluorescent lamps up to 125 watts for other types of discharge lamps.