Asymmetrical bus capacitors

#The present invention relates to capacitor networks comprising a plurality of capacitors arranged in series to achieve a desired voltage rating and capacitance value.

#The present invention is especially useful in electronic ballast circuits for lighting applications, but is also useful in other electronic circuits where space and cost savings are desired. Although the invention will be described in connection with electronic ballast circuits for lighting applications, it is applicable to other electrical and electronic circuits as well.

#In an electronic ballast which has an energy storage device, capacitors are often used to hold a charge. In many electronic ballasts, a dc bus voltage is created in order to efficiently drive the lamps. The bus voltage must be greater than the peak voltage of the input line in order for the power factor correction boost converter to work properly. For any input line, the peak voltage is the product of the nominal input line voltage and the square root of two (which is approximately 1.414). For a standard 120 Vac input line, which is a common household voltage, the peak voltage of the line is approximately 170 V. A typical dc bus voltage is 230 Vdc. The capacitors that support this voltage must therefore be rated for at least 230 Vdc. Capacitors are normally chosen so that the voltage across them is between 65% and 95% of their rated voltage. This range of values is recommended by the capacitor manufacturer. If the capacitor continually sees voltages above this range (i.e., voltages above 95% of rated voltage), the life of the capacitor is reduced. If the capacitor continually sees voltages below this range (i. e., voltages below 65% of rated voltage), the dielectric of the capacitor reforms and the capacitor's maximum voltage rating is decreased to the lower voltage value.

#For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

#FIG. 1 is a simplified schematic diagram of two capacitors in series.

#FIG. 2A is a top plan view, and FIG. 2B a side elevation view, of a printed circuit board having mounted thereon two capacitors of the same voltage rating and capacitance value, according to the prior art.

#Referring now to the drawings, wherein like numerals indicate like elements, FIG. 1 is a simplified schematic diagram of two capacitors C1 and C2, having the same voltage rating and connected in series. Typically, in a prior art ballast circuit using two capacitors in series, each capacitor has the same voltage rating and a capacitance value of twice the desired capacitance of the series circuit. As those skilled in the art will understand, two capacitors in series have a total series capacitance C.sub.S calculated using the formula ##EQU1## Thus, if both capacitors C.sub.1 and C.sub.2 are equal, ##EQU2##

#A typical arrangement of two capacitors connected in series on a printed circuit board according to the prior art is illustrated FIGS. 2A and 2B. In those figures, capacitors C.sub.1 and C.sub.2 are shown mounted on their side on printed circuit board 10. Each capacitor has two leads 12, which extend from the bottom of the capacitor. The leads 12 are bent at right angles and are inserted into corresponding openings in circuit board 10. Capacitors C.sub.1 and C.sub.2 may be secured to circuit board 10 by adhesive, tape, or ties, in known manner.

#For a typical prior art 220 Vac, 240 Vac, or 277 Vac input line ballast, with a bus voltage of 465 Vdc, it is desired that the breakdown voltages of capacitors C.sub.1 and C.sub.2 add up to about 500 Vdc, to provide a safety margin over the bus voltage of 465 Vdc. It is also desired that the capacitance value of capacitors C.sub.1 and C.sub.2 when connected in series be about 22 .mu.F, which is a standard capacitance value for off-the-shelf capacitors. In the prior art example illustrated in FIGS. 1, 2A, and 2B, capacitors C, and C.sub.2 are identical. That is, capacitors C.sub.1 and C.sub.2 are both rated at 250 Vdc and have a capacitance value of 47 .mu.F. The total voltage rating of both capacitors is 500 Vdc, and, according to formula above, the total capacitance is 23.5 .mu.F. Capacitors having a breakdown voltage of 250 Vdc and a capacitance of 47 .mu.F have a height h of 23 mm and a diameter d of 16 mm. When placed on their side on the printed circuit board 10, each capacitor takes up an area of 23 mm.times.16 mm, or 368 mm.sup.2. Both capacitors together, therefore, take up 736 mm.sup.2 of area on the printed circuit board. In addition, the leads 12 typically extend 5 mm or so before they are bent, requiring an additional 8 mm.sup.2 per capacitor of board area to accommodate the leads. Thus, both capacitors together require 896 mm.sup.2 of board area, and a total volume of 14,336 mm.sup.3. As to cost, each capacitor will cost about $0.40 in quantity. Thus, the cost of using two 250 Vdc 47 .mu.F capacitors is about $0.80.