NZ243218A - Power supply for fluorescent lamp has capacitor bank connected between rectifier input and output to reduce harmonic distortion of ac supply current - Google Patents

Description

24321H Priority Date(s): \ I Complete Specification Filed: Class: (6) VA.O'S.Sihi'J.l-'.ht".

Publication Data:. ...2..LFE0J99B. ■^.0. Journal No: Patents Form No. 5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION ELECTRICAL POWER SUPPLY WE, GOLDEN POWER ELECTRONICS PTY LTD., an Australian company of 5 St. Kilda Road, St. Kilda, Victoria, AUSTRALIA hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: (followed by page la) v; \ I oft * ~ 2 OCT 1992 i <•<">/ -la** ELECTRICAL POWER SUPPLY This invention relates to a power supply for an electrical circuit. The invention has been developed to 5 address problems that have become apparent with large scale usage of compact fluorescent lamps of the type that incorporate so-called electronic ballasts, and the invention is hereinafter described in the context of a power supply for such lamps. However, it will be 10 understood that the invention may have broader applications.

A compact fluorescent lamp may comprise two folded discharge tubes which project outwardly from a base. The lamp may be connectable to an AC mains supp-ly grid by 15 means of conventional bayonet or screw-type lamp sockets. The base of the AC lamp may incorporate a switch mode converter/inverter. The converter may rectify the mains supply voltage and the resultant direct current may be fed to the inverter which may be operated at a relatively high 20 frequency of the order of 20 to 50kHz. A block diagram of the above arrangement is shown in Fig. 1 of the accompanying drawings.

The converter stage may comprise AC to DC rectifying means such as a full-wave bridge rectifier (10) having a 25 pair of input terminals connectable directly to the AC supply grid and a pair of output terminals for providing direct current (DC). A filter capacitor (11) may be connected across the output or D.C. terminals of the rectifier. 3 0 As shown in Fig. 2 of the drawings, current (Ic) b in such a circuit is drawn from the mains supply grid in the form of a pulse during a relatively short period in each half-cycle of the AC mains supply voltage (Vg) in order to restore the charge which is drawn by the load 35 (rl) ^rom filter capacitor. Because the current is drawn from the supply in non-sinusoidal pulses, the supply grid retains an undesirable percentage of odd harmonics which have the effect of reducing the power factor, creating rf radiation and other p.robieqjs for supply 40 authorities. These problems will increase increasing * "il i _ 2 qQ VJ92 ;'f Tfollowed by page 2) \ , „<? *' 24.5218 I usage of compact fluorescent lamps in commercial industrial and domestic situations, particularly as they continue to replace incandescent lamps and conventional inductive ballasted fluorescent lamps. It is foreseeable 5 that supply of such non-sinusoidal current to compact fluorescent lamps may in time constitute a significant proportion of the total current drawn from the supply grid.

Figure 3 shows a prior art circuit for reducing harmonic distortion in the supply current. The circuit of 10 Figure 3 includes a rectifier bridge 20 providing full-wave rectification of the single phase AC supply. A filter capacitor 21 is connected across the output or DC terminals of bridge 20 with series resistor 22. Resistor 22 increases the time constant of the circuit and, thus 15 increases the period during which current is conducted by bridge 20. As a consequence, the supply current waveform Ig which is shown in Figure 4, more closely resembles a sinusoidal form (when compared with the current waveform in Figure 2). The circuit of Figure 3 provides a degree 20 of reduction of harmonic distortion in the supply current. However the improvement in harmonic distortion carries with it a penalty. As can be seen by comparing the load voltage waveform v_ of Figure 2 with that of ll Figure 4, the depth of ripple in the DC load voltage 25 increases following insertion of resistor 22. As the value (R^) of resistor 22 is increased the supply current waveform Ig is improved, but with consequent increase in depth of ripple and power loss attributable to resistor 22. Generally speaking, values (R1) of 30 resistor 22 which give acceptable harmonic distortion produce unacceptable power loss.

It may be shown that as the ratio R,/RT JL L increases, total harmonic distortion decreases at the expense of ripple voltage (which increases) and average 35 output voltage to the load (which decreases). Power loss in resistor 22 increases as R./RT increases and 1 ll efficiency of the power supply suffers because the ripple voltage appears across resistor 22.

A further disadvantage of the circuit of Figt^feM 3 40 arises from practical considerations. Because the^ input m i '-20CTi992 / J 24;s218 to rectifier bridge 20 is connected directly to the AC supply, any transients which exist on the supply side of bridge 20 could, in the absence of resistpr 22, be substantially bypassed by filter capacitor 21. However 5 the presence of series resistor 22 severely limits the ability of the power supply to bypass these transients which can cause damage to semiconductors and other sensitive components included in the load.

An object of the present invention is to alleviate 10 the disadvantages of the prior art.

The present invention seeks to increase the pulse width or conduction time of the supply current (Ig) so that supply current will more closely approximate a. sinusoidal form without increasing significantly 15 undesirable ripple and power loss in the circuit.

This may be achieved according to one aspect of the present invention by connecting a pair of bank capacitors each having a value CD between one AC or input terminal of the (bridge) rectifier and respective output or DC 20 terminals of the rectifier. The purpose of the addled bank capacitors is to provide a leading phase addition to the input current waveform thereby further extending conduction time of the rectifier and reducing harmonic distortion. Values of CD may be optimised to produce 25 the best overall current wave-shape for given values of R1, R^. When CD is correctly proportioned to suit Rj_ a leading phase is added to the input current which improves harmonic distortion and increases power to the load.

Research by the present applicant shows that if harmonic distortion in the input current is to be minimised, there should exist a substantially linear relationship between the discharge time constant (CdRl) associated with each bank capacitor and the 35 value of the resistance element (R1) relative to the load (Rl) . The relationship may be used to predict a value of CD> The research indicates that the following relationship may be used to minimize harmonic disto.r-t^p^ 40 in the input current: • ''' ■■-V o;, •V*'' - 2 OCT 1992 K:| o/ 2^21'd = 50 [Ckj Log Rj/1-5 + k2) - (*3 Log Rj^/1-5 + k4)] where RL are in kilohms, CD is in microfarads and f is the supply frequency in Hertz.

Empirical results indicate that for 0.02 ± ri/rl < 0.1 kl= 1 .33 k2 = 4 • CT> VO k3 = 14 .95 k4 = .8 and for 0.1 <_ R]/kl ±0.4 kl = .99 ** to 1 = 3 .45 k3 = 3 .32 = 3 .17 The above relationship may minimize harmonic distortion in the input current if the discharge time constant (ClRL> associated with the filter capacitor (C^) is greater than about 15 milliseconds and f is in the range of a supply frequency eg. 50-60 Hertz. Nethertheless the skilled person will appreciate that a window of CD values (eg + 80%) either side of predicted values may 30 produce acceptable results. Hence, the extent of adherence to predicted values may be dictated by a number of factors including efficiency, bulk, cost, industry standards etc.

According to the present invention there is' provided 35 a power supply circuit for supplying power to a load, said circuit being connectable directly to an AC supply and comprising: AC to DC rectifying means having a pair of input terminals connected to said AC supply and a pair of outputs '■./a** Q. 4 0 terminals; ^ , o| >2 OCT 1992*,? ' " o / 243218 a filter capacitance means and a non-capacitive impedance means connected in series between said output terminals, said non-capacitive impedance means increasing conduction time of input current drawn from said AC supply; a bank capacitance means connected between said output terminals and one of said input terminals; wherein said bank capacitance means further increases said conduction time of said input current to more closely approximate a sinusoidal form and reduce unwanted harmonic distortion in said input current.

The phrase "connectable directly to an AC supply" includes within its scope a connection made via RFI suppression and/or various protective devices and the like.

The power supply of the present invention functions to increase the conduction period of the rectifying means which reduces distortion of the supply current waveform and the proportion of undesirable harmonics in the supply.

The rectifying means may comprise a full-wave bridge rectifier. The non-capacitive impedance means located in series with the filter capacitor means may comprise a resistive and/or inductive element. The non-capacitive impedance means may comprise, for example, a resistor, an inductor or the primary winding of a transformer. The latter may have its secondary winding connected to a circuit associated with the load.

When the non-capacitive impedance means comprises a resistance it usually has the effect of increasing the depth of ripple which appears across the resistance, resulting in loss of power. However, the presence of the bank capacitance means reduces the depth of ripple across the resistance thereby reducing the power loss attributable to the resistance or non-capacitive impedance means. Put in other words, the bank capacitance effectively reduces the AC voltage to the load and raises the average or DC voltage to the load (VL) thus increasing average power to the load. Because the raised DC voltage is blocked by the filter capacitance means, less current flows to the resistance or non-capacitive • ... 2^321 impedance means thus reducing power dissipated in the resistance or non-capacitive impedance means. Power which would have been dissipated in the resistance or non-capacitive impedance means, were it not for the bank capacitance means is thus effectively transferred to the load, increasing 5 efficiency. Therefore the bank capacitance means alleviates the extent of power loss in the non-capacitive impedance means which would otherwise occur.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings wherein: Figure 1 shows a power supply for a fluorescent lamp of a type that 10 includes an inverter and a high frequency ballast; Figure 2 shows waveforms applicable to the supply voltage, supply current and load voltage in the circuit of Figure 1; Figure 3 shows a prior art power supply circuit suitable for use with the fluorescent lamp load shown in dotted outline in Figure 1; Figure 4 shows waveforms applicable to the supply votage, supply current and load voltage in the circuit of Figure 3; Figure 5 shows a power supply circuit in accordance with a preferred mode of the present invention, the circuit also being suitable for use with the fluorescent lamp circuit shown in dotted outline in Figure 1; Figure 6 shows waveforms applicable to the supply voltage, supply current and load voltage in the circuit of Figure 5; Figure 7 shows a variation of the power supply circuit illustrated in Figure ; Figure 8 shows a further variation of the power supply circuit illustrated in 25 Figure 5; and Figure 9 shows a power supply circuit which may be considered an electrical equivalent of the power supply circuit illustrated in Figure 5.

As shown in Figure 1, the power supply comprises a converter including bridge rectifier 10 and filter 243218 capacitor 11 providing full-wave rectification of a single phase AC supply. Filter capacitor 11 is connected across the output or DC terminals of bridge rectifier 10 and the DC output is fed to an inverter 12. Although not shown, 5 inverter 12 is of a conventional configuration and comprises a solid state high frequency switching circuit which generates a lamp excitation current at a frequency which may be of the order of 20 to 50kHz. The output from inverter 13 is fed to a gas discharge tube 13 via a high 10 frequency inductor 14 and a lamp starting circuit 15.

As previously described and illustrated in Figure 2 of the drawings, supply current Ig, which leads the supply voltage Vg, is drawn from the mains supply grid, in the form of a pulse during a relatively short period in 15 each half-cycle of the mains voltage Vg, to restore the charge which is drawn from filter capacitor 11 by load elements 12-15. As previously noted the current waveform Ig, or more particularly the high harmonic content of the current waveform, presents difficulties for the supply 20 authorities.

One preferred form of the present invention is illustrated in Figure 5 of the drawings. The circuit shown in Figure 5 uses components which may conveniently be located in the base moulding (not shown) of a compact 25 fluorescent lamp.

The power supply of Figure 5 comprises a rectifier bridge- 30 which provides full wave rectification of the single phase AC supply. A filter capacitor 31 is connected across the output or DC terminals of bridge 30. 30 A resistor 32 is connected in series with filter capacitor 31. A value (R-^) of resistor 32 may be chosen to provide a good compromise between unwanted distortion in the supply current Ig, undesirable ripple in the load voltage VL and power loss in resistor 32. Resistance 35 values (R^ of resistor 32 which have been found suitable in practical circuits may fall in the range of 1% to 40%, preferably 3% to 24% of the value of the load (RT ), ie. .01 < R-i/Rt < .40, and preferably .03v^.<._ < .24. 1 L . °*\ 4 0 A clamping diode 33 and bypass capacitor 34 may . "2GCTS992 7/ I 2432i8 optionally be connected in parallel with resistor 32. Diode 33 may be included in the circuit to clamp and prevent the low voltage side of filter capacitor 31 from going significantly below zero volts. Diode 33 serves to 5 bypass current in its forward bias direction thus reducing discharge time of capacitor 31 into the load and dissipation in resistor 32/ thereby improving efficiency of the circuit. Capacitor 34 serves to bypass high frequency (RF) currents produced in the circuit by high 10 frequency switching in the load and the like. Capacitor 34 may have a value which is significantly smaller than the value of filter capacitor 31 but which is sufficiently large to adequately bypass RFI but not appreciably affect^ harmonic distortion.

Bank capacitors 35 and 36 are connected between the output or DC terminals of bridge 30 and AC or input terminal 37 of bridge 30. Bank capacitors 35 and 36 are connected in series with each other and in parallel with filter capacitor 31.

Bank capacitors 35 and 3 6 increase the conduction time of bridge 30. This is achieved as a consequence of alternate charging of bank capacitors 35 and 36 from the AC supply. As one bank capacitor is charged during one-half cycle of the supply, the other bank capacitor 25 discharges into the load and/or through the circuit including filter capacitor 32. This causes current to flow through the circuit for an extended period and thus to be drawn from the input or AC side of the bridge. The two bank capacitors 35 and 36 are equal in capacitance 30 value to provide a balanced circuit, in which current attributable to charging and discharging thereof flows during each half-cycle of the AC supply. Figure 6 of the drawings shows the improvement which results in the supply current waveform Ig due to the circuit configuration 35 shown in Figure 5.

As can be seen from Figure 6, the load voltage waveform is also improved, having less ripple depth, due to the presence of bank capacitors 35 and 36 and 40 consequential flow of current through the load circuiJfe^|'^:::v The power supply illustrated in Figure 7 is/similar '~2 OCT 1992r 243218 in principle to that shown in Figure 5 but includes a primary winding of a transformer 38 located in series with filter capacitor 31 in place of resistor 32. The secondary winding of transformer 38 is arranged to provide current to the load, for example by locating it to provide power for the base drive circuit of switching 5 transistors (not shown) in the inverter. The reflected load on the secondary winding of the transformer provides the required time constant for the power supply circuit.

A further embodiment of the present invention is shown in Figure 8. The circuit shown in Figure 8 is self protecting when the load is removed, either 10 intentionally in the case of a two part lamp or unintentionally in the case of lamp failure. Figure 8 includes an optically-coupled triac 39 which is disabled when the load (Rl) is changed including when the load is removed or greatly increased. One effect of a reduced RL is a reduction in ripple current flowing through filter capacitor 31, resistor 32 and LED diode 40. This switches off LED 40 and 15 disables triac 39, disconnecting flow of AC supply grid current to capacitors 35, 36. LED diode 40 effectively senses a change in the load and acts to disconnect AC power flowing to capacitors 35, 36. This prevents voltage doubler action associated with capacitors 35, 36 which, if allowed to continue, could damage sensitive components (semiconductors, capacitors etc.) in the load due to voltage 20 rising above normal levels. Diode 33 in Figure 5 has also been replaced in Figure 8 with zener diode 41. Zener diode 41 has the advantage that it acts as a normal diode in its forward bias mode. However, if its reverse breakdown voltage is chosen just above normal peak operating voltage ie. the value of the ripple voltage, this may allow zener diode 41 to break down in the event of spikes and 25 other transients present in the AC supply grid, thus restoring the bypassing action of the power supply to protect voltage sensitive elements in the circuit. It is to be appreciated that means other than LED diode 40 may be used to sense a change in the load and that such means may be located elsewhere in the circuit so as to achieve this aim. It may also be appreciated that switch means other 243218 • . -10- than triac 39 may be used to disconnect AC power from capacitors 35, 36. The switch means also may be located elsewhere in the circuit so as to disconnect power from capacitors 35, 36.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.

For example the present invention is intended to encompass within its scope electrical circuits being electrical equivalents of the circuit of Figure 5. One such circuit is shown in Figure 9 but is not intended to. be exhaustive.

The arrangement of Figure 9 includes capacitors 42, 43 and resistors 44, 45 connected in series across the output or DC terminals of bridge 30. An impedance element 46 is connected between the AC or input terminal 37 of bridge 30 and the junction of resistors 44, 45. Impedance 20 element 46 may comprise an inductor (when space permits) or a capacitor.

The circuit of Figure 9 will be substantially electrically equivalent to the circuit of Figure 5 providing that: the value of each resistor 44, 45 is equal to half the value of resitor 32 in Figure 5 (ie. R1/2); the value of each capacitor 42, 43 is equal to twice the value _of capacitor 31 in Figure 5 (ie. 2C^; and impedance element 46 comprises a capacitor having a value substantially equal to twice the value of capacitor 35 or 36 in Figure 5 (ie. 2C^). It is apparent that resistors 44, 45 each being of value R^/2 and connected in series between the DC rails of bridge 30 are equivalent to resistor 32 in Figure 5 having a value R^. It is further apparent that capacitors 42, 43 each being of 35 value 2C1 connected in series between the DC rails of bridge 30 are equivalent to capacitor 31 in Figure 5 having a value C1.

The equivalence of impedance element 46 having a capacitance value 2C_ to capacitors 35, 36 in Figured 40 each having a value CD may be appreciated by coni^SerlngoHv j'/ 2 OCT 1992^1 243218 -li- element 46 as comprising notional first and second capacitors, each having a value CD, connected in parallel.

The first notional capacitor having a value CD may 5 be considered to be connected between AC or input terminal 37 and the positive DC rail of bridge 30 via resisto? 44 and capacitor 42 (or via capacitor 42). If the value of capacitor 42 is substantially greater than (say 10 times) the value CD attributed to the first notional capacitor, 10 the effective capacitance between terminal 37 and the positive DC rail of bridge 30 is about 91% CD, ie. it is substantially equivalent to capacitor 35 in Figure 5 having a value Cp. The second notional capacitor may. similarly be considered to be connected between AC or 15 input terminal 37 and the negative DC rail of bridge 30 via resistor 45 and capacitor 43 (or via capacitor 43) and for similar reasons is substantially equivalent to capacitor 36 in Figure 5 having a value Cp. 40 v'~2 OCT 1992*? u ... ■*£ & ft ?" % o.. •t ■■■"■■J — 05

Claims (13)

WHAT WE CLAIM IS: 12 24 3 2 18

1. A power supply circuit for supplying power to a load, said circuit being connectable directly to an AC supply and comprising: 5 AC to DC rectifying means having a pair of input terminals connected to said AC supply and a pair of output terminals; a filter capacitance means and a non-capacitive impedance means connected in series between said output terminals, said non-capacitive impedance means increasing conduction time of input current drawn from said 10 AC supply; a bank capacitance means connected between said output terminals and one of said input terminals; wherein said bank capacitance means further increases said conduction time of said input current to more closely approximate a sinusoidal form and 15 reduce harmonic distortion in said input current.

2. A power supply according to claim 1 wherein said filter capacitance means comprises a filter capacitor, one end of said filter capacitor being connected to said non-capacitive impedance element and the other end of said filter capacitor being connected to one of said output terminals. 20

3. A power supply according to claim 1 or 2 wherein said bank capacitance means comprises a pair of bank capacitors each having first and second ends, the first end of each bank capacitor being connected to said one of said input terminals and respective second ends of said bank capacitors being connected to respective said output terminals. 25

4. A power supply according to claim 1, 2 or 3 wherein said rectifying means comprises a full wave bridge rectifier.

5. A power supply according to any one of the preceding claims wherein values of bank capacitance means and non-capacitive impedance means relative to said load are chosen to provide a compromise between 10 15 20 25 2 4 3't i ^ -13 harmonic distortion in said input current and ripple voltage across said load.

6. A power supply circuit according to any one of the preceding claims, wherein a substantially linear relationship exists between the discharge time constant associated with said bank capacitance means and the value of said non-capacitive impedance means relative to said load.

7. A power supply according to any one of the preceding claims wherein said non-capacitive impedance means comprises a resistance means.

8. A power supply according to any one of claims 1-5 wherein said non-capacitive impedance means comprises a primary winding of a transformer and wherein a secondary winding of said transformer is connected to a circuit associated with said load.

9. A power supply according to any one of claims 1-7 wherein a clamping diode means is connected in parallel across said non-capactive impedance means.

10. A power supply according to any one of claim 1-7 wherein a zener diode means is connected in parallel across said non-capacitive impedance means.

11. A power supply according to any one of claims 1-7, 9 and 10, including switch means connected between said one of said input terminals and said bank capacitance means and load sensing means adapted to sense a change in said load, said load sensing means being associated with said switch means to cause said switch means to disconnect said bank capacitance means from said one of said input terminals in the event that said load sensing means senses a change in said load.

12. A power supply according to claim 11 wherein said switch means comprises an optically-coupled triac and said load sensing means comprises an enabling LED diode associated with said triac, said enabling diode being connected between said output terminals in series with said non-capacitive impedance means and said filter capacitance means. 30 Z. PATENT CrrlCB """ o 0 0EC 1995 J RECEI J*;24;i:ci o;-14-;

13. A power supply substantially as herein described with reference to Figure 5, 7, 8 or 9 of the accompanying drawings.;5;10;15;GOLDEN POWER ELECTRONICS PTY LTD;U/fnoimo a fay Their Attorneys I V BALDWIN SON & CAREY;25;30;35;40;-2 OCT J992 fj;"■ - * J