AU672796B2 - Power supply - Google Patents

Description

OPT DATE 15/08/94 APPIN. 10 43003/93 Il" AOPDATE 13/10/94 PCT NUMBER PCT/AU93/00315 1111W 11 I1111 1111111~I AUJ9343003 (51) Internaotional Patent Classllcatioti 5 1102M 1/12, 5/4S. 51451. 5/452. 1105B1 4 1/24 (11) International Publication Number.- (43) International Publication D)ate:

ICT)

WO) 94/16492 21 July 19)94 (l .107941 (21) International Application Number.

(22) International Filing Date; (30) Priority IData: PL 6811 14 January "IAV9310) 28 June 19911(2&06 1315 (81) Designated States: AT. AV. BB1. BG, BR. BY. CA, CIH. (7.

DEi, DK. [iS, Fl1, GO, flU. JP. Kr. KR. KZ, LK. Lit. MG.

93) M1N. M1W, NL. NO, NZ. PL. PT. RO. RV. SD. SE,. SKC.

L'A. VS. VN. European patent tAT, BF, CH. 13. DIC. ES, FR. OB. OR. 1W. IT. LU, MIC NL, PT. SH). QAPI patent CF, CGI. Cf. CM,. GA. ON. NIL, MR. NE. SN. TD.

AV TO). 1993 (14,01,9,1) (71) Applicant (for all de'signaed States el)e, L'S If 11P.M. IN- DUSTRIESB PTY UMJTED) (AU/AUJ; 4 Hll Street. Darlinghurst, NSW 2010 (AU).

(72) Inventor; and Inventor/Applicant (for iS onl*f HIAYASI. Koichi IP/AU];.

16 Ethel Street, Vaucluse, NSW 2030 (AU).

(74) Agent: GRIFFITH HACK CO,; G.P.O. Box 4164. Sydney.

NSW 2001 (AU).

(54) Title: POWER SUPPLY Published Mit inernaiwonal search repart.

V2) (S7) Abstract A power supply which incorporates a high frequency inveuter and which is suitable for use in conjunction with a fluorescent lamp, The power supply comprises a full wave rectifier (10) and a reservoir capacitor (12) connected across the rectifier for the purpose of providing successive half-cycles of the voltage waveform V2 across the capacitor with a minimum instantaneous voltage level greater than zero. A solid state switching device (14) and an oscillator (15) are connected in circuit with the reservoir capacitor. The oscillator generates a gating signal V3 at a frequency significantly greater than that of the voltage waveform V2 across the reservoir capacitor and applies the gating signal to the switching device. The switching device is arranged to provide an output voltage V4 which has a frequency determined by the applied gating signal V3 and which has a peak amplitude value which varies over successive cycles with the waveform of the voltage V2 across the reservoir capacitor.

WO 94116492 IICT/AIJ93100315 TECIINICA!L FIRML' Thlo invention e~t~ t powo- suppl a.electrical ci.rcuit and, i.n parrt.cula., tc a powe,. suppr, whi.ch incorporates a high trequency invertor which -o.

suitable for use in conjunction with a fluorescent~ lam.

The power supply has been developed primarily as a zoccalled electronic ballast for a compact fluorescent. lant and is hereinafter described in such context. However, the invention does in fact have broader applicatio.

BACKGROUND ART A compact fluorescent lamp is normally connected tc a mains voltage lighting circuit by way of an integral lamp base which incorporates a power supply in the torm of a switch mode converter -inverter. The supply voltage is full-wave rectified and the resultant direct current is fed to an inverter which produces alternating current at a frequency in the order of 40 to 50 kl~z.

The converter stage of the conventional power supply includes a bridge rectifier and a reservoir capacitor connected across the dc side of the rectifier. The capacitance value of the reservoir capacitor is chosen to minimise the ripple voltage at the dc side of the rectifier and, as a consequence, current is drawn from~ the mains supply in the form of a relatively narrow pulse during each half-cycle of the supply voltage, As a result, the supply current contains a large number of harmonics which have the effect of reducing the power factor, producing rf radiation and otherwise creating power generation and distribution problems for supply authorities in areas where large numbers of compact fluorescent lamps are used.

various proposals have been made for solving thE above mentioned problems, including those disclosed and referenced in European patent specification number 0296859 (application no. 88305743.2, dated June 23 1988', entitled "Power factor correction circuit". However, all known proposals for improving the shape of the mains -2current waveform and, hence, reducing the harmonic content, have involved circuits adapted to effect prolonged current flow through the bridge rectifier during each half-cycle of the supply voltage whilst maintaining the high value of the reservoir capacitor for the purpose of minimising the ripple voltage at the dc side of the rectifier.

DISCLOSURE OF THE INVENTION The present invention is based on an entirely different approach from that described above; one in which a high ripple voltage at the dc side of a rectifier is tolerated. This results in prolonged flow of current during each half-cycle of the supply voltage.

Thus, the present invention may be defined broadly as providing a power supply comprising a full wave ac-todc rectifier, a reservoir capacitor connected permanently in current conductive relationship across the dc side of the rectifier, an inverter stage comprising a solid state switching device and an oscillator connected in circuit with the reservoir capacitor, the oscillator being connected in circuit with the switching device, being arranged to generate a gating signal at a frequency significantly greater than that of the voltage waveform across the rectifier and being arranged to apply t.e gating signal to the switching device, the reservoir S• capacitor having a capacitance value which causes the successive half-cycles of the voltage waveform across the dc side of the rectifier to have a minimum instantaneous value which is significantly smaller than the maximum instantaneous value of the voltage waveform but greater than zero, and the switching device being arranged to provide an output voltage which has a frequency determined by the applied gating signal and which has a peak amplitude value which varies over successive cycles with the waveform of the voltage across the reservoir capacitor.

As a consequence of accepting that the peak S amplitude value of the output voltage may vary over successive cycles, the voltae at the dc side of the rectifier does not need to be smoothed and the reservoir capacitor may be employed effectively for the sole purpose of imparting a (positive or negative) value greater than zero to the instantaneous minimum level of the voltage across the rectifier. The magnitude of the minimum voltage and, hence, the value of capacitance required in the reservoir capacitor will depend upon the required application of the power supply. When used in conjunction with a fluorescent lamp, it has been determined that the reservoir capacitor should be selected to provide a voltage which has an instantaneous minimum level of not less than 30 volts in order to avoid sigaificant discernible flickering of the tube illumination.

The voltage supply for the oscillator is preferably derived as a proportion of the voltage across the S.reservoir capacitor, and the capacitance value of the reservoir capacitor will then need be sufficient to provide a voltage which has an instantaneous minimum level sufficient to sustain operation of the oscillator.

When the power supply is used for energising a compact fluorescent tube, the reservoir capacitor might typically have a value in the order of 10% of the value 25 of a capacitor which would otherwise be chosen to *e0 °9 minimise the ripple voltage across the reservoir capacitor. However, the value of the reservoir capacitor should be selected to be as small as possible whilst, at WO 94/16492 IICTIAU93100315 S4 t same time-, providini fey. instatnc-, minmut level of ripple voltage which is suff icientliv high f4 ci the dual purposes of maintaining supply to the oscillatcr and sustaining excitation of the fluorescent tube.

The switching device preferably comprises a fieldJ effect transistor and the oscillator, which has an output connected to the gate of the transistor, preferably comprises an integrated circuit oscillator. The period of oscillation will be selected to meet specific requirements of the power supply circuit and, when used for energising a compact fluorescent tube, the oscillator might be selected typically to provide an oscillation frequency in the order of 20 to 50 kHz, with a 2 to SjiS ON time in a total period of 20 to S5jtS. However, the oscillator is preferably controlled to provide an ON time which increases with decreasing amplitude of the ripple voltage across the reservoir capacitor. That is, the oscillator may be controlled to provide an ON time of 2yS in a total period of 25/IS (assuming an oscillator frequency of 40 kHz) when the amplitude of the ripple voltage is a maximum and to progressively increase the ON time to logS when the amplitude of the ripple voltage is a minimum.

The invention will be more fully understood from the following description of a preferred embodiment of a power supply for a compact fluorescent tube, the description being provided with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 shows a schematic representation of a circuit applicable to the power supply; Figure 2 shows an actual circuit diagram applicable to the power supply; Figure 3 shows a series of waveforms applicable to; A the voltage wavef orm at the ac side of a bridge rectifier WO 94116492 I'CT/AU93/00315 B the voltage waveform at the dc side of a bridairectifier to which a purely resistive load is connected C the current waveform into a bridge rectifier to which a resistive load is connected D the (ripple) voltage waveform at the dc side of a bridge rectifier across which a "high" value reservoii capacitor is connected E the current waveform into a bridge rectifier to which a high value reservoir capacitor is connected F the (ripple) voltage waveform at the dc side of a bridge rectifier across which a "low" value reservoir capacitor is connected G the current waveform into a bridge rectifier to which a low value reservoir capacitor is connected; Figure 4 shows voltage waveforms as measured at the points indicated by legends V 2

V

3 and V 4 in Figures 1 and 2; Figure 5 shows a modified form of the circuit diagram which illustrated in Figure 2; and Figure 6 shows voltage waveforms as measured at the points indicated by legends V 2 and V 3 in Figure MODE FOR CARRYING OUT THE INVENTION As illustrated in Figure 1, the power supply comprises a full-wave bridge rectifier 10, a low frequency current limiting inductor 11, a reservoir capacitor 12, a high frequency inductor 13, a solid state switching device 14 and an oscillator 15 which is employed to provide gating signals to the switching device 14. The voltage that appears across the switching device 14 is applied to a fluorescent tube 16, and a high pass capacitor 17 is located in series with the tube 16.

Alsc, a starter 18 in the form of a bimetal switch is connected between the filaments of the tube 16 in the usual way.

Although the voltage supply for the oscillator 15 is derived as a proportion of the voltage V 2 across the reservoir capacitor, the gating signal from the oscillator 15 is effectively independent of voltage WO 94/16492 PCT/AU93/00315 across the reservoir capacitor 12. The enerQising voltaQ.

V

4 which is applied to the tube 16 is obtained as a function of the voltage across the reservoir capacitor I.

and the frequency of the gating signal from the oscillator The capacitance value of the reservoir capacitor 12 is chosen to provide a high ripple voltage, that is to effect a relatively low level of smoothing, and at the same time to provide an instantaneous minimum level of ripple voltage which is sufficiently high to sustain operation of the oscillator 15 and maintain excitation of the tube 16. This latter aspect is described in more detail in the context of the circuit of Figure 2 and the waveforms shown in Figure 4 of the drawings. However, before proceeding with that description, brief reference is first made to the waveforms that are shown in Figure 3.

Waveform A represents the supply voltage V 1 which is applied to the ac side of the bridge rectifier Waveform B represents the voltage that would appear at the dc side of the rectifier if the current from the rectifier were to flow into a purely resistive load, and waveform C represents the current that would flow into the ac side of the rectifier with such a resistive load connected in circuit with the rectifier.

Waveform D represents the ripple voltage that would normally appear at the dc side of the rectifier of a prior art power supply circuit of a type that incorporates a reservoir capacitor having a typically "high" value of capacitance, and waveform E represents the current pulses that flow into the ac side of a rectifier of a power supply which produces a ripple voltage of the type shown in ".aveform D.

Waveform F represents the ripple voltage that appears at the dc side of the rectifier 10 of the power supply of the present invention, that is a power supply having a reservoir capacitor 12 with a relatively "low" value of capacitance, and waveform G represents typically

I

WO 94116492 IICT/AI9300315 7 the current that fiows into the ac side of the rcctifie as a consequence of the inclusion of a low value capacitance 12.

The power supply circuit which is shown schematically in Figure 1 is illustrated in more detail in Figure 2 and corresponding circuit elements are indicated by the same numerals in both figures. Further circuit components shown in Figure 2 include a voltage dropping resistor 20 a constant voltage power supply which comprises a diode 21, Zener Liode 22 and low pass capacitor 23.

The oscillator 15 shown in Figure 2 comprises a 555 integrated circuit to which the constant voltage supply is connected via pins 4,8. Voltage divider resisters 24 and 25, diode 26 and capacitors 27 and 28 are provided to control the operation of the oscillator, including the period of oscillation and the mark-space ratio. With a kHz output signal from the oscillator the circuit components may be chosen to provide an oscillator ON period of up to 5yS in a total period of The output signal V 3 from pin 3 of the oscillator is applied to a switching device 14 in the form of a field effect transistor (FET). The FET 14 is connected in circuit with the tube 16 in such manner that, when the FET is gated into conduction the instantaneous drain voltage V 4 applied to the tube 16 is zero and when the FET is gated OFF the instantaneous drain voltage applied to the tube rises to that which exists instantaneously at the drain side of the inductor 13.

With a 240 volt RMS supply V 1 at the ac side of the rectifier 10 and a resultant peak voltage level typically of 325 volts at the dc side of the rectifier, the value of the reservoir capacitor 12 may be selected to provide a minimum ripple voltage level in the order of 40 volts.

The peak and minimum ripple voltage levels are indicated as such in Figure 3F and these voltage levels increase to approximately 450 volts and 55 volts respectively at the FET drain when the FET is gated OFF, due to the influence WO 94/16492 PCT/AU93/00315 of the inductor 13. Thus, the voltage applied te tntube 16 has a peak amplitude which varies over successive cycles with the voltage waveform across the reservoir capacitor 12. This is indicated schematically in Figure 4 and in which, for ease of illustration, the period ct each cycle of the gate voltage V 3 and the tube voltage V 4 is expanded relative to the period of the voltage V, across the reservoir capacitor.

A reverse biased diode 29 and a metal oxide varistor (MOV) 30 are connected across the drain and source of the FET in order to protect the FET from any transient spikes that may appear in the power supply.

Circuit component types and values may be selected for specific applications but the following listing of circuit components is provided in respect of the power supply which is illustrated in Figure 2 and which has been developed for use as a so-called electronic ballast for a 15 watt compact fluorescent lamp.

Low frequency inductor 11 4.7mH High frequency inductor 13 1.3mH Capacitor 12 0.33yF Capacitor 17 0.01/F Capacitor 23 220/F Capacitor 27 1,500pF Capacitor 28 0.01/F Resistor 20 Resistor 24 1.2kQ Resistor 25 105kQ Zener diode 22 13 volts Oscillator 15 555IC FET 14 IRF 840 MOV 30 Philips Type 2322-592-64612 Figure 5 of the drawings illustrates a power supply which also meets the operating principles of the schematic circuit shown in Figure 1 and like reference numerals are employed in Figure 5 to identify components that are the same as those illustrated in Figures 1 and 2. However, in the power supply of Figure 5 the WO 94/16492 PCT; kU193/00315 9 oscillator 15 is controlled to provide an ON time which increases with decreasing amplitude of the ripple voltage across the reservoir capacitor 12. This is illustrated diagrammatically in Figure 6 which shows the output V 3 of the ostillator 15 plotted (on an expanued scale) with reference to the ripple voltage V,.

In the circuit shown in Figure 5 a further 555 oscillator 31 is controlled by circuit components 32 tc 34 to generate trigger pulses at pin 3 at a 40kHz rate.

The trigger pulses from oscillator 31 are applied to pin 2 of the oscillator 15, and oscillator 15 is in this case controlled by the divider network comprising resistors 36 to 38 and the RC network comprising components 39 and 40 to provide an ON time that increases from 2AS to 8AS with decreasing amplitude of the ripple voltage across the bridge rectifier 10. Thus, with a high instantaneous value of ripple voltage the capacitor will charge rapidly to provide a short ON time of the oscillator 15, and low instantaneous values of ripple voltage will result in relatively longer capacitance charging times and proportionately longer ON times of the oscillator Circuit component values that are applicable to the components in Figure 5 (but not in Figure 2) are listed as follows.

Oscillator 31 555IC Resistor 32 Resistor 33 Resistor 36 470kQ Resistor 37 680kQ Resistor 38 180kQ Resistor 39 68kQ Capacitor 34 1,000pF Capacitor 40 82pF As in the case of the circuit shown in Figure 2, that which is illustrated in Figure 5 has been developed as an electronic ballast for use with a 15 watt compact fluorescent lamp. Variations and modifications may be WO 94/1 6492 PCT/AU93/003 10 made in the circuit as above described in~ order taccommodate other applications of the circuit.

Claims (9)

1. A power supply comprising a full wave ac-to-dc rectifier, a reservoir capacitor connected permanently in current conductive relationship across the dc side of the rectifier, an inverter stage comprising a solid state switching device and an oscillator connected in circuit with the reservoir capacitor, the oscillator being connected in circuit with the switching device, being arranged to generate a gating signal at a frequency significantly greater than that of the voltage waveform across the rectifier and being arranged to apply the gating signal to the switching device, the reservoir capacitor having a capacitance value which causes the successive half-cycles of the voltage waveform across the dc side of the rectifier to have a minimum instantaneous value which is significantly smaller than the maximum instantaneous value of the voltage waveform but greater S than zero, and the switching device being arranged to provide an output voltage which has a frequency determined by the applied gating signal and which has a peak amplitude value which varies over successive cycl i with the waveform of the voltage across the reservoir .capacitor.

2. The power supply as claimed in claim 1 wherein the reservoir capacitor has a capacitance value w1 n causes successive half-cycles of the voltage wavetuim across the dc side of the rectifier to have a minimum instantaneous value which is not greater than 50% of the maximum instantaneous value of the voltage waveform.

3 3. The power supply as claimed in claim 2 wherein a voltage supply for the oscillator is derived as a proportion of the voltage across the reservoir capacitor and wherein the reservoir capacitor is selected to provide a voltage which has an instantaneous minimum level sufficient to sustain operation of the oscillator.

4. A power supply for a fluorescent lamp and which comprises a full wave ac-to-dc rectifier, a reservoir 'capacitor connected permanently in current conductive 'V. 12 relationship across the rectifier, in inverter stage comprising a solid state switel, device and an oscillator connected in circuit with the reservoir capacitor, the oscillator being connected in circuit with the switching device, being arranged to generate a gating signal at a frequency significantly greater than that of the voltage waveform across the reservoir capacitor and being arranged to apply the gating signal to the switching device, the reservoir capacitor having a capacitance value which causes successive half-cycles of the voltage waveform across the dc side of the rectifier to have a minimum instantaneous value which is significantly smaller than the maximum instantaneous value of the voltage waveform but greater than zero and sufficiently large to sustain excitation of the fluorescent lamp, and the switching device being arranged to provide an output voltage which has a frequency determined by the applied gating signal and which has a peak amplitude value which varies over successive cycles 20 with the waveform of the voltage across the reservoir capacitor.

5. The power supply as claimed in claim 4 wherein a voltage supply for the oscillator is derived as a proportion cf the voltage across the reservoir capacitor 25 and wherein the reservoir capacitor is selected to provide a voltage which has an instantaneous minimum level sufficient to sustain operation of the oscillator.

G. The power supply as claimed in claim 5 wherein the reservoir capacitor is selected to provide a voltage which has an instantaneous minimum level of not less than volts when the power supply is connected to a mains voltage power supply.

7. The power supply as claimed in any one of claims 4 to 6 wherein the oscillator is controlled to provide the gating signal at a rate of 20 to 50 kHz and with an ON time of 2 to 10S during each oscillation period.

8. The power supply as claimed in claim 7 wherein -i the oscillator is controlled to provide an ON time which 13 increases with decreasing amplitude of the voltage waveform across the reservoir capacitor.

9. The power supply as claimed in any one of claims 4 to 8 wherein the switching device comprises a FET in series with a high frequency inductor. The power supply substantially as hereinbefore described with reference to Figure 2 or Figure 5 of the accompanying drawings. DATED this 9th day of August 1996 H.P.M. INDUSTRIES PTY LIMITED By their Patent Attorneys GRIFFITH HACK CO .r 'R.Q 4, roe,