Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Patent
1994-09-13
1996-06-18
Wong, Peter S.
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
363 89, H02M 112
Patent
active
055284841
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a power supply for an electrical circuit and, in particular, to a power supply which incorporates a high frequency invertor which is suitable for use in conjunction with a fluorescent lamp. The power supply has been developed primarily as a so-called electronic ballast for a compact fluorescent lamp and is hereinafter described in such context. However, the invention does in fact have broader application.
BACKGROUND ART
A compact fluorescent lamp is normally connected to a mains voltage lighting circuit by way of an integral lamp base which incorporates a power supply in the form 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 kHz.
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 Jun. 23, 1988) entitled "Power factor correction circuit". However, all known proposals for improving the shape of the mains current 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 which comprises a full wave or bridge rectifier, means providing for successive half-cycles of the voltage across the dc side of the rectifier to have a minimum instantaneous value greater than zero, a solid state switching device connected in circuit with the rectifier and an oscillator located in circuit with the switching device. The oscillator is arranged to generate a gating signal at a frequency significantly greater than that of the voltage across the rectifier and to apply the gating signal to the switching device. The switching device is 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 rectifier.
A capacitor is preferably incorporated in the power supply to provide a minimum instantaneous voltage level greater than zero. Thus, the invention in its preferred form may be defined as a power supply comprising a bridge rectifier, a reservoir capacitor connected across the rectifier, a solid state switching device connected in circuit with the reservoir capacitor and an oscillator located in circuit with the switching device. The oscillator is arranged to generate a gating signal at a frequency significantly greater than that of the voltage across of the reservoir capacitor and to apply the gating signal to the switching device. The switching device is arranged to provide an output
REFERENCES:
patent: 4564897 (1986-01-01), Okamoto et al.
patent: 4745539 (1988-05-01), Nilssen
patent: 4945467 (1990-07-01), Bhagwat
patent: 5082998 (1992-01-01), Yoshioka
H.P.M. Industries PTY Limited
Han Y. Jessica
Wong Peter S.
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