GB2219877A

GB2219877A - Operating circuits for fluorescent lamps

Info

Publication number: GB2219877A
Application number: GB8909818A
Authority: GB
Inventors: Andrew Kisylia; Robert C Griffith
Original assignee: Magnetek Inc
Current assignee: Magnetek Inc
Priority date: 1988-05-03
Filing date: 1989-04-28
Publication date: 1989-12-20
Also published as: MX170570B; GB8909818D0; DE3914676A1; FR2631508A1; JPH0260097A

Description

BALLAST CIRCUITS This invention relates to electronic ballast circuits for

fluorescent lamps.

Those skilled in the art recognize that electronic ballasts impose a primarily capacitive load on the AC power line that supplies power to the lamp. Since the starting voltage of the lamp is typically greater than the line voltage, the lamp is typically coupled to a circuit having both inductive and capacitive reactance which cooperate to boost the line voltage until it is sufficiently high to light the lamp. Because the presence of these reactances adversely effect the power factor of the lamp circuit, and impose consequential economic penalties on the user, a great amount of effort has been invested in the design of lamp.circuits having power factors. as close to unity as possible.

Embodiments of the invention can provide a circuit and method for supplying starting voltage and operating current to fluorescent lamps, while maintaining a near- unity power factor. Briefly, the circuit may comprises first terminal means for coupling the circuit to an AC power source having an AC power frequency; second terminal means for electrically coupling the circuit to the lamp; means for generating a periodic reference 2 1 signal having a frequency substantially higher than the AC power frequency; switchable energy storing means responsive to the reference signal for alternately - storing energy from the power source during a first interval, and discharging at least some of said energy into the lamp during a second interval; and means responsive to fluctuations in the AC power voltage for varying the ratio of the first and second intervals to keep the magnitude of lamp current generally constant, e.g. between two limits.

In operation, the duty cycle of the switchable energy storing means is preferably responsive to a number of monitored conditions, including changes in the lamp current and changes in the power source voltage to vary the amount of energy stored during the first interval and discharged into the lamp during the second interval. Thus, lamp current is maintained generally constant during fluctuations in power voltage such as AC ripple which is present after rectification of the AC power voltage.

The magnitude of the input power current may be monitored by the switchable energy storage means and provided as a feedback to lock the phase of the input current to the phase of the power source by adjusting the duty cycle in response to errors between the two phases. Consequently, such a lamp and ballast circuit would be characterised by the unity power factor desired by both the user and the power company.

Other aspects fo the invention are given by way of example in the claims' For a better understanding of the invention and to show how the same may be carried into effect, reference is made, by way of example, to the accompanying drawing showing a schematic representation of a circuit for starting and operating a fluorescent lamp.

Referring to the Figure, a circuit is shown which 251- is coupled to a pair of serially connected fluorescent lamps 12 at circuit terminals 26, 28. The illustrated circuit comprises a ballast portion 24, a rectifier portion 10 for providing a source of power to the circuit and the lamp 12, and a 50 kHz sawtooth generator 14 for gen erating a periodic reference signal having a frequency substantially higher than the AC power frequency.

The ballast portion 24 compensates for the changing impedance of the lamps. It is generally known to those skilled in the art that the internal impedance of a fluorescent lamp exhibits decreasing impedance with C.

increasing lamp current. This negative impedance characteristic would cause a rapid and continuous increase in lamp current once the lamp is lit, leading to immediate burn out. Accordingly, a ballast provides a compensating resistance which limits lamp current to a value within its operating range which substantially maximizes lamp life.

The rectifier portion 10 of the circuit is a full wave bridge rectifier. The illustrated bridge rectifier receives 115 volt RMS, 60 Hz AC line voltage from a power source (not shown), and converts that input to DC power having an AC ripple component. The ripple component has a maximum amplitude of approximately 160 volts, and a frequently of twice the line voltage frequency, or 120 Hz. Naturally, other line voltage values with other AC frequencies are suitable with appropriate changes which are well known to circuit designers.

The output from the rectifier portion 10 is applied to a pair of series-connected inductances Ll, L2. The coiw-non node between the inductances is coupled to common by a capacitor C4. The combination of the inductor Ll, L2 and the capacitor C4 filters out high frequency switching components from the input waveform.

25.; As will be described in greater detail below, the remaining portion of the illustrated circuit functions to alternately store energy from the rectifier portion in a capacitor C5 during a first interval, and to discharge at least some of that energy into the lamps 12 during a second interval. The ratio of these two intervals is varied by varying the duty cycle of FET Q1; namely, the ratio of its ON time to OFF time.

The duty cycle of FET 11, in turn, is controlled by a pulse width modulator 20 which is responsive to a number of circuit conditions. The pulse width modulator 20 comprises a comparator 22 whose output is coupled to the base of a NPN transistor Q2. The collector of the transistor Q2 is coupled to the output of the rectifier portion 10, while its emitter coupled to the gate of the FET Ql.

The inverting input of the comparator 22 receive the sawtooth reference signal from the sawtooth reference generator 14. In addition, the inverting input of the comparator is electrically coupled at 24 through a resistor R20 to one side of the lamp pair 12 to monitor the lamp pair 12 to monitor the lamp voltage E2- The non-inverting input of the comparator 22 is electrically coupled through a pair of serially coupled resistors R8, Rl to the side of the ballast 24 opposite the lamps 12. The non-inverting input of the comparator is also electrically coupled through resistor R9 to the output of the rectifier portion 10.

The non-inverting input of the comparator 22 is coupled through a resistor R10 to the filtered rectifier output El through resistor R9. Additionally, the noninverting input of the comparator 22 is coupled through the resistor R10 to common through serially-connected resistors R8 and Rl to monitor input power current as described below. The voltage E3 applied to the comparator 22 through the resistor R10 is thereby the sum of voltages indicative of the instantaneous input power voltage El and lamp current.

The input DC power current is monitored through the coupling of the noninverting comparator input to resistor Rl to sense the voltage across the resister. Because the current in the return leg of the diode bridge is equal to the current in the positive leg (but of opposite polarity), the voltage across Rl is proportional to the current in the positive leg. The common node between the resistors R8 and Rl is coupled to the return side of the diode bridge, with circuit common coupled to the return side of the diode bridge through resistor Rl.

Accordingly, circuit common floats above the return side at a level generally equivalent to the voltage across the resistor Rl. Thus,the voltage at that common node is driven negative, with respect to common, as DC power current in the return leg of the diode bridge increases from zero.

Prior to the starting of the lamps, only a signal proportional to the rectified line voltage El is applied to the non- inverting input of the comparator 22. Since the maximum value of the voltage El is approximately 160 volts, the.values of R8 and R9 are selected to provide a maximum voltage E3 of approximately 1 volt.

Prior to the lighting of the lamps 12, the comparator 22 compares the magnitude of the sawtooth waveform at its inverting input with the signal applied to its non-inverting input, and produces a positive output only when the sawtooth signal is the greater of the two. The sawtooth signal, which has a peak-to-peak value of approximately 200 mv., is sufficiently large during approximately 50% of its cycle to drive the output of the comparator positive, turning on transistor Q2.

25- With the activation of transistor Q2, DC current flows via the transistor's collector-emitter junction to the gate of FET Ql via resistor R13. The FET Ql turns ON, coupling the output end of inductor L2 to common, and reverse basing diode D2. The capacitor C5 is prevented by reverse-biased diode D2 from discharging to common through the FET Ql. The voltage E2 stored in capacitor C5 is, however, applied as a negative feed back signal to the comparator 22 through a resistor R14 and capacitor C10.

Owing to the inductive "inertia" of the inductor Ll, the effect of switching the FET Ql on and off is to build the stored voltage in capacitor C5 to a value in excess of the rectified DC voltage El. Specifically, the voltage E2 on the capacitor is related to the rectified to the rectified DC voltage El as follows:

(Eq. 1) E2 = El/(l-duty cycle) where: duty cycle = % ON time of FET Ql Accordingly, the initial 50% duty cycle of FET Ql is sufficient to store an increasing in capacitor C5 which quickly provides a level of voltage sufficient to start the lamps.

Once the lamps have started, the lamp currentrelated voltage is developed across R1, and coupled to the non-inverting input of the comparator, is compared to the voltage El to produce an error signal unless the two voltages are in phase. The comparator 22 compares the error signal with the sawtooth waveform, and drives the circuit components Ql, Q2, C5 so that the input DC power current value (sensed as a voltage at its connection point B to Rl) is of opposite polarity to the rectified DC line voltage El. The inductor Ll, FET Ql, and capacitor C5 may be thought of as a voltage booster which is responsive to comparator 22. Together, the comparator 22 and booster circuitry function as a single error-correcting amplifier in which E3 is an error 25.1-' signal, and E2 the output signal, which is fed back as a negative input signal to input of the error-correcting amplifier.

A change in E3 from -200 mv to +200 mv. results in a respective change in the duty cycle of FET Ql from 0% to 100%. Accordingly, a change in duty cycle is imposed at a rate of 0. 25% mv change in E3. The error signal E3 is the net voltage resulting from any phase difference between the ripple valves of the input DC power current and the input DC power voltage. Since, as set forth in Equation 1 above, the output E2 fo the error-correcting amplifier is related to the input power voltage El by i-.

the duty cycle of the FET El, the duty cycle adjusts to minimize the "error" between input power current and input power voltage, thereby minimizing any phase difference between current indicative voltage across resistor Rl, and the power voltage sensed at the junction between resistors R8 and R9 by forcing the input power current to remain in phase with the input power voltage, the ballast circuitry and lamp exhibit an essentially unity power factor to the input line.

The self regulating nature of the disclosed circuit eliminates the need for series inductor, transformers and tuned circuits. Accordingly, most of the illustrated circuit can be designed using LSI techniques, with attendant cost advantages.

While the foregoing description of.the preferred embodiment is very specific, it is recognized that various changes and modifications may be made by those skilled in the art who have benefit of the description. Accordingly, the invention is not to be limited to the specific embodiment illustrated and described herein, but should be defined by the claims-appended hereto, and the claims should be interpreted as broadly as possible in light of the prior art to include all such changes, modifications, and equivalent devices.

-1

Claims

CLAIMS 1. A circuit for providing operating current to a fluorescent lamp,

comprising:

first terminal means for coupling the circuit to an AC power source having an AC power frequency; second terminal means for electrically coupling the circuit to the lamp; means for generating a periodic reference signal having a frequency substantially higher than the AC power frequency; switchable energy storing means responsive to the reference signal for alternately storing energy from the power source during a first interval, and for discharging at least some of said energy into the lamp during a second interval; means responsive to fluctuations in the AC power voltage for varying the ratio of the first and second intervals to keep the magnitude of lamp current generally con stant.
2. A circuit for providing operating current to a fluorescent lamp, comprising:

voltage booster means connectable to an input power source for producing an output voltage greater than the input voltage, the input power source providing input 2_5 voltage and current having an input phase relationship, the voltage booster means including switching means having two switching states for adjustably varying the output voltage in accordance with the ratio of the duration in states; 30 output terminal means for coupling the circuit to one or more lamps; capacitor means responsive to said output voltage for storing electrical energy during one of said two switch states and for discharging at least a portion of the energy through the terminal means into the lamp during the other of said switch states; and comparator means for controlling the ration of switch stage durations in accordance with the magnitudes of the voltage and current of the input power source and the output voltage to lock the lamp current essentially 5 in phase with the input power voltage.
3. The circuit of Claim 2 wherein the voltage booster means includes inductor means coupled to the input power source for maintaining instantaneous output voltage immediately after the switching means changes from first to second states; and means coupling the inductor means to the capacitor means for permitting only undirectional current flow from the inductor means to the capacitor means.
4. The circuit of Claim 2 including reference signal generating means coupled to the comparator means for establishing a reference ratio of switch state durations when the lamp current and input voltage are in phase.
5. The circuit of Claim 4 including means coupling the input voltage to the comparator means for varying the ratio of switch state durations in accordance with the magnitude of the input voltage.
6. A circuit for providing operating current to a fluorescent lamp, comprising:

first terminal means for coupling the circuit to a 25. source of AC power; the AC power source producing a voltage and current having a power frequency; second terminal means for electrically coupling-the circuit to the lamp; and switchable energy storing means for alternately storing energy from the power source during a first interval, and for discharging at least some of said stored energy into the lamp during a second interval, the switchable energy storing means being responsive to the phase difference between input power voltage and the input power current to vary the ratio of the first and second intervals so that the circuit and lamp exhibit an essentially unity power factor.
7. The circuit of Claim 6 including means for generating a periodic reference signal having a frequency substantially higher than the AC frequency; means for producing an error signal indicative of a phase difference between the input current and input voltage, the error signal having a magnitude between the maximum and minimum magnitudes of the periodic reference signal; comparator means having a first output signal state when the periodic reference signal is less than the error signal, and having a second output signal state when the periodic reference signal is greater than the error signal, the states defining the intervals.
8. In combination:

full wave rectification means coupled to a source of AC power for producing a D.C. signal having an AC ripple component; filter means for reducing the AC ripple component; inductor means having its input coupled to the filtered DC signal and having an output; undirectional conductor means for conducting an electrical current from its input to its output and for substantially blocking current flow in the reverse direction, the unidirectional conductor means being coupled at its input to the output of the inductor means; first output terminal means electrically coupled to the output of the unidirectional conductor means and adapted to be electrically coupled to one electrode of a fluorescent lamp; variable resistance means electrically coupled in series with the first output terminal means to compensate for the variable resistance of the fluorescent lamp; second output terminal means adapted to be L-, electrically coupled to the other electrode of the fluorescent lamp, one of the first and second output terminal means electrically coupling one of the lamp's electrodes to common; 5 first switching means responsive to a switch signal for selectively coupling the inductor means output and unidirectional conductor means input to common to define a first interval; means coupling circuit common to the return path of the rectifier means, said coupling means being responsive to the magnitude of current flowing from common to the return path to produce a current indicative voltage; comparator means responsive to the magnitude of the current indicative voltage and the magnitude of the filtered DC signal for applying the switch signal to the switching means.
9. A lamp ballast substantially as hereinbefore described with reference to the accompanying drawing.

Published 1989 at The Patent office, State House. 66 71. High HolbornLondon WCIR4TP- Further copies maybe obtained from The PatPntOffice. Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1/87