CA2059016A1 - Electronic ballast - Google Patents

Abstract

ABSTRACT

ELECTRONIC BALLAST

A circuit is disclosed to implement an energy saving electronic ballast for driving a fluorescent lamp or tube.
The circuit is characterized by a high power factor, low harmonic distortion and minimized radio frequency interference.
The circuit includes a unique DC power supply comprising a high pass filter, which receives an AC input signal, a diode bridge rectifier and a high speed diode rectifier which cooperates with the bridge rectifier to provide the power supply with first and second rectification stages. A current regulating capacitor for driving the fluorescent lamp or tube is connected to the DC power supply between the first and second rectification stages. A trigger and high frequency oscillator are provided after the second rectification stage of the power supply to provide the DC current necessary to operate the ballast circuit.

Description

2~59~16 ELECTRONIC BALLAST

Technical Field This invention relates to a compact, relatively low cost, energy-saving electronic ballast that is particularly useful for efficiently driving a fluorescent tube or a compact fluorescent lamp. The ballast is characterized by a high power factor, low total harmonic distortion and minimal radio frequency interference and is adapted to be used with compatible tubes and lamps from different manufacturing sources.

Backqround Art Fluorescent tube and compact fluorescent lamp assemblies are well known lighting devices. The fluorescent tube assembly includes a tube and a separate electronic ballast or adapter to drive the tube. In the case of the compact fluorescent lamp assembly, a lamp and ballast are combined as an integrated unit.
The conventional ballast that is associated with the fluorescent tube assembly is typically heavy and large.
Moreover, such a ballast is not very energy efficient and is characterized by a low power factor. The conventional ballast associated with the compact fluorescent lamp assembly is also characterized by a low power factor, unless size is sacrificed.
Such a ba last is also characterized by total harmonic distortion which, in many cases, is not in full compliance with government 2~9~ ~

regulations. Because the compact lamp and electronic ballast are integrated, the entire lighting assembly must be discarded when the lamp is damaged or reaches the end of its life. The foregoing results in waste and inefficient use of materials, particularly in view of the fact that the ballast is more costly to manufacture than the lamp and commonly has a longer expected life span.
It would therefore be desirable to overcome the aforementioned shortcomings to fluorescent lighting devices by providing a lightweight, compact, energy efficient electronic ballast that would have low total harmonic distortion, minimum radio frequency interference, and a high power factor (e.g. 0.9 or higher). Moreover, it would be desirable that the ballast be detachable from the fluorescent lamp so that the lamp can be replaced without the necessity of scrapping the ballast. In addition, the ballast must comply with all government regulations and be compatible with fluorescent tubes and lamps that are produced by different manufacturers.

SUMMARY OF THE INVENTION
In general terms, a circuit is disclosed for implementing a relatively low cost, lightweight, energy efficient ballast for driving a fluorescent tube or compact lamp. In accordance with the present invention, the ballast includes a unique DC power supply comprising a high pass filter and a pair of diode rectifiers, such that the input current is first filtered and then twice rectified to provide the DC current required to operate the ballast circuit. The ballast also includes a high 2 ~
frequency oscillator and power output section comprising the interconnection of a ferrite oscillator transformer, first and second power transistors arranged in a push-pull relationship, a ferrite choke coil, capacitors and resistors. Also included is a section to trigger the oscillator and thereby cause the fluorescent tube/lamp to ignite. The trigger section will become inactive once the tube/lamp has ignited.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a circuit having a unique DC power supply for implementing the electronic ballast of the present invention for efficiently driving a fluorescent tube or lamp;

FIG. 2a illustrates the input current waveform of the DC
power supply of a conventional electronic ballast;

FIG. 2b illustrates the input current waveform of the DC
power supply of the circuit of FIG. 1 after regulation;

FIG. 3a illustrates the output voltage waveform of the DC
power supply of a conventional electronic ballast;

FIG. 3b illustrates the output voltage waveform of the DC
power supply of the circuit of FIG. 1;

FIG. 4a illustrates the output current waveform of the DC
: power supply of a conventional electronic ballast; and 2~Ql~

FIG~ 4b illustrates the output current waveform of the DC

power supply of the circuit of FIGo 1 after rectification.

DETAILED DESCRIPTION
An electronic circuit 1 for implementing a relatively compact, lightweight, low cost, and energy efficient ballast which forms the present invention and which may be detachably connected to a fluorescent tube 2 is initially described while referring to FIG~ 1 of the drawings. While a tube 2 is shown and described, it is to be expressly understood that the ballast circuit 1 may also be used with a compact fluorescent lamp. The ballast circuit 1 includes a unique DC power supply 4 that is connected to receive a 120 volt AC, 60 Hz input line signal. The power supply 4 includes a pair of filter capacitors Cl and C2 that are connected in parallel with one another. Connected between filter capacitors Cl and C2 are a pair of ferrite choke coils LlA and LlB. Capacitors Cl and C2 and ferrite choke coils LlA and LlB are interconnected to form a high pass filt~r 8.
Note the reverse connection of ferrite choke coil LlB relative ; to coil LlA of filter 8 which contributes to producing a high power factor. The high pass filter 8 will advantageously limit the radio frequency interference produced by the AC input signal so as to comply with government regulations regarding such interference without consuming large amounts of power. Filter 8 is connected to a conventional diode bridge rectifier 6 by which to provide half wave rectification of the filtered AC input signal in the usual manner.

2~5~3 ~

The DC power supply 4 includes a pair of rectification stage output capacitors C3 and C5 that are connected in parallel with one another and the diode bridge rectifier 6. A high speed rectifier diode D2 is connected between output capacitors C3 and C5. The capacitance of output capacitor C5 is preferably very large relative to the capacitance of output capacitors C3 to achieve a high power factor, as will soon be described. By way of example only, the optimum values of the circuit components which form the DC power supply 4 of ballast circuit 1 are giv~n 0 as follows:
C1 = 0.1 to 0.22 ~F
C2 = 0.005 to 0.01 ~F
C3 = 0.005 to 0.01 ~F
C5 = 22 to 33 ~F
LlA = 0.01 to 0.015 Henries LlB = 0.01 to 0.015 Henries A first rectification stage output terminal 10 of power supply 4 is formed at a common electrical junction with rectifier 6, diode D2, output capacitor C3, a current regulating capacitor C4 and a resistor R1. Current regulating capacitor C4 is connected in series with the electrodes 12 and 14 of fluorescent tube 2 by way of a capacitor C7 which is connected between said electrodes 12 and 14. Capacitor C7 may be either an integral part of the fluorescent tube 2 (as shown) or part of the ballast circuit 1. Electrode 14 is also connected in series with a ferrite choke coil L2 and a coil L3A which forms a ferrite oscillator transformer 16. A second coil L3B of transformer 16 2 ~

is connected to the base of a first power transistor TRl via a current limiting resistor R3.
A second rectification stage output terminal 11 of power supply 4 is formed at a common electrical junction with rectifier diode D2, output capacitor C5 and the collector of transistor TRl. The diode bridge 6 and high speed diode D2 provide power supply 4 with double rectification to provide the circuit 1 with DC power necessary to obtain a high power factor. More particularly, the first rectification stage output terminal 10 functions as a current summing junction, such that diode D2 rectifies the input current rectified by diode bridge 6 and current from the tube 2 which passes through current regulating capacitor C4. Accordingly~ all of the DC current necessary for operating the re~ainder of ballast circuit 1 is available from the second rectification stage output terminal 11.
; The emitter of transistor TR1 is connected to a common electrical junction 18 with coils L3A and L3B of the ferrite oscillator transformer 16 and a resistor R7 via a resistor Rll.
Resistor Rll is connected in a feedback path with resistor R7 between the emitter and base of transistor TRl.
The collector of a second power transistor TR2 is also connected to common electrical junction 18. The emitter of transistor TR2 is connected to ground via a resistor 12.
P~esistor R12 is connected in a feedback path with a resistor R6 between the emitter and base of transistor TR2. The base of transistor TR2 is connected to a common electrical junction 20 by way of a current limiting resistor R4 and a diac D4. A coil L3C which is magnetically coupled to the coils L3A and L3B to 2 ~

provide reactance to the ferrite oscillator transformer 16, is connected through a resistor ~2 to a common electrical junction 24 formed with resistors R4 and R6 and th~ base of transistor TR2.
With fluorescent tube 2 connected to ballast circuit 1, the interconnection of power transistors TRl and TR2, ferrite choke coil L2, ferrite transformer coils L3A, L3B and L3C, capacitors C3 and C4 and rectifier diode D2 provides the ballast circuit 1 with high frequency oscillation and high power output for efficiently driving fluorescent tube 2. Wh~n tube 2 is removed from circuit 1, the aforementioned high frequency operation will cease.
The aforementioned resistor R1 is connected in series with ; a resistor R8 to form a voltage divider network. A resistor R9 is connected from the common electrical junction 20 with diac D4 to a point between voltage divider resistors R1 and R8. A
capacitor C6 is connected between common electrical junction 20 and ground. As will also be described in greater detail, the interconnection of resistors R1, R4 and R9, diac D4 and capacitor C6 provides the ballast circuit 1 with a trigger capability.
A diode D5 is connected across the interconnection of resistor R11 with the emitter and collector of transistor TR1, and a resistor R5 is connected across the diode D5. Another diode D6 is connected from common electrical ~unction 18 to ground. Thus, diode D6 is connected across the interconnection of resistor R12 with the emitter and collector of transistor TR2.
Diodes D5 and D6 are included to protect power transistors TR1 and TR2 against voltage surges. A diode D3 is connected from the 2 ~

common electrical junction 20 with diac D4, resistor R9 and capacitor C6 to the coil L3A of transformer 16 by way of a resistor R10. Diode D3 and resistor R10 are included to disable the aforementioned trigger after ballast circuit 1 begins its high frequency oscillation.
The operation of the ballast circuit 1 of FIG. 1 is now described. The 60 Hz source or line voltage signal initially passes through the high pass filter 8 of DC power supply 4. The line voltage signal is then rectified at the diode bridge rectifier 6. The half wave rectified 120 Hz signal is regulated by a high frequency current (in the order of tens of kHz) through current regulating capacitor C4 (best illustrated by the waveform of FIG. 4b). The majority of the high frequency current from capacitor C4 flows through the first rectification stage output terminal 10 of power supply 4 to output capacitor C5 by way of diode D2 at which said current is rectified. However, a minor portion of the high frequency current through capacitor C4 flows to output capacitor C3 via output terminal 10. Being that the inductance of high pass filter 8 is relatively low, as indicated above, little reactance is provided for the 60 Hz line voltage, but a high resistance is provided to the high frequency current added to the 120 Hz current at output terminal 10 ~see FIG. 4b).
With regard to the trigger operation of ballast circuit 1, positive current flowing through resistors R1 and R9 will be applied to capacitor C6. When the voltage across capacitor C6 increases to about 30 to 32 volts, the diac D4 will produce a pulsating current flowing through resistor R4 to the base of 2~ 0~

power transistor TR2 to trigger the oscillator of ballast circuit 1. When the voltage across capacitor C6 drops to about 3 volts, the trigger is disabled.
The current flowing in coil L3A of ferrite oscillator transformer 16 is magneticâlly coupled to coils L3B and L3C.
Thus, with power transistors TRl and TR2 working in standard push-pull fashion, an osci]lator is created which operates in the 40-70 kHz range (depending upon the particular fluorescent tube 2 being used). Being that an oscillator is common to conventional ballast circuits, a detailed description of the operation of the oscillator of ballast circuit 1 will not be described.
Ferrite choke coil L2, which is connected in series with electrode 14 and coil L3A of transformer 16, provides two important functions. Prior to ionization and the ignition of the fluorescent tube 2, ferrite choke coil L2 cooperates with capacitor C7 to provide the high voltage necessary between electrodes 12 and 14 to ionize the gas in tube 2. What is more, after the tube 2 is ignited, ferrite choke coil L2 limits the current, whereby to stabilize the operation of tube 2.
Considering now the formula RL = 2~fL, where:
RL = the reactance (equivalent resistance) of L
f = frequency L = inductance.
When 2L is known, RL is directly proportional to f. That is to say, when the frequency increases, reactance also increases, thereby resulting in corresponding changes to the waveform of the input current.

2 ~ 3~ ~
The foregoing is best illustrated while referring to FIGs.
2-4 of the drawings. FIGs. 2a, 3a and 4a show waveforms associated with the DC power supply of a typical electronic ballast, while FIGs. 2b, 3b and 4b show corresponding waveforms associated with the power supply 4 of ballast circuit l of FIG.
1. In praetice, the time to eharge the output eapacitor of the power supply of a conventional ballast circuit is very short.
Therefore, the waveform of the input current will be pulsating (best illustrated in FIG. 2a). Hence, the RMS eurrent is large, the power factor is only about 0.5, and the harmonic content will typieally exceed 90%.
In the DC power supply 4 of ballast circuit 1, the 120 Hz half waveform eurrent at output eapacitor C3 and the 40 to 70 kHz current produced by the oscillator of ballast circuit 1 regulate each other. Moreover, the high pass filter 8 and diode D2 cause the eurrent applied to output eapacitor C5 via second rectifieation stage output terminal 11 to be very smooth and even (best illustrated in FIG. 4b). Thus, and is best shown by eomparing FIGs. 2a and 2b, the RMS of the AC input eurrent is redueed by about 40% in the DC power supply 4 of FIG. l relative to the RMS of the AC input eurrent to the power supply of the eonventional ballast. Changing the pulsating input current waveform shown in FIG. 2a to the near sinusoidal input eurrent waveform of FIG. 2b greatly reduces the input current harmonics and advantageously raises the power factor from 0.5 to about 0.9.
The following list represents the optimum parameters of other eomponents of ballast circuit l:
C4 = 0.01-0.1 ~F

C6 = 0.04-0.068 ~F
R1 = 470k Ohm R2 = R3 = 12-18 Ohm R4 = 15 Ohm R5 = 22Ok Ohm R6 = R7 = 68 Ohm R8 = R9 = 33Ok Ohm Rl0 = 10k Ohm Rll = R12 = 0-2.7 Ohm TR1 = TR2 = Part No. LSE1305 or equivalent D2 = D3 = D5 = D6 = Part No. FR104 or equivalent D4 = Part No. HT-32 or equivalent The ballast circuit 1 may be detached from the fluorescent tube 2 at suitable plug-in connection terminals 24. Thus, ballast circuit 1 may be reused with a new tube in the event that an old tube is detached therefrom and discarded. The circuit 1 is advantageously of compact design and lightweight and, therefore, adapted to be used in conventional fluorescent lighting assemblies with the tubes or compact lamps which are available from different manufacturers and have a variety of configurations, such as conical, oval, rectangular or cylindriral.
It will be apparent that while a preferred embodiment of the invention has been shown and described, various modifications and changes may be made without departing from the true spirit and scope of the invention. Having thus set forth the preferred invention, what is claimed is:

Claims (11)

1. A circuit for forming an electronic ballast to drive a fluorescent lamp or tube, said circuit comprising a DC power supply connected to receive an AC input signal and to provide an output signal to ignite the lamp or tube, the DC power supply of said circuit including:
filter means to filter the AC input signal;
first rectification means to rectify the filtered input signal;
second rectification means connected to said first rectification means so that said input signal is rectified twice; and an output terminal connected to said second rectification means to provide a DC output current for operating said circuit.

2. The electronic ballast circuit recited in claim 1, wherein said first rectification means of said DC power supply is a diode bridge.

3. The electronic ballast circuit recited in claim 1, wherein said second rectification means of said DC power supply is a high speed diode.

4. The electronic ballast circuit recited in claim 1, further comprising means to regulate the input signal rectified by said first rectification means, said regulating means connected to said DC power supply between said first and second rectification means thereof.

5. The electronic ballast circuit recited in claim 4, wherein said regulating means is a capacitor connected between an electrode of the fluorescent lamp or tube and said DC power supply, such that a high frequency current flows from said lamp or tube to said second rectification means of said DC power supply by way of said current regulating capacitor.

6. The electronic ballast circuit recited in claim 1, wherein the filter means of said DC power supply is a high pass filter including a pair of ferrite choke coils, one of said coils being reverse connected relative to the other coil.

7. The electronic ballast circuit recited in claim 1, wherein said DC power supply also includes a first capacitor connected across said first rectification means and a second capacitor connected in parallel with said first capacitor, said second rectification means of said DC power supply being connected between said first and second capacitors.

8. The electronic ballast circuit recited in claim 7, wherein the capacitance of said first capacitor of said DC power supply is very small relative to the capacitance of said second capacitor.

9. The electronic ballast circuit recited in claim 1, further comprising a high frequency oscillator connected between said DC power supply and the fluorescent lamp or tube for operating said lamp or tube, said high frequency oscillator including a ferrite oscillator transformer having a plurality of coils which are magnetically coupled to one another, said ferrite oscillator transformer interconnected with an electrode of the fluorescent lamp or tube.

10. A circuit for forming an electronic ballast to drive a fluorescent lamp or tube, said circuit comprising a DC power supply to receive an AC input signal and to provide an output signal to ignite the lamp or tube, the DC power supply of said circuit including:
filter means to filter the AC input signal;
first rectification means to rectify the filtered input signal;
a first capacitor connected across said first rectification means;
a second capacitor connected in parallel with said first capacitor;
second rectification means to rectify the filtered input signal previously rectified by said first rectification means, said second rectification means connected between said first and second capacitors; and output terminal means connected to said second rectification means to receive the signal rectified thereby and to provide a DC output current for operating said circuit.

11. The electronic ballast circuit recited in claim 10, further comprising a current regulating capacitor connected between said lamp or tube and said DC power supply at a point between said first and second rectification means, such that said second rectification means receives the rectified current from said first rectification means and current from said lamp or tube by way of said current regulating capacitor.