NZ212779A - Electronic ballast for discharge tube - Google Patents

Description

Patents Form No. 5 Fee §100 Priority Date{s): 16. JuJy 3.fcbruQ.y.X9& Complete Specification Filed: . Class: H.Q5&4-J.j.Q3Q. k9 MAY 1986 ' Publication Date: ......7^; P.O. Journal, No: J38 1 277 Attorney's ref. P19/85L Under the provisions of i lation 23 (I) the . .<^sw-5 4 Specification has been ^nte-da£edS to I9<! Patents Act 1953 COMPLETE SPECIFICATION ELECTRONIC BALLAST SYSTEM We, INTENT PATENTS A.G., a Lichtenstein Corporation of C/-Timothy Elwes, 7 Storey's Gate, Westminster, London, SW1P3AT, United Kingdom 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 » ' ' 2 H 277 ELECTRONIC BALLAST SYSTEM BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention pertains to electronic ballast systems for gas discharge tubes. In particular, this invention relates to an electronic ballast system for fluorescent light sources which provides a high efficiency in transforming electrical energy into the visible bandwidth of the electromagnetic 10 spectrum. More particularly, this invention directs itself to a transistorized electronic ballast system for fluorescent light sources which provides for a minimal number of electrical components to provide low heat dissipation within a confined volume. Further, this invention relates to an 15 improved transistorized electronic ballast system which allows for low cost operation and minimizes the manufacturing expenses and labor costs associated with the application thereof. Still further, this invention provides for a single fluorescent lamp ballast system using a unique circuitry where 20 the gas discharge tube is incorporated within the circuit to provide the dual role of producing visible light as well as to dampen oscillations produced in the primary winding of a transformer when its current is interrupted as the transistor is switched to an "off" mode. ( PRIOR ART Ballast systems for gas discharge tubes and fluorescent lightbulbs in particular are known in the art. Additionally, ballast systems for both single and a plurality ot fluorescent lightbulbs are also known in the art. However, in many prior art electronic ballast systems, the number of electrical components contained within the circuit has been found to be 5 relatively large. Such large number of components has led to such prior art ballast systems having relatively large volumes. The large volumes have been due in part to a number of electrical components in combination with the components used for dissipation of heat because of the disadvantageous 10 thermal effects resulting from high heat dissipation factors when large numbers of components are being used.

Other types of prior art ballast systems generally operate at relatively low frequencies and have a low operating efficiency, which provides for approximately one-half the 15 visible light output found in the subject invention electronic ballast system for the substantially same electrical power input.

SUMMARY OF THE INVENTION An electronic ballast system connected to an AC power source for a gas discharge tube having a first and second filament, comprising: (a) a first capacitor electrically coupled to 25 said first filament of said gas discharge tube; (b) a transistor having a base, emitter, and collector, said collector being connected to said first capacitor; and, (c) transformer means having a primary winding coupled on opposing ends to said AC power source, and in series relation with said first capacitor and said.collector of said transistor, and a secondary winding coupled on opposing ends thereof in positive feedback relation to said 5 base of said transistor and said emitter of said transistor.

BRIEF DESCRIPTION OF THE DRAWING The drawing is an electrical schematic diagram of the 10 electronic ballast system network for a single gas discharge tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT There is shown an electronic ballast system 10 for operation of a single gas discharge tube 12, which may be a standard fluorescent tube to be further described in following paragraphs. As will be detailed, the gas discharge tube 12 is an integral part of the circuitry associated with the 20 electronic ballast system 10. The system 10 operates at an extremely high frequency compared to prior art fluorescent lighting systems. Such prior art fluorescent lighting systems operate at approximately twice the line frequency, or approximately 120 hertz. The subject electronic ballast 25 system 10 operates at approximately 20 kilohertz which provides the advantage of minimizing any type of flicker effect. Further, with the high frequency of operation, the average light output of the gas discharge tube 12 is substantially greater than that provided by prior art 2 1277 fluorescent lighting systems for a particular power source output. Further, as will be seen in following paragraphs, the duty cycle of the system 10 is minimized and thus, reliability is increased compared to the electronic components contained 5 therein. Further, with a low duty cycle as provided in the subject electronic ballast system 10, temperature gradients and temperature increases of the electronic components are minimized compared to prior art ballast systems. The minimization of temperature effects increases the overall 10 reliability of the ballast system 10 in that overheating problems are minimized.

The AC power source 14 is electrically coupled to a switch W through a power source output line 18. The AC power source 14 for purposes of this disclosure may be considered to 15 be a standard domestic 120 volt AC power source. It is to be understood that the .AC power source 14 may be a 220 volt AC power source or other power source, however, the basic invention concept as detailed in following paragraphs remains the same independent of the power source although electrical 20 component parameters may change. The 120 volt AC power source is used herein for illustration purposes. The switch W may be a standard off/on type switch, used merely for closing the overall circuit and coupling an electrical line 16 to the line 18 when closed. A diode input line 16 is connected to the 25 anode side of a diode D^, which is a commercially available diode. One such diode has the commercial designation 1N4004. The diode functions as a conventional half-wave rectifier to provide half-wave rectification of the AC signal coming in on the line 16, where such half-wave rectification is output 2 12779 on a line 20 on the cathode side of the diode .

Capacitor C^ is connected on opposing ends thereof to the output of the diode and a return power source line 34. Thus, the capacitor C^ is connected in parallel with the diode 5 and AC power source 14, as is clearly seen in the schematic diagram. For purposes of this disclosure, the capacitor C^ has a value approximating 100 microfarads, and functions as a filter which charges during the half-cycle that the diode passes current and discharges during the remaining portion of 10 the cycle. Thus, the voltage being input to a transformer T on a line 36 is a DC voltage having a small ripple at line frequency.

The pulsating DC current is applied to the transformer T on a transformer primary input line 36. The transformer T is 15 a ferrite core type transformer and has the characteristics of allowing the core to saturate relatively early in the voltage rise time and fall time of each pulse across a primary winding 22. The secondary voltage pulse amplitude is limited to a predetermined value by the turns ratio of primary and 20 secondary windings 22 and 24. However, it is to be understood that the energy to the base 44 of a transistor Tr is a function of both the voltage ratio and the differentiation of a capacitor C^ and the resistance of a second filament 32. The primary winding 22 includes terminals A and B and the 25 secondary winding 24 has associated therewith terminals C and D. The specific transformer T being used in the electronic ballast system 10 is conventional in nature and for purposes of this disclosure, the primary winding 22 is formed of 160 turns of number AWG 28 wire wrapped around a ferrite core.

The secondary winding 24 of the transformer T is formed of approximately 18 turns of AWG number 28 wire. As shown in the schematic diagram the transformer T is phased in such a manner that as a voltage change appears between the terminals A and B of the primary winding 22, there is produced a proportional voltage change between the terminals C and D of the secondary winding 24 of the transformer T; however, this proportional voltage change is of opposite polarity as measured between lines 51 and 34. Thus, when a voltage increase is applied to the collector 38 of the transistor Tr, a voltage of opposite polarity is applied to the base 44 of the transistor Tr.

The output of the primary winding 22 from the terminal B on a line 40 is coupled to the collector 38 of the transistor Tr on a line 60. Additionally, the primary winding 22 is similarly coupled to a capacitor through line connections 40 and 50. Thus, t-his type of coupling provides for parallel paths for current leaving the primary winding 22 for purposes and objectives to be seen in following paragraphs.

The transistor Tr is a commercially available transistor of the NPN type and includes the collector 38, base 44 and emitter 42. One particular transistor Tr which has been successfully used in the electronic ballast system 10 is a commercially available MJE13002 produced by Motorola Semiconductor, Inc. The transistor Tr operates as a switch in the ballast system 10 and the current path through the transistor Tr is provided when the voltage of the base 44 to the emitter 42 is greater than 0.7 volts for the particular transistor Tr being disclosed. The 0.7 voltage drop of the base 44 to emitter junction 42 is typical of this type of silicon transistor Tr.

Current flow through a second path from the terminal B or the primary winding 22 passes through the line 50 into the first capacitor C2« The first capacitor C2 is a commercially available capacitor having a value approximating 0.050 microfarads. As is the usual case, as current passes through the primary winding 22 of transformer T, the first capacitor C2 is charged to the voltage available at the terminal B. Output from the first capacitor C2 is provided on a first capacitor output line 70 to one end of the gas discharge tube first filament 30. When the first filament 30 is positive with respect to the second filament 32, electrons are attracted to the filament 30, and obviously when the filament 30 is negative, electrons are emitted when the negative filament 30 is heated by ion bombardment. When the transistor Tr is "on", the first and second filaments 30 and 32 are respectively a cathode and an anode, when transistor Tr is "off", the first filament 30 is an anode and the second filament 32 is a cathode. Initially, as the base 44 becomes more positive, electrons flow from the emitter 42 to the collector 38. This makes the output line 40 more negative than the terminal A. At the same time, electron current flows from the first filament 30 through the tube 12, the second filament 32, a line 80, the emitter 42, and the collector 38 into the lines 60 and 50 to the capacitor C2. Thus, the first filament 30 acts as a cathode connection during this phase of the cycle.

The gas discharge tube 12 may be a standard fluorescent tube which is commercially available. One such type tube bears the designation F20T12/CW 20 watt lamp. As can be seen, the gas discharge tube 12 becomes an integral part of the overall circuit of the electronic ballast system 10. The second filament 32 is coupled to the return power source line 5 34 of the AC power source 14 through the electrical line 80. Thus, during this phase of the lighting cycle, the second filament 32 acts as an anode for the gas discharge tube 12. As is evident, the discharging current of the first capacitor C2 flows through the gas discharge tube 12 which has a high 10 resistance during the initial phases of the lighting cycle. Specifically, the gas discharge tube 12 of the aforementioned type has a resistance of approximately 1100 ohms.

The second filament 32 in opposition to the first filament 30 does have a measurable current flowing 15 therethrough which is used to heat the filament 32 by Joule Effect and provides an aid in ionization of the contained gas in the gas discharge or fluorescent tube 12. Current flowing through the second filament 32 is provided by the secondary winding 24 of the transformer T. In the transformer T being 20 used, the secondary winding 24 is 18 turns of number 28 wire wound on the ferrite core, as previously described. The terminal D of the secondary winding 24 is coupled to the second capacitor C3 through a line 46. Current on a line 46 is differentiated by the capacitor C3 and leaves on a line 48 25 which is coupled directly to the second filament 32. The second capacitor C^ also acts to establish the desired duty cycle by the resonant frequency of the inductance of the secondary winding 24 coupled to the capacitor C^• Returning to the secondary winding 24 of the transformer T, it is noted that the secondary winding 24 is phased with respect to the primary winding 22 in a manner such that as the voltage increases across the primary winding 22 from terminal A to terminal B, the voltage at the secondary winding 24. is provided such that the voltage at terminal C increases with respect to that at terminal D.

Current passing through the second filament 32 is brought back to the secondary winding terminal C of the secondary winding 24 through the secondary filament output line 80 through either the diode element D2 or the base-emitter junction defined by the elements 42 and 44 of the transistor Tr, and then back through the line 51 to the terminal C of the secondary winding 24. The diode D^ is a commercially available diode element, one such being used is Model No. 15 IN4001. Determination of whether current passes through the diode D2 or the transistor Tr is made by the polarity of the secondary voltage of the secondary winding 24. Thus, there is a complete current path during each half-cycle of the secondary voltage being produced.

For possible ease of understanding the electronic ballast system 10, the overall system may be considered as having a primary circuit and a secondary circuit. The primary circuit provides for a charging current through the gas discharge tube 12 between the first and second filaments 30 and 32. The 25 primary circuit includes the primary winding 22 of the transformer T with the primary winding 22 being electrically coupled on opposing ends to the first filament 30 and AC power source 14. In detail, the primary circuit may be seen to provide a path from the AC power source 14 through the diode through the primary winding 22 of the transformer T into the first capacitor C2« Additionally, the current path from the first capacitor C2 passes into the first filament 30, through the resistance of the tube 12, into the filament 32, 5 and passes into the output line 80 and finally into the return line 34 and the AC power source 14. The primary circuit provides for a source of alternating positive and negative voltage pulses having different amplitudes. When the positive pulse is applied to the base 44 of the transistor Tr from the 10 secondary circuit, the transistor Tr is turned "on". The collector 38 is quickly brought to the potential of the emitter 42 and line 34 since there is substantially little resistance between the emitter 42 and line 34. Current then flows from the line 36 through the transistor Tr and primary 15 winding 22, to the line 34. This induces a voltage drop across the primary winding 22 opposing the applied voltage from the terminal A with the terminal B being more negative than the terminal A. The magnetic lines of force created by the current move outward from the core of the transformer T. 20 The drop of voltage across the primary winding 22 is substantially equal to the potential difference between the lines 36 and 34 because the collector 38 is substantially at the potential of the emitter 42.

As the transistor Tr ceases to conduct because of the 25 negative potential applied to the base 44, the DC current falls to substantially a zero value and the negative lines of force collapse back toward the coil which induces a voltage. The direction of the voltage is such as to try to maintain the same direction of current flow as previously described, due to -■ 7 § * , I 1 the fact that the induced voltage makes the primary winding 22 act as the source in which case the current flows from negative to positive within the source.

Thus, the terminal B now becomes more positive than the terminal A. Ordinarily, the induced voltage value L di/dt would make this vlotage greater than the source on the lines 34, 36; however, very importantly, the gas discharge in the tube 12 between the first and second filaments 30 and 32 becomes a bi-directional voltage limiter. Thus, the tube 12 acts as if it were constructed of two Zener diodes in back-to-back relation, thus preventing deleterious effects on the transistor Tr caused by large voltage peaks. The tube 12 thus produces light with energy which would otherwise have been dissipated as heat.

When the transistor Tr is in the "off" mode, there is a singular path of current flow. The transistor Tr does not draw current from the charge of the capacitor C2 by the voltage pulse L di/dt and the source line 36. With the line 50 more positive than the line 70, the first filament 30 will become an anode and the second filament 32 a cathode when the transistor Tr turns "on" again and the capacitor C2 discharges current into the tube 12.

The secondary circuit for actuating the primary circuit and the transistor Tr, and controlling gas discharge in the gas discharge tube 12, includes the secondary winding 24 of the transformer T coupled to the second capacitor and the second filament 32. The path of current of the secondary circuit passes through the output filament line 80 through either the diode or the transistor Tr into the line 51 and then into the terminal C of the secondary winding 24.

In overall operation, the electronic ballast system circuitry 10 provides for sufficient electrical discharge within the gas discharge tube 12 for transforming electrical energy from the power source 14 into a visible light output. Prior to a first closure of the switch W, there is obviously no potential drop across any portion of the ballast system 10, thus, as in all other portions of the overall circuit, the potential difference across the transistor Tr and between the lines 40 and 70 is substantially a zero value.

Upon an initial closure of the switch W, the AC power source 14 provides a current flow in the electronic ballast circuit 10 which is a half-wave rectified by the diode connected within the lines 16 and 20, as is shown in the drawing. The capacitor C^ is coupled between the line 20 and the return supply line 34 in parallel with the AC power source 14. The filter or capacitor C^ charges during the half-cycle that the diode passes current, i.e., during the positive half-cycle on the line 16, and is reverse biased during the other half preventing discharge back to the source 14. Thus, on the line 36 being input to the primary winding 22 of the transformer T, there is pulsating DC current.

At this time, the transistor Tr is not biased and there is not sufficient potential difference to cause a discharge in the gas discharge tube 12. The resistance of the collector 38 to the emitter 42 of the transistor Tr is extremely high, being for practical purposes infinite, with the exception of a small leakage. The transistor Tr for all practical purposes, has no voltage on the base 44 and emitter 42, and thus, the transistor Tr is in an "off" state and no current flows from the emitter 42 to the collector 38. The only current that flows is charging of the capacitor through the lines 40 and 50. The current flows from the line 36 to the line 70 through 5 both the primary winding 22 and the capacitor C^ an<3 is small and insufficient to induce a voltage in the secondary winding 24 of the transformer T.

The transformer T is a ferrite core type transformer, and is used because in this type of transformer Tr the core 10 becomes saturated in a rapid manner using less than one-tenth of the current needed to energize the tube 12. Thus, the core transmits the maximum magnetic flux to the secondary winding 24 prior to the voltage reaching its peak value on the primary winding 22. Prior to saturation, the difference in secondary 15 voltage is obtained as the primary voltage continually increases. The capacitor C^ charges at a rate determined by the capacitance value and resistance in the gas discharge tube 12 which for the tube 12 approximates 1100 ohms during the gas discharge and is greater prior to discharge, as is found in 20 the F20T12/CW 20 watt lamp being used for purposes of this disclosure.

When the switch W is then opened and closed for a second time, an impulse or secondary pulse is produced through the primary winding 22. The impulse provides for a current change 25 on the primary winding 22 which is large and the secondary winding 24 generates a current sufficient in the ultimate passage of current through the circuit 10 to turn the transistor Tr into an "on" state. With the transistor Tr turned to the "on" state, the voltage drop across the 2 12779 collector 38 to the emitter 42 is extremely small and the capacitor C2 on line 50 is coupled to the supply line 34 through the line 60 and the transistor Tr.

The capacitor C2 has been charged positively on the line 5 50 and negatively on the line 70 up to this point. A negative current is now output since the capacitor C2 is coupled to the return line 34 through the line 60 and the transistor Tr. Since there is a negative output on the line 7 0, the filament 30 becomes a cathode. The second filament 32 which is at the 10 potential of the return side of the power supply 14, thus becomes an anode. At this time, the capacitor C2 becomes the current source for the gas discharge tube 12 since one end of the capacitor C2 is coupled to the return line 34 through the lines 50, 6 0 and the transistor Tr and the opposing end of C2 15 is coupled to the discharge tube 12 through the first filament 30, and the return path from the filament 32 of the gas discharge tube 12 to the return line 34.

The end of the capacitor C2 coupled to the line 50 was charged positively and is at this time, coupled to the return 20 line 34. Negative current is applied to the discharge tube 12 on the line 70 and the voltage produced is greater than the approximate 85.0 volts which for this tube 12 is the breakdown voltage, there is produced the usual light output. As is evident, the plasma within the gas discharge tube 12 is 25 effectively an electrical resistor. The temperature of the filaments 30 and 32 of the gas discharge tube 12 are maintained at a sufficiently high value to insure emission of electrons as long as the pulses of voltage are applied from the capacitor C2« In the gas discharge tube 12, as used in 2 12779 this disclosure with a 20.0 watt dissipation, the electrical resistance of the tube 12 approximates 1100 ohms. Thus, the time constant of the capacitor C2 in series with the tube 12 represents a time constant approximating 50.0 microseconds. 5 The secondary winding 24 of the transformer T provides for a differentiated signal through the capacitor C3 to the base 44 of the transistor Tr. Thus, a narrow pulse is supplied to the transistor Tr and once the transistor Tr is turned to the "on" state, the current in the secondary winding 10 24 will become substantially zero and place the transistor Tr in the "off" state. The cycle is then repetitive and the capacitor C2 again charges as previously described.

Going back in the cycle, as the case of the transformer T is being saturated, a potential is applied across the diode D2 15 which is a positive pulse of voltage which is also applied across the base to emitter junction of the transistor Tr. This positive pulse is due to the fact that the line 40 to the transformer T is at a lower voltage than the line 36.

Thus, there is a positive signal pulse on the line 51 20 generated from the secondary winding 24.

Because the diode D2 is reverse biased, it does not conduct when the line 51 is positive. The base emitter junction is forward biased and conducts current and limits the voltage drop between the lines 51 and 62 which for the ballast 25 system 10, approximates 1.0 volts. The transistor Tr then goes to an "on" state and during the "on" state of the transistor Tr, voltage in the secondary winding 24 is induced with a potential on the line 40 being, approximately zero.

When the transistor Tr comes out of saturation, the line 51 becomes negative. This now forward biases the diode and reverse biases the base-emitter junction of the transistor Tr. Secondary current flows through the diode D2 and the voltage across D2 is clamped at minus 1.5 volts on the line 51 with respect to the line 62. The line 40 goes from substantially a zero value to a positive level. Thus, once again, current flows between the lines 40 and 36 and a pulse of positve polarity is applied to the line 70 across the capacitor C2. The positive polarity pulse is applied to the first filament 30 of the gas discharge tube 12 and the plasma ignition is maintained.

It is to be understood that a subsequent resistor may be placed between the lines 40 and 51 of the diagram shown in the drawing. With the placement of a subsequent resistor, the pulse necessary to be input to the secondary winding 24 will be accomplished through a single closing of the switch W. Thus, with the insertion of a subsequent resistor between the lines 40 and 51, once saturation has occured in the transformer T, a pulse is provided for initiation of the overall cycle of the ballast system 10.

Claims (6)

WHAT WE CLAIM IS:

1. An electronic ballast system connected to an AC power source for a gas discharge tube having a first and second 5 filament, comprising: (a) a first capacitor electrically coupled to said first filament of said gas discharge tube; (b) a transistor having a base, emitter, and collector, said collector being connected to said first 10 capacitor; and, (c) transformer means having a primary winding coupled on opposing ends to said AC power source, and in series relation with said first capacitor and said collector of said transistor, and a secondary winding coupled on 15 opposing ends thereof in positive feedback relation to said base of said transistor and said emitter of said transistor.

2. The electronic ballast system as recited in Claim 1 including means for applying a pulse voltage to said second filament of said gas discharge tube. 20

3. The electronic ballast system as recited in Claim 2 where said means for applying said pulse voltage includes means for providing said pulse voltage to said second filament of said gas discharge tube.

4. The electronic ballast system as recited in Claim 3 25 where said pulse voltage means includes a second capacitor in series connection with said secondary winding of said transformer means and a first end of said second filament of said gas discharge tube.

5. The electronic ballast system as recited in Claim 4 - 18 - 21277 where a second end of said second filament is coupled to a return side of said AC power source, and said emitter of said transistor.

6. An electronic ballast system substantially as herein described with reference to the accompanying drawing. 10 15 'g cq. Patent Attorneys for the Applicant(s). 20 25 - 19 -