EMI SUPPRESSION FOR AN ELECTRODELESS DISCHARGE LAMP

Abstract: A circuit arrangement for an electrodeless fluorescent lamp having EMI suppression characteristics and which is effective for producing an RF signal to excite a fill contained within a lamp envelope portion (12) of the electrodeless lamp (10), includes a ballast circuit arrangement (24) receptive of line power and effective for converting the line power into the RF signal for driving the discharge (23) within the lamp envelope (12). The ballast circuit arrangement (24) includes an excitation coil (16) which extends within a re-entrant cavity (15) formed in the lamp envelope (12). A first tuning capacitor (26) is connected on one end to a first end of the excitation coil whereas a second tuning capacitor (27) is connected on one of its ends to the other end of the excitation coil (16), the capacitive values of the first and second tuning capacitors being substantially the same. The split capacitive arrangement is effective in cooperation with the excitation coil for producing the RF signal and does so in a manner to minimize EMI emissions that would otherwise be produced by an unbalanced load across the excitation coil.

Description: EMI SUPPRESSION FOR AN ELECTRODELESS DISCHARGE LAMP FIELD OF THE INVENTION This invention relates to an EMI suppression arrangement for an electrodeless discharge lamp and more particularly, to such an EMI suppression arrangement as can be accomplished using a minimum number of added components to the ballast circuit and yet still provide a significant improvement in suppression capabilities regarding the amount of EMI emitted. BACKGROUND OF THE INVENTION Compact fluorescent lamps and particularly, electrodeless discharge fluorescent lamps are considered to be key elements in efforts to reduce energy demand stemming from the use of lighting products. Specifically, electrodeless discharge lamps offer significant energy efficiency advantages over a conventional incandescent lamp and further offer life expectance advantages even over the popular compact fluorescent lamps now being heavily promoted. An example of an electrodeless fluorescent lamp can be found in US Patent No. 4,010,400 in which the basic principles of such lamp are described. This patent discusses that an ionizable medium can be contained in a lamp envelope and excited to a discharge state by the introduction of an RF signal in close proximity to the lamp envelope, which lamp envelope contains the appropriate phosphor coatings to allow the discharge energy to be converted to visible light. This patent further discusses that an electric field generated by the RF signal initiates the discharge whereas a magnetic field then sustains continuous operation of such discharge thereafter. In order to generate this RF signal the electrodeless discharge lamp contains a ballast circuit arrangement disposed in the base of the lamp and which circuit includes a coil member extending into a cavity formed in the lamp envelope, the coil member being effective for outputting the RF signal. In order for the electrodeless discharge lamp to reach widespread commercial acceptance, it will be necessary to achieve this ballast circuit arrangement in a reliable and cost effective manner using as few a number of components as possible. Additionally, it will be necessary in the generation of the RF signal, radio frequency interference (RFI), which can have both conducted and radiated components, is kept below a level which is in compliance with Government regulatory standards. For instance, the International Electro-Technical Commission Standard dealing with Electromagnetic compatibility of lamps (CISPR 15) requires that the conducted component of RFI in the frequency range of between 0.5 and 5.0 megahertz, be less than 56 dB(microvolts). A number of proposals for the suppression of Electromagnetic Interference (EMI) have been made to alleviate this problem. One such proposal is to provide a capacitive arrangement by means of a conductive layer disposed on the inside of the lamp envelope and a conductive layer disposed on the outside of the lamp, such capacitive arrangement being coupled during lamp operation to the supply mains. Such a proposal is set forth in US Patent No. 4,727,294. This arrangement has the disadvantage of requiring additional material, the conductive coating layers, and of requiring an additional manufacturing step to implement this proposal, both of which add a measurable cost to the discharge lamp itself. Another proposal has been to connect one end of a parasitic coil to the exciter coil, that is, the coil member which outputs the RF signal. The other end of the parasitic coil would be allowed to float to a voltage equal and opposite to that developed across the exciter coil. This results in electric field cancellation which can significantly reduce the conducted component of RFI. Such an arrangement can be found in US Patent No. 4,710,678. Though effective in reducing RFI, this approach also suffers in that, by adding an additional relatively expensive component, the parasitic coil, the overall cost of the discharge lamp has again been increased by a measurable amount. Accordingly, it would be advantageous to provide a ballast circuit arrangement for an electrodeless discharge lamp which can be minimized in terms of the number and cost of components required and can meet the regulatory requirements relating to RFI suppression without then increasing the overall cost of the discharge lamp in order to comply with such regulatory requirements. SUMMARY OF THE INVENTION Based on the principles of the present invention, there is provided a circuit arrangement for an electrodeless discharge lamp, which circuit arrangement has EMI suppression features and which is effective for providing an RF signal which is inductively coupled to an ionizable medium contained in a lamp envelope portion of said electrodeless discharge lamp so as to excite such ionizable medium to a discharge state. The electrodeless discharge lamp further includes a base portion in which is housed a drive circuit which is receptive of line power and is effective for converting such line power into a drive signal for driving the circuit arrangement to thus generate the RF signal. The circuit arrangement of the present invention includes a core member which extends into a re-entrant cavity portion of the lamp envelope and has wound thereon, an excitation coil extending into the re-entrant cavity as well so as to be in close proximity to the ionizable medium. A first tuning capacitor is series connected on one end to a first end of the excitation coil whereas a second tuning capacitor having the same capacitive value as the first tuning capacitor is series connected on one end to a second end of the excitation coil opposite to the first end. When the drive signal is connected over the first tuning capacitor to the excitation coil, the RF signal is produced thereby. BRIEF DESCRIPTION OF THE DRAWINGS: This invention will be described more fully with reference to the drawings in which: Figure 1 is an elevational view in section of an electrodeless discharge lamp constructed in accordance with the present invention. Figure 2 is a circuit diagram illustrating the theory behind the cause of conducted radio frequency interference in an electrodeless lamp. Figure 3 is a circuit diagram of a prior art solution to the problem defined in fig. 2. Figure 4 is a circuit diagram of an EMI suppression arrangement constructed in accordance with the teachings of the present invention. DETAILED DESCRIPTION OF THE INVENTION As seen in fig. 1, a low pressure electrodeless fluorescent lamp 10 includes a lamp envelope 12 having a lower portion which fits within a housing base assembly 17. A conventional threaded screw base 19 is mounted on the housing base assembly 17 for connecting line power to a ballast circuit arrangement 24 disposed within housing base arrangement 17. The ballast circuit arrangement 24 includes an RF coil portion 16 which extends within a re-entrant cavity portion 15 of the lamp envelope 12. The RF coil portion includes a core and a winding portion which are disposed around the exhaust tube 14 extending down from the top of the re-entrant cavity 15 and into the region of the base housing assembly 17 in which the ballast circuit arrangement 24 is disposed. When energized, the ballast circuit arrangement 24 is effective for generating an RF signal which is inductively coupled to a fill contained within lamp envelope 12 so as to produce discharge 23. Discharge 23 is effective in a conventional manner for converting energy into visible light in cooperation with the fluorescent coating 20 disposed on the interior wall surface of lamp envelope 12. It can be appreciated that because of the nature of the operation of lamp 10, that a radio frequency field is generated, which field poses the problem of developing an arrangement for suppressing the amount of the emission which spills out beyond the space of the lamp itself. One of the main causes of the conducted radio frequency interference RFI being emitted from lamp 10 is due to the asymmetrical voltage, with respect to ground, that is developed across the excitation coil 16. The theory behind this occurrence will now be explained with reference to fig. 2. This figure shows a circuit with a loaded lamp coil 16' and its associated tuning capacitor 21 whose reactances are Z and -j ' Z respectively where j represents the imaginary number of the square root of -1. Due to the voltage formed between these two components, a capacitive current path (shown in dotted lines) is created. The capacitive reactance, -jX can result in excessive interference if Z and -jX have similar magnitudes. The conductive current (i) through excitation coil 16' and the displacement current to ground (i g ) are also indicated in fig. 2. In the following analysis, the effect of stray capacitive current has been neglected since for practical coils, the former is significantly less than the latter. By considering the leakage reactance, V1 can be expressed as: However, V1 is also the voltage across loaded coil 16' whose impedance is Z; thus: VI = Zi (2) By combining equations (1) and (2), i can be expressed as: It follows that if the stray capacitance is known, the leakage current i can be calculated. Conversely, a measure of i g will reveal the stray capacitance. One technique for reducing this source of interference is to use a balanced supply. A "push-pull" arrangement can be achieved at RF frequencies with a transformer 25 as indicated in fig. 3. The ends of the secondary of transformer 25 are connected to the excitation coil 16' via tuning capacitors 26 and 27. Capacitive currents flowing from each leg of the excitation coil 16' are equal in magnitude but since they are also in anti-phase, full cancellation occurs. However, the addition of transformer 25 adds to the complexity and cost of the circuit of fig. 3. As seen in fig. 4, an embodiment of the present invention presents a solution to the RFI problem without incurring the additional costs and complexity of the circuit of fig. 3. As seen in fig. 4, tuning capacitors 26 and 27, having the same capacitance value (-jX/2), are connected to the respective ends of excitation coil 16. Since one end of excitation coil 16 is no longer grounded as in the circuit of fig. 2, the two displacement paths shown in dotted lines in fig. 4 must be added together to give the total leakage current. Despite splitting the original single tuning capacitor 21, the present circuit remains electrically the same except for the additional stray capacitance. The voltage V2 and the potential drop across the loaded excitation coil 16 can be expressed by: v x - ι 2 = iZ (4) and since V2 = i(-jX/2), the displacement current can be expressed as: i(jZ * X}2) (5) V Assuming that the leakage reactances are equivalent; namely -jX g1 jX g2 = -jX g , then the total leakage current to ground is given by: + I., = * x (6) Hence, as a fraction of the original leakage current, the effect due to splitting the tuning capacitor can be expressed by: + I X (7) where i is the leakage current due to the completely unbalanced configuration shown in fig. 2. If the total reactance of the circuit is to be tuned out, then loaded coil reactance must be +JX, neglecting the effects of strays. Therefore, equation 7 can be simplified to: 'a *1 _ (8) v where loaded coil Q-factor is given by: (9) < - ! and R is the loaded coil resistance. Hence, from equation 8, it can be seen that any reduction in stray current to ground depends upon the loaded Q-factor of the excitation coil 16. Therefore, the reduction in leakage power, denoted by the term Shielding Effectiveness (SE) can be expressed by: SE = -10 log 10 (l +<? 2 ) (10) Using this equation and assuming a Q-factor of greater than 5, a significant improvement in the interference levels between the circuits of figures 2 and 4 can be realized. Although the above embodiment constitutes a preferred embodiment of the invention, it can be appreciated that modifications can be made thereto without departing from the scope of the claims as set forth in the appended claims.

Claims: C L AJ M S 1. A circuit arrangement having EMI suppression features and which is effective for providing an RF signal to an electrodeless discharge lamp having a lamp envelope containing an ionizable medium excitable to a discharge state upon introduction of said RF signal thereto, and wherein the electrodeless discharge lamp includes a base portion in which is housed a drive circuit receptive of line power and effective for converting the line power into a drive signal such that said drive circuit and said circuit arrangement together produce said RF signal, said circuit arrangement comprising: a core member extending into a re-entrant cavity formed in said lamp envelope; an excitation coil wound on said core member and being in close proximity to said ionizable medium; a first tuning capacitor connected on one end to a first end of said excitation coil; a second tuning capacitor series connected on one end to a second end of said excitation coil opposite to said first end, said first and second tuning capacitors having capacitive values which are split proportionately between each other so as to minimize radio frequency interference occurring in the production of said RF signal; and wherein said drive signal is connected to said excitation coil through said first tuning capacitor thereby producing said RF signal. 2. A circuit arrangement for an electrodeless discharge lamp as set forth in claim 1 wherein the capacitive value between said first tuning capacitor and said second tuning capacitor are split such that each respective capacitive value is substantially the same as the other capacitive value. 3. A circuit arrangement for an electrodeless discharge lamp as set forth in claim 1 wherein said excitation coil has a loaded Q-factor of greater than approximately 3. 4. A circuit arrangement for an electrodeless discharge lamp as set forth in claim 1 wherein capacitively coupled current leakage paths from either side of said excitation coil to ground are substantially the same.

Inventors: GREEN IAN MACDONALD, STEEL MARTIN CHRISTOPHER, TRUMBLE FRANCIS ROBERT, GREEN, IAN, MACDONALD, STEEL, MARTIN, CHRISTOPHER, TRUMBLE, FRANCIS, ROBERT
Applicants: GE LIGHTING LTD, GREEN IAN MACDONALD, STEEL MARTIN CHRISTOPHER, TRUMBLE FRANCIS ROBERT, GE LIGHTING LIMITED, GREEN, IAN, MACDONALD, STEEL, MARTIN, CHRISTOPHER, TRUMBLE, FRANCIS, ROBERT
Application year: 1995
Main classification: H05B41
IPC codes: H05B41