WO2001039555A1 - Self-tuning electrodeless lamps - Google Patents

Abstract

A self-tuned system includes an oscillator (30) and an oscillator-driven load which uses the oscillator-driven load as a frequency determining element of the resonant system. In one illustration, the oscillator-driven load is a lamp head circuit for an electrodeless lamp. The lamp head circuit of the electrodeless lamp comprises an excitation coil (40) proximate an envelope or bulb (42) enclosing a fill, the fill forming a plasma discharge when ignited. Various oscillator configurations can be used with the load, including modified Hartley (30A), modified Colpitts (Clapp) (30B), and modified Armstrong configurations (30C). Various embodiments of clamping circuits (100) can also be utilized to limit a feedback signal applied by the feedback system to a region tolerated by the amplifier. Of particular interest to the present invention are self-tuned systems which operate at high frequency and high power. Example implementations of high frequency and high power electrodeless lamps are provided, as well as various inventive techniques for, e.g., providing sufficient gain and for enhancing ignition/self-starting (e.g., reducing start time) of high frequency, high power self-tuned systems. Techniques and arrangements for ensuring sufficient gain for the self-tuned systems of the invention include choice of oscillator configuration, multi-stage amplification (when required), and feedback system configuration. Self-ignition of the load is achieved in numerous ways, such as (for example) using a special starting oscillator; using a low pressure rare gas fill for the lamp bulb; and increasing current in the excitation coil (e.g., by selectively positioning a juncture of a tap (60) to the excitation coil).

Description

SELF-TUNING ELECTRODELESS LAMPS

BACKGROUND

Field Of The Invention The present invention pertains to the operation of oscillator-driven loads which have changing resonant frequency, an example of which is an electrodeless lamp. Related Art And Other Considerations

A radio frequency (RF)-driven load may have changing characteristics which affect the resonant frequency. For example, the resonant frequency may vary considerably through a start up phase of operation of the load. Moreover, due to aging and other considerations, over the life time of the load the resonant frequency of the load changes in the post-start up phases of operation. Examples of such RF- driven loads include radio frequency antennae. Historically, various types of oscillators have been employed in connection with RF-driven loads operating in certain low power, low frequency environments. Such known types of oscillators include the Colpitts (Clapp) oscillator, the Hartley oscillator, and the Armstrong oscillator. These known types of oscillators were employed years ago, primarily for transmitters. See, e.g., Mandl, Electronics Handbook, Reston Publishing Company, Inc., 1983, pages 73 - 77.

An electrodeless lamp is an example of an RF-driven load which, by contrast, utilizes high power and high frequencies. Some electrodeless lamps are capable of generating high brightness levels over more than 10,000 hours of operation without requiring replacement. Electrodeless lamps have no internal electrodes, but rather rely upon external structures to achieve breakdown and excitation of a fill material for emission of light. Typically electrodeless lamps are classified as inductively coupled (H discharge), capacitance coupled (E discharge), microwave discharge, and traveling wave discharge. Certain basic principles of electrodeless lamps, as well as each of these classifications, are discussed in the literature. See, e.g., Wharmby, D.O., "Electrodeless Lamps For Lighting: A Review", IEEE Proceedings- A, Vol. 140, No. 6, November 1993, pp. 465 - 473. An inductively coupled electrodeless lamp can be analogized to an electrical transformer. In inductively coupled electrodeless lamps, the fill material (e.g., plasma) in a discharge vessel (bulb) serves as a single turn secondary coil, while a primary (exciter) coil is connected via suitable impedance matching to a power source. There are various forms of inductively coupled electrodeless lamps: the primary (exciter) coil can be outside the discharge vessel; inside the vessel; within a reentrant; or wrapped around part of the tubular lamp forming a torus. The magnetic field can be provided by a coil with an air core, or a magnetic core.

In an inductively coupled electrodeless lamp, an alternating current in the coil causes a changing magnetic field, which induces an electric field which drives a current in the plasma. Certain electrical properties and phenomena involved with inductively coupled electrodeless lamps have been documented, e.g., by Piejak, R.B. et al., "A Simple Analysis of An Inductive RF Discharge", Plasma Sources Sci. Technlol. 1 (1992), pages 179 - 186. Examples of structures and operating techniques for inductively coupled electrodeless lamps are provided in PCT Publication No. WO 99/36940, published July 22, 1999, entitled "High Frequency Inductive Lamp and Power Oscillator", as well as United States Patent 5,798,61 1 , both of which are incorporated herein by reference in their entirety. In an electrodeless lamp, the plasma-filled bulb comprises the RF-driven load.

After start up and until ignited, the cold plasma has an infinite impedance, with the power supply (oscillator) seeing a nearly reactive load. Once ignited, however, the resistance and inductance of the bulb's plasma significantly and continuously changes, thereby changing the resonant frequency of the lamp head. Since the resistance and inductance of the plasma are reflected back into the primary or driving circuit, both the frequency and Q of the driving circuit are affected. Typically this means that special frequency adjusting circuitry is required to minimize the effect of the changing resonant frequencies attributable to startup, operation, and aging. Otherwise, lamp-out or high reflected power due to poor matching can occur. In some electrodeless lamps, a RF source provides RF energy at a certain frequency to a lamp head, the lamp head being part of a separate resonant circuit which is designed to resonate at the frequency of the provided energy. In such lamps, both frequency matching (tuning) and impedance matching are important for optimizing overall system efficiency. PCT Publication No. WO 99/36940 describes, e.g., lamp systems of this type with a novel RF source which can efficiently provide RF power at high power (e.g., 70 watts or more) at frequencies above 300 MHz (e.g., 700 - 900 MHz) to the lamp head resonant circuit. Further, various RF control circuits are described which perform frequency tuning between the RF source and the lamp head, as well as novel coil structures for coupling energy to an electrodeless lamp bulb at high frequencies (e.g., 700 - 900 MHz or more). However, electrodeless lamps that employ separate RF source and control circuits include additional components and circuitry for, among other things, frequency tuning, with associated manufacturing costs and complexity.

Other electrodeless lamps are configured with the lamp being part of the RF source, e.g., part of a tank circuit of an oscillator, so that the lamp influences, at least in part, the operating frequency of the oscillator circuit. The lamp can be either inductively coupled or capacitively coupled. Respective examples of these coupling techniques are disclosed in United States Patent 4,010,400 to Hollister and United States Patent 4,485,333 to Goldberg. Self-tuning oscillator circuits (presuming the conditions for oscillation are met) are taught, but in the context of low frequency applications. For high frequency circuits, such as a high frequency electrodeless lamp, the effective integration of the load (e.g., the lamp head) as part of an oscillator circuit can depend on many other factors and be more complex. For example, impedance matching is important to optimizing overall system efficiency.

What is needed, and an object of the present invention, is a high power, high frequency inductive lamp with fewer components and consequentially reduced manufacturing costs and complexity.

BRIEF SUMMARY OF THE INVENTION

A self-tuned system includes a gain element (active element) together with an RF-driven load configured as an oscillator which uses the RF-driven load as a frequency determining element for the system. The gain element comprises, for example, an amplifier which may be single stage, dual stage, or other configuration. In one illustration, the oscillator-driven load is a lamp head circuit for an electrodeless lamp. The lamp head circuit of the electrodeless lamp comprises an excitation coil proximate an envelope enclosing a fill, the fill forming a plasma discharge when ignited. In addition, the lamp head circuit includes a high power capacitance and a matching capacitance. Various oscillator configurations can be used with the load, including modified Hartley, modified Colpitts (Clapp), and modified Armstrong configurations.

In addition to the gain element (e.g., amplifier), the oscillator comprises a feedback system connected between the oscillator-driven load and the amplifier. For voltage protection, various embodiments of clamping circuits can also be utilized to limit a feedback signal applied by the feedback system to a region tolerated by the amplifier, as well as various locations for the clamping circuits. In one configuration, the feedback system comprises plural segments having different impedances, with the clamping circuit being connected to one of the plural segments which has an impedance that overcomes limitations of an element of the clamping circuit (e.g., high dynamic impedance of a Shottky diode comprising the clamping circuit). The feedback system is, in one embodiment, a dual path feedback system.

The self-tuned systems of the invention operate at high frequency and high power. For facilitating operation of high frequency, high power self-tuned systems, various inventive techniques are disclosed for, e.g., providing sufficient gain and for enhancing ignition/self-starting (e.g., reducing start time).

Techniques and arrangements for ensuring sufficient gain for the self-tuned systems of the invention include choice of oscillator configuration, multi-stage amplification (when required), and feedback system configuration. For example, a Hartley oscillator configuration is preferred over a Colpitts oscillator configuration when the active element is a low gain device for maintaining oscillation at a high frequency while transferring significant power to bulbs of self-tuned lamps. Further, an additional stage(s) of amplification can be added. Moreover, a feedback system with dual feedback delay lines promotes greater gain.

The present invention also addresses improved ignition/reduced start times for the loads of self-tuned systems. Self-ignition can be achieved in numerous ways, such as (for example) using a low pressure rare gas fill for the lamp bulb. Another technique is to increase current in the excitation coil. For example, in a Hartley-type self tuning lamp, a position for juncture of a tap to the excitation coil is chosen to increase current in the excitation coil. Further, to assist improved ignition, a self- tuned system can include both a starting oscillator and a power oscillator, with the starting oscillator having an impedance/operating frequency matched/tuned to the cold tank. After the self-tuned system starts, the impedance of the tank has a better match to the power oscillator. The start oscillator is turned off when it is sensed that the power oscillator has started to draw high power.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

Fig. 1 is a schematic view of a resonant system including an oscillator and an oscillator-driven load.

Fig. 2 is a schematic view of a lamp head circuit for an inductively coupled electrodeless lamp. Fig. 3 is a schematic view of a lamp head circuit for a capacitively coupled electrodeless lamp.

Fig. 4 is a diagrammatic depiction of a partial cross-section of a lamp head for the inductively coupled electrodeless lamp of Fig. 2.

Fig. 5 - Fig. 7 are schematic views of resonant circuits which employ differing oscillators for driving an electrodeless lamp head circuit as a load.

Fig. 8 - Fig. 10 are schematic views of resonant circuits which correspond to the circuits of Fig. 5 - Fig. 7, but wherein two stage amplification is provided in the differing oscillators.

Fig. 11 , Fig. 12, and Fig. 13 are schematic views of clamping circuits according to differing embodiments the present invention.

Fig. 14, Fig. 15, and Fig. 16 show cross-sectional views of example differing coil configurations with which the invention can be used. Fig. 17 is a cross-sectional view showing a coil with integral tap for use in a modified Hartley oscillator included in a resonant system of the invention.

Fig. 18 is a schematic view of a circuit layout for one example implementation of a resonant system utilizing a modified Hartley type oscillator in conjunction with a electrodeless lamp head circuit.

Fig. 19 is a partial cross section representation of the oscillator taken along line 19-19 of Fig. 18.

Fig. 20 is a schematic view showing a first location for connecting a clamping circuit to the circuit layout of Fig. 18. Fig. 21 is a schematic view showing a second location for connecting a clamping circuit to the circuit layout of Fig. 18.

Fig. 22 and Fig. 23 are schematic views of yet other resonant circuit variations in accordance with the present invention.

Fig. 24 is a schematic view of a self-tuned system that includes both a power oscillator and a starting oscillator

Fig. 25, Fig. 26, Fig. 27, and Fig. 28 are schematic views showing equivalent circuits for various Colpitts-type oscillator variations.

Fig. 29 is a schematic view showing equivalent circuits for an Armstrong-type oscillator variation.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. Fig. 1 shows a self-tuning system which includes a generic oscillator 30 which drives a load 32. The load 32 is of a type which provides a reflection to oscillator 30. The oscillator 30 includes an amplifier 34 having an amplification factor A and a feedback delay or matching element 36 having a feedback factor B (the feedback factor B being phase change or delay adjustment as well as matching and attenuation). The load 32 provides a reflection at the amplifier plane. In the self- tuning system of Fig. 1 feedback comes from load 32 (as indicated by the dashed line) rather than directly from the oscillator output.

In accordance with the present invention, one example of a load which can be used in a self-turning system such as that of Fig. 1 is an electrodeless lamp. As mentioned above, electrodeless lamps can fall into various classifications. For example, Fig. 2 depicts basic aspects of a lamp head circuit for an inductively coupled (H discharge) electrodeless lamp. Fig. 3 depicts basic aspects of a lamp head circuit for a capacitance coupled (E discharge) electrodeless lamp. As a prelude to the ensuing discussion, a brief overview of these two types of electrodeless lamps is provided below.

The representative illustration of the lamp head circuit of an inductively coupled (H discharge) electrodeless lamp as shown in Fig. 2 includes an excitation coil 40 situated proximate an envelope or bulb 42 enclosing a fill 44. The lamp head circuit of the inductively coupled electrodeless lamp of Fig. 2 includes a high power capacitance C1 connected to a first end of the excitation coil 40 and a matching capacitance C2 connected to a second end of excitation coil 40 and to electrical ground.

In the inductively coupled electrodeless lamp of Fig. 2, RF input 43 is applied to the lamp head circuit at a point between the high power capacitance C1 and the matching capacitance C2. The inductively coupled electrodeless lamp of Fig. 2 operates at an elevated frequency in the range of 100 to 3000+ MHz. As illustrated in Fig. 4, when ignited by excitation coil 40 the fill 44 forms a plasma discharge 46, thereby providing illumination. Fig. 4, being a cross-sectional view of Fig. 2, shows plasma discharge 46 as having an essentially toroidal or annular shape. The excitation coil 40 has an inductance and resistance. Once ignited the plasma has a respective inductance and resistance. Certain techniques for driving the lamp head circuit of an inductively coupled electrodeless lamp are described in the aforementioned PCT Publication No. WO 99/36940. In the representative illustration of the lamp head circuit of a capacitively coupled electrodeless lamp as shown in Fig. 3, the bulb 52 having a fill is positioned between plates of a capacitor 50. Once the fill is ignited, a displacement current flows through the capacitance of the wall of bulb 52, causing plasma discharge. As indicated previously, once ignited the resistance and inductance of the bulb's plasma significantly and continuously change, whether the electrodeless lamp is inductively coupled (as, for example, in Fig. 2) or capacitively coupled (as, for example, in Fig. 3). The continuous change of the plasma's resistance and inductance changes the resonant frequency, since the resistance and inductance of the plasma are reflected back into the primary or driving circuit for the lamp head. Thus, both the frequency and Q are affected.

Fig. 5 - Fig. 7 illustrate three example embodiments showing how, in accordance with the present invention, the lamp head circuit of an electrodeless lamp is incorporated as load 32 into the self-tuning circuit of Fig. 1. The illustrations of Fig. 5 - Fig. 7 particularly show an inductively coupled lamp as being representative of electrodeless lamps generally (including, e.g., electrodeless lamps of other classifications). Although elements in Fig. 5 - Fig. 7 which are comparable to those in Fig. 1 and Fig. 2 have comparable numerical identifiers, alphabetical suffixes A, B, and C are provided in order to distinguish the elements as belonging to Fig. 5, Fig. 6, or Fig. 7, respectively.

Fig. 5 particularly shows oscillator 30A, which drives the lamp head circuit of the electrodeless lamp, taking the form of a modified Hartley oscillator. In the self- tuning circuit of Fig. 5, the oscillator 30A includes amplifier 34A which has an input terminal and an output terminal. The output terminal of amplifier 34A is connected between high power capacitance C1 and matching capacitance C2. A tap 60 is connected to the excitation coil 40. Feedback element 36A and blocking capacitance 62 are connected in series between tap 60 and the input terminal of amplifier 34A. The modified Hartley oscillator of Fig. 5 thus has a grounded source and grounded excitation coil 40. In feedback element 36A, B represents various tuning adjustments that are made including the length of the coil tap inductance. Advantageously, the oscillator 30A of Fig. 5 provides high feedback, which is beneficial for relatively lower gain single transistor amplifiers. Fig. 8 shows a modification, particularly oscillator 130A, which employs two stage amplifier 134A.

Fig. 6 shows oscillator 30B taking the form of a modified Colpitts (Clapp) oscillator. In the self-tuning circuit of Fig. 6, the oscillator 30B includes amplifier 34B. Amplifier 34B has an input terminal and an output terminal, the output terminal of the amplifier being connected between high power capacitance C1 and matching capacitance C2. A third capacitance C3 is connected between matching capacitance C2 and ground. Feedback element 36B has a first end connected between matching capacitance C2 and third capacitance C3, and a second end connected to the input terminal of amplifier 34B. A significant difference between oscillator 30B of Fig. 5 and oscillator 30A of Fig. 6 is that oscillator 30B has less feedback. The amount of B in Fig. 6 is not necessarily the same as in Fig. 5. A two stage amplifier version of the modified Colpitts (Clapp) oscillator embodiment, shown as oscillator 130B in Fig. 9 with two stage amplifier 134B, is preferred in view, e.g., of the feedback parameters.

Fig. 7 shows oscillator 30C taking the form of a modified Armstrong oscillator. In the self-tuning circuit of Fig. 7, the oscillator 30C includes amplifier 34C. Amplifier 34C has an input terminal and an output terminal, the output terminal of the amplifier being connected between high power capacitance C1 and matching capacitance C2. In addition, oscillator 30C features a pick-up coil 64 arranged co-axially to excitation coil 40. Pick-up coil 64 has a first end and a second end, the first end of pick-up coil 64 being connected to electrical ground. Feedback element 36C is connected between the second end of pick-up coil 64 and the input terminal of amplifier 34C. Thus, the modified Armstrong oscillator 30C of Fig. 7 obtains its feedback signal from pick-up coil 64. Fig. 10 shows an embodiment wherein the modified Armstrong oscillator 130C includes two stage amplifier 134C.

As understood from Figs. 5-7 and Figs. 8-10 described above, in accordance with the present invention the self-tuning RF oscillator for an oscillator-driven lamp head circuit follows the frequency requirements of the lamp head circuit by taking its input signal from the lamp head circuit. The modified Hartley (Fig. 5, 8), Colpitts (Fig. 6, 9), and Armstrong (Fig. 7, 10) oscillators are example oscillators which can be employed to fulfill this function. Fig. 11 , Fig. 12, and Fig. 13 respectively show first, second, and third embodiments of clamping circuits 100A, 100B, 100C that can be utilized with embodiments of the invention for voltage protection. The clamping circuits 100A, 100B, 100C limit the feedback signal of an oscillator 30 of the invention to a region tolerated by the active element(s) of the oscillator 30, e.g., amplifier 34. Such being the case, the clamping circuits 100A, 100B, 100C are connected either directly or via a feedback system to a gate of amplifier 34. In general terms, the clamping circuits should provide high current carrying capability, present low capacitance and inductance, and utilize short line lengths. In the ensuing discussion, general reference to clamping circuit 100 can refer to either clamping circuit 100A of Fig. 1 1 , clamping circuit 100B of Fig. 12, clamping circuit 100C of Fig. 13, or to equivalents or variations thereof.

As shown in Fig. 11 , clamping circuit 100A comprises a series pair of switching diodes, e.g., Shottky diodes 102 and 104, with the anode of Shottky diode 104 and the cathode of Shottky diode 102 being connected to fixed voltages, -V_B and +V_B, which should not exceed the maximum rated voltage for the active device and are typically 70 - 80% of such rating.. The clamping circuit 100B of Fig. 12 also has switching diodes (e.g., Shottky diodes 102, 104), but further has a regulating diode (e.g., Zener diodes 106, 108) connected between each Shottky diode 102, 104 and ground. In particular, the anode of Zener diode 108 is connected to the anode of Shottky diode 104, with the cathode of Zener diode 108 being connected to ground. The cathode of Zener diode 106 is connected to the cathode of Shottky diode 102, with the anode of Zener diode 106 being connected to ground. In the clamping circuits 100 above described, the Shottky diodes 102, 104 can be a series pair MA4CS102B (with a max C, of 1.0 pF); the Zener diodes 106, 108 can be 16 volt TVS/Zener IN6276, for example. The clamping circuit 100C of Fig. 13 differs from that of Fig. 12 in having a small capacitance connected in parallel with each of Zener diodes 106, 108.

The coils of an electrodeless lamp can take a variety of configurations. Fig. 14, Fig. 15, and Fig. 16 show three examples of coil cross-sectional configurations. In particular, Fig. 14 shows what has been termed an "Omega" coil 140 with leads bent tangential to the coil loop; Fig. 16 shows a "DCC" coil with leads extending approximately radially from the coil; and Fig. 15 shows a modified coil with one tangential lead and one radial lead. It should be understood that in the context of electrodeless lamps the present invention can be used with any type of coil including those illustrated. However, as explained below, when using a modified Hartley oscillator such as that shown in Fig. 5 or Fig. 18, for example, it is preferred that the tap 60 be located at a proper position on and formed integral with the excitation coil 40. Positioning the junction of the tap 60 to the excitation coil 40 can be selected to increase current in the excitation coil, thereby assisting with self-ignition of the electrodeless lamp. As explained above, the modified Hartley oscillator involves connection of the feedback system to the excitation coil via the tap (see, e.g., tap 60 in Fig. 5). Preferably, in order to overcome thermal transitions by the proximity of the excitation coil 40 to the bulb of the electrodeless lamp, the tap is embedded or connected to the excitation coil in one assembly, e.g., there being no welded or soldered connections of the tap to the excitation coil. In accordance with an aspect of the present invention, the excitation coil and tap can be formed from one solid piece (e.g., of copper). For example, the tap can be stamped and formed to the excitation coil, or the entire assembly formed by casting. Fig. 17 shows one such coil formed with the tap being integral. Generally speaking, the feedback voltage increases as the angle θ of the tap with respect to the ground lead increases and coil losses also increase as the angle θ increases. If the angle is too large, a high feedback signal is obtained but the losses may be too great to sustain a discharge. Accordingly, the tap preferably positioned at an angle which provides sufficient feedback voltage while minimizing the corresponding losses. Higher angles may be required for low gain devices. Preferably, in one embodiment, the tap 60 is positioned about 15-30% around the single turn of excitation coil 40 relative to the ground lead in order to facilitate sufficient feedback through the tap while maintaining satisfactory current through the excitation coil.

Fig. 18 shows a circuit board layout for one example implementation of a self- tuning system utilizing a modified Hartley type oscillator 230. Fig. 19 is a partial cross section representation of the oscillator 230 taken along line 19-19 of Fig. 18. Fig. 18 and Fig. 19 show excitation coil 240 having a first end connected (soldered) to a ground pad 280. A second end of excitation coil 240 is connected via high power capacitance C1 and a transmission line 281 (approximately 10 ohm) to a drain 282 of amplifier 234. The tap 260 of oscillator 230 extends from excitation coil 240 to blocking capacitance 262. The blocking capacitance 262 is situated on a feedback pad 284 which connects to a dual feedback system comprising feedback system 286A and feedback system 286B. The feedback system 286A and feedback system 286B are essentially identical mirror images of one another, each comprising various segments including a 15 ohm transmission line segment 288A,B; a 93 ohm transmission line segment 290A.B; an adjustable capacitance 292A.B; and a 24 ohm transmission line segment 294A.B. For each feedback system 286A and 286B, the transmission line segment 294A.B connects to the gate 283 of amplifier 234. In the oscillator 230 of Fig. 18, the 93 ohm transmission line segments 290A.B are of narrower width than the other transmission lines constituting the feedback systems 286A.B to give good electrical length (to get the correct phase). The adjustable capacitances 292A.B are approximately between 1 pF and 3 pF. The amplifier 234 can be, for example, Motorola part MRF184 or MRF373.

The feedback system 286A.B with its dual feedback delay line thus incorporates a section (93 ohm transmission line segment 290A.B) of higher impedance for delay adjustment, and a short low impedance section ( transmission line 281) from the transistor drain to the high voltage capacitor for coil-transistor separation. Advantageously, the duality aspect of feedback system 286A,B provides both stability and robustness.

While the clamping circuits 100 of the present invention can be connected on the gate of the amplifier 34 of the oscillators 30 of the present invention, it is preferred to locate the clamping circuits 100 on a part of the feedback system with a higher impedance. Whereas, in connection with Fig. 18, for example, gate 283 may be 2 to 3 ohms or even 0.3 to 1 ohm (e.g., for a Motorola MRF-184 amplifier in the range of 700 to 1000 MHz), the feedback transmission line segments 288, 290, and 294 have impedances of 15, 93, and 24 ohms, respectively. Fig. 20 thus shows how the clamping circuit 100 (which, as mentioned above, can be, e.g., either clamping circuit 100A of Fig. 1 1 , clamping circuit 100B of Fig. 12, or clamping circuit 100C of Fig. 13) can be connected to the 93 ohm transmission line segment 290A, B of feedback system 286A.B. Fig. 21 shows how the clamping circuit 100 can be connected to 24 ohm transmission line segment 294A.B. Thus, the clamping circuit 100 is connected to a point on the feedback system 286 at which the high dynamic impedance (approximately 5 ohms) of the Shottky diodes 102, 104 do not present a problem (e.g., the 93 ohm transmission line segment 290 in Fig. 20 and the 24 ohm transmission line segment 294 in Fig. 21).

Fig. 22 shows a modified Hartley-type oscillator variation 330 for bulb 342. The excitation coil 340 has a tap 360 formed therewith, which is connected via blocking capacitance 362 to a gate of amplifier 334. A clamping circuit 100B is also connected to the gate of amplifier 334, the clamping circuit 100B being essentially the same as that illustrated in Fig. 12. Element 390 shown in Fig. 22 is an RF choke (RFC), having 5 turns, 20 AWG, inner diameter of 0.126 inch.

Fig. 23 shows yet another self-tuning circuit variation. The excitation coil 440 is in proximity to bulb 442, and is connected via the high power capacitance C1 (on the order of about 6pF) to the output of amplifier 434. The capacitances 494 and 495 have values between about 20 and 40 pF; capacitance 496 is large enough to handle feedback. A driving current i₁ is shown. If needed, a clamping circuit 100A (similar to that shown in Fig. 1 1) can be connected to the gate of amplifier 434. The amplifier 434 must have sufficient gain, and may include plural stages, if necessary. However, the size must be small to preclude phase change around the loop.

Aspects of the invention also provide various arrangements and techniques for providing sufficient gain for the self-tuned lamps of the invention. For example, a Hartley oscillator configuration (such as that shown in Fig. 5 or Fig. 18) is preferred over a Colpitts oscillator configuration when the active element is a low gain device in view, e.g., of the fact that the Hartley oscillator configuration requires less feedback. Moreover, a feedback system (such as feedback system 486 in Fig. 18) with dual feedback delay lines promotes greater gain.

Moreover, in view of gain considerations, an additional stage(s) of amplification can be added. In this regard, while oscillators such as those shown in Fig. 5, Fig. 6, and Fig. 7 can have single stage amplification, a second stage amplification can also be added (as shown in Fig. 8 - Fig. 10, respectively) in order to boost the loop gain. The present invention is not limited to one or two stage amplification, as a greater number of amplification stages (e.g., three) can also be employed. Whatever the number of stages, the phase change around the loop is minimized from that required (e.g., n x 360 degrees, where n = 1) so that the lamp head circuit can operate over the required frequency range. As explained subsequently, the feedback signal can be clamped to a region tolerated by the active element(s).

Other aspects of the invention provide improved ignition for self-tuned lamps (e.g., /reduced start time). Various ones of these aspects include employment of a special start oscillator which is turned off when a power oscillator starts drawing high power; lower envelope mass; and increased coil current/reduced pressure fill. To assist improved ignition, as shown in Fig. 24, a self-tuned system can include both a power oscillator 501 and a starting oscillator 502, both connected to a tank circuit 503 of a self-tuned system such as a self-tuned lamp. Feedback 508 from tank circuit 503 is applied to power oscillator 501. Power is applied via power sense circuit 504 both to power oscillator 501 and starting oscillator 502. Starting oscillator 502 has a high impedance Z_out1 (on output 506) matched to the tank circuit 503 when tank circuit 503 is cold and an operating frequency tuned to tank circuit 503 when tank circuit 503 is cold. When the circuit (e.g., lamp) starts, Z,_n (on input 507) of the tank circuit 503 is pulled down so that it has a better match to power oscillator 501. Power sense circuit 504 turns off the starting oscillator 502 when power oscillator 501 starts drawing high power. In Fig. 24, Z_out2 (on output 505) is approximately equal to Z_ιn when the tank circuit 503 is hot and Z_out2 is much less than Z,_n when the tank circuit 503 is cold.

Regarding envelope or bulb mass, the bulb 42 may be formed of any suitable material, such as quartz, sapphire, or polycrystalline alumina (for example). While the bulb 42 has a spherical shape in the illustrated embodiments, other shapes for bulb 42 are possible, such as cylindrically shaped and pill-box shaped bulbs, for example. The bulb 42 can be an aperture bulb. An aperture bulb is surrounded with a reflecting ceramic jacket, except in a region of an aperture through which light is emitted.

Preferably, the amount of quartz or other material utilized for the bulb 42 is as small as possible. An electrodeless lamp bulb must reach a temperature such that the coldest part of the lamp quartz envelope is sufficiently hot so as to provide a vapor fill of correct density. The inventors have performed calculations and experiments pertaining to heat capacity of the quartz bulb, the results of which are shown in Table 1 - Table 4. Table 1 deals with bulbs having 6 cm inner diameters and various bulb wall thicknesses; Table 2 deals with bulbs having 7 cm outer diameters and various bulb wall thicknesses; Table 3 deals with bulbs having 6 cm outer diameters and various bulb wall thicknesses; Table 4 deals with bulbs having 5 cm outer diameters and various bulb wall thicknesses. In the Tables, the Power in is 90 watts; the heat coupled into the plasma in the initial three seconds is 18 Joules/sec; the initial temperature is 25°C; the final temperature is 900°C; the bulb cp is 1250 J/kg-C; the bulb density is 2.20E+03 kg/m³ (i.e., 2.20E-06 kg/mm³). From the results reflected in Table 1 - Table 4, the inventors have concluded that start-up time can be proportional to the mass of bulb 42. Therefore, an aspect of the present invention is utilization of a lower quartz mass to effect shorter start-up time of an electrodeless lamp.

Another way to assist self-igniting is to maximize the coil current just prior to ignition. This coil current maximization may be important, for example, in systems having a single stage amplification (e.g., 12 to 15 dB) which otherwise loads the lamp head circuit (low Q) and therefore provides lower initial coil voltage. One technique for maximizing coil current is to positionally lower (i.e. reduce the angle θ shown in Fig. 17) the location at which tap 60 (in a modified Hartley self-tuning lamp) joins excitation coil 40.

While the present invention is not fill-dependent, it appears preferable to use a low pressure rare-gas fill in order to assist self-igniting of the electrodeless lamp. For example, the pressure should preferably be in a range from about 5 to 30 Torr or less, and more preferably below 20 Torr. Example fills include Indium Bromide (1 mg/cc), Argon, and a small amount of Krypton 85.

In the illustrated embodiments coils such as the coil 40 have been illustrated as having a single turn. It should be understood, however, that the phenomena described herein and principles of the invention are equally applicable to coils having plural turns with the coil size/wavelength relationship being as discussed in the aforementioned PCT Publication No. WO 99/36940. Moreover, coils of differing types of geometries may be employed where advantageous (e.g., to minimize arcing, optimum sizing for quadrature driving, etc.).

The lamps described herein and lamps within the scope of the invention can operate in low, mid, and high power ranges. The signals applied to the lamps illustrated herein are preferably in the 200 MHz to 2000 MHz band, with 300 MHz to 900 MHz band being the most preferred frequency band.

Included in the inventors' work has been fabrication of two electrodeless lamps having a modified Hartley oscillator with a single stage amplifier. One of these lamps had a DCC 10 mm diameter coil such as that shown in Fig. 16; the other of these lamps had a omega configuration coil such as that shown in Fig. 14. In both of these lamps at least 2700 lux was obtained at a distance of 56.4 cm normal to an aperture of the lamps. The lamp with the omega configuration coil was preset to 26 volts and, with assisted starting, when switched on came to near 2700 lux in 7.5 seconds without additional intervention. The 7.5 second transition is relatively rapid, since 20 - 30 seconds is not unusual in other systems. After ignition, the resonant system transition smoothly through the blue mode to full output without the need for outside frequency adjustments.

Concerning the aspect of the invention pertaining to usage of the RF-driven load (e.g., the lamp head circuit) as a frequency determining element of the self- tuning system, the basic embodiments shown in Fig. 5, Fig. 6, and Fig. 7 are but examples. As explained below, other oscillator configurations and variations of these basic embodiments are within the scope of the present invention.

For example, Fig. 25, Fig. 26, Fig. 27, and Fig. 28 show various Colpitts-type oscillator variations. Fig. 25 shows an equivalent schematic where a high voltage input is added at point 505. The oscillator of Fig. 25 includes amplifier 534 and feedback element 536 connected around high voltage capacitance C1. The feedback element 536 can be a phase shifting element. In Fig. 25, the excitation coil 540 is shown in series with the load Z (the load being the bulb and plasma).

Fig. 26 shows an equivalent schematic for an oscillator driving an electrodeless lamp head circuit where a low voltage input is added at point 606. The oscillator of Fig. 26 primarily differs from that of Fig. 25 by placement of the capacitances and addition of a third capacitance (namely capacitance 664). In the oscillator of Fig. 26, the high voltage capacitance C1 is much less than the capacitances C2 and 664. The equivalent schematic of Fig. 27 is similar to that of Fig. 26, but shows that amplifier 734 is a high gain transistor or FET with emitter connected both between capacitances C2 and 764 and through resistance 766 to ground. The equivalent schematic of Fig. 28 is similar to that of Fig. 27, but further shows an RF input current path 870. In Fig. 28, a DC bias Vb is supplied, if needed, to provide 30 to 100 volts RF at point 872. It will be appreciated by the person skilled in the art that feedback must be at 0°, 360°, ... etc., phase for the embodiments of Fig. 25, Fig. 26, Fig. 27, and Fig. 28. Fig. 29 shows an Armstrong -type oscillator variation with its pick-up coil 964 which generates feedback from the resonant excitation coil 940. The amplifier 934 can be a one stage amplifier, or a two stage amplifier (as shown in Fig. 29 as including a first stage amplifier 980. Furthermore, a clamping circuit can be connected, e.g., at point 982 in Fig. 29, to protect one or more of the amplifiers of the oscillator. Thus, in the oscillator system of Fig. 29, feedback for oscillation is derived from a choke used in separating DC current from the RF current in the power oscillator. An extra winding is wound on the choke that is used to generate the feedback signal. The level of feedback is controlled by the ratio of this winding to the windings on the choke. An advantage of the Armstrong-type oscillator circuit is that both the resonant coil (i.e., excitation coil 940) and the last stage of the amplifier (i.e., amplifier 934) can be grounded. With a high frequency lamp utilizing a wedding ring type excitation coil, the pickup coil is preferably placed in close proximity to the wedding ring coil.

Thus, while the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. For example, while various embodiments have been illustrated in context of an inductively coupled electrodeless lamp, it should be understood that the techniques and principles of the invention are also utilizable with other classifications of electrodeless lamps, such as capacitively coupled lamps, for example. In addition, as explained above, the field of electrodeless lamps serves as just one example field to which the principles and techniques of the invention are applicable.

Aspects of the present invention have numerous advantages. The invention employs a self-tuning system to remain at a changing resonant frequency during starting and long term operation of the load (e.g., the lamp head circuit of an electrodeless lamp). The use of less quartz in the lamp bulb and a higher initial power contribute to self-ignition and reduced start up time. Further, reliable lamp ignition is achieved by increasing coil current prior to ignition. In addition, a lower pressure fill can be employed.

By eliminating the frequency determining elements of the lamp structure (independent of the series resonant circuit), the system is simplified and the need for frequency adjusting circuits during start up and operation is precluded. Moreover, the lamp will continue to operate with well matched conditions as the lamp ages. Such a self-tuned lamp system prevents the lamp from extinguishing due to the input power being out of resonance with the lamp system. A self-tuned lamp also allows wider manufacturing tolerances, thereby raising yield and lower manufacturing cost.

Table 1 bulb ID bulb wall quartz bbuullbb v weight energy to time to reach

(inner thickness volume (gram reach T final T final diameter) (mm) (mm³) (Joules) (seconds)

6 0.25 30.7 0.068 74 4.1

0.4 51.5 0.113 124 6.9

0.45 58.9 0.130 142 7.9

0.5 66.5 0.146 160 8.9

1 155.0 0.341 373 20.7

1.5 268.6 0.591 646 35.9 Table 2 bulb OD bulb wall quartz bulb weight energy to time to reach

(outer thickness volume (grams) reach T final T final diameter) (mm) (mm³) (Joules) (seconds)

7 0.25 35.8 0.079 86 4.8

0.4 54.8 0.121 132 7.3

0.45 607 0.134 146 8.1

0.5 66.5 0.146 160 8.9

1 114.1 0.251 275 15.3

1.5 146.1 0.321 352 19.5

Table 3 bulb OD bulb wall quartz bulb weight energy to time to reach (outer thickness volume (grams) reach T final T final diameter) (mm) (mm³) (Joules) (seconds) 6 0.25 26.0 0.057 63 3.5

0.4 39.5 0.087 95 5.3

0.45 43.6 0.096 105 5.8

0.5 47.6 0.105 115 6.4

1 79.6 0.175 192 10.6

1.5 99.0 0.218 238 13.2

Table 4 bulb OD bulb wall quartz bulb weight energy to time to reach (outer thickness volume (grams) reach T final T final diameter) (mm) (mm³) (Joules) (seconds) 5 0.25 17.7 0.039 43 2.4

0.4 26.7 0.059 64 3.6

0.45 29.4 0.065 71 3.9

0.5 31.9 0.070 77 4.3

1 51.3 0.113 123 6.9

1.5 61.3 0.135 147 8.2

Claims

CLAIMSWhat is claimed is:

1. A self-tuned system including a gain element and an RF-driven load configured as an oscillator, wherein the RF-driven load is a frequency determining element of the self-tuned system and wherein the oscillator oscillates at a frequency greater than 300 MHz.

2. The apparatus of claim 1 , wherein the oscillator-driven load comprises a lamp head circuit of an electrodeless lamp.

3. The apparatus of claim 2, wherein the electrodeless lamp is one of an inductively coupled electrodeless lamp and a capacitively coupled electrodeless lamp.

4. The apparatus of claim 2, wherein the lamp head circuit of the electrodeless lamp comprises: an excitation coil proximate an envelope enclosing a fill, the fill forming a plasma discharge when ignited; a high power capacitance connected to a first end of the excitation coil; a matching capacitance connected to a second end of the excitation coil and to electrical ground.

5. The apparatus of claim 4, wherein the oscillator comprises: an amplifier having an input terminal and an output terminal, the output terminal of the amplifier being connected between the high power capacitance and the matching capacitance; a tap connected to the excitation coil; a feedback element connected between the tap and the input terminal of the amplifier.

6. The apparatus of claim 5, wherein a junction of the tap to the excitation coil is positioned to provide sufficient current in the excitation coil so that the lamp self-ignites.

7. The apparatus of claim 5, wherein the amplifier is a two stage amplifier having a first amplification stage and a second amplification stage, the first amplification stage having a lower gain than the second amplification stage.

8. The apparatus of claim 4, wherein the oscillator comprises: an amplifier having an input terminal and an output terminal, the output terminal of the amplifier being connected between the high power capacitance and the matching capacitance; a third capacitance connected between the matching capacitance and ground; a feedback element having a first end connected between the matching capacitance and the third capacitance and a second end connected to the input terminal of the amplifier.

9. The apparatus of claim 8, wherein the amplifier is a two stage amplifier having a first amplification stage and a second amplification stage, the first amplification stage having a lower gain than the second amplification stage.

10. The apparatus of claim 4, wherein the oscillator comprises: an amplifier having an input terminal and an output terminal, the output terminal of the amplifier being connected between the high power capacitance and the matching capacitance; a pick-up coil arranged co-axially to the excitation coil, the pick-up coil having a first end and a second end, the first end of the pick-up coil being connected to electrical ground; a feedback element connected between the second end of the pick-up coil and the input terminal of the amplifier.

11. The apparatus of claim 10, wherein the amplifier is a two stage amplifier having a first amplification stage and a second amplification stage, the first amplification stage having a lower gain than the second amplification stage.

12. The apparatus of claim 4, wherein the lamp head circuit further comprises means for varying the matching capacitance in accordance with a control signal, the control signal being indicative of a stage of operation of the electrodeless lamp.

13. The apparatus of claim 4, wherein the envelope has a low quartz mass.

14. The apparatus of claim 1 , wherein the oscillator comprises: an amplifier; a feedback system connected between the oscillator-driven load and the amplifier; and a clamping circuit which limits a feedback signal applied by the feedback system to a region tolerated by the amplifier.

15. The apparatus of claim 14, wherein feedback system comprises plural segments having different impedances, and wherein the clamping circuit is connected to one of the plural segments which has an impedance that overcomes limitations of an element of the clamping circuit.

16. The apparatus of claim 14, wherein feedback system comprises plural segments having different impedances, and wherein the clamping circuit is connected to one of the plural segments which has an impedance that overcomes high dynamic impedance of a Shottky diode comprising the clamping circuit.

17. The apparatus of claim 14, wherein the feedback system comprises dual feedback paths, each of the dual feedback paths comprising plural segments having different impedances, wherein a first clamping circuit is provided for a first of the dual feedback paths and a second clamping circuit is provided for a second of the dual feedback paths, and wherein for each of the dual feedback paths the respective clamping circuit is connected to one of the plural segments which has an impedance that overcomes limitations of an element of the clamping circuit.

18. An electrodeless lamp comprising: a lamp head circuit comprising: an excitation coil proximate an envelope enclosing a fill, the fill forming a plasma discharge when ignited; a high power capacitance connected to a first end of the excitation coil; and a matching capacitance connected to a second end of the excitation coil and to electrical ground; and an oscillator for driving the lamp head circuit, the oscillator comprising: an amplifier; a feedback system connected between the oscillator-driven load and the amplifier; and a clamping circuit which limits a feedback signal applied by the feedback system to a region tolerated by the amplifier.

19. The apparatus of claim 18, wherein the feedback system comprises plural segments having different impedances, and wherein the clamping circuit is connected to one of the plural segments which has an impedance that overcomes limitations of an element of the clamping circuit.

20. The apparatus of claim 18, wherein the feedback system comprises plural segments having different impedances, and wherein the clamping circuit is connected to one of the plural segments which has an impedance that overcomes high dynamic impedance of a Shottky diode comprising the clamping circuit.

21. The apparatus of claim 18, wherein feedback system comprises dual feedback paths, each of the dual feedback paths comprising plural segments having different impedances, wherein a first clamping circuit is provided for a first of the dual feedback paths and a second clamping circuit is provided for a second of the dual feedback paths, and wherein for each of the dual feedback paths the respective clamping circuit is connected to one of the plural segments which has an impedance that overcomes limitations of an element of the clamping circuit.

22. An oscillator system, comprising: a power oscillator; a starting oscillator; a tank circuit receiving power from both the power oscillator and the starting oscillator and providing feedback to the power oscillator; and a power sense circuit providing power to both the power oscillator and starting oscillator, wherein the starting circuit is configured to be better matched to the tank circuit when the tank circuit is cold, the power oscillator is configured to be better matched to the tank circuit when the tank circuit is hot, and the power sense circuit is configured to turn off the starting oscillator when the power oscillator draws high power.

23. The apparatus of claim 22, wherein the tank circuit comprises a lamp head circuit of an electrodeless lamp.