GB2356081A - Electrodeless discharge lamp having self-resonant filter choke - Google Patents

Abstract

Low pressure electrodeless discharge lamp having Kr or Ar at pressure between 0.2-0.6 Torr. The lamp may have one or more ferrite toroidal cores mounted externally of the discharge and an RF coupler having a coupling efficiency of greater than 97%. Starting electrodes may be located beneath the toroidal core. The bulb is shaped to hide a significant part of the ferrite core from view and is compact. There is a small gap between the ferrite core and the glass bulb which serves to thermally insulate the ferrite core. The core is composed of two or more pieces of ferrite material. Electromagnetic interference (EMI) is minimised by installing a self resonant choke into the wires that carry the AC line current to the power supply electronics of the lamp. The choke is composed of two windings wound about a single ferrite core assembly.

Description

2356081 ELECM10DELESS DISCHARGE LAMP AND SELF-RESONANT FILTER CHOFE The

present invention relates to an electrodeless discharge lamp and selfresonant filter choke. Although patents for low pressure inductive discharge lamps using a ferrite core have been issued as long as 60 years ago (Bethonod, Patent # 2,030,957), most -patents granted in this area came generally after about 19"0. It is interesting to note that from all these granted patents, to our knowledge, only one commercial product has been made (PhWips "QLinduction lamp) from them. The general reason for the high ratio of granted patents to actual products is that almost all of the patents in this area have one, or more, practical problems that have kept them from becoming a product. Typical practical problems found in these patents are: 1) high lamp cost, 2) difficulty in manufacture, 3) overheating 4) electromagnetic interference (ENE), 5) unattractiveness, 6) low systern efficiency, and 7) poor maintenance. It is the intention of this invention to describe an electrodeless 1anip package that mitigates many of the problems listed above.

This invention differs fi-orn. previous inventions in the following respects: 1) the discharge bulb is intentional.]y shaped to be attractive and compact yet to hide a significant part of the ferrite core from view, 2) a small gap is introduced between the ferrite core and the glass bulb, 3) the primary induction coil appears electrically symmetric (because it is wound back upon itselo and 4) the ferrite core was chosen specifically so as to have decreased loss with increasing temperature.

The major effects of these differences are to reduce EMI, to improve the aesthetic3 of the discharge bulb, to reduce thermal problems, to reduce core loss and to improve maintenance of this device. These advantages are discussed in detail in what follows.

2 Detailed DesCription

One embodiment of the invention claimed here is shown in Figs. I (a & b). In this embodiment the bulb shape is such that two ferrite "U" cores.0) can h-- fitted into -Im limp structure forming a closed magnetic path that is encircled by the discharge plasma. In this configuration the "U" cores are mostly hidden from view. The hole in the-center of the discharge tube is slightly larger than the ferrite core forming an air gap b etween the ferrite care and the glass. The air gap serves to thermally insulate the ferrite core from the glass etwelcpe (2) which is hEted by tin plwm. Altbm;h tr=re as __cm cm h, dn to dissipation of power in the ferrite core, the major part of the core heating comes from the plasma heating the glass surface and that heat being conducted into the core. On the outer surfaces of the lamp this heating results in a relatively sni&U temperature rise due to convective cooling from the ambient air. Within the hole thmugh the center of the Larnp however, because the power density in these lamps is so high, the surface temperature can easily reach about 200C, or more, Thus, it is imperative to thermally isolate the ferrite core from the glam to prevent ferrite overheating andr hing of the ferrite!s Curie temperatum Postma ( Pat # 4,536,675) has successfally dealt with the issue of conducting heat away from a ferrite in a re-en=t cavity geometry. In the Postma lamp the ccre is a ferrite tube (which forms an open magnetic path) and heat is removed. from the ferrite with a copper rod or tube with a liquid coolant that conducts the heat fi-orn the ferrite to an external heat sink. In the lamp shown here ferrite overheating is avoided through a simpler and cheapr approach. Two specific things are done here: First, to minimize the tempm-ature of glaS3 which surrounds the ferrite, the internal surface of this glass is coated with material that is reflective to UV and visible radialion. Taus, th only beating on this surface is that of the plasma recombination and conductive heating from the hot gas that surrounds the in= rube. Second, about a one mm air gap is inn-oduced between the core and the glass surfke.

Although theferrite Material is not a particularly good conductor of heat, there is sufficient thermal resistance between the glass and the core so that the core temperature is considerably less than the local glass surface temperature.

The air gap slightly reduces the coupling coefficient and the discharge volume but these effects are heavily offset by the decrease in core temperature. Incidentally, some heating of the ferrite can be beneficial since loss in the ferrite decreases up to about 110 C The ferrite chosen for this work are those where this indeed the case. The ferrite used in this lamp was type "R" ferrite material, which has a relative permeability of about 2700. In general, any high permeability, manganese 2inc ferrite with low loss at the operating frequency can be used to make such a lamp.

In this specific embodiment the lower indentation which hides the lowim part of the ferrite also has a slight gap between the glass and the ferrite. Tlis gap thermally insulates the ferrite ftom the discharge (as does the center hole) but it also may serve another purpose depending on how the primary coil is wound on this leg of the ferrite. If the primary winding is a simple one layer coil, this gap also effectively significantly reduces capacitive coupling and thus eliminates the resulting capacitive discharge that niight have otherwise occurred between the two ends of the primary coil.

There is also a smaU gap between the lamp body and the ballas:t compartment to reduce heat flow from the discharge into the electronic components in the balut (3). 9Ms essmtiaLly serves to ' de-couple the heat of the discharge from the heat generated by the driving circuiL 7bis is very important to the circuit reliability since component reliabi1ity generally decreases as component temperatures increase.

-4 Starting of this lamp is provided by winding a primary coil around one leg of a "U" core and winding a second identical "starting" winding about the adjacent leg of the other "U" core. Essentia Ily the two symmetrical coils are wound around the core in the same direction are then wired such that the potential at each end is out of phase with t I he other end (pushpull). Thus the potential across the entire coil (both windings) is symmetric with respect to ground and for a voltage V applied to the primary, a voltage 2V appears across- the entire coil and thus the potential 2V is available for initiating starting. The starting potential is applied to copper foils wrapped around the two rubulations extending from the lower part of the lamp body (see Fig. 1, for ex.). The specific operation and advantages of this starting method is discussed in another disclosure. The electrical and physical arrangement for one embodiment of the coil assembly is shown in Figure 2.

It is essential that the primary and the starting windings are wound identically on the ferrite legs and connected so as to appear symmetrical with respect to ground. This has a very important consequence in thw the potential (and displacement current) from one end of the coil is just opposite to the potential (and displacement current) from the other end of the coil thus the total of the two currents is practically zem. This has the effect of minimizing RF potential of the discharge and minimizing conducted ENE currents. The conducted EW is low in these lamps for two reasons: a) the use of a low frequency to drive the discharge and 2) the electrically symmetric configuration of the coil; the radiated EM is pwtically elimirmted due to the closed magnetic path formed by the ferrite cores.

A second embodiment of this principle i3 shown in Fig. 3. In this figure the "U" of the cures (1) is vefticaUv a:iat-ated raU-r=r thEn hxizatany as in Fig. 1. Tm qpamticn cf this larrp is VirtUaLl).7 th-- sarre as that picb.3re in Fig. 1.

Another essential feature of this lamp is that the ferrite core be shielded by a reflective coating applied to the inner surface of the discharge vessel. The coating is usually made up of find grain aluminum oxide which form a scattering type reflector. Ilermocouple measurements in such a device show that the reflector coating substandally reduces the temperature of the glass surfaces that faces the ferrite core.

Detailed measurements have been perforrned with different versions of this lamp to determine the lamp discharge voltage, core loss data and core temperature. Data for the embodiment shown in Fig. I is given in Figs. 4 7. 7be data shown bere was measured for a lamp gas fill of Hg and a 0-5 Torr Krypton buffer gas. The lanip body diameter was 2-5" and its length was 2.5". Ballast efficiency was about 92% conversion from the AC line to the lamp 'driving frequency which was about 200 KHz. Fig. 4 shows the primary voltage as a function of power. The actual discharge voltage is about 20 times less dian. the primary voltage. Note that this results in a inductive discharge voltage of about 3.5 volts at 20 watts. Fig. 5 shows core loss as a function of total power delivered to the ferrite core. For a given material, core loss is a function of core flux density and core temperature. Fig. 6 shows power tr=sfer efficiency, that is, the ratio of power dissipated in the discharge to total power supplied to the ferrite core."Ibe efficiency above 20 watts is between 95%. and 97%. The temperatures at various places in this lamp is given in Fig. 7 for a lamp with no reflector coaling around the core. Note that 2t about 20 watts the ferrite core in the center of the lamp is about 125 C and the lower part of the ferrite core is about 100' C while at 30 watts the temperature of these two points is 165 and 120' C, respectively. As shown in Fig. 5 this range (between 20 and 30 watts of total power) corresponds to a region of minimum core loss.

A fm-dr=r eTbodurert, t1mt is a toroidal 1-rip with cm, two or tIzee fa=te toroiclal cxes (4), as RF oapkr, is shxn in Fig. 8.

Embodiment OD Outer Diameter: 106 mm Fill: Kr 0.3 Turr ID Inner Diameter: 30 mm Hg: 5 mg f5 mg) Glass bulb T12 (38 mm) Cdating: likc cmpEct fhnrescErt larps Discharge length S 18 cm Narrowest -lass bulb diarneEer: 20 mm C Tijlroidal 36 x 23 x 15 mmA, = I CM2 V, = 9.3 cm-1 Mat. H 325 (-VOGI) 7- The discharge voltage UE = LE - 0.4 V/cin 18 - 0.4 7.2 V Radio frequency primmy winding on toroidal core: N 12... 15 tums Neccss ary RIF supply voltage of the RF coupler U I = k - Up - N Ui = 1.05 - 7.2 - N = 90--.113 VIO Operating frequency: f, =- 400 kHz (2(X)...500 kliz) U.104 113-10" Inducdoninferritecorv. B = I = 42.4, rnT 4.44-fo-A,-N 4.44-400- Is 15 Specific core losses: at 42.4 mT and f = 4CO kHz Pv = 70 jnW/cM3 Core losses: P& = V. - Pv = 9.3 - 0.07 = 0.65 W Photaxnaric values: OL = 2000 lwnen Discharge power: PE = 22W Discharge efficiency: nE = 90-9 1MfW Losses in RF coupler P,,-Fc = Pt. + P., = 0.65 + 0.1 W Lamp power: P1. Pr, + Pac = 22 + 0.65 + 0.1 = 22-75 W Luminous cfricicncy of lamp: IIL OtJPL 2WO/22.75 = 87.9 1mIW EVO (Electronic ballast): Pew 2 W System mains power: PY. FL + Pzvo = 22.75 + 2 = 24.75 W System efficiency; T4 = Py;' OL = 2000/24.75 = 80.8 Irn/W.

A furt'her object of the invention is to disclose a self-resonant filter choke to reduce conducted line noise.

Electromagnetic interference (emi) from a high frequency (HF) circuit generally originates in two ways: either through conducted line noise -or through electromagnetic radiation. The intent of this disclosure is to address the conducted line noise aspect of emi. Conducted line noise can be classiflied as either common mode or differential mode. 7be device shown in this work can significantly reduce both types of conducted line noise from a HF power supply- in most modern electronic equipment, conducted line noise is reduced to an acceptable level by attacking the problems of differential mode noise and common -mode noise individually. Differential line noise generally results from some component of a HF generator finding a 0 way back through the circuitry and onto the power line. If this type of line noise cannot be reduced satisfactorily in the power circuitry itself it is generally reduced by installing a pinetworlI ow pass filter in between the line input and the rectifier circuitry. Common mode conducted line noise occurs when an HF voltage is generated and there is a current (displacement or conductive) path from a point of BF voltage to some point outside the sy.stem from which.there is a current path back to the power line. Common mode line noise is generally eliminated from many circuits by simply surroundina the circuit which contains Lhe BF voltage with a metal sheet, i.e., by shielding the HF circuit.

C: 0 In a HF lighting device, a high voltage generally is present somewhere in the lamp bulb and shielding the bulb from common mode conductive line noise can be accomplished by.surrounding the bulb with a metal sheet, howeve.r. this defeats the purpose of the lamp cince in this case no licht would be einitted outside the metal enclosure! 1nstead of a metal sheet, a screen capacitively coupled to the line might be emploved, however, this solution;,s far from perfect from the aesthetic and the practical point of view. Another approach to 9 solve this problem is to apply an optically transparent conductive coatino to a lamp bulbs 0 inner (or outer) surface. This can and is being, done in some instances but it requires special lamp processing steps to create the conductive coating and to electrically connect the coating to the AC line.

The invention described here reduces conducted line noise from a BF lamp by instilling a self-resonant choke into the wires (2) that carry the AC. line current to the lamps power supply electronics. The choke is composed of two windings which are of sufficiently low inductance to practically be a short circuit at the power line frequency (6011z). The two windincys of the choke have the same number of turns and are wound similarly about a single ferrite core assembly. They are wired such that the magnetic flux density induced into the core by 60 Hz current flow through one coil winding essentially cancels the maone tic flux density induced by current flow through the other winding. 71e self-resonant frequency of each winding is chosen to be at or near the frequency of the common mode current (to be suppressed). A separate choke is required for each frequency to be suppressed.

Essentially, each of the coil windings forms a parallel resonant circuit consisting of the coil in parallel with the distributed capacitance of the coil windincy itself. At resonance the impedance of the common mode current path through the AC lines (and the filter choke) is high and the common mode current (and emi) is low. It should be appreciated that a resonant or near resonant condition is vital to this approach because it is the (parallel) resonant condition thatr'aises the coil impedance from WL to Q(OL, where Q is the coil Qfactor, co is the resonant frequency in radians an d L is the choke inductance. A selfresonant choke not only allows one to reach a relatively high impedance (Qo)L) with a physically small packa2e but also allows one to reach a hi aher impedance than might be reached with a very lar2e inductor because of the inherent stray capacitance of large inductors. The conducted line noise emi reduction scheme suggested here is applicable in a frequency range where QwL is appreciable in comparison to the stray reactance (mocstay). -1be greater the value of QcdL the more effective this approach will be. It is readily applicable at 2.6 MHz as demonstrated below.

A schematic diagram of the resonant filter in the AC line and the path of common mode current is shown in Fig. 9 The magnitude of the HF.common mode current depends on the L- 4:

impedance of the current path from the hig ,h voltage point on the bulb (5) tircugh th-- strW capacitance and the filter to the HF generator circuit (6). IF thim is m MtEr thm th-- aram mode current is simply the HF voltage dividedby the reactance of the stray capacitance but with the filter in the circuit the common mode current is the HF voltage divided by the total reactance of the return path. Thus, the filter reduces common mode current (and thus common mode line noise) by the ratio of the impedance of the entire circuit (filter impedance plus stray impedance) divided by the stray impedance. One can show that the Tf voltage on the ac line with this resonant filter is reduced G times where G = QCsumy/CD, Cstmy is the total lamp to ground capacitance and Co is the total capacitance ofthe filter (distributed capacitance across the coil winding). To reduce differential mode line noise, two capacitors are placed across the line so as to form a pi-type low pass filter network as shown in Fig. 9.

To demonstrate this approach, two coils with about 90 turns each were wound around a COB form and a typeW2135 pot core (.5" OD,.3125" width) made of 3F3 material was fitted about the coil form with a small air gap between the halves of the pot core. An air gap was introduced between the two pot core halves so that the final inductance of the coils would be relatively insensitive to changes in core permeability. The reactance of each winding was about 330 uH and the distributed capacitance of the winding itself was about 9 pF. The cincuit used to demonstrate the effectiveness of this technique for common mode emi is shown in Fig. 10-rhe filter was intended to reduce common mode emi at 2.6 MHz (the self resonant frequency of each winding in the filter choke). 7be 6 pF capacitance (7) sbam in series with tIn filter (8) was dram to rEpresat StEW UPEXatz-m f ran the high w1tage-, paint that finds its way back to the power lines. Voltage applied to the 6 pF capacitor represents the BF voltage causing the common mode line noise. Voltage applied to the 6 pF capacitor was compared with voltage measured across the 50 ohm viewing resistor (9) between a frequency of I MHz and I I MHz. Tle 50 ohm viewing resistor is representative of the circuit usually used to determine line conducted emi.

Fig.11compares the voltage on a 50 ohm viewing resistor with the source voltage driving' the circuit for two cases: with and without a filter. The relative magnitude of ratio of the voltages with no filter is irrelevant and simply represents the voltage division between a 6 pF and a 50 ohm resistor over a frequency range of I to I I MHz. What is important to note is that the resonant filter provides almost 28 dB (more than 20 times) of common mode line noise reduction at 2.6 MHz which is assumed to be the frequency of the HF generator. Ilus at this frequency the common mode signal would be 28 db less than it would have been without a filter. No doubt a better filter could be built by a professional filter designer but this simple de.monstration illustrates the principle of the concept disclosed here. IncidentaBy, Roll also shows that the line conducted emi is enhanced by the filter in a Z frequency range about 2.1 MBz. 7bis is just a manifestation of the series resonance between the filter choke (with its distributed capacitance) and the stray capacitance. This behavior suggests that the distributed capacitance of the coil should be as small as possible with respect to the stray capacitance to insure that the the series resonant frequency is always removed from the parallel resonant frequency of the coil windings of the choke.

in summary, a very compact device has been built and tested that shows a significant

C reduction in the common mode line emi at a given frequency. Although not shown, it also 12 - is effective in reducing differential mode line noise. With such a device it may be possible to reduce emi to acceptable levels without external shielding. In any case the method proposed here wiU reduce (and in some cases practically eliminate) conducted line noise emi from high voltage circuits that cannot be external]y shielded.

Claims (6)

1. CLAIMS:

   I An electrodeless lamp with low buf f er gas pressure Kr, Ar in range 0.2-0.6 Torr.
2. 2. A lamp as claimed in claim 1, wherein the energy is coupled in by one or more ferrite toroidal cores mounted externally of the discharge on a narrow portion of the discharge bulb.
3. 3. A lamp as claimed in claim 1 or 2, wherein the operating frequency and the magnetic loading of the RF couple r are selected such that the losses of the RF coupler are at a minimum, whereby a coupling efficiency of higher than 97 is achieved.
4. 4. A lamp as claimed in any preceding claim, wherein the lamp dimensions are so selected that a thermal balance is attained in continuous operation, that is optimal temperature for cold spot.
5. S. A lamp as claimed in claim 2, wherein starting electrodes are located underneath the toroidal core which are supplied with the required ignition voltage by the RF coupler coil.
6. 6. A lamp as claimed in any preceding claim, wherein the system - as described -.is suitable for lamps of IOW to 100OW, with corresponding dimensioning of the discharge bulb and of the RF coupler.