Symmetrical RF power supply for inductively coupled electrodeless lamps

A radio frequency (RF) power supply for an electrodeless lamp includes a pair of DC rails, an RF inverter having power input terminals connected between the rails, a first inductor arranged to inductively couple with an electrodeless lamp, first and second resonance capacitors that each connects a respective one of two input terminals of the first inductor to a same first rail of the pair of DC rails, and a second (ballasting) inductor connecting an output of the RF inverter to one of the two input terminals of the first inductor. Thus, the first inductor is connected in a symmetrical π-filter and supplied by two equal but phase-opposite voltages whose sum is the lamp voltage. The inductance of the ballasting inductor is significantly reduced so that the RF efficiency of the power supply is 96%.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of U.S. Provisional Application 60/928,603 filed May 10, 2007, and PCT Application No. PCT/US08/61867 filed Apr. 29, 2008, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The present invention is directed to a radio frequency (RF) power supply for operating an electrodeless lamp, such as a fluorescent, molecular, or high intensity discharge electrodeless lamp. An RF power supply converts a DC voltage to a suitable radio frequency for the lamp and is typically part of the electronic ballast of the lamp. The RF power supply includes a ballasting inductor that is coupled to the electrodeless lamp to ignite and maintain the plasma in the lamp's discharge gas, without providing electrodes in the lamp bulb.

Because the complete electronic ballast includes numerous components in addition to the RF power supply (e.g., EMI filter, rectifier, PFC boost stage, DC bus electrolytic capacitors), the efficiency of the RF power supply is desirably 95% or more, which has not been achievable in a commercially available power supply. It has been found that one of the key factors in improving efficiency is reducing power loss in the ballasting inductor that is coupled to the lamp.

FIG. 1shows a known circuit for an RF power supply whose efficiency is about 91.7%. DC power source E delivers a DC voltage to a pair of DC rails, with a electrolytic capacitor (parasitic inductance) C0. During operation, first inductor L1is inductively coupled to lamp D. Transistors S1and S2are driven with a sinusoidal voltage (8-9Vp) delivered by driving transformer Dt that is tuned to a specific frequency (2.6 MHz) by capacitors CP, CG, and Ciss. Feedback capacitor Cicouples driving transformer Dt with the output voltage V1. Resonance capacitor CRis parallel to the first inductor L1and coupling capacitor CCconnects the output of the driving transformer Dt to one of the input terminals of first inductor L1through the ballasting inductor LL. The resonant circuit is tuned on a frequency fRS(about 2.45 MHz) that is slightly lower than the resulting operation frequency (f0≈2.5 MHz). This RF power supply has a 13.5 W loss, of which 7.8 W are attributed to the ballasting inductor LL. This circuit is further explained in U.S. Pat. No. 5,962,987. The particular parameters for this circuit are shown in Table 1 (inFIG. 5) that includes operating characteristics for RF power supplies of the prior art (FIGS. 1-2) and of the present invention (FIGS. 3-4) for a same set of input parameters so that results can be easily compared.

FIG. 2shows a variation of the circuit ofFIG. 1in which the voltage viewed by the half bridge (the voltage VGon CR) is reduced by inserting an additional capacitor CSin series with the first inductor L1, thereby avoiding the bulky coupling capacitor CC. This reduces the inductance of ballasting inductor LLand thereby reduces the losses in the ballasting inductor LL. The voltage drop on CSis VCS=I1XCS, which in this instance is about 190V. This reduces the viewed voltage VGon CRfrom 550V to 360V, which is a 35% reduction. This, in turn, reduces the inductance of ballasting inductor LLby 35% from 37 μH to 24 μH. The current in ballasting inductor LLcan also be reduced from 3.8 App to 3.4 App by reducing the phase angle between ILand the fundamental sine wave V0fcontained in the half bridge midpoint voltage, which is trapezoidal in consequence of ZVS. As a result, the loss in the ballasting inductor is reduced to about 4.4 W (with a further 3.6 W loss in transistors S1and S2) so that the total loss is 9.4 W, thereby increasing the efficiency from 91.7 to 94.1%. This circuit is further explained in U.S. Pat. No. 5,446,350. The particular parameters for the circuit ofFIG. 2are also shown in Table 1.

SUMMARY

An object of the present invention is to provide a novel RF power supply for an electrodeless lamp that has an efficiency of at least 95%.

A further object of the present invention is to provide a novel RF power supply for an electrodeless lamp in which the lamp's induction coil (the first inductor L1) is connected in a symmetrical π-filter to further reduce the loss in the ballasting inductor LL.

A yet further object of the present invention is to provide a novel RF power supply for an electrodeless lamp that includes a pair of DC rails, an RF inverter having power input terminals connected between the rails, a first inductor arranged to inductively couple with an electrodeless lamp, where the symmetrical π-filter includes first and second resonance capacitors that each connects a respective one of two input terminals of the first inductor to a same first rail of the pair of DC rails, and a second (ballasting) inductor connecting an output of the RF inverter to one of the two input terminals of the first inductor.

These and other objects and advantages of the invention will be apparent to those of skill in the art of the present invention after consideration of the following drawings and description of preferred embodiments.

DETAILED DESCRIPTION

With reference now toFIG. 3, in the present invention an RF power supply for an electrodeless lamp D includes a pair of DC rails receiving DC power from DC power source E, an RF inverter having power input terminals connected between the pair of DC rails (the inverter including driving transformer Dt and transistor switches S1and S2), a first inductor L1inductively coupled with lamp D, first and second resonance capacitors C1and C2that each connects a respective one of two input terminals of first inductor L1to a same first rail of the pair of DC rails, and a second (ballasting) inductor LLconnecting an output of the RF inverter to one of the two input terminals of first inductor L1. The RF inverter may be either a full bridge or a half bridge inverter.

Operating characteristics for the embodiment ofFIG. 3are shown in Table 1 (inFIG. 5) for the same input parameters asFIGS. 1 and 2so that a direct comparison can be made. The operating characteristics listed in Table 1 will be appreciated by those of skill in the art and need not be explained in detail. However, it should be noted that the loss in the ballasting inductor is reduced to 2.7 W (and the loss in switches S1and S2to 2.4 W) so that the efficiency increases to 96.0%

As is apparent, the lamps inductor, first inductor L1, is connected in a symmetrical π-filter and thereby supplied by two equal but phase-opposite voltages VC1and VC2. Their sum is the lamp voltage V1. Lamp current is the current in second resonance capacitor C2; i.e., I1=IC2. In the example with the input parameters from Table 1, the half bridge sees only half of V1(277V) and the second (ballasting) inductor LLhas only 18.4 μH. Continuing this example and with further reference to Table 1, the current IL=1.13 A is the vectorial sum of IC1=3.1 A and I1=2.25 A, but is the smallest one, which is 3.2 App. In this configuration with 2.7 W loss in second inductor LL, 2.4 W loss in switches S1and S2, 0.4 W loss in Dt, and 0.3 W loss in resonance capacitors C1and C2, the total loss is 6.3 W, so that efficiency reaches 96%.

This arrangement is particularly suited for electrodeless lamps with a low power factor (PF=cos φ1<0.2) because of the low magnetic coupling between the induction coil and the plasma. The suitability may also be enhanced by the low coil inductance and the low operation frequency.

The present invention affords a further advantage in that the HF potentials applied to the first inductor L1are halved so that the ion bombardment of the phosphors in the lamp are reduced fourfold. This provides a longer life for the lamp and reduces lamp maintenance. One additional advantage related to EMI suppression is that only half the RF potential is against ground, which eases the common-mode interference suppression within the lamp ballast. Thus, in some lamps, the E-field compensating bifilar induction coil can be avoided.

FIG. 4shows a further embodiment of the RF power supply of the present invention. In this embodiment, the resonance capacitors C1and C2are split and connected to respective DC rails. That is, the power supply includes third and fourth resonance capacitors that each connects a respective one of the two input terminals of the first inductor L1to a same second rail of the pair of DC rails (different than the rail to which C1and C2are connected in the first embodiment.) In a similar manner, the feedback capacitor Cican be split and connected to opposite rails. This arrangement reduces the high frequency ripple current in the electrolytic capacitor C0and eases once more the EMI suppression.

Further, a low-pass filter, including capacitor Cfand inductor Lf, can be added to filter the remaining interference at 2.5 MHz due to ESR so that the parasitic inductance C0can be filtered to make the RF power supply neutral from the conducted EMI point of view.

Significantly, the circuit ofFIG. 4also reduces the considerable losses in C0by 0.5 W so that the efficiency is yet further improved to 96.3%.

In a variation of the circuit ofFIG. 4shown inFIG. 6, a further capacitor C5is connected between a first node between the pair of feedback capacitors Ci/2 and a second node between resonance capacitors C1/2. The further capacitor C5is optional and can be used to reduce the dead time between the switching-ON gate controls of S1and S2(Q1and Q2inFIG. 6.)

The symmetrical topology of the present invention permits implementation of low loss and long lifetime by minimizing the amount of energy stored in the ballasting inductor, reducing ion bombardment by the lamp's induction coil, reducing the stress in the resonance capacitors, and lowering interference levels to ease EMI suppression.

While embodiments of the present invention have been described in the foregoing specification and drawings, it is to be understood that the present invention is defined by the following claims when read in light of the specification and drawings.