WO2002030160A1

WO2002030160A1 - A power supply for driving a discharge lamp

Info

Publication number: WO2002030160A1
Application number: PCT/GB2000/003836
Authority: WO
Inventors: Colin Julian Seymour
Original assignee: Central Research Laboratories Ltd
Current assignee: Central Research Laboratories Ltd
Priority date: 2000-10-06
Filing date: 2000-10-06
Publication date: 2002-04-11
Anticipated expiration: 2003-04-06

Abstract

A power supply for driving a discharge lamp comprises a half bridge inverter having an external gate drive circuit, the output of the two transistors of the inverter are coupled together via a transformer, and are connected to the lamp coil and capacitor in use.

Description

A POWER SUPPLY FOR DRIVING A DISCHARGE LAMP

The present invention relates to a power supply for driving a discharge lamp. It relates particularly, though not exclusively, to a power supply for driving an inductively coupled discharge lamp, such as for example an electrodeless lamp.

A known electrical power oscillator designed to drive inductively coupled lamps has previously been described in GB 2,322,019, and is essentially as shown in Figure 1. Although this circuit is very efficient, there are areas in which improvements are desired, mainly for cost and size reduction:

1. Wound components: there are 4, plus the lamp winding. These are all custom- made, except possibly LI, therefore introduce significant costs and PCB area requirements.

2. The circuit must be adjusted by C_adj. This component is relatively expensive compared to a fixed capacitor, invokes production costs, and is susceptible to drift, for example if contaminated by sealant. Eventually excessive ageing can occur shortening the lifetime of the circuit. It is desirable to eliminate the tuning capacitor or allow the use of a trimming resistor instead, which is.less costly. US 5,519,285 "Electrodeless Discharge Lamp" cites 25 patent references.

These patents mainly refer to the electrodeless lamp itself and methods for coupling RF energy into the plasma via an externally wound coil or coils.

Circuits for electrodeless discharge lamps differ somewhat from those for electroded lamps. US 5,252,891 describes an "uninterruptible fluorescent lamp circuit available for emergency lighting" comprised of a circuit which generates a high-frequency, high-voltage power, and a current-limiting circuit which can provide enough high-voltage power to activate the lamp without the need to use any conventional starter and stabiliser. Electrodeless lamps must be driven by an external coil, and to obtain sufficient coupling of energy into the plasma, magnetic induction must be used since electric field coupling would require impractical voltage levels to couple equivalent energy into the plasma through the lamp shell.

Because of a frequency dependency, the use of magnetic coupling is not practical below a few MHz. Practical coupling circuits are probably limited at the low end to approximately 5 MHz. Frequencies of 10, 13.6 and 27.1 MHz are common. Higher frequencies (e.g. 2450 MHz) may be used but are not cost-effective for lamps of the order of 10 W power levels. US 5,852,339 "Affordable Electrodeless Lighting" describes a solid-state 2450MHz lamp (and see also US 4,070,630). For electroded lamps or low efficiency lamps, drive circuits may be operated at much lower frequencies corresponding to common practice in switching power supplies, i.e. up to about 1 MHz (for example US 4,712,170 describes an electroded neon tube power supply at 40kHz) or even at DC (for example US 5,675,220: "Power supply for vehicular neon light" describes a DC- DC converter utilising a Royer inverter circuit at 30 kHz to drive a neon bulb so as to avoid electromagnetic interference radiation from the bulb). The principal general requirements for an inductive electrodeless lamp driver circuit are therefore, as described in GB 2,322,019:

1. Higher frequency operation- order of magnitude or more (compared with lamps having electrodes).

2. High frequency devices: e.g. fast switching, low distributed gate resistance power FETs.

3. Circuit must accommodate the reactive component of the gate impedance at the driving frequency (this is far more important at higher frequencies of operation).

4. A suitable phase shift response must be provided in the oscillator's feedback loop.

5. The circuit must generate sufficiently large electric field to strike the plasma discharge, then maintain a sufficiently high (but controlled) current in the induction coil to maintain an inductive plasma discharge. According to the present invention, there is provided a power oscillator as specified in the claims.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic diagrams, in which:- Figure 1 shows a known circuit arrangement,

Figure 2 shows a circuit arrangement according to the present invention, Figure 3 shows an FET lamp driver circuit arrangement whose inputs are driven by the drive circuit of Figure 2.

Figure 4 shows a further circuit arrangement according to the present invention, and

Figure 5 shows a coupling coil suitable for use with the circuit of Figure 4.

The present invention is suitable for driving inductively coupled gas discharge lamps with air-cored coils, and due to the relatively low inductance and the frequency dependency of coupling between the magnetic field and plasma, the coils must be driven with frequencies of at least a few MHz. A frequency of 9- 10 MHz has been found a good compromise between the coil-plasma response which improves with frequency, and the capability of present-day low cost transistors to supply power, which declines with frequency. The circuit must handle the particular characteristics of inductively coupled discharge lamps, that is to say a resonant circuit must generate a high order of voltage multiplication (Q) to strike the gas discharge and must therefore be driven from a circuit which has a low source impedance throughout the full RF cycle, and not be damaged by the high voltages and currents present, and must also continue to supply drive power efficiently when the initial 'E' mode shifts to Η' mode (power transferred by magnetic field to effective circulating current in the plasma) which results in an increase in effective loss resistance in the lamp resonant circuit. In any oscillator, assuming that a feedback loop can be identified and broken at some point in order to measure energy flow as flowing from an output node on one side of the break to an input node on the other side of the break, the phase shift through the whole network must be zero, (or n times 360 degrees where n is an integer), and the gain magnitude must be 1. These are the

Barkhausen criteria. In practical circuits, the unity gain results from the amplitude of oscillation growing until non-linear compression causes the gain magnitude to fall to unity.

The circuit arrangement according to the invention employs power FETs not as a self-oscillating circuit but driven by an external gate driver circuit. This has the advantage of allowing the output coupling network to be optimised for maximum efficiency without loading by feedback coupling and without necessarily having to meet the restrictions placed on phase shifts by the Barkhausen criteria. The diagram in Figure 2 shows this external drive circuit.

An astable oscillator is used as a primary variable frequency source, from 8 to 30 MHz, and its output is divided in frequency by 2 to provide 50% duty cycle outputs at 4 to 15 MHz, and then supplies complementary outputs to power amplification stages. The pulse widths are individually adjustable using pre-set resistors VR2, VR3. The gates G5, G6 are provided to allow pulse width modulation of the oscillator via an external PWM input.

To improve frequency stability, a phase locking approach may be used to lock the master oscillator frequency to the phase shift in the lamp coil/capacitor resonant circuit. Ql, Q6 form the basis of fast Schottky saturated switch stages to increase the pulse amplitude to reference the Vdd voltage which should be between about twice the Vg threshold of 4V (i.e. Vdd>=8V), and Vdd<=20V which is constrained by the gate-source breakdown voltage - these figures apply only to the specific FET type shown. Sufficient current levels are obtained (of the order of 0.5 A peak) to drive the FET gate capacitance by high slew-rate current amplification stages Q2/Q3, Q7/Q8, Q4/Q5, and Q9/Q10. R4 and R5 suppress instabilities. R6, C3 is a DC blocking network that allows variable DC bias to be fed to the power FET gates elsewhere as required while providing (through R6,

1M) a DC path to allow the output of the pulse amplifier to set the DC bias point by default if there is no other DC bias source.

The diagram in Figure 3 shows the FET lamp driver circuit arrangement whose inputs are driven by the drive circuit of Figure 2.

Transistors Q_x and Q₂ provide active gain in push-pull mode. The resonant frequency (with which the FET inputs must be driven) is predominantly controlled by LI (lamp) and Cl. Diode Dl, D2, D3, D4 protect the gates against transient voltage spikes which must not exceed the Vgs absolute maximum limit. Transient voltage spikes may occur during switch-on or switch-off; but normally these diodes should not conduct. DC is supplied to the oscillator through bihlar RF choke T2 and power supply decoupling is provided by C4, C5 and C6. T2 couples the two FETs together such that the impedance seen at the drains is kept low throughout the cycle. T2 is air-cored to eliminate losses in ferrite (It was found that with an additional ferrite-core in T2, the light output decreased by approximately 4 percent). When Ql is off, the low impedance of Q2 (being on) is reflected via transformer coupling to Ql. This is essential in order for sufficient voltage multiplication to occur in the high-Q resonant circuit of Ll(lamp) and Cl, to provide high E fields to initiate an E mode plasma in the lamp. C2 and C3 provide compensation for the gate-drain capacitance Cgd of each FET. This produced approximately 7.5 percent increase in light output compared to the same circuit with C2, C3 omitted. The use of these compensation capacitors is not recommended for self-oscillating circuits as there is a tendency to oscillate at VHF which is suppressed under external drive conditions. A fixed bias circuit Rl, R2, D5 allows the DC bias on the transistor gates to be pre-set for optimum power output conditions. The inhibit function is included in the driver circuit.

A light output of 18700 lux was obtained close to the bulb for a 44% duty cycle at Vdd=12.8V, I(Vdd)ave=0.8A, and operating frequency of approximately 9 MHz. This compares with the prior art oscillator /lamp circuit of GB 2,322,019, which gave 11960 lux at close range for a 65% duty cycle at V_dd=12.8V, O

I(V_dd )ave=l.lA, and operating frequency of 9.3 MHz.

A further example of a circuit arrangement according to the present invention is shown in Figure 4. This circuit is similar to that of Figure 2, and uses the same external driver, but differs in the output coupling network between the FETs and the lamp. In Figure 2 the output was by direct coupling of the lamp LC network between the two drains, with DC supplied via a bifilar wound inductor. In this second circuit, the bifilar wound inductor is eliminated by the use of an inductively coupled winding on the lamp coil former. By doing this it is believed that a cost saving may be possible.

To improve frequency stability, a phase locking approach may be used to lock the master oscillator frequency to the phase shift in the lamp coil/capacitor resonant circuit, as before. Such a method is described in our co-pending application reference PQ 12,885E. The diagram in Figure 5 shows the mechanical details for the output coupling coil for the circuit arrangement of Figure 4. Lie consists of 12 turns of 0.4 mm diameter PTFE covered wire (e.g. BICC size 1/0.40 ref. BSG210), close-wound on a 28.6 mm -diameter gas discharge lamp envelope using a former, for experimental use only, of 100 micron thick plastic film as commonly used for overhead projection. Lla and Lib are each one turn (i.e. two turns close-wound centre-tapped) of the same wire positioned with a gap of between 5 and 6mm from the end of the main lamp coil. It is important to keep the lead lengths between FETs and lamp coil proper as short as possible; in the prototype, the leads to the FET drains were 10 to 15mm long, and the centre-tap was a length of about 5mm twisted into a pigtail, soldered along its length, and soldered to two lOOnF ceramic capacitors with their other leads connected directly to the ground plane.

A light output of 16750 lux was obtained close to the bulb for a 65% duty cycle at Vdd=12.2V, I(Vdd)ave=l.lA, and operating frequency of 6.96 MHz. The invention can provide the following potential advantages:

• Eliminating need for purpose built wound components or tuning capacitor.

• Interfaces to the same power supply voltage rail and pulse width modulation inputs as previous EDL circuits.

• If a phase locking method is used the self-oscillating frequency is controlled by lamp tuned circuit and hence is more tolerant of drift in component values over time (compared with the sensitivity of the previous circuit to drift in the gate circuit tuning capacitor C_adjand related wound components L_ad)andT₂). • Lowered cost of manufacture.

• More easily modified for different types and sizes of lamp envelope.

Claims

1. A power supply for driving a discharge lamp, comprising a half bridge inverter having an external gate drive circuit, the output of the two transistors of the inverter are coupled together via a transformer, and are connected to the lamp coil and capacitor in use.

2. A power supply as claimed in claim 1 having an operating frequency of between 3 and 30 MHz.