Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Silicon controlled rectifier ignition
Patent
1999-09-07
2000-12-26
Wong, Don
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Silicon controlled rectifier ignition
315284, 323214, 323215, H05B 3702
Patent
active
061664944
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a drive circuit arrangement for a gas discharge lamp, comprising a self resonating inverter for supplying a drive voltage across a load having an impedance with a reactive component, the inverter having a pair of field effect transistors which operate in anti-phase iri a first resonant circuit. It relates particularly, though not exclusively, to a drive circuit arrangement for inductively coupled backlights for displays, hazard beacons or vehicle signals.
2. Background of Related Art
Inductively coupled backlights having air cored coils must be driven at frequencies in the Megahertz range to avoid excessive coil losses. At these frequencies the gate impedance of suitable field effect transistors (FET's) is low, so that the gate drive currents are high in consequence. To minimize the drive power a resonant tank circuit may be provided which cancels the reactive component of the gate impedance at the driving frequency, and makes the gate input impedance look resistive.
A difficulty with known drive circuits occurs when the inverter is supplied by a d.c. voltage which is not constant--for example when supplied from a vehicle battery or the like. It has been observed that optimum circuit efficiency is not obtained under such conditions.
EP-A-0 696 157 discloses a low frequency ballast circuit for an arc discharge lamp. A difficulty with such known drive circuits relates to the loop phase shift of self-oscillating circuits having integral loads. A stable oscillator has a loop phase shift of either 0 or a multiple of 2.pi.. Since any combination of lossless components has a phase shift of either 0 or .pi., and since the load, being resistive, contributes a phase shift which is neither 0 nor .pi., an extra phase shift is required which may be, for example, .pi./4. A problem is how to provide this phase shift without incurring extra losses.
A third difficulty relates to discharge lamps as loads in a resonant circuit. Before the discharge strikes the lamp acts as a high impedance and the resonant circuit will have a high Q factor. The current and voltage from this resonant circuit can cause damage circuitry.
BRIEF SUMMARY OF THE INVENTION
According to the present invention, there is provided a drive circuit arrangement for a gas discharge lamp as specified in the claims. This arrangement can minimize circuit losses.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a circuit diagram of a preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying circuit diagram of a preferred embodiment of the invention labelled FIG. 1.
Referring to FIG. 1, it shows a drive circuit arrangement for a gas discharge lamp, comprising a self resonating inverter for providing a drive voltage across a load. The inverter is constituted by field effect transistors 1 and 2 which operate in anti-phase together with a first transformer comprising inductances 3, 4 and 5, the load 6 comprising an impedance having a reactive component and a capacitor 7 which forms a resonant circuit together with the load, components 8 and 9 which form part of a feedback loop and gate coupling components 10, 11 and 12. These gate coupling components comprise a tank circuit for cancelling the reactive component of the impedance of a load (6) coupled to the inverter in use. A voltage division circuit comprising capacitors 7 and 8 provides a given fraction of the said drive voltage to the gates of the field effect transistors 1 and 2 in use. Phase shifting means for shifting the phase of the given fraction of the said drive voltage is provided by the inductance 9 feeding into the gate tank circuit. Diodes 13 and 14 comprise a voltage limiter which acts to restrict the peak resonant voltage as explained below. The circuit arrangement of FIG. 1 is also provided with an autobias circuit 15 which biases the transistor gates positive until the o
REFERENCES:
patent: 5574335 (1996-11-01), Sun
patent: 5781418 (1998-07-01), Chang et al.
patent: 6040661 (2000-03-01), Bogdan
Bollman William H.
Central Research Labs, Ltd.
Tran Thuy Vinh
Wong Don
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