WO1989010679A1

WO1989010679A1 - Electronic ballast circuit for gas discharge lamp

Info

Publication number: WO1989010679A1
Application number: PCT/GB1989/000438
Authority: WO
Inventors: Luke Theodossiou
Original assignee: Active Lighting Controls Limited
Priority date: 1988-04-25
Filing date: 1989-04-25
Publication date: 1989-11-02
Also published as: GB2236921B; GB8809726D0; GB2236921A; GB9023237D0

Abstract

An electronic ballast circuit for a gas discharge lamp includes a driven inverter (52) the high frequency oscillatory ouput of which is supplied to a gas discharge lamp (164) through a current limited inductor (154) with a capacitor (160) in parallel with the lamp. A protection circuit includes an additional winding (152) on the inductor (154) which monitors the peak voltage and current of the lamp (164). The signal from the additional winding (152) is fed to a threshold detector and latch circuit (170) which operates to remove the drive from the inverter (52) and de-energise the lamp (164) if the voltage across the lamp exceeds a first predetermined value. Additional protection is provided by a voltage clamping circuit (166) which shorts out the additional winding (152) if the voltage across the lamp exceeds a second predetermined value higher than the first before the lamp is de-energised, thereby to damp the resonant circuit of the inductor (154) and capacitor (160). The additional winding (152) also supplies power for a low-voltage stabilised supply for parts of the ballast circuit. The ballast circuit can supply a number of lamps (164, 264, 364, 464) connected in parallel to the driven inverter (52), a respective inductor (154, 254, 354, 454) with additional winding (152, 252, 352, 452), and a respective capacitor (160, 260, 360, 460) being provided for each lamp.

Description

ELECTRONIC BALLAST CIRCUIT FOR GAS DISCHARGE LAMP

This invention relates to electronic ballast circuits for gas discharge lamps.

Electronic ballast circuits, of the kind comprising an inverter having an oscillatory output for connection to a gas discharge lamp, a current limiting inductor in series with the output of the inverter and a capacitor for connection in parallel with the lamp, are well known. The circuits operate at a frequency at or close to the resonant frequency of the circuit including the current limiting inductor and parallel capacitor, to apply a voltage to the lamp which is high enough to effect ignition. In order to avoid damage to the ballast and/or lamp and unsafe operating conditions, it is necessary to provide protection means for limiting the voltage applied to the lamp.

It is an object of this invention to provide an improved electronic ballast circuit.

This invention consists in an electronic ballast circuit for a gas discharge lamp, the circuit comprising an inverter having an oscillatory output for connection to a gas discharge lamp, a current limiting inductor in series with the output of the inverter, and a capacitor for connection in parallel with the lamp, in which there are provided protection circuit means comprising a monitoring circuit inductively coupled to the current limiting inductor and means for inhibiting the voltage applied to the lamp on receipt of a signal from the monitoring means if the voltage exceeds a predetermined value.

Preferably, the inverter is a driven inverter and the means for inhibiting the voltage applied to the lamp comprises means for removing the drive from the inverter on receipt of a signal from the monitoring circuit thereby to de-energise the lamp.

In one form of the invention, the monitoring circuit includes an additional winding on the current limiting inductor, and a threshold detector circuit arranged to supply a signal to the inverter if the voltage across the additional winding exceeds a predetermined value. In accordance with, another aspect of the invention, additional protection means are provided to connect the additional winding in a closed circuit if the voltage applied to the lamp exceeds a second predetermined value higher than the first predetermined value, thereby to damp the resonant circuit of the current limiting inductor and parallel capacitor to reduce the voltage applied to the lamp. The additional protection means preferably comprises a voltage clamping circuit connected in parallel with the additional winding.

In accordance with a further aspect of the invention, a low-voltage supply for parts of the circuit is derived from the monitoring circuit inductively coupled to the current limiting inductor.

Suitably, means are provided to control the output frequency of the inverter on switch-on of the circuit so that during an initial preheating period a voltage lower than the striking voltage is applied to the lamp, following which a rising voltage is applied to the lamp until ignition is effected or the protection circuit operates.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic block diagram of an electronic ballast circuit in accordance with the invention, and

Figure 2 is a circuit diagram of part of the circuit of Figure 1.

Referring to the drawings, the electronic ballast circuit is divided into two sections, a first section 10 referred to hereinafter as the "line conditioner" and a second section 50 referred to hereinafter as the "ballast unit". The two sections are mounted in a common housing 200, provided with suitable terminals by means of which the input of the line conditioner can be connected to a mains supply 2 and the output of the ballast unit can be connected to a number of discharge lamps. In the illustrated embodiment, the circuit supplies four lamps, 164, 264, 364 and 464, though any number of lamps could be connected to the circuit.

The line conditioner 10 comprises a radio frequency interference (RFI) filter 12, a full wave rectifier circuit 14, and an active harmonic filter 16. The RFI filter 12 provides a high impedence to signals at the operating frequency of the driven inverter 52 and active harmonic filter 16, and also provides primary surge protection against mains derived transients. The full wave rectifier circuit 14 provides both AC rectification and secondary mains surge protection, including overvoltage protection for the active harmonic filter 16. The active harmonic filter 16 is used to maintain a near sinusoidal input current from the mains supply 2. This enables the circuit to comply with regulations governing equipment drawing power from the public mains, where these apply. The elements of the line conditioner 10 may be of generally conventional form and therefore will not be described further.

The ballast unit 0 comprises a driven inverter 52, which is supplied with the smoothed DC output of the line conditioner 10. The high frequency output of inverter 2 is supplied via an inductor circuit 154 and a capacitor l6θ to the lamp 164. In addition, the driven inverter 2 receives inputs from a frequency control circuit and a protections circuit 55_*

Additional inductor circuits 24, 35^ and 54, and capacitors 260, 360 and 460 respectively are provided to supply the output of the inverter 52 to the lamps 264, 364 and 464, a common return for the lamps being provided through line 53•

The circuit of the ballast unit 0 is shown in more detail in Figure 2. For convenience, only one lamp 164 is shown connected to the circuit in Figure 2. The driven inverter 52 is a conventional half-bridge formed by capacitors 66 and 68, power MOSFETs 62 and 64, and transformer 70. The gate electrodes of transistors 62 and 64 are voltage clamped by zener diodes 63 and 65 and are connected to the secondary windings 72 and -i of the transformer 70, the primary winding 76 of which, is connected to output terminals of the integrated circuit 78.

The integrated circuit 78 includes components forming parts of the frequency control circuit 56 and protection circuit 55• The remaining components of the frequency control circuit are provided by resistors 80, 82, 84 and 86, transistor 100, capacitor 106 and diode 88. The circuit is arranged to provide an initial pre-heat period, preferably greater than 4θOms, during which a sufficiently high pre-heating current flows through the lamp. On switch-on of the ballast circuit the capacitor 106 begins to charge through resistors 84 and 86, from a low-voltage supply, which is provided through transistor 110 as described below. The transistor 102 is therefore rendered conducting, placing resistor 80 in parallel with resistor 82. This has the effect of^*increasing the frequency of the signal applied to the primary 76 of transformer 70 and therefore the operating frequency of the driven inverter. Since the impedance of the inductor circuit 1 4 at this higher frequency is greater, the combination of the higher frequency in conjunction with inductor circuit 1 4 determines the preheating current of the heater electrodes of lamp 4. The value of the parallel capacitor l6θ is chosen so as to prevent the parallel combination of inductor circuit 1 4 and capacitor l6θ from being sufficiently close to resonance at the higher frequency, thereby preventing the generation of a high voltage which would otherwise appear across the lamp and ignite it.

As the capacitor 106 charges, the voltage at the base of transistor 100 falls until the transistor is rendered non-conducting. When capacitor 106 is fully charged, any residual current flow due to leakage from the capacitor is diverted via resistor 86, effectively preventing transistor 100 from conducting. The frequency of the driven inverter is now solely determined by the resistor 82. The transition between the two states is not instantaneous, and transistor 100 operates in the linear mode for a period of times. The frequency of the driven inverter decreases over this period, so that the parallel combination of the inductor 154 and parallel capacitor 160 moves towards resonance, thereby gradually increasing the voltage across the lamp 164, until it reaches a sufficient voltage to cause the lamp to ignite.

The protection circuit 55 includes an additional winding 152 on the inductor-circuit 154. One terminal of the winding 152 is connected to ground whilst the other is connected through diode 153 and resistors 156 and I58 to the input of threshold detector and latch circuit 170. The input is also connected to ground through resistor 157. across which is connected capacitor 159• The output of the latch circuit 170 is connected to the integrated circuit 78.

The junction of resistors 156 and 158 is connected to the collector of transistor 110 forming part of a stabilised low-voltage supply, as described below. The base of the transistor 100 is connected to ground through zener diode 162, and a reservoir capacitor 164 is connected between the collector of transistor 110 and ground.

A voltage clamping circuit 166, which may comprise a zener diode, a voltage-dependent resistor or any other suitable device, is connected in parallel with capacitor 164.

The winding 152 effectively monitors the peak lamp voltage and current. The voltage across the winding 152 is fed to the threshold sensing and latch circuit 170 via the voltage divider formed by resistors 157 and 158. If the voltage applied to circuit 170 exceeds a predetermined value, a latch is activated, and the output voltage supplied to integrated circuit 78 rises, causing the output to transformer 70 to be turned off, removing the drive to the inverter and de-energising the lamp. The circuit is arranged so that the latch circuit 170 will be actuated if the voltage applied to the lamp 164, after the preheating period, rises to a value (typically in the region of 1200 volts) above that at which the lamp is expected to ignite. The latch circuit 170 includes a reset circuit to reset the circuit and enable the lamp to be re-started after it has been switched off.

The capacitor 159 in parallel with resistor 157 prevents the latch circuit 170 responding to spurious transient voltages in the circuit. The slight delay thus introduced may in some circumstances allow the voltage applied to the lamp to increase above the predetermined value before the lamp is de-energised. This may occur for example when an additional lamp is connected to the ballast circuit whilst it is operating, so that the additional lamp is not subjected to the preheating current. In some countries, statutory regulations set an absolute maximum for the voltage which may be applied to the lamp. To ensure that the voltage cannot exceed such a limit, additional protection is provided by the voltage clamping circuit 166. This is set to conduct if the voltage at the junction of resistors 1 6 and I58 exceeds a value corresponding to the maximum voltage to be applied to the lamp. Conduction of the device 166 effectively short-circuits the additional winding 152, damping the resonant circuit formed by inductor 1 4 and capacitor 160 and reducing the voltage applied to the lamp 164. The transistor 110 and associated components provide a stabilised low-voltage supply for various control circuits, including the protection circuit 55 and frequency control circuit 56, the power for the low-voltage supply being drawn from the additional winding 152, and from the additional windings on the other inductor circuits 254, 35^ and 454, if lamps are connected to those circuits.

It- ill therefore be appreciated that the use of an additional winding-car the.- current limiting inductor, in accordance with the invention, to monitor the peak lamp voltage and current has the advantage of enabling three functions to be performed, namely monitoring the peak voltage and current to enable the.lamp to be shut down if a predetermined threshold is reached, providing a voltage clamp to prevent the lamp voltage exceeding a maximum permissible level, irrespective of other circuit conditions, and providing power for low voltage control circuits of the ballast.

It will be appreciated that modifications can be made in the described embodiment. For example, in an alternative form of the invention, the protection circuit may be arranged to increase the frequency of the output of the driven inverter, to reduce the voltage applied to the lamp, instead of removing the drive to the inverter as in the described embodiment.

Claims

1. An electronic ballast circuit for a gas discharge lamp, the circuit comprising an inverter having an oscillatory output for connection to a gas discharge lamp, a current limiting inductor in series with the output of the inverter, and a capacitor for connection in parallel with the lamp, in which there are provided protection circuit means comprising a monitoring circuit inductively coupled to the current limiting inductor and means for inhibiting the voltage applied to the lamp on receipt of a signal from the monitoring means if the voltage exceeds a predetermined value.

2. A circuit as claimed in Claim 1, in which the inverter is a driven inverter and the means for inhibiting the voltage applied to the lamp comprises means for removing the drive from the driven inverter on receipt of a signal from the monitoring circuit, thereby to de-energise the lamp.

3- A circuit as claimed in Claim 1 or Claim 2, in which the monitoring circuit includes an additional winding on the current limiting inductor, and a threshold detector circuit arranged to supply a signal to the inverter if the voltage across the additional winding exceeds a predetermined value.

4. A circuit as claimed in Claim 3. in which additional protection means are provided to connect the additional winding in a closed circuit if the voltage applied to the lamp exceeds a second predetermined value higher than the first predetermined value, thereby to damp the resonant circuit of the current limiting inductor and parallel capacitor to reduce the voltage applied to the lamp.

5'-. A circuit as claimed in Claim 4, in which the additional protection means comprises a voltage clamping circuit connected in parallel with the additional winding.

6. A circuit as claimed in any preceeding claim, in which a low-voltage supply for parts of the circuit is derived from the monitoring circuit inductively coupled to the current limiting inductor.

7. A circuit as claimed in any preceding claim, in which means are provided to control the output frequency of the inverter on switch-on of the circuit so that during an initial preheating period a voltage lower than the striking voltage is applied to the lamp, following which a rising voltage is applied to the lamp until ignition is effected as the protection circuit operates.