WO2003005778A1

WO2003005778A1 - Controlling apparatus

Info

Publication number: WO2003005778A1
Application number: PCT/AU2002/000825
Authority: WO
Inventors: Brian Charles Webb; Terry William Norris; Wayne Anthony Coyte
Original assignee: Briter Electronics Pty Ltd
Priority date: 2001-07-04
Filing date: 2002-06-28
Publication date: 2003-01-16
Also published as: AUPR610801A0

Abstract

The present invention provides apparatus for controlling the operation of a discharge lamp. The apparatus includes an input (11, 12) for receiving an electrical supply. The input is coupled to a power converter (20) which generates a controlled output, which is supplied to the lamp in use. The apparatus also includes a control system for controlling the operation of the power converter in accordance with the operation of the lamp to thereby control the power drawn by the lamp.

Description

CONTROLLING APPARATUS

Background of the Invention

The present invention relates to apparatus for controlling the operation of a gas discharge lamp, and in particular to apparatus capable of acting as a ballast for discharge lamps such as a fluorescent lamps, or the like.

Description of the Prior Art

The reference to any prior art in this specification is not, and should not be taken as an acknowledgment or any form of suggestion that the prior art forms part of the common general knowledge in Australia.

Gas discharge lamps, such as fluorescent lamps operate by applying a potential difference to electrodes contained in a suitable gas. The potential difference causes electrons to be emitted from the negative electrode and accelerated toward the positive electrode. As the electrons are accelerated through the gas, they collide with gas atoms or molecules, causing radiation to be emitted. This is in turn typically used to excite a fluorescing material, causing the fluorescing material to emit visible radiation.

Accordingly, in order to operate a gas discharge type lamp, it is necessary to be able to strike the lamp to cause a current to flow between the two electrodes. This can be achieved by applying a large enough voltage to the electrodes to initiate an arc between the two electrodes. Alternatively, the lamp electrodes can be pre-heated, which causes the ionisation of gas near the electrodes. This increases the conductivity of the lamp, thereby reducing the voltage needed to strike the lamp.

Once current flow has started the resistance of the lamp drops dramatically and it is therefore also necessary to be able to limit the flow of current through the lamp.

This functionality is generally achieved by the use of ballast circuits. Currently, there are two main types of ballast circuit, namely conventional and electronic ballasts.

Conventional ballasts typically operate at mains frequencies and use an iron cored choke to limit the current. In use, the conventional iron core ballasts use either a filament transformer or a starter circuit to heat the filaments of the lamp.

In the switch start type a starter shorts out the lamp by connecting the electrode filaments from each end of the lamp and the ballast in series across an AC supply. The power is passed through the starter and the filaments, ionising the gas near the filaments until a bi-metallic strip in the starter heats up and breaks the circuit. When the circuit is broken the voltage across the lamp increases rapidly as the ballast tries to keep the current flowing. When the voltage across the lamp reaches the breakdown voltage of the lamp the lamp starts to conduct, causing further ionisation of the gas and causing the voltage across the lamp to fall, with the ballast now limiting the current through the lamp.

In the filament transformer case, a separate transformer is provided to supply a pre-heat current to the lamp electrode filaments. When the power is applied the filament transformer passes current through the filaments, heating the filaments. The gas near the filaments starts to ionise, lowering the impedance of the lamp until it is low enough for the mains voltage to strike the lamp. When this occurs the impedance of the lamp decreases as more of the gas in the lamp is ionised by the current in the lamp. This causes the voltage of the lamp to decrease and the ballast limits the current flow through the lamp.

Turning now to electronic ballasts, these typically operate at much higher frequencies, 20 to 70Khz and are currently available in two major forms, namely an instant start and warm start versions. In both cases, the lamp is coupled to an inverter to allow current flow through the lamp to be controlled.

In the former, instant start ballast, the voltage from the high frequency inverter ramps up until the lamp strikes without pre-heating the lamp electrodes. After the lamp strikes the voltage collapses and an inductive or capacitive impedance limits the current.

Warm start ballasts, however, pre-heat the filaments before the voltage ramps up to start the lamps. This is usually controlled by electronic means, typically using a thermistor in the ballast that heats up as current is supplied to the electrodes.

Gas discharge lamps are available in a number of output power levels and in a number of different shapes. In general each lamp has different properties and it is therefore necessary to ensure that the operational parameters of the ballast are matched to the lamp which is to be operated. As a large number of lamps are available, this also means that a large number of ballasts must be produced in order to allow the lamps to be operated.

A further problem is that currently available ballasts often fail to pre-heat the lamp correctly. However, if a lamp has been recently used, it is possible for the thermistor to still be warm even if the lamp itself has cooled. In this case, the thermistor will shorten the pre-heat time and strike the lamp before it has been properly pre-heated. The use of non-optimum preheating results in damage to the lamp heaters, and this may be due to too low a heater temperature and/or too high lamp voltage applied, resulting in excessive lamp glow current and/or too slow a transition between preheat and lamp ignition.

It is also possible for the heaters to fail, usually by going open circuit. Normally if a lamp heater becomes open circuited this denotes the end of lamp life, as conventional or warm start electronic ballasts are unable to restart the lamp even though the lamp can still theoretically operate.

A further problem is that as lamps age, an increase in the lamp operating voltage occurs. Normally if the lamp voltage increases beyond a pre-set limit, electronic ballasts will trip out so as to protect the ballast and lamp installation against overloading. In particular, most electronic ballasts operate the lamp at near constant current. Accordingly, any significant increase in the lamp voltage results in a corresponding increase in the lamp power and ballast load. Accordingly, as the lamp ages, it is often not possible to drive it using the intended ballast.

A major deficiency of many currently available electronic ballast's, is their inability to withstand high levels of mains borne transient energy. Transients of up 6kV at lkA have been measured on the 240V 50Hz mains supply. These transients may have a duration of several hundred micro seconds. However, this energy level is several orders of magnitude greater than the present mandatory design requirements for electronic ballast transient protection. A transient may be caused by, for example; lightning strikes, sub-station and generation switching, and sudden heavy loads applied or removed from the supply.

A further known problem with current ballast circuits is the undesirably high inrush current taken from the mains supply (or alternative DC supply) on switching on the ballast. This is important in practice since the inrush current is a limiting factor that determines the maximum number of ballasts that may be connected to an electrical circuit. In particular, it s important to limit the number of ballasts to prevent the circuit breakers, which are installed to protect against dangerous or potentially dangerous fault conditions, from tripping out when the circuit is switched on. Currently available ballasts can have inrush currents exceeding 30 amps.

Summary of the Present Invention

In a first broad form the present invention provides apparatus for controlling the operation of a discharge lamp, the apparatus including: a) An input for receiving an electrical supply; b) A power converter coupled to the input for generating a controlled output, the power converter being coupled to the lamp in use; and, c) A control system for controlling the operation of the power converter in accordance with the operation of the lamp to thereby control the power drawn by the lamp.

Accordingly, the present invention provides apparatus that can act as a ballast circuit to supply power to gas discharge lamps, such as fluorescent lamps, or the like.

The power controller is usually controlled in accordance with the power drawn by the lamp.

Preferably, the power controller generates a controlled electrical current, to thereby control the current drawn by the lamp.

Typically the power converter includes: a) An inverter coupled to the input for generating an alternating current; and, b) A reactor for coupling the lamp to the inverter in use.

A power converter could encompass a range of switched mode techniques that could be used to convert the supply voltage into a form that was suitable for powering a discharge lamp. Accordingly, the power converter could take the form of an inverter, as mentioned above, or a step- down forward converter or any number of other well know switch mode converters.

Furthermore, whilst the reactor may be implemented as an integral part of the power converter, as opposed to being provided as an element separate to the inverter, as described above. Thus, for example the converter could utilise a high leakage inductance transformer, which is fundamental to the operation of the converter circuit, but also provides the current limiting because of the nature of the transformer. However, the reactor is usually at least one of a capacitor or an inductor.

Operation of the power converter is preferably controlled by controlling the frequency of the alternating current generated by the inverter. However, the inverter could be controlled by alternative means such as pulse width or phase modulation. The remainder of this document describes only the frequency-controlled embodiment of the invention.

The control system usually includes a control system input for setting the maximum power to be used by the lamp .

The control system preferably includes: a) A first sensor for sensing the voltage supplied to the inverter; and, b) A second sensor for sensing the current drawn by the inverter.

Typically, the control system input includes a variable resistor coupled to the inverter, the second sensor being adapted to measure the voltage across the variable resistor. In this case, the variable resistor is used as the control system input to allow the maximum power to be used by the lamp to be set.

The control system generally includes a first controller adapted to: a) Compare the voltages measured by the first and second sensors; and, b) Generate a reference signal in accordance with the result of the comparison.

The control system usually also includes: a) A third sensor for sensing the current supplied to the lamp in use; and, b) A regulator, the regulator being adapted to control : i) Compare the current measured by the third sensor to the reference signal; and, ii) Adjust the frequency of the inverter to vary the output current/power of the inverter in accordance with the result of the comparison.

The control system generally includes an oscillator adapted to generate a signal having a selected frequency, the inverter being responsive to the signal to generate an alternating current having the selected frequency.

In this case, the regulator is preferably coupled to the oscillator, the oscillator being responsive to the regulator to adjust the frequency of the generated signal.

Accordingly, the oscillator is usually a voltage controlled oscillator responsive to a control signal generated by the regulator, with the voltage of the control signal being adjusted depending on the difference between the voltage measured by the third sensor and to the reference voltage.

The lamp usually includes lamp heaters, with the apparatus usually further including a transformer coupled to the inverter for supplying a pre-heat current to the lamp heaters in use.

In this case, the apparatus usually further includes a timer for controlling the length of time for which the pre-heat current is supplied to the lamp heaters. Typically this is achieved by having the timer coupled to the oscillator to cause the oscillator to generate a signal at a predetermined frequency for a predetermined time period when the apparatus is initially activated.

The transformer is then typically coupled to the inverter via a capacitor so that in use the transformer and the capacitor form a tuned circuit having a resonant frequency equal to the predetermined frequency.

The regulator is then usually adapted to generate a signal having a second predetermined frequency when the predetermined time period has expired.

In this case, the inductor is coupled to the inverter via a second capacitor, so that in use the inductor and the second capacitor form a second tuned circuit having a resonant frequency equal to the second predetermined frequency.

A current limiter is also preferably provided for coupling the electrical supply to the inverter, the current limiter limiting the current that can be drawn from the power supply.

Preferably the current limiter is adapted to: a) Determine if the current drawn from the power supply exceeds a predetermined threshold; and, b) In response to a successful comparison increase the impedance of the apparatus thereby limiting the current.

In this case, the current limiter usually including a capacitor coupled to the inverter and a resistor coupled to the capacitor in series, and wherein, in use, any current exceeding the predetermined threshold flows through the resistor to charge the capacitor, the resistor operating to limit the current flow. Accordingly, the resistor and the capacitor represent the additional impedance.

After the capacitor has been charged and the inverter starts to operate the imush current limiting resistor is shorted out to minimise the power dissipated by the resistor and to improve the overall efficiency of the ballast.

However, the current limiter is usually adapted to operate while the apparatus is in use. Accordingly, should a large current be detected during use, current will be redirected through the resistor to provide protection against mains borne transient surges, and the like. In the case in which the electrical supply supplies an alternating current, the apparatus typically further includes a rectifier for coupling the electrical supply to the inverter, the rectifier being adapted to convert the alternating current to a direct current in use.

The apparatus also usually further includes a power factor correction circuit. The power factor correction circuit being adapted to control the current drawn from the supply to give a power factor close to unity. This is achieved by boosting the rectified voltage high enough to allow the current being drawn to be controlled and controlling the current being drawn to closely match the phase and waveform of the supplied voltage, resulting in unity power factor.

The apparatus also preferably includes a protection system, the protection system being adapted to deactivate the lamp if: a) The current power usage of the lamp is more than a predetermined threshold; and, b) Instabilities are detected in the lamp operation.

The apparatus usually includes a fourth sensor, the fourth sensor being adapted to measure the voltage across a capacitor coupled to the lamp to thereby detect the instabilities. In this case, the instabilities are usually detected as random variations in the voltage measured by the fourth sensor.

The predetermined threshold is also usually being determined in accordance with the set maximum power

In a second broad form the present invention provides apparatus for controlling the operation of a discharge lamp, the apparatus including: a) An input for receiving an electrical supply; b) A power converter coupled to the input for generating a controlled output, the power converter being coupled to the lamp in use; c) A transformer; and, d) A timer adapted to cause the transformer to couple the power converter to lamp heaters for a predetermined time period when the apparatus is initially activated, thereby preheating the lamp.

In accordance with a third broad form the present invention provides apparatus for controlling the operation of a discharge lamp, the apparatus including: a) An input for receiving an electrical supply; b) A power converter coupled to the input for generating a controlled output, the power converter being coupled to the lamp in use; and, c) A current limiter for coupling the electrical supply to the power converter, the current limiter operating to limit the current that can be drawn from the power supply.

In a fourth broad form the present invention provides, apparatus for controlling the operation of a discharge lamp, the apparatus including: a) An input for receiving an electrical supply; b) A power converter coupled to the input for generating a controlled output, the power converter being coupled to the lamp in use; and, c) A protection system, the protection system being adapted to deactivate the lamp if: i) The current power usage of the lamp is more than a predetermined threshold; or, ii) Instabilities are detected in the lamp operation.

The second, third and fourth broad forms of the invention which usually also include a control system adapted to control the frequency of the alternating current in accordance with the amount of power used by the lamp to thereby control the current drawn by the lamp.

Furthermore the power controller of the second, third and fourth broad forms of the invention usually generates a controlled electrical current, to thereby control the current drawn by the lamp.

Accordingly, it will be appreciated that any one or more of the second, third and fourth broad forms of the invention may be combined with the first broad form of the invention in a single apparatus configuration.

Brief Description of the Drawings Examples of the present invention will now be described with reference to the accompanying drawing in which:-

Figure 1 is a block diagram of a ballast circuit according to the present invention; Figure 2 is a circuit diagram of a first example of a current limiter for use in the ballast circuit of Figure 1; and,

Figure 3 is a circuit diagram of a second example of a current limiter for use in the ballast circuit of Figure 1.

Detailed Description of the Preferred Embodiments Figure 1 is an example of a ballast circuit for controlling the operation of a fluorescent lamp or the like. The ballast includes a filter 10, a rectifier 15, a current limiter 20, a power factor correction circuit 25, an inverter 30, a lamp current regulator 35, timer 40, an oscillator 45, a lamp power controller 55, and a protection circuit 65. In addition to this, the circuit is also provided with a transformer 75, which is used to couple the circuit to a gas discharge lamp 85, such as a fluorescent lamp or the like.

The filter circuit 10 includes inputs 11, 12 that in use are coupled to a power supply (not shown) to receive a supply of alternating current, such as "Mains" electricity. The filter circuit 10 filters the received AC supply, and transfers the filtered AC supply via outputs 13, 14 to inputs 16, 17 of the rectifier 15.

The rectifier 15 rectifies the received current, transferring the rectified supply via an output 18 to an input 21 of the current limiter 20.

The current limiter 20 includes an output 23 that is coupled to an input 26 of the power factor correction circuit 25 and a second input 24. In use, the current limiter operates to limit the amount of current that can be drawn from the rectifier, and hence from the power supply, by the remaining elements in the circuit, as will be described in more detail below.

The power factor correction circuit 25 boosts the rectified supply received at the input 26 and transfers the boosted DC supply to an input 31 of the inverter 30, via an output 28. The power factor correction circuit 25 also includes a ground connection 27 to ground A, as shown.

The inverter 30 includes a ground connection 32, which is connected to ground B, as well as a drive input 33 and an output 34. The inverter converts the DC supply received from the power factor correction circuit 25 into an alternating current supply having a controlled frequency. The frequency is controlled in accordance with a signal received at the drive input 33, so that the resulting AC supply provided at the output 34 has the same frequency as the signal applied to the drive input 33.

The AC supply generated at the output 34 is used to drive the lamp by having the output 34 coupled to the lamp by an inductor L2 and a capacitor C 1.

In addition to this, the output 34 is coupled to a capacitor C2 and then to the input 76 of the primary winding LID of a transformer 75. The output 77 from the primary winding LID of the transformer 75 is connected to the ground B, as shown. The secondary windings of the transformer 75 which act as respective inductors LI A, LIB are coupled to lamp heaters 81, 82, via respective impedances. In this example, the impedances are formed from capacitors C5, C6, as shown, however alternatively resistors may also be used, as will be appreciated by persons skilled in the art. Accordingly, in use, the transformer provides a pre-heat current to the lamp heaters 81, 82, to preheat the lamp, as will be explained in more detail below.

The transformer can include a third secondary winding (not shown) which can be used to allow the circuit to light two lamps connected together in series. In this case, one end of each lamp will be connected to a respective one of the capacitors C5, C6. As will be appreciated by a person skilled in the art, the other end of the lamps are then coupled either: i) In parallel to a third capacitor (not shown) which couples the other ends to the third secondary winding (not shown); or ii) In series with suitable series capacitor and third secondary winding

The output 34 is also coupled the second input 24 of the current limiter 20 to switch the current limiter into its low resistance state as soon as the inverter 30 starts up. Prior to this the current limiter 20 is bypassed by a resistor R13 that limits the input current taken from the power supply at switch on.

The lamp current regulator includes an AC input 37, a control output 38 and control input 39. In use, the regulator 35 is adapted to determine the current taken by the lamp and compare this to a reference signal received at the control input 39. The regulator 35 then generates a control signal based on this comparison. The control signal is output to a frequency control input 47 of the oscillator 45, from the control output 38.

The oscillator 45 generates an alternating signal the frequency of which is controlled by the control signal received at the control input. The alternating signal is output from an output 48 and transferred to the drive input 33 of the inverter 30, to control the frequency of the AC supply generated by the inverter.

In addition to this the oscillator also includes a synchronisation input 49 which is coupled to the resistor R10 and the capacitor C9, and an inhibit input 50 which is coupled to the inhibit output 67 of the lamp protection circuit 65, as shown.

The lamp power controller 55 includes a current sense input 58 and a voltage sense input 59 which are coupled to a variable resistor VR1 and the DC output 28 of the power factor correction circuit 25, as shown. The lamp power controller generates the reference signal at a control output 57 in accordance with values measured at the current sense input 58 and a voltage sense input 59. The lamp power controller also includes a trip output 60 that is coupled to trip power input 68 of the protection circuit 65.

The protection circuit also includes a trip volts input 69 which is coupled to the capacitors C9, Cl, as well as a ground connection 66, as shown. In use the protection circuit operates to generate an inhibit signal at an inhibit output 67 if the lamp is deemed to be functioning incorrectly, as will be described in more detail below.

Operation of the circuit will now be described in more detail.

In use, an alternating current supply is received from the power supply (not shown). The AC supply is filtered by the filter 10, and transferred to the rectifier 15 via the inputs 16, 17. The filter 10 operates to attenuate any high frequency signals that may be generated by the ballast to prevent them being injected back into the power supply. Accordingly, the filter may be any form of filter such as an EMC filter.

The rectifier 15, which may be any form of rectifier known in the art, operates to rectify the alternating current to generate a direct current that is transferred via the outputs 18,19 to the current limiter circuit 20.

The supply of DC current drawn by the remaining circuitry is controlled using the current limiter 20, a first example of which is shown in more detail in Figure 2.

In this case, the circuit shown in Figure 2 also includes the filter 10 and the rectifier 15, for ease of explanation. Accordingly as shown, the filter is foπned from a variable resistor VDRl, which is coupled across the filter inputs 11, 12. The variable resistor is coupled to an inductor L4, and a capacitor C15, via a resistor R14. A capacitor C14 and an inductor L3 are connected to the inductor L4 and the capacitor C15, as shown.

In use, the outputs 1, 8 of the inductor L3 are coupled to four diodes D9, D10, Dll, D12, which form the rectifier 15. Thus the inductor L3 forms part of the filter 10, with the inductor outputs 1,8 forming the outputs 13, 14 of the filter 10.

The current limiter is formed from a variable resistor VDR2, which is coupled to the rectifier output 18 to form the current limiter input 21. The variable resistor VDR2 also forms the current limiter output 23, as shown, with the current limiter output 23 being connected to the ground com ection 19, via a capacitor C34. Two resistors R13, R58 are arranged in series across the variable resistor VDR2. A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) Q3 is arranged in parallel with the resistor R13, with the source connected to the junction of the resistors R13, R58, and the drain connected to the input 21. Two resistors R56, R57 are coupled across the MOSFET drain and source as shown. A diode D7 also connects the MOSFET source to the gate.

The junction between the resistors R56, R57 is coupled to the base of a transistor Q10. The transistor emitter is coupled to the output 23, with the collector being coupled to the gate of the MOSFET Q3. The collector and emitter of the transistor Q10 are coupled to a resistor R70 and a capacitor C36, which are in turn connected to two diodes D34, D2, arranged in series across the capacitor C36. The junction between the two diodes D34, D2, is coupled to a capacitor C4, which is in turn coupled to the current limiter input 24. The transistor base is coupled to the output 23 via a capacitor C42.

This circuit operates to limit the current in a number of ways.

Firstly, when the device is initially activated an inrush current is generated as the capacitor C13 is charged. In order to limit the inrush current, the current limiter includes a resistor R13 that is in series with the capacitor C13. The resistor R13 is selected so that current flow into the capacitor is limited to 1 amp, thereby preventing the ballast from tripping out circuit breakers when it is initially activated.

After the capacitor C13 is fully charged the inverter circuit 30 starts and provides an input to the diodes D2, D34, via the input 24. The resulting dc voltage is limited by the zener diode D7 to 15 volts, which switches on the MOSFET Q3, providing a low impedance path for the input current. Accordingly, the inrush current limiting resistor is shorted out to minimise the power dissipated by the resistor and to improve the overall efficiency of the ballast.

In the event of a mains voltage transient occurring the voltage dependent resistor VDRl limits the mains voltage to a maximum of 1000 volts peak across the input terminals (L,N) 11 and 12 on Figure 1. This voltage would result from a mains transient current of 1000 amps.

The inductors L4, L3 and an inductor L5 (which forms part of the power factor correction circuit) delay the rise in current that would charge the capacitor C13 from the mains transient. When this rise in current exceeds 3 amps the voltage drop across the resistor R58 allows the transistor Q10 to turn on via the resistor R57, resulting in the MOSFET Q3 being turned off. The voltage across the MOSFET Q3 rises and the transistor Q10 is held on via the current flow in the resistor R56. The mains transient current is now limited by the resistor R13 to typically below 1 amp. Accordingly, the circuit resets when the peak mains voltage reduces to below the voltage across the capacitor C13 (400volts). The voltage dependent resistor VDR2 protects the MOSFET Q3 from excess voltage.

The current limiter 20, which can be used with other types of circuit, introduces a resistor R13 into the path of the transient/surge current, as opposed to the more common method of shunting the transient mains current to ground by using a voltage dependent resistor or similar device.

Accordingly, the current limiter 20 provides protection both during startup and during the normal operation of the ballast. In order to achieve this, the current limiter is adapted to introduces the resistor R13 into the path of the transient/surge current, if the current exceeds the set current, regardless of whether the circuit has just been started up, or has been operating for a period of time.

The current limiter 20 is connected to the power factor correction circuit boost circuit 25 that operates to generate a dc voltage from the rectified supply received from the current limiter 20. The power factor correction circuit 25 generates a DC supply at the output 28, with the DC supply being stabilised at a voltage of approximately +400 volts. This is achieved using conventional boost circuitry that is capable of generating the required output from the rectified mains supply at near unity power factor over a full range of load powers (which in this example is 20 to 80 Watts) over a mains input voltage range of 200 to 265 volts.

However, the power factor correction circuit is not an essential component in the ballast design. Thus for cases in which power factor correction is not required, such as for low wattage lamps, the power factor correction circuit 25 can be omitted and the inverter 30 supplied from the rectifier 15 via the current limiting circuit 20. This may require some component value changes to allow the inverter 30 to operate at the lower dc supply voltage.

The DC supply is received by the inverter 30, which is typically a high voltage MOSFET driver/oscillator circuit formed from a half bridge driver integrated circuit configured to drive the associated MOSFETs in a half bridge configuration. The inverter operates to repeatedly reverse the polarity of the DC supply as it is transferred from the input 31 to the output 34, thereby generating an AC supply. The frequency at which the inverter 30 reverses the polarity is controlled by the alternating signal received from the oscillator 45. Accordingly, the frequency of the AC supply generated at the output 34 is controlled by frequency of the alternating signal, which is in turn controlled by the regulator 35, as will be explained in more detail below.

The alternating current generated by the inverter 30 is supplied to the transformer 75 via the input 76 and the capacitor C2. The transformer 75 effectively acts an inductor LI that forms a first tuned circuit with the capacitor C2. The transformer 70 and the capacitor Cl are selected to have a resonant frequency of approximately 120 kHz.

The output 34 is also coupled to the lamp 80 via an inductor L2 and capacitor Cl which together form a second tuned circuit having a resonant frequency in the region of 60 kHz. The operation of these first and second tuned circuits will now be described in more detail below.

In use, when the inverter 30 generates an AC supply this causes oscillation of both the first and second tuned circuits, with the magnitude of the oscillation depending on the driving frequency of the AC supply.

Accordingly, if the AC supply has a frequency in the region of 120kHz, this will cause the first tuned circuit to undergo large magnitude oscillations, with the second tuned circuit undergoing relatively small oscillations. If however the frequency of the AC supply is in the region of 60kHz then the opposite will be true with the first tuned circuit undergoing small magnitude oscillations, and the second tuned circuit undergoing relatively large oscillations.

Oscillation of the first tuned circuit will cause the generation of a current in the secondary windings LI A, LIB of the transformer 75. This in turn causes a current to flow through the capacitors C5, C6 and through the lamp heaters 81, 82. If the first tuned circuit is driven at 120kHz, the magnitude of the current induced in the secondary coils L1A, LIB is sufficiently large to be able to drive the lamp heaters to pre-heat the lamp.

In order to achieve this, the capacitance of the capacitors C5, C6, (or the resistance of the resistors, if resistors Rl, R2 are used instead of the capacitors C5, C6) should be selected to achieve optimum lamp heater temperatures during pre-heat. In addition to this, the first transformer input 76 is coupled to the rectifier output 28 via a diode Dl and a resistor R72. This ensures that the voltage applied to the transformer 75 is clamped to the inverter dc supply voltage (+400V), thereby regulating the voltage applied to the lamps during pre-heat. As a result, at a driven frequency of 120kHz, the first tuned circuit will cause the lamp heaters 81, 82 to generate 600°C over a full range of lamp types, and heater resistance values (typically between 1.5 to 25 ohms).

Typical properties include:

Lla=3.4mH, with a turns ratio L1A/L1B/L1C:L1D = 50:1 C2 = 470Pf. C5, C6 = 82nF.

If resistors are used instead of the capacitors C5, C6, typical values are: R1,R2,R3 = 8.2 ohms

At this frequency, oscillation of the second tuned circuit is negligible and accordingly, only minimal voltage is applied across the lamp at this time.

In contrast however, if the inverter 30 generates a signal having a frequency of approximately 60khz, this causes large oscillations of the second tuned circuit and only small oscillations of the first tuned circuit.

In this case, if current is not flowing through the lamp, the large oscillations of the second tuned circuit will cause a large voltage to be generated across the lamp. At this time, the voltage across the resistor R10 will follow the oscillations of the second tuned circuit. Accordingly, the oscillator measures the voltage across the resistor R10 at the synchronisation input 49 and synchronises the generated alternating signal to these oscillations. This ensures that the AC supply is generated at the resonant frequency of the second tuned circuit, thereby increasing the voltage generated across the lamp.

In this scenario, the lamp voltage will increase until the potential across the lamp is sufficient to strike the lamp and induce a lamp current. This will typically require a voltage of up to 1500 Volts.

In this case however, as the AC supply is significantly different to the resonant frequency of the first tuned circuit, the current generated in the secondary windings LI A, LIB of the transformer is only small, and accordingly, only a minimal voltage is applied to lamp heaters 81, 82. This continues to supplement the heating effect of the lamp current, as will be understood by a person skilled in the art. Once the lamp has been activated in this manner it is necessary to control the flow of current through the lamp, and this is achieved by controlling the frequency of the AC supply generated by the inverter 30, as will now be described.

Thus, in order to ignite the lamp 80, it is necessary for the inverter 30 to initially generate an AC supply having a frequency of 120kHz to pre-heat the lamp 80, and then generate an AC supply having a frequency of 60kHz to ignite the lamp.

This is achieved by the use of the timer circuit 40 and the oscillator 45, which in this example is a voltage controlled oscillator which generates an alternating signal the frequency of which varies depending on the magnitude of the control signal received at the control input 47.

Accordingly, when the circuit is initially powered up, the timer operates to generate a signal having a predetermined reference voltage for a time period of approximately 2 seconds, which is transferred to the oscillator 45. The predetermined voltage is selected to cause the oscillator 45 to generate an alternating signal having a frequency of 120kHz. This is in turn causes the inverter 30 to generate an AC supply at 120kHz, thereby causing the lamp heaters to activate.

At the end of this two second interval, the timer resets.

At this point with no current flowing through the lamp, the regulator 35 is adapted to generate a signal having a second predetermined voltage which in turn causes the oscillator to generate an alternating signal having a frequency of 60kHz. This causes the inverter to generate an AC supply having a frequency of 60kHz that causes a strike voltage to be generated across the lamp, as described above. In this case, the oscillator also uses the synchronisation input 49 to ensure that the alternating signal has a frequency identical to the resonant frequency of the second tuned circuit.

Once the lamp has struck, the lamp current regulator 35 detects the current flow through the lamp.

In order to achieve this the regulator 35 monitors the current in the lamp 80 by detecting the voltage across the resistor R21. In the example shown in Figure 1, all the lamp current flows through the resistor R21. However, in alternative configurations a capacitor (not shown) can be used to bypass the resistor R21, thereby reducing the dissipation in R21. In this case, typically about 10% of the lamp current flows through the resistor R21. However, this can be compensated for during the initial set up of the circuit.

Once current is flowing, the current flow is controlled using the lamp current regulator 35, the lamp power controller 55 and the variable resistor VR1. The variable resistor VRl is used to control the maximum power that is used by the lamp. This allows the ballast circuit to be used with lamps having different power characteristics, as well as allowing the output brightness of lamps to be adjusted.

Furthermore, as the inverter circuit is operated from a stabilised 400Volt supply generated by the power factor correction circuit 25, the power taken by the inverter 30 is proportional to the current flow through the variable resistor VRl . As the power used by the lamp is equal to the power used by the inverter 30 (less inverter losses which are small and are allowed for in the initial configuration of the circuit), the current flow through the variable resistor VRl is indicative of the current power usage by the lamp.

Accordingly, the lamp power controller 55 operates to monitor the voltage supplied to the inverter 30 by detecting the DC voltage across the capacitor C13 at the voltage sense input 59. In addition to this the lamp controller 55 also monitors the current taken by the inverter 30 by detecting the voltage across the variable resistor VRl at the current sense input 58.

The voltages detected at the voltage and current sense inputs 59, 58 are used to generate a reference voltage at the control output 57. The reference voltage is transferred to the regulator 35 where it is compared to the voltage measured across the resistor R21.

The difference between the reference voltage and the voltage across the resistor R21 is used to generate a second reference voltage that is transferred to the frequency control input of the oscillator 45.

Accordingly, the regulator 35 and the controller 50 measure:

1) The voltage across R21 - which is representative of the lamp current;

2) The voltage across the variable resistor VRl - which is representative of the current used by the inverter; and, 3) The voltage supplied to inverter 30.

Accordingly, by monitoring the voltage supplied to the inverter 30 and the current drawn by the inverter 30, this allows the lamp power controller to determine the current power usage of the lamp compared to the power level set by the variable resistor VRl .

It will be appreciated that the power taken by the inverter 30 is proportional to the dc current flowing through VRl, assuming that the inverter supply is exactly +400v dc. Accordingly, the voltage sense 59 monitors the inverter dc supply voltage to compensate for any deviation away from the nominal +400volts.

This allows the controller 55 and regulator 35 combination to determine whether the power used by the lamp is equal to, above, or below the power usage indicated by the setting of the variable resistor VRl.

Accordingly, if the lamp power (inverter power) is lower than required, as set by VRl, the lamp current is increased by decreasing the oscillation frequency of the inverter.

Thus, for example, if the power used by the lamp at the current settings is less than the power indicated by the variable resistor VRl, then the regulator 35 will generate a reference signal having a reduced voltage at the control output 38. This will lead to a decrease in the frequency of the alternating signal generated by the oscillator 45, which will in turn lead to a decrease in the frequency of the AC supply from the inverter 30, leading to an increase in the flow of current through the lamp.

As an alternative to controlling the supply of current to the lamp using the frequency control outlined above, the supply of current can also be achieved using pulse width or phase modulation. In these cases, instead of varying the operating frequency of the inverter 30, the system operates to vary the time the voltage is applied to the lamp.

In the case of pulse width modulation the duty cycle of the inverter is varied from 0% to 50%, causing the power through the lamp to be varied from 0 to 100%.

In the phase modulation technique both sides the lamp are switched by the inverter 30. Both sides of the lamp are switched at a 50%o duty cycle and at the same frequency, with the power throughput being varied by varying the phasing between the switching of the two ends of the lamp. When the two ends of the lamp are in phase, both ends are connected to the same voltage at the same time, and as a result no current flows through the lamp.

However, when the switching at each end is 180 degrees out of phase, each side of the lamp is connected to the opposite side of the supply voltage, resulting in the maximum voltage across the lamp and maximum power transfer. Varying the phasing between these two points will vary the power transferred between 0 and 100%.

In any event, the remainder will focus on the frequency-controlled operation of the inverter. Finally the protection circuit 65 operates to monitor the voltage supplied to the lamp by measuring the voltage across the capacitor C9 as shown. In normal operation, the voltage should initially rise when the pre-heat cycle is finished. Once the lamp has struck, the voltage will then fall and remain constant as the resistance of the lamp falls.

Accordingly, the protection circuit 65 operates to monitor the voltage and generates an inhibit signal at the inhibit output 67 if:

1) The lamp fails to strike, in which case the voltage will initially rise and then remain at a high level (approx. 1.5kV);

2) The lamp voltage rises above a pre-set level; or,

3) The lamp becomes defective and generates an unstable lamp voltage.

In addition to this, the protection circuit also monitors the current used by the lamp. When lamps become older, they typically suffer loss of emission from the lamp heaters. As a result, the lamp voltage increases. To counteract this, the circuit will automatically (through operation of the regulator 35 and the controller 55) reduce the lamp current to control the lamp power at a constant level. This is performed until the lamp current equals a lower preset lamp current (150mA in this example). At this point the current is not reduced further and the power used is allowed to increase above the limit specified by the variable resistor VRl .

The process is allowed to continue until the lamp power has risen to 30% above the indicated lamp power set by the variable resistor VRl . At this point the protection circuit 65 detects this difference and generates an inhibit signal at the inhibit output 67.

The generation of the inhibit signal at the inhibit output 67 causes the oscillator to inhibit the alternating signal generated at the output 48, which in turn causes the inverter 30 to stop generating an AC supply. As a result, this causes the ballast to stop supplying current to the lamp, thereby acting as a fail-safe, should a fault occur.

Accordingly, the above described circuit provides a ballast which automatically operates to pre-heat the lamps correctly irrespective of the lamp heater specification. This is achieved by ensuring atwo second heating period which is voltage controlled to ensure the correct heater temperature is used.

The ballast then generates a voltage to strike the lamp, even if the lamp is dead with the heaters being stuck open circuit. This is achieved by having the voltage controlled to continue to rise to a pre-set limit so that the voltage is sufficient to strike even dead lamps. The ballast then allows the power usage of the lamps to be controlled allowing different lamps to be used with the same ballast. This also allows two lamps to be connected to the ballast in series.

A current limiter is provided to protect the lamp and circuitry from mains born transients, as well as to ensure a limited inrush current of below one amp.

Finally the ballast includes a protection circuit which operates to shut down the lamp should a fault occur.

It will be appreciated by persons skilled in the art that numerous variations and modifications will become apparent. All such variations and modifications that become apparent to persons skilled in the art, should be considered to fall within the spirit and scope of the invention as broadly hereinbefore described.

Thus, for example, an alternative arrangement for the current limiter of Figure 2 is shown in Figure 3. In this example, the inductor L4 is no longer connected to the resistor R14, but is instead connected to the junction of the diodes D9, Dl l. The inductor L4 is also coupled to the variable resistor VDR2, and accordingly, the inductor L4 now forms the output 18 of the rectifier 15.

Accordingly, during use, the inductor L4 now operates to smooth the current output from the rectifier 15, as well as help limit and delay the rise in current that would charge the capacitor C13 from the mains transient.

Operation of the second example of the current limiter circuit will then be substantially as described above with respect to Figure 2, and will not therefore be described in any further detail.

Other modifications can also be made to the general ballast circuit shown in Figure 1, as will be appreciated by those skilled in the art.

Thus, for example, the inverter could be replaced by a power converter that uses switched mode techniques to convert the supply voltage into a form that was suitable for powering a discharge lamp. This power converter could take the form of an inverter, as discussed above, or a step-down forward converter or any number of other well know switch mode converters. In the case of the forward converter, this could be used to convert the power DC supply received from the current limiter into a pulsating DC supply that could be used to power the lamp. Such a circuit could limit the lamp current by varying the pulse width of the pulses supplied to the lamp.

A converter of this form may or may not include a separate reactor. Thus for example, the converter may incorporate a high leakage inductance transformer in its operation. In this case, the high leakage inductance transformer is fundamental to the operation of the converter circuit, but also provides the current limiting effect normally provided by the reactor, because of the nature of the transformer.

Persons skilled in the art will appreciate that numerous variations and modifications will become apparent. All such variations and modifications which become apparent to persons skilled in the art, should be considered to fall within the spirit and scope that the invention broadly appearing before described.

Claims

THE CLAIMS:-

1) Apparatus for controlling the operation of a discharge lamp, the apparatus including: a) An input for receiving an electrical supply; b) A power converter coupled to the input for generating a controlled output, the power converter being coupled to the lamp in use; and, c) A control system for controlling the operation of the power converter in accordance with the operation of the lamp to thereby control the power drawn by the lamp.

2) Apparatus according to claim 1, the power controller being controlled in accordance with the power drawn by the lamp. 3) Apparatus according to claim 1 or claim 2, the power controller generating a controlled electrical current, to thereby control the current drawn by the lamp. 4) Apparatus according to claim 3, wherein the power converter includes: a) An inverter coupled to the input for generating an alternating current; and, b) A reactor for coupling the lamp to the inverter in use. 5) Apparatus according to claim 4, wherein the reactor is at least one of a capacitor and an inductor.

6) Apparatus according to claim 4 or claim 5, the operation of the power converter being controlled by controlling the frequency of the alternating current generated by the inverter.

7) Apparatus according to claim 6, the control system including a control system input for setting the maximum power to be used by the lamp.

8) Apparatus according to claim 7, the control system further including: a) A first sensor for sensing the voltage supplied to the inverter; and, b) A second sensor for sensing the current drawn by the inverter.

9) Apparatus according to claim 8, wherein the control system input includes a variable resistor coupled to the inverter, the second sensor being adapted to measure the voltage across the variable resistor.

10) Apparatus according to claim 9, the control system includes a first controller, the first controller being adapted to: a) Compare the voltages measured by the first and second sensors; and, b) Generate a reference signal in accordance with the result of the comparison.

11) Apparatus according to claim 10, the control system further including: a) A third sensor for sensing the current supplied to the lamp in use; and, b) A regulator, the regulator being adapted to control : i) Compare the current measured by the third sensor to the reference signal; and, ii) Adjust the power output of the inverter in accordance with the result of the comparison. 12) Apparatus according to claim 11, the control system including an oscillator adapted to generate a signal having a selected frequency, the inverter being responsive to the signal to generate an alternating current having the selected frequency.

13) Apparatus according to claim 12 wherein the regulator is coupled to the oscillator, the oscillator being responsive to the regulator to adjust the frequency of the generated signal.

14) Apparatus according to claim 13, wherein the oscillator is a voltage controlled oscillator responsive to a control signal generated by the regulator, the voltage of the control signal being adjusted depending on the difference between the voltage measured by the third sensor and to the reference signal. 15) Apparatus according to any of claims 1 to 14, the lamp including lamp heaters, the apparatus further including a transformer coupled to the inverter for supplying a pre-heat current to the lamp heaters in use. 16) Apparatus according to claim 15, the apparatus further including a timer for controlling the length of time for which the pre-heat current is supplied to the lamp heaters. 17) Apparatus according to claim 15 or claim 16, when dependent on at least claim 12, wherein the timer is coupled to the oscillator to cause the oscillator to generate a signal at a predetermined frequency for a predetermined time period when the apparatus is initially activated.

18) Apparatus according to claim 17, wherein the transformer is coupled to the inverter via a capacitor, and wherein in use the transformer and the capacitor form a tuned circuit having a resonant frequency equal to the predetermined frequency.

19) Apparatus according to claim 18, when dependent on at least claim 10, the regulator being adapted to generate a signal having a second predetermined frequency when the predetermined time period has expired.

20) Apparatus according to claim 19, when dependent on claim 6, wherein the inductor is coupled to the inverter via a second capacitor, and wherein in use the inductor and the second capacitor form a second tuned circuit having a resonant frequency equal to the second predetermined frequency.

21) Apparatus according to any of claims 1 to 20, the apparatus further including a current limiter for coupling the electrical supply to the inverter, the current limiter limiting the current that can be drawn from the power supply.

22) Apparatus according to claim 21, the current limiter being adapted to: a) Determine if the current drawn from the power supply exceeds a predetermined threshold; and, b) In response to a successful comparison increase the impedance of the apparatus thereby limiting the current.

23) Apparatus according to claim 22, the current limiter including a capacitor coupled to the inverter and a resistor coupled to the capacitor in series, and wherein, in use, any current exceeding the predetermined threshold flows through the resistor to charge the capacitor, the resistor operating to limit the current flow. 24) Apparatus according to any of claims 21 to 23, the current limiter being adapted to operate while the apparatus is in use. 25) Apparatus according to any of claims 1 to 24, the electrical supply being adapted to supply an alternating current, the apparatus further including a rectifier for coupling the electrical supply to the inverter, the rectifier being adapted to convert the alternating current to a direct current in use.

26) Apparatus according to any of claims 1 to 25, the apparatus further including a power factor correction circuit, the power factor correction circuit being adapted to control the current drawn from the power supply to thereby provide a power factor close to unity.

27) Apparatus according to any of claims 1 to 26, the apparatus including a protection system, the protection system being adapted to deactivate the lamp if: a) The current power usage of the lamp is more than a predetermined threshold; and, b) Instabilities are detected in the lamp operation.

28) Apparatus according to claim 27, wherein the system includes a fourth sensor, the fourth sensor being adapted to measure the voltage across a capacitor coupled to the lamp to thereby detect the instabilities.

29) Apparatus according to claim 28, wherein instabilities are detected as random variations in the voltage measured by the fourth sensor.

30) Apparatus according to any of claims 27 to 29, when dependent on at least claim 7, the predetermined threshold being determined in accordance with the set maximum power.

31) Apparatus for controlling the operation of a discharge lamp substantially as hereinbefore described with reference to the accompanying drawings. 32) Apparatus for controlling the operation of a discharge lamp, the apparatus including: a) An input for receiving an electrical supply; b) A power converter coupled to the input for generating a controlled output, the power converter being coupled to the lamp in use; and, c) A transformer; and, d) A timer adapted to cause the transformer to couple the power converter to lamp heaters for a predetermined time period when the apparatus is initially activated, thereby preheating the lamp. 33) Apparatus according to claim 32, the power controller generating a controlled electrical current, to thereby control the current drawn by the lamp. 34) Apparatus according to claim 33 the apparatus further including a control system adapted to control the frequency of the alternating current in accordance with the amount of power used by the lamp to thereby control the current drawn by the lamp. 35) Apparatus according to claim 34, the apparatus being apparatus in accordance with any of claims 1 to 31.

36) Apparatus for controlling the operation of a discharge lamp, the apparatus including: a) An input for receiving an electrical supply; b) A power converter coupled to the input for generating a controlled output, the power converter being coupled to the lamp in use; and, c) A current limiter for coupling the electrical supply to the power converter, the current limiter operating to limit the current that can be drawn from the power supply.

37) Apparatus according to claim 36, the power controller generating a controlled electrical current, to thereby control the current drawn by the lamp.

38) Apparatus according to claim 37, the current limiter being adapted to: a) Determine if the current drawn from the power supply exceeds a predetermined threshold; and, b) In response to a successful comparison increase the impedance of the apparatus thereby limiting the current.

39) Apparatus according to claim 38, the current limiter including a capacitor coupled to the inverter and a resistor coupled to the capacitor in series, and wherein, in use, any current exceeding the predetermined threshold flows through the resistor to charge the capacitor, the resistor operating to limit the current flow. 40) Apparatus according to any of claims 37 to 39, the current limiter being adapted to operate while the apparatus is in use. 41) Apparatus according to any of claims 37 to 40, the apparatus further including a control system adapted to control the frequency of the alternating current in accordance with the amount of power used by the lamp to thereby control the current drawn by the lamp. 42) Apparatus according to claim 41, the apparatus being apparatus in accordance with any of claims 1 to 31.

43) Apparatus for controlling the operation of a discharge lamp, the apparatus including: a) An input for receiving an electrical supply; b) A power converter coupled to the input for generating a controlled output, the power converter being coupled to the lamp in use; and, c) A protection system, the protection system being adapted to deactivate the lamp if: i) The current power usage of the lamp is more than a predetermined threshold; or, ii) Instabilities are detected in the lamp operation.

44) Apparatus according to claim 43, the power controller generating a controlled electrical current, to thereby control the current drawn by the lamp.

45) Apparatus according to claim 44, when dependent on at least claim 7, the predetermined threshold being determined in accordance with the set maximum power 46) Apparatus according to claim 44 or claim 45, wherein the instabilities are detected as random variations in a voltage measured by a fourth sensor, the fourth sensor being adapted to measure the voltage across a capacitor positioned in series with the lamp.

47) Apparatus according to any of claims 44 to 46, the apparatus further including a control system adapted to control the frequency of the alternating current in accordance with the amount of power used by the lamp to thereby control the current drawn by the lamp.

48) Apparatus according to claim 47, the apparatus being apparatus in accordance with any of claims 1 to 31.