GB2201307A - Electronic starter for discharge lamps - Google Patents

Abstract

A starter circuit for a gas discharge lamp T, particularly fluorescent lamps with heated cathodes, uses a thyristor-type of electronic starter switch device F having a high holding current and which can be made to drop out of conduction at current below the holding value by grounding the gate terminal G. The grounding is controlled by a switch TR2 connected to the cathode K of the device through a delay circuit R4, C2. The cathode circuit includes a current sensing resistor R3, such that during an A. C. supply half-cycle the device F reacts with the switch TR2 and ballast L to provide a multiplicity of ignition pulses for the lamp T using the drop out mode of device F. To initially heat the cathodes before ignition the device F is used as an ordinary thyristor. A delay circuit C1, R5 provides a preheating period by delaying operation of the switch TR2 to prevent initiation of the drop out mode. A circuit R7, C1 is provided to ensure that circuit operation is terminated if the lamp T fails to strike. The cathode heating phase may alternatively be provided by making the sensing resistor R3 a PTC type, such as a filament lamp, so that the switch TR2 cannot close until the resistor has heated up, (Fig. 8) <IMAGE>

Description

Title: Electronic Starter for Discharge Lamps This invention relates to a circuit for starting a discharge lamp, particularly a fluorescent lamp, and which makes use of a three terminal device known as a fluorescent lamp electronic starter switch.

The main circuit elements of å fluorescent lamp electronic starter switch are shown in Fig. 12 of the accompanying drawings. It comprises a main thyristor SCR1 and an auxiliary thyristor SCR2 which are connected in a Darlington pair configuration between anode and cathode terminals A and K. The gate of the auxiliary SCR provides the external control terminal or gate G of the device. The anode and cathode terminals are bridged by a voltage-clamping device, such as Zener diode Z. The gate and cathode of SCRl are bridged by a resistor Ra of low value, typically Sn and up to a few tens of ohms. The gate and cathode of SCR2 are bridged by a resistor Rb typically of several hundred ohms, e.g. 700n.

The device as a whole exhibits particular switching characteristics and will be considered as a three-terminal entity F having the aforementioned terminals A, K and G.

The auxiliary thyristor SCR2 operates in the normal way but the main thyristor SCRl can onlyelatch in, and remain latched, if the current flowing in the low value resistor is sufficient to provide a base-emitter voltage to the internal NPN transistor portion ofCRl that will maintain the transistor turned on. The result is that the device has a high holding current value as compared with normal SCR devices. This characteristic can be put to good advantage in starting fluorescent lamps.

The basic device F and particular constructional features to obtain its desired switching characteristics are disclosed in patent specification EP 0118309A2 of Texas Instruments Limited where the device F is referred to as a fluoractor. In the present specification the device will be referred to as a starter switch device.

Particular attention is drawn to the description of the fluoractor 12 given with reference to Fig. 1. While the device could be realised as a unit using discrete circuit elements, the just-mentioned specification also discloses with particular reference to Figs. 5 and 6 how the circuit elements are effectively realised in integrated circuit form. Semiconductor devices of this kind are currently available as fluorescent lamp starter switches from Texas Instruments Limited under the type numbers Yllll and Y1112.

Specification EP 0118309A2 also discloses starter circuits for fluorescent lamps using the special characteristics of the starter switch device. Before turning to circuit detail, the characteristics of the device will be further discussed. The three terminal starter device has switching actions similar to a thyristor but provides additional features: 1) having initiated conduction as in a thyristor by triggering the control terminal, it is possible to call up a mode in which conduction will automatically cease at a finite and fixed current level (or holding current IH) typically 150 to 200mA. This mode is facilitated by drawing current from the control terminal G during conduction, or at least providing a low impedance path to the common negative rail (ground of the circuit in which the device is used.If there is no low impedance path from control terminal to common, the starter switch device acts as a thyristor and conducts to a low value of current before dropping out but if a low impedance path is provided, the device drops out of conduction next time it reaches the specified value of holding current.

2) Because the drop out of conduction of the starter switch device is to be used in conjunction with an inductor to generate high voltages, the starter switch device includes the on-chip Zener diode Z or the equivalent of a Zener diode which clamps the high voltage so generated to a finite value (typically 1200 volts) in order to prevent damage to the thyristor device and to any other circuit components. The clamping current of the Zener diode also flows between the anode and cathode 'terminals of the starter switch device.

The drop-out mode can arise in two circumstances.

The device gate is effectively grounded while the device current is below the holding current, in which case the device turns off. The device gate is effectively grounded while the device current is above the holding current, in which case the device turns off when the current drops below the holding current. These drop out of conduction modes are discussed in more detail below.

The starting switch device thus provides the two essential functions for starting a fluorescent lamp.

Acting as a thyristor it passes heater current to heat the cathodes, then using the drop out of conduction feature a high voltage is generated, in conjunction with the ballast inductor, to strike a conductive arc between the tube ends. This is illustrated diagrammatically in Figs. 1, 2, 3 and 4 of the accompanying drawings.

Fig. 1 shows the circuit of the fluorescent lamp T, its ballast choke L and starter switch device F connected in the starter configuration. Device F is connected through a full wave bridge B so as to be responsive to all half-cycles of the A.C. supply. Fig. 2 shows the heating mode of the device F with attendant current and voltage waveforms. Figs. 3 and 4 show the drop out modes in which the gate is clamped to ground to draw current from the gate prior to the device current reaching IH and after the device current has exceeded IH.Note that if the clamp switch is operated before the thyristor reaches IH (Fig. 3) then conduction ceases either immediately or at some value between the immediate value and IH (dependent on the amount of current drawn from the gate terminal by the clamp); whereas if the clamp switch operates after the device F has passed IH then conduction continues until IH is reached on the downward direction of current (Fig. 4). The voltage waveforms shown are obtained with the device F connected in the ballasted circuit of Fig. 1.

In the practice of this invention we are dealing chiefly with the interruption of the rising current waveform (Fig.

3).

Fig. 5 shows a complete starter switch circuit- (except for the lamp and ballast inductor) typical of those used in existing designs. The circuit is connected between rails 10 and 12 fed by the rectifier bridge B. When first energised, timing capacitor C1 is at zero voltage and the clamp thyristor TH1 is inactive. The device F is therefore triggered via R1 and conducts in the thyristor mode to give heating current. The forward voltage drop across diodes D1 and D2 slowly charges the timing capacitor C1 via R2 until the capacitor voltage reaches a level nearly equal to the turn on voltage of thyristor TH1. The presence of R3 causes a voltage component present at TH1 trigger which is proportional to the voltage - across D1 and D2 in addition to the steady voltage ramp (Vcl) on C1.

Fig. 6A shows the current (IF) in device F. Fig. 6B shows the waveform on TH1 trigger terminal. When the waveform (Vc1 plus superimposed ripple) reaches the trigger level, thyristor TH1 turns on and clamps the gate of device F thus selecting the drop-out (IH) mode. This first of all occurs on the trailing part of the current waveform (see Fig. 4), and then on the leading part (Fig.

3). C1 then continues to charge until TH1 remains clamped at all times preventing any further triggering and hence conduction of the starter switch device. This condition is held by the current now in R4 and the starter becomes quiescent. Normally the fluorescent lamp starts when the high voltage pulses VT occur (Fig. 6C).

The circuit described is typical of those of the prior art which are characterised by providing just one high voltage pulse each half cycle of applied mains voltage.

This puts a limit on the energy available to initiate conduction in the tube and, as a result, it is essential to provide a) a very high voltage pulse and b) a safe amount of heating time to ensure that the cathodes are well heated under the most adverse conditions.

With the highest practical voltage starter switch devices, there is still insufficient energy to start a 100 watt 2400 mm Krypton filled tube reliably and the prior art starter circuits are not recommended for this application.

There will be described hereinafter starter circuits embodying the present invention intended to provide a higher energy function which is sufficient to start 100w Krypton tubes. It may also be used with lower power tubes where a reduction in heating, and therefore -faster starting, can be achieved if required.

Whilst greater energy can be provided by increasing either or both the Zener clamp voltage in the starter switch device F and the holding current, these methods demand a new design of device which (if a compact integrated circuit device is to be used) is likely to be expensive both in development and manufacture. Instead the starter circuit now proposed has been developed on the basis of using existing starter switch devices. It is now proposed to provide new circuitry to give not one but a multiplicity of high voltage pulses each half cycle of the applied mains supply. Because the pulses follow one another quickly (typically at less than 1 milli-second intervals) the ionisation effect on the tube becomes cumulative with each successive pulses and the total effective energy is greatly increased. This effect is referred to as multipulsing.

Electronic starter circuits have previously been proposed in which efforts have been made to provide multiple -starting pulses in one or more half-cycles of the supply voltage. In this connection reference is made to patent specifications GB 1 418 080, GB 1 445 291, GB:l 569 045 and GB 1 574 518.

According to the present invention there is provided a starter circuit for a gas discharge lamp connected in circuit with a ballast inductor, comprising: an electronic starter switch device having its anode and cathode connected between rails for receiving rectified half-cycles of an A.C. supply, means connected from the rail associated with the anode of the starter switch device to its gate to trigger the device into conduction, switch means connected between the gate of said device and the rail to which its cathode is connected and having a control terminal for controllably putting the switch means into open or closed state, current sensing means responsive to the current flow through the starter switch device and connected to the control terminal of said switch means to cause closure thereof when the current sensed exceeds a predetermined level, and the circuit arrangement being such that the starter switch device is operable to be triggered on and turned off a number of times in one half-cycle.

The starter circuits to be described are particularly intended for use with fluorescent tubes having heated cathodes. The starter circuit rails are connected to the respective tube cathodes through a rectifier, preferably a full-wave rectifier bridge to use all half-cycles of the supply. The starter circuit is provided with timing or delay means to ensure that on initial switch-on the starter switch device is used as a thyristor conducting a substantial portion of each half-cycle to provide cathode heating prior to the controllable switch means becoming active to cooperate with the starter switch device to produce a number of ignition pulses in a following halfcycle. The delay may be provided through the current sensing means if the latter is made a PTC resistor in the main current path of the starter switch device.

Provision may also be made to terminate the generation of pulses if eventually the tube fails to strike, e.g. the tube may be faulty. The circuits to be described are rendered non-operational once the tube has struck and is running normally. It has been found preferable to provide some delay in the interaction of the starter switch device and the controllable switch to ensure that the required pulse operation is achieved and avoid the circuit going into uncontrolled oscillation. The delay can be provided in the current sensing means to delay response to sensed current changes of at least one polarity.

Having discussed various aspects of starter switch devices and existing circuits using them with reference to Figs. 1 to 6 and 12 of the accompanying drawings, the present invention and its preferred practice will now be described with reference to Figs. 7 to 10 of the accompanying drawings. In the drawings: Fig. 1 shows a basic fluorescent lamp starter circuit using a starter switch device: Fig. 2 illustrates the control of the starter switch device in the heating mode for the lamp with the current and voltage waveforms: Figs. 3 and 4 show respective modes in which the gate of the starter switch device is clamped to interrupt conduction through the device, relevant current -and voltage waveforms being shown in each case:: Fig. 5 shows an existing starter circuit employing a starter switch device; Fig. 6 shows waveforms of device current (A), gate voltage (B) and lamp triggering voltage pulses (C); Fig. 7 shows the principle of a multipulse triggering waveform; Fig. 8 shows a starter circuit incorporating a starter switch device in accord with the present invention for cold starting fluorescent tubes; ; Fig. 9 shows waveforms relating to the operation of the circuit of Fig. 8, a) relating to the applied supply voltage, b) the load current in the circuit, and c) the voltage at the starter switch device anode including the pulses for triggering the lamp, the number of such pulses being reduced for ease of illustration, Fig. 10 shows a preferred starter circuit incorporating a starter switch device in accord with the present invention providing a heating phase in the starting of a fluorescent tube and automatic shut-down if the tube fails to start, Fig. 10A being a table of component values for the circuit; Fig. 11 shows waveforms a) - d) relating to the operation of the circuit of Fig. 10.

Referring firstly to the prior art circuit of Fig. 5 again and the associated waveforms of Fig. 6, it will be seen that the triggering of thyristor TH1 results in one high voltage pulse occurring as IH is reached.

Thereafter, TH1 remains conductive until the applied mains voltage returns to zero thus preventing any further conduction of the starter switch device F for the rest of the half cycle.

In contrast to the operation of the circuit of Fig. 5, a multipulse mode of operation is proposed as is diagrammatically illustrated in Fig. 7 Fig. 8 shows a simple multipulse circuit in which the clamp function is now provided by a non-latching device (i.e. a simple transistor TR1) which is controlled in such a way as to remove the clamp action on the control terminal G of device F once the high voltage pulse has occurred, thus enabling the device to retrigger and repeat the high voltage pulse generation sequence.

In more detail, and referring to the complete circuit of lamp, ballast and basic multipulse circuit as shown in Fig. 8, and associated waveforms in Fig. 9, the-;operation is as follows: when the A.C. supply voltage Vs (Fig. 9a) reaches 'a sufficient level, starter switch device F is triggered' via R1 and Zener diode Z1. Current then begin to build up in ballast inductor L as shown in the waveform diagram Fig. 9b.Because of the thyristor chåracteristic of the device, the current will continue to build-up even though the trigger voltage has been removed due to the conduction of device F - see the voltage waveform of Fig. 9c which shows the voltage VT at the device anode and represents the voltage across the cathodes of the lamp T. If however the control terminal is now clamped, the current will either stop immediately or only rise to some later value below IH governed by the amount of current drawn from the trigger terminal. Clamping is therefore applied by turning on transistor TR1 by feeding its base drive from a current sensing resistor R3 in the cathode path of the starter switch device. (The value of R3 is small but must be sufficient to apply the clamp action before IH is reached.Typically R3 = 5.6fl.) When the main current is cut off in this way inductor L is caused to develop a large back EMF which appears at the device anode (voltage VT). The peak value of VT is limited by the high voltage of the Zener diode in the starter switch device and therefore the load current ramps down at a rate given by dl = Vz.

dt L The Zener current also flows in R3 thus keeping the clamp on and preventing retriggering of the device. The result is a substantial pulse P (Fig. 9c) for triggering the lamp T, the pulse having the Zener voltage (say 1000 - 1500V) and the duration of the ramp down interval.

When the current due to the inductor L diminishes sufficiently the clamp transistor TR1 stops conducting and the device F can retrigger whereupon the cycle of current increase to IH etc. is repeated. In practice it is necessary to delay the drive to the clamping transistor.

Without a delay the commencement of a change in the main current can bring about an immediate change in drive condition to the device F such as to restore the main current. If this happens high frequency oscillation is likely to result. The delay is provided by R4 and C1.

Depending on the value of delay, it may cause the removal of the clamp to occur, d significant time after the current has returned to zero giving a dwell period before the next trigger point. This is manifest in a dwell on the multipulse waveform as shown in Fig. 9c. The diode D1 in the circuit of Fig. 8 is to give a comparatively large component of feedback voltage (the forward voltage drop of D1) which is substantially current independent. This overcomes the forward voltage drop of the base-emitter diode of TR1. R3 then only has to be such as to provide the additional voltage necessary to overcome the drop in R4 in order to turn on TR1. With appropriate choice of components, the number of high voltage pulses per half cycle of the mains supply can be typically 10 to 15.

(Only three are shown in Fig. 9 for simplicity). Note that the timing of the multipulse cycle period is governed principally by the inductance value of the ballast inductor L and the value of current flowing when conduction ceases, the applied mains voltage and the voltage of the Zener diode in the device F.

The circuit of Fig. 8 is essentially an ignitor i.e.

it does not provide for a heating period for the tube cathodes. The high voltage pulse energy produced is so considerable that it will cold start most types of fluorescent tube. Whilst this is not recommended because of the injurious effect on the cathodes which cold starting causes, it does demonstrate that in the event of too short a heating period (such as might be caused by low applied mains voltage and/or low ambient temperature) a successful start can still be achieved once the multipulse phase commences.

The circuit of Fig. 8 can be modified very simply to provide a heating phase prior to the multipulse phase by making R3 a temperature dependent resistor (PTC). By an appropriate choice of cold and hot values for R3, the voltage fed back to the clamp transistor is initially below its turn-on threshold so that the starter switch device acts as . a simple thyristor conducting over a substantial portion of each half-cycle. As the current heats R3 its value increases until the clamp operates and multipulsing commences. The thermal capacity of R3 is chosen to give the required heating time for the lamp cathodes. R3 can be a filament lamp.

Another multipulse starter which provides a heating phase followed by multipulsing, followed by shut down if a tube fails to strike is shown in Fig. 10. The waveforms occurring at various points in the circuit of Fig. 10 are shown in Fig. 11.

The feedback of a voltage derived from the main current to operate a transistor clamp as described for the circuit of Fig. 8 is again used. To reduce the drive current required for the clamp however, a series pair of transistors (Darlington pair) TR2 is used for the clamp.

As the input voltage to the Darlington has to be twice the normal base-emitter diode voltage before current flows, two diodes D1 and D2 are used in the main current path from the starter switch cathode terminal K. Resistor R3 in series with D1 and 2 provides the current dependent component of clamp drive voltage and diode D3 shunting R3 is added to limit the maximum voltage occurring across R3.

The timed heater current phase of the starter is provided by the function of the additional components R5 and C1 and the automatic shut down facility (in the event of the tube failing to start) by Zener diode Z2 and R7 in conjunction with C1. The essential delay in the clamp drive path is now provided by C2 connected from collector to base of the Darlington pair. In this connection C2 serves in addition to give high frequency gain stability to the very high gain circuit and also provides a low output impedance at the Darlington collector.

The circuit operates as follows: on first switching on there is no charge on C1. The values of R4 and R5 are chosen so that the voltage fed back from the common (cathode) terminal K of the device F to the Darlington base is insufficient to turn on the Darlington (R4 is typically twice the ohmic value of R5).

Therefore the clamp does not operate and the starter switch device F behaves as a thyristor being triggered via R1, Z1 and Z2 and passing heater current through the lamp cathodes. C1 charges up through R4 and R5 during this phase which lasts about 1 second. (The time being determined mainly by the values of C1, R4 and R5).

When C1 reaches a sufficiently high voltage, the proportion of the voltage across D1, D2 and R3 reaching the Darlington base becomes sufficient to turn on the Darlington pair just before the device current reaches 1H The circuit now enters the multipulse phase. Referring to the waveforms of Fig. 11, the device F is triggered producing near zero voltage across the starter (Fig. lla) and causing the current in the ballast inductor and tube heaters to rise linearly (Fig. llb). Early in the current ramp before IH is reached, the voltage across D1, D2 and R3 (Fig. lld) reaches a level sufficient to drive current (via R4) into the Darlington base. The Darlington base can be considered as a virtual earth and the Darlington collector ramps down at a rate determined by C2 and the current in R4 less than that in R5 (Fig.

llc). Just before the main current reaches IH theterminal G is fully clamped at about 0.75V i.e. below the voltage of the common terminal which is at about 2V.

Under these conditions it is found that the starter switch device drops out of conduction at a current level just below IH and a high voltage pulse is generated by the current flowing in the ballast inductor L (Fig. lla). A decreasing ramp of current flowing via the internal Zener diode of the starter switch device (Fig. llb) now occurs.

This causes the voltage across D1, D2 and R3 to decrease and turn off the Darlington at a rate determined by the falling current and the lag capacitor C2. When the Darlington unclamps, the device F re-triggers and the pulse generation cycle repeats. Pulses continue to be produced until the applied mains voltage reaches a level in the half cycle which is too low to trigger the device F through R1, Z1 and Z2 Pulsing then ceases until the following half cycle. Z1 + Z2 voltages are chosen such that an onset of conduction in the tube T reduces the voltage across the starter to a level at which it will not trigger thus automatically- stopping any further starter action once the tube is- running.During the multipulse phase, C1 does not charge up further (the limited amount of charging being balanced by the discharge via R5, R4 and R6) so that if a tube fails to start the multipulse phase would continue indefinitely. To prevent this, R7 is connected to charge C1 slowly during multipulsing. It does so only if current is flowing in R1 either to the starter switch device gate G or through the clamp, and Z2 provides a constant voltage to R7 when this is the case.

Time out of the multipulse phase occurs in about g to 1 second by this means.

R6 is present in order to facilitate the discharge of C1, either when the lamp is running or when the system is switched off, ready for the next start cycle. R2 is selected in order to arrive at a practical value for Z1 (say 200v) and C3 is added to attenuate high value re ignition pulses which occur on some tubes in the running condition and which might otherwise give unwanted triggering of the starter.

This starter is suitable, and operates in a similar manner, for both leading and lagging ballast types (lagging - ballast inductor only, leading - consisting of inductor plus series capacitor CB as shown dotted in Fig.

10).

The multipulse system described can also be used to ignite other types of gas discharge lamp requiring the application of high voltage energy to initiate conduction.

Claims (14)

1. A starter circuit for a gas discharge lamp connected in circuit with a ballast inductor, comprising: an electronic starter switch device having its anode and cathode connected between rails for receiving rectified half-cycles of an A.C. supply, means connected from the rail associated with the anode of the starter switch device to its gate to trigger the device into conduction, switch means connected between the gate of said device and the rail to which its cathode is connected and having a control terminal for controllably putting the switch means into open or closed state, current sensing means responsive to the current flow through the starter switch device and connected to the control terminal of said switch means to cause closure thereof when the current sensed exceeds a predetermined level, and the circuit arrangement being such that the starter switch device is operable to be triggered on and turned off a number of times in one half-cycle.

2. A starter circuit as claimed in Claim 1 including means for introducing a delay in the interaction of the starter switch device and the switch means to define an interval between pulses.

3. A starter circuit as claimed in claim 1 in which current sensing means includes a delay circuit for delaying the response of the switch means to changes of the sensed current of at least one polarity.

4. A starter circuit as claimed in claim 1, 2 or 3 in which said gate triggering means includes a threshold voltage device for establishing a minimum, triggering voltage.

5. A starter circuit as claimed in claim 1, 2, 3 or 4 for use with a lamp having heated cathodes, comprising means to delay the closure of the switch means for a plurality of half-cycles upon initial energisation of the circuit such that the starter switch device is triggered in each half-cycle of that plurality to be conductive for a substantial portion of the half-cycle to provide a heating current for such a lamp.

6. A starter circuit as claimed in claim 5 in which said closure delay means comprises a capacitor connected to the control terminal of said switch means to receive a charge in each of said plurality of half-cycles, the capacitor voltage level preventing closure of the switch means until a predetermined voltage is achieved.

7. A starter circuit as claimed in claim 1, 2, 3 or 4 for use with a lamp having heated cathodes in which said current sensing means includes a resistive element connected in the main current path of the starter switch device and having a temperature coefficient such that, on initially energising the starter circuit, the switch means is inactive 'until said element is heated to reach a predetermined resistance, whereby on initial energisation the starter switch device is triggered to conduct for a substantial portion of each of a plurality of halfcycles to provide heating current for the lamp and the resistive element.

8. A starter circuit as claimed in any preceding claim comprising means for inhibiting the triggering of the starter switch device when the rail voltage is at a value consistent with that obtained when the starter circuit is connected to a lamp that has struck.

9. A starter circuit as claimed in any preceding claim comprising means for terminating the operation of the starter circuit on continued pulse generation-over a plurality of half-cycles.

10. A starter switch circuit as claimed in any preceding claim further comprising a full-wave bridge circuit to the output terminals of which said rails are connected to receive rectified half-cycles.

11. A gas discharge lamp circuit comprising a gas discharge lamp; a ballast inductor and a starter circuit as claimed in any preceding claim connected such that the starter circuit generates pulses to apply across the lamp by repetitively drawing increasing current through the ballast inductor upon triggering on of-^the'-starter switch device followed by a ramping down of ifihecurrent through its internal Zener diode upon closure of switch means, a pulse being generated during the interval off the ramping down.

12. A gas discharge lamp circuit as claimed in Claim 11 in which the lamp is a fluorescent tube having heated cathodes, the ballast inductor being connected in series with the cathodes and said starter circuit, and said starter circuit being connected between the heated cathodes.

13. A starter circuit for a gas discharge lamp or a gas discharge lamp circuit incorporating same substantially as hereinbefore described with reference to Figs. 8 and 9 or Figs. 10 and 11 of the accompanying drawings.

14. A fluorescent lamp circuit substantially as hereinbefore described with reference to Figs 8 and 9 or to Figs. 10 and 11 of the accompanying drawings.