GB2028019A - Circuit arrangements for converting A.C. voltages into higher A.C. voltages - Google Patents

Abstract

In order to convert an alternating- current voltage into a substantially higher alternating voltage a load (9) e.g. a gas discharge lamp is connected to alternating current power supply input terminals (3) via the series combination of a choke (inductor) (4) and a capacitor (5) forming a resonant circuit. A starting device (8) including a full-wave rectifier (10), to which a voltage divider constituted by resistors (12, 13) is connected, is connected in parallel with the load (9). A further capacitor (14) is connected in parallel with one resistor (13) of the voltage divider and is connected by way of a connecting diode (15) to the control electrode of a controllable switching element (1), which is also connected to the output of the full-wave rectifier (10). The thyristor may be replaced by a transistor, or a triac in which case the rectifier could be located after the triac. In operation, when the thyristor 1 is fired, capacitor 5 charges and its voltage is added to the supply voltage to provide a high ignition pulse for the discharge lamp. A fusing element 2 protects the circuit against overload. <IMAGE>

Description

SPECIFICATION Circuit arrangements for converting a.c. voltages into higher a.c. or d.c. voltages This invention relates to circuit arrangements for converting alternating current (a.c.) voltages into substantially higher alternating current or direct current (d.c.) voltages, particularly (but not exclusively) for igniting gas discharge lamps with a high ignition voltage requirement.

The term 'alternating current (a.c.) voltages', as used herein, relates not only to periodically varying voltages, but, in particular, also to voltage peaks which occur irregularly both in amplitude and also in polarity.

For the conversion of a.c. voltages into higher a.c.

voltages, transformers with a high turns ratio are generally used. However, if the secondary voltage is to be as high as possible, difficulties arise. These difficulties are not only due to the fact that the number of secondary windings becomes very great.

Apart from special measures to prevent internal flash-over, such transformers become very expensive and heavy. Instead of transformers, arrangements are known which work with inductive current surges but which have the disadvantages of being technically extremely complicated, expensive and of very extensive dimensions.

Such problems occur interalia, and in particular, in the ignition of gas discharge lamps.

Gas discharge lamps, with or without pre-heatable electrodes, can be ignited in all sorts of ways. Lamps with pre-heatable electrodes require only low ignition voltages, while those with non-pre-heatable electrodes require higher ignition voltages. While the problem of igniting the first-mentioned type of lamp would appear to have been satisfactorily resolved, the problem of an inexpensive spacesaving and functionally reliable ignition means for lamps having a high ignition voltage requirement has not yet been satisfactorily resolved.

In the past, for the ignition and operation of such lamps, transformers have mainly been used. The transformers suffer from the disadvantages of the considerable space they occupy, the high prime costs, the high connected loads, and the fact that they do not shut down automatically in the event of a lamp faiiure. Continued generation of the high secondary voltage of the transformer in the event of a lamp failing results in this high voltage being constantly available at the lamp terminals or mountings, so that special protective measures are re quiredwith effect from a certain level of ignition voltage.

Furthermore, ignition devices for high ignition voltages have become known which are based on the principle of the sudden variation of a current by an inductor with the help of a switch and, connected therewith, the induction of a voltage peak. Also known are circuit arrangements for generating voltage peaks by closing and re-opening a switch parallel with a capacitor which is connected in series with a coil.

In the case of the last-mentioned arrangements, the switches for opening and closing the circuit are in most cases mechanical (relay contacts, manually actuated push-buttons, contacts which can be actuated by automatically flexing bimetal strips, and so on) or electronic (thyristors, triacs, transistors, varistors). Mechanical switches have the disadvantage that, upon switching high voltages, they wear out relatively quickly and, furthermore, disturbing noises occur during the switching process. Furthermore, appropriately designed relays, for example for igniting gas discharge lamps, can scarcely be housed in conventional starter housings.

Hitherto known electronic switching devices for generating higher voltages in most cases require additional transformers (e.g. German Patent No. 2 060 472, Austrian Patent No. 256 991 and German Patent No. 2 009 442), which increase the amount of space required and also the prime costs, or which are otherwise very complicated in their construction.

A simple electronic circuit arrangement with a switch (thyristor) in parallel with a capacitor and in series with a coil, but not for very high voltages (twice the mains voltage) is described in German Patent 1 764995. However, this circuit not only has the disadvantage of limited magnitude of ignition voltage but also the disadvantage that it does not switch itself off if the gas discharge lamp is not ignited.

A further circuit arrangement, though certainly one which is conceived just for relatively low ignition voltages, is described in German Patent No. 1 957 672. With this circuit arrangement, pre-heatable electrodes are heated up sufficiently and then ignited by the mains voltage. In the case of the starter device in this circuit arrangement, in conjunction with a capacitor which is in series or parallel with a choke coil, undesired oscillation phenomena occur, and, connected therewith, voltage peaks, since the capacitor, which breaks off the ignition process after a few cycles, has to be designed for this voltage.However, in order to be able to fulfil its function, this capacitor must have a capacitance of the order of microfarads, i.e. according to the current state of the art, for higher voltages, its geometrical dimensions are such that it, together with the other elements of the ignition device, cannot be accommodated in conventional starterhousingsforfluores- cent lamps, which are generally cylindrical and have a diameter of approximately 20 mm and a length of 35 mm. Also, the prices of such capacitors are currently still very high. Furthermore, this large capacitor represents a virtual short-circuit for voltage peaks generated by an inductor. The thyristor envisaged in this circuit arrangement is ignited again at once by such a pulse through a trigger diode, i.e.

the voltage peak cannot exceed a certain value, and the width (duration) of the pulse is very small. The rapid re-ignition of the thyristor after reaching a certain voltage level is further assisted by a voltage divider which, to guarantee functioning, must be so designed that the circuit arrangement ignites already at the commencement of the first applied half-wave of the mains voltage, in other words switches even more rapidly when higher voltages are applied.

According to the present invention there is pro vided a circuit arrangement for converting an a.c.

voltage into a substantially higher a.c. or d.c.

voltage, comprising input terminals for connection to a supply of the a.c. voltage to be converted, output terminals for connection to a load, a series combination of a choke and a capacitor forming a resonant circuit and interconnecting the input and output terminals to connect the supply to the load when the arrangement is in use, and a starter arrangement connected across the output terminals and comprising a full-wave rectifier, the input of which is substantially in parallel with the output terminals and the output of which is connected to a controllable switching element which can be controlled via a switching device by a voltage applied to a further capacitor and which is connected to a divider point of a resistive voltage divider which is connected to the output of the full wave rectifier, wherein the voltage divider is constituted by two resistors which are connected substantially directly to the full wave rectifier and said further capacitor is connected in parallel with one only of the resistors of the voltage divider which supplies a control voltage for the controllable switching element, the further capacitor being connected via the switching device to a control electrode of the controllable switching element.

In a circuit embodying the invention and described in detail hereinbelow, the controllable switching element interrups the current in the resonant circuit once or several times and is in series with the resonant circuit comprising the choke and the capacitor. The controllable switching element is in parallel with a load, e.g. a gas discharge lamp having a high ignition voltage requirement is so connected that in the conductive condition of the switching element, the charge of the capacitor in the resonant circuit changes its polarity every time and the absolute amount of the voltage present at this capacitor, starting at zero first of all, during the course of a few of these switch conditions which conduct current, reaches a maximum value adjustable by selection of the values of control circuitry of the switching element.In the intervals when the controllable switching element does not conduct, the sum of the voltage attained at the capacitor in the conductive condition of the switch, plus a possible slight drop in voltage due to minor discharge currents in the control circuit or in the load, and the mains voltage present at the load is available as a useful voltage. The circuit arrangement, for generating voltage peaks, has a major advantage that the height of these peaks can firstly be adjusted almost as desired and is limited in the upward direction only by the dielectric strength of the component used.

Furthermore, the width of the voltage peaks is substantially greater than with conventional arrangements. This width corresponds to the conduction or opening time of the controllable switching element. A further advantage is the energy available in the capacitor, for example for igniting a gas discharge lamp which for conventional magnitudes of inductance and capacitance is substantially greater than the energy stored in a coil in some of the hitherto known ignition devices by rapid current fluctuations in the coil. The low price and the small geometrical dimensions of the few component parts can be indicated as further advantages of the circuit arrangement.Above all, the dielectric strength of the controllable switchable element, without planning for any reserve, can be fully utilised since also, as is indicated hereinafter, non-destructive voltage flashover in the controllable switching element does not adversely affect the functioning of the arrangement.

This fact brings with it the great advantage that the controllable switching element can be of relatively small dimensions, since the price and the dimensions of commercially available switching elements (e.g. thyristors triacs, and possibly transistors) increase quite sharply with the dielectric strength required. By a corresponding dimensioning of the control circuit, the ignition device is switched off upon completion of the process of igniting the gas discharge lamp, i.e. when the latter has reached its burning voltage which is low in comparison with the mains voltage.

Conveniently, there is in the circuit upstream of the controllable switching element a fusing element which switches off or at least greatly reduces the current, subject to a time lag. By reason of this fusing element, the ignition device can be switched off automatically, after a time lag, in the event of failure of the gas discharge lamp for example, in other words in the case of its failing to ignite. Furthermore, the fusing element also safeguards the controllable switching element and acts in the same way as an over-current protection means.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 shows a circuit arrangement embodying the invention; and Figure 2 shows voltage and current diagrams for the circuit arrangement of Figure 1 as a function of time t, diagram a showing a mains voltage E, diagram b indicating a voltage u14 occurring at a capacitor 14, diagram c showing a voltage uL occurring at a choke 4, diagram dshowing a voltage u0 occurring at a capacitor 5, diagram e showing a voltage uv occurring at a load 9, and diagram f showing a current iflowing through a controllable switching element.

As Figure 1 shows, a load 9, e.g. a gas discharge lamp, is connected via a choke 4 and a capacitor 5 to alternating current mains supply input terminals 3. A starter device 8 is connected in parallel with the load 9 via connection terminals 6, 7. The starter device 8 comprises a full-wave rectifier 10, a controllable switching element 1 in the form of a thyristor, a fusing element 2 and a control circuit 11 for the controllable switching element 1.

The fusing element 2, which after a time lag switches off or reduces the current, can for example be a ballast resistor, an over-current protector, a resistor which becomes destroyed and nonconductive after current has flowed through it for a time, a bimetallic strip which, upon being heated by a heating resistor in the circuit, will after a time open a contact which can be re-set manually, a relay, or some other electronic component by which current through the controllable switching element 1 is interrupted so long as the mains voltage is connected. The fusing element 2 can also be a heating wire, the length of which is altered when it is heated and which, as a result of this variation in length, opens a contact which can be re-set.

The control circuit 11 comprises a voltage divider constituted by voltage divider resistors 12, 13 which can be connected either before or after the fusing element 2 (points A or B), a capacitor 14, a trigger element (diode) 15, and, optionally, a resistor 16. An RC network 18, may be connected in parallel with the series combination of the controllable switching element 1 and the fusing element 2.

The operation of the circuit arrangement will now be described with reference to Figure 2. If, at a moment in time t = O, a mains voltage E (Figure 2a) is applied to the input terminals 3, then Figure 2b) the capacitor 14 is charged via the choke 4, the capacitor 5, the rectifier 10 and the resistor 12. The voltage divider resistors 12 and 13 are chosen to be large enough that it requireds several cycles of the mains voltage E for the capacitor 14 to be sufficiently charged for the ignition voltage U15 of the trigger diode 15 to be attained. It can be demonstrated, both mathematically and experimentally, that the ignition voltage u15 of the trigger diode 15 is always reached in the vicinity of the maximum value of an incoming rectified voltage half-wave u5 (shown by broken lines in Figures 2e).

During the process of charging of the capacitor 14 from the point in timet = O tot = to, on account of the extremely low current through the reistors 12, 13 and the capacitor 14, and since the lamp 9 is not yet ignited, virtually no voltage is present at the choke 4 or at the capacitor 5 (see Figures 2c and d). The voltage uL at the choke 4 and the voltage uc at the capacitor 5 are virtually nil. The load voltage uv (Figure 2e) is practically equal to the mains voltage E, whereby here and in the following, the ohmic resistance of the choke 4 and of the capacitor 5 and the resistance of the fusing element 2 are not taken into account, since the sum of these resistances has no influence until after a certain magnitude, which does not arise in practice, has been reached.At the moment to, the ignition voltage u15 of the trigger diode 15 is reached, the capacitor 15 discharges through the trigger diode 15 and, if fitted, the resistor 16, and ignites the thyristor 1. The capacitor 14 discharges only down to a voltage level Ur4U (Figure 2b) determined by the characteristic values of the trigger diode 15 and the resistors 16,17 and the resistance between the gate and cathode of the thyristor 1. By virture of the controllable switching element 1 becoming conductive, the switch voltage u5 and the load voltage uv become equal to zero.

Immediately after to, the same voltage uLO (Figure 2c) as at the mains input terminals 3 is present at the choke 4. A current i, restricted principally by the choke 4, starts to flow through the controllable switching element 1. The result is a voltage uc (Figure 2d) at the capacitor 5, the voltage of the choke 4 altering its polarity. The voltage U14U at the capacitor 14 (Figure 2b) remains virtually the same since the discharge takes place only very slowly, due to the magnitude of the resistors 12 and 13.

At a point in time ta, the current i becomes nil and the controllable switching element (thyristor) 1 stops conducting. The voltage peaks induced at the choke 4 by the in reality earlier switch-off, due to a short fall in the holding current of the thyristor, are of no importance as regards the principle of operation of the circuit. Since the current through the choke 4 and the capacitor 5 is now virtually nil again, the voltage uL at the choke 4 (Figure 2c1) disappears, the voltage u01 (Figure 2d) remains at the capacitor 5, and the voltage uv (Figure 2e) is then, due to the fact that E = ucl + uv, the difference between the mains voltage E and the voltage ucl at the capacitor (uv = E - uci).

The voltage uv varies between the time t1 and a time t2 in accordance with the mains voltage E (in the example according to Figure 2 it becomes greater) and is substantially greater than the latter. It is also present at the lamp 9. If the lamp 9 is not yet ignited by this long-lasting (several ms) voltage pulse, then the process continues as follows: the high voltage uv, rectified (us = | uV I ), is also present at the voltage divider 12, 13, the capacitor 14 is re-charged by the voltage U14U relatively quickly, approximately in half the duration of one cycle of the mains voltage, to the ignition voltage u15 of the trigger diode 15, the thyristor 1 ignites again at the time t2, the voltage uv becomes zero again, and the current i, due to the higher voltage present at the choke 4 and capacitor 5, becomes higher than with the first switching cycle.

Shortly after the time t2, the difference between the mains and the almost constant capacitor voltages is present at the choke 4 and diminishes as the current i increases and changes its polarity. The voltage at the capacitor 5 likewise diminishes and changes its polarity. At a moment in time t3, the absolute amount of the capacitor voltage Uc3 is in general substantially higher than at the point in times2. After the current is switched off at the time ts, once again the difference between the mains voltage E and the capacitor voltage Uc3 (rectified as voltage u5) is again present at the thyristor 1 and at the same time as a voltage uv at the lamp 9. Its magnitude is now substantially higher than in the preceding cycle and will generally ignite the lamp 9.If the lamp is not ignited this time either, the process is repeated, the voltage uv then becoming higher again. How great the voltage uv may be at maximum depend upon several factors: first and foremost, it depends on how the resistors 12, 13, the capacitor 14 and the trigger diode 15 (due to the magnitude of the voltage U15) are dimensioned, since the charging time of the capacitor 14 depends on this. The higher the voltage uv is, the more rapidly the capacitor 14 will be charged and the sooner the following ignition of the thyristor 1 will take place, i.e. the relevant moment of ignition will be displaced with reference to the phase situation of the mains voltage which is present. Also dependent upon this phase situation are how great the current i becomes and how the voltage of the capacitor 5 varies therewith in the conductive condition of the thyristor 1. There are phase situations of the thyristor ignition time in the vicinity of the point at which the mains voltage passes through zero, at which the capacitor voltage, after switching off of the thyristor, is smaller in its amount than before the preceding switch-on. The maximum increase in the amount of the capacitor voltage in the time during which the thyristor is conductive is achieved at thyristor ignition times in the vicinity of the mains voltage amplitudes. Furthermore, the level of voltage attained naturally depends also upon the dielectric strength of the components used. In general, with correct dimensioning, a stable state of oscillation will be adjusted at mains frequency.If the voltage uv becomes higher, then the moment of ignition will move closer to the passage of the alternating current voltage through zero and thus the capacitor voltage uc will become smaller, and vice versa. The value of this alternating current voltage which is present at the load 9 can be adjusted by the choice of the resistors 12,13 or by the capacitor 14.

For example, the greater the capacitance of the capacitor 14, the longer the charging process lasts and the higher the voltage uv must be so that this capacitor 14 can be charged in half the duration of one period of the mains voltage. The moment of ignition of the thyristor 1 moves in the direction of the maximum of the mains voltage, the amplitudes of the voltage uv becomes greater. In this way, the voltage uv may be made so great that the zero breakovervoltage of the thyristor 1 is reached so that it will conduct independently of the control circuit. Also in this way, it is possible to achieve a stable oscillation condition but after a prolonged period, if the fusing element 2 does not shut down previously, it will lead to over-heating of the thyristor 1 and alteration of its properties.

The circuit arrangement described above operates in its stable oscillation condition as a transformer. If the fusing element 2 is omitted, then in the case of high ohm loads, the circuit arrangement can also be used for purposes other than for igniting gas discharge lamps. Furthermore, it is also possible to rectify and smooth the voltage uv so that a high direct current voltage is available for various pur poses.

A situation may however, also occur where no stable oscillation conditions are achieved. It can in fact happen that the rapid rise in voltage at the thyristor 1 after the current passes through zero once again ignites the thyristor or the rectified current i, after passage through zero, again reaches the lock down (switching-on) current value within the circuit commutated turn-off time. This is readily the case at lower inductance values of the choke 4.These instabilities can be eliminated by various measures, for example, by increasing the inductance of the choke 4, by incorporating an additional inductance 20 shown in broken'lines in Figure 1 upstream or downstream of the rectifier 10, before the thyristor 1, by using higher speed thyristors, by using the RC network 18, 19 to diminish the steepness of voltage parallel with the thyristor, the capacitor 19 then also acting as a suppressor capacitor, or by incorporating the resistor 17 to increase the switching-on (lock down) current. In the case of use for igniting gas discharge lamps, however, they generally do not have a disadvantageous effect.Then, namely, as described, the thyristor will unintentionally ignite a number of times until some time, normally after two or three ignitions, upon a passage through zero, a lower voltage with a lesser voltage rise and a smaller current will now be present at the capacitor and thus at the thyristor. However, in most cases, this voltage will still be adequate to ignite the lamp. Furthermore, in approximately half the cases, the voltage during the course of the reverse recovery time of the thyristor 1 is increased by subtraction of the mains voltage drop from the charging voltage of the capacitor 5. In this time, then, the lamp 9 is ignited.If the fusing element 2 is correspondingly dimensioned, i.e. if it is likely to respond only after a prolonged period, then even in this unstable oscillation instance, sufficiently energy-rich voltage pulses of sufficiently high amplitude will reach the lamp 9.

However, the aim will generally be so to dimension the arrangement that the stable (transformer) oscillation condition is reached at clearly defined and high voltage amplitudes.

With regard to the voltage waveforms generated by the circuit arrangement described, it should also be noted that due to the processes in the case of normal switch-off (e.g. at t1 in Figure 2) of the thyristor 1 as a result of the brief and rapid rise in current two below the lock-down current, due to the voltage rising rapidly at the thyristor, additional voltage peaks are induced at the choke 4which also reach the consumer voltage uv. They are shown by broken lines in Figures 2c and 2e. The voltage peaks additionally have a favourable effect in relation to the ignition of a gas discharge lamp. When the lamp 9 ignites, the voltage at the terminals 6,7 drops, the capacitor 14 is no longer charged to the level of the flash-over voltage of the trigger diode 15, and the entire ignition device cuts out.

If the lamp 9 cannot be ignited at all (for example due to a fault), then the ignition device continues its attempts to start the lamp until the current 1 flowing through the controllable switching element 1 and the fusing element 2 causes the fusing element 2 to respond and to shut down the installation. If the fusing element 2 is for example a ballast tube, then it is more favourable to connect the conrol circuit 11 to point A instead of to point B (Figure 1), since then the thyristor 1 will always be switched through and the current flowing through the thyristor 1 will keep the ballast tube warm. With other types of fusing elements 2 (e.g. fusible safety devices, bimetal swiches, etc.) which switch off the current after a time lag, it is more favourable to connect the control circuit 11 to point B since then no current at all any longer flows in the entire starting device. In the latter case, the fusing element 2 can naturally also be connected upstream of the rectifier 10 and be operated on alternating current.

Thus, it is the task of the fusing element 2, in the case of the lamp suffering destruction, to switch off the ignition device, and it can have an electrical resistance which may be as small as desired. In addition, the fusing element 2 interrupts the current in the event of a fault in the starting device 8.

Instead of using a thyristor as the controllable switching element 1, it is also possible to use a triac.

In this case, the rectifier 10 could be located after the controllable switching element 1. Also, with the same type of functioning of the circuit arrangement, a transistor with a similar response characteristic could be used as the controllable switching element 1, in which case the ignition pulse from the capacitor 14 would need to be made sufficiently wide as to correspond approximately to the time ti - to according to Figure 2, which can be done by corresponding dimensioning of the capacitor and the resistor 16.

Claims (16)

1. A circuit arrangement for converting an a.c.

voltage into a substantially higher a.c. or d.c.

voltage, comprising input terminals for connection to a supply of the a.c. voltage to be converted, output terminals for connection to a load, a series combination of a choke and a capacitor forming a resonant circuit and interconnecting the input and output terminals to connect the supply to the load when the arrangement is in use, and a starter arrangement connected across the output terminals and comprising a full-wave rectifier, the input of which is substantially in parallel with the output terminals and the output of which is connected to a controllable switching element which can be controlled via a switching device by a voltage applied to a further capacitor and which is connected to a divider point of a resistive voltage divider which is connected to the output of the full wave rectifier, wherein the voltage divider is constituted by two resistors which are connected substantially directly to the full wave rectifier and said further capacitor is connected in parallel with one only of the resistors of the voltage divider which supplies a control voltage for the controllable switching element, the further capacitor being connected via the switching device to a control electrode of the controllable switching element.

2. A circuit arrangement according to claim 1, wherein the two resistors are connected to the full-wave rectifier means via an inductor.

3. A circuit arrangement according to claim 1 or claim 2, wherein the switching device is a trigger element.

4. A circuit arrangement according to claim 3, wherein the trigger element is a diode.

5. A circuit arrangement according to any one of the preceding claims, wherein a fusing element is disposed upstream of the controllable switching element to shut down the current or at least intensely reduce it after a time delay.

6. A circuit arrangement according to claim 5, wherein the fusing element is connected into an a.c.

part of the circuit upstream of the full wave rectifier.

7. A circuit arrangement according to claim 5, wherein the fusing element is in series with the controllable switching element, this series combination being connected to the output of the full-wave rectifier.

8. A circuit arrangement according to claim 7, wherein the series combination of the two resistors forming the voltage divider is in parallel with the series combination of the fusing element and the controllable switching element.

9. A circuit arrangement according to claim 7, wherein the series combination of the two resistors forming the voltage divider is connected in parallel with the controllable switching element.

10. A circuit arrangement according to any one of the preceding claims, wherein the control electrode of the controllable switching element is connected to the divider point of a further voltage divider comprising resistors connected in parallel with the further capacitor via the switching device.

11. A circuit arrangement according to any one of the preceding claims, wherein an RC network is connected in parallel with the output of the full wave rectifier.

12. A circuit arrangement according to any one of the preceding claims, the arrangement being operative, in order to produce a stable voltage of high amplitude at the load, such that the capacitor of the resonant circuit achieves a charging voltage identical in amount in each switching cycle, and that switching-on of the controllable switching element occurs at equal intervals from an integral multiple of half the period of the mains voltage supply.

13. A circuit arrangement according to any one of claim 1 to 12, wherein the controllable switching element is a thyristor.

14. A circuit arrangement according to any one of claim 1 to 12, wherein the controllable switching element is a triac.

15. A circuit arrangement according to any one of claim 1 to 12, wherein the controllable switching element is a switching transistor.

16. A circuit arrangement for converting an a.c.

voltage into a substantially higher a.c. or d.c.

voltage, the arrangement being substantially as herein described with reference to the accompanying drawings.