GB2131236A - Inverter circuit - Google Patents

Abstract

In an inverter circuit for the operation of a load (1) with high- frequency voltage it should be ensured that the high voltage does not rise too high in the case of a defect of the load, because the insulation of the circuit can be endangered by the high and rapid voltage rise. In order to avoid an extraordinarily rapid and undefined voltage rise on account of the use of the almost loss-free oscillating circuits (5, 6) in such circuits, two series-connected rectifiers (10, 11) having the same forward direction are arranged in parallel to the inverter 7 and between the two lines (3, 8) of the direct-current voltage source V+-V-. The connecting conductor (12) of these rectifiers is connected, with interposition of a further capacitor (13), to the supply line (14) to the load and/or to the circuit members (5, 6) of the series oscillating circuit. <IMAGE>

Description

SPECIFICATION inverter circuit This invention relates to an inverter circuit for operation of a load at high-frequency, comprising an inverter connectable between two lines of a DC source to provide an output between an output terminal of the inverter and either source line, a series LC circuit having one side connected to the inverter output terminal so that a load can be connected between the other side ofthe LC circuit and one or other ofthe source lines, and a capacitor connected to be in parallel with the load.

Circuit arrangements of this kind are known, for example in the form of network circuit parts. If in such networkthe output load changes suddenly, for example on unloading by an unforeseen interruption of a conductor or iffor any reason the output load should not be connected, the voltage rises quickly and steeply and thus endangers the circuit, especially its insulation.

It is further known to operate fluorescent lamps with hig h-frequency voltage, for exam ple 20 kHzor more.

By the operation of fluorescent lamps with high frequency voltages it is possible to increase the light output of such a lamp and in this case the components forthe operation ofthe lamp also have lower power losses and therefore can be of smaller construction than those serving for the operation of fluorescent lamps with ordinary mains frequency. In orderto obtain the high voltages necessary for starting the lamps, in the above-mentioned circuit arrangements series oscillating circuits are allocated to the lamps, which permit the voltage to rise extraordinarily quickly until the lamp starts. In this case lamps with pre-heated or unpre-heated electrodes can be used. If for any reason, despite reaching the start voltage, the lamp does not start, here again the voltage rises to values which endangerthe circuit arrangement.This is also the case ofthe lamp should not be inserted into a lightfitting which is operated with such a circuit arrangement, or if the inserted lamp does not have adequate contact with its socket. If a lamp with pre-heated electrodes is used, here again the voltage rises so extraordinarily quickly thatthe electrodes of the lamp do not have sufficient time to become heated, but rather the lamp starts by reason of the rapid voltage rise and the high voltages with cold electrodes. This is detrimental to the life of the lamps.

In order to retard this rapid voltage rise caused by the use of the low-loss series oscillating circuits, that is to say in orderto detune or damp the oscillating circuit, it would in principle be possible to insert an ohmic resistor in series with the circuit members forming the series oscillating circuit. While this would produce the desired result, that is a sufficiently retarded voltage rise so that the electrodes would have sufficient time to heat up, nevertheless the use of such a resistor would involve constant losses in the operation of the lamp.

Now the invention comes into effect here, having the aim of limiting the inherently extraordinarily rapid and undefined voltage rise on account of the use ofthe almost loss-free series oscillating circuits and if necessary retarding it so farthat this voltage rise does not proceed out of control but assumes definable values. The aim is furtherthat, if fluorescent lamps with pre-heated electrodes are used as loads, during this voltage rise the current flowing through the electrodes has sufficient time to heat up the electrodes so that consequentlythe lamp starts with hot electrodes, and that the consequent detuning or damping ofthe series oscillating circuit works practically without loss.

According to the invention the solving ofthis complex problem surprisingly succeeds duetothe provision of a pair of series connected rectifiers connected in parallel with the inverter so as to be between the source lines, theforward directions of the rectifiers being arranged in the same direction from the lower potential to the higher potential of the source lines, and the connection between the rectifiers being connected via a further capacitor to a connection between the LC circuit and the load and/orto a connection between the elements ofthe LC circuit.

By providing the mentioned rectifiers, which lie through a network parallel to the load feed, the rise of the voltage on switch-on is limited, in the no-load condition ofthe voltage so to speak. If the circuit has reached its normal working point, the apex factor of the current is somewhat limited, which has a positive effect upon the life ofthe load.

Bythe arrangement a transformer between LC circuit and load it is possible to adapt the achievable impedances to the load.

In orderto keep the construction expense for the circuit arrangement as low as possible it is proposed according to a furtherfeature ofthe invention that the inductance ofthe LC circuit is at least in part provided by the primarytransformerwinding.

In order that the several consu mers with negative resistance characteristics may be connected to one single inverter it is proposed in accordance with the invention that a respective such LC circuit and a respective such load capacitor are provided for each load, andfurthermorea respective such further capacitor is provided for each load, the further such capacitors all being connected to the connection between the pair of rectifiers.

If the load, especially when a lamp, has a defect and for this reason cannot ignite at all, the ignition voltage would rise more and more until the insulation of the circuit arrangement breaks through at its weakest point, whereby the circuit arrangement would be destroyed. In orderto avoid this, the inverter circuit may be provided with a voltage divider preferably comprising a time delay member including an R-C member, which is in operative connection with the inverterthrough a switch-off device, is connected to the connection between the rectifiers orto a connection between the elements ofthe LC circuit. This provides a possibility of regulation or of switching off if the ignition voltage reaches too high a value or too high a voltage is present on the circuit arrangement throughout a certain adjustable or freely selectable time.

Fortheadjustmentand selection ofthestartvoltage for a load with negative resistance characteristic it is provided according to the invention that the capacita tive reactance ofthe capacitor lying in series between theterminals ofthe lamp and arranged parallel tothe gas discharge path of the lamp is greater, for example greater by about 2 to 5 times, than the capacitative reactance ofthe capacitor ofthe LC circuit.

To illustrate the invention eight examples of embodimentwill be described in greater detail by reference to the accompanying drawings, wherein: Figures 1-3 showthree basic circuit arrangements forthe operation of a fluorescent lamp; Figure 4 shows a circuit arrangement according to Figure 1, butforthe operation of two lamps; Figure 5 shows a circuit arrangement according to Figure 1 forthe operation of a lamp with a switch-off device; Figure 6 shows a circuit arrangement according to Figure 5 with a switch-off device, but for the operation oftwo lamps;; Figure 7 shows a circuitarrangementforthe operation of a fluorescent lamp without pre-heated electrodes and Figure 8 shows a circuit arrangement with a transformer and a load, which can be formed as a combinatorial circuit part.

In all the figures the same reference numerals are used to designate the same parts.

According to Figure 1 a fluorescent lamp 1 has one electrode 2 connected to a positive line 3 of a direct-current voltage source V±V and can be switched on and off by means of a switch S. The other electrode 4 of the lamp 1 is connected, through a series oscillating circuit 5-6 consisting of the choke 5 and the capacitor 6, to an output 17 of an electronic inverter7 which in turn is connected to the two lines 3, 8 of the direct-current voltage source.The two electrodes 2-4 of the lamp 1 are connected in series through a capacitor 9.Parallel to the inverter7 and between the two lines 3 and 8 of the direct-current voltage source V±V there lie two series-connected rectifier diodes 10-11 having the same forward direction. the connecting conductor 12 of these two rectifier diodes 10-11 is connected, with interposition of a further capacitor 13, to the conductor connecting lamp 1 and a series oscillating circuit 5-6. The inverter 7 is designed for exampleforaworking frequency of 20 kHz. The series oscillating circuits 5-6, which consists oftheinductance orchoke 5 and the capacitor6, is dimensioned for minimum possible loss. Its resonance frequency can correspond to the working frequency of the inverter 7.

In Figures 1 to 6the lamp 1,1' is drawn substantially smallerthan the rectangular symbol forthe inverter 7.

This is solely for reasons of clarity of illustration and is irrelevanttothe invention,forthe lamp 1, 1' is always considerably larger than the mentioned inverter, which as an electronic component has only small dimensions. This should be stated here for the sake of completeness.

If now in the circuit arrangement according to Figure 1 the switch S is closed and thus the inverter7 is connected to the direct-current source V±V~, the voltage on the choke Sand the capacitor 6 ofthe series oscillating circuit 5-6 rises very fast, whereby a correspondingly high voltage builds up on the capaci tor9, which is in series with the electrodes ofthe lamp 1.By reason of the manner of action of the inverterthe potential ofthe conductor 14 drops so that it reaches a value lying belowthat ofthe conductor 8, which consequently leads to a currentflowing through the capacitor 13 and the rectifier 10, the circuit ofwhich is given by the rectifier diode 10, the capacitors 13 and 6, the choke 5, the inverter7 and the conductor 8 which connects the latter and the rectifier diode 10. Thus the oscillating circuit is damped, since a part of the current flowing through the choke 5 is conducted bywayof the capacitor 13 and the rectifier diode 10, so that only a current reduced in relation to it is availableforthe capacitor 9 on the lamp 1, and therefore only a small voltage can build up on this capacitor 9.This is valid in a fully analogous manner of the potential of the conductor 14 reaches a highervaluethan that of the conductor 3. In this case again a part ofthe resonance current is branched off so to speak from the lamp 1 and its capacitor so that on the latterthe voltage rises only relativley slowly and thus sufficienttime is available to heat up the electrodes 2 and 4, so thatthe start voltage is reached only when the electrodes 2 and 4 are heated. This resonance part current branched offfrom the lamp 1 flows through the choke 5, the capacitors 6 and 13, the rectifier diode 11 and through the connecting conductor3 to the inverter7.

In this case the lamp 1 is in shuntto the current path formed bythe capacitor 13 and the rectifier diode 11.

During the start operation the rectifier diodes 10 and 11 in combination with the capacitor 13 associated with them form, alternately and in dependence upon the potential drop on the conductor sections, a shunt to the lamp 1, byway ofwhich in each case a partofthe high resonance current can be derived.

If nowthe lamp 1 has lit up, the capacitor9 is thereby practicallyshort-circuited and the oscillating circuit 5-6 is damped bythe resistance ofthe gas discharge path ofthe lamp 1.The resistance behaviourofthis gas discharge path is at least approximate to that of an ohmic resistance. This resistance of the gas discharge path nowdampsthe oscillating circuit, the oscillation behaviourofwhich is in principle namely notthereby altered, butthevoltagevalues no longer reach the original high levels.As soon as the lamp 1 is started, no more current flows through the rectifier diodes 10 and 11, wherebythe flow ofcurrentthrough the capacitor 13 is also interrupted, so that the lamp circuit is no longer influenced by these "start-aid circuit members" 10,11, 13. The choke current is thus available exclusivelyto the lamp, the current absorbed by the capacitor 9 in operation is here very slight.

In lamps of this kind the ratio of startvoltage to burning voltage is in the range of 3:1 to 7:1.

Without limiting the invention, somevalues may be noted by way of example, namely for a lamp of 58 watts with a start voltage of about 500 volts, the heating currentforthe electrodes ofwhich is lessthan 2A and which is operated with an operating frequency of above 80 kHz.

Capacitor 6 = 9.0 nF Capacitor 9=3.3nF Capacitor 13 = 4.7 nF Inductance (choke) S = 0.85 mH.

Due to this dimensioning ofthe circuit members both the heating current and the start voltage assume definable values, so that cross-discharges on the lamp, which occur of excessively high heating cur rentsflowand excessively high voltages occur on the as yet unstarted lamp, are avoided.

Nowthecircuitarrangement according to Figure 2 corresponds in all essential points to that according to Figure 1, but with the difference that here the capacitor 13 is connected to the conductor 16 of the series oscillating circuit 5-6 which connects the inductance (choke) 5 and capacitor 6. If in the circuit arrangement according to Figure 1 the voltage peaks are directly clipped in the apex during the start operation, in the circuit arrangement according to Figure 2 the voltage rise is interrupted between the respective zero pas sage ofthevoltage and its apexvalue, sothatthetime course of the voltage displaysashoulderofstepped form whereby the voltage is reduced as regards its effective value.A behaviour analogous to this circuit arrangement is displayed by the circuit arrangement according to Figure3, where the inductance or choke of the oscillating circuit has been divided so to speak into two component chokes 5' and 5" and the capacitor 13 is connected between these two components 5' and 5".

The circuit arrangement according to Figure4 serves for the operation of two lamps 1 and 1'. Here series oscillating circuits 5-6 and 5'-6' lying parallel with one another are connected to the output 17 of the inverter 7. As was setforth in connection with the circuit examples explained above, each of these series oscillating circuits is connected with one ofthe lamps 1,1 ' through a conductor 14 and 14' respectively. The two rectifier diodes 10 and 11 here again lie between the two lines 3 and 8 ofthe supply voltage and their connecting conductor 12 is connected,through capacitors 13 and 13' provided as a pairto the conductors 14 and 14' which connectthe lamps 1 and 1 ' and their series oscillating circuits 5-6; 5'-6'.This connection of the capacitors 13 and 13' can also be produced in the same manner as was explained in connection with Figure 2 or Figure 3. The behaviour of this circuit arrangement correspondsto that of the circuit arrangement according to Figure 1.

If despite reaching of the start voltage the lamps were still notto start,forwhatever reason, the circuit arrangement according to Figures 1-4 as described would drive the voltage even higher until the insulation is broken through at its weakest point by the ever rising voltage orthe voltage reaches its very high terminal value dueto thefinite quality of the series oscillating circuit.In order to prevent this voltage divider 18, which is in operative connection through a switch-off device 19 with the inverter 7, is connected with this circuit arrangementthrough a time-delay member (R-C member) (Figure 5). When the switch S is closed and the lamp 1, afterthe elapse ofthe intended time, does not start, then after the elapse of the time interval determined by the time-delay mem brand afterthe reaching of a pre-determined upper voltage limit, the inverter 7 is switched off by means of the switch-off device 19. In the example of embodiment as shown according to Figure 5the voltage present in the damping circuit (10-11-13) is used for the controlling of the switch-off device. When the switch-off device has responded the circuit arrangement remains out of operation until the defect has been removed.

Figure 6 shows a circuit arrangementforthe operation of two lamps which is assembled in principle like the circuit arrangement according to Figure 4. Here again a switch-off device is provided.

The voltage forthe control ofthe switch-off device is here tapped from the two series oscillating circuits, namely from the connecting conductor 16 between the circuit members 5 and 6 and 5' and 6' of the series oscillating circuits. If one ofthe lamps does not start the entire circuit arrangement is set out of operation.

Thanks to the equipment and circuit arrangement according to the invention the voltage rise during the starting phase of the lamps on the one hand and the time interval necessaryforthe sufficient heating up of the electrodes on the other are optimally attuned to one another, withoutthe consequent damping or detuning ofthe series oscillating circuit causing losses of effect during the operation of the lamp.

Figure 7 shows a circuit arrangement having a fluorescent lamp 1 which corresponds substantially to the circuit arrangement as shown in Figure 1, with the exception thatthe capacitor 9 allocated to the lamp 1 lies in parallel with the lamp as an entirety,thatisto say is not connected with the electrodes of the lamp. It is here a matter of a lamp which is started with cold electrodes. Here again due to the measure according to the invention if the lamp is damaged the voltage cannot rise out of control and furthermore the start voltage is definable.

In the examples of embodiment as shown the lamps 1, ' were in each case connected between the lines 3 ofthe direct-cu rrent voltage source and the output 17 ofthe inverter 7. Fundamentally it is naturally also possible to connect the lamps 1, 1' between the output 17 ofthe inverter 7 and the line 8 ofthe direct-current source. This is indicated in Figure 1 by the chain line 20. What has been shown here in Figure 1 by the chain line 20 is valid equally in completely analogous mannerforthe othercircuitarrangements according to Figures 2 to 7.

Figure 8 shows a circuit arrangement with a load 1 which is here formed as network circuit partforthe supply of an electric circuit arrangement, which is not illustrated further. Here a transformer 21, which can be formed as isolating transformer or auto-transformer, lies between the load 1 and the series oscillating circuit 5-6. It is also possible to form a part of the primary winding of the transformer 21 as inductance ofthe series oscillating circuit.

In the examples of embodiment as described, fluorescent lamps with cold or pre-heated electrodes or network circuit parts were listed as loads. In place of such loads others can also be connected with the circuit arrangement according to the invention.

The chain line 25 in Figure 1 indicates yet a further connection possibilityforthe capcitor 13 on the series oscillating circuit 5-6. It is also possible to exchange the positions ofthe circuit elements ofthe series oscillating circuit, namely inductance 5 and capacitor 6. Such an arrangement appears by way of example from Figure 2, but this arrangement can also be used forthe other circuit examples.

Claims (8)

1. An inverter circuitfor operation of a load at high-frequency, comprising an inverterconnectable between two lines of a DC source to provide an output between an output terminal ofthe inverter and either source line, a series LC circuit having one side connected to the inverteroutputterminal so that a load can be connected between the other side of the LC circuit and one or other ofthe source lines, a capacitor connected to be in parallel with the load, a pairofseries connected rectifiers connected in parallel withthe inverterso asto be betweenthe source lines, the forward directions ofthe rectifiers being arranged in the same direction from the lower potential to the higher potential ofthe source lines,and the connec- tion between the rectifiers being connected via a further capacitorto a connection between the LC circuit and the load and/orto a connection between the elements ofthe LC circuit.

2. An invertercircuit according to Claim 1, wherein a transformer is connected between the LC circuit and the load.

3. An inverter circuit according to Claim 2, wherein the inductance ofthe LC circuit is at least in part provided bytheprimarytransformerwinding.

4. An invertercircuit according to one of Claims 1 to 3 for operating more than one load, wherein a respective such LC circuit and a respective such load capacitor are provided foreach load, and furthermore a respective such further capacitor is provided for each load,thefurthersuch capacitors all being connected to the connection between the pair of rectifiers.

5. An inverter circuit according to one of Claims 1 to 4, wherein a voltage divider preferably comprising a time delay member including an R-C member, which is in operative connection with the inverterthrough a switch-off device, is connected to the connection between the rectifiers to a connection between the elements ofthe LCcircuit.

6. An invertercircuit according to one of Claim 1 to 5, wherein a fluorescent lamp is provided as a load.

7. An inverter circuit according to Claim 6, wherein the capacitative reactance ofthe capacitor lying in series between the terminals of the lamp and arranged parallel to the gas discharge path ofthe lamp is greater, for example about 2 to 5 times greater, than the capacitative reactance ofthe capacitor ofthe LC circuit.

8. An invertercircuit substantially as hereinbefore described with referenceto and as illustrated in any of the accompanying drawings.