GB2071949A - D.C.-A.C. Inverter circuit - Google Patents

Abstract

A high frequency inverter circuit for lighting a fluorescent lamp 7 comprises two transistors Q1 and Q2 and a series resonant circuit comprising a capacitance 5, 6 and an inductance LO. The transistors Q1 and Q2 are connected in series across two D.C. terminals a, c and the resonant circuit is linked with the series connection point between the two transistors Q1 and Q2. The lamp 7 is connected across the capacitance 5, 6 of the series resonant circuit. A plurality of lamps may be connected to the inverter in series or parallel via respective resonant circuits. <IMAGE>

Description

SPECIFICATION A high frequency inverter circuit This invention relates to a high frequency inverter circuit and particularly, but not exclusively, to a high freqency inverter circuit which may be used as an electronic ballast for a fluorescent lamp.

According to the present invention there is provided a high frequency inverter circuit comprising two transistors and a series resonant circuit comprising a capacitance and an inductance, the transistors being connected in series across two D.C. terminals and the resonant circuit being linked with the series connection point between the two transistors for driving a load which is connected across said capacitance or inductance of the series resonant circuit when the inverter circuit is in use.

Though the history of lamps is almost as long as that of electricity and various kinds of lamps have been developed, lamps with high efficiencies have not been invented yet.

Among others, the fluorescent lamp is in the limelight due to its relatively reasonable efficiency and, thus, is most widely used nowadays. Fluorescent lamps, however, require high voltages at the beginning of the lighting instant, as in the cases of the other discharge tubes, and maintain almost constant voltages during the lighted interval, which makes it difficult to light the lamps with good efficiency.

Many approaches have been developed to light fluorescent lamps with good efficiency since the invention of the lamps; however, the conventional choke ballasts developed at the early stage are still considered to be the most popular ones nowadays, in spite of their many shortcomings, for the reasons of the technical difficulties and economics of other methods.

As high voltage and high speed transistors are available with low cost, the development of highly efficient and reliable electronic ballasts can be considered nowadays using semiconductor devices.

Furthermore, the necessity of the electronic ballast with better efficiency has been ever increasing as the cost of energy increases day by day.

Therefore a preferred embodiment of the present invention relates to a transistor inverter for lighting the fluorescent lamp with very good efficiency. Firstly, we will briefly examine the characteristics of an embodiment of the invention in comparison with conventional choke ballasts.

A connection diagram using a choke ballast is shown in Figure 1. Several modified configurations are possible, but the characteristics are almost the same for all of those using choke ballasts. The major demerits of the method are as follows: 1) Power loss of the choke which can be classified into the following two cases: The one is the resistance of the copper wire, so-called "copper loss", and the other is the hysterisis loss of the iron core, so-called "hysteresis loss". It generally becomes 25 percent or more of the total power due to the above two factors; 2) The weight of the choke ballast is very heavy due to the existence of the iron core.

3) Audible noise (50 or 60 Hz hum) is generated due to the vibration of the iron core.

4) Line power factor becomes low which causes redundant power loss in the transmission line, if not compensated by inserting external capacitors, resulting in additional loss.

These demerits may be reduced or eliminated by using a transistor inverter instead of the conventional choke ballast.

The inverter is an apparatus capable of converting D.C. source into A.C. source, and many methods were developed. But, they possess merits and demerits in accordance with the objects of applications.

In this case, the DC-source for the inverter must be provided by rectifying the AC line source using diodes.

Therefore, the design condition of the inverter circuit becomes very difficult for lighting fluorescent lamps since the voltage ripple on the DC-source is generally very large and the lighting characteristics are very severe, which may be the common problem for all of the inverter schemes to light the fluorescent lamp.

For a better understanding of the invention and to show how it may be put into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 (already described) is a connection diagram for lighting a fluorescent lamp using a conventional choke ballast; Figure 2 is an example of a well-known conventional inverter circuit for lighting a fluorescent lamp; Figure 3 is a basic circuit of an electronic ballast for lighting a fluorescent lamp, in accordance with the present invention; Figure 4 is a simplified equivalent circuit of Figure 3 for illustrating the basic driving mechanism of the circuit; Figure 5 is a waveform sketch representing the switching operation of the circuit shown in Figure 3; Figure 6 is an example of a full bridge construction of an electronic ballast in accordance with the invention;; Figure 7shows examples of connection diagrams for lighting one fluorescent lamp using a basic circuit in accordance with the invention; Figure 8 shows examples of connection diagrams for lighting two fluorescent lamps in series using a basic circuit in accordance with the invention; Figure 9 shows examples of the connection diagrams for lighting three fluorescent lamps in series using a basic circuit in accordance with the invention; and Figure 10 shows examples of the connection diagrams for lighting multiple fluorescent lamps in parallel using a basic circuit in accordance with the invention.

Before a description of embodiments of the present invention, we will examine a typical conventional inverter using a ferrite transformer.

Figure 2 shows a typical example of inverter circuit to light a fluorescent lamp.

The DC-source V5 is converted to alternating current by the self oscillation of the transistors Q, and Q2 with high freqency of about 20-50 KHz, where arbitrary high voltage can be obtained by adjusting the turn ratio between the primary and secondary windings of the transformer T1, thus the fluorescent lamp can be lighted.

In this case, as the secondary voltage is made higher, the value of choke Ldl should be greater; however, this value is negligible compared to the value of choke Ldo shown in Figure 1. Due to the presence of Ldl, high voltage spikes occur at the collectors of the transistors Qi and Q2, and large amounts of reactive current continuously oscillate between source and load during the operation of the inverter. The amount of oscillating current largely depends on the value of Ldl and the transformer turn ratio N2/N1.

The transistor switching between Qi and Q2 during the operation occurs at every maximum peak point of the collector current, which gives rise to considerable power loss due to the turn-off delay of conducting state transistor. Therefore, in principle, it is very difficult to obtain high efficiency using this type of inverter circuit.

If a capacitor Cdi is inserted in series with the choke Ld, to form a series resonance circuit as shown in Figure 2, the inverter efficiency can be somewhat improved. However, this method is also disadvantageous in cost since it requires not only large and expensive ferrite transformers but also high speed, high voltage and high power switching transistors.

The present invention is reiated to a new transistor inverter which may reduce or almost completely eliminate the problems discussed above and maybe simply realisable with low cost having high efficiency and high reliability.

The basic circuit of one embodiment of the invention is shown in Figure 3.

Figure 4 is a simplified equivalent circuit of Figure 3, where the transistors Q1 and Q2 are replaced by two switches S and S2, respectively. In Figure 4, if the switches Si and S2 are on and off alternately synchronized to the natural frequency of the series resonant circuit composed of Lo and 2C1, when the fluorescent lamp is not lighted yet, a very high voltage appears across the capacitor 2C1 due to the circuit resonance; in this case, the capacitor 2Co does not influence to the circuit at all since the lamp can be regarded as an open switch.When the resonant voltage across 2C1 becomes higher than that of the initial discharging value of the fluorescent lamp, the lamp is lighted and the voltage across the lamp gradually decreases to reach a certain value which is different according to the sort of lamp. In this case, the power supplied to the lamp can be controlled by adjusting the capacitance value 2Co.

Therefore in Figure 3, the switches of Figure 4, are replaced by two transistors Q and Q2 and the states of the transistors at an instant during the operation must be one of the two cases: that is, (Q1 :on, Q2:off) or (Qi :off, Q2:on). Hence, the maximum peak voltage accross the off-state transistor is just the source voltage V5 and a high spike voltage problem does not appear in this case. Thus, reliable operation can be guaranteed even in the case using low voltage transistors.

Waveform sketches at the collector and base for one of the two transistors during the operation are as shown in Figure 5.

The transformer To is a current transformer which plays an important role to turn on and off the two transistors (Q1, Q2) alternately in synchronization with the natural resonant frequency of the circuit. By the operation of To, the two transistors can not be turned on simultaneously, that is, only one can be in the conducting state at an instant, and in addition, the current of N1/N2 times that of the collector is supplied to the base of the conducting transistor during a certain period. Hence, the turn-off of the on-state transistor occurs when the collector current (which is equal to the inductor L0 current) decreases to zero due to the role of the current transformer To. In the actual operation, this occurs automatically since the inductor current automatically decreases to zero due to the presence of the capacitors (C0 and C1).In this case, the capacitors are overcharged due to the role of L0, which is one of two main factors (another one is the magnetizing effect of the current transformer To) to turn on the opposite transistor the next time by forcing the reverse current through the off-state transistor due to their overcharged voltages. In this way, the oscillation of the inverter is sustained. As can be seen from the above procedure, since the mechanism of the oscillation is closely related to the series resonant of the circuit, a very high voltage of Qo.Vs across the capacitor C1 appears if the lamp is disconnected and the circuit quality factor is defined as 00. The fluorescent lamp is lighted when the capacitor voltage exceeds the peak discharging value of the lamp.

Once the lamp is lighted, the current flowing through the lamp can be controlled by the capacitor 2Co, and the oscilation frequency is lowered since the total value of the capacitance is increased from 2C1 to 2C1 + 2Co.

In this case, if the discharge initiating capacitor 2C1 is directly connected to the inductor L0 bypassing the filament of the fluorescent lamp as shown in Figure 4, it is advantageous to generate high voltage for the initial discharging of the lamp, since the quality factor 00 becomes large as the circuit is composed of L0 and 2C1 only. However, taking into consideration of the worst case, a dangerous high voltage problem always exists when the lamp is removed on the condition of power-on state. The oscillation is continued even after the lamp is disconnected, which is an undesirable phenomenon. Furthermore, if the resonance voltage exceeds the maximum operating value of the capacitor C1, the capacitor will be broken down.

On the other hand, as shown in Figure 7, if the capacitor Ci is connected through the filament of the fluorescent lamp, the quality factor of the resonance circuit is lowered due to the filament resistance to the lamp, and the oscillation is stopped when the lamp is removed. As a result, the resonance voltage Qo.Vs is lowered but the discharge initiating voltage of the lamp is also much lowered due to the effect of electron emission from the heated filament.

Figure 7 shows an example of the connection diagrams for one fluorescent lamp. The capacitance values are all equal to 2C1 and 2C, and the inductance value is Lo for the cases of Figure 1 (i)-(iv). Hence, the oscillation frequencies are also the same for all of these cases. If we let r be the equivalent filament resistance (which may vary according to its temperature), the equivalent resistances become r for (i), r for 2 for (ii) and (iii), and 2r for (iv), thus the circuit Q is different for each case.

For the cases (ii) and (iii), the equivalent resistance is relatively low and thus Q0 is higher compared to the others. Hence, the initial resonance voltage may be adequately increased to a proper value, which makes these schemes more preferable than the others, since the initial discharging of the lamp can occur even at relatively low source voltage. The circuits shown in Figures 8 and 9 which are the cases of multiple connection of the lamps are, in principle, similar to those of the Figure 7 as long as the equivalent relation described above are maintained even though the configurations of the filament connections seem to be much different to one another. The equivalent resistances are 4r for (i), r for (ii), in Figure 8, and they are 6r for(i),4r/3for(ii) in Figure 9.

On the other hand, for the cases (iii) to (vi) of Figures (8) and (9), parts of the filaments are heated by connecting to the secondary winding of the inductor Lo. This is particularly desirable for the series multiple connections of the lamps since all the lamps can be lighted simultaneously, otherwise, as in the cases of (i) and (ii), they are sequentially lighted with some delays due to somewhat different characteristics of the lamps. One disadvantage of the series connections is the fact that all of the other lamps can not be lighted when one of them is removed. Such a problem can be solved by connecting the lamps in parallel. In the parallel connection, removal of one lamp does not nearly affect the operations of the others. Thus see Figure 10.

Again in Figure 3, the diodes 10 and 11 are only necessary for the cases where the sum of the voltage drops across the lamps in the circuit is considerabiy lower than the source voltage at the steady state operation, otherwise, they can be eliminated as shown in the Figure 7-9.

In these cases, the circuit parameters can be approximately determined as the following if we denote V as the average source voltage since the voltage ripple of the DC source is not negligible.

In this case, assuming the ripple voltage is not so great, then the total power supplied to the load (P5) can be approximately represented as (exact values must be given by rms values); P5=V5.15 (1) Where 15 is the average source current.

Denoting f as the circuit resonant frequency and considering the fact that the source current must be equal to the load current (or the capacitor current), the following relation can be obtained:

where

Ro A equivalent resistance of the lamp.

In this case, we introduced an equivalent resistance R0 of the lamp since it is believed to be valid for high frequency operation. And the resonant freqency is given by

Therefore, from (2) and (4), we obtain

where

From (1) and (6), another representation of Z is given by

Energy consumption cof in the fluorescent lamp per one cycle of operation can be represented as:

From the condition that the lamp power must be equal to the source power and applying (4) and (9), the following relation can be written as:

Rearranging (10), we obtain

Hence, from (8) and (11), we also obtain

From (5) and (7), the parameter values can be represented as:: Lo=Z/co0 (13) C=1iZco0 (14) In this case, o)O can be given by a function of from (4) and (11) thus the parameter value Lo and C can be determined for a desirable operating frequency f when the power rating P5 of the lamp and the average source voltage Us are given. These calculations are only approximately valid if and when the average value of the supply voltage is approximately equal to the rms value, that is, the voltage ripple of the source is not so great.

Thus, the parameter values obtained from (10) and (11) must be properly corrected through the actual experiments. They must almost completely rely upon the experiments when the diodes 10 and 11 are inserted as shown in Figure 3.

Up to this point, we have described a half bridge configuration of the inverter as shown in Figure 3, but this can be constructed with full bridge form. Figure 6 shows an example of the full bridge type which is particularly useful for lighting the high voltage and high power lamps with relatively low line voltage.

The operating principles of the full bridge inverters are basically the same as those of the half bridge ones except some differences in configuration. In the full bridge inverter, four isolated secondaries with identical number of turns are wound on a same current transformer and connected to the transistor bases having polarities for driving a pair of transistors (1 and 15 or 2 and 14) simultaneously at a time as shown in Figure 6.

In conclusion, the distinctive features of embodiments of the present invention can be summarized as follows: A) Efficiency of the inverter is very good since the ON/OFF switching of the transistors occurs near the zero crossing of the collector current due to the role of the current transformer, which results in reliable and smooth operation at each transition even though the switching characteristics of the transistors are not so good.

B) Relatively low voltage transistors can be used since there is no spike voltage problem at the collectors of the transistors and the maximum voltage across the collector-to-emitter of the off-state transistor is equal to the maximum value of the source voltage.

C) Small size and low cost ferrite inductors can be used, thus low cost realisation is possible and the transformer loss which is indispensible for the conventional method is eliminated.

D) Instantaneous lighting is achieved at the instant of switch-on.

E) It is possible to light the lamps even at considerably low voltage, and the realible operation of the inverter is obtained notwithstanding a wide voltage variation (+ 30%).

F) Small size and light weight of the ballast are achieved.

G) The power factor can be greatly improved, and thus the reactive power loss can be minimized.

Claims (11)

1. A high frequency inverter circuit comprising two transistors and a series resonant circuit comprising a capacitance and an inductance, the transistors being connected in series across two D.C. terminals and the resonant circuit being linked with the series connection point between the two transistors for driving a load which is connected across said capacitance or inductance of the series resonant circuit when the inverter circuit is in use.

2. An inverter circuit according to claim 1, which is operable to utilize high voltage induced on said capacitance or inductance for lighting a discharge lamp (e.g. a fluorescent lamp) as said load by switching the transistors alternately in synchronization with the natural frequency of the series resonant circuit.

3. An inverter circuit according to claim 1 or 2, wherein the load power and the operating frequency are controllable by changing the capacitance and/or inductance values of the said series resonant circuit.

4. An inverter circuit according to any one of the preceding claims, comprising multiple resonant circuits in order to drive multiple loads in parallel using the one inverter.

5. An inverter circuit according to any one of the preceding claims, which is operable such that one or a multiple of discharge lamps may be connected in series or in parallel to the said inverter circuit so that the filaments of the or each discharge lamp is heated during the operation of the said inverter in order to facilitate lighting of the lamp.

6. An-inverter circuit according to any one of the preceding claims, which is a half-bridge inverter circuit.

7. An inverter circuit according to any one of the preceding claims, which is a full-bridge inverter circuit, comprising two additional transistors.

8. An inverter circuit according to claim 7, comprising more than one said resonant circuit.

9. A high frequency inverter circuit substantially as hereinbefore described with reference to Figures 3 and 4, or Figure 6, of the accompanying drawings.

10. A high frequency inverter circuit according to any one of the preceding claims, when connected to light one or more discharge lamps (e.g. fluorescent lamps).

11. A high frequency inverter circuit according to claim 10, wherein the lamp is connected, or the lamps are connected, substantially as hereinbefore described with reference to Figures 3 and 4, or Figure 6, or any one of Figures 7(i) to 7(iv), or of Figures 8(i) to 8(iv), or of Figures 9(i) to 9(iv), or of Figures 10(i) and 10(ii), of the accompanying drawings.