GB2071950A

GB2071950A - DC-AC inverter circuit

Info

Publication number: GB2071950A
Application number: GB8107057A
Authority: GB
Original assignee: KUMPA INST ELECTRONIC RES; Kumho Electric Inc
Current assignee: KUMPA INST ELECTRONIC RES; Kumho Electric Inc
Priority date: 1980-03-18
Filing date: 1981-03-06
Publication date: 1981-09-23
Also published as: ES500448A0; ES8202667A1; IT8120218A0; GB2071950B; IT1211004B; KR810001421B1; DE3107031A1; FR2478933A1; FR2478933B1

Abstract

A high frequency inverter circuit comprises two transistors 1, 7 and a series resonant circuit comprising a capacitance 15, 16 and an inductance 14. The transistors 1, 7 are connected in series across a D.C. source 25 and the resonant circuit is linked with the series connection point between the two transistors 1, 7 for driving a load 26 which is connected across the capacitance 15, 16 in order to utilize high voltage induced on the capacitance 15, 16 by switching the transistors 1, 7 alternately in synchronisation with the natural frequency of the resonant circuit. The magnetization of the current transformer 13 may be controlled by adjusting the turns ratio of the windings or the values of the base drive resistors (3, 5, 9, 11) in order to reduce power losses due to both transistors (1, 7) being conductive simultaneously. Additional transistors (2, 8) are also provided to improve switching. <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 frequency inverter circuit which may be used as an electronic ballast for lighting 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, in order to utilize high voltage induced on the capacitance or inductance by switching the transistors alternately in synchronization with the natural frequency of the 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, those 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. The fluorescent lamps. however, require high voltages at the beginning of the lighting instant as in the cases of other discharge tubes, and maintain almost constant voltages during their lighting interval, which makes it difficult to light the lamps with good efficiency.

Many approaches have been developed to light the 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 voltages 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.

We, the inventors devote ourselves to study the ballast to meet the above requirements. As a result, we have invented a new electronic-ballast using transistors having satisfactory experimental results.

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 'shows a basic circuit of electronic ballast for a fluorescent lamp in accordance with the invention; Figure 2 is a detailed circuit diagram of an electronic-ballast having an NPN auxiliary transistor at each main transistor base; Figure 3 is a modified circuit diagram of Figure 2 using a PNP auxiliary transistor instead of NPN type; Figure 4shows waveform sketches of the voltage at the collector and the base referred to emitter of a main transistor for different operating conditions; and Figure shows waveform sketches of the collector current and the magnetizing current of the current transformer compared to the voltages at the collector and the base referred to the emitter in accordance with the invention.

Figure 1 shows that a ballast according to the invention is a new high frequency inverter mainly composed of two transistors connected in series to a DC source, an inductor, a current transformer and some capacitors In this case, assuming that the load is open before the beginning of lighting on account of the high initial discharging voltage of the lamp, a circuit being consisted of L0, 2C1, Q1 and Q2 is formed. If ON OFF switchings of the transistors Q1 and Q2 occur forcedly and alternately in synchronization with the resonant frequency (fq = 1I2vT7o.C1) of the circuit, a high voltage of Q (= circuit quality factor) times the input voltage is generted across C1 since the said circuit is a series resonant circuit.Accordingly, it is possible to light the fluorescent lamp having hign initial discharging voltage even with relatively low supply voltage.

Due to the role of the current transformer, only one of the transistors Q1 and Q2 is ON state and the other is OFF state in accordance with the direction of the current flowing through the inductor LO. And, the current which is equal to N1/N2 (which is the winding ratio of the current transformer) times that of the inductor (leo) flows into the base of the transistor under the "ON" state. In this case, ON/OFF switchings of the transistors Q1 and Q2 occur continuously and automatically in synchronization with the resonance frequency due to the effects of the magnetizing of the current transformer To and the overcharged voltage of C1 by the operation of the series resonance. As a result, self oscillation of the inverter is sustained.

Here in this case, it should be noted that the transistors, Q1 and Q2 are not ideal switches; that is the ON time transition is very fast, but the OFF time delay is fairly long and depends upon the characteristics of the transistors and the currents flowing through the transistors during ON state. Hence, bypassing current through the transistors can occur during the switching transient since the OFF state transistor can be turned on in advance before the turn-off of the conducting state transistor due to the effects of the magnetization of the current transformer and the turn-off delay of the transistor, which leads to low efficiency of the inverter or, in the worst case, brings about second thermal break-down of the transistors.

The above problems are successfully solved in this invention using such configurations of Figures 2 and 3, and thus high efficiency is obtained.

The transistors Q1 and Q2 shown in Figure 1 correspond to the numbers 1 and 7 of Figures 2 and 3, respectively, where the construction of the base circuit of the transistors are shown in detail.

The operating principles of the base circuit of the main transistors based on Figure 2 are described in detail hereinbelow.

Two voltage waveforms, i.e., the collector-to-emitter and the base-to-emitter voltage waveforms of the transisitor which are varied with the changes of the parameter values of the circuit are shown in (i) and (ii) of Figure 4. In particular, the voltage waveforms in Figure 4 (i) are shown together with the relationship of the inductor Lo current and the magnetizing current of the transformer To in FigureS.

The auxiliary transistors 2a and 8a which are connected between the base and emitter of the transistors 1 and 7, respectively, take a role to aid the switching operation of the main transistors since the base of each auxiliary transistor is connected to the other terminal of the secondary winding of which the polarity is the same as that of the base of the opposite main transistor and thus operates in synchronization with the opposite main one to shorten the turn-off delay. Accordingly, the transistor 1 performs the ON/OFF operation in synchronization with the transistor 8a and so does the transistor 7 and 2a.

Therefore, as shown in the voltage waveform of the base in Figure 4 (i) the base voltage of the transistors 1 and 7 varies between 0V and 0.6V. The time interval T occurs due to the magnetization of the current transformer T,, which represents that the current is flowing into the base of the transistor 7 when the transistor 1 is still "ON" state. Therefore, it means that the current simultaneously flows through the two transistors 1 and 7 during this interval.

Such a phenomenon will be described in detail hereinbelow referring to Figure 5. The transistor 1 is "ON" state at the time interval ranging from "t "to "t4", The current 1Lo of the inductor L0 is closely coincided with the collector current of the transistor 1, of which the waveform approximately depicts a sine curve.

On the other hand, since the current iLo of the inductor L0 flows through the primary winding N1 of the transformer T,, this current also includes the magnetizing current in addition to the current flowing into the base of the transistor 1 which is connected to the secondary winding N2.

Therefore, denoting i5 as the secondary winding (N2) current of the transformer T,, i.e., the current on the base side of the main transistor and 1Tom as the magnetizing current of the primary winding (N1), then the inductor current iLo can be represented as::

And we also obtain = = 1B + 1R ...... (2) where = = base current of the main transistor 1 (or 7) 1R = bypassing current through the resistor 5 (or 11) The magnetizing current 1Tom is generally varied with a sine curve as shown in Figure 5, and the time duration that the current is flowing into the base of the main transistor during the interval of to t is approximately to~t3. Because, from the above equations (1) and (2),

Accordingly, the zero crossing point of BB occurs before t3 when iLo = iTom. However, in this case the collector current of the main transistor is delayed and decreased to zero at tS as plotted in Figure 5 by dotted line. Such a phenomenon is due to the fact that the transistors operate at high frequency and since the transistors are not ideal switches, that is, the switching delay of the transistors which is mainly the functions of their characteristics and the amount of satu ration of the previous conducting state.

If the time for the transistors 1 and 7 to change its state from "OFF" to "ON" is t4, the following condition should be satisfied:

where ,8 is the current gain of the main transistors. Specifically assuming the shorted or no load conditions for a simple description of the operation, the inductor current iLo can be represented as another form: L0 (S) ill =Sin o (t - to) where

C = 2C1 for no load 2 (C1 + CO) for shorted load V = peak value of the voltage swing on the capacitor C.

Therefore, the magnetizing current iTom of the primary winding can be given by

where Lm = primary inductance of the transformer To when all of the secondary windings (N2) are open lo = initial currentatt = 0 VTm = instantaneous voltage across the primary winding. And, vTm can be approximately written as:

Wherein, Rx represents the value of resistance X. Accordingly, vTm can be changed by adjusting Rg and R (or R3 and Rg) from the above equation (7) and thus iTom can also be changed from the equations (6) and (7).

Therefore, from the above equations (3) and (5) and for given voltages and the values of Lo and C, the time when the base current i5 becomes zero can be properly adjusted. It is noticeable that a higher effect can be obtained by changing Lm rather than the resistance value from the above equation (6), which means that the adjustment of the number of primary turns N1 of the current transformer To is more effective.

If the number of turns are well adjusted in accordance with the load conditions, the delay of the collector current due to the slow switching speed of the transistor may be decreased as the same amount or even faster than the load current, iL0, does.

By the way, on the one hand, the turn-off delay of the conducting transistors can be controlled by the magnetizing effect but, on the other hand, the transistor under OFF-state also can be turned on in advance on account of the above magnetizing effect. In other words, if the magnetizing current exceeds the load current, the base of the transistor under OFF-state becomes positive (+). Such effects can be controlled and suppressed by adjusting the values of the resistors. By doing so, it is possible to avoid the transistor under OFF-state being converted to ON-state in advance.

It is a difficult problem that we should magnetize the transformer To and eliminate the wrong effects mentioned above. But, such problems can be solved simultaneously by some proper adjustments of the values of Lm and the resistors.

The waveform in such a case is shown in Figure 4 (ii). The negative spike in the base waveform is due to the fact that the inductor (Lo) current is decreased to flow inversely through the base-collector junction of the main transistor 7 (or 1) and a diode 28 (or 27).

in this case, the main transistor is driven into the reverse mode wherein the collector and the emitter are changed and the transistor maintains "ON" state even until the inductor (Lo) current decreases to zero and the current polarity changes again. This is due to the abundant current being flowed through the base-collector junction during reverse mode operation. This makes a new transition condition of the opposite transistor by the role of the transformer i,. Accordingly, a stable self-oscillation can be continuously maintained.

The auxiliary transistors 2a and 8a take a role of rapid drawing out of the current from the base so as to turn off the main transistors rapidly. In Figure 3, the auxiliary transistors are changed from NPN of Figure 2 to PNP, and are connected to the bases of the main transistors. The emitters of PNP transistors are connected to the bases of the main transistors in the direction capable of rapid drawing out of the base charge of the main transistors 1 and 7 during transition condition.

Accordingly, the main operation of the circuit shown in Figure 3 is in principle the same as the one of Figure 2 except that the total number of turns of the secondary windings are halved.

It is proved through the experiments that, when the fluorescent lamps are lighted using the new inverters, the characteristics of instantaneous lighting, high efficiency and high reliability are obtained.

Claims

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, in order to utilize high voltage induced on the capacitance or inductance by switching the transistors alternately in synchronization with the natural frequency of the resonant circuit when the inverter circuit is in use.

2. An inverter circuit according to claim 1, comprising a current transformer for the purpose of synchronizing the said transistors with the natural frequency of the said resonant circuit, by the primary winding of the transformer being connected in series to the series resonant circuit, and the secondary winding of the transformer being connected to the base of each said transistor.

3. An inverter circuit according to claim 2 which, when in use, utilizes the magnetizing effect of the said current transformer in order to keep the self-oscillation of the inverter and to reduce the power loss by minimizing the bypassing current through the said series connected transistors during each switching transient due to the turn-off delay of the conducting transistor.

4. An inverter circuit according to claim 2 or 3, which is designed to control the magnetization of the said current transformer by adjusting the number of primary turns thereof and by providing a resistor and adjusting the values between each secondary winding and the base of each said transistor.

5. An inverter circuit according to any one of claims 2 to 4, having a resistor between two terminals of each secondary winding of the said current transformer in order to control the turn-on point of the off-state transistor by reducing the magnetizing effect of the said transformer which is indispensible for compensating the turn-off delay of the conducting-state transistor and for keeping self-oscillation of the said inverter.

6. An inverter circuit according to any one of the preceding claims, having a PNP (or NPN) auxiliary transistor across the base to emitter of each said transistor which is connected for the purpose of aiding ON/OFF switchings of the said transistors.

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

8. An inverter circuit according to any one of the preceding claims, which is a full bridge inverter circuit comprising two half bridge inverter circuits.

9. A high frequency inverter circuit substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

10. A high frequency inverter circuit substantially as hereinbefore described with reference to Figure 2 or 3 of the accompanying drawings.

11. A high frequency inverter circuit according to any one of the preceding claims, when connected to a fluorescent lamp.