WO2010070582A2 - Electronic ballast for low wattage lighting applications - Google Patents

Abstract

A failsafe inexpensive electronic ballast for low wattage lighting applications comprising of a pair of complementary bipolar junction transistors (Q1, Q2) arranged in anti-parallel mode, said pair of complementary bipolar junction transistors biased partly in common base and mainly in common emitter mode thereby providing a self trigger to the said pair of complementary bipolar junction transistors to drive the said low wattage lighting application in an optimal frequency range while depriving inrush current due to elimination of electrolytic capacitor.

Description

TITLE

Electronic ballast for low wattage lighting applications

BACKGROUND OF THE INVENTION Fluorescent lamps are preferred over filament bulb in most lighting applications as they consume less power for providing equivalent light output and also have a longer lifetime than the incandescent but they have their own set of limitations.

When a mains voltage is applied across a fluorescent lamp directly, normally consisting of vapor filled sealed glass tube to ionize the resident vapor it results in radiation of light which is fine in principle but in operation the initial triggering voltage required is greater than the applied mains voltage, therefore a starting element is required to provide the high initial voltage pulse. Also after gas get ionized its impedance turn negative therefore unless the current is limited; the tube will draw excessive current and damage itself and/ or the supply circuit.

Typical practice to limit the current is to provide a damping circuit or a current limiter called" ballast," that operates to ignite the gas tube while also limiting the supply current. The most conventional ballast used was a big heavy inductor as the current limiter and a starter element normally a capacitor connected across the lamp to generate the large overvoltage required for initial starting of the lamp.

The main drawbacks with this arrangement is as there is no synchronization of the current drawn by the ballast elements with the mains supply, therefore the overvoltage generated may not be sufficient to ionize the vapor in the main tube every time leading to condition known a false starts. Secondly as the frequency of the mains supply is limited to the 50/60 Hz cycles, when the current in the tube falls to zero in the 50/60 Hz cycle, the gas vapor in the tube de-ionises resulting in a flicker. Also the size & weight of the inductor required to generate the desired overvoltage to cause the ionization of the gas vapor in the main lamp is a physical handicap in industrial applications.

The present state of the art uses electronic ballast to substantially substitute the starting and inductive elements of the conventional ballast. The aim is to increase the operating frequency of the lamp above the 50/60 Hz determined by the main s thereby reducing the power consumption as the gas vapor in the lamp doesn't get the time to deionise. Thus a constant RMS supply to the lamp exists resulting in a longer lamp life and secondly a reduction in the cost, weight and size of the ballast circuit by using a smaller inductive element. Also it satisfies the need to match the line voltage to the operating voltage of the lamp.

The different topologies commonly used for electronic ballast can be broadly classified as voltage fed half bridge quasi resonant circuit, current fed half bridge resonant circuits and push-pull resonant circuits among various other variations. In context to the present specification which generally addresses low wattage application the topology that is most relevant in the state of the art is the voltage fed half bridge quasi resonant circuit which is used in the low wattage range of 7W to 32W.

Modern ballast designs use transistors to regulate more precisely the current flowing through the lamp circuit. The basic blocks that constitute present day electronic ballast are power source converting circuit comprising of a rectifier bridge circuit for rectifying the conventional AC voltage, a signal conditioner like an electrolytic capacitor for smoothening of the ripple factor that still remains in the rectified voltage supplied from the rectifying circuit. The electrolytic capacitor also couples this voltage feed to the next stage which in this kind of ballast is a lamp driving circuit. It also stores energy so that the capacitor can continuously provide enhanced DC voltage to the driving stage during the time interval when the rectifying circuit does not provide the rectified voltage to the said driving circuit directly. Thus basically it provides for the initial trigger of the switching elements of the lamp driver circuit.

The driving circuit is a switching section or commonly known as a inverter section comprising of a pair of switching devices normally a pair of NPN BJTs or MOSFETs of high voltage rating which are alternately executing the switching operation and enhancing amounts of currents flowing through the pair of the switching devices by amplification provided because of the biasing to each of the switching devices. Normally common emitter biasing is implemented and the two transistors are in series.

Energy discharge from the said coupling electrolytic capacitor along with the biasing elements forms the first base current pulse for the BJT connected at the lower half of this switching section to start the oscillation. After start-up, this generator is made inoperative by diode which prevents the voltage across the biasing elements from becoming high enough to allow this device to remain on and the circuit is maintained in oscillation by feedback to the gates of the transistors from the output circuit via the coupled secondary winding of the transformer. The tube is fed by generating an overvoltage across capacitor which is connected either in series or parallel across the said lamp means. Combination of the inductor and in some topologies primary winding of the said transformer and the said capacitor constitutes the resonant circuit responsible for selecting a fundamental frequency component from the discharge current which flows through the half- bridge switching section so as to provide the selected fundamental component to the lamp driving section.

In normal operation, when the transistor is first turned on, the current through transformer increases until its core saturates. At this point the feedback to the base of the transistor is removed, and after the storage time of the transistors has passed, it turns off. In this way, apart from the value of capacitor, the operating frequency of the circuit is also defined by the storage times of the transistors which is characterized by the biasing elements of the circuit as they decide whether the said transistor goes into saturation region and for what time lapse the other switching element remains in the cut off region and thus delays its entering the active operational region which in itself can be a drawback in deciding the switching frequency of the inverter section.

The second switching section comprises a second bipolar transistor to conduct for the second half of the switching cycle also having a feedback from the secondary winding of the said transformer connected between the emitter and base of the second bipolar transistor for enhancing the base driving current of the second bipolar transistor based on a variation of an emitter current of the second bipolar transistor.

The present state of the art suffers from a number of drawbacks that can complicate the use of the design especially in low wattage lighting applications. For example as we have seen from the description that all electronic ballast in the existing state of the art have a very high initial current surge. This is commonly known as the initial rush current which is much more than the steady state operative lamp current and which the electrolytic capacitor present at the input of the inverter section suffers in the initial phase of the lamp ionization and therefore a high voltage and obviously expensive (as compared to the cost of the rest of the components used in the ballast circuit) has to be used but as the lamp ages and thus takes more time to light up initially the electrolytic capacitor has to withstand it for a longer time period. It is therefore inevitable that the said electrolytic capacitor will succumb leading to the unreliability of the complete ballast.

Secondly because of the use of only common emitter biasing and high value of voltage that the transistors which are connected in series have to withstand in the initial ionization of the lamp, higher rated switching components have to be used leading to higher switching losses.

Also as we have seen that the switching frequency of the transistors is determined by the biasing used to keep the switching elements in active region and which in present state of the art does allow the transistor storage time to limit the switching frequency resulting in a visible flicker which is again an undesirable feature of the lamp.

The bipolar design is also not self starting, but rather requires additional circuitry to start the lamp, control the gate drive which results in number of redundant components which again add to the cost and reliability of the ballast circuit.

OBJECT OF THE INVENTION

Therefore there is a need for a lamp ballast circuit which is reliable and also requires less components for generating flicker less light output for and optimizing the component operational parameters in low wattage lighting applications. In addition the ballast circuit should be self starting without the dependence on capacitor discharge to start the oscillation.

The object of this invention is to provide failsafe electronic ballast for low wattage lighting applications.

Another object of the instant invention is to provide for inexpensive and reliable electronic ballast.

Yet another object of the instant invention is to develop an electronic ballast that meets all international specification that is compatible with the existing fixture.

SUMMARY OF THE INVENTION In order to overcome the drawbacks in the prior act, the instant invention provides for failsafe electronic ballast for low wattage lighting applications. This application is novel insofar as it eliminates the dependency of the electronic ballast circuit on the highly vulnerable expensive high voltage electrolytic capacitor.

The instant invention is failsafe inexpensive electronic ballast for low wattage lighting applications. It comprises a pair of complementary bipolar junction transistors arranged in anti-parallel mode. This arrangement of bipolar junction transistors along with novel biasing means at the main input supply self triggers the oscillation in the lamp driving circuit instantly eliminating the need of a high voltage electrolytic capacitor thereby providing a self sustaining reliable electronic ballast for low wattage lighting application operating in an optimal frequency range. The prior art required the rectified input voltage to be relayed to the switching section using the electrolytic capacitor as the coupling means to trigger the oscillation in the said switching section.

Accordingly the present invention provides for a failsafe inexpensive electronic ballast for low wattage lighting applications comprising of a pair of complementary bipolar junction transistors arranged in anti-parallel mode, said pair of complementary bipolar junction transistors biased in partly common base and primarily in common emitter, thereby providing a self trigger to the said pair of complementary bipolar junction transistors to drive the said low wattage lighting application in an optimal frequency range. The pair of complementary bipolar junction transistors (BJTs) comprises of a first BJT in npn class and a second BJT in pnp class. The biasing elements to self trigger the said first BJT comprises of a high inductive value resistor connected between the base and collector of the said first BJT.

The optimal frequency range is such that the switching range of the pair of the said BJTS is exactly half of the resonating frequency of the said low wattage lighting application. The optimal frequency range is between 75 to 125 kilohertz.

BRIEF DESCRIPTION OF THE DRAWINGS:

Figure 1 depicts a circuit diagram of the existing prior art. The legend describing the various parts are as follows: UlP = bridge rectifier, TR1PTR2P= pair of npn BJTs ,

L4P = an inductor being part of series resonant circuit, LLP = lamp load, ClP=high value electrolytic capacitor, C2P = one of the biasing elements to initially trigger TR2P, C3P =one of the components for controlling the operating frequency of the lamp LLP, C4P =low value capacitor connected in parallel across the lamp load LLP &is part of the series resonant circuit completed by the other part, an inductor L4P & C3P as described above, DlP=diode to prevent TR2P from running away with forward bias by providing a bypass for the base pulse, D2P, D3P diodes connected across collected emitter points of the two transistors TRIP & TR2P , TlP=Primary winding of the transformer TP connected in series with the said lamp load LLP,T'1P & T"lP=secondary windings of the transformer TP coupled to the two bases of the two transistors TRIP & TR2P Respectively.

Figure 2 depicts a circuit diagram depicting the novel implementation of an electronic ballast schematic to achieve failsafe electronic ballast for low wattage lamp applications.

The legend describing the various parts are as follows: Ul = bridge rectifier, Ql = npn, Q2 = pnp, R2 = high inductive resistance responsible for the initial triggering of Ql, R3 = is the resistance that is controlling the lamp current, Ll = negative feedback inductor, LL = lamp load, Cl, C2 = are two capacitors connected to the complementary pair of transistors, C3 = capacitor part of the series resonant circuit, LlA, LIB = represent the primary and secondary winding respectively of the transformer coupled with the output of the circuit, Dl, D2 = represent package of two diodes each in series.

DETAILED DESCRIPTION OF THE INVENTION IN RELATION TO DRAWINGS AND EXAMPLES

A fail safe and cost efficient electronic ballast for low wattage lighting application is disclosed where a novel arrangement of the switching elements of the electronic ballast i.e. the BJTs is introduced to eliminate the dependency of the said electronic ballast on the expensive and yet vulnerable high voltage electrolytic capacitor, thereby achieving a reliable and self-sustaining electronic ballast for low wattage lighting applications.

The said switching elements are pair of complimentary BJTS having a matched pnp and npn class configured in an anti-parallel arrangement to keep the switching section of the ballast in a self sustaining mode and further eliminating the use of the high value electrolytic capacitor which was primarily used for triggering of the switching section components in present state of the art as well as withstand the high initial inrush current which occurs in the initial ionization of the lamp. Thus the proposed arrangement is capable of maintaining its state of readiness over a long period without having to depend on the capacitor retaining charge over a period of time and then discharging to start the oscillation of the switching section.

The said BJTs are in partly common based biasing mode and primarily in common emitter biasing mode to offset some of the amplification problem, hence there is appreciable reduction in heating problems related to the switching of higher rated components and thus reducing the overall cost.

The bridge rectifier has also been relocated to a new position as now there is no high voltage electrolytic capacitor to store, filter and finally couple the high voltage rectified voltage to the switching section of the electronic ballast.

In the instant invention the high inductive value resistor connected to the base of BJT placed in top half of the switching circuit (NPN) substitutes the functionality of the high value vulnerable electrolytic capacitor by limiting the initial inrush current as well as triggering the said BJT.

The instant invention is explained below by way of an example which is the best mode of working this invention. However, this example is in no way limiting this invention.

The schematic for the proposed electronic ballast for low wattage lighting applications as depicted in fig 2 consists of a bridge rectifier Ul for rectification of the input mains, whose rectified output is connected across a high value resistor R2 at one end and packages of two diodes each in series Dl and D2 as the other end.

Conventionally high voltage electrolytic capacitor TR2 as shown in the prior art fig 1 was used for the initial triggering of transistor and set the transistors in oscillation as well as smoothing of the ripples that still remains in the rectified output of the bridge rectifier. It was however vulnerable to high inrush current and was a major reason for the ballast failure.

The proposed schematic of fig 2 introduces the high value resistor R2. R2 is connected between the base and collector of transistor Ql for controlling the initial triggering of Ql. This constitutes one half of the proposed novel arrangement of a pair of complimentary bipolar junction transistors Ql and Q2 where Ql belongs to npn class and Q2 to pnp class of transistor families instead of pair of transistors QlP and Q2P belonging to the same class of transistor family. In the existing prior art both transistors are of npn class as depicted in figl.

Therefore the said resistor Rl limits the initial inrush current that directly flows to the base of the transistor Ql. In the process Rl switches on this transistor Ql because it is a resistor which highly inductive in nature. The resistor Rl does not allow the initial high inrush current to rise quickly thus negating the problem of high inrush current of the prior art. It eliminates the need of the presence of high voltage electrolytic capacitor in fig 1. of the prior art.

In view of the above feature, the high value resistor R2 of the instant invention helps the electronic ballast circuit achieve self triggering without depending on the coupling means of high voltage electrolytic capacitor of prior art and thus maintaining a state of readiness without having to depend on the discharge of the said electrolytic capacitor to trigger the transistors TRl or TR2.

Also we observe from figl of prior art that the pair of transistors TRl, TR2 were connected in series and being operated in Common Emitter biasing mode resulting in large current gain. Thus higher rated transistors TRl and TR2 had to be used, further resulting in high switching losses.

The novel arrangement of the instant invention operates the said complementary pair of transistors Ql and Q2 in anti parallel mode and also biases in partly common base and primarily common emitter mode to offset some of the high switching losses faced due to high current gain since the common base bias has inherently low current amplification as compared to common emitter biasing method.

Smoothing of ripples is intrinsically accomplished by operating the complementary pair of transistors Ql and Q2 in high frequency of 75 to 125 Kilohertz needed for flicker-less lighting phenomenon. Therefore there is no need of any Electrolytic capacitor thus preventing any inrush current associated with it which is of very high amplitude and thus not desirable as it can disrupt the electronic ballast functionality Initially when circuit is switched on base of transistor Ql is triggered by the current passing through the resistor Rl as described above which in turn triggers a capacitor C3 and the lamp load LL connected in series with the primary winding LlA of the transformer Ll. The secondary winding of the said transformer Ll is referred to as LIB. LIB is connected to an inductor Ll as shown in fig 2. The LIB charges the said inductor L2 connected to the base of the BJT Ql through a base capacitance Cl to stop the conduction of first BJT Ql. The Ql turns off immediately using the negative feedback charge from the said inductor Ll.

At the same instant the second BJT Q2 of the switching section is triggered by base capacitor C2 connected to the base of the second BJT Q2. The base capacitor C2 is powered from the negative feedback action created by the inductor TRIA. The capacitor C3 forms a series resonant circuit with the primary winding of the transformer LlA which is further connected serially to the lamp load LL. Therefore it can be deduced that the negative feedback charging of the inductor Ll through the secondary winding LIB of transformer Ll helps to create switching on-off cycle of the switching elements i.e. BJTs Ql and Q2 at a predetermined frequency. The inductor L2 helps to regulate the power in the lamp load LL.

The instant switching of the said BJTs Ql, Q2 at zero current switching and zero voltage switching is obtained using appropriate values of base capacitance Cl connected at the base of BJT Ql and C2 connected at base of Q2. The above mentioned inductor L2 is connected serially to the said capacitance Cl, C2 and is further coupled to the secondary winding LIB of the said transformer Ll as depicted in fig 2. Therefore the said two BJTs Ql, Q2 are not allowed to enter the cut-off or the saturation region as zero voltage and zero current switching mechanism is used to maximize the active region operation of the said two BJTs Ql, Q2.

Thus the switching frequency of the BJTs Ql, Q2 is optimized to be exactly half of the resonant frequency of the lamp so that they conduct for the full duration of the complete signal cycle resulting in an almost flicker free luminescence output. Another Resistor R3 connected between the bases of the two BJTs Ql, Q2 is responsible for controlling the lamp load LL current and ensuring oscillation of the above mentioned switching section Ql, Q2 under all circumstances.

The present invention meets and can meet or exceed all international standards made or in the making. The conceived design provides a power factor of above 85%, and Total Harmonic Distortion less than 40% with the third harmonic distortion at less than 20% because of the use negative feedback and also lower values of inductors Ll and L2 and higher operating lamp load frequency. Bibliography:

Millman and Halkias "Electronic devices and circuits" published by Tata McGraw-Hill Publishing Company Limited New Delhi.

Claims

We claim:

1. A failsafe inexpensive electronic ballast for low wattage lighting applications comprising of : a pair of complementary bipolar junction transistors (Ql, Q2) arranged in anti-parallel mode, said pair of complementary bipolar junction transistors biased partly in common base and mainly in common emitter mode

thereby providing a self trigger to the said pair of complementary bipolar junction transistors to drive the said low wattage lighting application in an optimal frequency range while depriving inrush current due to elimination of electrolytic capacitor.

2. A failsafe inexpensive electronic ballast as claimed in claim 1 wherein the said pair of complementary bipolar junction transistors (BJTs) comprises of a first

BJT in npn class (Ql) and a second BJT in pnp class (Q2).

3. A failsafe inexpensive electronic ballast as claimed in claim 1 wherein the said biasing elements to self trigger the said first BJT (Ql) comprises of a high value resistor connected between the base and collector of the said first BJT

Qi.

4. A failsafe inexpensive electronic ballast as claimed in claim 1 wherein the said optimal frequency range is such that the switching range of the pair of the said BJTS is half or same of the resonating frequency of the said low wattage lighting application.

5. A failsafe inexpensive electronic ballast as claimed in claim 4 wherein the said transistor optimal frequency range is between 75 to 125 Kilohertz.

6. The conceived design lets operate the disposable lamp at a frequency in the range higher than those used in the present state of art (which is around 40-50 KHz) and can exceed upto 250 KHz thus avoiding any flickering. The conceived design provides a power factor of above 85%, and Total Harmonic Distortion less than 40% with the third harmonic distortion at less than 20% because of the use negative feedback and also lower values of inductors Ll and L2 and higher operating lamp load frequency.