Abstract:
A circuit to supply an AC voltage to a fluorescent lamp load from an AC supply converts the supply to DC, boosts the voltage in a converter and then drives an inverter from the smoothed boosted voltage in such a manner that the inverter will not start if the load is too low and the inverter frequency is almost constant. The power factor of the circuit is maintaining very close to unity.

Description:
TECHNICAL FIELD 
       [0001]    The invention generally relates to a driver for fluorescent lamps. 
         [0002]    More particularly the invention relates to a driver for fluorescent lamps which allows control of the lumen output of a fluorescent lamp and consumes power efficiently. 
       BACKGROUND ART 
       [0003]    Drivers for fluorescent lamps are known and it is also known to provide controls on these so that they produce a required light output despite varying input voltage. Such known driver circuits often provide other than a unity power factor and will still consume power even if a fluorescent lamp is not working. 
         [0004]    Therefore a need exists for a solution to the problem of controlling the light output of a fluorescent lamp while providing a close to unity power factor and limiting current consumption where a lamp fails to strike. 
         [0005]    The present invention provides a solution to this and other problems which offers advantages over the prior art or which will at least provide the public with a useful choice. 
         [0006]    All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country. 
         [0007]    It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process. 
       SUMMARY OF THE INVENTION 
       [0008]    In one exemplification the invention consists in an apparatus for providing a supply voltage to an AC load, the apparatus being connected to an AC supply and providing the AC current to a bucking transformer under repetitively switched control, the bucking transformer output being rectified and supplying a half bridge inverter having at least two output transistors in half bridge arrangement, the inverter haying a current output including a transformer winding, the transformer winding being coupled to two other windings operable to drive the inverter, whereby if no output current can be provided because of output circuit conditions, the inverter will not operate characterised in that the other windings drive the bases of the half bridge transistors and are DC isolated, and the base/emitter voltage of the transistors in the turn off direction is limited by a unidirectional component network. 
         [0009]    Preferably the unidirectional component network is the series combination of a diode and a resistor. 
         [0010]    Preferably only one of the output sides of the half bridge has a transistor emitter resistor and only the other side of the bridge has a base pull up resistor. 
         [0011]    Preferably there are two parallel transistors in each side of the half bridge arrangement. 
         [0012]    A method of providing an AC output from an AC input comprising rectifying the AC input to power a bucking converter, the converter being of controllable power factor, the smoothed converter output supplying an inverter, the inverter output current passing through a transformer winding, other windings of the transformer being operable to drive inverter control switches such that if the output circuit conditions reduce the output current below a minimum, the inverter will not operate, characterised in that the transformer windings driving the inverter control switches are DC isolated and the switch control electrode is regulated to limit the applied control electrode voltage when the switch is cut off. 
         [0013]    Preferably the switch control electrode limiting is provided by a series resistor and diode. 
         [0014]    Preferably the smoothed converter output is regulatable for voltage. 
         [0015]    Preferably the frequency of the inverter is substantially constant within the working voltage range. 
         [0016]    Preferably the inverter is a half bridge inverter. 
         [0017]    These and other features of as well as advantages which characterise the present invention will be apparent upon reading of the following detailed description and review of the associated drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a circuit diagram of an embodiment of the invention. 
           [0019]      FIG. 2  shows the layout of the majority of the circuit components on a printed circuit board. 
           [0020]      FIG. 3  shows the layout of the circuit board track. 
           [0021]      FIG. 4  shows the layout of most of the remaining circuit components on a daughter board. 
           [0022]      FIG. 5  shows the layout of the track on the daughter board. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0023]    Referring now to  FIG. 1  AC mains of 50 Hz or 60 Hz at a voltage from 120 to 277 volts is applied to the circuit at  101 ,  102 , and passes via a fuse  103 , spike eliminating bifilar inductor  104  and suppression capacitors  105 ,  106  which assist in eliminating noise on the input AC line, to a bridge rectifier of diodes  107 . The output is smoothed by capacitor  108  and applied via bucking transformer  109  to FET transistor  112  and thence via load sampling resistors  113 ,  114  returns. 
         [0024]    The buck circuitry and phase angle of conduction at transistor  112  is controlled by integrated circuit  111 , typically a Motorola MC3326P or L6563N, via resistor  110 . The integrated circuit is supplied with power via resistor  115  and smoothing capacitor  118  at startup. A voltage sample input is provided from the voltage divider of resistors  122 ,  123  and integrating capacitor  124 . Input via resistor  121  triggers the control of transistor  112  assisted by quickstart capacitor  124 . A bootstrap supply of about 18 volts is provided via resistor  117 , capacitor  119  and diode  116  provides power to the integrated circuit  111  once the bucking supply is running. Resistor chain  127 ,  128 ,  130  and  132  controls the output voltage via the integrated circuit  111  and the buck converter and resistors  128 ,  130  may be switched by switches  129 ,  131  to set a desired output voltage level controlling the lumen level emitted by the attached fluorescent lamps  163 ,  164 . Rather than using switches and fixed resistors the output voltage may be varied continuously if a variable resistor is used as the adjustment element. 
         [0025]    The fluorescent lamps are driven by a half bridge inverter circuit fed from the output of bucking transformer  109  via high frequency rectifier  126  and smoothing capacitors  137 ,  138  which have bleed resistors  135 ,  136 . 
         [0026]    The inverter has paralleled upper and lower transistors  146 ,  147  and  154 ,  155  forming a half-bridge arrangement. The lower of a pair of inverting transistors  154 ,  155  is triggered into conduction by resistor  159  and capacitor  160  at startup via pullup resistor  153 . Trifilar windings  139 ,  140 ,  141 , preferably on a ferrite core, act to bootstrap the inverter into oscillation, with the proviso that if no current flows in output winding  141  the oscillation will not be maintained. Hence no load produces no oscillation. Capacitors  142 ,  143  DC isolate the windings  139 ,  140  and with the aid of resistors  148 ,  156  and reverse conducting diodes  149 ,  157  act to regulate the turn-on voltage at the transistor bases. Inductors  145  and  152  act to set the frequency of operation of the inverter in conjunction with capacitors  142 ,  143 . Resistors  148 ,  156  and diodes  149 ,  157  assist in waveshaping while snubbing diodes  151  and  158  act to protect the transistors from reverse voltages. This combination of components provides a stable frequency of operation despite changes in the input voltage to the inverter and allows control of the light output from the lamps by varying the applied voltage to the inverter, thus varying the output voltage to the fluorescent lamps and the light output from those lamps. 
         [0027]    Inverter output via winding  141  and ballast chokes  161 ,  162  which define the operating frequency of the inverter is supplied to the lamp filaments via capacitors  165 ,  166  and acts to strike and maintain the lamps. Current return is via balanced capacitors  137 ,  138 . 
         [0028]    The inverter typically runs at between 30 KHz and 40 KHz. Because the inverter frequency is set by fixed components and the load is virtually constant both the frequency and voltage of the output are stably controlled despite changes in the AC input voltage over a 120 to 277 volt range. 
         [0029]    Light output of the fluorescent lamps is controlled with switches  129 ,  131  which set the supply voltage for the inverter and maintain it sensibly constant despite variations in mains supply voltage without varying the frequency of operation of the inverter. This stability also allows use of fluorescent lamps of several differing types, for instance compact or linear styles. 
         [0030]    Preferred component values are: 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 PART 
                 Ref. 
                 DESCRIPTION 
                 VALUE 
               
               
                   
               
             
             
               
                 103 
                 F1 FIG. 2 
                 Fuse 
                 4 A 300 V 
               
               
                 104 
                 T1 FIG. 2 
                 Bifilar line filter 
                 2 × 10 mH 
               
               
                 105 
                 CY FIG. 2 
                 Capacitor 
                 2n2F 
               
               
                 106 
                 CX FIG. 2 
                 Capacitor 
                 0.33 uF 
               
               
                 107 
                 D1, D2, D3, D4 
                 Diode 
                 1N4007 
               
               
                   
                 FIG. 2 
               
               
                 108 
                 C1 
                 Capacitor 
                 33 nF 
               
               
                 109 
                 T2 FIG. 2 
                 Transformer 
                 EE25/13/7 90 turn 
               
               
                   
                   
                   
                 1 D  7 turn 2 D   
               
               
                 110 
                 R5 FIG. 2 
                 Resistor 
                 10E 
               
               
                 111 
                 IC1 FIG. 2 
                 IC 
                 L6562N 
               
               
                 112 
                 Q1 FIG. 2 
                 FET 
                 IRFBC40 
               
               
                 113, 114 
                 R5, R6 FIG. 2 
                 Resistor 
                 1E 
               
               
                 115 
                 R3 FIG. 2 
                 Resistor 
                 240K 
               
               
                 116 
                 D3 FIG. 2 
                 Diode 
                 1N4150 
               
               
                 117 
                 R2 FIG. 2 
                 Resistor 
                 100E 
               
               
                 118 
                 C2 FIG. 2 
                 Capacitor 
                 22 uF 
               
               
                 119 
                 C6 FIG. 2 
                 Capacitor 
                 12 nF 
               
               
                 120 
                 D2 FIG. 2 
                 Diode 
                 1N5248B 
               
               
                 121 
                 R1 FIG. 2 
                 Resistor 
                 68K 
               
               
                 122 
                 R9 FIG. 2 
                 Resistor 
                 1.24M 
               
               
                 123 
                 R10 FIG. 2 
                 Resistor 
                 10K 
               
               
                 124 
                 C7 FIG. 2 
                 Capacitor 
                 10 nF 
               
               
                 125 
                 C3 FIG. 2 
                 Capacitor 
                 0.68 uF 
               
               
                 126 
                 D1 FIG. 2 
                 Diode 
                 MUR160 
               
               
                 127 
                 R7 FIG. 2 
                 Resistor 
                 1M 
               
               
                 128 
                 RA FIG. 2 
                 Resistor 
                 100K 
               
               
                 129, 131 
                 SW1 
                 DIP switch 
                 Dual on/off 
               
               
                 130 
                 RB FIG. 2 
                 Resistor 
                 100K 
               
               
                 132 
                 R8 FIG. 2 
                 Resistor 
                 5.6K 
               
               
                 133 
                 C4 FIG. 2 
                 Capacitor 
                 47 uF 
               
               
                 134 
                 C5 FIG. 2 
                 Capacitor 
                 47 uF 
               
               
                 135, 136 
                 R11, R12 FIG. 2 
                 Resistor 
                 1.2M 
               
               
                 137, 138 
                 C8, C9 FIG. 3 
                 Capacitor 
                 0.22 uF 
               
               
                 139, 140, 
                 TA, TC, TB 
                 Trifilar inductor 
                 resp. 5, 5, 7 
               
               
                 141 
                 FIG. 3 
                   
                 turns on T10 core 
               
               
                 142, 143 
                 C1, C2 FIG. 3 
                 Capacitor 
                 0.33 uF 
               
               
                 145, 152 
                 L1, L2 FIG. 3 
                 Inductor 
                 56 uH 
               
               
                 146, 154 
                 Q1, Q2 FIG. 3 
                 Transistors 
                 MJE13007A 
               
               
                 147, 155 
                 Q2, Q3 FIG. 2 
                 Transistors 
                 MJE13007A 
               
               
                 148 
                 R1 FIG. 3 
                 Resistor 
                 100E 
               
               
                 149, 157 
                 D1, D2 FIG. 3 
                 Diode 
                 1N4148 
               
               
                 150 
                 R4 FIG. 3 
                 Resistor 
                 0.62E 
               
               
                 151, 158 
                 D3, D4 FIG. 3 
                 Diode 
                 1N4007 
               
               
                 153 
                 R2 FIG. 3 
                 Resistor 
                 470K 
               
               
                 156 
                 R3 FIG. 3 
                 Resistor 
                 100E 
               
               
                 159 
                 R5 FIG. 3 
                 Resistor 
                 470K 
               
               
                 160 
                 C3 FIG. 3 
                 Capacitor 
                 0.33 uF 
               
               
                 161, 162 
                 L1, L2 FIG. 2 
                 Inductor 
                 5.2 mH air gap 
               
               
                 163, 164 
                 not shown 
                 Fluorescent lamp 
               
               
                 165, 166 
                 C10, C11 FIG. 2 
                 Capacitor 
                 0.01 uF 
               
               
                   
               
             
          
         
       
     
         [0031]    The L6562N controller may be set up in known manner to provide a power factor on the supply of better than 0.99. Other controllers may be used to provide a stable inverter supply voltage while maintaining a high power factor. 
         [0032]    The layout of the components on the printed circuit boards of the apparatus is shown in  FIGS. 2 and 4  using the references shown in Table 1, while the corresponding layout of the single layer printed circuit board tracks is shown in  FIGS. 3 and 5 . Changes in the component or track layout may result in changes in the performance of the circuit. 
         [0033]    Other component values may be used to accomplish the aims of the invention. 
         [0034]    The circuit efficiency is high because of low circuit losses through the switching transistors and via the ballast chokes. Waveform control plus the stopping of the inverter when the output is open circuit assists in increasing the efficiency. 
         [0035]    It is to be understood that even though numerous characteristics and advantages of the various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and functioning of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail so long as the functioning of the invention is not adversely affected. For example the particular elements of the circuit may vary dependent on the particular application for which it is used without variation in the spirit and scope of the present invention. 
         [0036]    In addition, although the preferred embodiments described herein are directed to a supply for fluorescent lamps, it will be appreciated by those skilled in the art that the teachings of the present invention can be applied to other systems such as fan motors or other constant AC loads, without departing from the scope and spirit of the present invention. 
       INDUSTRIAL APPLICABILITY 
       [0037]    The apparatus of the invention is used in the provision of a power supply for lamps or similar loads while providing a high power factor to the supply. The present invention is therefore industrially applicable.