Abstract:
An energy saving electronic control circuit for fluorescent tubes, where it controls the tubes&#39; light output. This system uses a half-bridge topology, which is supplied from an AC to DC current source obtained from the mains supply. The switching transistors are driven at rather high frequencies. An inductor is connected in series with a capacitor, forming an LC circuit, which is connected to the junction between the switching transistors and these resonant at the same switching frequency of the transistors. At least one tube is connected across one capacitor, in other words in series with the inductor at one end and at the center junction of a passive half-bridge formed by two capacitors in series.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to an electronically controlled power saving, power supply system for fluorescent tubes. 
     More particularly, the present invention relates to a system which drives fluorescent tubes with a much higher efficiency than a traditional setup would, the comprehensive protective features of the circuit enabling the designer to design ballasts with protection against over voltages, removed tubes and over-currents. It is also possible to enhance the circuit by employing a thermal cutout as well. 
     The following disclosure describes an electronic driver for fluorescent tubes, better known as an “Electronic Ballast”. 
     It is known that hot cathode fluorescent tubes require an inductor, better known as choke, to limit the current and voltage to the tube from the incoming mains voltage. This setup also requires a switching element, better known as starter, to give the initial strike till the cathodes become warm. These starters have a relatively short life and give the annoying prolonged flashing during the starting phase. 
     The traditional setup arrangement produces another problem, which is somewhat more annoying or rather dangerous. When the tube is supplied from a 50 Hz source, this produces a 100 Hz flicker which is hardly noticed but in the presence of rotating machinery, becomes very hazardous since the machinery might seem to be stationary while rotating. This effect is better known as the “Stroboscopic Effect”. Much has been done to try to solve the various problems, but others always arise. 
     The electronic driver system (electronic ballast) according to the present invention tends to solve practically all the problems of the prior art. 
     The characteristics of the device according to the invention will be stated in the characterizing part of the attached claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described with reference to currently preferred embodiments, illustrating and not limiting the invention and according to the figures of the attached drawings, in which: 
     FIG. 1 shows the schematic diagram of a first embodiment of the electronic ballast according to the invention; 
     FIG. 2 shows a second embodiment of the schematic diagram of FIG. 1 above; 
     FIG. 3 shows the schematic diagram of a block diagram shown in FIGS.  1  and  2 ; 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIGS. 1 and 2, the proposed circuit shall be used in conjunction with a half bridge topology comprising MOSFETs or IGBTs as switching elements. The following circuit forms part of an overall strategy towards designing more environmentally friendly fluorescent tube ballasts. The main benefit of the system according to the invention is energy saving which is hard to beat with conventional electronic ballasts. The comprehensive protective features of the circuit enable the designer to design ballasts with protection against over-voltages, over-current, over-temperature and removed tubes. 
     FIGS. 1 and 2 show typical layouts, which are practically identical, and therefore only FIG. 1 will be described. 
     The disclosure starts with the detailed description of the controller. A discussion of practical tips, which make the realization of a superior energy saving electronic ballast possible, follows. 
     Fuse F 1  protects the incoming installation from any possible short circuit caused by the electronic ballast in the case of malfunction. C 1 , L 1  and C 2 , L 2  form a double filter that block any incoming or outgoing electrical noise in a wide band of frequencies. C 3 , connected with the traditional inductor already found in the light fixture, acts to correct the power factor and as an additional filter for the lower frequency range. C 4  and C 5  are class Y capacitors to filter out the very high frequencies generated by the ballast&#39;s oscillator and its switching devices. 
     C 6 , C 7 , C 8 , R 1 , R 2  and BR 1  form the ballast controller&#39;s power supply. It is basically a capacitive power supply, which dissipates only a fraction of the power dissipated if a dropper resistor where to be used. C 8  smoothes the 100/120 Hz ripple. 
     An alternative power supply can be easily obtained by substituting C 6 , C 7 , C 8 , R 1 , R 2  and BR 1  by a dropper resistor and connected to D 3  and D 4  cathodes. This way the circuit can be supplied by AC and DC input supplies. DC input supplies are usually found in emergency circuits and in naval installations. 
     Diodes D 1 ˜D 4  rectify the input supply for the power section of the electronic ballast. Resistors R 3  and R 4  assure that the DC bus supply is equally distributed across the reservoir capacitors C 9  and C 10 . C 11  and C 12  further limit the current of the fluorescent tubes. 
     MOSFETs T 1  and T 2 , which are controlled by the circuit IB 1  and will be described later with reference to FIG. 3, supply two resonant circuits each made of a capacitor in series with an inductor (L 3 , C 13 A and L 4 , C 14 A). Each capacitor has one fluorescent tube in parallel, and the return of the resonant circuit is connected to C 11  and C 12  junction, which are connected in a half-bridge configuration. The preferred drive frequency used is around 40 kHz. 
     Resistor R 5  monitors the current passing through the switching devices (T 1 , T 2 ), and triggers the protection circuit in circuit IB 1  in case of over current due to a fault or bad installation. 
     The gap of the ferrites, which constitutes the output chokes L 3  and L 4 , plays a very important role in reducing the energy consumed. A gap size of 1.2 mm for the N 27  core provides the best compromise between leakage flux and the heat build up in the ferrite itself The windings are based on single stranded wire, since the effect of the skin effect at around 40 kHz is not too pronounced with wires having a diameter of less than 0.6 mm. The standard choke design procedure may be followed bearing in mind that the size of the gap should be around 1.2 mm. 
     It is important to keep the capacitance value of the capacitor shunting the fluorescent tubes FL 1  and FL 2  to a minimum; thus ensuring that most of the current flowing through the choke is that required by the tubes themselves. One must, however, bear in mind that the LC combination of the output inductor (or choke) puts limit on the capacitance value of the capacitor, since the smaller it is, the larger the inductance of the choke has to be. A too large value of inductance (L 3 , L 4 ) may necessitate the use of a larger ferrite, which may not be possible due to physical and financial constraints and reduced light output. 
     Capacitors C 13 A and C 14 A can be fitted in starter cases, which will eventually substitute the traditional mechanical starters, thus reducing installation wiring modifications. 
     The use of MOSFET transistors reduces drastically most losses, in practice the overheating of the transistors does not exceed 43° C. in a 25° C. ambient temperature and with an input supply of 300 Volts AC and having a load of 116 W (2×58 W), thus reducing size and cost. 
     The circuit can also easily handle a wide range of input supply from 140 V to 380 V. At the high end of the range the over voltage protection is triggered. When this protection triggers, the ballast can be reused once it is reset. 
     The hybrid circuit IB 1  together with some auxiliary circuits will be described with reference to FIG.  3 . 
     IC 1 , preferably IR 2153 of International Rectifier, is a self-oscillating half bridge driver for the MOSFET transistors T 1  and T 2 . The frequency of operation is programmed by the combination of R 7  and C 4  and is approximately given by 
     
       
           f =1/(1.4×R 1 ×C 1 ) Hz 
       
     
     An external DC source of 12 V @ 10 mA meets the power requirements of the circuit. Zener diode ZD 1  ensures that the voltage is clamped at 12 V. Resistors R 8  and R 9  in conjunction with the parallel high-speed diodes limit the rate at which the switching element, either a MOSFET or IGBT, switches on. This has the benefit of reducing electromagnetic emissions by reducing the rate of change of the Drain Voltage. The high-speed diodes ensure that switch off takes place at a faster rate than switch on. This further increases the inherent dead time of IC 1 . Diode D 2  and capacitor C 5  form the bootstrapping circuit, which provides the upper, floating switching element with enough voltage to switch on. 
     The protection circuit may be divided in two parts 
     the over-voltage 
     the over-current sections. 
     Over voltage protection is achieved by continuously sampling the bus voltage by means of resistors R 1 , R 2 , R 3  and R 4 . Capacitor C 2  smoothes any ripple which might be present. If the sampled voltage, which is that voltage across R 4 , exceeds the break-over voltage of the DIAC SCR 2 , and the trigger voltage of SCR 1 , the latter latches on and the voltage at pin  1  of IC 1  is reduced to practically ground potential. Once power is removed from the IC 1 , the circuit and hence the ballast switches off. Operation is automatically resumed once the power to the ballast is switched off and on again. 
     Over current protection is achieved by D 1 , R 5 , C 3 , R 6  and SCR 1 . A current to voltage converter feeds D 1  with a voltage equivalent to the instantaneous load current of the ballast. This voltage is rectified by diode D 1 . R 5  and R 6  form a potential divider, which samples the voltage across the current sensor. Capacitor C 3  serves as a spike filter as well as delays the sampled voltage before it reaches the gate of SCR 1 . This ensures that the circuit does not trip on start up. Once the sampled voltage exceeds the trigger voltage of SCR 1 , which is about 0.7 volts, the thyristor latches on and the ballast stops functioning. The circuit is self-starting once power is switched off and on. 
     A thermal protection may be added by simply attaching a quick changeover thermal device, such as the TO-92 enclosed solid state fuses supplied by RS components, between the supply of the 12 V circuit and the ground. Needless to say, the power supply must be current limited. 
     The following tables 1 and 2 show, by illustrative, but not limiting way, the parts list referring to FIGS. 1 and 3. Needless to say that the components of FIG. 2 are the same as FIG.  1  and do not need to be disclosed into detail. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Manufacturer 
               
               
                 Component 
                 Description 
                 number 
               
               
                   
               
             
             
               
                 D1, D2, 
                 High speed diodes 
                 BYM11-800 
               
               
                 D3, D4 
                 &lt;=115 nS-1.5 A-1000 V 
               
               
                 R3, R4 
                 Resistor 
                 470k W 
               
               
                 R1, R2 
                 Resistor 
                 120 W 
               
               
                 R5 
                 Resistor 
                 0.33 W 
               
               
                 BR1 
                 Bridge rectifier 
                 MB8S 
               
               
                 C8 
                 Electrolytic capacitor 
                 100 mF-25 V 
               
               
                 C9, C10 
                 Electrolytic capacitor 
                 22 mF-250 V 
               
               
                 C1 
                 Polypropylene Capacitor Class 
                 470 nF-275 V 
               
               
                   
                 X2 
               
               
                 C2, C3 
                 Polypropylene Capacitor Class 
                 68-100 nF-630 V 
               
               
                   
                 X2 
               
               
                 C6, C7 
                 Metalized polypropylene 
                 470 nF-250 V 
               
               
                   
                 Capacitor 
               
               
                 C11, C12 
                 Metalized polypropylene 
                 100-220 nF-630 V 
               
               
                   
                 Capacitor 
               
               
                 C4, C5 
                 Class Y Capacitor/High 
                 1 nF-275 V/1 nF-1 kV 
               
               
                   
                 Voltage 
               
               
                 T1, T2 
                 MOSFET transistors 
                 IRF840 
               
               
                 L1, L2 
                 Common mode chokes 
                 45 mH 
               
               
                 L3, L4 
                 Output chokes 
               
               
                 C13a, C14a 
                 Metalized polypropylene 
                 10 nF-630 V 
               
               
                   
                 Capacitor 
               
               
                   
               
               
                 C13a, C14a: 10 nF = 36 Watt; 12 nF = 58 Watt; 15 nF = 2 × 18 Watt  
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Manufacturer 
               
               
                 Component 
                 Description 
                 number 
               
               
                   
               
             
             
               
                 D2, D3, D4 
                 High speed diodes 
                 BYM 37 M 
               
               
                   
                 &lt;=115 nS-1.5 A-1000 V 
               
               
                 D1 
                 Diode 1N4007 type or the like 
                 BYM 10-1000 
               
               
                 ZD1 
                 Zener diode 12 Volts 
                 BZD 27 C 12 
               
               
                 SCR2 
                 Diac 
                 BR 100/03 LLD 
               
               
                 SCR1 
                 Thyristor 
                 BT 148 W-400 R 
               
               
                 IC1 
                 IC-product of International 
                 IR 2153 
               
               
                   
                 Rectifier 
               
               
                 R8, R9 
                 Resistor 
                 47 W 
               
               
                 R7 
                 Resistor 
                 18 kW 
               
               
                 R4, R6 
                 Resistor 
                 27 kW 
               
               
                 R5 
                 Resistor 
                 56 kW 
               
               
                 R1, R2 
                 Resistor 
                 120 kW 
               
               
                 R3 
                 Resistor 
                 150 kW 
               
               
                 C4 
                 Capacitor 
                 1 nF 
               
               
                 C1, C2, C3, C5 
                 Capacitor 
                 100 nF 
               
               
                   
               
             
          
         
       
     
     The above description refers to an electronic ballast for hot cathode fluorescent tubes, having an efficiency in the region of 40% without any penalty on the light flux. This electronic ballast has a superior efficiency than other electronic ballasts found in the market. 
     This documentation has been described referring to the diagrams and parts list above, however it is not limited to such, as in practice some components can be substituted providing the same or maybe even a slightly better result. A person skilled in the art can easily carry out these modifications.