Abstract:
A drive architecture for electric loads, and in particular for loads of light sources is presented. The architecture includes first and second drive circuit blocks connected in series with each other into a half-bridge configuration between first and second terminals of a rectified electric power supply network for the light source. Each drive circuit block has a respective secondary winding of a transformer associated therewith and includes at least a power device and a control circuit portion for controlling the power device. Each control circuit portion of each drive circuit block is subjected to a trigger action directly by its associated secondary winding to generate a high-frequency AC current for driving the light source.

Description:
TECHNICAL FIELD 
     This invention relates to a variable frequency self-oscillating half-bridge drive architecture, and, more particularly to a drive architecture for electric loads, such as light sources and the like, that include first and second drive circuit blocks connected in series with each other into a half-bridge configuration between first and second terminals of a rectified power supply network for the light source. 
     BACKGROUND OF THE INVENTION 
     A halogen lamp or fluorescent lamp can be driven by an electronic circuit capable of generating signals at a very high frequency compared to the frequency of the power supply network. In particular, frequencies in the 30 to 50 kHz range can be generated compared to the 50-60 Hz of the power supply network. 
     In this way, the quality of the emitted light and the efficiency of the emitting source can be improved substantially. 
     This amplified frequency is usually obtained by interposing, between the power supply network and the light emitting source or lamp, a circuit effective to perform a first conversion from AC voltage [50/60 Hz] to essentially DC voltage, with only a limited oscillation or ripple. A subsequent conversion from DC voltage to AC voltage brings the signal up to a higher frequency [30-50 kHz], as shown schematically in FIG.  1 . 
     In particular, FIG. 1 shows a drive circuit  1  which comprises first  3  and second  4  stages cascade connected with each other between a supply network terminal TR and a light source  2 . 
     The AC voltage is rectified and filtered through the first stage  3  to produce a DC voltage which is input to the second stage  4  for conversion to a suitable high-frequency AC voltage for driving the source  2 . 
     In actual practice, there exist several ways of obtaining this conversion from low-frequency to high-frequency AC voltage. In general, two switches SW 1  and SW 2  are used, suitably driven and connected into a half-bridge configuration, and will be discussed with reference to FIGS. 2A-2D. 
     More particularly, the switches SW 1  and SW 2  are connected in series with each other between the terminals T 1 ′ and T 2 ′ of the rectified supply network, which terminals are connected together by a series of a first C′ and a second C″ capacitor. The second terminal T 2 ′ of the rectified supply network is connected to a voltage reference, such as a signal ground GND. 
     The halogen or fluorescent source  2  is placed between a first interconnection node of the switches SW 1 , SW 2  and a second interconnection node of the capacitors C′, C″, it being connected in series with a winding or the primary winding of a transformer  4 . 
     Lately the trend among manufacturers of halogen or fluorescent apparatus has been toward increasingly smaller and low-cost designs. Accordingly, a primary concern has become the design of circuits which can be driven using a minimum of components, while being reliable and inexpensive. 
     In this framing, different design circuits are currently available for driving such apparatus, as shown schematically in FIGS. 2A to  2 D. 
     FIG. 2A shows a conventional drive architecture  1 A which comprises an integrated circuit  5  arranged to drive both switches SW 1  and SW 2  directly. 
     This prior architecture is quite effective to minimize the number of on-board components, but is highly expensive on account of the high cost of the integrated circuit, and disallows feedback between the working state of the lamp and an oscillator contained in the integrated circuit  5 ; the oscillator operates, therefore, at a fixed frequency regardless of the operating phase of the light source  3 . 
     A second conventional design is shown schematically in FIG. 2B, wherein a drive architecture  1 B drives the switches SW 1  and SW 2  with the intermediary of two L-C oscillators  6  and  7  which are connected in parallel with the switches SW 1 , SW 2  and triggered by first  8  and second  9  secondary windings wound around the same core of transformer  4 . 
     The drive architecture  1 B includes a DIAC circuit connected to the input of the second switch SW 2 , and an internal circuit node X which is formed between a resistor R and a capacitor C connected in series with each other between the terminals T 1 ′ and T 2 ′of the rectified supply network. 
     The drive architecture  1 B also includes a diode D, connected between the node X and the intermediate node of the switches SW 1  and SW 2 . 
     It should be noted that the DIAC circuit and diode D are only useful at startup of the drive architecture because, afterwards, the oscillations of the oscillators  6 ,  7  support themselves automatically. 
     A prior art modification of the drive architecture  1 B is shown in FIG. 2C, generally at  1 C in schematic form, and comprises a single oscillator  10  having a respective trigger secondary winding  11 . The drive architecture  1 C further comprises a driver block  12  connected to the second terminal T 2 ′ of the rectified supply network, and connected to the second switch SW 2  directly and the first switch SW 1  via a voltage shifter  13 . 
     FIG. 2D shows another state-of-art drive architecture  1 D which is widely used because of its low cost. The drive architecture  1 D comprises first  14  and second  15  drive circuits connected to the inputs of the switches SW 1 , SW 2  and triggered by first  16  and second  17  secondary windings which are connected to a saturated-core transformer  18 , itself connected to the light source  3  by a winding  19 . 
     The frequency of oscillation of the drive architecture  1 D is set by the saturated-core transformer  18 , which is incapable, however, of ensuring ready repeatability of its characteristics. To achieve stable operation of this transformer, its ferrite components must be carefully selected. 
     In general, working frequencies are obtained, however, which differ between devices, resulting in the lamp being supplied different power levels. 
     There has yet to be developed a drive architecture that has adequate structural and functional features to overcome the drawbacks of conventional architectures. 
     SUMMARY OF THE INVENTION 
     Embodiments of this invention have an oscillation generated within the drive architecture using a trigger winding, rather than by a true oscillator. 
     Presented, therefore, is a drive architecture for electric loads, in particular light sources and the like, that includes first and second drive circuit blocks connected in series with each other into a half-bridge configuration between first and second terminals of a rectified electric power supply network. Each drive circuit block has a respective secondary winding of a transformer associated therewith, and each drive circuit block includes at least a power device and a control circuit portion for controlling the power device. In each control circuit portion of each drive is a circuit block being subjected to a trigger action directly by its associated secondary winding to generate a high-frequency AC current for driving the light source. 
     The features and advantages of the architecture according to embodiments of the invention will be apparent from the following description of one of the embodiments thereof, given by way of non-limitative example with reference to the accompanying drawings. Although this description covers an architecture adapted to drive light sources, e.g. halogen or fluorescent lamps the invention is not limited to this exclusively, and the description covers this field only for convenience of illustration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of the general layout of a voltage conversion arrangement according to the prior art. 
     FIGS. 2A to  2 D show examples of conventional drive architectures for halogen or fluorescent sources. 
     FIG. 3 is a schematic diagram of a drive architecture according to an embodiment of the invention. 
     FIG. 4 is a detailed schematic view of the drive architecture of FIG. 3, as used for driving a fluorescent lamp. 
     FIG. 5 is a detail schematic view of the drive architecture of FIG. 3, as used for driving a halogen lamp. 
     FIG. 6 shows a detail of the drive architectures of either FIG. 4 or  5 . 
     FIG. 7 shows another embodiment of the detail represented in FIG.  6 . 
    
    
     DETAILED DESCRIPTION 
     Referring in particular to FIG. 3 of the drawings, a drive architecture according to an embodiment of the invention is generally shown at  20  in diagramatic form. This architecture  20  is intended, particularly but not exclusively, for driving light sources, such as halogen lamps, fluorescent lamps, and the like. 
     The drive architecture  20  includes first  21  and second  22  circuit blocks which are connected in series with each other between first T 1  and second T 2  terminals of a rectified power supply network. These terminals T 1 , T 2  are also connected together by a series of a resistor R 1  and a first capacitor C 1 , upstream of the circuit blocks  21 ,  22 , and by of a series of second C 2  and third C 3  capacitors, downstream of the blocks  21 ,  22 . The second terminal T 2  is connected to a voltage reference, e.g., a signal ground GND. 
     A series of a light source  23  and a transformer  24  are connected between a first intermediate circuit node X 1 , between the blocks  21 ,  22 , and a second intermediate circuit node X 2 , between the capacitors C 2 , C 3 . 
     Advantageously in this embodiment, the circuit blocks  21  and  22  are triggered by first Ls 1  and second Ls 2  secondary windings wound around the same core or primary winding Lp of the transformer  24 . Furthermore, a third circuit node X 3 , between the resistor R 1  and the first capacitor C 1 , is connected to the second circuit block  22 . 
     The series of the second capacitor C 2 , inherent capacitance of the light source  23 , and primary winding Lp form a resonant series portion within the drive architecture  20 . 
     Specifically, with reference to FIG. 4, the circuit block  21  comprises a power device  25  which is connected between the first terminal T 1  of the rectified supply network and the first intermediate circuit node X 1  and has an input connected to an output terminal of a control circuit portion  25 ′, itself connected to one end of the first secondary winding Ls 1  and the first circuit node X 1 . 
     This control circuit portion  25 ′, in particular, comprises an operational amplifier  26  which is connected between one end of the first secondary winding Ls 1  and the first intermediate circuit node X 1  and has an output terminal connected to the input of the power device  25 . 
     The operational amplifier  26  also has an inverting input terminal and a noninverting input terminal which are connected to the node X 1  respectively through a capacitor Cin′ and a generator G 1 ′ of a voltage reference Vref′, and has a drive terminal connected to one end of the first secondary winding Ls 1 , in turn connected with the other end to the first node X 1 . 
     The control circuit portion  25 ′ further comprises a second generator G 2 ′ of a current I′ which is connected between the drive terminal and the inverting input terminal of the operational amplifier  26 . 
     Advantageously in this embodiment, the control circuit portion  25 ′ also includes a switch SW′, connected across the capacitor Cin′ and controlled by a voltage presented at one end of the first secondary winding Ls 1 . 
     Finally, the first circuit block  21  includes a diode D 1  connected, in parallel with the power device  25 , between the first terminal T 1  of the rectified supply network and the first intermediate circuit node X 1 . 
     The circuit block  22  likewise comprises a power device  27  which is connected between the first intermediate circuit node X 1  and a further reference circuit node X 1 ′ being coincident with the second rectified supply network terminal T 2  and having an input connected to an output terminal of a control circuit portion  27 ′, itself connected to one end of the second secondary winding Ls 2  and the second terminal T 2 . 
     This control circuit portion  27 ′, in particular, comprises an operational amplifier  28  which is connected between one end of the second secondary winding Ls 2  and the second terminal T 2  and has an output terminal connected to the input of the power device  27 . 
     The operational amplifier  28  also has an inverting input terminal and a noninverting input terminal which are connected to the second terminal T 2  respectively through a capacitor Cin″ and a generator G 1 ″ of a voltage reference Vref″′, and has a drive terminal connected to one end of the second secondary winding Ls 2 , in turn connected with the other end to the second terminal T 2 . 
     The control circuit portion  27 ′ further comprises a second generator G 2 ″ of a current I″ which is connected between the drive terminal and the inverting input terminal of the operational amplifier  28 . 
     Advantageously in this embodiment invention, the control circuit portion  27 ′ also includes a switch SW″, connected across the capacitor Cin″ and controlled by a voltage presented at one end of the second secondary winding Ls 2 . 
     Finally, the second circuit block  22  includes a diode D 2  connected, in parallel with the power device  27 , between the first intermediate circuit node X 1  and the second terminal T 2  of the rectified supply network. 
     Advantageously in this embodiment invention, the circuit block  22  fer includes a DIAC device  29  connected between the third intermediate circuit node X 3  and the output terminal of the operational amplifier  28 , and includes a diode D 3  connected between the third intermediate circuit node X 3  and the first X 1 . 
     In a preferred embodiment, the values of the capacitors Cin′ and Cin″, the values of the reference voltages Vref′ and Vref′, and the values of the currents I′ and I″ are chosen to be the same, i.e.: 
     
       
         Cin′=Cin″=C 
       
     
     
       
         Vref′=Vref″=Vref 
       
     
     
       
         I′=I″=I 
       
     
     The operation of the drive architecture according to the described embodiment for driving a light source in a halogen or fluorescent apparatus will now be described. It is important to observe that, whereas prior solutions used different circuit designs for fixing the working frequency of the halogen or fluorescent apparatus, the drive architecture of this circuit uses no true oscillator, but obtains oscillation from a circuit capable of establishing the “off” point in time of the apparatus upon receiving an “on” signal through the secondary winding of the transformer. 
     At the start-up of the drive architecture  20 , the rectified supply network voltage is presented at the first terminal T 1 , the second terminal T 2  being connected to ground GND. Current begins to flow through the resistor R 1  and charges the capacitor C 1 ; upon the voltage across this capacitor C 1  reaching the trigger threshold of the DIAC device of the circuit block  22 , the power device  27  contained in said circuit block  22  is turned on. 
     Thus, a current is caused to flow to ground from the terminal T 1 , along a path which includes the capacitor C 2 , source  23 , transformer  24 , and power device  27 . 
     Advantageously, the secondary windings Ls 1  and Ls 2  are wound to respectively apply negative and positive voltages to the control circuit portions  25 ′ and  27 ′. 
     In particular, the positive voltage from the winding Ls 2 , besides acknowledging the “on” state of the corresponding power device  27 , also activates the generator G 2 ″ of the current I″ to charge the capacitor Cin″, having the same capacitance as C. When the voltage across the capacitor Cin″ equals the value of the voltage Vref, the power device  27  is turned off by the operational amplifier  28 , and the current present in the circuit block  22  will continue flowing through the diode D 1  of the circuit block  21  until exhausted. 
     During this phase, the voltage at the secondary windings Ls 1  and Ls 2  is inverted to become positive at Ls 1  and negative at Ls 2 , thus allowing the charge built up within the capacitor Cin″ of the circuit block  22  to be discharged, and initiating the same process as previously described for the circuit block  21 . 
     The capacitor Cin″ will be discharged through the switch SW″. This switch SW″, being connected in parallel with the capacitor Cin″, is controlled directly by its connection to one end of the second secondary winding Ls 2 , and in particular, is turned on by the voltage at the secondary winding Ls 2  becoming negative. 
     Advantageously, the blocks  21  and  22  are push-pull driven from the secondary windings Ls 1  and Ls 2 , so that simultaneous conduction of the two blocks is prevented. 
     The working frequency of the drive architecture  20  of this embodiment is, therefore, set by the values of the capacitor C, the current I, and the reference voltage Vref, which are all internal parameters of the blocks  21  and  22 . 
     A major advantage of this drive architecture is that it does not constrain the system working frequency when the latter is higher than that set by the blocks  21  and  22 . In this case, the working frequency of the whole apparatus is the frequency established by the series resonant portion of the drive circuit  20  comprising the capacitor C 2 , an additional capacitor C 4  connected in parallel with the source  23 , and the primary winding Lp. 
     This is what takes place in the apparatus at start-up, and with the source comprising essentially a fluorescent lamp, as shown in FIG. 4, connected in series with the primary winding Lp and having an additional capacitor C 4 . 
     When turned on, a fluorescent lamp has the character of a series circuit, and the working frequency is dependent on the values of the capacitors C 2 , C 4  and the primary winding Lp. Thereafter, the fluorescent lamp may be regarded as the equivalent of a resistor forming the series resonant portion in combination with the capacitor C 2  and the primary winding Lp. 
     Thus, an elevated frequency (on the order of 70 kHz), above the working frequency set by the blocks  21 ,  22  for normal operation, is obtained at start-up. 
     Advantageously, the drive circuit  20  provides, therefore, an adaptative type oscillating system, having its trigger frequency set by the system itself automatically changing the value of the working frequency at the end of the turn-on phase. 
     When, on the other hand, the source  23  comprises a halogen lamp, as shown in FIG. 5, the drive circuit should be designed for a low-voltage supply to the lamp. For this purpose, the halogen lamp is to be related to a third secondary winding of the transformer  24 . 
     Shown in FIG. 6 is another possible embodiment of the control circuit portion  27 ′ of the block  22 , which includes a bipolar transistor B 1  having its collector terminal connected to the input of the power device  27 , and having its emitter terminal and base terminal connected to the second terminal T 2  of the rectified network directly and through a first resistor R 2 , respectively. 
     The control circuit portion  27 ′ further includes a second resistor R 3  connected between one end of the secondary winding Ls 2  and the collector terminal of the bipolar transistor B 1 , and has a series of a third resistor R 4  and a capacitor C 5  connected across the secondary winding Ls 2  and defining an intermediate circuit node Z, in turn connected to the base of the bipolar transistor B 11  through a Zener diode DZ. 
     In addition, the block  22  may be an emitter-switching configuration, so as to allow the amount of charge stored in the block  22  to be recovered. 
     In this way, a more accurate drive circuit can be obtained, since a supply reference would be used which is relatively stable and the apparatus as a whole is unconstrained by voltage variations in the secondary winding while generating the charge current to the capacitor C 5 . 
     FIG. 7 shows a preferred embodiment of the control circuit portion  27 ′ which comprises a further switching block B 2  connected in parallel across the capacitor C 5  and, via a resistor R 5 , to one end of the secondary winding Ls 2  for discharging the capacitor C 5 . 
     Advantageously, a decoupling diode D 4  is connected in series with the resistor R 4  to force the capacitor C 5  to discharge through the switching block B 2 . Thus, the initial charge conditions of the capacitor C 5  are set at the end of each charge/discharge cycle independently of the drive provided by the secondary winding Ls 2 . 
     To summarize, this drive architecture can be implemented using discrete components, in combination with a smart-power technology, to provide the advantages of improved reliability, guaranteed repeatable performance, as well as reduced cost from fewer components. 
     Changes can be made to the invention in light of the above detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all methods and devices that are in accordance with the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined by the following claims.