Abstract:
A driving device for a lamp, in particular an HID lamp, the device including a first circuit to convert a network input voltage into a output direct voltage, a second circuit that receives the direct voltage as an input and converts the direct voltage into an alternating signal for supplying the lamp. The first circuit includes a transformer provided with a secondary winding elements a center tap. The driving device further includes at least two capacitive elements connected to the center tap of the secondary winding of the transformer and coupled with the ends of the secondary winding and with the input of the second circuit.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a device for driving lamps, in particular HID lamps. 
     2. Description of the Related Art 
     There are known electronic devices suitable for driving lamps, in particular HID lamps. These particular lamps have a gas within the bulb, for example metal halide or mercury vapor; the lamps require a voltage even higher than 20 KV in order to be ignited for a period of a few seconds and a voltage between 80 V and 110 V in order to be maintained turned on. HID lamps work at a low frequency, from 150 to 800 Hz, in order to avoid damage due to acoustic resonance. 
     The device normally used to drive HID lamps is the ballast. Ballasts are formed with circuit topologies that make use of microcontrollers and rather complex configurations of power transistors. Typically, four power switches in a bridge configuration are provided, two of which work at a high frequency (80-100 KHz) to regulate the current across the lamp, whereas the other two work at a low frequency (150-400 Hz) to meet requirements of a mechanical nature of the lamp itself. 
     Therefore, an HID lamp requires a very particular and precise control that renders the circuit design rather complex. 
     BRIEF SUMMARY 
     In view of the present state of the art, the present disclosure provides a driving device for lamps, in particular HID lamps, that is different from prior devices. The driving device has a simpler circuit configuration while maintaining the same good quality of operation as the known devices. 
     According to one embodiment of the present disclosure, a device for driving a lamp, an HID lamp in particular, is provided, the device having a first circuit adapted to convert an input network voltage into an output direct voltage, a second circuit having at the input said direct voltage and adapted to convert the direct voltage to an alternated signal to supply the lamp. Ideally, the first circuit includes a transformer that has a secondary winding with a center tap. The device has at least two capacitive elements connected to the center tap of the secondary winding of the transformer and coupled with the ends of said secondary winding and with the input of the second circuit. 
     In accordance with another embodiment of the present disclosure, a circuit is provided that includes a first converter circuit having an input to receive an input voltage and generating on an output a filtered and rectified voltage; a second converter circuit coupled to the first converter circuit to receive the filtered and rectified voltage and to output an alternating voltage, the second converter circuit comprising a transformer having a secondary winding with a center tap on which is output an alternating signal that is received at a half-bridge circuit, and the second converter circuit further comprising a first capacitance and a second capacitance coupled respectively to first and second terminals of the secondary winding and to first and second terminals of the half-bridge circuit, the half-bridge circuit generating a driving current with an alternating square-wave voltage. 
     In accordance with another aspect of the foregoing embodiment, the circuit includes a control circuit coupled to the secondary winding and adapted to detect and sum together a detected voltage and a detected current of the alternating signal on the center tap of the secondary winding as a sum signal and to maintain the sum signal constant. 
     In accordance with another aspect of the foregoing embodiment, the circuit includes a controller device coupled to the control circuit and adapted to compare the sum signal to a constant reference signal and to generate an error signal that is used to drive the first converter circuit. 
     In accordance with another aspect of the foregoing embodiment, the circuit includes a circuit for recovering leakage energy on an inductance of the transformer, the leakage energy recovery circuit comprising a capacitor and two diodes coupled to a primary winding of the transformer to obtain a re-flux of the leakage current in the primary winding of the transformer when a transistor of the first converter circuit is turned off. 
     In accordance with another aspect of the foregoing embodiment, the circuit includes a protection circuit having a capacitor coupled to receive the filtered and rectified voltage and connected with a Zener diode so that when the filtered and rectified voltage across the capacitor of the protection circuit overcomes a threshold voltage of the Zener diode, the protection circuit sends a signal for turning off the driving current with an alternating square-wave voltage. 
     In accordance with another aspect of the foregoing embodiment, the circuit includes an HID lamp that is coupled to the half-bridge circuit and is driven by the driving current with an alternating square-wave voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The characteristics and advantages of the present disclosure will become apparent from the following detailed description of an embodiment thereof, illustrated solely by way of non-restrictive example in the appended drawings, in which: 
         FIG. 1  is a block diagram of a device for driving HID lamps according to the present disclosure; 
         FIG. 2  is a simplified circuit diagram of a part of the device for driving HID lamps according to the present disclosure; 
         FIG. 3  is a circuit diagram of the device of  FIG. 1 ; 
         FIGS. 4 and 5  are time diagrams of the voltage and current across the lamp respectively during striking and after striking of the lamp. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , there is shown a block diagram of the device for driving a lamp, in particular an HID lamp, according to the present disclosure. The driving device or ballast includes a block  1  having an EMI filter and a bridge rectifier of the network input voltage Vin, a stage  2  having a DC-DC converter and a control device and, preferably, a PFC circuit with a boost converter, a DC-AC converter  3  that supplies the HID lamp  20  and an igniter circuit  4 . 
     As may be better seen in  FIGS. 2 and 3 , the DC-AC converter  3  is provided with a transistor half-bridge  21 , preferably an IGBT half-bridge, with an associated driving device  22 . 
     The block  1  is of a known type whereas the block  2  has a flyback-type DC-DC converter  100  provided with a transformer  10  having a primary winding  11  and a secondary winding  12 ; the secondary winding is of the center-tapped type. Preferably, the input voltage Vf to the flyback converter  100  is supplied by a PFC stage  28  receiving as input the voltage Vin filtered and rectified by the block  1 ; this in order to assure a very stable input voltage for the flyback converter  100 . 
     The secondary winding  12  of the transformer  10  has the center tap  13  connected to a first capacitance C 1  and a second capacitance C 2  coupled respectively to the terminals  14  and  15  of the secondary winding  12  and connected to the input terminals  17  and  18  of the transistor half-bridge  21 ; also the HID lamp  20  and the central terminal  212  of the IGBT half-bridge  21  are coupled to the center tap  13  of the secondary winding  12 . The IGBT half-bridge  21  receives the voltage Vout deriving from the secondary winding as input and supplies the HID lamp  20  with a current having constant modulation and amplitude whose ripple is minimized by the capacitance C 1  and C 2 . Said capacitances are not of the electrolytic type, but have a low value, on the order of a few hundred nanofarads; in this manner it is possible to drive the HID lamp at around 200 Hz without the use of electrolytic capacitances, which would preclude obtaining control of the lamp current. The use of the low value capacitances C 1  and C 2  is possible due to the center tap of the secondary winding of the transformer, which enables the closing of the circuit for charging and discharging the capacitances irrespective of whether the two IGBT transistors of the half-bridge  21  are turned off or on. The IGBT half-bridge  21  supplies a square wave voltage to the HID lamp  20  and is suitably driven by a device  22 . The half-bridge has two IGBTs  210 ,  211 . 
     The current that flows inside the lamp is preferably controlled by means of a device  30  that detects the current by means of the sensing resistor Ri and detects the voltage Vout of the center-tapped secondary winding  12  across the sensing resistor Rv. The two detected signals are processed in order to construct the error signal, which enables the voltage and current across the lamp to be regulated from the time of ignition until the steady state operating condition is reached. In particular, the control function initially assures a square wave voltage of +/−280V across the lamp (nearly four times the steady state value) with a frequency of around 200 HZ; the lamp  20  in turn also receives voltage peaks of 2.5-3 KV from the igniter circuit  4 . Once ignition has occurred, the lamp voltage rapidly drops to very low values (40% of the steady state value, i.e., approximately 110 V) and then the current control function takes over, which allows the power to be initially adjusted to 60% of the rated power and then to reach, in just over a minute of lamp warm-up, the steady state condition. The graphs in  FIGS. 4 and 5  show the time diagrams of the voltage across the lamp Vlamp and the lamp current Ilamp during the ignition ( FIG. 4 ) and after the ignition ( FIG. 5 ) with the lamp in the steady state condition  20 . 
     The device  30  allows the detected lamp voltage Vlamp and the detected lamp current Ilamp to be summed together and maintained constant. Considering the same value X for Vlamp and Ilamp and letting SUM indicate the output value of the device  30 , it follows that SUM=X+X=K, where K is a constant. The maximum possible variation in either of the two would be 10%, i.e., SUM=(X+10% X)+(X−10% X)=K, so that, correspondingly, the power PLAMP=(X+0.1X)×(X−0.1X)=X 2 −0.01 X 2 =P LAMPtyp −1%. Therefore 10% variations in V LAMP  are controlled with a 1% variation in P LAMP . The device  30  transmits the SUM signal to the input of a controller device  102 ; within the device  102 , the SUM signal is compared, preferably by means of a comparator (non visible in the figures), with a constant reference signal K in order to produce the error signal Se. The device  102  is used to drive the transistor  101  of the flyback converter based on the error signal Se obtained. Preferably, the device  102  is a PFC controller, for example the STMicroelectronics device L6562D, to whose input INV the SUM signal is transmitted. 
     The transistor  101  of the flyback converter is preferably driven by the controller device  102  for the PFC stage, for example the STMicroelectronics device L6562D, in which a constant current is input to the MULT input of the multiplier in place of the traditional current envelope of the sinusoidal type. The controller device  102  is used as the controller for the PFC stage  28 , in particular for controlling the power transistor of the boost converter. 
     Preferably, the ballast device includes a circuit  31  for recovering the leakage energy on the inductance of the transformer  10 ; and the circuit  31  includes the capacitor C 3  and the diodes D 2  and D 4  coupled with the primary winding  11  of the transformer  10  in such a way as to obtain a recirculation of the current leaked from the transformer in the same primary winding of the transformer when the transistor  101  is turned off. 
     Preferably, the ballast has a protection circuit  40  in absence of a load for no-load protection. The circuit includes a capacitance C 4  having one terminal connected to ground GND and another terminal connected to a Zener diode D 21 ; the voltage present across the capacitor C 4  is proportional to the voltage present across the secondary winding  12 . When the voltage across the capacitance C 4  exceeds the threshold voltage of the Zener diode D 21 , a pulse is transmitted in order to turn off the driving device of the lamp  20  by means of the control device  102 , which turns off the transistor  101 . 
     Preferably, the ballast includes a circuit  41  for setting the period of time in which the ignition pulse delivered by the igniter device  4  must be transmitted. 
     The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.