Patent Publication Number: US-7723920-B2

Title: Drive circuit for a switchable heating transformer of an electronic ballast and corresponding method

Description:
This Application is a National Phase Application filed under 35 U.S.C. 371 claiming the benefit of an international application PCT/EP2006/067786 filed Oct. 26, 2006, having a priority benefit from an application Germany 102005052525.3 filed Nov. 3, 2005. 
   TECHNICAL FIELD 
   The present invention relates to a drive circuit for a switchable heating transformer of an electronic ballast with a circuit input terminal for picking up an oscillating inverter voltage (DC/AC converter), which has a variable inverter frequency, and a switching device, to whose output terminal the heating transformer can be connected. Furthermore, the present invention relates to a corresponding method for switching a heating transformer. 
   PRIOR ART 
   Depending on the application area, various preheating concepts for ballasts for gas discharge lamps are conventional. These include, for example, preheating via the resonant capacitor of the load circuit, via an auxiliary winding on the lamp inductor, via a resonant heating transformer and via a switchable heating transformer. The most cost-intensive but also most efficient solution for the preheating consists in a switchable heating transformer. 
   A corresponding drive signal and a driver or level converter, which are generally provided by an ASIC, are required for driving a switchable heating transformer. This ASIC conventionally also implements the entire sequence control. However, there are also less expensive ASICs on the market which do not provide a drive signal for a heating transformer. 
   In principle, it has been possible to drive the switchable heating transformer by a delay element instead of by the ASIC. With this delay element, for example a PTC thermistor, a signal can be produced which is only active for a short time directly after the device has been switched on. This method of driving using a delay element does not allow for any synchronization with remotely controlled sequence control, however. 
   DESCRIPTION OF THE INVENTION 
   The object of the present invention therefore consists in providing a simple drive circuit for a switchable heating transformer, where synchronization with remotely controlled sequence control should be possible. A corresponding method should also be made available. 
   According to the invention, this object is achieved by a drive circuit for a switchable heating transformer of an electronic ballast with a circuit input terminal for picking up an oscillating inverter voltage, which has a variable inverter frequency, and a switching device, to whose output terminal the heating transformer can be connected, as well as a frequency evaluation device, which is connected downstream of the circuit input terminal and with which the inverter frequency can be converted into a drive signal for the switching device. 
   Furthermore, the invention provides a method for switching a heating transformer of an electronic ballast, by pickup of an oscillating inverter voltage, which has a variable inverter frequency, conversion of the inverter frequency into a drive signal, and switching of the heating transformer (HT) as a function of the drive signal. 
   The invention is based on the concept that, prior to starting of the gas discharge lamp, the frequency in the load circuit is higher than the nominal operating mode, in which the lamp is lit and therefore the difference in frequency can be used to drive the heating transformer prior to starting of the lamp. If, therefore, the oscillating inverter voltage, which is produced, for example, by the mid-point potential of a half-bridge or full-bridge, is used for producing a drive signal for the heating transformer, synchronization with remotely controlled sequence control of the ballast is possible. 
   Preferably, the frequency evaluation device has a charge pump. This makes it possible, using simple means, to convert the frequency into a drive signal. 
   A voltage divider can be connected downstream of the charge pump. As a result, the current produced by a charge pump can be converted into a desired voltage. Favorably, this switching device comprises a MOSFET transistor. This component is distinguished as a reliable switching unit. 
   If the drive circuit according to the invention is installed in an electronic ballast, a half-bridge, for example, produces the oscillating inverter voltage. It is advantageous here if the amplitude of the oscillating inverter voltage is kept invariable since in this case the output signal of the charge pump is directly proportional to the frequency of the oscillating inverter voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will now be explained in more detail with reference to the attached drawing, which reproduces a circuit diagram of a drive circuit according to the invention. 
   

   PREFERRED EMBODIMENT OF THE INVENTION 
   The exemplary embodiment outlined in more detail below represents a preferred embodiment of the present invention. 
   The FIGURE illustrates a drive circuit for a heating transformer HT. A square-wave oscillating inverter voltage, which originates from a half-bridge mid-point (not illustrated), is present at the input E of the circuit. A charge pump is fed via the input E. Said charge pump comprises the two capacitors C 1  and C 2  and the two diodes D 1  and D 2 . The capacitor C 1  is connected at one terminal to the input E and at the other terminal to the cathode of the diode D 1 . The anode of the diode D 1  is connected to ground. The cathode of the diode D 1  is also connected to the anode of the diode D 2 . Finally, the capacitor C 2  is connected on one side to the cathode of the diode D 2  and on the other side to ground. 
   In the event of a positive input voltage, the capacitors C 1  and C 2  are charged via the diode D 2 . In this case, the magnitude of C 1  determines the amount of charge supplied to C 2 . Given an input voltage of zero, the diode D 2  turns off and the capacitor C 1  is discharged via the diode D 1 . This operation is repeated with each period of an oscillating inverter or input voltage. The mean current transferred by the charge pump is directly proportional to the frequency of the inverter (not illustrated) since, as the frequency increases, the charging operation to the capacitor C 2  takes place more and more often, with the result that its voltage increases. 
   The voltage present at the capacitor C 2  is adjusted in a suitable manner via a resistive load. The resistive load can be in the form of an individual resistor R 2  or in the form of a voltage divider R 1 , R 2  for the more precise adjustment of the voltage. For this purpose, the voltage divider R 1 , R 2  is positioned between the cathode of the diode D 2  and ground and therefore in parallel with the capacitor C 2 . The center tap between the two resistors R 1  and R 2 , i.e. the output of the voltage divider, is used for controlling a MOSFET transistor S 1 , for which reason its gate is connected to the center tap. In order to improve the switching response, the gate is also connected to ground via a capacitor C 3 . The source of the MOSFET transistor is likewise connected to ground, while the drain is connected to the heating transformer HT. 
   The voltage present at the output of the voltage divider is directly proportional to the frequency of the square-wave input voltage, presupposing that its amplitude is constant. Since the MOSFET transistor has a defined switching threshold, the transistor is switched on and off as a function of the frequency of the input voltage. This means that the heating transformer HT is connected via the MOSFET transistor S 1 , which acts as the switching element, at a high inverter frequency (preheating phase) and is disconnected at a low inverter frequency (lamp operation phase). The drive signal therefore precisely follows the frequency of the inverter and therefore predetermined sequence control, which is implemented, for example, by an ASIC.