Abstract:
An electronic ballast is provided, including first a second input connections for connecting an alternating supply voltage; an EMC filter; a rectifier; a capacitor for providing the DC operating voltage for the output stage; and a power factor correction device, which comprises a number n of partial devices for power factor correction, as well as a control device for controlling the number n of partial devices, the power to be provided at the output of the power factor correction device being a total power currently to be provided, the power to be provided by the partial device i being a currently to be provided partial power of the partial device i. The control device is configured to control at least first and second partial devices as a function of the currently to be provided total power in such that their currently to be provided partial powers differ from one another.

Description:
RELATED APPLICATIONS 
     The present application is a national stage entry according to 35 U.S.C. §371 of PCT application No.: PCT/EP2007/057844 filed on Jul. 30, 2007. 
     TECHNICAL FIELD 
     Various embodiments relate to an electronic ballast for operating at least one discharge lamp, including an input stage and an output stage, the output stage having a first output connection and a second output connection for connecting the at least one discharge lamp, as well as a control input for varying the output power provided between the first output connection and the second output connection, the input stage being configured to provide at its output a DC operating voltage for the output stage, and including the following: a first input connection and a second input connection for connecting an alternating supply voltage; an EMC filter (EMC=electromagnetic compatibility), a rectifier, the EMC filter being coupled between the first input connection and the second input connection and the rectifier; a capacitor for providing the DC operating voltage for the output stage; and a power factor correction device that is coupled between the rectifier and the capacitor, the power factor correction device including a number n of partial devices for power factor correction that are interconnected in parallel, as well as a control device for controlling the number n of partial devices, the power to be provided at the output of the power factor correction device being a total power currently to be provided, the power to be provided by the partial device i being a currently to be provided partial power of the partial device i. Furthermore, it relates to a method for operating at least one discharge lamp on such an electronic ballast. 
     BACKGROUND 
     Various embodiments relate to dimmable electronic ballasts, that is to say electronic ballasts that can be controlled in such a way that the total power provided by them can be varied in a wide range. For example, an electronic ballast that is configured for a maximum power to be provided of 300 W provides a total power of 180 W at 60%, and of approximately 60 W at 20%. 
       FIG. 4  carries the variation in the switching frequency f PFC  (PFC=power factor correction) of, for example, three parallel connected PFC stages of a generic electronic ballast as a function of the partial power to be provided by each PFC stage. Thus, given a provided total power P Ges  of 300 W, that is to say each PFC stage provides 100 W, the switching frequency f PFC  is 74.6 kHz. Given a total power to be provided of 240 W, that is to say each PFC stage provides a partial power of 80 W, the switching frequency f PFC  is 93.2 kHz. Given a total power to be provided of 120 W, that is to say each PFC stage provides a partial power of 40 W, the switching frequency f PFC  already rises to a considerable 186 kHz. Finally, the switching frequency f PFC  even increases to 372.9 kHz given a total power to be provided of 60 W, that is to say each PFC stage provides 20 W. 
     The following, further parameters that are reproduced in  FIG. 4  relate to the following variables: U netz  is the rms value of the alternating supply voltage connected between the two input connections. I netz  is the rms value of the current flowing via the input connections. I netz   max  is the maximum value of the current flowing via the input connections U rail  is the DC operating voltage provided at the output of the rectifier. The middle block of  FIG. 4  reproduces variables that relate to the PFC stages, that is to say the partial devices for the power factor correction. Thus, L is the quantity of the inductor used in a PFC stage. I L,max  is the maximum current flowing through this inductor. T on  is the switch on time of the switch of a PFC stage, T off  is the switch off time of the switch of a PFC stage. Finally, the last column of  FIG. 4  specifies how many PFC stages are simultaneously in operation in order to attain the respective provided total power. 
     In order to be able to satisfy the EMC standard decisive for electronic ballasts, presently EN55015, it is required to design the EMC filter such that it is effective for the entire power range, that is to say for a very wide range of the switching frequency f PFC . There is a consequential increase in the complexity and costs of the EMC filter. Moreover, the switching frequency f PFC  increases considerably for very small total powers that are to be provided, such that in this case the efficiency of the power factor correction device decreases markedly in an undesired way. 
     SUMMARY 
     Various embodiments further develop a generic ballast and/or a generic method in such a way that the use of a cost effective EMC filter is enabled. 
     The present invention is based on the finding that this object can be achieved when the control device is designed to control at least a first and a second partial device as a function of the currently to be provided total power in such a way that their currently to be provided partial powers differ from one another. This basic idea opens up various possibilities of implementation, and without anticipating the following description of preferred embodiments mention is to be made, in particular, of two preferred possibilities which consist in completely turning off at least one of the partial devices, or dimensioning different partial devices for different partial powers to be provided. It can thereby be ensured in any case that the respective partial devices can always be operated near their optimum operating point with reference to the switching frequency f PFC  and the partial power to be provided. On the one hand, this enables a more favorable design of the EMC filter, since the switching frequency of the partial devices remains within a markedly narrower range. Furthermore, because of the operation near its respectively optimum operating point, the efficiency of the power factor correction device is improved in the case of dimmed operation. 
     In a preferred embodiment, the control device is designed to control the partial devices as a function of the currently to be provided total power in such a way that only a number m of partial devices are activated in order to provide the currently to be provided total power, it holding true that m≦(n−1), and n expressing the number of all the partial devices. As already mentioned, this opens up the possibility of operating each partial device near its optimum operating point. It is thereby possible to keep the switching frequency f PFC  to low values. 
     The total power currently to be provided by the power factor correction device is preferably determined by evaluating an analog or digital control signal, in particular a dimming signal. Particular mention is to be made here of the so called DALI signal. Consequently, such a signal, which is usually coupled to the output stage in order, in particular, to be evaluated there for the purpose of controlling the switches of the inverter, is also fed to the control device that controls the partial devices of the power factor correction device. 
     Alternatively, the total power currently to be provided by the power factor correction device can be determined by evaluating electrical variables determined in the electronic ballast, in particular by evaluating current and voltage at the input of the power factor correction device, by evaluating current and voltage at the output of the power factor correction device, and/or by evaluating current and voltage at the output of the output stage. It is particularly advantageous here that such electrical variables can be determined in any case for other control purposes in the electronic ballast. These can therefore also be used without a large outlay for the purpose of determining the total power currently to be provided by the power factor correction device. 
     The partial powers of the partial devices can be assigned a phase shift at the beginning of a half wave of the current flowing via the first input connection. The control device is then designed with particular preference to vary the phase shift between the active partial devices as a function of the currently to be provided total power and/or of the number of the active partial devices. For example, if the power factor correction device includes three partial devices that output the same partial power and are operated with a phase shift of 120°, after one of these partial devices has been turned off the phase shift is set between the two remaining partial devices to 180°. If the partial devices are designed for different partial powers to be provided, other phase shifts than 360° divided by the number of the active partial devices can be advantageous. 
     A particularly preferred embodiment is distinguished in that the control device is configured to control at least two active partial devices such that the latter provide different current partial powers. For example, in the case of a power factor correction device that include three partial devices, it can be provided that the partial devices divide up the total power to be provided between themselves into 50%, 30% and 20%. Since at least only one partial device is operated in this case to provide a low partial power, it is possible hereby to keep the efficiency of the power factor correction device at a high level overall. 
     Alternatively or in addition, depending on the number of the partial devices present, the control device can furthermore be configured to control at least two active partial devices such that the latter provide equal current partial powers. The total power to be provided can, for example, already be divided up into 40%, 40%, 20% in the case of three partial devices. It is also possible hereby to attain a higher efficiency of the power factor correction device than in the case of the mode of procedure known from the prior art. 
     Furthermore, it is preferred when one partial device is designed for a maximum current partial power, the operating point for switching off a first or further partial device being selected as follows in the case of a reduction in the currently to be provided total power: (currently to be provided total power)/(number of active partial devices−1)=factor A*(maximum current partial power), it holding true for the factor A that: 0.8&lt;=A&lt;=1. Thus, one of the active partial devices is turned off when in the event of a drop-by dimming the total power to be provided drops so far that it is possible by turning off a first or further partial device to operate the remaining partial devices in a range between 80 and 100% of their maximum current partial powers. 
     By contrast, it can be provided that when one partial device is designed for a maximum current partial power the operating point for switching on a further partial device is selected as follows in the case of an increase in the currently to be provided total power: 
     (currently to be provided total power)/(number of active partial devices)=factor B*(maximum current partial power), it holding true for the factor B that: 0.8&lt;=B&lt;=1. Thus, when for a currently to be provided total power the active partial devices are operated in the range from 80 to 100% of their maximum current partial power, a further partial device is activated given a further increase in the currently to be provided total power. A continuous turning on and off of partial devices can be prevented by appropriately selected hystereses. 
     The two last named measures ensure that the respective partial devices are always operated in the range for which they are designed. On the other hand optimum turning off and/or on of a partial device is fixed such that the power factor correction device is always operated with as optimum an efficiency as possible and in a range that is distinguished by as low a switching frequency f PFC  as possible. 
     Furthermore, it is preferred when it holds for the maximum total power to be provided that: maximum total power to be provided=factor C*(sum of the maximum partial powers to be provided), it holding true for the factor C that: 0.8&lt;=C&lt;=1. 
     Each partial device preferably includes an electronic switch, the control device being configured to operate the partial devices in discontinuous mode, the control device being configured to vary the switch on time of the electronic switch as a function of the currently to be provided total power. This measure constitutes a possibility, which is particularly easy to implement, of realizing the basic idea of the present invention without a large outlay. In this case, the partial devices can be configured identically for the purpose of simplicity, a partial device being turned off and/or the partial device being operated for the purpose of providing different partial powers only by driving the switch of each partial device differently. 
     In this case, the control device can, in particular, be configured to deactivate a partial device when the switch on time of one or each electronic switch has dropped below a prescribable threshold. This can be implemented particularly easily by a micro-processor, use being made of a register to determine the switch on time. The content of the register can be compared very easily with a prescribable threshold stored in a further register. 
     The control device can correspondingly be designed to activate a further partial device when the switch on time of one or each electronic switch has risen above a prescribable threshold. 
     The preferred embodiments, presented above with reference to an inventive electronic ballast, and their advantages are valid correspondingly, to the extent that they can be applied, for the inventive method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings, in which: 
         FIG. 1  is a schematic of the design of an exemplary embodiment of an inventive electronic ballast; 
         FIGS. 2A and 2B  show comparisons of the switching frequency f PFC  against the total power to be provided for a first exemplary embodiment of the invention and for the prior art; 
         FIGS. 3A and 3B  show comparisons of the switching frequency f PFC  against the total power to be provided for a second exemplary embodiment of the invention and the prior art; 
         FIG. 4  shows various electrical variables for a generic electronic ballast known from the prior art; and 
         FIGS. 5 and 6  show the corresponding electrical variables for the first and second exemplary embodiments of an inventive electronic ballast. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. 
       FIG. 1  is a schematic of the design of an exemplary embodiment of an inventive electronic ballast  10 . It includes an input stage  12  and an output stage  14 . The output stage  14  includes a multiplicity of electronic components (not illustrated), including, in particular, an inverter and a lamp inductor. It further includes a first output connection A 1  and a second output connection A 2  for connecting a discharge lamp LA. The input stage  12  includes a first input connection E 1  and a second input connection E 2  between which the alternating supply voltage U netz , in particular the line voltage, can be connected. Following next is an EMC filter  16  to which a rectifier device  18  is connected. Following thereupon is a power factor correction device  20  that in the present case includes three parallel connected PFC stages  20   a  to  20   c  that are controlled via a control device  22 . A partial power P teil1  to P teil3  is provided at the output of each partial device. Provided at the output of the power factor correction device  20  is a total power P Ges  that is formed from the sum of the partial powers P teil1  to P teil3  in provided by the partial devices  20   a  to  20   c . Capacitor C 1  provided at the output of the power factor correction device  20  provides the so called intermediate circuit voltage U ZW  as DC operating voltage at the output stage  4 . The output stage  14  has a control input St optionally varying the output power P A  provided between the first output connection A 1  and the second output connection A 2 . Also illustrated, by way of example, is the design of a partial device  20   c , other embodiments of PFC stages likewise being suitable straightaway for applying the present invention. In the present exemplary embodiment, a PFC stage includes a PFC inductor L through which the current I L  flows. It includes a switch S 1 , which is controlled by the control  22 , as well as a diode D 1  and a capacitor C 2 . 
     Specified in  FIG. 5  for a first exemplary embodiment of an inventive electronic ballast are a multiplicity of electrical variables that correspond to those which have already been introduced in conjunction with  FIG. 4 . In this exemplary embodiment, the partial devices  20   a  to  20   c  are designed for a maximum partial power to be provided of P teili  of 120 W. All three PFC stages are operated in order to provide a total power P Ges  of 300 W, the power provided by each PFC stage being 100 W, see line one of  FIG. 5 . In accordance with line two, as in the prior art all three PFC stages are operated in order to provide a total power P Ges  of 240 W such that each PFC stage provides a partial power of 80 W. The switching frequencies f PFC  in accordance with the first and second lines of  FIG. 5  therefore correspond to the switching frequencies f PFC  of the first and second lines of  FIG. 4 . In accordance with line three of  FIG. 5 , one PFC stage is now turned off in order to provide a total power of 240 W, that is to say only two PFC stages continue in operation, respectively providing a partial power P teil  of 120 W. Whereas during operation of three PFC stages the phase shift of the currents through the respective PFC inductor L are mutually offset by 120°, during operation of two partial devices this phase shift is 180° and is set by the control device  22  by appropriate control. As is to be gathered from line  3  of  FIG. 5 , the switching frequency f PFC  drops to 62.1 kHz. In accordance with line four, in order to provide a total power P Ges  of 120 W, a switching frequency f PFC  of 124.3 kHz is reached during activation of two PFC stages that respectively provide a partial power of 60 W. If a total power P Ges  of 120 W is provided by operating a single PFC stage—see line five of FIG.  5 —, the switching frequency f PFC  drops to 62.1 kHz. When a total power P Ges  of 60 W was provided during operation of three PFC stages, in comparison herewith the switching frequency f PFC  was 186.4 kHz in accordance with line three of  FIG. 4 , and thus approximately three times more. 
     In order to provide a total power P Ges  of 60 W and operation of only one PFC stage, in accordance with line six of  FIG. 5  it is possible to reach a switching frequency f PFC  of 124.3 kHz. 
     A switching frequency f PFC  of 372 kHz is required when providing the same total power P Ges  during operation of three PFC stages in accordance with line four of  FIG. 4 . 
     For the purpose of illustration,  FIG. 2B  shows the profile of the switching frequency f PFC  against the total power to be provided P Ges , for the first exemplary embodiment (dashed line) and the associated prior art (solid line). The corresponding total powers P Ges  and switching frequencies f PFC  are given in  FIG. 2A . 
       FIG. 2B  illustrates how greatly the switching frequency f PFC  rises in the case of small total powers P Ges  to be provided in the case of the prior art, whereas it is only at most 124 kHz in the present exemplary embodiment. Since the lowest switching frequency f PFC  is still approximately 60 kHz for the present invention, the result is a small bandwidth of the required switching frequencies f PFC  and this results in an extremely simple, and thereby cost effective design of the EMC filter  16 . 
     The partial devices  20   a  to  20   c , that is to say the individual PFC stages for providing a maximum partial power of 100 W are set out in the exemplary embodiment in accordance with  FIG. 6 . In order to provide a total power P Ges  of 300 W all three PFC stages are in operation, each PFC stage providing a partial power of 100 W—see line one of  FIG. 6 . When a total power P Ges  of 200 W is provided and all three PFC stages are operated in accordance with line two of  FIG. 6 , the result is a switching frequency f PFC  of 112.0 kHz. If the same total power P Ges  of 200 W is provided by activating only two PFC stages, of which each contributes 100 W—see line three of FIG.  6 —, the result is a switching frequency f PFC  of only 74.6 kHz. 
     Providing a total power P Ges  of 100 by operating two PFC stages in such a way that each PFC stage contributes a partial power of 50 W produces a switching frequency f PFC  of 149.1 kHz. If the same total power P Ges  is provided by activating only one PFC stage—see line five of FIG.  6 —, the result is a switching frequency f PFC  of only 74.6 kHz. A switching frequency f PFC  of 124.3 kHz—see line  6  of FIG.  6 —results for providing a total power P Ges  of 60 W by activating only one PFC stage. 
       FIG. 3B  shows a comparison of the profile of the switching frequency f PFC  against the total power P Ges  provided for the second exemplary embodiment (dashed line) and the corresponding prior art (solid line).  FIG. 3A  gives the associated values of the switching frequency f PFC  and of the total power P Ges  to be provided. It is clearly to be seen once again that in the prior art the switching frequency F PFC  rises steeply given small total powers P Ges , while in the case of the second exemplary embodiment it lies in a window between approximately 75 and 150 kHz. This window is certainly slightly wider than in the case of the first exemplary embodiment, but the second exemplary embodiment is already enabled by a more cost effective design of the individual PFC stages, that is to say in the first exemplary embodiment the individual PFC stages are designed for providing a partial power of 120 W, while for the individual PFC stages a design of 100 W suffices in the second exemplary embodiment. 
     In the exemplary embodiments illustrated, the power provided by the respective PFC stage is effected by varying the switch on time T on  and the switch off time T off  of the switches of the individual PFC stages. Independently of the two exemplary embodiments illustrated, the present invention also covers dimensioning the partial devices for different partial powers, or controlling identical partial devices for the purpose of providing different partial powers by means of the control device  22 , in particular by varying the switch on and switch off times of the respective switch S 1 . Given a reduction in the total power P Ges , a partial device is preferably turned off whenever it transpires that after a partial device has been turned off the remaining partial devices can be operated with a partial power that is approximately 80 to 1001 of its maximum partial power to be provided. Conversely in a total power P Ges  to be provided that is increasing a partial device is switched on when it transpires that after the partial device is switched on the partial devices then activated have to provide between 80 and 1001 of its maximum current partial power. 
     The total power currently to be provided by the power factor correction device can be determined by evaluating a signal applied to the control input St, and also by evaluating electrical variables determined in the electronic ballast, consideration being given here, in particular, to the current and the voltage at the input of the power factor correction device  20 , the current and the voltage at the output of the power factor correction device  20  and/or the current and the voltage at the output A 1 , A 2  of the output stage  14 . 
     While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.