Patent Application
20110316434
The present invention relates in general to the switching of discharge lamps.
It is generally known that gas discharge lamps, for example the well-known TL-lamps, are driven by an electro magnetic ballast (EM ballast).
In a more sophisticated design, the mechanical switch is replaced by a controllable semiconductor switch, operated by an intelligent control device such as for instance a micro controller.
The electronic switching circuit 20 further comprises a control device 28, having a control output 28a connected to the control terminal of the switch 26. The control device 28 may derive its power from the terminals 24, 25, or may derive its power from an external circuit (not shown). The control device 28 may be responsive to external command signals, transmitted over an external circuit (not shown), via a wired or wireless link, e.g. RF.
Assume that the mains power is switched on while the switch 26 is OFF, i.e. non-conductive. The voltage from the mains is insufficient to start the lamp. Starting the lamp is done by the controller 28 in two steps. The first step involves switching the switch 26 ON, i.e. generating a control signal Sc for the switch 26 such as to render the switch 26 conductive. Now, an AC current will flow through the inductor L and the lamp electrodes 5, heating the lamp electrodes 5. In a second step, the controller 28 switches the switch 26 OFF again, i.e. it generates its control signal Sc for the switch 26 such as to render the switch 26 non-conductive. As a result of this interruption, the inductor L develops a high voltage causing breakdown and ignition of the lamp, so that lamp current flows between the electrodes 5 within the lamp.
The magnitude of the ignition voltage induced by the inductor L depends on the amount of energy E(L) stored in the inductor at the moment of interrupting the current circuit, which can be expressed as E(L)=0.5·L·I2.
A problem is associated with the fact that the voltage induced by the inductor L is also applied to the switch 26, which is after all connected in parallel to the lamp 2. Normally, the lamp ignites before the induced voltage reaches its maximum, but it may be that the lamp does not ignite immediately. In such case, the maximum value of the induced voltage will be applied to the switch, that is not capable to resist this voltage and will conduct a current in avalanche mode. Such current may cause the switch to be destroyed. In order to prevent this, the controller 28 may be programmed to set the timing of the interruption of the preheat current so that it does not coincide with the maximum current: a suitable timing is for instance 86% of the current period. In that case, for an exemplary situation of a 70 W lamp, where the coil has an inductance of 600 mH while the momentary coil current is about 0.75 A, the energy E(L) stored in the inductor is about 170 mJ. For normal switches, the amount of avalanche energy they can resist is about 350 mJ.
However, it is also possible a user to switch off the lamp 2 by switching the mains. Or, it may be that the lamp fails and stops working. In both cases, the above scenario also takes place, with the difference that the timing with respect to the current phase is now random so it may coincide with the maximum lamp current and thus may result in a very high voltage peak over the switch. In the example mentioned above, the maximum lamp current may be about 1.6 A and the energy applied to the switch is about 770 mJ.
An object of the present invention is to provide a ballast with an electronic switching circuit wherein the above-mentioned problems are overcome, particularly, wherein the electronic switch is protected against high induction voltage pulses.
According to the present invention, the controller 28 is adapted to monitor whether a current flows through the switch while it is OFF, and if so, to switch the switch to its ON condition. Now the current, which continues to flow, does not harm the switch any more, and the switch may dissipate some of the energy on the basis of its small resistance RDSon.
Further advantageous elaborations are mentioned in the dependent claims.
These and other aspects, features and advantages of the present invention will be further explained by the following description of one or more preferred embodiments with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which:
During the stages of preheating and ignition, the enable signal Se is LOW, and the output signal from the AND gate 42 is LOW. Thus, the switching state of the switch 26 is only determined by the control signal Sc, which is HIGH for closing the switch 26 to generate the preheat current and which is switched to LOW for opening the switch to trigger ignition.
Then, the controller 28 enters a normal operation mode, during which the lamp is burning normally. In this mode, the enable signal Se is HIGH and the control signal Sc is LOW. As long as no current is flowing through the switch 26, the output signal from the AND gate 42 remains LOW and the switch remains open. As soon as a current in the switch 26 (which must be an avalanche current because the switch is open) reaches a sufficient magnitude, the comparator 41 outputs a HIGH signal, causing the AND gate 42 to output a HIGH signal, which in turn causes the OR gate 43 to output a HIGH signal so that the switch 26 is closed. Note that the switch 26 is opened automatically when the current in the switch has extinguished.
In step 51, the controller 28 checks whether it is operating in a mode in which current through the switch is allowed, such as the preheat phase or ignition. If yes, no further action needs to be taken.
In step 52, the controller 28 checks whether any current is flowing through the switch. If no, the controller 28 sets or maintains a control signal for the switch 26 such as to turn or maintain the switch OFF in step 53a. If yes, the controller 28 sets or maintains a control signal for the switch 26 such as to turn or maintain the switch ON in step 53b.
It is noted that in the above embodiments the rectifier 21 allows the use of relatively cheap MOSFETs, which should be operated to conduct current in one direction only. Instead, it is in principle possible to another type of controllable switch, capable to be operated with current in two directions, in which case the rectifier can be omitted.
Summarizing, the present invention provides an electro magnetic ballast 110 for a gas discharge lamp 2, which comprises:
When operating in a normal mode, the control circuit 28 is responsive to a current sense signal received from the current sensor to switch the controllable switch 26 ON if said current sense signal indicates a current flowing in the controllable switch 26 and to switch the controllable switch 26 OFF if said current sense signal indicates that no current is flowing in the controllable switch 26.
While the invention has been illustrated and described in detail in the drawings and foregoing description, it should be clear to a person skilled in the art that such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments; rather, several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. For instance, the capacitor C may be absent. Further, the inventive gist of the present invention can also be applied to protect other semiconductor switches against avalanche currents, i.e. even in other applications not being a lamp ballast application.
Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.
In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09152745.7 | Feb 2009 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2010/050576 | 2/9/2010 | WO | 00 | 8/10/2011 |
