Abstract:
In an inverter for operating a discharge lamp by means of an AC current comprising two switching elements, the effect of hard switching is counteracted by means of a snubber comprising two inductive elements and at least two diodes.

Description:
This invention relates to a circuit arrangement for feeding a lamp comprising 
     a first input terminal K 1  and a second input terminal K 2  which are to be connected to a supply voltage source supplying a DC voltage, 
     an inverter for generating a square-wave periodic voltage from said DC voltage, which inverter is provided with a series arrangement of a first switching element S 1 , a first inductive element L 1 , a second inductive element L 2  and a second switching element S 2 , and which inverter interconnects the input terminals, 
     a control circuit which is coupled to a control electrode of the first switching element S 1  and to a control electrode of the second switching element S 2 , which control circuit is used to generate a control signal for rendering the first and the second switching element alternately conducting and non-conducting, 
     a load branch comprising a third inductive element L 3 , lamp terminals for connecting the lamp, and a first capacitive element C 1 , 
     a first unidirectional element D 1  having an anode coupled to the second input terminal K 2  and a cathode coupled to a point between the first switching element S 1  and the first inductive element L 1 , 
     a second unidirectional element D 2  having a cathode coupled to the first input terminal K 1  and an anode coupled to a point between the second switching element S 2  and the second inductive element L 2 . 
     Such a circuit arrangement is disclosed in WO-9902020. In the known circuit arrangement, the control circuit is also provided with a dimmer circuit for dimming the lamp by regulating the duty cycle of the control signal. In addition, the self-inductances L 1 ′, L 2 ′ and L 3 ′ of, respectively, the first, the second and the third inductive element L 1 , L 2  and L 3  are chosen so as to be substantially equal to each other. The first and the second inductive element are magnetically coupled to each other and hence jointly form a transformer. As a result of said values of the self-inductances and by virtue of this magnetic coupling, it is achieved that the shape of the current through the lamp during dimming the lamp comes fairly close to a sine shape. In other words, the lamp current comprises comparatively few higher harmonic terms, as a result of which the amount of disturbance generated by the lamp is limited. In addition, in the known circuit arrangement, acoustic resonances are effectively suppressed. In a part of the range wherein the duty cycle of the control signal can be regulated “hard switching” occurs. This means that each one of the switching elements is rendered conducting while a comparatively high voltage is present across the switching element. This may give rise to a comparatively high power dissipation in the switching elements. In the known circuit arrangement, this power dissipation is counteracted to a limited extent only as a result of the fact that the first and the second inductive element are arranged in series with the switching elements. In addition, a drawback of the known circuit arrangement resides in that the transformer formed by the first and the second inductive element is a comparatively expensive and bulky component. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a circuit arrangement wherein the power dissipation caused by “hard switching” is effectively counteracted using comparatively straightforward, inexpensive and small components. 
     To achieve this object, a circuit arrangement as mentioned in the opening paragraph is characterized, in accordance with the invention, in that with respect to the self-inductances L 1 ′, L 2 ′ and L 3 ′ of, respectively, the first, second and third inductive element, the following relationship applies; 
     
       
           L   3 ′&gt;5* L   1 ′ and  L   3 ′&gt;5* L   2 ′. 
       
     
     In a circuit arrangement in accordance with the invention, power dissipation in the switching elements due to “hard switching” is substantially suppressed in spite of the comparatively small self-inductances of the first and the second inductive element. Power that would be dissipated in the switching elements, if the first and the second inductive element and the first and the second unidirectional element were absent, is effectively fed back to the supply voltage source or used to generate a current through the lamp. It has been found that this applies if the first and the second inductive element are magnetically coupled, but also if the inductive elements are not coupled. 
     It has been found that in many cases power dissipation is very effectively counteracted if with respect to the self-inductances L 1 ′, L 2 ′ and L 3 ′ of, respectively, the first, second and third inductive element, it applies that 
       L   3 ′&gt;10 *L   1 ′ and  L   3 ′&gt;10* L   2 ′. 
     It has also been found that power dissipation can be further reduced if the circuit arrangement is additionally provided with a third unidirectional element D 3  and a fourth unidirectional element D 4 , with a cathode of the third unidirectional element D 3  being coupled to the first input terminal K 1 , an anode of the fourth unidirectional element D 4  being coupled to the second input terminal K 2  and an anode of the third unidirectional element D 3  and a cathode of the fourth unidirectional element D 4  each being coupled to a point between the first inductive element L 1  and the second inductive element L 2 . 
     As the circuit arrangement comprises parasitic capacitances, oscillations occur which are brought about by the first and the second inductive element and said parasitic capacitances. By means of the third and the fourth unidirectional element it is achieved that the amplitude of voltages caused by these oscillations, particularly of the voltage on the point between the first and the second inductive element, remains limited. A further reduction of the power dissipation is thus achieved. In addition, the unidirectional elements D 3  and D 4  form part of current paths for “reverse” currents having a small impedance. As a result, in the case of “hard switching”, the third unidirectional element D 3  carries current, not the second unidirectional element D 2 , for rendering the second switching element S 2  conducting. Correspondingly, the fourth unidirectional element D 4  carries current, not the first unidirectional element D 1 , for rendering the first switching element S 1  conducting. By virtue thereof, power dissipation in the first and the second unidirectional element and the switching elements is limited substantially when the switching elements are becoming conducting. 
     Field effect transistors such as MOSFETs are often used as the switching elements in a circuit arrangement in accordance with the invention. Such field effect transistors comprise an internal diode that is capable of guiding the current in a direction that is in opposition to the direction in which the field effect transistor carries current in the conducting state. These internal diodes play an important part in the functioning of the circuit arrangement since they carry current during specific operational phases of the circuit arrangement. If these internal diodes are comparatively slow, then a comparatively high power dissipation occurs when said internal diodes become non-conducting. This contribution to the power dissipation can be reduced substantially if the circuit arrangement is additionally provided with a fifth unidirectional element D 5  which is arranged in series with the first switching element S 1 , a sixth unidirectional element D 6  which is arranged in series with the second switching element S 2 , a first shunt branch which comprises a seventh unidirectional element D 7  and shunts the series arrangement of the fifth unidirectional element D 5  and the first switching element S 1 , and a second shunt branch which comprises an eighth unidirectional element D 8  and shunts the series arrangement of the sixth unidirectional element D 6  and the second switching element S 2 . Said unidirectional elements D 5 -D 8  being chosen so as to operate at a comparatively high speed with respect to the internal diodes of the switching elements S 1  and S 2 . 
     As indicated hereinabove, “hard switching” occurs particularly in a circuit arrangement wherein the control circuit is provided with a dimmer circuit for regulating the duty cycle of the control signal. Consequently, the invention can very advantageously be used in such circuit arrangements. 
     Controlling the luminous flux of the lamp by means of a dimmer circuit for regulating the duty cycle of the control signal can be very advantageously applied in circuit arrangements which are intended to feed lamps of a different type, since the relation between the duty cycle of the control signal and the luminous flux of the lamp is very similar for lamps of a different type. Such circuit arrangements intended to feed lamps of different types are generally provided with a circuit part for recognizing the type of lamp connected to the lamp terminals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Examples of a circuit arrangement in accordance with the invention will be explained in greater detail with reference to the accompanying drawing. In the drawing, 
     FIG.  1  and 
     FIG. 2 show, respectively, a first and a second example of a circuit arrangement in accordance with the invention to which a lamp is connected. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, K 1  and K 2  are input terminals which are to be connected to a supply voltage source supplying a DC voltage. Such a supply voltage source can be, for example, an AC source, such as the mains, provided with a rectifier. Input terminals K 1  and K 2  are connected to each other by means of a buffer capacitance Cbuf. The buffer capacitance Cbuf is shunted by a series arrangement of diode D 5 , switching element S 1 , coil L 1 , coil L 2 , diode D 6  and switching element S 2 . A junction paint of coil L 1  and switching element S 1  is connected to input terminal K 2  by means of diode D 1 . A junction point of coil L 2  and switching element S 2  is connected to input terminal K 1  by means of diode D 2 . Circuit part SC is a control circuit for generating a control signal for rendering switching element S 1  and switching element S 2  alternately conducting and non-conducting. For this purpose, a first output of circuit part SC is coupled to a control electrode of switching element S 1 , and a second output of circuit part SC is coupled to a control electrode of switching element S 2 . The circuit part SC is provided with a dimmer circuit DC for regulating the duty cycle of the control signal. The series arrangement of diode D 5  and switching element S 1  is shunted by diode D 7 . The series arrangement of diode D 6  and switching element S 2  is shunted by diode D 8 . A junction point of coil L 1  and coil L 2  is connected to input terminal K 2  by means of a series arrangement of coil L 3 , lamp terminal K 3 , lamp La, lamp terminal K 4  and capacitor C 1 . Lamp terminal K 3  is connected to input terminal K 2  by means of capacitor C 2 . Diodes D 5 -D 8 , switching elements S 1  and S 2 , and coils L 1  and L 2  jointly form an inverter for generating a square-wave periodic voltage from the DC voltage supplied by the supply voltage source. Coil L 3 , lamp terminals K 3  and K 4 , lamp LA and capacitors C 1  arid C 2  form, in this example, a load branch. Diodes D 1 , D 2  and D 5 -D 8  form, respectively, a first, a second and a fifth to an eighth unidirectional element. The self-inductances L 1 ′, L 2 ′ and L 3 ′ of coils L 1 , L 2  and L 3  are chosen such that the following applies: 
       L   3 ′&gt;10* L   1 ′ and  L   3 ′&gt;10 *L   2 ′. 
     Next, a description is given of the operation of the example shown in FIG.  1 . When the input terminals K 1  and K 2  are connected to a supply voltage source supplying a DC voltage, then the circuit part SC renders the switching elements S 1  and S 2  alternately conducting and non-conducting. As a result, a substantially square-wave voltage is present across the load branch. Under the influence of this substantially square-wave voltage, an alternating current flows through the load branch, which feeds the lamp and the frequency of which is equal to that of the substantially square-wave voltage. The lamp can be dimmed by regulating the duty cycle of the control signal by means of the dimmer circuit DC. In a part of the range in which the duty cycle can be regulated “hard switching” occurs, i.e. each switching element is rendered conducting while a comparatively high voltage is present across the switching element. However, as the coils L 1  and L 2  are arranged in series with the switching elements, the current through each switching element can increase only to a limited extent when said switching element is becoming conducting, as a result of which the amount of power dissipated in the switching element remains limited. The electric energy stored in the coil L 1  when the switching element S 1  is in the conducting state causes a current to flow from a first end of coil L 1 , which is formed by a junction point of coil L 1  and coil L 2 , via the load branch and diode D 1  to a second end of coil L 1 . In this manner, the electric energy stored in coil L 1  is used, when the switching element S 1  is in the conducting state, to generate a current through the lamp. The electric energy stored in coil L 2  when the switching element S 2  is in the conducting state causes a current to flow from a first end of coil L 2 , which is formed by a junction point of coil L 2  and diode D 2 , via diode D 2  and capacitor Cbuf and the load branch to a second end of coil L 2 . In this manner, the electric energy stored in coil L 2  is partly transferred, when the switching element S 2  is in the conducting state, to the supply voltage source, and is partly used to generate a current through the lamp. In the case of “hard switching”, the diodes are conducting also before the switching elements become conducting. The current through coil L 3  flows in the direction of lamp terminal K 3  during a time interval before the first switching element S 1  becomes conducting. This current flows partly through diode D 1  and coil L 1 , and partly through diode D 8  and coil L 2 . During a time interval before the second switching element S 2  becomes conducting, the current flows through coil L 3  in the direction of the junction point of coil L 1  and coil L 2 . This current flows partly through coil L 1  and diode D 7 , and partly through coil L 2  and diode D 2 . 
     In FIG. 2, components and circuit parts that correspond to components and circuit parts shown in the example of FIG. 1 are indicated by means of the same reference numerals. The only difference between the example shown in FIG.  2  and the example shown in FIG. 1 is that the circuit arrangement of FIG. 2 additionally comprises diodes D 3  and D 4 , which, in the example shown in FIG. 2, form, respectively, a third and a fourth unidirectional element. Diode D 3  connects a junction point of coils L 1  and L 2  to input terminal K 1 . Diode D 4  connects input terminal K 2  to a junction point of coils L 1  and L 2 . 
     The operation of the example shown in FIG. 2 corresponds substantially to the operation of the example shown in FIG.  1 . However, the presence of diodes D 3  and D 4  substantially limits the amplitude of, in particular, the voltage on the junction point of coil L 1  and coil L 2 , which is caused by an oscillation of parasitic capacitances in the circuit arrangement and the coils L 1  and L 2 . As a result, a further reduction of the power dissipation in the circuit arrangement is achieved. 
     In addition, the unidirectional elements D 3  and D 4  form part of current paths for “reverse” currents having a small impedance. If, for example, the current through coil L 3  flows in the direction of the junction point of coils L 1  and L 2  before the switching element S 2  is rendered conducting, then this current flows through diode D 3 , and not, or hardly, through coil L 1  and diode D 7 , and coil L 2  and diode D 2 . When the switching element S 2  becomes conducting, the amount of current that flows in the reverse direction through diode D 3  remains limited by virtue of the presence of coil L 2  between diode D 3  and switching element S 2 . As a result, power dissipation in diode D 3  and switching element S 2  is limited. However, in the absence of diode D 3 , as in the example shown in FIG. 1, the current flows through coil L 3 , before the switching element S 2  becomes conducting, and through coil L 1  and diode D 7 , and through coil L 2  and diode D 2 . When the switching element S 2  becomes conducting, in this case, a comparatively high reverse current flows through diode D 2  causing a comparatively large power dissipation in diode D 2  and switching element S 2 . When the current through coil L 3  flows in the direction of the lamp terminal K 3 , before the switching element S 1  becomes conducting, diode D 4  carries current, while diode D 8  and coil L 2 , or diode D 1  and coil L 1  do not carry current. When the switching element S 1  becomes conducting, the reverse current through diode D 4  is limited by the presence of coil L 1  between switching element S 1  and diode D 4 . As a result, power dissipation in diode D 4  and switching element S 1  is limited. In the absence of diode D 4 , however, the current flows through coil L 3  before the switching element S 1  becomes conducting, and through coil L 1  and diode D 1 , and through coil L 2  and diode D 8 . When the switching element S 1  becomes conducting, in this case, a comparatively large reverse current flows through diode D 1  causing a comparatively large power dissipation in diode D 1  and switching element S 1 . 
     For practical embodiments of the examples shown in FIG.  1  and FIG. 2, and of a circuit arrangement wherein the coils L 1  and L 2 , and the diodes D 1 -D 4  are not provided, the following results were found. In all cases, the power consumed by the lamp was 1 Watt. Coils L 1  and L 2  had a self-inductance of 100 μH, coil L 3  had a self-inductance of 1.1 mH. The buffer capacitance had a capacitance value of 22 nF. Capacitor C 1  had a capacitance of 220 nF and capacitor C 2  had a capacitance of 6.8 nF. Power dissipation was highest in the circuit arrangement wherein coils L 1  and L 2  as well as diodes D 1 -D 4  had not been provided. The power dissipation of the practical embodiment of the example shown in FIG. 1 was 1.3 Watt lower, while the power dissipation of the practical embodiment of the example shown in FIG. 2 was approximately 1 Watt lower than that of the practical embodiment of the example shown in FIG.  1 .