Circuit arrangement

In an electronic ballast for operating a discharge lamp, the voltage across the heater windings during preheating the lamp electrodes is used to detect whether a lamp is present.

BACKGROUND OF THE INVENTION
 The invention relates to a circuit arrangement for feeding a discharge
 lamp, comprising
 lamp clamps for holding the discharge lamp,
 a main inverter coupled to the lamp clamps for generating, during
 stationary operation, a current which is fed to the discharge lamp,
 an auxiliary inverter for preheating electrodes of the discharge lamp,
 provided with
 an oscillator for generating an alternating voltage with a frequency f1,
 a transformer provided with a primary winding coupled to the oscillator,
 and with a first and a second secondary winding which each shunt a lamp
 electrode during operation of the lamp,
 a control circuit coupled to the main inverter and the auxiliary inverter
 for controlling the operating state of the circuit arrangement,
 a first circuit part coupled to an input of the control circuit for
 generating a first signal which is a measure of the voltage difference
 between a first end of the first secondary winding and a first end of the
 second secondary winding.
 Such a circuit arrangement is well-known. After putting the known circuit
 arrangement into operation, the control circuit ensures that, if a
 discharge lamp is connected to the lamp clamps, the circuit arrangement is
 successively brought into a number of operating states. In the first
 operating state, the lamp electrodes are preheated by means of the
 auxiliary inverter. Subsequently, in a second operating state, an ignition
 voltage is generated across the discharge lamp by means of the main
 inverter. If the discharge lamp ignites under the influence of this
 ignition voltage, the control circuit brings the circuit arrangement into
 a third operating state wherein the discharge lamp is fed so as to remain
 in the stationary mode of operation. The first signal, which is a measure
 of the voltage difference between a first end of the first secondary
 winding and a first end of the second secondary winding, represents the
 voltage across a discharge lamp connected to the circuit arrangement. The
 first signal is used by the control circuit to preclude that the voltage
 across the discharge lamp becomes too high during ignition, and to
 establish whether the discharge lamp has ignited.
 As mentioned hereinabove, it is first checked whether a discharge lamp is
 present. For this purpose, the known circuit arrangement also comprises
 means for establishing whether a discharge lamp is connected to the lamp
 clamps. These means generally include a circuit part which generates a
 current which flows through one of the lamp electrodes and is subsequently
 detected. The detection, or non-detection, of this current affects the
 form of a lamp-presence signal which is present at an input of the control
 circuit. If said lamp-presence signal indicates that no discharge lamp is
 connected to the circuit arrangement, the control circuit keeps the
 circuit arrangement in a state of rest. A drawback of the known circuit
 arrangement resides in that the control circuit must be provided with an
 input where the lamp-presence signal is present and which input is used
 exclusively to determine whether a discharge lamp is connected to the
 circuit arrangement. Since the control circuit often comprises an IC, the
 total number of inputs and outputs of the control circuit is determined to
 a substantial degree by the number of pins of the IC. In the known circuit
 arrangement, the number of pins of the IC is relatively large in the
 control circuit. As a result, the control circuit is relatively expensive
 and difficult to manufacture.
 SUMMARY OF THE INVENTION
 It is an object of the invention to provide a circuit arrangement for
 feeding a discharge lamp, wherein the means for determining whether a
 discharge lamp is connected to the lamp clamps are relatively simple, and
 the control circuit need only comprise a relatively small number of
 inputs.
 To achieve this, a circuit arrangement of the type mentioned in the opening
 paragraph is characterized in that a second end of the first secondary
 winding and a second end of the second secondary winding are
 interconnected by a first conducting branch and in that, during operation
 of the circuit arrangement, the polarity of the voltage across the first
 secondary winding is equal to the polarity of the voltage across the
 second secondary winding.
 An equal polarity of the voltages across the first and the second secondary
 winding can be readily obtained by suitably choosing the sense of winding
 of the first and the second secondary winding. If the oscillator in a
 circuit arrangement in accordance with the invention generates an
 alternating voltage with a frequency f1, then, consequently, a voltage is
 present across the first and the second secondary winding of the
 transformer. If a lamp is connected to the lamp clamps, the amplitudes of
 both said voltages are very small because substantially all of the
 electric power generated by the oscillator is dissipated in the lamp
 electrodes. As a result, also the voltage between the first end of the
 first secondary winding and the first end of the second secondary winding
 has a very low amplitude. If, however, no discharge lamp is connected to
 the lamp clamps, the amplitude of the voltage across the first secondary
 winding and the amplitude of the voltage across the second secondary
 winding are relatively high. As the voltages exhibit the same polarity,
 also the amplitude of the voltage between the first end of the first
 secondary winding and the first end of the second secondary winding is
 relatively high. Consequently, in a circuit arrangement in accordance with
 the invention, the presence of a lamp can be detected during the first
 operating state by means of the first signal. In a circuit arrangement in
 accordance with the invention, the first signal is used to determine
 whether a discharge lamp is connected to the lamp clamps as well as to
 monitor the voltage across the lamp. As a result, the number of inputs of
 the control circuit can be relatively low.
 To preclude that, during stationary operation of the lamp, a relatively
 large amount of power is dissipated in the lamp electrodes, it is
 desirable that impedance is present in the first conductive branch.
 Satisfactory results have been obtained in examples wherein the impedance
 comprises a first capacitive element.
 Preferably, the main inverter comprises a second conductive branch
 including a series arrangement of a first inductive element and a second
 capacitive element, and the second capacitive element forms part of a
 third conductive branch connecting the first end of the first secondary
 winding and the first end of the second secondary winding to one another.
 Such an embodiment of the main inverter enables the discharge lamp to be
 ignited in a relatively simple manner. However, in practice, the second
 capacitive element constitutes a relatively small impedance relative to
 the first signal generated by the auxiliary inverter. To preclude that
 this relatively small impedance causes a relatively small amplitude of the
 first signal, the value of f1 is chosen to be close to the resonance
 frequency of the first inductive element and the second capacitive
 element. More particularly, satisfactory results have been obtained if f1
 is chosen in the range between 0.8*f0 and 1.2*f0, wherein f0 is the
 resonance frequency of the first inductive element and the second
 capacitive element. If the main inverter comprises a switching element
 which shunts the second conductive branch, then the control circuit is
 preferably provided with a circuit part for maintaining the switching
 element in the conducting state during preheating the electrodes of the
 discharge lamp. The switching element and the second conductive branch
 thus form a circuit of which the first inductive element and the second
 capacitive element form part.
 To preclude that power is dissipated in the lamp electrodes during
 stationary operation, it is desirable that the first conductive branch
 exhibits an impedance which is at least hundred times the impedance of the
 second capacitive element.
 It is noted that, dependent upon the construction of the circuit
 arrangement in accordance with the invention, the main inverter and the
 auxiliary inverter are built up, either entirely or partly, from the same
 components.
 These and other aspects of the invention will be apparent from and
 elucidated with reference to the embodiments described hereinafter.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 In FIG. 1, K3 and K4 are the input terminals which are to be connected to a
 direct voltage source. Input terminal K3 is connected to the terminal K4
 by means of a series arrangement of two switching elements T1 and T2.
 Control electrodes of the switching elements T1 and T2 are connected to
 respective outputs of a control circuit SC1 for rendering the switching
 elements T1 and T2 alternately conducting and non-conducting. The
 switching element T2 is shunted by a series arrangement of a capacitor C3,
 coil L1 and capacitor C2. In this example, this series arrangement forms a
 second conductive branch. Coil L1 forms, in this example, a first
 inductive element. Capacitor C2 forms a second capacitive element, in this
 example, and also a third conductive branch. Capacitor C3 is a DC blocking
 capacitor. Capacitor C2 is shunted by a series arrangement of a secondary
 winding L2a, capacitor C1 and secondary winding L2b. In this example,
 capacitor C1 forms first capacitive means. The secondary winding L2a is
 coupled to the lamp clamp K1, and the secondary winding L2b is coupled to
 the lamp clamp K2. A discharge lamp TL1 is connected to the lamp clamps K1
 and K2 in such a manner that a first lamp electrode El1 is shunted by the
 first secondary winding L2a, and a second lamp electrode E12 is shunted by
 the second secondary winding L2b. Switching elements T1 and T2, control
 circuit SC1, capacitors C3 and C2 and coil L1 jointly form a main inverter
 for generating a current with which the lamp TL1 is fed. Input terminals
 K3 and K4 are also interconnected by means of a series arrangement of
 switching elements T3 and T4. Control electrodes of switching element T3
 and switching element T4 are connected to respective outputs of a control
 circuit SC2 for rendering switching elements T3 and T4 alternately
 conducting and non-conducting. Switching element T4 is shunted by a series
 arrangement of capacitor C4 and primary winding L2. Primary winding L2 is
 magnetically coupled to secondary windings L2a and L2b. Switching elements
 T3 and T4, control circuit: SC2 and capacitor C4 jointly form an
 oscillator for generating an alternating voltage of frequency f1. Primary
 winding L2 and secondary windings L2a and L2b jointly form a transformer.
 The oscillator and the transformer jointly form an auxiliary inverter for
 preheating electrodes of the lamp TL1. CC is a control circuit for
 controlling the operating state of the circuit arrangement. A first output
 of control circuit CC is connected to an input of control circuit SC1. A
 second output of control circuit CC is connected to an input of control
 circuit SC2. A common point of capacitor C2 and coil L1 forms, in this
 example, a first circuit part and is connected to an input of control
 circuit CC.
 The operation of the example shown in FIG. 1 is as follows.
 Immediately after input terminals K3 and K4 have been connected to the
 poles of a direct voltage source, the control circuit activates a first
 operating state wherein the control circuit SC2 renders the switching
 elements T3 and T4 alternately conducting and non-conducting with a
 frequency f1. In addition, during this first operating state, the control
 circuit CC renders the switching element T2 conducting and the switching
 element T1 non-conducting via the control circuit SC1. An alternating
 voltage with a frequency f1 is present across the primary winding L2. As a
 result, voltages with a frequency f1 are also present across secondary
 windings L2a and L2b. Since the secondary windings are interconnected by
 means of capacitor C1, a voltage is present across capacitor C2 the
 amplitude of which is equal to the sum of the voltages across both
 secondary windings L2a and L2b and the voltage across capacitor C1. This
 voltage across capacitor C2 forms, in this example, a first signal. If the
 discharge lamp TL1 is present, almost all the electric power generated by
 the auxiliary inverter is dissipated in the lamp electrodes E11 and E12.
 As a result, the amplitudes of the voltages across the secondary windings
 are relatively low. For this reason, the amplitude of the first signal
 present at the input of the control circuit CC is also low, and the
 control circuit maintains the circuit arrangement in the first operating
 state. If, however, no discharge lamp is connected to the circuit
 arrangement, the amplitudes of the voltages across the secondary windings
 are relatively high. Since, as a result of a suitably chosen sense of
 winding of both the first and the second secondary winding, the polarity
 of the voltage across the first secondary winding is equal to the polarity
 of the voltage across the second secondary winding, also the amplitude of
 the first signal is relatively high. This can be contributed to the fact
 that in the absence of the discharge lamp, no power is dissipated in the
 lamp electrodes. This is partly caused by the fact that the frequency f1
 is chosen to be close to the resonance frequency of coil L1 and capacitor
 C2. If the first signal present at the input of the control circuit CC is
 high, the control circuit CC brings the circuit arrangement into a state
 of rest, wherein the control circuits SCI and SC2 maintain all switching
 elements in the non-conducting state. During ignition of the lamp, the
 voltage across capacitor C2 is equal to the ignition voltage, and during
 stationary operation of the lamp, the voltage across capacitor C2 is equal
 to the working voltage of the discharge lamp. For this reason, the first
 signal in a circuit arrangement in accordance with the invention can be
 used in different operating states of the circuit arrangement to monitor
 the operating state, and the control circuit CC requires relatively few
 inputs. This means that, if the control circuit CC comprises an IC, the
 number of pins of this IC can be relatively small.