Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Silicon controlled rectifier ignition
Reexamination Certificate
2000-04-05
2001-09-11
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Silicon controlled rectifier ignition
C315S224000, C315S307000
Reexamination Certificate
active
06288500
ABSTRACT:
The invention relates to a circuit arrangement for operating at least one discharge lamp in accordance with the preamble of Patent Claim
1
.
PRIOR ART
A circuit arrangement corresponding to the preamble of Patent Claim
1
is disclosed, for example, in European Laid-Open Specification EP 0 753 987 A1. This circuit arrangement has a half-bridge inverter with a switch-off device which switches off the half-bridge inverter in the case of an anomalous operating state—for example a lamp which is unwilling to start or defective. The switch-off device has a field effect transistor whose drain-source path is arranged in the control circuit of a half-bridge inverter transistor and switches the control circuit between a low-resistance and a high-resistance state. Upon the occurrence of an anomalous operating state, the switching off is performed synchronously with the blocking phase of that half-bridge inverter transistor in whose control circuit the field effect transistor is arranged. The switch-off device of this circuit arrangement certainly switches off the half-bridge inverter reliably in the case of a lamp unwilling to start, but it reacts generally not sensitively enough to the occurrence of the so-called rectifier effect of the discharge lamp, which will be explained in more detail below.
A possible cause of failure of discharge lamps, in particular of low-pressure discharge lamps, is due to an electron emissivity of the lamp electrodes which is reduced over the service life of the lamp. Since the loss in emissivity generally progresses with different intensity over the service life of the lamp in the case of the two lamp electrodes, a discharge lamp operated with alternating current has developed a preferred direction of the discharge current through the discharge lamp by the end of its service life. The discharge lamp in this case develops a current-rectifying effect. This effect is denoted as the rectifier effect of the discharge lamp. The occurrence of the rectifier effect in the discharge lamp causes extreme heating of the lamp electrode which is incapable of emission, with the result that impermissibly high temperatures can occur, which can even cause melting of the lamp bulb glass.
In the case of discharge lamps which are operated on a half-bridge inverter, the rectifier effect of the discharge lamp causes a conspicuous deviation in the voltage drop across the coupling capacitor or across the coupling capacitors from the normal value, which is usually half as large as the value of the input voltage of the half-bridge inverter. In the case of self-oscillating half-bridge inverters, this deviation in the voltage drop across the coupling capacitor or the coupling capacitors has the effect that the oscillation of the half-bridge inverter is stopped, because the supply voltage of one of the two half-bridge arms is in this case too low to maintain the feedback. However, the oscillation of the half-bridge inverter is restarted immediately after it has been interrupted by the starting circuit of the half-bridge inverter if the switch-off device is not triggered. As a result, the discharge lamp affected by the rectifier effect is not reliably switched off, but flickers instead.
SUMMARY OF THE INVENTION
It is the object of the invention to provide an improved circuit arrangement for operating at least one discharge lamp, which does not have the disadvantages of the prior art. In particular, the circuit arrangement is intended to detect the occurrence of the rectifier effect of the at least one discharge lamp and in this case to switch off the half-bridge inverter permanently or at least to ensure a limitation of the voltage and/or the current in the load circuit to safe values.
This object is achieved according to the invention by means of the characterizing features of Patent Claim
1
. Particularly advantageous designs of the invention are described in the subclaims.
The circuit arrangement according to the invention, which has a half-bridge inverter with a drive device and downstream load circuit, at least one coupling capacitor connected to the load circuit and the half-bridge inverter, and terminals with at least one discharge lamp, has a reference voltage source and a detector circuit which compares the voltage drop across the at least one coupling capacitor or the voltage drop, divided downwards by a voltage divider, across the at least one coupling capacitor with the reference voltage of the reference voltage source, and generates an output signal for driving the half-bridge inverter.
As already mentioned further above, the occurrence of the rectifier effect of the at least one discharge lamp causes a conspicuous deviation of the voltage drop across the at least one coupling capacitor from its normal value, which is half as large as the input voltage of the half-bridge inverter. With the aid of the reference voltage source and the detector circuit, the occurrence of the rectifier effect of the at least one discharge lamp is detected by using these means to determine deviations in the voltage drop across the at least one coupling capacitor from its desired value, and generating a corresponding output signal and feeding it to the drive device of the half-bridge inverter, in order either to switch off the half-bridge inverter or, for example by means of a rise in frequency, to regulate the voltage and/or the current in the load circuit down to safe values. For this purpose, the detector circuit of the circuit arrangement according to the invention advantageously has at least two voltage inputs and a voltage output connected to the drive device of the half-bridge inverter, a first voltage input being connected to the reference voltage source, and a second voltage input being connected to the at least one coupling capacitor. In order to be able to switch off the half-bridge inverter upon the occurrence of an anomalous operating state, that is to say in the case of a defective lamp or given the occurrence of some other malfunction, the drive device advantageously has a switch-off device which is fed the output signal of the detector circuit.
The reference voltage source is advantageously designed as a voltage divider which is connected in parallel with the DC voltage input of the half-bridge inverter, and at whose centre tap the reference voltage is provided. Thus, the reference voltage is generated with the aid of simple means from the supply voltage of the half-bridge inverter. The detector circuit advantageously comprises at least two transistors and a voltage divider. The transistors are advantageously pnp bipolar transistors. The voltage divider of the detector circuit advantageously has a first and a second terminal, as well as a first and a second centre tap, the first terminal being connected to the at least one coupling capacitor, and the second terminal being connected to the reference voltage source, and the first centre tap being connected to the emitter of the first pnp transistor and to the base of the second pnp transistor, while the second centre tap is connected to the emitter of the second pnp transistor and to the base of the first pnp transistor. The collector terminals of the pnp transistor are advantageously connected to the voltage output of the detector circuit.
REFERENCES:
patent: 5703439 (1997-12-01), Nerone
patent: 5777861 (1998-07-01), Shimizu et al.
patent: 6060843 (2000-05-01), Primisser et al.
patent: 0681414 (1995-11-01), None
patent: 0753987 (1997-01-01), None
patent: 0886460 (1998-12-01), None
patent: 9743879 (1997-11-01), None
Bessone Carlo S.
Lee Wilson
Patent Truhand-Gesellschaft fuer elektrische Gluehlampen mbH
Wong Don
LandOfFree
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