Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
1999-12-27
2001-01-09
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C323S282000, C315S2090SC
Reexamination Certificate
active
06172890
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an operating circuit for a load, in particular a low-pressure discharge lamp.
In this case, the invention is based on an oscillator circuit for high-frequency operation of the load, in which case the circuit is supplied with a voltage with a dominant DC element. Specifically, a free-running oscillator circuit is considered, in which a bipolar transistor bridge is driven by means of a switching control device such that the process of switching the bipolar transistors on and off allows the potential of a bridge tap to oscillate at the operating frequency. To do this, with free-running oscillator circuits, the load current is fed back through the switching control device in order to switch and control the bipolar transistors.
A major problem in the design and—with regard to the tolerances—in the production of such a circuit is to consider three functional criteria simultaneously, namely radio suppression, relief of the transistor switching load and minimizing switching losses, and, finally, the power which is passed to the load. The relationship between these three criteria in conventional circuits is highly complex. Taking account of all three aspects at the same time is feasible either only on the basis of a very complex circuit design and with correspondingly tight tolerances with regard to the various electrical variables and operating parameters, or frequently cannot be achieved at all in a satisfactory manner.
SUMMARY OF THE INVENTION
The invention is based on the technical problem of developing the described circuit further such that the design of the circuit with regard to the said criteria is simplified, and the tolerances with regard to the electrical variables and operating parameters are widened.
This problem is solved according to the invention by means of a free-running oscillator circuit for operating a load having a bipolar transistor bridge and having a switching control device for transmitting the load current, with feedback, to the bases of the bipolar transistors wherein the base connecting lines of the bipolar transistors are in each case connected to one another, between the switching control device and the bipolar transistor, by means of at least one “base bridging capacitor”.
The solution adopted with the circuit according to the invention is aimed, in particular, at the aspect of the collector initial current peak. This is an undesirable collector-emitter current which flows for a short time but with a relatively high amplitude when the base of a switching transistor under consideration is driven too early, that is to say the transistor is switched on too early. Too early in this case means that the collector-emitter voltage in the change in potential during the last oscillator half-cycle has not disappeared completely and thus still produces a short current surge if the transistor is switched on too early. If the transistor is switched on at the right time, that is to say starting only once the collector-emitter voltage has disappeared, no positive collector-emitter current flows whatsoever, and only a very much smaller base-collector current.
This collector initial current peak can occur in particular with base drives whose losses have been minimized, and in which the nominal switching-on time occurs very shortly after the collector-emitter voltage has disappeared and, correspondingly, small fluctuations in the electrical variables or operating parameters are a sufficient cause on their own.
The so-called “base bridging capacitor” which is added according to the invention between the base connecting lines of the bipolar transistors now results in the current, which possibly starts too early, for driving the base and switching on the transistor being “sucked away” so to speak. This is done by means of the base bridging capacitor which has not yet been entirely discharged or charged if the switching-on current starts too early.
To illustrate this, it can be stated that the connection points of the base bridging capacitor to the base connecting lines in each case occur essentially at a potential of the supply voltage or at the potential of the center tap of the bipolar transistor bridge owing to the fact that the impedances of the switching control device and of any further base drive circuits are generally low in comparison to the change in voltage of the oscillator circuit. In consequence, the time when the potential at the center tap and the respective potential of the supply voltage are identical can in practice be equated to the time when the base bridging capacitor is completely discharged or charged.
Even if the switching control device supplies a switching-on current which is possibly somewhat too early, this thus ensures that the transistor under consideration is switched without any voltage (with respect to the collector-emitter voltage), and thus with optimized losses.
Avoiding the current surge pulse of the collector initial current peak in general also results in improved electromagnetic compatibility.
The described method of operation means that the requirements for the switching mode criterion are considerably less stringent for the design of the overall circuit and ensure simplified and improved capabilities with regard to the two other said criterion of radio suppression and design for power in the load. In other words, the described protective mechanism of the base bridging capacitor according to the invention allows, in particular, the power in the load to be “decoupled”, so to speak, from the two other criteria by appropriate dimensioning of the relevant components.
In one preferred embodiment, the bipolar transistor bridge is a half-bridge comprising two bipolar transistors. The switching control device is preferably a control transformer, whose secondary winding currents drive the transistor bases.
The connection points of the base bridging capacitor can be arranged directly at the respective transistor base or directly at the output on the base side of the switching control device, for example on a transformer secondary winding. The second solution is preferable if a relatively high-impedance drive is connected between the switching control device and the transistor base since, otherwise, an excessively high alternating-current short-circuiting effect could occur immediately between the base connections, at the operating switching frequency, through the base bridging capacitor. Two capacitors are also conceivable, which are each arranged in one of the two circuits. In general, the invention is not limited to a single base bridging capacitor, particularly in the case of a full-bridge circuit.
It has been found that the (total) capacitance of the base bridging capacitor or capacitors is best matched to the (total) capacitance of the conventional trapezoidal capacitor or capacitors in the overall circuit. In this case, the term trapezoidal capacitor refers to a capacitor which is (effectively) connected in parallel with a collector-emitter path for a switching transistor in order to reduce the gradient—on the potential/time graph—of the sudden potential changes at the bridge tap resulting from the corresponding charge-reversal processes. A preferred range for the capacitance of the base bridging capacitor is in this case 10% to 100% of the trapezoidal capacitor capacitance and, in particular, 10% to 50% of this capacitance.
A preferred range for the sum of the said (total) trapezoidal capacitor capacitance and the (total) capacitance of the base bridging capacitor or capacitors is between 680 pF and 2.2 nF. This applies to a wide range of load power levels between several watts and few kilowatts.
REFERENCES:
patent: 4279011 (1981-07-01), Nilssen
patent: 4959591 (1990-09-01), Hirschmann
patent: 5309350 (1994-05-01), Kijima
patent: 5402087 (1995-03-01), Gorczak
patent: 2553266 (1977-06-01), None
patent: 0682464 (1995-11-01), None
patent: 2016846 (1979-09-01), None
patent: 2086164 (1982-05-01), None
Bessone Carlo S.
Nguyen Matthew
Patent-Treuhand-Gesellschaft Fuer Elektrische Gluehlampen MBH
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