Electric lamp and discharge devices: systems – Current and/or voltage regulation
Utility Patent
1998-10-16
2001-01-02
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S224000, C315S307000, C315SDIG005, C315SDIG007
Utility Patent
active
06169375
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable.
FIELD OF THE INVENTION
The present invention relates to circuits for energizing one or more loads and more particularly to a circuit that regulates the amount of energy flowing to at least one load.
BACKGROUND OF THE INVENTION
As is known in the art, there are many of types of artificial light sources such as incandescent, fluorescent, and high-intensity discharge (HID) light sources. Fluorescent and HID light sources or lamps are generally driven with a ballast which includes various inductive, capacitive and resistive elements. The ballast circuit provides a predetermined level of current to the lamp which causes the lamp to emit light. To initiate current flow through the lamp, the ballast circuit may provide relatively high voltage levels, e.g., a strike voltage, that differ from operational levels.
One type of ballast circuit is a magnetic or inductive ballast. One problem associated with magnetic ballasts is the relatively low operational frequency which results in a relatively inefficient lighting system. Magnetic ballasts also incur substantial heat losses thereby further reducing the lighting efficiency. Another drawback associated with magnetic ballasts is the relatively large size of the inductive elements.
To overcome the low efficiency associated with magnetic ballasts, various attempts have been made to replace magnetic ballasts with electronic ballasts. One type of electronic ballast includes inductive and capacitive elements coupled to a lamp. The ballast provides voltage and current signals having a frequency corresponding to a resonant frequency of the ballast-lamp circuit. As known to one of ordinary skill in the art, the various resistive, inductive and capacitive circuit elements determine the resonant frequency of the circuit. Such circuits generally have a half bridge or full bridge configuration that includes switching elements for controlling operation of the circuit.
Conventional ballasts generally provide particular voltage and current levels adapted for a single lamp size. Thus, a ballast is only useful for one particular lamp. As known to one skilled in the art, the diameter of the lamp determines the level of current that flows through the lamp. That is, lamps of eight feet, four feet, two feet and one foot all pass about the same amount of current, provided that the lamps have the same diameter. The voltage drop across the lamp, however, varies in accordance with the length of the lamp. The longer the lamp, the greater the voltage drop across the lamp. It would be desirable to provide a ballast that can energize any lamp in a family of lamps where each lamp has the same diameter and a different length.
Another drawback to some known ballast circuits is associated with initiating, or attempting to initiate, current flow through the lamps. One type of ballast initially operates in a so-called rapid start mode to establish current flow through the lamp and thereby cause the lamp to emit light. In rapid start mode, the ballast heats the lamp filaments with a predetermined current flow through the filaments prior to providing a strike voltage to the lamp. Thereafter, the ballast provides operational levels of voltage and current to the lamp as it emits visible light. However, in the case there a lamp does not light, such as a lamp that is only marginally operational, excessive energy levels can be generated by the circuit. High voltages and currents can stress the circuit components and thereby reduce the useful life of the ballast. It would, therefore, be desirable to provide a ballast that detects and eliminates excessive signal levels that can occur when a lamp fails to start. It would also be desirable to provide ballast circuit that, when attempting to light the lamp, applies a strike voltage to the lamp at predetermined intervals to reduce stress on the ballast circuit components.
SUMMARY OF THE INVENTION
The present invention provides a circuit for regulating the amount of energy flowing to one or more loads and detecting excessive energy levels. Although primarily shown and described as a ballast circuit that controls the energy flow to at least one lamp, it is understood that the circuit is applicable to other circuits and loads as well, such as power supplies and electrical motors.
In one embodiment, a ballast circuit includes an inverter circuit for energizing at least one lamp. The inverter circuit includes first and second switching elements coupled to a resonant inductive element. A first control circuit controls the conduction state of the first switching element and a second control circuit controls the conduction state of the second switching element. In one particular embodiment, the inverter circuit is a resonant inverter with the first and second switching elements coupled in half bridge configuration. During resonant operation of the circuit, the first switching element is conductive while current to the load flows in one direction and the second switching element is conductive as the load current flows in the opposite direction.
In an exemplary embodiment, the duty cycle of the second switching element is selectively reduced to achieve desired power levels at the lamp. However, it is understood that the duty cycle of the first switching element can be altered in addition to or instead of the duty cycle of the second switching element.
To control the duty cycle of the second switching element, the second control circuit includes a third switching element coupled to the second switching element and a third control circuit for controlling the conductive state of the third switching element. The third switching element is effective to transition the second switching element to a non-conductive state when the third switching element transitions to a conductive state. In one embodiment, an inductive bias element, which is inductively coupled with the resonant inductive element, is coupled to the second and third switching elements for biasing the switching elements to a conductive state. In particular, when the voltage polarity at the bias element switches to a first polarity corresponding to current flow through the second switching element, the bias element biases the second and third switching elements to a conductive state. However, a delay circuit coupled to the third switching element delays the transition of the third switching element to the conductive state. Thus, the second switching element is conductive until the delay time expires and the third switching element becomes conductive thereby causing the second switching element to transition to the non-conductive state.
In one feature of the invention, excessive energy levels generated by the resonant circuit are detected and eliminated. Excessive voltages can occur when a lamp fails to light and the power to the lamp continues to increase without being consumed by the lamp. In one embodiment, the circuit includes a first threshold circuit coupled to the third switching element for detecting a voltage at the bias element that is greater than a first predetermined threshold. When a voltage at the bias element exceeds the first predetermined threshold, the third switching element is biased to the conductive state which transitions the second switching element to the non-conductive state. When the second switching element is non-conductive, power to the load is reduced.
In one particular embodiment, the first threshold circuit includes a zener diode for providing the first predetermined threshold. In other embodiments, the circuit can include further threshold circuits coupled to further switching elements, such as a fourth switching element described below, for detecting further excess voltage conditions.
Another feature of the invention includes duty cycle modification of the second switching element to adjust the power supplied to the load. In an exemplary embodiment, the third control circuit further includes a fourth switching element coupled to the third
Electro-Mag International, Inc.
Nutter & McClennen & Fish LLP
Tran Thuy Vinh
Wong Don
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