Electric lamp and discharge devices: systems – Current and/or voltage regulation – Automatic regulation
Reexamination Certificate
2000-03-30
2001-08-21
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
Automatic regulation
C315S224000, C315S308000, C363S041000, C363S098000
Reexamination Certificate
active
06278245
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of The Invention
The subject invention relates to electronic ballasts for driving a high intensity discharge lamp, and more particularly, to controlling the output power in such electronic ballasts.
2. Description of The Related Art
It is typical to drive a high intensity discharge (HID) lamp with a low frequency square current waveform. An electronic ballast is used to generate the required drive waveform for the lamp and to provide power factor correction for the utility line. The present state of the art for electronic ballasts is the three-stage topology shown in FIG.
1
. This topology consists of a boost converter for power factor correction, a buck converter for regulating lamp power/current, and a commutator for converting the regulated DC voltage into an AC square waveform. In this topology, two controllers are required. One controller is used in the boost converter to regulate the bus voltage, Vbus and to shape the line voltage waveform to follow the input voltage sinusoidal waveform for power factor correction. The other controller is used in the buck converter for regulating the lamp current and power. This topology is well known to those skilled in the art of power electronics. It is simple and straightforward to sense its control variables, because the line voltage is rectified and the sensed parameters can be referenced to the same potential throughout all phases of circuit operation. For example, a simple current sense resistor, R
1
placed in series between ground and switch Q
2
, can be used to monitor the DC lamp current. However, the primary drawbacks for this topology are high parts count and low efficiency due to its multi-stage cascading nature. As shown in
FIG. 1
, the known three-stage converter ballast includes two controller switches, one for the boost converter and one for the buck converter, as well as four switches Q
1
-Q
4
for controlling the commutation of the high intensity discharge lamp, for a total of six switches.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a combined buck-boost function type electronic ballast in which the parts count is significantly reduced
It is a further object of the invention to provide a combined buck-boost function type electronic ballast in which the number of switches is reduced.
It is a still further object of the invention to provide a combined buck-boost function type electronic ballast which exhibits a high efficiency.
These and other objects are achieved in an electronic ballast for high intensity discharge lamps having combined boost and buck converters and a simple current sensing technique, wherein said electronic ballast comprises an AC voltage source having a first terminal and a second terminal; a series arrangement of a first inductance, a second inductance and a high intensity discharge lamp connected to said first terminal of said AC voltage source; a first capacitor arranged in parallel with said high intensity discharge lamp; a first series arrangement of a first high frequency switch having a body diode, and a first low frequency switch arranged in parallel with said second inductance and said high intensity discharge lamp; a second series arrangement of a second high frequency switch also having a body diode, and a. second low frequency switch arranged in parallel with said second inductance and said high intensity discharge lamp, a junction between said second high frequency switch and said second low frequency switch being connected to ground; a series arrangement of two diodes, a cathode of one of said two diodes being connected to a junction of said first high frequency switch and said first low frequency switch, and an anode of the other of said two diodes being connected to the junction of said second high frequency switch and said second low frequency switch, a junction between said two diodes being connected to a second terminal of said AC voltage source; and a series arrangement of a capacitor and a current sensing resistor arranged in parallel with said series arrangement of two diodes, wherein a current through said sensing resistor is used to detect an input buck current of said electronic ballast for indirectly controlling an output power generated by said electronic ballast.
The electronic ballast of the subject invention combines the buck and boost functions and combines the line voltage rectification as well as the load commutation. This circuit arrangement is suitable for driving HID lamps with power requirements less than 200 watts. When the boost converter is running in discontinuous conduction mode (DCM), the AC current waveform will naturally follow the AC voltage waveform without any feedback control. Based on this DCM boost property, the circuit can be rearranged into a form where both the boost converter and the buck converter can share the same active switch. Such an arrangement not only offers reduction in parts count over the conventional approach, but also allows for simplification in control. Only one controller is needed in the electronic ballast of the subject invention. The controller regulates the lamp power by modulating the duty cycle of the high frequency switches. However, if the conventional control philosophy is used in this circuit where lamp power is regulated directly by sensing and controlling the lamp current, then the implementation of sensing the lamp current can be complicated and expensive.
The concept behind the present invention is to control the lamp power indirectly by regulating the input power to the buck converter instead of the conventional lamp current. The buck input current can be easily sensed via a current sense resistor that is strategically placed in series with the bus capacitor. This method works because of two reasons:
(1) The voltage polarity across the bus capacitor is fixed throughout the whole circuit operation; and
(2) There is a unique current flowing pattern through the bus capacitor.
In general, it is not easy to separate the boost and the buck currents flowing in and out of the bus capacitor as in the three-stage topology of the conventional buck-boost converter power supply. However, when the buck and boost functions are combined, the boost current will only flow into the capacitor during the off time of the active switch (the second high frequency switch during the positive half line cycle, and the first high frequency switch during the negative half line cycle), and the buck current will only flow out of the capacitor during the on time of the active switch. Hence, the input current to the buck converter can be easily filtered out by simple circuitry. With this control method, the electronic ballast of the subject invention not only offers a reduction of parts count and an improvement in efficiency in its power stage, but also has a simple resistive current sensing method comparable to the three-stage approach, with the reduction of one controller.
REFERENCES:
patent: 4626979 (1986-12-01), JaQuay
patent: 5039921 (1991-08-01), Kakitani
Li Faye
Shen Eric B.
Goodman Edward W.
Philips Electronics North America Corporation
Vu David
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