High intensity discharge lamp ballast

Details High intensity discharge lamp ballast High intensity discharge lamp ballast

1999-06-11

2001-02-13

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Current and/or voltage regulation

Automatic regulation

C315S224000, C315S291000, C315SDIG002, C315SDIG007

Reexamination Certificate

active

06188183

ABSTRACT:

This application claims foreign priority under 35 U.S.C. §119(a)-(d) or 356(b) of Great Britain Applications Nos. 9812703.8 (pending) and 9904913.2 (pending), filed Jun. 13, 1998 and Mar. 3, 1999, respectively.
DESCRIPTION
1. Technical Field
This invention relates to a power control circuit which is particularly, though not exclusively, suited to the ballasting of low and high pressure sodium, mercury arc and metal halide discharge lamps (high intensity discharge lamps or HID lamps). Typically such systems can be used for highway lighting, architectural floodlighting, warehouse and industrial lighting etc.
2. Background of the Invention
Traditionally, ballasting for HID lamps is by use of inductors or chokes capable of controlling the lamp current through the impedance they present in series with the mains supply voltage. With some types of HID lamp a high striking voltage, typically 4-5 kV, is required to ionize the gas filling the tube and initiate the arc.
In prior art systems for ballasting HID lamps, the lamp ballasting means and the lamp striking means are typically discrete circuit elements. Historically, HID lamps have been ballasted by using the impedance of a series connected inductor for controlling the lamp current and a separate starter or igniter module to provide the necessary high voltage to strike the lamp.
FIG. 1
illustrates a typical arrangement for prior art electronic ballasts for HID lamps. A conventional power factor controller is formed by transistor TR
1
, inductor L
1
, diode D
1
and capacitor C
1
. Alternating positive and negative output voltage is provided to the lamp by a full bridge arrangement comprising four transistors TR
3
, TR
4
, TR
5
, TR
6
. The transistors are alternately switched on and off in complementary pairs TR
3
, TR
6
and TR
4
, TR
5
at a low frequency, typically 100-200 Hz.
Connected in series with the lamp across the bridge is an igniter circuit comprising pulse transformer TX
1
, a Sidac, capacitor C
3
and resistor R
1
. When the igniter circuit operates, the capacitor C
3
charges through resistor R
1
to a voltage at which the Sidac device switches on, discharging the capacitor C
3
into the primary winding of the transformer TX
1
. The voltage applied to the transformer primary is multiplied by the high turns ratio of the transformer and is sufficient to ionize the gas filling the lamps arc tube, thereby initiating an arc.
Since the voltage is AC, the arc will be extinguished when the lamp current approaches zero and the voltage applied to the tube is subsequently reversed. Therefore the igniter must operate again in the opposite voltage half cycle to re-strike the arc for the flow of current in the opposite direction. This ignition cycle is repeated until the lamp electrodes are sufficiently heated by the arc current for thermionic emission to take place. Then the arc voltage in the tube falls below the threshold voltage of the Sidac and arc current is maintained without operation of the igniter circuit.
A further transistor TR
2
controls the flow of current in the output bridge circuit and consequently controls the lamp current. Transistor TR
2
is turned on until the current in inductor L
2
reaches a preset threshold value, then the transistor TR
2
is turned off. Current continues to flow via a diode D
2
until the current has decayed to another preset threshold value, then the transistor TR
2
is turned on again.
Because of the high rate of rise of voltage, the capacitance and inductance of the wiring to the lamp act to attenuate the high voltage ignition pulse to the lamp, so limiting the practical length of the wiring between the igniter circuit and the lamp.
In recent years it has become known to use high frequency (>20 kHz) electronic ballasts to supply lamp current for fluorescent lamp installations, giving longer tube life due to lower tube current crest factor and higher overall efficiencies due to reduced power losses in the ballast and tube. Attempts have been made to design high frequency electronic ballasts for HID lamps with some level of success but these are fraught with problems due mainly to the predisposition of many HID lamps to acoustic arc resonance when operated at frequencies substantially above line frequency. It is in some cases possible to design high frequency ballasts specifically for one type and size of HID tube if the operating frequency of the ballast is chosen carefully but if the gas pressure in the tube changes substantially during the life of the tube the resonance characteristics will also change and could cause catastrophic failure of the arc tube. This will be at best alarming and could be dangerous if the arc tube fragments are not sufficiently contained within the lamp fitting (luminaire).
SUMMARY OF THE INVENTION
The invention provides a high intensity discharge lamp ballast circuit comprising: a high intensity discharge lamp connected between a first lamp terminal and a second lamp terminal; a resonant circuit, to which the first lamp terminal is connected; first switching means operable to connect the resonant circuit to a positive rail of a source of high voltage; second switching means operable to connect the resonant circuit to a negative rail of the source of high voltage; control means for alternately operating the first and second switching means to supply current to the resonant circuit, the alternation occurring in a first mode at a first switching frequency that causes the resonant circuit to resonate and in a second mode at a second switching frequency that does not cause the resonant circuit to resonate; and current limiting means for limiting the electrical current through the lamp.
By operating in two modes, respectively at high and low frequency, the invention overcomes many of the problems associated with prior art HID lamp ballasts. The first, high frequency mode is operated at typically greater than 20 kHz while the lamp is cold and uses resonance to cause the lamp to strike. Once the lamp has warmed up sufficiently for thermionic emission from the lamp electrodes, the second mode of operation may be employed at typically less than 1 kHz and the lamp arc will be maintained. Because resonance in the first mode of operation is used to provide sufficient voltage to strike the lamp, there is no requirement for a separate igniter circuit and consequently the expense of components such as the Sidac can be avoided.
Preferably, a first return capacitor is connected between the second lamp terminal and the positive high voltage rail and a second return capacitor is connected between the second lamp terminal and the negative high voltage rail. Because the lamp voltage is boosted by resonance in the first mode of operation and because a comparatively low lamp voltage is required to maintain the arc in the second mode of operation, it is possible to run the lamp according to the invention using only half the voltage from the split high voltage supply. Thus a full transistor bridge circuit is not necessary and return capacitors may replace two of the transistors, with consequent cost savings.
In a preferred embodiment of the invention, the current limiting means comprises sensing means for measuring the current through the lamp; means for representing the measured current as a voltage signal; means for comparing the voltage signal with a reference voltage; and means for disabling the operation of the first and second switching means by the control means if the compared voltage signal is greater than the reference voltage. By effecting current limiting through disabling operation of the first and second switching means, the need for a separate current control transistor (such as transistor TR
2
in
FIG. 1
) is avoided.
Thus, the present invention makes possible control of the necessary lamp striking voltage, the warm-up current and the steady state running current using only two power switching transistors operating in two discrete modes, which represents a significant saving of components compared with the prior art.
A further preferred feature of the present i

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