Electric lamp and discharge devices: systems – Current and/or voltage regulation – Automatic regulation
Reexamination Certificate
2000-05-25
2001-09-11
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
Automatic regulation
C315S247000, C315S224000
Reexamination Certificate
active
06288501
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ballast for a discharge lamp, and more particularly, an electronic ballast provided with a voltage converter which provides a DC power from a DC source voltage for operating the discharge lamp.
2. Description of the Prior Art
Japanese Patent No. 10-511220 discloses a ballast for a discharge lamp which includes a voltage converter providing a DC power from a DC source voltage for operating the discharge lamp, and a controller for controlling DC power in a feedback manner based upon an output condition of the voltage converter. The voltage converter includes an inductance and a switching element which is controlled by the controller to turn on and off in order to accumulate an energy in the inductance and allow the energy to release from the inductance for applying the resulting power to operate the discharge lamp. The controller is basically configured to give a varying OFF-period of the switching element for effecting a mode in which the switching transistor is turned on upon seeing a zero-current flowing through the inductance, thereby enhancing switching efficiency. The controller is also configured to give a forced OFF-end limit for the varying OFF-period so as to turn on the switching element forcibly after an elapse of a predetermined period even if the current from the inductance does not reduce to zero, whereby avoiding a condition where the OFF-period becomes too long due to a prolonged time by which the current from the inductance reduces to zero, for prevention of a switching frequency of the switching element becoming too low. Further, the controller is configured to turn off the switching element to terminate the ON-period only after the current being fed to the inductance increases to a predetermined high level. However, this may pose a problem that the switching element should fail to turn off when the current to the inductance does not increase to the predetermined high level, as seen in a case where the DC source voltage gives a high impedance. Further, if the switching element is turned off after a long period, i.e., the ON-period becomes considerably long, the switching frequency would goes too low into an audible range, which is not acceptable for the ballast.
SUMMARY OF THE INVENTION
In view of the above problems, the present invention has been achieved to provide an improved ballast for a discharge lamp. The ballast in accordance with the present invention comprises a voltage converter providing a DC power from a DC source voltage. The voltage converter includes a switching element and an energy storing element having an inductance. The switching element is controlled to operate for repetitively switching the DC source voltage in order to store an energy into the energy storing element. A power converter is included in the ballast to receive the energy and convert it into an operating power for driving the discharge lamp. Also included in the ballast is a controller which provides a command value and turns on and off the switching element for varying time periods in accordance with the command value in order to regulate the output of the voltage converter for generating a necessary power to the discharge lamp. The controller gives a variable OFF-period in which the switching element is turned off and a variable ON-period in which the switching element is turned on. The controller gives a minimum OFF-end limit and a forced maximum OFF-end limit for terminating the OFF-period, and also gives a minimum ON-end limit and a forced maximum ON-end limit for terminating the variable ON-period. The minimum OFF-end limit is defined at a timing when a secondary current flowing from the inductance decreases to zero, while the minimum ON-end limit is defined at a timing when a primary current flowing through the switching element or a primary voltage developed across the switching element reaches a level corresponding to the command value. The controller acts to terminate the OFF-period at the minimum OFF-end limit or at the forced maximum OFF-end limit, whichever comes earlier, and to terminate the ON-period at the minimum ON-end limit or at the forced maximum ON-end limit, whichever comes earlier. By provision of the forced maximum OFF-end limit as well as the forced maximum ON-end limit respectively for the variable OFF-period and ON-period, the switching element can operate at an admissible switching frequency, while flowing a suitable current through the switching element without causing a substantial lowering of switching efficiency, which is therefore a primary object of the present invention.
Preferably, the controller gives the maximum OFF-end limit which varies in accordance with the output condition of the voltage converter so that the OFF-period can terminate at an optimum timing for keeping the suitable switching frequency even in the absence of the secondary current decreasing to zero. For this purpose, the controller monitors an output power of the voltage converter as indicative of the output condition and provides the command value based upon the output power. The controller causes the forced maximum OFF-end limit to vary in accordance with the command value in a direction of shortening the OFF-period as the command value demands to flow an increased current through the switching element. Thus, it is possible to increase the output power without accompanying the lowering the switching frequency. This is particularly suitable for rapidly increasing lamp luminosity at a so-called cold start condition where the lamp voltage is low.
The controller may include a voltage detector which detects the primary voltage across the switching element and compares the primary voltage with a reference so as to judge that the secondary current decreases to zero when the primary voltage decreases to the reference. Alternatively, the controller may include a voltage variation detector which detects a variation in the voltage across the switching element and compares the variation with a reference so as to judge that the secondary current decreases to zero when the variation reaches the reference.
Preferably, the controller may include a comparator which compares an output voltage of the voltage converter with an allowable maximum voltage and issues a limit signal when the output voltage exceeds the allowable maximum voltage. In response to the limit signal, the controller operates to prolong the subsequent OFF-period until the output voltage goes below the allowable maximum voltage, thereby preventing the voltage converter from providing an excess output voltage at the time of starting the lamp.
It is also preferred that the controller gives the forced maximum ON-end limit which varies in accordance with the DC source voltage so that the ON-period terminates earlier as the DC source voltage becomes higher. Whereby, the ON-period can be suitably adjusted in a condition where the DC source voltage varies to a large extent.
Further, the controller may be configured to give the command value having an upper limit which defines a maximum current allowed to flow through the switching element. Thus, it is possible to avoid an excess current flowing through the switching element.
The controller may include a delay circuit which delays an increase of the command value when the voltage converter starts operating, thereby enabling a soft-starting of the ballast for reducing stresses applied to the switching element and other components of the voltage converter.
Further, the controller may include a restrictor which reduces the upper limit of the command value to a low level for a predetermined starting period from the start of operating the ballast and restores the upper limit to a high level thereafter, thereby reducing stresses applied to the switching element and other components of the voltage converter at the start of operating the ballast.
Also, the controller may include an output voltage monitor which monitors an output voltage of the voltage converter so as to reduce the upper limit to
Konishi Hirofumi
Nakamura Toshiaki
Shiomi Tsutomu
Arent Fox Kintner & Plotkin & Kahn, PLLC
Lee Wilson
Matsushita Electric & Works Ltd.
Wong Don
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