Ballast for a discharge lamp

• US 6,288,501 B1
• Filed: 05/25/2000
• Issued: 09/11/2001
• Est. Priority Date: 05/26/1999
• Status: Expired due to Term

• Current Assignee: Panasonic Electric Works Company Limited (Panasonic Holdings Corporation)
• Original Assignee: Asahi Matsushita Electric Works Ltd. (Panasonic Holdings Corporation)
• Inventors: Nakamura, Toshiaki, Konishi, Hirofumi, Shiomi, Tsutomu
• Primary Examiner(s): Wong, Don
• Assistant Examiner(s): Lee, Wilson
• Application Number:
  US09/577,490
• Time in Patent Office: 474 Days
• Field of Search: 315/307, 315/224, 315/247, 315/244, 315/291, 315/225, 315/308
• US Class Current: 315/307
• CPC Class Codes: H05B 41/2886 comprising a controllable preconditioner, e.g. a booster
• CPC Class Codes: H05B 41/392 using semiconductor devices, e.g. thyristor
• CPC Class Codes: Y02B 20/00 Energy efficient lighting technologies, e.g. halogen lamps or gas discharge lamps

Abstract

A ballast includes a voltage converter providing a DC power from a DC source voltage. The 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 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. A controller provides a command value and turns on and off the switching element for variable periods, respectively in accordance with the command value for generating a necessary power to the lamp. The controller gives a variable OFF-period and a variable ON-period to the switching element. 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 reaches the command value. The controller terminates the OFF-period at the minimum OFF-end limit or at the forced maximum OFF-end limit, whichever comes earlier, and terminates the ON-period at the minimum ON-end limit or at said forced maximum ON-end limit, whichever comes earlier.

Claims

1. A ballast for a discharge lamp comprising:

a voltage converter providing a DC power from a DC source voltage, said voltage converter including a switching element and an energy storing element having an inductance, said switching element operating to repetitively switch the DC source voltage in order to store energy into said energy storing element;

a power converter receiving said energy and converting it into an operating power for driving said discharge lamp;

a controller which provides a command value and turns on and off said switching element for varying time periods in accordance with the command value in order to regulate the output of said voltage converter for generating a necessary power to said discharge lamp;

said controller giving a variable OFF-period in which said switching element is turned off and a variable ON-period in which said switching element is turned on,

said controller providing a minimum OFF-end limit and a forced maximum OFF-end limit for terminating said variable OFF-period,

said controller providing a minimum ON-end limit and a forced maximum ON-end limit for terminating said variable ON-period,

said minimum OFF-end limit being defined at a timing when a secondary current flowing from said inductance decreases to zero,

said minimum ON-end limit being defined at a timing when a primary current flowing through said switching element or a primary voltage developed across said switching element reaches a level corresponding to said command value,

said controller terminating the OFF-period at said minimum OFF-end limit or at said forced maximum OFF-end limit, whichever comes earlier

said controller terminating the ON-period at said minimum ON-end limit or at said forced maximum ON-end limit, whichever comes earlier.

2. The ballast as set forth in claim 1, wherein said controller monitors an output condition of said voltage converter and provides said command value which varies with the output condition.

3. The ballast as set forth in claim 1, wherein said forced maximum OFF-end limit varies in accordance with the output condition of said voltage converter.

4. The ballast as set forth in claim 3, wherein said controller monitors an output power of said voltage converter as indicative of said output condition, and provides said command value based upon said output power, and

said controller causing said forced maximum OFF-end limit to vary in accordance with said command value in a direction of shortening the OFF-period as the command value demands to flow an increased current through the switching element.

5. The ballast as set forth in claim 1, wherein said controller includes a voltage detector which detects said primary voltage across the switching element and compares said primary voltage with a reference so as to judge that the secondary current decreases to zero when said primary voltage decreases to said reference.

6. The ballast as set forth in claim 1, wherein said controller includes a voltage variation detector which detects a variation in the voltage across the switching element and compares said variation with a reference so as to judge that the secondary current decreases to zero when said variation reaches said reference.

7. The ballast as set forth in claim 1, wherein said controller includes a comparator which compares an output voltage of said voltage converter with an allowable maximum voltage and issues a limit signal when the output voltage exceeds said allowable maximum voltage, said controller, in response to said limit signal, prolonging the subsequent OFF-period until said output voltage goes below said allowable maximum voltage.

8. The ballast as set forth in claim 1, wherein said forced maximum ON-end limit varies in accordance with said DC source voltage so as to be earlier as said DC source voltage becomes greater.

9. The ballast as set forth in claim 1, wherein said command value has an upper limit which defines a maximum current allowed to flow through said switching element.

10. The ballast as set forth in claim 9, wherein said controller includes a delay circuit which delays an increase of said command value when said voltage converter starts operating.

11. The ballast as set forth in claim 9, wherein said controller includes a restrictor circuit which reduces said upper limit to a low level for a predetermined starting period from the start of operating the ballast and restores said upper limit to a high level after the elapse of said starting period.

12. The ballast as set forth in claim 9, wherein said controller includes an output voltage monitor which monitors an output voltage of said voltage converter so as to reduce said upper limit to a lower level as said output voltage becomes greater.

13. The ballast as set forth in claim 1, wherein said controller includes an output voltage monitor which monitors an output voltage of said voltage converter and provides a signal when said output voltage exceeds a predetermined voltage which is greater than an operating voltage for operating the lamp, said controller, in response to said signal, varying said minimum ON-end limit of said ON-period in a direction of elongating the ON-period than in the absence of said promote signal.