Driving circuit for piezoelectric transformer, cold-cathode tube light-emitting apparatus, liquid crystal panel and device with built-in liquid crystal panel

• US 20030102776A1
• Filed: 11/27/2002
• Published: 06/05/2003
• Est. Priority Date: 11/29/2001
• Status: Active Grant

• Current Assignee: Matsushita Electric Industrial Company Limited (Panasonic Holdings Corporation)
• Original Assignee: Matsushita Electric Industrial Company Limited (Panasonic Holdings Corporation)
• Inventors: Nakatsuka, Hiroshi, Takeda, Katsu, Moritoki, Katsunori
• Granted Patent: US 7,038,354 B2
• Time in Patent Office: Days
• US Class Current: 310/318
• CPC Class Codes: H05B 41/2822 using specially adapted components in the load circuit, e.g. feed-back transformers, piezoelectric transformers; using specially adapted load circuit configurations
• CPC Class Codes: H05B 41/2855 against abnormal lamp operating conditions
• CPC Class Codes: H05B 41/392 using semiconductor devices, e.g. thyristor
• CPC Class Codes: H10N 30/804 for piezoelectric transformers

Abstract

A driving circuit for a piezoelectric transformer is provided, which ensures lighting of all the cold-cathode tubes connected to the piezoelectric transformer, and reduces the difference in brightness between the cathode tubes during steady lighting, thereby enhancing reliability and performance. A plurality of cold-cathode tubes connected to a secondary side of the piezoelectric transformer, and a cold-cathode tube output detector circuit connected in series to a plurality of cold-cathode tubes, for detecting an output state of the respective cold-cathode tubes are provided, and the driving of the piezoelectric transformer is controlled based on a detection signal from the cold-cathode tube output detector circuit. Because of this, the piezoelectric transformer performs the same operation as that with respect to one cold-cathode tube.

Claims

1. A driving circuit for a piezoelectric transformer including a primary side electrode and a secondary side electrode formed on a piezoelectric body and converting an AC voltage input from the primary side electrode to output the converted AC voltage from the secondary side electrode, the driving circuit comprising:

a plurality of loads connected to a secondary side of the piezoelectric transformer; and

a load state detector portion connected in series to the plurality of loads, for detecting an output state of each load,

wherein driving of the piezoelectric transformer is controlled based on a detection signal from the load state detector portion.

2. The driving circuit for a piezoelectric transformer according to claim 1, wherein the piezoelectric body has a rectangular shape and the secondary side electrode of the piezoelectric transformer is provided on each end face of the piezoelectric body in a longitudinal direction, and polarization directions of the piezoelectric body in a vicinity of the secondary side electrodes are the same in the longitudinal direction.

3. The driving circuit for a piezoelectric transformer according to claim 1, wherein the load is a cold-cathode tube.

4. The driving circuit for a piezoelectric transformer according to claim 3, wherein the number of the cold-cathode tubes is 2n (n is an integer of 1 or more).

5. A driving circuit for a piezoelectric transformer including a primary side electrode and a secondary side electrode formed on a piezoelectric body and converting an AC voltage input from the primary side electrode to output the converted AC voltage from the secondary side electrode, the driving circuit comprising:

a plurality of loads connected to a secondary side of the piezoelectric transformer; and

a load state detector portion connected in series to the plurality of loads, for electrically separating and detecting an output state of each load,

wherein driving of the piezoelectric transformer is controlled based on a detection signal from the load state detector portion.

6. The driving circuit for a piezoelectric transformer according to claim 5, wherein the piezoelectric body has a rectangular shape and the secondary side electrode of the piezoelectric transformer is provided on each end face of the piezoelectric body in a longitudinal direction, and polarization directions of the piezoelectric body in a vicinity of the secondary side electrodes are the same in the longitudinal direction.

7. The driving circuit for a piezoelectric transformer according to claim 5, wherein the load is a cold-cathode tube.

8. The driving circuit for a piezoelectric transformer according to claim 7, wherein the number of the cold-cathode tubes is 2n (n is an integer of 1 or more).

9. The driving circuit for a piezoelectric transformer according to claim 5, wherein the load state detector portion includes an optical isolator composed of a light-emitting diode and a phototransistor.

10. A driving circuit for a piezoelectric transformer including a primary side electrode and a secondary side electrode formed on a piezoelectric body and converting an AC voltage input from the primary side electrode to output the converted AC voltage from the secondary side electrode, the driving circuit comprising:

a plurality of loads connected to a secondary side of the piezoelectric transformer;

a load state detector portion connected in series to the plurality of loads, for detecting an output state of each load; and

a frequency selection portion for selectively outputting only a frequency component in a vicinity of a driving frequency of the piezoelectric transformer in a detection signal from the load state detector portion,

wherein driving of the piezoelectric transformer is controlled based on a signal from the frequency selection portion.

11. The driving circuit for a piezoelectric transformer according to claim 10, wherein the piezoelectric body has a rectangular shape and the secondary side electrode of the piezoelectric transformer is provided on each end face of the piezoelectric body in a longitudinal direction, and polarization directions of the piezoelectric body in a vicinity of the secondary side electrodes are the same in the longitudinal direction.

12. The driving circuit for a piezoelectric transformer according to claim 10, wherein the number of the cold-cathode tubes is 2n (n is an integer of 1 or more).

13. A cold-cathode tube light-emitting apparatus, comprising:

a piezoelectric transformer including a primary side electrode and a secondary side electrode formed on a piezoelectric body and converting an AC voltage input from the primary side electrode to output the converted AC voltage from the secondary side electrode;

a plurality of cold-cathode tubes connected to a secondary side of the piezoelectric transformer;

a cold-cathode tube output detector portion connected in series to the plurality of cold-cathode tubes, for electrically separating and detecting a current flowing through the respective cold-cathode tubes; and

a control portion for controlling light-emission of the plurality of cold-cathode tubes based on a detection signal from the cold-cathode tube output detector portion.

14. The cold-cathode tube light-emitting apparatus according to claim 13, wherein the piezoelectric body has a rectangular shape and the secondary side electrode of the piezoelectric transformer is provided on each end face of the piezoelectric body in a longitudinal direction, and polarization directions of the piezoelectric body in a vicinity of the secondary side electrodes are the same in the longitudinal direction.

15. The cold-cathode tube light-emitting apparatus according to claim 13, wherein the number of the cold-cathode tubes is 2n (n is an integer of 1 or more).

16. The cold-cathode tube light-emitting apparatus according to claim 13, wherein the cold-cathode tube output detector portion includes an optical isolator composed of a light-emitting diode and a phototransistor.

17. The cold-cathode tube light-emitting apparatus according to claim 13, wherein the piezoelectric transformer and the control portion are mounted on a first substrate placed in proximity to one electrode of each of the plurality of cold-cathode tubes, and the cold-cathode tube output detector portion is mounted on a second substrate placed in proximity to the other electrode of each of the plurality of cold-cathode tubes.

18. A liquid crystal panel whose brightness is controlled by a built-in cold-cathode tube light-emitting apparatus, the cold-cathode tube light-emitting apparatus comprising:

a piezoelectric transformer including a primary side electrode and a secondary side electrode formed on a piezoelectric body and converting an AC voltage input from the primary side electrode to output the converted AC voltage from the secondary side electrode;

a plurality of cold-cathode tubes connected to a secondary side of the piezoelectric transformer;

a cold-cathode tube output detector portion connected in series to the plurality of cold-cathode tubes, for electrically separating and detecting a current flowing through the respective cold-cathode tubes; and

a control portion for controlling light-emission of the plurality of cold-cathode tubes based on a detection signal from the cold-cathode tube output detector portion.

19. The liquid crystal panel according to claim 18, wherein the piezoelectric body has a rectangular shape and the secondary side electrode of the piezoelectric transformer is provided on each end face of the piezoelectric body in a longitudinal direction, and polarization directions of the piezoelectric body in a vicinity of the secondary side electrodes are the same in the longitudinal direction.

20. The liquid crystal panel according to claim 18, wherein the number of the cold-cathode tubes is 2n (n is an integer of 1 or more).

21. The liquid crystal panel according to claim 18, wherein the cold-cathode tube output detector portion includes an optical isolator composed of a light-emitting diode and a phototransistor.

22. The liquid crystal panel according to claim 18, wherein the piezoelectric transformer and the control portion are mounted on a first substrate placed in proximity to one electrode of each of the plurality of cold-cathode tubes, and the cold-cathode tube output detector portion is mounted on a second substrate placed in proximity to the other electrode of each of the plurality of cold-cathode tubes.

23. A device with a built-in liquid crystal panel whose brightness is controlled by a built-in cold-cathode tube light-emitting apparatus, the cold-cathode tube light-emitting apparatus comprising:

a piezoelectric transformer including a primary side electrode and a secondary side electrode formed on a piezoelectric body, for converting an AC voltage input from the primary side electrode to output the converted AC voltage from the secondary side electrode;

a plurality of cold-cathode tubes connected to a secondary side of the piezoelectric transformer;

a cold-cathode tube output detector portion connected in series to the plurality of cold-cathode tubes, for electrically separating and detecting a current flowing through the respective cold-cathode tubes; and

a control portion for controlling light-emission of the plurality of cold-cathode tubes based on a detection signal from the cold-cathode tube output detector portion.

24. The device with a built-in liquid crystal panel according to claim 23, wherein the piezoelectric body has a rectangular shape and the secondary side electrode of the piezoelectric transformer is provided on each end face of the piezoelectric body in a longitudinal direction, and polarization directions of the piezoelectric body in a vicinity of the secondary side electrodes are the same in the longitudinal direction.

25. The device with a built-in liquid crystal panel according to claim 23, wherein the number of the cold-cathode tubes is 2n (n is an integer of 1 or more).

26. The device with a built-in liquid crystal panel according to claim 23, wherein the cold-cathode tube output detector portion includes an optical isolator composed of a light-emitting diode and a phototransistor.

27. The device with a built-in liquid crystal panel according to claim 23, wherein the piezoelectric transformer and the control portion are mounted on a first substrate placed in proximity to one electrode of each of the plurality of cold-cathode tubes, and the cold-cathode tube output detector portion is mounted on a second substrate placed in proximity to the other electrode of each of the plurality of cold-cathode tubes.