DC-AC converter, and method for supplying AC power

• US 7,126,831 B2
• Filed: 01/26/2006
• Issued: 10/24/2006
• Est. Priority Date: 05/23/2003
• Status: Expired due to Fees

• Current Assignee: ROHM Co., Ltd.
• Original Assignee: ROHM Co., Ltd.
• Inventors: Fujita, Hiroyuki, Fukumoto, Kenichi, Toda, Hiroki, Aoyagi, Yosuke
• Primary Examiner(s): Vu, Bao Q.
• Application Number:
  US11/340,396
• Publication Number:
  US 20060120117A1
• Time in Patent Office: 271 Days
• Field of Search: 363/24, 363/26, 363/56.06, 363/133, 363/134
• US Class Current: 363/26
• CPC Class Codes: H02M 1/0048 Circuits or arrangements for reducing losses using snubbers H02M1/34
• CPC Class Codes: H02M 3/3372 of the parallel type
• CPC Class Codes: H02M 7/5381 Parallel type
• CPC Class Codes: H05B 41/2824 using control circuits for the switching element H05B41/2822 takes precedence
• CPC Class Codes: H05B 41/3927 by pulse width modulation H05B41/3922 takes precedence
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

First and second semiconductor switches which are activated alternately are provided between ends of a primary winding and a common potential point, wherein a DC power supply voltage is supplied to a center tap. An electric current flowing into a load is fed back to thereby subject the semiconductor switches to PWM control. Series circuits consisting of capacitors and semiconductor switches are connected between the center tap of the primary winding and the ends of the same. The semiconductor switches are activated in synchronism with the first and second semiconductor switches, thereby preventing occurrence of an anomalous high voltage, which would otherwise be caused at the time of switching operation.

Claims

1. A light source used for a fluorescent lamp comprising:

a transformer having a primary winding with a center tap, and at least one secondary winding, in which the center tap is connected to a first potential point of a DC power source;

a first semiconductor switch which is connected between one end of the primary winding and a second potential point of the DC power source and causes an electric current to flow through the primary winding in a first direction;

a second semiconductor switch which is connected between the other end of the primary winding and the second potential point and causes an electric current to flow through the primary winding in a second direction;

a first capacitor and a third semiconductor switch, which are connected in series between the other end of the primary winding and the center tap;

a second capacitor and a fourth semiconductor switch, which are connected in series between the one end of the primary winding and the center tap;

a current detection circuit to be used for detecting an electric current which flows into a load connected to the secondary winding;

a pulse width modulation circuit which produces a pulse width modulation signal by comparing a feedback signal formed on the basis of an electric current detected by the current detection circuit with a triangular wave signal train;

a logic circuit to be used for outputting a switch drive signal and which produces, on the basis of the pulse width modulation signal, a first switch drive signal to be used for turning on the first semiconductor switch, a second switch drive signal to be used for turning on the second semiconductor switch, a third switch drive signal to be used for turning on the third semiconductor switch, and a fourth switch drive signal to be used for turning on the fourth semiconductor switch

a third capacitor which is connected to one end of the secondary winding, and a cold cathode fluorescent lamp which is connected to said one end of the secondary winding, wherein

the first through fourth switch drive signals are produced at timings of an off-state period where the first through fourth semiconductor switches are turned off, said timings being produced during interval periods such that a first semiconductor switch group, comprising the first semiconductor switch and the third semiconductor switch, and a second semiconductor switch group, comprising the second semiconductor switch and the fourth semiconductor switch, are alternately made on-state synchronously with triangular wave signals of the triangular wave signal train.

2. The cold cathode fluorescent lamp according to claim 1, wherein the first and third semiconductor switches are turned on at every other peak of the triangular wave signal of the triangular wave signal train and remain on-sate until an immediately-subsequent triangular wave signal becomes equal to the feedback signal;

and further wherein

the second and fourth semiconductor switches are turned on at every other peak of the triangular wave signal of the triangular wave signal train, which is different from that of being generated when the first and third semiconductor switches are turned on, and remain on-state until an immediately-subsequent triangular wave signal becomes equal to the feedback signal.

3. The cold cathode fluorescent lamp according to claim 1, wherein the first semiconductor switch is turned on at every other peak of the triangular wave signal of the triangular wave signal train and remains on-state until an immediately-subsequent triangular wave signal becomes equal to the feedback signal;

the second semiconductor switch is turned on at every other peak of the triangular wave signal of the triangular wave signal train, which is different from that of being generated when the first semiconductor switch is turned on, and remains on-state until an immediately-subsequent triangular wave signal becomes equal to the feedback signal;

the third semiconductor switch is turned on at a timing before the first semiconductor switch is turned on, but after lapse of a predetermined period of time since the second semiconductor switch completes the on-sate period thereof and remains on-state during a period in which the first semiconductor switch remains on-state; and

the fourth semiconductor switch is turned-on at a timing before the second semiconductor switch is turned on, but after lapse of a predetermined period of time since the first semiconductor switch completes the on-state period thereof and remains on-state during a period in which the second semiconductor switch remains on-state.

4. The cold cathode fluorescent lamp according to claim 1, wherein the first through fourth semiconductor switches are MOS field-effect transistors.

5. The cold cathode fluorescent lamp according to claim 1, wherein a burst control signal in the form of a pulse train, which allows a ratio of on-state period and off-state period to be controlled, whereby the first through fourth switch drive signals can be produced or stopped.