Discharge lamp lighting circuit

• US 20070007913A1
• Filed: 07/10/2006
• Published: 01/11/2007
• Est. Priority Date: 07/11/2005
• Status: Active Grant

• Current Assignee: Koito Manufacturing Company Limited
• Original Assignee: Koito Manufacturing Company Limited
• Inventors: Ichikawa, Tomoyuki, Muramatsu, Takao
• Granted Patent: US 7,291,990 B2
• Time in Patent Office: Days
• US Class Current: 315/291
• CPC Class Codes: H05B 41/2883 the controlled element being a DC/AC converter in the final stage, e.g. by harmonic mode starting

Abstract

A discharge lamp lighting circuit 1 is provided with a DC-AC conversion circuit 3 having a plurality of switching elements 5H, 5L and a series resonance circuit (8, 9, 7p), and control means 17 for preventing a situation in which the drive frequency of the switching element remains less than its specified minimum frequency. When the discharge lamp is lit, driving control of the switching element is carried out in a frequency range higher than the series resonance frequency. By using a driving situation detection circuit 15, a driving situation of the switching element is monitored based on a relation with a phase of a lamp current which flows through the discharge lamp. If the drive frequency of the switching element becomes less than the specified minimum frequency, the drive frequency is increased, and thereby, a lower limit of the drive frequency is automatically restricted.

Claims

1. A discharge lamp lighting circuit comprising a DC-AC conversion circuit having a plurality of switching elements and a series resonance circuit, and a control circuit for preventing continuation of a situation in which a drive frequency of the switching element is less than its specified minimum frequency,

wherein when the discharge lamp is lit, the switching element is driven at a frequency in a frequency range which is higher than a resonant frequency of the series resonance circuit, and a driving situation of the switching element is monitored based on a relation with a phase of a lamp current that flows through the discharge lamp, and wherein, upon detection that the drive frequency of the switching element becomes less than the specified minimum frequency, the drive frequency is increased.

2. The discharge lamp lighting circuit of claim 1, wherein the specified minimum frequency is set at or near the resonant frequency, and wherein the discharge lamp lighting circuit includes a driving situation detection circuit to detect whether or not the switching element is driven in a frequency range lower than the resonant frequency.

3. The discharge lamp lighting circuit of claim 2, wherein the driving situation detection circuit is configured to detect a phase difference between any one of a signal for driving the switching element, an output of the DC-AC conversion circuit and a detection signal corresponding to a lamp voltage of the discharge lamp, and a detection signal corresponding to the lamp current, and wherein it is determined whether or not the switching element is driven in a frequency range lower than the resonant frequency.

4. The discharge lamp lighting circuit of claim 3 wherein the driving situation detection circuit is configured to detect an amount of deviation from the resonance situation.

5. The discharge lamp lighting circuit of claim 1, wherein, if it is detected that the switching element is driven in a frequency range lower than the specified minimum frequency, polarity of a signal for driving the switching element is inverted.

6. The discharge lamp lighting circuit of claim 1, wherein if it is detected that the switching element is driven in a frequency range lower than the specified minimum frequency, a target value of electric power applied to the discharge lamp is reduced depending on an amount of deviation from the specified minimum frequency.

7. The discharge lamp lighting circuit of claim 1, wherein if it is detected that the switching element is driven in a frequency range lower than the specified minimum frequency, the drive frequency of the switching element is raised in accordance with a predetermined time constant.

8. The discharge lamp lighting circuit of claim 2, wherein, if it is detected that the switching element is driven in a frequency range lower than the specified minimum frequency, polarity of a signal for driving the switching element is inverted.

9. The discharge lamp lighting circuit of claim 2, wherein if it is detected that the switching element is driven in a frequency range lower than the specified minimum frequency, a target value of electric power applied to the discharge lamp is reduced depending on an amount of deviation from the specified minimum frequency.

10. The discharge lamp lighting circuit of claim 2, wherein if it is detected that the switching element is driven in a frequency range lower than the specified minimum frequency, the drive frequency of the switching element is raised in accordance with a predetermined time constant.