text: This invention relates to a power supply apparatus for light sources, for example, a power supply apparatus for a lamp for use in a projector or a power supply apparatus for illuminating equipment.
  The arrangement of an example of prior art power supply apparatuses for light sources is as follows. The power supply apparatus includes an input-side rectifier which converts a commercial AC voltage applied thereto to a DC voltage. The DC voltage is then applied to a high-frequency inverter where it is converted to a high-frequency voltage. The high-frequency voltage is voltage-transformed by a transformer, and the voltage-transformed voltage is converted to a DC voltage by an output-side rectifier. This DC voltage is applied to a load, e.g. a lamp. Current supplied to the load is detected by a current detector, which develops a current representative voltage representative of the magnitude of the detected current. The current-representative voltage is applied to a summing circuit which receives also a reference voltage set in a reference voltage circuit. The summing circuit develops a signal representative of the difference between the current representative voltage and the reference voltage, and the signal is applied through an error amplifier to a control circuit. The control circuit operates to control the high-frequency inverter so as to make the current representative voltage equal to the reference voltage. In other words, this power supply apparatus is constant current controlled.
  Another type of power supply apparatuses for lamps is constant-power controlled, in which power supplied to a load or lamp is detected, and a high-frequency inverter is so controlled as to make the detected power to the load become equal to a predetermined value.
  Prior art constant-current controlled power supply apparatuses cannot provide constant power to a lamp, and, therefore, the brightness of the lamp fluctuates. With this type of power supply apparatus, when input current decreases, the output power supplied to the load also decreases, resulting in insufficient current to the lamp, which, in turn, may cause the lamp to be turned off.
  As for the above-described constant-power controlled power supply apparatus, even when an input voltage decreases, constant-power controlling is continued. This means that when an input voltage decreases below a rated voltage, input current increases. If the input current increases above allowable current values of an electric outlet and lines to which the input-side rectifier circuit is connected, the outlet and lines may be broken.
  An object of the present invention is to provide a power supply apparatus for light sources, which can power the light sources in such a manner that the light sources can provide constant brightness, does not cause interruption of the operation of the light sources even when input current decreases, or does not cause an outlet, a connector and wires to be broken.
  According to the present invention, a power supply apparatus for a light source includes an AC-to-DC converter for converting an input AC signal into a DC signal. A DC-to-high-frequency converter converts the DC signal to a high-frequency signal. Output providing means converts the high-frequency signal to an output signal for the light source, either in the form of DC output signal or AC output signal, which is to be applied to the light source. A high-frequency-to-DC converter or a high-frequency-to-AC converter may be used as the output providing means. Power detecting means detects the power which is supplied to the light source through the application of the DC output signal or AC output signal thereto, and develops a power-representative signal. Control means controls the DC-to-high-frequency converter in such a manner that the power supplied to the light source can have a predetermined value, in accordance with the power representative signal. Input current detecting means detects a current value of the AC input signal and develops an input current representative signal. If the input current representative signal value is below a predetermined input current reference value, input current control means provides such a control signal to the DC-to-high-frequency converter control means as to maintain the AC input signal current at a predetermined value.
  The power supply apparatus for light sources described above is constant power controlled by detecting the power being supplied to the light source and controlling the DC-to-high-frequency converter in such a manner that the detected power can be at a predetermined value. Accordingly, if the input voltage at the rated value is being applied to the apparatus, the power supplied to the light source and, hence, the brightness of the light source can be constant. If the input voltage decreases below the rated value when the apparatus is being constant power controlled, the input current will increase. In such a case, if a source of the AC input signal is provided with a breaker, the breaker will be tripped and the light source will be turned off. According to the invention, in order to prevent such tripping, the power supply apparatus is subjected to constant current controlled instead of being constant power controlled if the input voltage decreases, so that the light source can continue to emit light.
  The DC-to-high-frequency converter control means may include first error amplifying means which provides a first error signal representative of the difference of the power-representative signal from a predetermined constant-power reference value. In this case, the input current control means includes second error amplifying means which provides the first error amplifying means with a second error signal representative of the difference of the input current representative signal from the input current reference value. For example, the first error amplifying means may be so configured as to output, as the first error signal, a signal representing the difference between the sum of the second error signal and the power-representative signal and the constant-power reference signal.
  The DC-to-high-frequency converter may be controlled in such a manner that the input current can assume a predetermined value when the input voltage becomes lower than a predetermined value. However, in order to realize it, a complicated arrangement will be required, which includes an input voltage detector, a comparator for judging whether the detected input voltage is lower than a predetermined value, an input current detector, an error amplifier developing an error signal representing the difference between the input current and a predetermined reference current value, and a switching arrangement causing the error signal from the error amplifier to be coupled to the control means so that it can operate in accordance with the error signal. In contrast, with the above-described arrangement according to the present invention, when the value of the input current is larger than the input current reference value, the second error signal gives little effect on the first error signal, which permits constant power control to be performed, and, if the input current value becomes smaller than the input current reference value, the second error signal gives effect on the first error signal so that the power supply apparatus can be constant current controlled to make the input current constant. As is understood, the circuit arrangement can be simpler than the above-described input voltage detecting arrangement.
  A power factor improving circuit may be disposed between the AC-to-DC converter and the DC-to-high-frequency converter. With such arrangement, the phase difference between the voltage and current of a signal applied to the DC-to-high-frequency converter is reduced so that the power factor is improved. Thus, the DC signal can be converted to the high-frequency signal at a high efficiency.
  A low-frequency transformer may be disposed between the AC-to-DC converter and the DC-to-high-frequency converter. With this arrangement, harmonic noise generated when the AC input signal is converted to the DC signal in the AC-to-DC converter can be removed.