The present invention relates to a lighting device and associated circuitry for lighting and dimming a semiconductor light-emitting element such as a light emitting diode (LED) or a plurality of the same forming an LED module. More particularly, the present invention relates to an LED lighting device effective to provide a dimming operation based on externally provided PWM dimming signals, and illumination fixtures or video cameras utilizing the same.
An exemplary LED lighting device as conventionally known in the art, and as represented in FIG. 7, may include a switching element Q1 which is coupled in series to a direct-current (DC) power source 2, and an inductor L1 coupled in series with the switching element Q1 and to which an current flows from the DC power source 2 when the switching element Q1 is turned on. Energy stored in the inductor L1 (or equivalent inductive element) when the switching element Q1 is turned on is discharged to a semiconductor light-emitting element 4 via regeneration diode D1 when the switching element is turned off. A current detector R1 detects an current flowing to the switching element Q1. A control circuit is configured to turn the switching element Q1 off when the detected current reaches a predetermined value (i.e., an on-voltage of transistor Tr1) and to turn the switching element Q1 on when the energy discharge of the inductor L1 is completed (when a diode D2 is turned off). However, a LED lighting device so configured does not typically support a dimming function.
FIG. 13 shows an example of a conventional LED lighting device which is capable of supporting a dimming function and which (a) controls a switching element Q1 to be turned off when an current flowing to an inductor L1 reaches a predetermined value, and (b) controls the switching element Q1 to be turned on when an current is fully discharged from the inductor L1 to a semiconductor light-emitting element 4 via a regeneration diode D1. The lighting device controls the average value of the current flowing to the LED 4 to be constant based on a reference voltage Vref1 by improving the input power factor. Dimming of the LED 4 can also or alternatively be controlled by adjusting the reference voltage Vref1. A lighting device so configured can control the average current flowing to the semiconductor light emitting element 4 to be a constant value based on the reference voltage Vref1 even when a power source voltage and an ambient temperature have changed, and can accordingly reduce input current distortion.
Since the control circuit can be configured by using a commercially available IC for power factor improvement, it can be produced at relatively low cost. Among the commercially available ICs for power factor improvement are low-cost ICs which include an error amplifier EA, a multiplier circuit 52, a comparator CP1, a flip-flop FF1, and a driving circuit 54 in one chip. However, as previously noted, the semiconductor light-emitting element 4 is dimmed by setting the reference voltage Vref1 to be adjustable, and dimming control is substantially more difficult where the reference voltage Vref1 is incorporated in the IC.
Another conventional dimming method includes converting an externally-provided PWM signal into a second PWM signal having a different pulse width, interrupting current flowing to a semiconductor light-emitting element in accordance with the converted PWM signal, and adjusting the amplitude of the current flowing to the semiconductor light-emitting element in accordance with a direct-current voltage obtained by smoothing the PWM signal. In such techniques, the on-resistance of the transistor is variably controlled to adjust the amplitude of the current flowing to the semiconductor light-emitting element, and thus a large loss of electric power is generally incurred. To reduce power loss, the amplitude of the current flowing to the semiconductor light-emitting element is adjusted by using a switching power source circuit such as a chopper circuit, which has high power conversion efficiency when operating in a critical mode where the switching element is controlled to be turned on after detecting a zero-cross point for the regeneration current.
An exemplary configuration for controlling the current flowing to the semiconductor light-emitting element to be constant with use of a step-down chopper circuit operating in the critical mode includes PWM-controlling a drive signal of a switching element of the step-down chopper circuit on the basis of a dimming signal.
However, when the duty cycle (on-pulse width) of the switching element is controlled by the PWM, the zero-crossing time for the regeneration current, and thus the switching frequency, may vary in a wide range when control of the duty cycle is arbitrarily variable on the basis of the dimming signal in the switching power source circuit operating in the critical mode, as shown for example by the dashed line in FIG. 15(e)). Also, even if the high frequency switching operation is intermittently stopped on the basis of a low-frequency PWM signal, dimming cannot be sufficiently realized only within the range where the PWM signal can be varied, and accordingly the dimming range is limited.
Referring now to FIG. 19, a conventional operating method for an LED lighting device is represented wherein current flowing to a semiconductor light-emitting element (or LED module) is controlled via a step-down chopper circuit operating in a so-called critical mode. Energy stored in an inductor during an on-period TON of the switching element is discharged during an off-period TOFF, the switch is turned on again when the energy discharge is completed, and thus the power conversion efficiency becomes greater in comparison with other control modes. In addition, since substantially half the peak value of the switching current is an effective value of a load current, constant current control can be easily realized.
An example of the critical mode operation may be further described with reference to a switching element Q1 of a step-down chopper circuit 3a as shown in FIG. 5(a). A DC voltage is supplied across input terminals A and B, having conventionally been obtained by stepping-up a commercially-available alternating-current (AC) power source. An LED series circuit or a load circuit is coupled across output terminals C and D. When the switching element Q1 is turned on, current IQ1 (referring again to FIG. 19) flows via a switching element Q1, an inductor L1, and a capacitor C2, and thus energy is stored in the inductor L1. When the switching element Q1 is turned off, a counter-electromotive force is generated due to the energy stored in the inductor L1, and thus a regeneration current ID1 flows from the inductor L1 to the capacitor C2 and then to a diode D1. When the switching element Q1 is turned on again (upon the regeneration current ID1 returning to zero), switching loss is relatively small and accordingly the power conversion efficiency becomes higher in comparison with other control modes.
It would be desirable to provide a circuit configuration for PWM control of the on-period TON of the switching element Q1 based on an external dimming signal, and which could be implemented prior to factory shipment via easy and low-cost output adjustments to variations in LED characteristic variations or circuit constants such as inductance, temperature variations, age deterioration, etc.