Patent Publication Number: US-2010123410-A1

Title: Driving and Dimming Control Device for Illuminator

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention generally relates to illuminators, particularly to dimmable drivers for illuminator. 
     2. Related Art 
     In comparison with incandescent lamps, light emitting diodes (LEDs) have absolute ascendancy. Lifetime of the LEDs can reach 100,000 hours. The LEDs also have advantages of saving energy and fast response. Thus LED lamps are gradually replacing traditional lamps. 
     The LED can only be driven by direct current (DC) power, so it can not directly connect with alternating current (AC) power. An AC-to-DC converter must be added between the LED and AC power. 
     Furthermore, making the LED lamps constantly glow can not completely satisfy requirements of users because they can not be dimmed. Conventional incandescent lamps can perform dimming control by adjusting AC power phase. But AC power phase control can not be applied to the LED lamps. To make the LED dimmable, the AC-to-DC converter must not only perform dimming control, but also output stable DC current. 
     SUMMARY OF THE INVENTION 
     A primary object of the invention is to provide a driving and dimming control device for illuminator which can dimmably convert AC power to DC power with stable current output. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the invention; and 
         FIG. 2  is a schematic diagram of the low-voltage-input locking circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , the driving and dimming control device of the invention includes: 
     an AC power input end  2  and a DC power output end  3 , wherein the AC power input end  2  is used for connecting an external AC power  20  and connects to a phase dimmer  21  such as an SCR (Silicon Controlled Rectifier) dimmer, and the DC power output end  3  is used for connecting a lighting load  30  such as an LED lamp or module; 
     a rectifier  4  connecting to the AC power input end  2  for converting AC power into DC power; 
     a transformer  5  whose primary winding connects to an output of the rectifier  4  and whose secondary winding connects to the DC power output end  3  for transforming the input voltage into a voltage required by the lighting load  30 ; 
     a pulse width modulator  5  connecting to the primary winding of the transformer  5  for outputting power signals for the lighting load  30 ; 
     a voltage/current detector  8  connecting to the secondary winding of the transformer  5  for detecting the voltage and current being input to the lighting load  30 ; 
     a photo-coupler composed of an LED  7  and a phototransistor  60 , wherein the LED  7  connects to the voltage/current detector  8  and the phototransistor  60  connects to the pulse width modulator  6 ; and 
     a low-voltage-input locking circuit  9  connecting between an output of the phase dimmer  21  and pulse width modulator  6 . 
     Referring to  FIG. 2 , the low-voltage-input locking circuit  9  includes a locking switch Q 7  and a reset switch Q 2 . The signals input to the pulse width modulator  6  are locked by the locking switch Q 7  to prohibit the pulse width modulator  6  from hiccupping when dimming. 
     The pulse width modulator  6 , photo-coupler and voltage/current detector  8  jointly constitute an isolating feedback circuit for supplying constant current to the lighting load  30 . This can prevent current from varying when voltage varies for dimming control. When the voltage/current detector  8  detects a variation of voltage or current of the lighting load  30 , for example the voltage of the lighting load  30  exceeds a predetermined reference voltage, the LED  7  lights up to turn on the phototransistor  60 , then the duty cycle of the pulse width modulator  6  is lowered. Therefore, the output current is stably decreased to drive the lighting load  30 . 
     To make dimmable, the capacitor C 2  between the primary winding of the transformer  5  and the output of the rectifier  4  is a metalized polypropylene (MPP) film capacitor. This kind of capacitor has very small capacitance, so it is hard to form the charge effect. It cam improve the power factor correction (PFC) value and satisfy the requirement of the phase dimmer  21 . 
     Additionally, when the phase dimmer  21  controls the phase of AC power down to a low phase (such as smaller than 15 degree), the pulse width modulator  6  tends to hiccup because of insufficient voltage and current. This problem can be solved by the low-voltage-input locking circuit  9  connecting between an output of the phase dimmer  21  and pulse width modulator  6 . Further referring to  FIG. 2 , the output of the phase dimmer  21  is filtered, divided and regulated to generate voltages of control nodes. The difference of the voltages can control transistors Q 2  and Q 7 . Q 7  may make circuit of part A lock to control Q 5  and Q 4  lock the output of the FB signal, so that the pulse width modulator  6  can be prevented from hiccupping. Q 2  functions as reset to release the lock of the circuit of part A and then to restore the function of dimming. 
     It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.