High efficiency constant current led driver

Abstract

The present invention discloses a high efficiency constant current LED driver, which comprises a rectification bridge, a PFC main circuit, an isolated DC/DC converter, a PFC controller and a PFC bus control circuit. Since the input voltage is an intermediate PFC bus voltage, which varies with the output voltage of the DC/DC converter. When the isolated DC/DC converter is an LLC resonant circuit, the operating frequency of the LLC circuit is close to the resonant frequency within a wide output voltage range. Thus, the gain range and the operating frequency is narrow, and can enable the constant current module to work with a high efficiency at a wide output voltage range. When the isolated DC/DC converter is a symmetric half bridge, or an asymmetric half bridge or a full bridge circuit, the duty cycle of DC/DC circuit is close to 50% within a wide output voltage range. Thus, the changing range of the duty cycle of the DC/DC converter will be narrow and can improve the efficiency dramatically.

1 . A circuit which can improve a power factor of a power supply at a light load, the circuit comprising
 a filter configured with at least one filter capacitor;   a light load detecting circuit;   a light load;   a logic control and power drive circuit;   a switching transistor;   a rectifier bridge; and   a power factor correction circuit;   wherein the filter is configured to be placed before and/or after the rectifier bridge, the light load detecting circuit is configured to output a control signal to the logic control and power drive circuit which controls the switching transistor to conduct at heavy load and to cut off at light load in order to control a working status of the filter capacitor.   
     
     
         2 . The circuit according to  claim 1 , wherein input terminals of the rectifier bridge are connected with a power line voltage, while output terminals of the rectifier bridge are connected in parallel with the power factor correction circuit and a branch circuit in which a capacitor and the switching transistor are connected in series. 
     
     
         3 . The circuit according to  claim 2 , wherein an output of the light load detecting circuit is connected with an input of the logic control and power drive circuit, and an output of the logic control and power drive circuit is connected with a third terminal of the switching transistor. 
     
     
         4 . The circuit according to  claim 1 , wherein the filter further comprises a second capacitor and an inductor with two terminals of a power line voltage is connected with an input terminal of the rectifier bridge, a positive output of the rectifier bridge is connected with one terminal of the inductor and one terminal of the first capacitor, another terminal of the inductor is connected with one terminal of the second capacitor and the positive input terminal of the power factor correction circuit, while a negative input terminal of the power factor correction circuit is connected to ground. 
     
     
         5 . The circuit according to  claim 4 , wherein another terminal of the second capacitor is connected with a second terminal of the switching transistor, a first terminal of the switching transistor is connected to ground, an output terminal of the light load detecting circuit is connected with an input terminal of the logic control and power drive circuit, and an output of the logic control and power drive circuit is connected with a third terminal of the switching transistor. 
     
     
         6 . The circuit according to  claim 1 , wherein the capacitor in the filter is connected in series with a switching transistor to constitute a series loop, two terminals of the series loop are connected in parallel with a power line, the rectifier bridge is connected in parallel with the power line, output terminals of the rectifier bridge are connected with the power factor correction circuit, an output terminal of the light load detecting circuit is connected to the input of the logic control and power drive circuit, and an output of the logic control and power drive circuit is connected with a third terminal of the switching transistor.