Patent Document

FIELD OF THE INVENTION 
       [0001]    The present invention is related to low power non-isolated driver used for LED lighting and other non-isolated power supply appliance. 
       BACKGROUND OF THE INVENTION 
       [0002]    LED lighting market is growing very fast and becoming more and more important. For the AC input source, most LED drivers adopt switching mode power supply (SMPS) based on flyback topology. The flyback SMPS circuit usually consists of AC input, PWM control circuit, transformer and constant voltage/current control circuit, wherein the output voltage/current control circuit are coupled through an optical coupling element to the primary side PWM control circuit. The PWM control circuit adjusts the switching duty cycle when line voltage or load is changed, so constant output current or voltage can be realized. 
         [0003]    However, the cost of flyback SMPS circuit is relatively high and the total system size is big. For home appliance, it is ideal that the LED lighting driver is compact and can be placed in the lamp holder like CFL (compact fluorescent lamp). Thus the whole LED lighting system&#39;s installation will be easy. Also for compact LED lighting driver, isolating may not be needed if plastic lamp holder is used. 
         [0004]    Therefore, it is necessary to provide new cost down solutions with less component count, small print circuit board size and better price/performance ratio. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is to provide basic cost down solutions for low power non-isolated LED driver application with higher system reliability, better line/load regulation, and short circuit protection characteristic. 
         [0006]    The present invention provides a non-isolated LED lighting solution based on buck topology, in which the input side is connected to an AC or DC input, a PWM control circuit is connected to the buck converter switch, a capacitor filters the output voltage ripple and an output voltage/current control circuit provides feedback signal to the PWM control circuit. 
         [0007]    The present invention is based on a low cost PWM control circuit with emitter switched architecture. The current mode PWM control circuit contains P 1 , P 2  and P 3  terminals. The P 1  terminal is to produce switching pulse which can be connected to the emitter of NPN transistor or the source of MOSFET, the P 2  terminal is used for both bias supply and feedback control, the P 3  terminal is the reference ground of the PWM control circuit. 
         [0008]    The present invention provides an output voltage/current control circuit employing a low voltage PNP transistor and zener diode to compose voltage sense error signal amplification circuit, a resistor and a low voltage NPN transistor to compose current sense and amplification circuit, or adopt two resistors to form output voltage sensing circuit. 
         [0009]    The present invention has such features of less component number, low total cost, high reliability, and better line/load regulation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The present invention detailed is illustrated by way of example and not limitation in the accompanying figures. 
           [0011]      FIG. 1  shows one embodiment of low power non-isolated driver that has an approximately constant output current and output voltage clamping characteristic in accordance with the teachings of the present invention. 
           [0012]      FIG. 2  shows the function block of the PWM control circuit. 
           [0013]      FIG. 3  shows another embodiment of low power non-isolated driver that has an approximately constant current and output voltage clamping characteristic in accordance with the teachings of the present invention. 
           [0014]      FIG. 4  shows the embodiment of low power non-isolated driver that the transistor, PWM control circuit and the output voltage/current control circuit are integrated. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    The present invention is a low power non-isolated driver which provides a basic cost down solutions for low power non-isolated LED driver application with higher system reliability, better line/load regulation, and short circuit protection characteristic. In particular, the present invention provides a non-isolated LED driver with approximately constant current and constant voltage characteristic based on a PWM control circuit. 
         [0016]      FIG. 1  shows the first embodiment of a low power non-isolated driver that has an approximately constant current and output voltage clamping characteristic of the present invention. The AC or DC input  100  is rectified by diode  101  and filtered by capacitor  103 , inductor  102  and capacitor  104 . Then the high DC voltage is converted to a low voltage DC output  150  by a switch  108 , diode  129 , inductor  128  and filtered by the output capacitor  130 . The switch can be a high voltage transistor or a MOSFET which emitter terminal or source terminal is connected to a PWM control circuit  109 . 
         [0017]      FIG. 2  is the function block of the mentioned PWM control circuit. Its main function circuits include: under voltage lockout with low startup current; precise voltage reference for internal comparators; PWM comparator with current limit control, feedback signal and band-gap input; short circuit comparator. 
         [0018]    The current mode PWM controller contains P 1 , P 2  and P 3  terminals. The P 1  terminal is to produce switching pulse which can be connected with the emitter of NPN transistor or the source of MOSFET, the P 2  terminal is used for both bias supply and feedback control, the P 3  is the reference ground of the PWM control circuit. 
         [0019]    See  FIG. 1  and  FIG. 2 . During start up, start up current through resistors  105  charges capacitor  110 , and also brings up P 1  pin voltage of the PWM control circuit  109  through transistor  108  BE junction. The internal regulator of the PWM control circuit  109  will charge P 2  pin voltage from P 1  pin. When P 2  voltage reaches the start up voltage, the regulator sourcing current will be stopped by internal UVLO comparator. Then the PWM control circuit  109  starts to output PWM signal at P 1  pin and controls the switch  108  turning on and off. 
         [0020]    When the switch  108  turned on, input current will flow through rectifier  101 , switch  108 , PWM control circuit  109 , current sense resistor  114  and inductor  128  to the output. The current flow through PWM control circuit  109  is converted to a voltage by an internal resistor, and this voltage will compare with VREF by short circuit comparator. If output short circuit happens, the output current is limited by the PWM control circuit  109 . When the switch  108  turned off, the inductor  128  current will flow through fast recovery diode  129  and transfer energy to the output. 
         [0021]    The output voltage/current control circuit is composed of components from  114  to  127 . When the switch  108  turned off, the diode  127  will turn on and voltage on capacitor  126  will be almost equal to the output DC voltage. The capacitor  126  voltage is compared with zener diode  125  clamping voltage and amplified by the low voltage small signal PNP transistor  122 , then filtered by compensation resistor  120 , capacitor  119 , capacitor  121  and feedback to the P 2  of the PWM control circuit  109 . If the DC output  150  voltage is higher than the reference value, the P 2  voltage of PWM control circuit  109  will also becomes higher and the PWM control circuit  109  will reduce the converter switching duty cycle or comes into skip cycle mode and low down the DC output  150  voltage. So the constant output voltage control is realized. The inductor  128  current is sensed by resistor  114 , filtered by resistor  115 , capacitor  116  and drives the low voltage small signal NPN transistor  117  BE junction. If the resistor  114  voltage higher than transistor  117  BE junction conduction voltage (about 0.7V), the transistor  117  will turn on and pull down the transistor  122  base junction voltage and output DC voltage will be reduced. So the output current is limited and approximately constant current control is realized. 
         [0022]    When the switch  108  turned off, the inductor  128  energy will also charge capacitor  110  through diode  127  and  112 . The capacitor  110  will discharge and provide driving energy to the switch  108  when the PWM control circuit  109  turns on the switch  108 . The capacitor  110  voltage is clamped by zener diode  111  to prevent high voltage on  110  at very low or no load condition. 
         [0023]      FIG. 3  shows another embodiment of a non-isolated power supply of the present invention. The difference between  FIG. 3  and  FIG. 1  is the output voltage/current control circuit. In  FIG. 3 , the output DC voltage is sensed by two resistors  224  and  225 . There is no output voltage sensing error amplification circuit, so the output voltage precision of line/load regulation is not as good as  FIG. 1 . 
         [0024]      FIG. 4  is the solution with the integrated transistor, PWM control circuit and output voltage/current control circuit. The integrated circuit  360  has five terminals p 1 , p 2 , p 3 , p 4  and p 5 . The integration of the switch  308 , PWM control circuit and output voltage/current control circuit  309  can reduce the whole system size and improve the system reliability. 
         [0025]    It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

Technology Category: 5