Abstract:
Disclosed is a method of taking electric power from a low voltage bypass for providing low voltage power supply for an integrated circuit. Also provided is an alternating current (AC) directly driven LED integrated circuit adapted to use the method; the integrated circuit includes a voltage stabilizing circuit, a low voltage electronic switch circuit, an under-voltage control circuit, and a comparative amplification circuit, and has three pins, a positive power supply terminal, a zero potential reference terminal, and a common terminal for current sampling and for the low voltage electronic switch. The method of taking electric power from a low voltage bypass for providing low voltage power supply for an AC directly driven LED integrated circuit, and the integrated circuit adapted to use the method have the advantages of low power consumption and cost, high efficiency and reliability, and fewer pins and external devices, and are easy to use.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national stage application under 35 U.S.C. §371 of International Application No. PCT/CN2014/072539, filed Feb. 26, 2014, which in turn claims the benefit of Chinese Application No. 201310147248.6, filed Apr. 25, 2013, the content of each of which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to electronics, and in particular, relates to a method of taking power with low-voltage bypass by an integrated circuit (IC) for alternating current (AC) direct driving LEDs and the IC. 
     BACKGROUND OF THE INVENTION 
     The technologies of AC direct driving light emitting diodes (LEDs) have attracted a wide attention for their characteristics of simple structure, low cost, long lifetime and the like, and IC for AC direct driving LEDs is the key technology. However, in the existing technologies of IC for AC direct driving LEDs, all methods of supplying power for the IC use a method of taking power directly from an AC high-voltage supply that converts mains high-voltage AC supply of 220V RMS (Root Mean Square) or 110V RMS into a low-voltage DC and supplies the low-voltage DC power to the IC, which mainly includes power resistor voltage drop, high voltage nonpolar large-capacity capacitor voltage drop, transformer and switching power supply, etc. The existing resistor voltage drop technology has an apparent disadvantage that the useless power dissipated on the voltage-dropping power resistor is large. The capacitor voltage drop technology has disadvantages of poor anti-surge performance, and low reliability. The transformer technology has disadvantages of low efficiency and large volume. The switching power supply technology has disadvantages of complex circuits and high cost. Moreover, such an AC high voltage power taking technology has an especially critical technical disadvantage that, in the practical application, a large number of long AC high-voltage circuit wires will cause difficulty in the wiring between LEDs and ICs on printed circuit board (PCB), an increase in the area of the PCB and lower reliability. Especially, for the chip on board (COB) technology that has been widely used and is continuously developing rapidly, namely, the technology of bonding LED chips and IC chips for AC direct driving LEDs on a ceramic or aluminum board, the existing AC high voltage power supplying technology for the IC for AC high voltage direct driving LEDs usually requires a large number of long and high-voltage connecting wires and electronic components with a high voltage of hundreds of volts, which significantly increases the area of the board for LED chips and IC chips, lowers the reliability of the products and increases the manufacturing cost. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of taking power with low-voltage bypass by an integrated circuit (IC) for AC direct driving LEDs, and the IC for AC direct driving LEDs that is suitable for applying the method of taking power with low-voltage bypass. 
     Provided here in is a method of taking power with low-voltage bypass by an IC for AC direct driving LEDs, wherein: the IC for AC direct driving LEDs comprises three pins, i.e., a positive power-supply terminal, a zero potential reference terminal and a common terminal for current sampling and low-voltage electronic switching; and a LEDs-load unit, comprising a group of LEDs, together with a current sampling resistor, is suitable for being provided external to the IC, wherein the LEDs-load unit comprises several or dozens of LEDs connected in series in the same direction, and has a positive end with a positive power-taking node and a negative end with a common terminal node for current sampling and low-voltage electronic switching, where the negative end is connected to one end of a current sampling resistor Rs, and the other end of the current sampling resistor Rs has a zero potential reference terminal node. The method of taking power by the IC or the usage of the IC comprises: connecting the positive power-supply terminal of the IC to the positive power-taking node of the LEDs-load unit; connecting the common terminal of the IC for current sampling and low-voltage electronic switching to the common terminal node of the LEDs-load unit for current sampling and low-voltage electronic switching; and connecting the zero potential reference terminal of the IC to the zero potential reference terminal node of the LEDs-load unit; so that a voltage obtained by the method is a unidirectional pulsating low voltage, wherein the unidirectional pulsating low voltage has a maximum peak voltage, typically 10V to 60V, which is equal to a transient voltage across the LEDs-load unit and is much lower than a peak voltage of AC mains, such as a peak voltage 311V of 220V (rms) AC mains, and has a frequency, such as 100 Hz or 120 Hz, twice of that of AC mains. The IC comprises a voltage stabilizing circuit, a low-voltage electronic switching circuit, an under-voltage control circuit and a comparing and amplifying circuit. The voltage stabilizing circuit has a stabilized output voltage of about 2.4V; the low-voltage electronic switching circuit functions to control switching-in or short-circuiting of the LEDs-load unit according to current intensity of a sampled current; the under-voltage control circuit has a threshold of about 3.0V, and is suitable for causing the low-voltage electronic switching circuit to become an open circuit so as to switch in the LEDs-load unit when a transient voltage across the IC obtained by taking power with the low-voltage bypass is lower than the threshold voltage; the comparing and amplifying circuit has a reference voltage of about 1.2V, and is suitable for outputting a control level to control switching of the low-voltage electronic switching circuit upon comparing and amplifying a voltage across the current sampling resistor with the reference voltage, so that the low-voltage electronic switching circuit switches in the LEDs-load unit when the voltage across the current sampling resistor is greater than the reference voltage, and the low-voltage electronic switching circuit short-circuits the LEDs-load unit when the voltage across the current sampling resistor is less than the reference voltage. 
     The method of taking power by the IC is a method of taking power with low-voltage bypass, comprising: connecting the positive power-supplied terminal of the IC to the positive power-taken node of the LEDs-load unit; connecting the common terminal of the IC for current sampling and low-voltage electronic switching to the common terminal node, of the LEDs-load unit, for current sampling and low-voltage electronic switching; and connecting the zero potential reference terminal of the IC to the zero potential reference terminal node of the LEDs-load unit; so that a voltage obtained by the method is a unidirectional pulsating low voltage, wherein the unidirectional pulsating low voltage has a maximum peak voltage, typically 10V to 60V, which is equal to a transient voltage across the LEDs-load unit and is much lower than a peak voltage of AC mains such as peak voltage 311V of 220V (rms) AC mains, and has a frequency, such as 100 Hz or 120 Hz, twice of that of AC mains. 
     The IC is an IC suitable for AC direct driving LEDs by taking power with low-voltage bypass, comprising a voltage stabilizing circuit  1 , a low-voltage electronic switching circuit  2 , an under-voltage control circuit  3  and a comparing and amplifying circuit  4 , and provided with three pins, which are a positive power-supply terminal, a common terminal for current sampling and low-voltage electronic switching and a zero potential reference terminal, respectively. A unidirectional pulsating voltage of 10V to 60V is taken by the positive power supply terminal; the positive power-supply terminal is connected to each of the voltage stabilizing circuit  1 , the low-voltage electronic switching circuit  2  and the under-voltage control circuit  3 ; an output of the voltage stabilizing circuit  1  is connected to an input end of the under-voltage control circuit  3 ; an output end of the under-voltage control circuit  3  is connected to an input end of the low-voltage electronic switching circuit  2 ; another output of the voltage stabilizing circuit  1  is connected to an input end of the comparing and amplifying circuit  4 ; an output end of the comparing and amplifying circuit  4  is connected to the input end of the low-voltage electronic switching circuit  2 ; the common terminal for current sampling and low-voltage electronic switching is connected both to the low-voltage electronic switching circuit  2  and to the comparing and amplifying circuit  4 ; the zero potential reference terminal is connected, by connecting the current sampling resistor Rs in series, with the common terminal for current sampling and low-voltage electronic switching; the voltage stabilizing circuit  1  supplies voltage-stabilized power to the comparing and amplifying circuit  4 ; the low-voltage electronic switching circuit  2  has two working states that correspond to switching-in and short-circuiting of the LEDs-load unit, respectively; the under-voltage control circuit  3  has a fixed threshold voltage, and is suitable for causing the low-voltage electronic switching circuit  2  to become an open circuit so as to switch in the LEDs-load unit when the transient voltage across the IC obtained by taking power with low-voltage bypass is lower than the threshold voltage of the under-voltage control circuit  3 ; the comparing and amplifying circuit  4  has two differential output terminals, of which a non-inverting output terminal is connected to the input end of the under-voltage control circuit  3  and an inverting output end is connected to the input control terminal of the low-voltage electronic switching circuit  2 ; and the comparing circuit  4  is provided with a reference voltage and amplifying the voltage difference, and is suitable for outputting a control level to control switching of the low-voltage electronic switching circuit  2  upon comparing and amplifying the voltage across the current sampling resistor with the reference voltage, so that the low-voltage electronic switching circuit  2  switches in the LEDs-load unit when the voltage across the current sampling resistor is greater than the reference voltage, and the low-voltage electronic switching circuit  2  short-circuits the LEDs-load unit when the voltage across the current sampling resistor is less than the reference voltage. 
     The voltage stabilizing circuit  1  comprises a transistor Q 15 , a transistor Q 16 , a transistor Q 14 , a transistor Q 17 , a transistor Q 18 , a transistor Q 19 , a transistor Q 20 , a transistor Q 21  and a resistor R 4 . Emitter of the transistor Q 15 , emitter of the transistor Q 16  and collector of the transistor Q 14  each are connected to the positive power-supply terminal of the IC; base of the transistor Q 15 , base of the transistor Q 16  and collector of the transistor Q 16  each are connected to one end of the resistor R 4 , the other end of which is connected to the zero potential reference terminal of the IC; collector of the transistor Q 15 , base of the transistor Q 14 , base of the transistor Q 17  and collector of the transistor Q 17  are connected to one another; emitter of the transistor Q 17 , base of the transistor Q 18  and collector of the transistor Q 18  are connected to one another; emitter of the transistor Q 18  is connected both to base of the transistor Q 19  and to collector of the transistor Q 19 ; emitter of the transistor Q 19  is connected both to base of the transistor Q 20  and to collector of the transistor Q 20 ; emitter of the transistor Q 20  is connected both to base of the transistor Q 21  and to collector of the transistor Q 21 ; and emitter of the transistor Q 21  is connected to the zero potential reference terminal. The transistor Q 15  and the transistor Q 16  are PNP transistors, and the transistor Q 14 , the transistor Q 17 , the transistor Q 18 , the transistor Q 19 , the transistor Q 20  and the transistor Q 21  are NPN transistors. 
     The low-voltage electronic switching circuit  2  comprises a transistor Q 23 , a transistor Q 25 , a transistor Q 26  and a transistor Q 27 . Emitter of the transistor Q 23  is connected to the zero potential reference terminal, and collector of the transistor Q 23  is connected both to collector of the transistor Q 25  and to base of the transistor Q 26 ; emitter of the transistor Q 25 , collector of the transistor Q 26  and collector of the transistor Q 27  are all connected to the positive power-supply terminal; emitter of the transistor Q 26  is connected to base of the transistor Q 27 ; and emitter of the transistor Q 27  is connected to the common terminal for current sampling and low-voltage electronic switching. The transistor Q 25  is a PNP transistor, and the transistor Q 23 , the transistor Q 26  and the transistor Q 27  are NPN transistors. 
     The under-voltage control circuit  3  comprises a transistor Q 24  and a transistor Q 22 . Emitter of the transistor Q 24  is connected to the positive power-supply terminal; base of the transistor Q 24 , collector of the transistor Q 24  and collector of the transistor Q 22  are connected to one another; and emitter of the transistor Q 22  is connected to the zero potential reference terminal. The transistor Q 24  is a PNP transistor, whereas the transistor Q 22  is an NPN transistor. 
     The comparing and amplifying circuit  4  comprises a transistor Q 1 , a transistor Q 2 , a transistor Q 3 , a transistor Q 4 , a transistor Q 10 , a transistor Q 11 , a transistor Q 5 , a transistor Q 6 , a transistor Q 7 , a transistor Q 8 , a transistor Q 9 , a transistor Q 12 , a transistor Q 13 , a resistor R 1 , a resistor R 2 , a resistor R 3  and a resistor R 5 . Base of the transistor Q 1 , base of the transistor Q 2 , base of the transistor Q 11 , base of the transistor Q 10 , collector of the transistor Q 10  and one end of the resistor R 3  are connected to one another; the other end of the resistor R 3  is connected both to collector of the transistor Q 9  and to base of the transistor Q 9 ; collector of the transistor Q 1 , collector of the transistor Q 2 , emitter of the transistor Q 3  and emitter of the transistor Q 4  are connected to one another; base of the transistor Q 3 , collector of the transistor Q 5  and one end of the resistor R 1  are connected to one another; collector of the transistor Q 3  is connected to the zero potential reference terminal; base of the transistor Q 4 , collector of the transistor Q 6  and one end of the resistor R 2  are connected to one another; emitter of the transistor Q 5 , emitter of the transistor Q 6 , collector of the transistor Q 7  and collector of the transistor Q 8  are connected to one another; base of the transistor Q 5 , collector of the transistor Q 11 , collector of the transistor Q 12  and base of the transistor Q 12  are connected to one another; emitter of the transistor Q 12 , collector of the transistor Q 13  and base of the transistor Q 13  are connected to one another; base of the transistor Q 6  is connected to one end of the resistor R 5 , the other end of which is connected to the common terminal for current sampling and low-voltage electronic switching; base of the transistor Q 7 , base of the transistor Q 8  and the base of the transistor Q 9  are connected to one another; and emitter of the transistor Q 13 , emitter of the transistor Q 7 , emitter of the transistor Q 8  and emitter of the transistor Q 9  each are connected to the zero potential reference terminal. The transistor Q 1 , the transistor Q 2 , the transistor Q 3 , the transistor Q 4 , the transistor Q 10  and the transistor Q 11  are all PNP transistors, whereas the transistor Q 5 , the transistor Q 6 , the transistor Q 7 , the transistor Q 8 , the transistor Q 9 , the transistor Q 12  and the transistor Q 13  are all NPN transistors. 
     The above four function circuits are connected in a manner such that emitter of the transistor Q 14  for stabilizing voltage output inside the voltage stabilizing circuit  1  offers voltage-stabilized output, and the emitter of the transistor Q 14  is connected to each of emitter of the transistor Q 1 , emitter of the transistor Q 2 , emitter of the transistor Q 10 , emitter of the transistor Q 11 , the other end of the resistor R 1  and the other end of the resistor R 2  in the comparing and amplifying circuit  4 ; the base and the collector of the transistor Q 21  in the voltage stabilizing circuit  1  are together connected to base of the transistor Q 22  in the under-voltage control circuit  3 ; the collector of the transistor Q 4  in the comparing and amplifying circuit  4  is connected to base of the transistor Q 23  in the low-voltage switching control circuit  2 ; the other end of the resistor R 5  in the comparing and amplifying circuit  4  is connected both to the emitter of the transistor Q 27  of the low-voltage switching circuit  2  and to the common terminal for current sampling and low-voltage electronic switching; and the base and the emitter of the transistor Q 24  in the under-voltage control circuit  3  are connected to each other and are then together connected to base of the transistor Q 25  in the low-voltage electronic switching circuit  2 . 
     The present invention has the following beneficial effects. 
     The present invention provides a method of taking power with low-voltage bypass by an IC for AC direct driving LEDs and the IC for AC direct driving LEDs that is suitable for applying the method of taking power with low-voltage bypass. The method of taking power with low-voltage bypass has the characteristics of simplicity in power-taking, low voltage and high efficiency, thus the areas of the board for LED chips and IC chips may be significantly reduced, and the reliability may be greatly improved. The IC of the present invention not only meets the requirements of low-voltage bypass power-taking technologies, but also has the advantages of low power consumption, high efficiency, high reliability, low cost, less pins, less external components and convenient use, etc. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a circuit diagram illustrating an application of an IC in accordance with the present invention along with a method of taking power with low-voltage bypass by the IC; 
         FIG. 2  shows a block diagram illustrating a circuit configuration of the IC in accordance with the present invention; 
         FIG. 3  shows an internal circuit diagram of a bipolar IC in accordance with the present invention; and 
         FIG. 4  shows an internal circuit diagram of a BiCMOS IC in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention will be further described in conjunction with the accompanying drawings. 
     Embodiment 1 
       FIG. 1  shows a circuit diagram illustrating an application of an IC in accordance with the present invention along with a method of taking power with low-voltage bypass by the IC. The circuit diagram as shown by this figure includes: a bridge rectifier D 1 ; eight ICs (from a first IC U 1  to an eighth IC U 8 ) in accordance with the present invention; a LEDs-load unit comprising forward biased LED 1  to LED 40  connected in series which are not controlled by the IC in accordance with the present invention and are always switched in; a LEDs-load unit comprising forward biased LED 41  to LED 50  connected in series which are controlled by the IC in accordance with the present invention; a first current sampling resistor Rs 1  as an external current sampling resistor of the first IC U 1 ; and, by analogy, tO an eighth controlled LEDs-load unit comprising an eighth group of forward biased LED 111  to LED 120  connected in series, along with an eighth current sampling resistor Rs 8  as an external current sampling resistor of the eighth IC U 8 . AC mains with a frequency of 50 Hz and with a RMS voltage of 185V to 265V may be full-wave rectified via the bridge rectifier D 1  to obtain a unidirectional pulsating voltage with a frequency of 100 Hz and with a peak voltage, i.e., about 261V to 375V, which is about 1.414 times higher than AC input RMS voltage. The resistances of the eight external sampling resistors (the first current sampling resistor Rs 1  to the eighth current sampling resistor Rs 8 ) corresponding to the eight ICs have resistance values having a stepwise distribution, which are 10Ω, 11Ω, 12Ω, 13Ω, 15Ω, 16Ω, 18Ω and 20Ω, respectively. Each of LED 1  to LED 120  has a rated operational current of 100 mA, and has a forward voltage of 3.5V under the rated operational current of 100 mA. In this embodiment, each of the ICs (i.e., the first IC U 1  to the eighth IC U 8 ) has three pins, i.e., a positive power-supply terminal V+, a zero potential reference terminal V− and a common terminal Vs for current sampling and low-voltage electronic switching. 
     As can be seen from  FIG. 1 , the method of taking power with low-voltage bypass in accordance with the present invention is illustrated as follows. The positive power-supply terminal V+ of the first IC U 1  is connected to a positive electrode of the LED 41 . The LED 41  to LED 50  are forward biased and connected in series. The negative electrode of the LED 50  is connected both to one end of the current sampling resistor Rs 1  and to the common terminal Vs for current sampling and low-voltage electronic switching of the first IC U 1 . The other end of the Rs 1  is connected to the zero potential reference terminal V− of the first IC U 1 . The supply voltage of the IC U 1  is also a unidirectional pulsating voltage with a frequency of 100 Hz, the peak value of which is equal to a transient voltage across the first controlled LEDs-load unit (LED 41  to LED 50  being forward biased and connected in series) and the sampling resistor R 1 . When the low-voltage electronic switch of the first IC U 1  is in an open-circuit state, the first controlled LEDs-load unit (LED 41  to LED 50  being forward biased and connected in series) which is controlled by the first IC U 1  will be correspondingly in a switched-in state with a forward voltage of about 35V, where a voltage drop across the sampling resistor Rs 1  is about 1.0V, and the voltage of power taken with low-voltage bypass by the IC U 1  has a frequency of 100 Hz, and has a peak voltage of 36V. When the low-voltage electronic switch of the first IC U 1  is in a short-circuited state, this first controlled LEDs-load unit (LED 41  to LED 50  being connected in forward series) which is controlled by the first IC U 1  will be correspondingly in a short-circuited state with a short-circuit voltage drop of about 2.0V, where a voltage drop across the sampling resistor Rs 1  is about 1.0V and the voltage of power taken with low-voltage bypass by the IC U 1  is less than or equal to 3.0V. 
     The circuit illustrated by  FIG. 1  is an 8-stage-switch-controlled circuit for AC direct driving LEDs, in which the ICs of the present invention are applied. In the circuit, the first IC U 1  to the eighth IC U 8  are eight ICs in accordance with the present invention, each of which is supplied by taking power with the low-voltage bypass, and controls switching between switching-in and short-circuiting of the corresponding one group of eight groups of LEDs connected in series in the same direction, respectively. It is especially important that the resistances of the eight external current sampling resistors (i.e., the first current sampling resistor Rs 1  to the eighth current sampling resistor Rs 8 ) external to the eight ICs have different resistance values having a stepwise distribution, for example, 10Ω, 11Ω, 12Ω, 13Ω, 15Ω, 16Ω, 18Ω and 20Ω, respectively. 
     The 8-stage-switch-controlled circuit for AC direct driving LEDs, in which the ICs of the present invention are applied, has the main technical specifications as follows: AC input voltage in the range of 185V (rms) to 265V (rms); and efficiency of the driving circuit greater than 94%, Power Factor (PF) of greater than 0.96 and Total Harmonic Distortion (THD) of less than 25%, under the AC input voltage of 220V (rms). 
     Embodiment 2 
       FIG. 2  shows a block diagram illustrating a circuit configuration of the IC in accordance with the present invention. The IC comprises a voltage stabilizing circuit  1 , a low-voltage electronic switching circuit  2 , an under-voltage control circuit  3  and a comparing and amplifying circuit  4 . The IC has three pins, i.e., a positive power-supply terminal V+, a common terminal Vs for current sampling and low-voltage electronic switching, and a zero potential reference terminal V−. The LED 1  to LEDN connected in series in the same direction represent external loads, forming an external LEDs-load unit. Rs represents an external current sampling resistor of the IC. The voltage stabilizing circuit  1  has an input voltage corresponding to V+ for the IC, and has an output voltage of about 2.4V, which supplies power to the comparing and amplifying circuit  4 . The voltage stabilizing circuit provides a low level output under an under-voltage input, and is connected to the input end of the under-voltage control circuit  3 . The output end of the under-voltage control circuit  3  is connected with an input terminal of the low-voltage electronic switching circuit  2 . When V+ is lower than 3.0V, the low-voltage electronic switching circuit  2  will remain in an open-circuit state so that the external LEDs-load unit is in a switched-in state. The input end of the comparing and amplifying circuit  4  and a switching control terminal of the low-voltage electronic switching circuit  2  are connected to each other, as the common terminal Vs of the IC for current sampling and low-voltage electronic switching. The output end of the comparing and amplifying circuit  4  is connected to another input terminal of the low-voltage electronic switching  2 . The comparing and amplifying circuit  4  outputs a high level or a low level depending on the value of the voltage across the current sampling resistor, and the low-voltage electronic switching circuit  2  controls switching between switching-in and short-circuiting of the external LEDs-load unit. 
     Embodiment 3 
       FIG. 3  shows an internal circuit diagram of the IC in accordance with the present invention applying a bipolar IC technology, which is an internal circuit diagram of an IC that is compatible with a bipolar IC technology. As shown in  FIG. 3 , the IC is an IC for AC direct driving LEDs suitable for taking power with low-voltage bypass, comprising a voltage stabilizing circuit  1 , a low-voltage electronic switching circuit  2 , an under-voltage control circuit  3  and a comparing and amplifying circuit  4 , and provided with three pins, i.e., a positive power-supplied terminal, a common terminal for current sampling and low-voltage electronic switching, and a zero potential reference terminal. The positive power-supply terminal is connected to each of the voltage stabilizing circuit  1 , the low-voltage electronic switching circuit  2  and the under-voltage control circuit  3 ; an output of the voltage stabilizing circuit  1  is connected to an input end of the under-voltage control circuit  3 , the output end of which is connected to an input end of the low-voltage electronic switching circuit  2 ; another output of the voltage stabilizing circuit  1  is connected to the input end of the comparing and amplifying circuit  4 , the output end of which is connected to the input end of the low-voltage electronic switching circuit  2 ; the common terminal for current sampling and low-voltage electronic switching is connected both to the low-voltage electronic switching circuit  2  and to the comparing and amplifying circuit  4 ; and the zero potential reference terminal is connected, by connecting the current sampling resistor Rs in series, with the common terminal for current sampling and low-voltage electronic switching. The voltage stabilizing circuit  1  supplies voltage-stabilized power to the comparing and amplifying circuit  4 . The low-voltage electronic switching circuit  2  has two working states that correspond to switching-in and short-circuiting of the LEDs load, respectively. The under-voltage control circuit  3  has a fixed threshold voltage, and causes the low-voltage electronic switching circuit  2  to become an open circuit so as to switch in the corresponding LEDs-load unit when the transient voltage across the IC obtained by taking power with low-voltage bypass is lower than the threshold voltage of the under-voltage control circuit  3 . The comparing and amplifying circuit  4  has two differential output terminals, a non-inverting output terminal of which is connected to the input terminal of the under-voltage control circuit  3  and an inverting output end of which is connected to the input control terminal of the low-voltage electronic switching circuit  2 . The comparing and amplifying circuit  4  is provided with a reference voltage, and outputs a control level to control switching of the low-voltage electronic switching circuit  2  upon comparing the voltage across the current sampling resistor with the reference voltage and amplifying the voltage differential. When the voltage across the current sampling resistor is greater than the reference voltage, the low-voltage electronic switching circuit  2  switches in the LEDs-load unit, and when the voltage across the current sampling resistor is less than the reference voltage, the low-voltage electronic switching circuit  2  short-circuits the LEDs-load unit. 
     The voltage stabilizing circuit  1  includes a transistor Q 15 , a transistor Q 16 , a transistor Q 14 , a transistor Q 17 , a transistor Q 18 , a transistor Q 19 , a transistor Q 20 , a transistor Q 21  and a resistor R 4 . Emitter of the transistor Q 15 , emitter of the transistor Q 16  and collector of the transistor Q 14  are all connected to the positive power-supplied terminal of the IC; base of the transistor Q 15 , base of the transistor Q 16  and collector of the transistor Q 16  each are connected to one end of the resistor R 4 , the other end of which is connected to the zero potential reference terminal of the IC; collector of the transistor Q 15 , base of the transistor Q 14 , base of the transistor Q 17  and collector of the transistor Q 17  are connected to one another; emitter of the transistor Q 17 , base of the transistor Q 18  and collector of the transistor Q 18  are connected to one another; emitter of the transistor Q 18  is connected both to base of the transistor Q 19  and to collector of the transistor Q 19 ; emitter of the transistor Q 19  is connected both to base of the transistor Q 20  and to collector of the transistor Q 20 ; emitter of the transistor Q 20  is connected both to base of the transistor Q 21  and to collector of the transistor Q 21 ; and emitter of the transistor Q 21  is connected to the zero potential reference terminal. The transistor Q 15  and the transistor Q 16  are PNP transistors, and the transistor Q 14 , the transistor Q 17 , the transistor Q 18 , the transistor Q 19 , the transistor Q 20  and the transistor Q 21  are NPN transistors. 
     The low-voltage electronic switching circuit  2  includes a transistor Q 23 , a transistor Q 25 , a transistor Q 26  and a transistor Q 27 . Emitter of the transistor Q 23  is connected to the zero potential reference terminal, collector of the transistor Q 23  is connected both to collector of the transistor Q 25  and to base of the transistor Q 26 ; emitter of the transistor Q 25 , collector of the transistor Q 26  and collector of the transistor Q 27  are all connected to the positive power-supplied terminal; emitter of the transistor Q 26  is connected to base of the transistor Q 27 ; and emitter of the transistor Q 27  is connected to the common terminal for current sampling and low-voltage electronic switching. The transistor Q 25  is a PNP transistor, and the transistor Q 23 , the transistor Q 26  and the transistor Q 27  are NPN transistors. 
     The under-voltage control circuit  3  includes a transistor Q 24  and a transistor Q 22 , wherein emitter of the transistor Q 24  is connected to the positive power-supplied terminal; base of the transistor Q 24 , collector of the transistor Q 24  and collector of the transistor Q 22  are connected to one another; and emitter of the transistor Q 22  is connected to the zero potential reference terminal. The transistor Q 24  is a PNP transistor, whereas the transistor Q 22  is an NPN transistor. 
     The comparing and amplifying circuit  4  includes a transistor Q 1 , a transistor Q 2 , a transistor Q 3 , a transistor Q 4 , a transistor Q 10 , a transistor Q 11 , a transistor Q 5 , a transistor Q 6 , a transistor Q 7 , a transistor Q 8 , a transistor Q 9 , a transistor Q 12 , a transistor Q 13 , a resistor R 1 , a resistor R 2 , a resistor R 3  and a resistor R 5 . Base of the transistor Q 1 , base of the transistor Q 2 , base of the transistor Q 11 , base of the transistor Q 10 , collector of the transistor Q 10  and one end of the resistor R 3  are connected to one another; the other end of the resistor R 3  is connected both to collector of the transistor Q 9  and to base of the transistor Q 9 ; collector of the transistor Q 1 , collector of the transistor Q 2 , emitter of the transistor Q 3  and emitter of the transistor Q 4  are connected to one another; base of the transistor Q 3 , collector of the transistor Q 5  and one end of the resistor R 1  are connected to one another; collector of the transistor Q 3  is connected to the zero potential reference terminal; base of the transistor Q 4 , collector of the transistor Q 6  and one end of the resistor R 2  are connected to one another; emitter of the transistor Q 5 , emitter of the transistor Q 6 , collector of the transistor Q 7  and collector of the transistor Q 8  are connected to one another; base of the transistor Q 5 , collector of the transistor Q 11 , collector of the transistor Q 12  and base of the transistor Q 12  are connected to one another; emitter of the transistor Q 12 , collector of the transistor Q 13  and base of the transistor Q 13  are connected to one another; base of the transistor Q 6  is connected to one end of the resistor R 5 , the other end of which is connected to the common terminal for current sampling and low-voltage electronic switching; base of the transistor Q 7 , base of the transistor Q 8  and base of the transistor Q 9  are connected to one another; and emitter of the transistor Q 13 , emitter of the transistor Q 7 , emitter of the transistor Q 8  and emitter of the transistor Q 9  each are connected to the zero potential reference terminal. The transistor Q 1 , the transistor Q 2 , the transistor Q 3 , the transistor Q 4 , the transistor Q 10  and the transistor Q 11  are all PNP transistors, whereas the transistor Q 5 , the transistor Q 6 , the transistor Q 7 , the transistor Q 8 , the transistor Q 9 , the transistor Q 12  and the transistor Q 13  are all NPN transistors. 
     The above four function circuits are connected in the following manner. Emitter of the transistor Q 14  for stabilizing voltage output inside the voltage stabilizing circuit  1  offers voltage-stabilized output, and the emitter of the transistor Q 14  is connected to each of emitter of the transistor Q 1 , emitter of the transistor Q 2 , emitter of the transistor Q 10  and emitter of the transistor Q 11 , the other end of the resistor R 1  and the other end of the resistor R 2  in the comparing and amplifying circuit  4 . The base and the collector of the transistor Q 21  in the voltage stabilizing circuit  1  are together connected to base of the transistor Q 22  in the under-voltage control circuit  3 . The collector of the transistor Q 4  in the comparing and amplifying circuit  4  is connected to base of the transistor Q 23  in the low-voltage switching control circuit  2 . The other end of the resistor R 5  in the comparing and amplifying circuit  4  is connected both to the emitter of the transistor Q 27  of the low-voltage switching circuit  2  and to the common terminal for current sampling and low-voltage electronic switching. The base and the emitter of the transistor Q 24  in the under-voltage control circuit  3  are connected to each other and then are together connected to base of the transistor Q 25  in the low-voltage electronic switching circuit  2 . 
     The resistor R 1  has a resistance of 50KΩ; the resistor R 2  has a resistance of 50KΩ; the resistor R 3  has a resistance of 50KΩ; the resistor R 4  has a resistance of 300KΩ; and the resistor R 5  has a resistance of 2KΩ. The IC has a working voltage in a range of typically 0V to 60V. The 8-stage-switch-controlled circuit for AC direct driving LEDs by taking power with low-voltage bypass, which comprises the IC of the present invention, has the following main technical specifications: AC input voltage in the range of 185V (rms) to 265V (rms); and driving circuit having efficiency of greater than 94%, PF of greater than 0.96 and THD of less than 25%, under 220V (rms) AC input. 
     Embodiment 4 
       FIG. 4  shows an internal circuit diagram of an IC in accordance with the present invention applying a BiCMOS IC technology. In comparison with the  FIG. 3  of Embodiment 3, the IC shown by  FIG. 4  has the same circuit configuration, except for the transistor Q 27  as Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) in replace of the transistor Q 27  as NPN transistor, and has an internal circuit compatible with the BiCMOS technology. Since drain-to-source turn-on voltage of a MOSFET is typically lower than Collector-Emitter (CE) saturation voltage of a bipolar transistor, this IC of the present invention has higher circuit efficiency, but is fabricated with a relatively complex technology and a higher manufacturing cost. The IC has a working voltage in the range of typically 0V to 60V. The 8-stage-switch-controlled circuit for AC direct driving an LED by taking power with low-voltage bypass, which comprises the ICs of the present invention, has the following main technical specifications: AC input voltage in the range of 185V (rms) to 265V (rms); and efficiency of the driving circuit greater than 96%, PF of greater than 0.96 and THD of less than 25%, under 220V (rms) AC input.