Abstract:
A pulse width modulation (PWM) dimming circuit comprises a switch unit, a current generation unit, a mirror current source, a multi-path output unit, a plurality of current balance units and a plurality of LED lamp strings. The switch unit receives a PWM signal which controls the switch on or off. The current generation unit is connected with the switch unit and generates a current of a predetermined magnitude when the switch unit is on. The mirror current source is connected with the current generation unit and receives the current to generate a mirror current. The multi-path output unit is connected with the mirror current source and receives and output the mirror current in multiple paths. The current balance units are connected between the multi-path output unit and the LED lamp strings respectively. The PWM dimming circuit decreases both the difficulty and the manufacturing cost and is easy to operate.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to pulse width modulation (PWM) dimming technologies, and more particularly, to a PWM dimming circuit. 
         [0003]    2. Description of Related Art 
         [0004]    Currently, there are mainly three kinds of dimming technologies: PWM dimming, analog dimming and digital dimming. Many light emitting diode (LED) drivers on the market can support one or more of the dimming technologies. The PWM dimming is a dimming technology that utilizes simple digital pulses to switch on and off a white-light LED driver repeatedly. By use of the PWM dimming technology, the output current can be changed by simply providing digital pulses of different widths so as to adjust brightness of the white-light LED. 
         [0005]    In conventional LED driving circuits, a balance integrated circuit (IC) is usually used for PWM dimming or analog dimming. Specifically, brightness of the LED is adjusted by using an internal module of the balance IC to adjust a duty ratio of the output current. Usually, the balance IC has a very complex circuit and requires use of a corresponding dimming module in the balance IC. The dimming module mainly functions to multiply an input PWM signal with a driving signal of a MOSFET which is used for current balance so that the MOSFET switches on or off according to the PWM signal to make a duty ratio of the LED consistent with that of the PWM signal. In this way, the PWM dimming or analog dimming can be achieved. However, this adds to both difficulty in design of the circuit and the manufacturing cost. 
       BRIEF SUMMARY 
       [0006]    The primary objective of the present disclosure is to provide a PWM dimming circuit which has a simplified circuit design and reduced manufacturing cost. 
         [0007]    To achieve the aforesaid objective, the present disclosure provides a PWM dimming circuit, including a switch unit, a current generation unit, a mirror current source, a multi-path output unit, a plurality of current balance units and a plurality of LED lamp strings. The switch unit is configured to receive a PWM signal and is controlled to turn on or off by the PWM signal; the current generation unit is connected with the switch unit and is configured to generate a current of a predetermined magnitude when the switch unit is turned on; the mirror current source is connected with the current generation unit and is configured to receive the current generated by the current generation unit and to generate a mirror current; the multi-path output unit is connected with the mirror current source and is configured to receive the mirror current and output the mirror current in multiple paths; the number of the current balance units is equal to that of the LED lamp strings, and the plurality of current balance units are connected between the multi-path output unit and the plurality of LED lamp strings respectively and are configured to adjust respective resistances to keep balance of currents among the LED lamp strings. 
         [0008]    Preferably, the switch unit is a metal oxide semiconductor (MOS) transistor or a relay. 
         [0009]    Preferably, the switch unit is an MOS transistor ( 11 ) which has a gate for receiving the PWM signal, a drain connected with the current generation unit, and a source connected to the ground. 
         [0010]    Preferably, the current generation unit includes a resistor (RSET), an operational amplifier (OP 1 ) and an MOS transistor (Q 1 ); the resistor (RSET) has one end thereof connected to the switch unit and the other end thereof connected to an inverting input terminal of the operational amplifier (OP 1 ) and a source of the MOS transistor (Q 1 ); the operational amplifier (OP 1 ) has a positive input terminal connected to a first reference voltage and an output terminal connected to a gate of the MOS transistor (Q 1 ); and the MOS transistor (Q 1 ) has a drain connected to the mirror current source. 
         [0011]    Preferably, the mirror current source includes an MOS transistor (Q 2 ) and an MOS transistor (Q 3 ); the MOS transistor (Q 2 ) has a drain connected to the drain of the MOS transistor (Q 1 ); the MOS transistor (Q 2 ) has a source and a gate connected to a source and a gate of the MOS transistor (Q 3 ) respectively; the MOS transistor (Q 3 ) has a drain connected to the multi-path output unit; and the drain and the gate of the MOS transistor (Q 2 ) are connected together through a conductor. 
         [0012]    Preferably, the multi-path output unit includes an operational amplifier (OP 2 ), an MOS transistor (Q 4 ) and a plurality of MOS transistors (Q 6 ); an inverting input terminal of the operational amplifier (OP 2 ) and a drain of the MOS transistor (Q 4 ) are connected to the drain of the MOS transistor (Q 3 ) respectively; an output terminal of the operational amplifier (OP 2 ) is connected to gates of the MOS transistor (Q 4 ) and the plurality of MOS transistors (Q 6 ) respectively; a positive input terminal of the operational amplifier (OP 2 ) is connected to a second reference voltage; a source of the MOS transistor (Q 4 ) and sources of the plurality of MOS transistors (Q 6 ) are connected to the source of the MOS transistor ( 11 ) in the switch unit respectively; and the number of the MOS transistors (Q 6 ) is equal to that of the current balance units, and the plurality of MOS transistors (Q 6 ) have respective drains connected to the plurality of current balance units respectively. 
         [0013]    Preferably, each of the current balance units includes an MOS transistor (Q 5 ) and an operational amplifier (OP 3 ); an inverting input terminal of the operational amplifier (OP 3 ) and a source of the MOS transistor (Q 5 ) are connected to the drain of one of the MOS transistors (Q 6 ), and an output terminal of the operational amplifier (OP 3 ) is connected to a gate of the MOS transistor (Q 5 ); and a positive input terminal of the operational amplifier (OP 3 ) is connected to the second reference voltage, and a drain of the MOS transistor (Q 5 ) is connected to one of the LED lamp strings. 
         [0014]    Preferably, the MOS transistors ( 11 ), (Q 1 ), (Q 2 ), (Q 3 ), (Q 4 ), (Q 5 ) and (Q 6 ) are all NMOS transistors. 
         [0015]    Preferably, the first reference voltage is 1.2 V and the second reference voltage is 0.3 V. 
         [0016]    Preferably, the PWM dimming circuit is an integrated circuit (IC), and the source of the MOS transistor (Q 1 ) is a reset terminal of the IC. 
         [0017]    The present disclosure further provides a PWM dimming circuit, including a switch unit, a current generation unit, a mirror current source, a multi-path output unit, a plurality of current balance units and a plurality of LED lamp strings. The switch unit is an MOS transistor ( 11 ) which has a gate for receiving a PWM signal, a drain connected with the current generation unit, and a source connected to the ground, and the switch unit is controlled to turn on or off by the PWM signal; the current generation unit is connected with the switch unit and is configured to generate a current of a predetermined magnitude when the switch unit is turned on; the mirror current source is connected with the current generation unit and is configured to receive the current generated by the current generation unit and to generate a mirror current; the multi-path output unit is connected with the mirror current source and is configured to receive the mirror current and output the mirror current in multiple paths; the number of the current balance units is equal to that of the LED lamp strings, and the plurality of current balance units are connected between the multi-path output unit and the plurality of LED lamp strings respectively and are configured to adjust respective resistances to keep balance of currents among the LED lamp strings. 
         [0018]    Preferably, the current generation unit includes a resistor (RSET), an operational amplifier (OP 1 ) and an MOS transistor (Q 1 ); the resistor (RSET) has one end thereof connected to the switch unit and the other end thereof connected to an inverting input terminal of the operational amplifier (OP 1 ) and a source of the MOS transistor (Q 1 ); the operational amplifier (OP 1 ) has a positive input terminal connected to a first reference voltage and an output terminal connected to a gate of the MOS transistor (Q 1 ); and the MOS transistor (Q 1 ) has a drain connected to the mirror current source. 
         [0019]    Preferably, the mirror current source includes an MOS transistor (Q 2 ) and an MOS transistor (Q 3 ); the MOS transistor (Q 2 ) has a drain connected to the drain of the MOS transistor (Q 1 ); the MOS transistor (Q 2 ) has a source and a gate connected to a source and a gate of the MOS transistor (Q 3 ) respectively; the MOS transistor (Q 3 ) has a drain connected to the multi-path output unit; and the drain and the gate of the MOS transistor (Q 2 ) are connected together through a conductor. 
         [0020]    Preferably, the multi-path output unit includes an operational amplifier (OP 2 ), an MOS transistor (Q 4 ) and a plurality of MOS transistors (Q 6 ); an inverting input terminal of the operational amplifier (OP 2 ) and a drain of the MOS transistor (Q 4 ) are connected to the drain of the MOS transistor (Q 3 ) respectively; an output terminal of the operational amplifier (OP 2 ) is connected to gates of the MOS transistor (Q 4 ) and the plurality of MOS transistors (Q 6 ) respectively; a positive input terminal of the operational amplifier (OP 2 ) is connected to a second reference voltage; a source of the MOS transistor (Q 4 ) and sources of the plurality of MOS transistors (Q 6 ) are connected to the source of the MOS transistor ( 11 ) in the switch unit respectively; and the number of the MOS transistors (Q 6 ) is equal to that of the current balance units, and the plurality of MOS transistors (Q 6 ) have respective drains connected to the plurality of current balance units respectively. 
         [0021]    Preferably, each of the current balance units includes an MOS transistor (Q 5 ) and an operational amplifier (OP 3 ); an inverting input terminal of the operational amplifier (OP 3 ) and a source of the MOS transistor (Q 5 ) are connected to the drain of one of the MOS transistors (Q 6 ), and an output terminal of the operational amplifier (OP 3 ) is connected to a gate of the MOS transistor (Q 5 ); and a positive input terminal of the operational amplifier (OP 3 ) is connected to the second reference voltage, and a drain of the MOS transistor (Q 5 ) is connected to one of the LED lamp strings. 
         [0022]    Preferably, the MOS transistors ( 11 ), (Q 1 ), (Q 2 ), (Q 3 ), (Q 4 ), (Q 5 ) and (Q 6 ) are all NMOS transistors. 
         [0023]    Preferably, the first reference voltage is 1.2 V and the second reference voltage is 0.3 V. 
         [0024]    Preferably, the PWM dimming circuit is an integrated circuit (IC), and the source of the MOS transistor (Q 1 ) is a reset terminal of the IC. 
         [0025]    According to the PWM dimming circuit of the present disclosure, dimming of an LED lamp string is achieved through use of a simple circuit without having to use the dimming module as in the prior art. This decreases both the difficulty in design of the circuit and the manufacturing cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a schematic structural view of a first embodiment of a PWM dimming circuit according to the present disclosure; and 
           [0027]      FIG. 2  is a detailed circuit diagram of the PWM dimming circuit of the first embodiment. 
       
    
    
       [0028]    Hereinafter, implementations, functional features and advantages of the present disclosure will be further described with reference to embodiments thereof and the attached drawings. 
       DETAILED DESCRIPTION 
       [0029]    It shall be understood that, the embodiments described herein are only intended to illustrate but not to limit the present disclosure. 
         [0030]    Referring to  FIG. 1 , there is shown a schematic structural view of a PWM dimming circuit  10  of a first embodiment according to the present disclosure. The PWM dimming circuit  10  includes a switch unit  11 , a current generation unit  12 , a mirror current source  13 , a multi-path output unit  14 , a plurality of current balance units  15  and a plurality of LED lamp strings  16 . The switch unit  11  is configured to receive a PWM signal and is controlled to turn on or off by the PWM signal so as to adjust a duty ratio of the current passing through an LED lamp string  16 . The current generation unit  12  is connected with the switch unit  11  and is configured to generate a current of a predetermined magnitude when the switch unit  11  is turned on. The mirror current source  13  is connected with the current generation unit  12  and is configured to receive the current generated by the current generation unit  12  and to generate a mirror current. The multi-path output unit  14  is connected with the mirror current source  13  and is configured to receive the mirror current and output the mirror current in multiple paths. The number of the current balance units  15  is equal to that of the LED lamp strings  16 . The plurality of current balance units  15  are connected between the multi-path output unit  14  and the plurality of LED lamp strings  16  respectively and are configured to adjust respective resistances to keep balance of currents among the LED lamp strings  16 . 
         [0031]    The PWM dimming circuit  10  can be used in an LED driving circuit and is connected to a PWM signal generation circuit (not shown). By adjusting a duty ratio of the PWM signal, the PWM dimming circuit  10  can adjust a duty ratio of the current generated by the current generation unit  12  so as to adjust a duty ratio of the mirror current. Consequently, a duty ratio of the current passing through the LED lamp strings  16  can be adjusted to achieve the purpose of PWM dimming 
         [0032]    Referring to  FIG. 2 , there is shown a detailed circuit diagram of the PWM dimming circuit of the first embodiment. The switch unit  11  is an MOS transistor  11  in this first embodiment. However, in other embodiments, the switch unit  11  may also be other switching devices such as a relay. The MOS transistor  11  has a gate for receiving the PWM signal, a drain connected with the current generation unit  12 , and a source connected to the ground. The MOS transistor  11  may be an NMOS transistor. 
         [0033]    The current generation unit  12  includes a resistor RSET, an operational amplifier OP 1  and an MOS transistor Q 1 . The resistor RSET has one end thereof connected to the drain of the MOS transistor  11  and the other end thereof connected to an inverting input terminal of the operational amplifier OP 1  and a source of the MOS transistor Q 1 . The operational amplifier OP 1  has a positive input terminal connected to a first reference voltage and an output terminal connected to a gate of the MOS transistor Q 1 . The first reference voltage is 1.2 V. The MOS transistor Q 1  has a drain connected to the mirror current source  13 . 
         [0034]    The mirror current source  13  includes an MOS transistor Q 2  and an MOS transistor Q 3 . The MOS transistor Q 2  has a drain connected to the drain of the MOS transistor Q 1 . The drain and a gate of the MOS transistor Q 2  are connected together through a conductor. A source and the gate of the MOS transistor Q 2  are connected to a source and a gate of the MOS transistor Q 3  respectively. The MOS transistor Q 3  has a drain connected to the multi-path output unit  14 . 
         [0035]    The multi-path output unit  14  includes an operational amplifier OP 2 , an MOS transistor Q 4  and a plurality of MOS transistors Q 6 . An inverting input terminal of the operational amplifier OP 2  and a drain of the MOS transistor Q 4  are connected to the drain of the MOS transistor Q 3  respectively. An output terminal of the operational amplifier OP 2  is connected to gates of the MOS transistor Q 4  and the plurality of MOS transistors Q 6  respectively. A positive input terminal of the operational amplifier OP 2  is connected to a second reference voltage. The second reference voltage may be 0.3 V. A source of the MOS transistor Q 4  and sources of the plurality of MOS transistors Q 6  are connected to the source of the MOS transistor  11 . The number of the MOS transistors Q 6  is equal to that of the current balance units  15 , and the plurality of MOS transistors Q 6  have respective drains connected to the plurality of current balance units  15  respectively. 
         [0036]    Each of the current balance units  15  includes an MOS transistor Q 5  and an operational amplifier OP 3 . An inverting input terminal of the operational amplifier OP 3  and a source of the MOS transistor Q 5  are connected to the drain of one of the MOS transistors Q 6 , and an output terminal of the operational amplifier OP 3  is connected to a gate of the MOS transistor Q 5 ; and a positive input terminal of the operational amplifier OP 3  is connected to the second reference voltage. A drain of the MOS transistor Q 5  is connected to one of the LED lamp strings  16 . The MOS transistors ( 11 ), Q 1 , Q 2 , Q 3 , Q 4 , Q 5  and Q 6  are all NMOS transistors. 
         [0037]    Each of the LED lamp strings  16  has an anode which is used to receive an input voltage Vin and a cathode which is connected to one of the current balance units  15 . 
         [0038]    The PWM dimming circuit  10  may be an integrated circuit (IC), and the source of the MOS transistor Q 1  is a reset terminal of the IC. 
         [0039]    The operational principle of the PWM dimming circuit is as follows. 
         [0040]    The MOS transistor  11  is turned on when the PWM signal is at a high level, and then the positive input terminal and the inverting input terminal of the operational amplifier OP 1  in the current generation unit  12  are approximately short circuited because of the virtual short-circuit characteristics between the positive input terminal and the inverting input terminal. Therefore, the voltages of the inverting input terminal of the operational amplifier OP 1  and the source of the MOS transistor Q 1  may be regarded as 1.2 V. In addition, the voltage difference between the gate and the source of the MOS transistor Q 1  meet the conditions for turning on the MOS transistor Q 1 , so the MOS transistor Q 1  is turned on. The magnitude of the current flowing through the MOS transistor Q 1  is 1.2 V/Rset (Rset is the resistance value of the resistor RSET). The MOS transistor Q 2  and the MOS transistor Q 3  form the mirror current source  13 , which makes the magnitude of the current flowing through the MOS transistor Q 1  the same as that of the current flowing through the MOS transistor Q 4 . Furthermore, as both the positive input terminals of the operational amplifiers OP 2  and OP 3  are connected to the reference voltage of 0.3 V, voltages of the inverting input terminals of the operational amplifiers OP 2  and OP 3  are also 0.3 V respectively because of the virtual short-circuit characteristics between the positive input terminal and the inverting input terminal. In addition, as the drains of the MOS transistors Q 4  and Q 6  are connected to the inverting input terminals of the operational amplifiers OP 2  and OP 3  respectively, the voltage of the drain of the MOS transistor Q 4  is the same as that of the MOS transistor Q 6 . Furthermore, because a gate voltage and a source voltage of the MOS transistor Q 4  are the same as those of the MOS transistor Q 6 , both impedances of and the magnitudes of the currents flowing through the MOS transistors Q 4  and Q 6  are the same as each other. 
         [0041]    The MOS transistor Q 5  of each of the current balance units  15  serves as a resistor, and the MOS transistor Q 5  and one of the MOS transistors Q 6  of the multi-path output unit  14  are connected in series to form a voltage divider for the LED lamp string  16 . As a turn-on voltage drop Vf of each of the LED lamp strings  16  may be different from each other, the voltage of each of the current balance units  15  may be different because of the voltage drop of the same input voltage Vin across each of the LED lamp strings  16 . However, the source voltage of the MOS transistor Q 5  of each of the current balance units  15  is 0.3 V, so the operational amplifier OP 3  will automatically adjust the resistor of the MOS transistor Q 5  of each of the current balance units  15  by controlling the voltage of the output terminal thereof so that all the currents flowing through the LED lamp strings  16  connected in parallel are the same. 
         [0042]    The MOS transistor  11  is turned off when the PWM signal is at a low level, and then no current will flow through the MOS transistor Q 1 . Likewise, the MOS transistor Q 4  and the MOS transistor Q 6  are turned off, so the LED lamp strings  16  will not emit light. Therefore, the PWM dimming circuit can adjust the duty ratio of the current flowing through each of the LED lamp strings  16  by adjusting the duty ratio of the PWM signal. In this way, the PWM dimming is achieved. 
         [0043]    According to the PWM dimming circuit  10 , dimming of an LED lamp string is achieved through use of a simple circuit without having to use the dimming module as in the prior art. This decreases both the difficulty in design of the circuit and the manufacturing cost. If the PWM dimming circuit  10  is packaged into an integrated circuit, volume of the PWM dimming circuit  10  will also be reduced to make it more convenient for use. 
         [0044]    What described above are only preferred embodiments of the present disclosure but are not intended to limit the scope of the present disclosure. Accordingly, any equivalent structural or process flow modifications that are made on basis of the specification and the attached drawings or any direct or indirect applications in other technical fields shall also fall within the scope of the present disclosure.