Abstract:
An exemplary embodiment of the invention provides a set of an initial value of an error amplifying signal in the feedback control circuit for feedback control, so as to reduce the time and the amplitude of oscillation of the error amplifying signal. Accordingly, a feedback control circuit and a power converting circuit provided in an exemplary embodiment of the invention not only reduce the degree and the time of overshoot but also provide accurate and stable feedback control.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority benefit of China application serial no. 201010170215.X, filed on May 11, 2010. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a feedback control circuit and a power converting circuit, and particularly to a feedback control circuit and a power converting circuit capable of reducing overshoot. 
         [0004]    2. Description of Related Art 
         [0005]      FIG. 1  is a schematic view of a conventional light emitting diode (LED) driving circuit. Referring to  FIG. 1 , the LED driving circuit includes a controller  10 , a converting circuit  50 , and an LED module  60 . The converting circuit  50  is coupled to an input voltage source Vin. The controller  10  generates a control signal Sc to control the converting circuit  50  to transmit an electric power from the input voltage source Vin to an output end. The output end of the converting circuit  50  is coupled to the LED module  60  to apply an output voltage Vout to the LED module  60 , such that the LED module  60  emits light due to an output current Iout flowing through the LED module  60 . The output current Iout also flows through a current detecting resistor  65  to generate a current feedback signal IFB. 
         [0006]    The controller  10  includes an error amplifier  11 , a ramp generator  12 , an error compensating circuit  13 , a pulse width modulation (PWM) comparator  18 , and a driving circuit  19 . The error amplifier  11  receives the current feedback signal IFB and a reference signal Vr and accordingly generates an output signal. After the error compensating circuit  13  compensates the output signal, the output signal becomes an error amplifying signal Vcomp. The ramp generator  12  generates a ramp signal to the PWM comparator  18 . The PWM comparator  18  also receives error amplifying signal Vcomp and accordingly generates a PWM signal to the driving circuit  19 . 
         [0007]    Generally, the controller  10  stabilizes the output current Iout at a predetermined output current Io, and at this time, the output voltage Vout is also stabilized at a predetermined output voltage Vo. However, before output current Iout and output voltage Vout being stabled, the level of the error amplifying signal Vcomp is adjusted by the error amplifier  11  comparing the current feedback signal IFB and the reference signal Vr, and the error compensating circuit  13 , compensating the output of the error amplifier  11 . During the feedback control process, the output current Iout and the output voltage Vout respectively oscillate about the predetermined output current Io and the predetermined output voltage Vo and gradually approximate thereto, i.e. the amplitudes of oscillation become small. 
         [0008]      FIG. 2  illustrates signal waveforms of the LED driving circuit shown in  FIG. 1  during dimming process. The driving circuit  19  receives a dimming signal DIM and determines whether to output the control signal Sc according to the dimming signal DIM. During the period from the time point T 1  to the time point T 4 , the dimming signal DIM represents “ON”, and at this time, the driving circuit  19  outputs the control signal Sc; and during the period from the time point T 4  to the time point T 1 , the dimming signal DIM represents “OFF”, and at this time, the driving circuit  19  stops outputting the control signal Sc. During the period from the time point T 4  to the time point T 1 , because the driving circuit  19  stops outputting the control signal Sc, the converting circuit  50  stops transmitting the electric power to the LED module  60 . As a result, the output voltage Vout gradually falls down to the threshold voltage Vf of the LED module  60  at the time point T 5 , and at this time, the output current Iout also falls down to zero. It causes the current feedback signal IFB and the reference signal Vr to maintain a positive error, such that the level of the error amplifying signal Vcomp is raised up to a maximum level. At time point T 1 , when the driving circuit  19  outputs the control signal Sc again, because the level of the error amplifying signal Vcomp is at the maximum level, the duty cycle of the control signal Sc is also maximum. 
         [0009]    After the time point T 2 , the output current Iout is higher than the predetermined output current Io, such that the error amplifier  11  starts to pull down the level of the error amplifying signal Vcomp. However, due to the error compensation of the error compensating circuit  13 , the error amplifying signal Vcomp can not directly fall down to an error stable value Vcompo. This value corresponds to the level of the error amplifying signal Vcomp when the output current Iout is stabilized at the predetermined output current Io. It causes the duty cycle of the control signal Sc is over large at this time. Accordingly, the output current Iout is still raised up until the error amplifying signal Vcomp is lower than the error stable value Vcompo, such that the duty cycle of the control signal Sc is over small. Next, the output current Iout is lower than the predetermined output current Io again, such that the error amplifying signal Vcomp is raised up and higher than the error stable value Vcompo again. The foregoing process proceeds until the time point T 3 , and the output current Iout, the output voltage Vout, and the error amplifying signal Vcomp respectively converge on the predetermined output current Io, the predetermined output voltage Vo, and the error stable value Vcompo. 
         [0010]    Accordingly, when the LED module starts or burst dimming is performed, an obvious and serious overshoot phenomenon occurs in the output current Iout and the output voltage Vout. An over large overshoot phenomenons occurring in current and voltage cause the LEDs immediately emit light with over high brightness, so as to affect human eyes. Besides the stability of the circuit is lowered, the lifespan of the LEDs is also reduced, and the probability of the circuit or the LEDs being burnt down is increased. 
       SUMMARY OF THE INVENTION 
       [0011]    In the foregoing related art, the serious overshoot phenomenon due to the error compensation of feedback control reduces the stability of the circuit and increases the probability of the circuit being burnt down. Accordingly, an exemplary embodiment of the invention provides a set of an initial value of an error amplifying signal in the feedback control circuit for feedback control, so as to reduce the time and the amplitude of oscillation of the error amplifying signal. Accordingly, a feedback control circuit and a power converting circuit provided in an exemplary embodiment of the invention not only reduce the degree and the time of overshoot but also provide accurate and stable feedback control. 
         [0012]    An exemplary embodiment of the invention provides a feedback control circuit adapted to control a converting circuit to convert an input voltage into an output voltage to drive a load. The feedback control circuit includes a feedback unit, an integrating unit, a pulse width control unit, a first switch, and a level setting unit. The feedback unit receives a feedback signal and a reference signal and accordingly generating an error signal, wherein the feedback signal represents a state of the load. The integrating unit is coupled to the feedback unit to generate an integration signal according to the error signal. The pulse width control unit generates a control signal according to the integration signal and accordingly controls the converting circuit to convert the input voltage into the output voltage. The first switch is coupled between the feedback unit and the integrating unit and controls the error signal to be transmitted to the integrating unit. The level setting unit is coupled to the integrating unit, determines a set level according to the integration signal when the first switch is conducted, and adjusts a level of the integration signal to the set level when the first switch is cut off. 
         [0013]    Another exemplary embodiment of the invention provides a power converting circuit adapted to drive a light emitting diode module. The power converting circuit includes a converting circuit and a controller. The converting unit is coupled to the LED module and converts an electric power of a power source to drive the LED module. The controller performs a feedback control to generate a pulse width modulation signal (PWM signal) according to a feedback signal representing the amount of a current flowing through the LED module, so as to control the power converting circuit to stabilize the current at a predetermined current value. Herein, the controller receives a dimming signal, stops the converting circuit converting the electric power of the power source when the dimming signal is in a first state, and starts to perform the feedback control to modulate a duty cycle of the PWM signal starting from a predetermined duty cycle when the dimming signal is in a second state. 
         [0014]    It is to be understood that both the foregoing general descriptions and the following detailed descriptions are exemplary, and are intended to provide further explanation of the invention as claimed. In order to make the features and the advantages of the invention comprehensible, exemplary embodiments accompanied with figures are described in detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
           [0016]      FIG. 1  is a schematic view of a conventional LED driving circuit. 
           [0017]      FIG. 2  illustrates signal waveforms of the LED driving circuit shown in  FIG. 1  during dimming process. 
           [0018]      FIG. 3  is a schematic view of a power converting circuit according to a first embodiment of the invention. 
           [0019]      FIG. 4  is a schematic circuit of a level setting unit according to an embodiment of the invention. 
           [0020]      FIG. 5  illustrates signal waveforms of the circuits shown in  FIG. 3  and  FIG. 4  during dimming process. 
           [0021]      FIG. 6  is a schematic view of a power converting circuit according to a second embodiment of the invention. 
           [0022]      FIG. 7  illustrates signal waveforms of the power converting circuit shown in  FIG. 7  during dimming process. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0023]      FIG. 3  is a schematic view of a power converting circuit according to a first embodiment of the invention. Referring to  FIG. 3 , the power converting circuit includes a controller  100  and a converting circuit  150  to drive an LED module  160 . The controller  100  receives a feedback signal FB and performs feedback control to generate a control signal Sc to control the converting circuit  150 . The input end of the converting circuit  150  is coupled to an input voltage source Vin, and the output end thereof is coupled to the LED module  160 . The converting circuit  150  adjusts the value of the electric power from the input voltage source Vin according to the control signal Sc and converts it into a suitable output voltage Vout to drive the LED module, such that an output current Iout flowing through the LED module is stabilized at a predetermined current value. The output current Iout also flows through a current detecting resistor  165  to generate the current feedback signal FB representing the amount of the output current Iout. 
         [0024]    The controller  100  includes a feedback unit  111 , an integrating unit  113 , a first switch  114 , a level setting unit  115 , and a pulse width control unit  116 . The feedback unit  111  may be an error amplifier. The non-inverting input end receives a reference signal Vr, and the inverting input end receives the feedback signal FB. The feedback unit  111  accordingly generates an error signal. The integrating unit  113  generates an integration signal Vea according to the error signal and generally includes a capacitor and a resistor. In practice, the relationship of the voltage gain of the integrating unit  113  versus the frequency is adjusted to have a better transient response in difference circuit design. The first switch  114  is coupled between the feedback unit  111  and the integrating unit  113  and controls the error signal to be transmitted to the integrating unit  113  according to a dimming signal DIM. When the state of the dimming signal DIM represents “ON”, the first switch  114  is conducted. The error signal generated by the feedback unit  111  is transmitted to the integrating unit  113  through the first switch  114 . When the state of the dimming signal DIM represents “OFF”, the first switch  114  is cut off. The error signal generated by the feedback unit  111  stops being transmitted to the integrating unit  113 . The level setting unit  115  is coupled to the integrating unit  113 . When the first switch  114  is conducted, the level setting unit  115  determines a set level Vset according to the integration signal Vea, and when the first switch  114  is cut off, the level setting unit  115  adjusts the level of the integration signal  114  to the set level Vset. 
         [0025]    The pulse width control unit  116  generates a control signal Sc according to the integration signal Vea and accordingly controls the converting circuit  150  to perform the voltage conversion. The pulse width control unit  116  includes a ramp generator  112 , a PWM comparator  118 , and a driving circuit  119 . The ramp generator  112  generates a ramp signal to the inverting input end of the PWM comparator  118 , and the non-inverting input end of the PWM comparator  118  receives the integration signal Vea. The PWM comparator  118  accordingly generates a PWM signal to the driving circuit  119 . The driving circuit  119  also receives the dimming signal DIM. When the state of the dimming signal DIM represents “ON”, the driving circuit  119  generates the control signal Sc according to the PWM signal of the PWM comparator  118 ; and when the state of the dimming signal DIM represents “OFF”, the driving circuit  119  stops generating the control signal Sc. 
         [0026]      FIG. 4  is a schematic circuit of a level setting unit according to an embodiment of the invention. Referring to  FIG. 4 , the level setting unit includes a voltage divider  101 , a first amplifier  102 , a third switch  103 , a voltage storage element  104 , a second amplifier  105 , an inverter  106 , and a second switch  107 . Referring to  FIG. 3  and  FIG. 4 , when the state of the dimming signal DIM represents “ON”, the first switch  114  is conducted, such that the integration signal Vea is inputted through a connecting point Se to the non-inverting input end of the first amplifier  102  after being divided by the voltage divider  101 . The inverting input end of the first amplifier  102  is coupled to the output end thereof, and the output end thereof is also coupled to the voltage storage element  104 . The voltage storage element  104  may be a capacitor coupled to the first amplifier  102  through the third switch  103 . The third switch  103  is controlled by the dimming signal DIM and conducted when the state of the dimming signal DIM represents “ON”, such that the voltage storage element  104  and the first amplifier  102  form a voltage follower to store the level of the integration signal Vea which has been divided by the voltage divider  101  in the voltage storage element  104  to form the set level Vset. At the same time, the dimming signal DIM which has been inverted by the inverter  106  controls the second switch  107  such that the second switch  107  is cut off. 
         [0027]    When the state of the dimming signal DIM represents “OFF”, the first switch  114  and the third switch  103  are cut off, the voltage storage element  104  has stored the set level Vset by referring to the integration signal Vea and before. The dimming signal DIM which has been inverted by the inverter  106  controls the second switch  107  to be conducted. At this time, the output end of the second amplifier  105  is coupled to the connecting point Se, i.e. coupled to the integrating unit  113 , the output end of the second amplifier  105  is also coupled to the inverting input end thereof, and the non-inverting input end thereof is coupled to the voltage storage element  104  to receive the set level Vset. Accordingly, the second amplifier  105  adjusts the level of the integration signal Vea to be the same as the set level Vset. 
         [0028]    In the present embodiment, the voltage storage element  104  sets the set level Vset according to the level of the integration signal Vea which has been divided by the voltage divider  101 . Accordingly, the set level Vset is lower than the level of the integration signal Vea. In practice, the ratio of the set level Vset and the level of the integration signal Vea can approximate to the value 1, e.g. 1.2 or 0.8, and it will not affect the advantage of the invention. 
         [0029]      FIG. 5  illustrates signal waveforms of the circuits shown in  FIG. 3  and  FIG. 4  during dimming process. In this embodiment, the set level Vset is lower than the level of the integration signal Vea, for example. At the time point t 1 , the dimming signal DIM just changes from the low level representing “OFF” to the high level representing “ON”. At this time, the output voltage Vout is rising from the threshold voltage Vf, the output current Iout is rising from zero, and the integration signal Vea is rising from the set level Vset. Accordingly, the duty cycle of the control signal Sc starts to perform feedback control from a predetermined duty cycle (corresponding to the set level Vset). At the time point t 2 , the output current Iout rises to the predetermined output current Io, and the integration signal Vea rises to a peak value at this time. Because the integration signal Vea is raised up from the set level Vset instead of the maximum of the integration signal Vea, like in the foregoing related art, the peak value is, ib general, lower than the maximum of the integration signal Vea, ie, the driving voltage applied to the error amplifier  111 . Accordingly, the output voltage Vout, the output current Iout, and the integration signal Vea can become stable faster compared with those in the foregoing related art. At the time point t 3 , the dimming signal DIM just changes from the high level representing “ON” to the low level representing “OFF”. The level of the integration signal Vea is then adjusted to be the same as the set level Vset, and the control signal Sc stops being generated such that the converting circuit stops transmitting the electric power. At this time, the output voltage Vout and the output current Iout start to fall down. At the time point t 4 , the output voltage Vout falls down to the threshold voltage Vf, and the output current Iout also falls down to zero at this time. 
         [0030]      FIG. 6  is a schematic view of a power converting circuit according to a second embodiment of the invention. Referring to  FIG. 6 , compared with that of the embodiment of  FIG. 3 , the power converting circuit further includes a driving switch  170  coupled to the LED module  160  in the present embodiment.  FIG. 7  illustrates signal waveforms of the power converting circuit shown in  FIG. 7  during dimming process. Referring to  FIG. 6  and  FIG. 7 , when the state of the dimming signal DIM represents “ON”, the driving switch  170  is conducted, the operation of the power converting circuit is the same as that of the circuit shown in  FIG. 3 . When the state of the dimming signal DIM represents “OFF”, the driving switch  170  is cut off, such that the output current Iout can not flow to the ground through the current detecting resistor  165 . That is, the path of which the converting circuit  150  provides the electric power to the LED module  160  is cut off. Accordingly, the level of the output voltage Vout can still maintain at that of the predetermined output voltage Vo. When the dimming signal DIM changes to the level representing “ON” in the next period, the level of the output voltage Vout becomes stable faster. 
         [0031]    As the above description, the invention completely complies with the patentability requirements: novelty, non-obviousness, and utility. It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the invention without departing from the scope or spirit of the invention. In view of the foregoing descriptions, it is intended that the invention covers modifications and variations of this invention if they fall within the scope of the following claims and their equivalents.