Abstract:
The present invention discloses a driver circuit and a method for driving a load circuit. The driver circuit includes: a primary side circuit receiving rectified AC power; a transformer coupled to the primary side circuit and converting a primary voltage to a secondary voltage which is supplied to a load circuit; and a secondary side circuit coupled to the transformer, the secondary side circuit detecting current flowing through the load circuit and feedback controlling the primary side circuit accordingly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a driver circuit and a method for driving a load circuit; particularly, it relates to a light emitting diode (LED) driver circuit and a method for driving LEDs, which require less number of circuit devices and are able to control LED brightness from the AC side. 
     2. Description of Related Art 
     Referring to  FIG. 1 , conventionally, to provide power to an LED circuit from an AC power supply, it requires an AC-DC power regulator  10  to convert an AC voltage to a DC voltage, and an LED driver circuit  20  to provide electrical power to the LED circuit  50  and control current through the LEDs. Besides a transformer, the AC-DC power regulator  10  further comprises a primary side circuit  11 , a secondary side circuit  12 , and other discrete devices such as a capacitor C 2 , a diode D 2 , etc. The secondary side circuit  12  detects the output voltage and provides a feedback signal to the primary side circuit  11  by means of opto-coupling to control the operation of a power switch P in the primary side circuit  11 . 
     The aforementioned prior art has the following drawbacks. Because it requires the AC-DC power regulator  10  to generate a regulated voltage and the LED driver circuit  20  to control current through the LED circuit  50  according to the regulated voltage, the prior art circuitry needs at least three integrated circuit (IC) chips: the primary side circuit  11 , the secondary side circuit  12 , and the LED driver circuit  20 ; this is not cost-effective. Besides, there is not a method to control the LED brightness directly from the AC side. 
     In view of the above drawbacks, it is desired to provide a driver circuit and a method for driving a load circuit. 
     SUMMARY OF THE INVENTION 
     The first objective of the present invention is to provide a driving circuit, which for example can be applied to driving an LED circuit. 
     The second objective of the present invention is to provide a method for driving a load. 
     To achieve the objectives mentioned above, from one perspective, the present invention provides a driver circuit comprising: a primary side circuit receiving rectified AC power; a transformer coupled to the primary side circuit and converting a primary voltage to a secondary voltage which is supplied to a load circuit; a secondary side circuit coupled to the transformer, the secondary side circuit detecting current flowing through the load circuit and feedback-controlling the primary side circuit accordingly; a first capacitor coupled to the secondary side circuit for providing an operation voltage to the secondary side circuit; and a second capacitor coupled to the load circuit for providing an operation voltage to the load circuit and an optocoupler device, wherein the primary side circuit has an enable input for receiving an input duty signal, and when the input duty signal dose not enable the primary side circuit, the first capacitor still provides the operation voltage to the secondary side circuit at least for a predetermined period. 
     In one embodiment of the aforementioned driver circuit, the duty signal is generated by an AC signal extraction and conversion circuit according to a TRIAC signal. The duty signal can be used to control current flowing through the load circuit. 
     From another perspective, the present invention provides a method for driving a load circuit comprising: providing a primary side circuit receiving rectified AC power; providing a transformer coupled to the primary side circuit and converting a primary voltage to a secondary voltage which is supplied to the load circuit; providing a secondary side circuit coupled to the transformer, the secondary side circuit supplying the secondary voltage to the load circuit; enabling the primary side circuit according a duty signal; and keeping the secondary side circuit in an operation mode at least for a predetermined period when the duty signal does not enable the primary side circuit. 
     In one embodiment of the aforementioned method, the step of keeping the secondary side circuit in an operation mode includes: coupling the secondary side circuit with a first capacitor; and coupling the load circuit and an optocoupler device with a second capacitor. 
     In the aforementioned method, the capacitance of the second capacitor can be adjusted according to the damping status when driving the load circuit. 
     The objectives, technical details, features, and effects of the present invention will be better understood with regard to the detailed description of the embodiments below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art circuitry which includes an AC-DC power regulator  10  to convert an AC voltage to a DC voltage, and an LED driver circuit  20  to provide electrical power to an LED circuit  50 . 
         FIG. 2  shows a first embodiment of the present invention. 
         FIG. 3  shows a second embodiment of the present invention. 
         FIG. 4  explains that the precision of average brightness of the LED circuit  50  is lower than desired. 
         FIG. 5  shows one method of the present invention to solve the problem of the lower precision of average brightness of the LED circuit  50 . 
         FIG. 6  to  FIG. 10  show several other embodiments of the present invention. 
         FIG. 11  and  FIG. 12  show two embodiments of generating an EN signal according to a TRIAC signal. 
         FIG. 13  shows the relationship between the two capacitors C 3  and C 4  and the damping status of the circuit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  shows a first embodiment of the present invention. In this embodiment, it is not required to provide two IC chips, the secondary side circuit  12  and the LED driver circuit  20 , as in the prior art. As shown in  FIG. 2 , the driver circuit of this embodiment comprises: a primary side circuit  11  receiving rectified AC power; a transformer  13  coupled to the primary side circuit  11  and converting a primary voltage to a secondary voltage; and a secondary side circuit  32  coupled to the transformer  13  for providing a secondary side voltage to the load circuit  50 . The load circuit  50  is shown to be an LED circuit for example, but it can be any other circuit which requires current control. In this embodiment, the secondary side circuit  32  directly detects current through the load circuit  50 , and generates a feedback signal thereby; the feedback signal is sent to the primary side circuit  11  by means of opto-coupling. There are various ways to detect current through the LED circuit  50 ; as one example,  FIG. 2  shows that a resist R can be provided and connected with the LED circuit in series, and the voltage across the resist R is an indicator of the current through the LED circuit  50 . By comparing the voltage difference between two ends of the resist R, information related to the current through the LED circuit  50  can be obtained. 
     The embodiment of  FIG. 2  further comprises an AC signal extraction and conversion circuit (TRIAC/Duty)  40  for generating a duty signal according to an inputted AC signal. With minor circuit modification, the AC signal extraction and conversion circuit (TRIAC/Duty)  40  can extract AC signal from the node either before or after the Bridge Rectifier. (TRIAC: TRIode for Alternating Current, which means to extract a portion of an AC signal and generate a positive truncated semi-sinusoidal wave; for more details, please refer to  FIGS. 11 and 12 ). The duty signal outputted from the TRIAC/Duty  40  is inputted to an enable input terminal EN of the primary side circuit  11 , to serve as a dimming signal for adjusting the brightness of the LED. More specifically, when the TRIAC signal is ON such that the duty signal received by the input terminal EN is at high level or ON, the primary side circuit  11  is enabled, and the circuit supplies electrical power to the LEDs; when the TRIAC signal is OFF such that the duty signal received by the input terminal EN is at low level or OFF, the primary side circuit  11  is disabled, and the LEDs are OFF. As such, the duty ratio of the duty signal controls the average current through the LEDs, that is, the brightness of the LED (what human eyes observed is the average brightness). 
       FIG. 11  and  FIG. 12  show two examples as to how the AC signal extraction and conversion circuit  40  generates the duty signal according to the TRIAC signal. An AC signal is processed to obtain the TRIAC signal as shown in  FIG. 11 ; the TRIAC signal is a truncated AC signal, or a signal with a positive correlation to the truncated AC signal, such as a voltage-divident signal thereof. The AC signal extraction and conversion circuit  40  for example includes a comparator, which compares the TRIAC signal with a reference signal to generate the duty signal (EN signal). Or as shown in  FIG. 12 , for another example, the AC signal extraction and conversion circuit  40  includes a low-pass filter  41  and a voltage-to-duty (V-to-D) conversion circuit  42 , in which the low-pass filter  41  obtains the DC level (or an average) of the TRIAC signal, and the V-to-D conversion circuit  42  converts the DC level to the duty signal. 
     In the embodiment shown in  FIG. 2 , when the duty signal (i.e., the EN signal) inputted to the enable input terminal EN of the primary side circuit  11  switches from low level to high level, it takes some time for the capacitor C 2  to be charged, and therefore as shown in  FIG. 4 , the secondary side circuit  32  (same for the secondary side circuit  12  in the prior art) does not start operating until the voltage across the capacitor C 2  reaches a specific level. And after the secondary side circuit  32  starts operating, a circuit settling time is required for the LED circuit  50  to illuminate stably. In other words, the average brightness of the LED circuit  50  does not precisely correspond to the duty of the EN signal. One way to resolve this issue is to separate the supply voltage for the secondary side circuit  32  from the supply voltage for the LED circuit  50 , as depicted below. 
     Referring to  FIG. 3 , in this embodiment, the circuit further comprises a capacitor C 3  and a diode D 3 . The operation voltage of the secondary side circuit  32  in this embodiment is from the capacitor C 2 , and the operation voltage of the LED circuit  50  and the optocoupler  34  in this embodiment is from the capacitor C 3 . As shown in  FIG. 5 , because the feedback control is based on the current through the LED circuit  50 , the voltage waveform of the capacitor C 3  is as the fourth waveform in the figure, and this waveform is exactly the illumination status of the LED circuit  50 . With regard to the secondary side circuit  32 , since it is an IC which requires not much current (far less than what the LED circuit  50  requires), a low capacitance capacitor C 2  is enough to sustain the voltage required for keeping the secondary side circuit in the operation mode. In other words, regardless whether the EN signal is at high level or low level, the secondary side circuit  32  is kept in the operation mode. Therefore, when the EN signal changes from low level to high level, the secondary side circuit  32  only needs a very short response time, so the LED circuit  50  can illuminate at a more precise timing. Under such arrangement, when the capacitor C 3  is fully discharged and the operation voltage of the LED circuit  50  and the optocoupler  34  is totally lost, because the secondary side circuit  32  is kept in the operation mode, the voltage at critical nodes connected to the secondary side circuit  32  can be designed to be sustained such that the circuit can respond quickly in the next cycle. More specifically: 
     Taking the embodiment of  FIG. 3  for example, to make the circuit response quick in the next cycle, the most important node whereat voltage needs to be sustained is the negative terminal of the optocoupler  34 . When the operation voltage of the LED circuit  50  and the optocoupler  34  is lost, the operational amplifier  33  turns off the transistor Q because no current through the LED circuit is detected; thus, the current flowing through the optocoupler  34  becomes zero, and because there is no current, the voltage at the negative terminal of the optocoupler  34  is kept. If the positive terminal of the optocoupler  34  is connected to the capacitor C 2 , to share a common voltage source with the secondary side circuit  32 , the aforementioned advantage cannot be achieved, and the capacitance of the C 2  must be increased. 
       FIG. 6  shows another embodiment of the present invention, which is similar to the embodiment shown in  FIG. 3  except that the diodes D 2  and D 3  are placed in different paths. 
       FIG. 7  shows another embodiment of the present invention. Because the LEDs themselves are current rectifiers, this embodiment omits the diode D 3 . In circuit operation, the LEDs only illuminate when the secondary side of the transformer is conducting. Preferably, the lower frequency bandwidth of the feedback signal is decreased in this circuit. 
       FIG. 8  shows another embodiment of the present invention. This embodiment connects the lower end of the capacitor C 4  to the left end of the resistor R, not the right end of the resistor R (the lower end of the LED circuit  50 ). The difference between connections to the right end and left end of the resistor R is thus. A Zero is generated in the circuit if a capacitor (the capacitor C 3 ) is connected to the right end of the resistor R, and a Pole is generated in the circuit if a capacitor (the capacitor C 4 ) is connected to the left end of the resistor R. To the basic purpose of the present invention, Zero or Pole makes no difference. The effect to the circuit by Zero or Pole is shown in  FIG. 13 . When the capacitance of the capacitor C 3  is too low or the capacitance of the capacitor C 4  is too high, the circuit will operate in an under-damping condition. When the capacitance of the capacitor C 3  is too high or the capacitance of the capacitor C 4  is too low, the circuit will operate in an over-damping condition. When the capacitor C 3  or C 4  has a proper capacitance, the circuit will operate in a critical-damping condition, in this case the circuit will reach its steady status through an optimum approach. In other words, when the capacitor C 3  is used and under-damping is found, the capacitance of the capacitor C 3  should be increased. When the capacitor C 3  is used and over-damping is found, the capacitance of the capacitor C 3  should be decreased. When the capacitor C 4  is used and under-damping is found, the capacitance of the capacitor C 4  should be decreased. When the capacitor C 4  is used and over-damping is found, the capacitance of the capacitor C 4  should be increased. 
     Certainly, the present invention is not limited to using only one of the capacitors C 3  and C 4 ; they can both be used as shown in  FIG. 9 . 
     In all the aforementioned embodiments, there is shown only one LED path; however, the present invention is not limited to this. The LED circuit  50  can include more than two LED paths as shown in  FIG. 10 , wherein a current mirror can be used to duplicate current from one LED path to another. In this case, the resistor R for setting the LED current also functions as a degeneration resistor for the current mirror.  FIG. 10  shows that, in this embodiment, the lower end of the capacitor C 4  is connected to the left end of the resistor R, and the diodes D 2  and D 3  are placed in the lower paths, but they certainly can be modified to any other arrangement as described in the above. 
     The present invention has been described in considerable detail with reference to certain preferred embodiments thereof. It should be understood that the description is for illustrative purpose, not for limiting the scope of the present invention. Those skilled in this art can readily conceive variations and modifications within the spirit of the present invention. For example, the EN signal for dimming control can be generated by other ways, not from the AC signal. As another example, the bipolar transistor Q in the secondary side circuit  32  can be substituted by a field effect transistor. All such variations and modifications should be interpreted as being included within the scope of the present invention.