Abstract:
Integrated circuits for controlling power supplies and relevant control methods are disclosed. A controller generates a control signal to control a power switch. A feedback pin of an integrated circuit receives an external feedback signal representing an output voltage signal of a power supply. Controlled by the control signal, a transferring circuit transfers the feedback signal to the controller when the power switch is off. When the power switch is on, a clamping circuit clamps the voltage of the feedback signal at a predetermined value to avoid the controller from being influenced by the feedback signal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a power control integrated circuit and the related control methods, and more particularly, to a power control integrated circuit of a power supply and the related control methods. 
         [0003]    2. Description of the Prior Art 
         [0004]    Power supplies such as AC-to-DC converters or DC-to-DC converters are common electronic devices for generating constant voltage source or constant current source to power electronic devices that require specific power management. Since the upgrade for the energy efficiency has been demanded in recent years continuously, the electrical energy conversion competence of the power supplies has become a major subject. How to avoid unnecessary power consumption during power conversion is a goal the circuit designers pursue. 
         [0005]      FIG. 1  is a diagram illustrating a conventional power supply with an architecture of a flyback converter. Power control integrated circuit  100  controls power switch Q 1  through pin GATE. When power switch Q 1  is turned on, power signal V IN  starts charging transformer T 1  causing the current flowing through the primary winding of transformer T 1  to increase over time. When power switch Q 1  is turned off, the stored electrical energy in transformer T 1  starts being released through the induced current in the secondary winding of transformer T 1 , charging output capacitor C O . It is defined in this specification that a power supply operates in an energizing state if the energy of an inductive device, such as an inductor or a transformer, is increasing, and in a de-energizing state if the energy of the inductive device is decreasing. 
         [0006]    Resistors R 1  and R 2 , and pin FB together provides a feedback mechanism; power control integrated circuit  100  can monitor the magnitude of output power signal V OUT  to control power switch Q 1  and thus decide the charging energy through transformer T 1  to output capacitor C O . Generally speaking, the feedback mechanism is to maintain output power signal V OUT  to be as close to an expected value as possible. 
         [0007]    However, as shown in  FIG. 1 , resistors R 1  and R 2  also provide a power leakage path, through which the charge in output capacitor C O  leaks to ground. Regardless of whether power control integrated circuit  100  turns on power switch Q 1  or not, the stored electrical energy of output capacitor C O  is constantly and unnecessarily wasted through the power leakage path. Hence, such the power leakage path should be eliminated as much as possible. 
         [0008]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a diagram illustrating a conventional power supply. 
           [0010]      FIG. 2  is a diagram illustrating a power supply of an embodiment according to the present invention. 
           [0011]      FIG. 3  is a timing diagram illustrating the relation between signals of FIG.  2 . 
           [0012]      FIG. 4  is a diagram illustrating a power supply of an embodiment according to the present invention. 
           [0013]      FIG. 5  is a diagram illustrating a power supply of an embodiment according to the present invention. 
           [0014]      FIG. 6  is a timing diagram illustrating the timing relation between signals of  FIG. 4  and  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Further objects of the present invention and more practical merits obtained by the present invention will become more apparent from the description of the embodiments which will be given below with reference to the accompanying drawings. For explanation purposes, components with equivalent or similar functionalities are represented by the same symbols. Hence components of different embodiments with the same symbol are not necessarily identical. Here, it is to be noted that the present invention is not limited thereto. 
         [0016]    In the following descriptions, VXX represents the voltage of signal V XX , and RX represents the impedance of resistor R X . 
         [0017]      FIG. 2  is a diagram illustrating a power supply of an embodiment according to the present invention. Controller  202  of power control integrated circuit  200  generates signal V G  for controlling power switch Q 1  to turn on/off through pin GATE. Switch Q 2  is coupled between controller  202  and pin FB, and is controlled by signal V G2 . Signal V G2  is generated by inverter INV which receives the signal V G . Capacitor C F  is coupled to controller  202  and switch Q 2 . 
         [0018]    Similar to the operations in  FIG. 1 , when power switch Q 1  of  FIG. 2  is turned on, power supply of  FIG. 2  operates in an energizing state. When power switch Q 1  of  FIG. 2  is turned off, power supply of  FIG. 2  operates in a de-energizing state. 
         [0019]    Different from  FIG. 1 , feedback resistors R 1  and R 2  in  FIG. 2  provide a feedback mechanism by monitoring node N CON , which is the connection node between diode D O  and the secondary winding of transformer T 1 . Diode D O , acting as a rectifier, blocks the reverse current flowing from output capacitor C O  to resistor R 1 . Hence the constant power leakage path of  FIG. 1  does not exist in  FIG. 2 . 
         [0020]    When the power supply of  FIG. 2  operates in the de-energizing state, the energy stored in the secondary winding of transformer T 1  is released to charge output capacitor C O , causing diode D O  to be turned on or forward biased. Hence, signal V COM  on node N CON  is constantly higher than output voltage signal V OUT  by around 0.7 volt, the forward-biased voltage of a diode. The higher the output voltage signal V OUT , the more representative the signal V COM  on node N CON  to be output voltage signal V OUT . Feedback signal V FB  is the result of signal V COM  passing through the voltage divider composed of resistors R 1  and R 2 . Therefore, in the de-energizing state, feedback signal V FB  varies in response to the variation of output voltage signal V OUT , or in other words, feedback signal V FB  approximately represents output voltage signal V OUT . 
         [0021]    Please refer to both  FIGS. 2 and 3 .  FIG. 3  is a timing diagram illustrating the relation between signals V G , V G2 , V FB  and V FB2  of  FIG. 2 , wherein signal V FB2  represents to the voltage across capacitor C F . In the de-energizing state, power control integrated circuit  200  controls signal V G  to be at logic “0” and turn off power switch Q 1 . Meanwhile, V G2  is at logic “1”, due to inverter INV, and turns on switch Q 2 . Therefore, switch Q 2  functions to provide a signal path from pin FB to controller  202 , allowing controller  202  to switch power switch Q 1  according to feedback signal V FB . As shown in interval INT 1  of  FIG. 3 , in the de-energizing state, the voltage of feedback signal V FB  is deemed to be a constant positive value and can be approximately represented by the formula below: 
         [0000]        VFB=V OUT× R 2/( R 1+ R 2)   (1) 
         [0022]    It is presumed that, at the start of interval INT 1 , signal V FB2  is at a lower voltage level compared to signal V FB . As shown in  FIG. 3 , due to the current flowing through the current path provided by switch Q 2 , the voltage level of signal V FB2  increases with time and approaches the voltage level of signal V FB  gradually. In other words, switch Q 2  passes on feedback signal V FB  to generate signal V FB2  forwarded to controller  202 , and then controller  202  generates signal V G  according to signal V FB2  to control power switch Q 1 . 
         [0023]    Interval INT 2  of  FIG. 3  indicates power control integrated circuit  200  operating in the de-energizing state. In the de-energizing state, power control integrated circuit  200  controls signal V G  to be high for turning on power switch Q 1  and signal V G2  low for turning off switch Q 2 . In the meantime, feedback signal V FB , equivalent to the induced voltage across the secondary winding of transformer T 1 , is a constant negative value. In the de-energizing state, the voltage of feedback signal V FB  can be approximately represented by the formula below: 
         [0000]        VFB=−N×V IN× R 2/( R 1 +R 2)   (2) 
         [0000]    where N represents the winding ratio of the secondary winding to the primary winding of transformer T 1 . The intention of turning off switch Q 2  is to isolate feedback signal V FB  and signal V FB2 , maintaining signal V FB2  to approximately equal to feedback signal V FB  at the end of interval INT 1 . However, as shown in  FIG. 2 , Bipolar Junction Transistor (BJT) B Q2  parasitizes in switch Q 2 . In interval INT 2 , feedback signal V FB , which is at a negative voltage level in the charging operation, is likely to trigger BJT B Q2  to turn on, causing capacitor C F  to release the stored charge. Hence, as shown in interval INT 2  of  FIG. 3 , signal V FB2 , the voltage drop across capacitor C F , declines gradually over time. 
         [0024]    If signal V FB2  can retain the same voltage level as feedback signal V FB  in the energizing state, signal V FB2  can correctly represent output voltage signal V OUT  and provide controller  202  with correct feedbacks, allowing the feedback mechanism to function properly. However, as shown in  FIG. 3 , due to the leakage generated by a BJT, signal V FB2  is not a correct representation of output voltage signal V OUT , possibly resulting in an improperly functioning feedback mechanism of power control integrated circuit  200 . Consequently, output voltage signal V OUT  of  FIG. 2  may be unable to retain the desired value. 
         [0025]      FIG. 4  is a diagram illustrating a power supply of an embodiment according to the present invention. For brevity, further discussion on the repeating components between  FIG. 2  and  FIG. 4  thereof is omitted. Different from  FIG. 2 , power control integrated circuit  400  comprises an additional Zener diode D 1 , coupled between pin FB and ground end. Zener diode possesses a relatively low forward-biased voltage, such as 0.1 volt, and is utilized as a clamp circuit. In the energizing state, Zener diode D 1  can clamp signal V FB  to be not lower than the forward-biased voltage, in negative magnitude, of Zener diode D 1 . For instance, in the charging operation, if the forward bias voltage of Zener diode D 1  is 0.1 volt, feedback signal V FB  is then clamped and fixed at −0.1 volt. Hence, the base-to-emitter voltage (V BE ) of BJT B Q2  which parasitizes in switch Q 2  is only 0.1 volt, being not able to trigger BJT B Q2 , which generally needs V BE  to be higher than 0.7 volt for triggering. Therefore, when operating in the energizing state, signal V FB2  or controller  402  can avoid being influenced by feedback signal V FB , and signal V FB2  can approximately retain the level of feedback signal V FB  at the end of the previous de-energizing state. 
         [0026]    When in the de-energizing state, the reverse breakdown voltage of Zener diode D 1  of  FIG. 4  is preferred to be set at a level that is higher than feedback signal V FB . Hence, when in the de-energizing state, Zener diode of  FIG. 4  will not be broken down, forming an open circuit. An artisan of ordinary skill in the art can easily extrapolate the operation principle and functional behavior of the power supply in the de-energizing state of  FIG. 4 , according to the technical description of the power supply of  FIG. 2 . 
         [0027]    When in the de-energizing state, signal V FB2  in  FIG. 4  continues to increase and approach to the level of feedback signal V FB . When in the energizing state, signal V FB2  is approximately equal to the level of feedback signal V FB  at the end of the previous de-energizing state. Hence, signal V FB2  of  FIG. 4  can be extrapolated to correctly reflect feedback signal V FB  in the de-energizing state. In other words, signal V FB2  of  FIG. 4  is an accurate representation of output voltage signal V OUT , providing proper feedbacks to controller  402  for switching power switch Q 1 . Subsequently output voltage signal V OUT  is able to retain an expected value. 
         [0028]      FIG. 5  is a diagram illustrating a power supply of another embodiment according to the present invention. For brevity, further discussion on the repeating components between  FIG. 4  and  FIG. 5  thereof is omitted. Zener diode D 1  in  FIG. 4  is replaced by switch Q 3  in  FIG. 5 . The control end of switch Q 3  is coupled to controller  502 ; hence signal V G  controls the on/off states of switch Q 3 . Switch Q 3  functions as a clamp circuit. In the energizing state, switch Q 3  is turned on along with power switch Q 1 , causing pin FB to be short-circuited to ground end GND, and consequently feedback signal V FB  is being clamped to 0 volt. As a result, the base-to-emitter voltage of BJT B Q2  which parasitizes in switch Q 2  is also 0 volt; hence BJT B Q2  is kept turned off. Therefore, when in the energizing state, signal V FB2  or controller  502  are not influenced by feedback signal V FB . 
         [0029]    When in the de-energizing state, switch Q 3  of  FIG. 5  is kept turned off, forming an open circuit. An artisan of ordinary skill in the art can easily extrapolate the operation principle and functional behavior of the power supply of  FIG. 5  in the de-energizing state, according to the technical description of the power supply of  FIG. 2 . 
         [0030]    Similar to  FIG. 4 , signal V FB2  in  FIG. 5  correctly reflects the voltage level of feedback signal V FB  in the discharging operation. In other words, signal V FB2  of  FIG. 5  is a proper representation of output voltage signal V OUT . Signal V FB2  provides proper feedbacks to controller  502  for controlling the on/off operation of power switch Q 1  and subsequently allowing output voltage signal V OUT  to retain an expected value. 
         [0031]    In integrated circuits, the regions where the negative voltage exists are usually prone to emit electrons and the component characteristics of other regions are being affected accordingly. Hence, in  FIG. 5 , by clamping feedback signal V FB  to 0 volt, the component characteristics of power supply integrated circuit  500  are stabilized accordingly. 
         [0032]      FIG. 6  is a timing diagram illustrating the timing relation between signals V G , V G2 , V FB  and V FB2  in  FIG. 4  and  FIG. 5 . In  FIG. 6 , when in the charging operation, feedback signal V FB  is being clamped to a value that is close to 0 volt due to Zener diode D 1  in  FIG. 4  or switch Q 3  in  FIG. 5 , no longer at a negative voltage level like that in  FIG. 3 . As the base-to-emitter voltage of BJT B Q2  is lower than 0.7 V, BJT B Q2  is prevented from being turned on. In  FIG. 6 , signal V FB2  does not drift up and down like that in  FIG. 3  and can approximately be retained at a level that is equal to feedback signal V FB  of the de-energized state, which is VOUT×R 2 /(R 1 +R 2 ). Hence feedback signal V FB2  in  FIG. 4  and  FIG. 5  can correctly represent voltage output signal V OUT , for providing appropriate feedbacks to the controller. 
         [0033]    Even the invention is exemplified by flyback converters, it is not limited to and can be applied to converters with other architectures, such as buck converters, boost converters, buck-boost converter, and the like. 
         [0034]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.