Abstract:
A start-up circuit includes a switching-device control circuit arranged to receive an input voltage and to provide a switching-device control signal, a switching device arranged to be controlled by the switching-device control signal and to provide a start-up signal, a power-converter control circuit arranged to receive the start-up signal and to provide a power-converter control signal, and a power converter arranged to receive the power-converter control signal and to provide an auxiliary output signal. The switching control circuit is arranged to receive the auxiliary output signal such that, when the auxiliary output signal reaches a predetermined level, the switching-device control circuit stops providing the switching-device control signal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to power converters. More specifically, the present invention relates to start-up circuits that provide start-up voltage to a control system of a power converter during a power-up process when the output voltage rises from zero to a nominal level. 
         [0003]    2. Description of the Related Art 
         [0004]    According to one conventional start-up technique, an energy storage capacitor placed across the input terminals of a control circuit is charged up to the start-up level through a resistor connected to the input voltage. One drawback of this conventional technique is that the capacitance value and the physical size of the energy storage capacitor must both be relatively large to provide sufficient energy for the startup process to commence. An electrolytic or other polarized type capacitor is typically used for this purpose. Another drawback of this conventional technique is that the current charging the energy storage capacitor, and consequently the start-up time, depends on the input voltage, which limits this conventional technique to being usable only in power converters with relatively narrow input voltage ranges. Moreover, power losses in the charging resistor under steady state conditions of this conventional technique will increase monotonically with the input voltage. 
         [0005]    A modification of the above-described start-up technique based on an energy storage capacitor is shown in FIG. 4 of U.S. Pat. No. 5,812,385. The circuit  100  in  FIG. 1  is similar to FIG. 4 of U.S. Pat. No. 5,812,385. The start-up circuit  100  in  FIG. 1  operates in the following manner. DC voltage is applied to terminals V in+  and V in−  and charges input filter capacitor  101 . Initially, transistor  103  is turned ON by pull-up resistor  106  and charges energy storage capacitor  105  through starting resistor  102  and diode  104 . When voltage across energy storage capacitor  105  reaches a specific level, control circuit  112  is activated. The activated control circuit  112  supplies a CONTROL signal to converter  113  that starts the converter  113 , which in turn supplies a control circuit bias voltage though diode  111 . This control circuit bias voltage reverse biases diode  104  and supplies current to the control circuit  112 . Control circuit  112  then supplies a VREF signal to transistor  107  through base current limiting resistor  109 . The VREF signal turns the transistor  107  ON, which in turn turns transistor  103  OFF, saving power from being dissipated through resistor  102 . During the power saving mode of control circuit  112 , the CONTROL signal supplied to the converter  113  turns OFF, which causes the voltage supplied through diode  111  to capacitor  105  to begin to decay. When the voltage across capacitor  105  drops to a predetermined level, the control circuit  112  turns OFF and the VREF signal drops to zero, turning transistor  107  OFF and transistor  103  ON to restore power supplied from the DC input. This operation occurs in a cycle-by-cycle basis during the power saving mode of operation. 
         [0006]    However, this conventional technique does not fully eliminate start-up power dissipation. Charging resistor  102  does not dissipate power under a steady state condition because, when transistor  103  is OFF, resistor  106  is still connected across the input DC voltage of capacitor  101  though closed transistor  107 . If the input voltage range is relatively narrow, power dissipation in resistor  106  will be relatively low and can be discounted. However, if the input DC voltage has a wide range, then power dissipation in resistor  106  cannot be neglected because power dissipation increases directly proportional to the square of the input DC voltage. Assuming, for example, a 10:1 input voltage range and a 100 mW dissipation in resistor  106  at low input voltage, then the power dissipated in resistor  106  at high input voltage will be 0.1 W*(10) 2 =10 W. This is a significant change in the power dissipated by resistor  106 . 
         [0007]    Accordingly, the above conventional technique is undesirable because it requires the capacitor  105  to be relatively expensive, large value, and physical size, because it does not have a fixed start-up time, and the power dissipation losses under steady state conditions limit start-up circuits based on energy storage capacitors to only those that have a relatively narrow input voltage range. 
         [0008]    Another conventional start-up technique is illustrated in  FIG. 2 . A start-up circuit  200  includes a start-up transistor  201 , first diode  202 , resistor  203 , zener diode  204 , filter capacitor  205 , second diode  206 , and power converter  207  with control circuit  208  and with auxiliary output terminals  209 ,  210 . 
         [0009]    The start-up circuit  200  in  FIG. 2  operates in the following manner. After the input voltage V in  is applied to terminals V in+ , V in− , resistor  203  supplies current to zener diode  204  and to the base of transistor  201 . Transistor  201  supplies a start-up voltage at the input of the control circuit  208  and across the filter capacitor  205  equal to the zener voltage V z  of the zener diode  204  minus the combined voltage drops of transistor  201  and first diode  202 . The start-up voltage reverse biases the second diode  206  and is supplied to the control circuit  208 , which initiates the start-up process of the power converter  207 . During this start-up process, the output voltage supplied by power converter  207  to the LOAD and the auxiliary voltages supplied by power converter  207  to auxiliary output terminals  209 ,  210  rise to their nominal levels. Because the start-up current for the control circuitry  208  is supplied by the transistor  201  that is controlled by the fixed zener voltage V z  of the zener diode  204 , the control circuitry  208  functions independently of the input voltage. Thus, the start-up time is independent of the input voltage V in . 
         [0010]    Another significant difference is that filter capacitor  205  functions as a filter capacitor rather than an energy storage capacitor as the energy storage capacitor  105  depicted in  FIG. 1 . It should be noted that this capacitor  205  is not essential to circuit operation and is solely used for noise reduction. Because the filter capacitor  205  has a different function than energy storage capacitor  105 , the value and size of filter capacitor  205  can be significantly smaller than energy storage capacitor  105 . Additionally, capacitor  205  also can be a multi-layer ceramic capacitor, which provides savings in product cost in comparison to the cost of the energy storage capacitor  105 . 
         [0011]    When the auxiliary voltage across auxiliary output terminals  209 ,  210  exceeds the start-up voltage at the input of the control circuit  208 , second diode  206  is forward biased, first diode  202  is reversed biased, transistor  201  switches to the OFF state, and auxiliary power from output terminals  209 ,  210  is supplied to the control circuit  208 . 
         [0012]    The resistance value R of resistor  203  is selected in accordance with the following equation: 
         [0000]        R =( V   in min   −V   z )/ I   min   (1) 
         [0000]    where V in min  is the minimum input voltage and I min  is the minimum current in the resistor R needed both to activate zener diode  204  and to supply base current to the transistor  201 . At input voltages greater than V in min , the current I through the resistor  203  increases according to the formula: 
         [0000]        I =( V   in   −V   z )/ R   (2) 
         [0013]    Power dissipation P in the resistor  203  at high input voltage V in max  is defined by the following formula, which is based on the equations (1) and (2): 
         [0000]        P=I   min*(   V   in max   −V   z ) 2 /( V   in min   −V   z )  (3) 
         [0014]    If the input voltage range is narrow, the power dissipation P in the resistor  203  is not significant and can practically be neglected. However, if the input voltage range is wide, the power dissipation P requires a physically larger resistor size, thus causing overall efficiency deterioration and an increasing no-load current. For example, consider a power converter with output power of Po=100 W and efficiency η=90% at minimum input voltage V in min =16 V, with V z =12 V and I min =1.5 mA. Power dissipation P in the resistor  203  calculated according to the formula (3) and efficiency η, for V in max  levels of 36 V, 75 V, and 150 V are shown in Table 1. 
         [0000]    
       
         
               
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 V in max  (V) 
               
             
          
           
               
                   
                 36 
                 75 
                 150 
               
               
                   
                   
               
             
          
           
               
                   
                 P (W) 
                 0.22 
                 1.49 
                 7.14 
               
               
                   
                 η (%) 
                 89.83 
                 88.82 
                 84.57 
               
               
                   
                   
               
             
          
         
       
     
         [0015]    The above example demonstrates that the conventional start-up circuit  200  of  FIG. 2  is efficient enough in a relatively narrow input voltage range and is not efficient in a wide input voltage range. In the above example, at high input voltage V in =150 V, the efficiency drops by (90%-84.57%)=5.43%, and the rated power of the resistor  203  must be increased to tolerate power dissipation of 7.14 W. 
         [0016]    Thus, there is a need in the power conversion field for a more efficient start-up circuit for power converters with a wide input voltage range. 
       SUMMARY OF THE INVENTION 
       [0017]    To overcome the problems described above, preferred embodiments of the present invention provide a more efficient start-up circuit for power converters with wide input voltage range. 
         [0018]    According to preferred embodiments of the present invention, a start-up circuit includes a switching-device control circuit arranged to receive an input voltage and to provide a switching-device control signal, a switching device arranged to be controlled by the switching-device control signal and to provide a start-up signal, a power-converter control circuit arranged to receive the start-up signal and to provide a power-converter control signal, and a power converter arranged to receive the power-converter control signal and to provide an auxiliary output signal. The switching control circuit is arranged to receive the auxiliary output signal such that, when the auxiliary output signal reaches a predetermined level, the switching-device control circuit stops providing the switching-device control signal. 
         [0019]    The switching-device control circuit preferably includes a resistor arranged to receive the input voltage, a second switching device connected in series with the resistor, and a constant-voltage device connected to the second switching device and arranged to provide the switching-device control signal. The constant-voltage device is preferably a zener diode. When the auxiliary output signal reaches a predetermined level, the second switching device is preferably turned off such that no current flows through the resistor. The switching-device control circuit further preferably includes a third switching device arranged to receive the auxiliary output signal and to provide a second-switching-device control signal to the second switching device. The switching-device control circuit further preferably includes a delay circuit connected to the third switching device and arranged to delay the third switching device from stopping providing the second-switching-device control signal. The delay circuit is preferably an RC circuit. 
         [0020]    The start-up circuit preferably includes a filter capacitor connected to the switching device. When the auxiliary output signal reaches a predetermined level, the switching device preferably stops providing the start-up signal and the power-converter control circuit receives the auxiliary output signal. A start-up circuit preferably includes a first diode connected between the switching device and the power-converter control circuit and a second diode connected between the power-converter control circuit and the power converter. The power converter is preferably arranged to supply power to a load when the power converter receives the power-converter control signal. 
         [0021]    Other features, elements, characteristics, methods, steps and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  shows a conventional start-up circuit based around an energy storage capacitor. 
           [0023]      FIG. 2  shows a conventional start-up circuit based around a start-up transistor. 
           [0024]      FIG. 3  shows a start-up circuit according to a first preferred embodiment of the present invention. 
           [0025]      FIG. 4  shows a start-up circuit according to a second preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]      FIG. 3  shows a start-up circuit  300  according to the first preferred embodiment of the present invention that includes start-up transistor  301 , first diode  302 , resistor  303 , zener diode  304 , capacitor  305 , second diode  306 , power converter  307  with control circuit  308  and with auxiliary output terminals  309 ,  310 , and switch Q with control circuit  311 . In contrast to  FIG. 2 ,  FIG. 3  includes a switch Q that is controlled by the voltage across auxiliary output terminals  309 ,  310  and that is connected in series with resistor  303 . 
         [0027]    The start-up circuit  300  in  FIG. 3  operates in the following manner. After the input voltage V in  is applied across terminals V in+ , V in− , zener diode  304  is activated through resistor  303  and switch Q that is initially ON. Transistor  301  supplies a start-up voltage at the input of the control circuit  308  and across filter capacitor  305  equal to the zener voltage V z  of the zener diode  304  minus the combined voltage drops of transistor  301  and first diode  302 . The start-up voltage reverse biases the second diode  306  and is applied to the control circuit  308 , which initiates the start-up process of the power converter  307 . During this start-up process, the output voltage supplied by the power converter  307  to the LOAD and auxiliary voltages supplied by the power converter  307  to auxiliary output terminals  309 ,  310  rise to their nominal levels. The start-up current for the control circuit  308  is supplied by the transistor  301  controlled by the fixed zener voltage V z  of the zener diode  304  independent of the input voltage V in , resulting in a fixed start-up time over the entire input voltage range. It is possible to use any constant voltage element, e.g. voltage reference, shunt regulator, etc., instead of zener diode  304 . 
         [0028]    When the auxiliary voltage across auxiliary output terminals  309 ,  310  reaches a predetermined level, control circuit  311  turns switch Q to the OFF state, causing transistor  301  to also switch OFF. Once the transistor  301  switches OFF, the first diode  302  becomes reversed biased, second diode  306  becomes forward biased, and auxiliary power from output terminals  309 ,  310  is supplied to the control circuit  308 . 
         [0029]    Accordingly, the resistor  303  now only conducts during the start-up process (typically for about a couple of milliseconds) when switch  311  is in the ON state. At steady state power, power dissipation in the start-up circuit  300  shown in  FIG. 3  is eliminated or nearly eliminated because the resistor  303  is not conducting. Thus, each of increasing efficiency, decreasing a required physical size of the resistor  303 , and decreasing the level of no-load current can be achieved with the arrangement shown in  FIG. 3 . 
         [0030]    The start-up current for control circuitry  308  is supplied by the transistor  301  that is controlled by the fixed zener voltage V z  of the zener diode  304  such that the transistor  301  operates independent of the input voltage V in . Accordingly, because the transistor  301  operates independently from the input voltage V in , the start-up time is also independent of the input voltage V in . 
         [0031]    Accordingly, because the start-up circuit  300  does not create any losses at steady state, the start-up circuit in  FIG. 3  is suitable for power converters working in wide input voltage ranges. 
         [0032]    Another significant advantage is that the capacitor  305  functions as a filter capacitor rather than an energy storage capacitor as the energy storage capacitor  105  depicted in  FIG. 1 . Because the capacitor  305  has a different function than the energy storage capacitor  105 , the value and size of capacitor  305  can be significantly smaller than energy storage capacitor  105 . Additionally, capacitor  305  can also be a multi-layer ceramic capacitor, which provides a savings in product cost in comparison to the cost of the energy storage capacitor  105 . Further, it should be noted that capacitor  305  is not essential to circuit operation and is solely used for noise reduction. 
         [0033]      FIG. 4  shows start-up circuit  400  according to a second preferred embodiment of the present invention that includes a start-up transistor  401 , first diode  402 , resistor  403 , first zener diode  404 , filter capacitor  405 , second diode  406 , power converter  407  with control circuit  408  and with auxiliary output terminals  409 ,  410 , switch  411  (which is preferably a depletion mode N-channel MOSFET, but could be any other desirable switching element), additional switch  418  (which is preferably a N-channel MOSFET, but could be any other desirable switching element), second zener diode  416 , RC circuit formed by resistor  414  and capacitor  415 , third diode  413 , and resistors  412 ,  417 ,  419 . 
         [0034]    The start-up circuit  400  in  FIG. 4  is a variant of the circuit in  FIG. 3 , where switch Q is preferably provided by a depletion mode N-channel MOSFET and where control circuit  311  is preferably provided by a combination of MOSFET  418  and various passive components. The start-up circuit  400  in  FIG. 4  operates in the following manner. After input voltage V in  is applied to terminals V in+ , V in− , first zener diode  404  is activated through resistor  403  and switch  411  that is normally ON, transistor  401  supplies a start-up voltage at the input of control circuit  408  and across filter capacitor  405 , second diode  406  and third diode  413  become reverse biased, and the start-up voltage is applied to the control circuit  408 , which initiates the start-up process of the power converter  407 . During this start-up process, the output voltage supplied by the power converter  407  to the LOAD and the auxiliary voltages supplied by the power converter  407  to auxiliary output terminals  409 ,  410  rise to their nominal levels. The start-up current for control circuit  408  is supplied by transistor  401  controlled by the fixed zener voltage V z  of the zener diode  404  independent of the input voltage V in , resulting in a fixed start-up time over the entire input voltage range. It is possible to use any constant voltage element, e.g. voltage reference, shunt regulator, etc., instead of zener diode  404 . 
         [0035]    When the auxiliary voltage across auxiliary output terminals  409 ,  410  reaches the combined level determined by the gate-to-source turn-on threshold of switch  418  and zener voltage V z  of the second zener diode  416 , switch  418  switches to the ON state, the gate-to-source voltage of the switch  411  becomes negative and switch  411  switches OFF, causing transistor  401  also to switch OFF. When the transistor  401  switches OFF, the first diode  402  becomes reverse biased, diode  406  becomes forward biased, and auxiliary power from output terminals  409 ,  410  is supplied to control circuit  408 . 
         [0036]    Accordingly, resistors  412 ,  419  are arranged to define the negative gate-to-source voltage of the switch  411 . The RC circuit defined by resistor  414  and capacitor  415  and second zener diode  416  is arranged to create a delay in the turn-on process of the switch  418 , which allows sufficient time for the system to start-up and become stable. Resistor  417  is arranged to ensure that the switch  418  will be OFF in an initial state. 
         [0037]    Resistor  403  is similar to resistor  303  in  FIG. 3  in that resistor  403  only conducts during the start-up process (typically for about a couple of milliseconds) when switch  411  is in the ON state. At steady state power, power dissipation in the start-up circuit  400  shown in  FIG. 4  is eliminated or nearly eliminated because the resistor  403  is not conducting. Thus, each of increasing efficiency, decreasing a required physical size of the resistor  403 , and decreasing the level of no-load current can be achieved with the arrangement shown in  FIG. 4 . 
         [0038]    The start-up current for control circuitry  408  is supplied by the transistor  401  that is controlled by the fixed zener voltage V z  of the first zener diode  404  such that the transistor  401  operates independent of the input voltage V in . Accordingly, because the transistor  401  operates independently from the input voltage V in , the start-up time is also independent of the input voltage V in . 
         [0039]    Accordingly, because the start-up circuit  400  does not create any losses at steady state, the start-up circuit in  FIG. 4  is suitable for power converters working in wide input voltage ranges. 
         [0040]    Similar to the capacitor  305  in  FIG. 3 , capacitor  405  functions as a filter capacitor rather than an energy storage capacitor like the energy storage capacitor  105  shown in  FIG. 1 . Since the capacitor  405  has a different function than energy storage capacitor  105 , the value and size of capacitor  405  can be significantly smaller than energy storage capacitor  105 . Additionally, capacitor  405  can also be a multi-layer ceramic capacitor, which provides a savings in product cost in comparison to the cost of the energy storage capacitor  105 . Further, it should be noted that this capacitor  405  is not essential to circuit operation and is solely used for noise reduction. 
         [0041]    It should be understood that the foregoing description is only illustrative of the present invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the present invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications, and variances that fall within the scope of the appended claims.