Abstract:
The invention generally relates to the field of power factor correction and specifically to generation of a reference waveform which is proportional to line voltage and is controllable in amplitude.

Description:
TECHNICAL FIELD 
       [0001]    The invention generally relates to the field of power factor correction and specifically to generation of a waveform which is proportional to a voltage waveform and which is controllable in amplitude. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is generally desirable to operate an AC electrical system at a high power factor for the efficient transfer of electrical power. Highest power factor is attained when line current and line voltage are proportional. 
         [0003]    An electronic load on an AC electrical system, such as an offline switching power supply, may attain operation at high power factor through a variety of measures. 
         [0004]    One such measure is the closed loop control of the line current waveform. The closed loop control regulates the line current waveform in accordance with a line current reference waveform, the line current reference waveform being proportional to the line voltage waveform. In addition, it is generally desirable that the line current reference waveform is controllable in amplitude for the control of power level. 
         [0005]    In the practice of implementing a closed loop control of the line current waveform, a general simplification of circuitry is attained by processing the rectified versions of the line current waveform and the line voltage waveform. Accordingly, it is generally desired that the line current reference waveform is proportional to the rectified line voltage waveform. For the sake of brevity the mention of line voltage and line current in the following implicitly refers to the mention of their rectified versions, unless stated otherwise. 
         [0006]    There is a general need to provide the line current reference waveform at lowest cost and size. 
         [0007]    A simple and straightforward solution of the prior art provides the line current reference waveform in two steps, a first step whereby a signal proportional to line voltage is derived by means of a resistive voltage divider, and a second step whereby the signal is controlled in amplitude by means of an analog multiplier circuit. 
         [0008]    Drawbacks of the conventional approach are the general expense of a high voltage resistive divider, and the general expense of an analog multiplier. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention provides an approach for generating the desired line current reference waveform which is based on gauging the current rise of an inductor, the current rise being proportional to the line voltage and being proportional to the gauging period. 
         [0010]    The desired line current reference waveform is generated in two steps. In a first step, a sampled line current reference waveform is generated by sampling once per switching cycle the current rise of an inductor, the current rise being gauged during a period where the inductor is connected to the line voltage and being gauged over a gauging period dependent on an amplitude control signal. Accordingly, the sample rate of the sampled line current reference waveform is equal to the switching frequency and the line current reference waveform samples are proportional to the line voltage and are controllable in amplitude. In a second step a continuous line current reference waveform is constructed from the sampled line current reference waveform by means of a sample and hold circuit or by some other means of interpolation known in the art. 
         [0011]    A closed loop control of line current waveform may be implemented as a sampled data system. In such a situation, the second step of converting the sampled waveform into a continuous waveform can obviously be omitted. 
         [0012]    By the very nature of an inductor, the current rise, associated with the application of a voltage across an inductor over a given time period, is proportional to the voltage across the inductor and is proportional to the given time period. Accordingly, a sampled line current reference waveform, being proportional to the line voltage and being controllable in amplitude, is provided by sampling the current rise of an inductor over a gauging period, where the inductor is connected to the line voltage during the gauging period and where the gauging period is dependent on an amplitude control signal. 
         [0013]    The approach is generally suited to power supply topologies where an inductor of the power supply is subjected to the line voltage during some period of the switching cycle. A number of common offline power supply topologies, such as the boost topology and the flyback topology, feature an inductor, or a coupled inductor, which is subjected to the line voltage for some period of the switching cycle. 
         [0014]    An advantage of the approach is that the measurement of the inductor current, for purpose of gauging the inductor current rise, often can be provided with few or no additional means, and that an analog multiplier circuit is not required. Particularly advantageous are power supply control circuit implementations which include the measurement of switch current, the switch current measurement providing the measurement of inductor current at no extra cost. 
         [0015]    The present invention is suited to both the discontinuous and the continuous mode of operation. According to the principles of the invention the only requirement is that an inductor of the switching power supply is subjected to the line voltage for a time period within the switching cycle, and that a measurement of inductor current can be made within the time period for purpose of gauging a current rise of the inductor over a gauging period. 
         [0016]    Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in combination with the accompanying drawings, illustrating by way of example the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  shows a schematic diagram of a line current reference regulator of the prior art. 
           [0018]      FIG. 2  shows a schematic diagram of an exemplary embodiment of the invention in the setting of a boost regulator. 
           [0019]      FIG. 3  shows a waveform diagram of an exemplary embodiment of the invention in the setting of a boost regulator. 
           [0020]      FIG. 4  shows a schematic diagram of an exemplary embodiment of the invention in the setting of a flyback regulator. 
           [0021]      FIG. 5  shows a waveform diagram of an exemplary embodiment of the invention in the setting of a flyback regulator 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]      FIG. 1  shows an exemplary embodiment of a line current reference generator of the prior art in the setting of an offline boost regulator. 
         [0023]    Line voltage waveform  100  is applied to the input of boost regulator  10 . Bridge rectifier  11  converts line voltage waveform  100  into rectified line voltage waveform  101 . Line current reference waveform  103  is established in a two step process. In a first step, line voltage waveform  101  is scaled down proportionally into intermediate reference waveform  102  by resistive voltage divider  21 . In a second step, intermediate reference waveform  102  and amplitude control signal  120  are multiplied by analog multiplier  22 , thus producing line current reference waveform  103 . 
         [0024]    Accordingly, the exemplary embodiment of  FIG. 1  is capable of providing line current reference waveform  103 , the waveform being proportional to line voltage waveform  101  and being controllable in amplitude by amplitude control signal  120 . The exemplary embodiment of the prior art requires the expense of resistive voltage divider  21  and an analog multiplier  22 . 
         [0025]      FIG. 2  shows a schematic diagram and  FIG. 3  shows a waveform diagram of an exemplary embodiment of a line current reference generator according to the current invention, again, in the setting of an offline boost regulator. Shown is an exemplary embodiment where the inductor current is measured by way of measuring the boost switch current. Furthermore, shown is an exemplary embodiment of a boost regulator operating in the continuous conduction mode. The circuitry is arranged primarily for highlighting the principles of operation and not necessarily for use of fewest resources. 
         [0026]    Line current reference waveform  103 , being proportional to line voltage  101  and being controllable in amplitude, is generated as follows. 
         [0027]    Shown is  FIG. 2  is offline boost regulator  10  which includes switch  12  being a switch of the FET type. Gate control signal  130  controls the state of switch  12  in typical fashion, closing and opening switch  12  once per switching cycle  200 . Switch  12  is in the closed state during a leading part of switching cycle  200 , the leading part generally known as on time period  201 , and switch  12  is in the open state during a trailing part of switching cycle  200 , the trailing part generally known as off time period  202 . 
         [0028]    Shown in  FIG. 3  are waveforms covering two consecutive switching cycles  200 , where the line voltage  101  and amplitude control signal  120  are, by way of example, assumed to be of constant magnitude, thus resulting in an identical inductor current rise  300  in both switching cycles and ultimately in a line current reference waveform  103  of constant amplitude. 
         [0029]    Inductor  13  is subjected to line voltage  101  during on time period  201  causing inductor current  140  to rise. During on time period  201 , inductor current  140  follows a path by way of switch  12  and current sense resistor  15 . Accordingly, switch current measurement signal  150  as provided by current sense resistor  15  may serve as an inductor current sense signal for purpose of gauging inductor current rise  300  over gauging period  301  during on time period  201 . 
         [0030]    Continuous time reference waveform  103  is produced with the aid of three sample and hold circuits, SH 1 , SH 2 , SH 3  and subtractor circuit SUB. Sample and hold circuit SH 1  establishes the value of switch current  150  at the start of gauging period  301  by way of signal  151  at time instant  251 . Sample and hold circuit SH 2  establishes the value of switch current  150  at the end of gauging period  301  by way of signal  152  at time instant  252 . Subtractor circuit SUB establishes sampled reference waveform  153  comprising the magnitude of inductor current rise  300  by subtracting the starting value as provided by signal  151  from the ending value as provided by signal  152 . Sample and hold circuit SH 3  facilitates the conversion of sampled reference waveform  153  into continuous reference waveform  103  at time instant  253 . One of ordinary skill in the art will understand that sample and hold circuit SH 3  is optional and need not be used if a sampled reference waveform, such as sampled reference waveform  153 , is desired instead of a continuous reference waveform  103 . 
         [0031]    The gauging operation is controlled by the timers TM 1 , TM 2  and inverter INV. 
         [0032]    Timer TM 1  operates sample and hold circuit SH 1  by way of control signal  131  and establishes starting value  151  of inductor current rise  300  at time instant  251 . Pulse period  350  of timer TM 1  may be selected so as to locate the start of gauging period  301  in a part of on time period  201  where switching artifacts arising from the closing of switch  12  have subsided. 
         [0033]    Timer TM 2  operates sample and hold circuit SH 2  by way of control signal  132  and establishes ending value  152  of inductor current rise  300  at time instant  252 . The pulse period  301  of timer TM 2  establishes the gauging period and is determined by amplitude control signal  120 . Timer TM 2  defines the relationship between gauging period  301  and amplitude control signal  120 . The relationship may be proportional or otherwise. A proportional relationship provides a line current amplitude control characteristic which is identical to the one of analog multiplier  22  of the prior art circuit in  FIG. 1 . 
         [0034]    Inverter INV operates sample and hold circuit SH 3  by way of control signal  133 , converting sampled reference waveform  153  into continuous reference waveform  103  at time instant  253 . Time instant  253  corresponds with the start of off time period  202  by virtue of the use of inverter INV, but may be arranged for any other instant after time instant  252  and before time instant  251  of the directly following switching cycle using some other timer or logic arrangement. 
         [0035]    Accordingly, the exemplary embodiment of  FIG. 2  and  FIG. 3  is capable of providing continuous line current reference waveform  103  which is proportional to the line voltage waveform  101  and which is controllable in amplitude by amplitude control signal  120 . 
         [0036]    One skilled in the art may devise many variations in defining a circuit for accomplishing the task of sampling a starting and an ending value of an inductor current rise over a gauging period, defining a starting location for the gauging period, defining circuitry for measuring the inductor current within the path of current flow, making adaptations specifically for either the discontinuous or the continuous conduction mode, defining circuitry for converting a sampled waveform into a continuous waveform, and choosing a suitable relationship between the amplitude control signal and the gauging period. 
         [0037]      FIG. 4  shows a schematic diagram and  FIG. 5  shows a waveform diagram of an exemplary embodiment of a line current reference generator in the setting of an offline flyback regulator. Shown is an exemplary embodiment where the inductor current is measured by way of a sense resistor in series with the bridge rectifier. Furthermore, shown is an exemplary embodiment where the switching regulator operates in the discontinuous mode of operation. 
         [0038]    Shown is  FIG. 4  is offline flyback regulator  30  which includes switch  12  and inductor  13 . Operational similarities of flyback regulator  30  and boost regulator  10  and their respective line current reference waveform generators  20  are not repeated here for the sake of brevity. Items, labeled with the same numbers shown in  FIG. 4  and  FIG. 5 , perform the same function described previously for  FIG. 2  and  FIG. 3 . 
         [0039]    Inductor current  140  is measured in an alternative location with the help of current sense resistor  16 , the location being the return path of bridge rectifier  11 . Sense resistor  16  provides an inductor current measurement signal  141  with inverted sense. Inverting sense amplifier ISA is provided to invert the sense of measurement signal  141 , thus providing an inductor current measurement signal  150  with the same sense as in  FIG. 2  and  FIG. 3 . 
         [0040]    Flyback regulator  30  is shown to operate in the discontinuous mode. Discontinuous mode operation permits simplification of the line current reference waveform generation circuitry. Shown is a simplified circuit where sample and hold circuit SH 1 , timer circuit TM 1  and subtractor circuit SUB are not present. Operational differences with the exemplary embodiment of  FIG. 2  and  FIG. 3  are as follows. 
         [0041]    Discontinuous mode operation is characterized in that inductor current  140  is zero or substantially near zero at the start of the switching cycle  200 . Through locating the start of gauging period  301  at the start of switching cycle  200  the need for sampling and subtracting the starting value of inductor rise  300  is rendered unnecessary since the starting value is inherently zero or substantially near zero. Timer circuit TM 2  provides sample and hold control signal  132  to sample and hold SH 2  as before, thus providing ending value of the inductor current rise  300  as sampled reference waveform  154  at time instant  252 . Inverter INV operates sample and hold circuit SH 3  by way of control signal  133 , converting sampled reference waveform  154  into continuous reference waveform  103  at time instant  253  as before. 
         [0042]    Accordingly, the exemplary embodiment of  FIG. 4  and  FIG. 5  is capable of providing line current reference waveform  103  which is proportional to the line voltage waveform  101  and which is controllable in amplitude by amplitude control signal  120 . 
         [0043]    Although the embodiments described above involved a boost regulator or flyback regulator, one of ordinary skill in the art will understand that the invention applies to many types of regulators which include an inductor that is connected to the line voltage for some period of time in each switching cycle. 
         [0044]    References to the present invention herein are not intended to limit the scope of any claim or claim term, but instead merely make reference to one or more features that may be covered by one or more of the claims. Materials and processes described above are exemplary only, and should not be deemed to limit the claims.