Abstract:
A control device for controlling a switching power supply adapted to convert an input voltage into an output voltage according to a switching rate of a switching element. The control device includes first control means for switching the switching element in a first working mode at a constant frequency and second control means for switching the switching element in a second working mode at a variable frequency, under a maximum frequency, in response to the detection of a predefined operative condition of the switching power supply. The control device further includes means for selecting the first working mode or the second working mode.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments of the present invention relate generally to the field of electronics, and more specifically to the control of switching power supplies. 
       BACKGROUND 
       [0002]    Switching power supplies are widely used to provide regulated voltages and currents to circuit loads. A switching power supply converts an input voltage by temporarily storing energy corresponding to the input voltage and then releasing that energy to the load, so as to obtain a DC output voltage. The storage of energy (carried out with either magnetic components, like inductors and transformers, or capacitors) is controlled by means of a switching element (such as a power MOSFET). 
         [0003]    Different circuit topologies are known within the class of the switching power supplies. A well-known topology is that referred to as “flyback topology”. As known to those skilled in the art, a switching power supply having the flyback topology, also referred to as “flyback converter”, may be directly connected to a main (AC) voltage source, and its (DC) input voltage is obtained by rectifying and filtering the main voltage by means of an input network including a rectifier bridge and a capacitive filter. A flyback converter includes an electric transformer, with the switching element that periodically connects a primary winding of said transformer to the input network (providing the input voltage), in such a way to modulate the energy that is transferred to a secondary winding coupled to the load. 
         [0004]    Generally, the flyback converter, like all the switching power supplies, requires a control circuit that carries out the task of properly driving the switching element. 
         [0005]    The simplest control circuits of such type—herein referred to as “constant frequency (CF) control circuits”—are designed for driving the switching element with a constant frequency. In this case, the control circuit has a simple circuit architecture, including for example an oscillator and a flip-flop that controls a driver connected to the switching element. Thus, the switching element is opened and closed according to the frequency of the oscillator; by regulating the frequency of the oscillator, it is possible to regulate the transfer of energy, and, thus, the level of the output voltage. The CF control circuit has the advantage of allowing the flyback converter to work according to two different modes. 
         [0006]    Particularly, the flyback converter may work in the so-called Continuous Conduction Mode (CCM) or in the so-called Discontinuous Conduction Mode (DCM), depending on the behavior of a current flowing through the secondary winding during the switching cycle of the switching element: in the CCM, the current flows through the secondary winding for the whole period occurring between the opening of the switching element and the closure thereof. Conversely, in the DCM, the current in the secondary winding diminishes so as to reach a null value before the ending of said period. 
         [0007]    The DCM is usually exploited when the powers to be managed are low, since in this case it is possible to reduce the sizes of the magnetic components of the converter. For higher powers the CCM is instead preferable, since it allows obtaining a more advantageous current&#39;s shape factor. 
         [0008]    A more complex family of control circuits for flyback converters consists of control circuits—herein referred to as “Quasi-Resonant (QR) control circuits”—designed for driving the switching element with a frequency that varies over time, depending on the magnetization status of the transformer (which in turn depends on the input voltage and the output current). Particularly, the QR control circuit synchronizes, with a proper delay, the switching of the switching element with the instant at which the current flowing in the secondary winding reaches a null value, in such a way that the switching element is activated when a voltage at a terminal of the switching element connected to the primary winding is near zero. The latter condition is also known as Zero Voltage Switching (ZVS) condition. 
         [0009]    Examining in greater detail the behavior of the switching element during the operation of the flyback converter, the voltage at the terminal of the switching element connected to the primary winding—herein referred to as “switching voltage”—starts to oscillate with damped oscillations immediately after the instant at which the current flowing in the secondary winding reaches the null value. In this way, the switching voltage exhibits a sequence of falling edges followed by corresponding rising edges, which determine a corresponding sequence of minimum values, each one representing a possible ZVS conditions. 
         [0010]    Since the switching element driven by the QR control circuit is activated very near the instant at which the current of the secondary winding has reached the null value, the operation of the flyback converter corresponds to the boundary between the CCM and DCM. Usually, a flyback converter driven by the QR control circuit has to work with powers under 100 Watts. 
         [0011]    Compared to the CF control circuit, the QR control circuit has several advantages. 
         [0012]    The most important advantage regards the electromagnetic compatibility. Indeed, being the working frequency variable depending on the input voltage and the output current, the energy irradiated by the converter belongs to a plurality of energy bands. In this way, instead of being concentrated at a single frequency with a high intensity, the electromagnetic emission is distributed among different energy bands with much lower intensities. 
         [0013]    Another significant advantage given by the utilization of the QR control circuit regards the possibility of switching the switching element in the ZVS condition. Indeed, since the switching element is usually a MOSFET connected between the primary winding and a ground terminal, in the ZVS condition the voltage across the MOSFET is near to zero, and thus the capacitive losses due to its drain-to-source parasitic capacitance are negligible. 
         [0014]    In the variable-frequency operation of the flyback converter, the switching frequency increases when the output load is low ant the input voltage is high. This increase can drastically increment the losses due to parasitic elements of the flyback converter. 
         [0015]    This situation is usually addressed by introducing a circuit (implementing a so-called valley skipping technique) that sets a minimum period during which the switching element has to remain activated. In this way, if the ZVS condition occurs before the end of the minimum period, the ZVS condition is ignored, and the switching element remains deactivated. More particularly, the switching element is activated at the occurrence of the ZVS condition corresponding to the first minimum value—in the sequence determined by the oscillations of the switching voltage—after the expiration of the minimum period. 
         [0016]    At the present time, the choice of which type of control circuit to be used—between the CF type and the QR type—depends on several and different factors, like for example the personal skill of a technician, the complexity and the cost of the circuit and the amount of power that has to be transferred. 
         [0017]    More complex structures of the control circuit have also been proposed. 
         [0018]    An example of a control circuit for flyback converters known in the art is the device TEA1654 by Philips. This device is a QR control circuit that normally operates at a variable switching frequency. However, if the switching frequency exceeds a predetermined value, the switching frequency is locked to the value and the device operates in the CF mode. This solution has the great drawback that in the latter condition the switching element may be activated when the switching voltage has a value that is significantly higher than zero (with significant capacitive losses). 
         [0019]    Another control circuit for flyback converters known in the art is the device TDA16846/TDA16847 by Infineon. This device is again a QR control circuit that operates at a variable switching frequency. Moreover, the device is capable of always operating in the QR mode, but in such a way to control the switching element with a switching frequency that is as close as possible to a predetermined value. However, since the device always drives the switching element following the occurrences of the ZVS condition, it is not possible to use it for controlling the flyback converter in the CCM. 
       SUMMARY 
       [0020]    More specifically, an embodiment of the present invention provides a control device for controlling a switching power supply. The switching power supply is adapted to convert an input voltage into an output voltage according to a switching rate of a switching element. The control device includes first control means for switching the switching element in a first working mode at a constant frequency, and second control means for switching the switching element in a second working mode at a variable frequency, under a maximum frequency, in response to the detection of a predefined operative condition of the switching power supply. The control device further includes means for selecting the first working mode or the second working mode. 
         [0021]    In an embodiment of the invention, the constant frequency and the maximum frequency depend on a timing frequency of a common timing signal. 
         [0022]    The switching power supply may have a flyback architecture including a transformer, and the control device may include means for detecting a zero current condition in a secondary winding of the transformer. 
         [0023]    According to an embodiment of the present invention, means is provided for detecting stationary points in an auxiliary voltage of an auxiliary winding (oscillating in response to the zero current condition). 
         [0024]    In an embodiment of the invention, the first control means comprises means for switching the switching element in response to the timing signal, and the second control means comprises means for switching the switching element in response to a first stationary point in the auxiliary voltage after a minimum period corresponding to the maximum frequency. 
         [0025]    According to an embodiment of the invention, an oscillator is provided for generating the timing signal. Furthermore, the second control means includes means for resetting said oscillator in response to the switching of the switching element. 
         [0026]    The switching element may be switched by the second control means after a maximum period (that depends on the timing frequency) without the detection of any stationary point. 
         [0027]    Means for switching the switching element in response to a power-on of the power supply may be included in the second control means. 
         [0028]    According to an embodiment of the present invention, the control device comprises a first input terminal for receiving a frequency indication for setting the timing frequency. In an embodiment of the invention, a second input terminal for receiving a working mode indication of the control device is provided. The second input terminal may be further adapted to receive the auxiliary voltage. Another embodiment of the present invention provides a power conversion system including the control device and a switching power supply. A further embodiment of the present invention provides an electronic apparatus including the system. A still further embodiment of the present invention provides a corresponding method for controlling a switching power supply. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The features and advantages of the present invention will be made apparent by the following detailed description of an embodiment thereof, provided merely by way of non-limitative example, which will be conducted making reference to the annexed drawings, wherein: 
           [0030]      FIG. 1  schematically shows the main circuit components of a flyback converter that can be driven by a control circuit according to an embodiment of the present invention; 
           [0031]      FIG. 2  is a timing diagram illustrating how the flyback converter of  FIG. 1  is driven when the control circuit works in the CF mode; 
           [0032]      FIGS. 3A and 3B  are timing diagrams illustrating how the flyback converter of  FIG. 1  is driven when the control circuit works in the QR mode; and 
           [0033]      FIG. 4  is a schematic diagram of the main functional blocks included in the control circuit of  FIG. 1  according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    The following discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
         [0035]    With reference to the drawings,  FIG. 1  schematically shows the main circuit components of a flyback converter  100  that can be driven by a control circuit  105  (in the form of a corresponding device, for example integrated in a chip of semiconductor material) according to an embodiment of the present invention. 
         [0036]    For the sake of convenience, in the following of the description the voltages and the circuit nodes providing them will be denoted with the same references. 
         [0037]    The flyback converter  100  includes an input rectifier block  110 , which receives an AC voltage Vac from an AC main source (not shown in the figure) and provides a corresponding DC input voltage Vin. More particularly, the voltage Vac is fed to an input of a rectifier circuit  115 —for example, a diode bridge—whose output is coupled to a filter circuit  120 —for example, a capacitive filter. The rectifier circuit  115  rectifies the voltage Vac into an output voltage with a fixed polarity but with a pulsating magnitude. The voltage is then filtered by means of the filter circuit  120 , so as to smooth the variation in the voltage provided by the rectifier circuit  115  and obtain the input voltage Vin of which the amplitude is substantially constant. 
         [0038]    The flyback converter  100  further includes a transformer  125  having the purpose of transforming the input voltage Vin into a corresponding output voltage Vout with a different (particularly, lower) value. The transformer  125  comprises a primary winding  130  magnetically coupled with a secondary winding  135 . The primary winding  130  has a first terminal connected to the rectifier block  110  for receiving the input voltage Vin, and a second terminal that is brought to a switching voltage Vsw. The secondary winding  135  has instead the terminals connected to an output stage  140  for providing the output voltage Vout thereto. 
         [0039]    The output stage  140  has the purpose of generating a DC regulated output voltage Vrout from the output voltage Vout. The output stage  140  includes a diode  145  having an anode terminal connected to a first terminal of the secondary winding  135  and a cathode terminal connected to a first terminal of an output capacitor  147 . The second terminals of the secondary winding  135  and of the output capacitor  147  are connected together to a reference (or ground) terminal. The output capacitor  147  provides the regulated output signal Vrout to an output load (not shown in the figure). 
         [0040]    The flyback converter  100  further includes a switching element  150 , for example, a power MOSFET. The MOSFET  150  has a drain terminal connected to the second terminal of the primary winding  130  for receiving the switching voltage Vsw, and a source terminal connected to a first terminal of a feedback resistor Rf and to an input terminal of the control circuit  105  for providing an open voltage Vop, whose purpose will be described in the following of the description. The MOSFET  150  further includes a gate terminal that receives (from an output terminal of the control circuit  105 ) a driving signal Vdr whose value determines the opening or the closure of the MOSFET  150 . The feedback resistor Rf has a second terminal connected to the ground terminal. 
         [0041]    When the MOSFET  150  is closed, i.e., when it is activated and the drain terminal is electrically coupled with the source terminal, the primary winding  130  receives (from the input rectifier block  110 ) a current that generates a corresponding magnetic flux increasing the energy stored in the transformer  125 . During this operating phase, and according to the reciprocal polarities of the windings  130  and  135  that are shown in the figure, the voltage applied to the diode  145  reverse biases it. Moreover, during this phase, the output capacitor  147 , which stores electric charge received in a previous phase, discharges onto the load so as to provide the requested current. 
         [0042]    When the MOSFET  150  is opened, i.e., when it is deactivated, and the drain terminal is electrically insulated from the source terminal, the current flowing into the primary winding  130  goes to zero. Moreover, the energy stored in the transformer  125  is transferred to the output load, which receives a current flowing from the secondary winding  135  through the diode  145  (being now forward biased). At the same time, the capacitor  147  is charged by the current coming from the secondary winding  135 . 
         [0043]    Consequently, the regulated output voltage Vrout takes a value that strongly depends on the duty cycle of the driving signal Vdr. 
         [0044]    The transformer  125  further includes an auxiliary winding  155  that is magnetically coupled with the primary winding  135  in such a way to be traversed by the same magnetic flux flowing therethrough. The voltage between the terminals of the auxiliary winding is provided to a voltage divider circuit  160  (for example, a resistive divider) to obtain a corresponding auxiliary voltage Vaux. According to the reciprocal polarities of the windings  130  and  155  that are shown in the figure, and according to the voltage ratio determined by the divider circuit  160 , the amplitude of the auxiliary voltage Vaux is proportional to the voltage between the first and second terminals of the primary winding (equal to Vin−Vsw) and has an opposite polarity. The auxiliary voltage Vaux is applied to another input terminal of the control circuit  105 . 
         [0045]    The control circuit  105  further includes an input terminal for receiving a setting signal SET whose purpose will be apparent in the following. 
         [0046]    According to an embodiment of the present invention, the control circuit  105  is capable of driving the flyback converter  100  according to two different working modes (being selectable by a user of the flyback converter  100 ). 
         [0047]    More particularly, as will be described in greater detail in the following description, according to a first working mode—referred to as “constant frequency (CF) mode”, in relationship with the operation of the CF control circuits described in the introduction of this document—the MOSFET  150  is driven by the control circuit  105  in such a way to switch between the open and the closed conditions with a constant switching frequency. Moreover, according to a second working mode—referred to as “Quasi-Resonant (QR) mode”, in relationship with the operation of the QR control circuits previously described—the MOSFET  150  is driven by the control circuit  105  with a switching frequency that varies over time, depending on the magnetization status of the transformer  125 . 
         [0048]    In this way, the same flyback converter  100  can be used in different scenarios. Indeed, the user of the flyback converter  100  is able to set the preferred working mode, based on the characteristics of the load and the voltage to be converted. 
         [0049]    In this way, the flyback converter  100  can provide the best performance in every scenario. For example, it is possible to work normally in the QR mode (between the CCM and the DCM). Moreover, it is possible to switch to the CF mode, so as to work in the CCM for high powers. 
         [0050]    Reference will be now made to  FIG. 1  together with  FIGS. 2 ,  3 A and  3 B, which are timing diagrams illustrating how the flyback converter  100  is driven when the control circuit  105  works in the CF and QR modes. 
         [0051]    As will be described in greater detail in the following description, all the operations carried out by the control circuit  105 —both in the CF and in the QR modes—are based on a clock signal OSC, which is internally generated in the control circuit  105  itself. 
         [0052]      FIG. 2  shows how the control circuit  105  operates in the CF mode. 
         [0053]    In the CF mode, the clock signal OSC oscillates with a period Tcf, which directly determines the switching frequency at which the MOSFET  150  of the flyback converter  100  switches between the open and the closed conditions. More particularly, according to this mode, the driving signal Vdr is generated based on the rising edges of the clock signal OSC: every time the clock signal OSC exhibits a rising edge, the driving signal Vdr is brought to a high value, in such a way to close or turn ON the MOSFET  150 . The driving signal Vdr remains at the high value for a period Tclosed that depends on the current flowing through the primary winding  130  of the transformer  125 . In particular, as far as the MOSFET  150  is in the closed condition, the current flowing through the primary winding  130  of the transformer  125  increases, so that the open voltage Vop accordingly increases. When the open voltage Vop reaches a predetermined value, the driving signal Vdr is brought to a low value, in such a way to open or turn OFF the MOSFET  150 . The MOSFET  150  then remains open (for a period Topen) until the next rising edge of the clock signal OSC. Consequently, by varying the frequency of the clock signal OSC it is possible to vary the switching frequency at which the MOSFET  150  switches, and, consequently, the transfer of electric power from the primary winding  130  to the secondary winding  135 . 
         [0054]      FIGS. 3A and 3B  show two different examples of how the control circuit  105  operates in the QR mode. 
         [0055]    Referring in particular to  FIG. 3A , the clock signal OSC oscillates between a high value and a low value with a variable duty cycle, which depends on the switching voltage Vsw. More particularly, the period at which the clock signal OSC is at the high value—referred to as high period Th—is kept constant, and can be set to a desired value by the user of the flyback converter  100 . On the contrary, the period at which the clock signal OSC is at the low value—referred to as low period Tl—varies, since it is determined by the occurrence of a ZVS condition. 
         [0056]    More specifically, when the MOSFET  150  is in the closed condition, i.e., when the driving signal Vdr is at the high value, the switching voltage Vsw is low, being substantially equal to the ground voltage. As in the CF mode, the driving signal Vdr remains at the high value for a period Tclosed that depends on the open voltage Vop, and then on the current flowing through the primary winding  130  of the transformer  125 . When the driving signal Vdr reaches the low value, the MOSFET  150  switches to the open condition. In this condition, the switching voltage Vsw rises, reaching—after a transient period characterized by the presence of oscillations—a stable value that depends on the input voltage Vin. During this period, the energy stored in the transformer  125  is transferred to the load, which is fed by a current provided by the secondary winding  135 . As known to those skilled in the art, when the current flowing in the secondary winding  135  reaches the null value—this condition is referred to as “Zero Current” (ZC) condition—, the switching voltage Vsw starts to oscillate with damped oscillations. Consequently, the switching voltage Vsw exhibits a sequence of falling edges followed by corresponding rising edges—which determine a corresponding sequence of minimum values. Since each minimum value of the sequence corresponds to a substantially null voltage, a corresponding sequence of possible ZVS conditions is established. 
         [0057]    As will be described in the following description, the control circuit  105  is able to identify the ZVS conditions by analyzing the behavior of the auxiliary voltage Vaux, since the amplitude of the auxiliary voltage Vaux is proportional to Vin−Vsw. At the instant at which the switching voltage Vsw has the first minimum value in the sequence, the control circuit  105  checks if the clock signal OSC is at the low value or not. 
         [0058]    In case the clock signal OSC is at the low value—that is, in case the high period Th is expired—the driving signal Vdr is driven to the high value, in such a way that the MOSFET  150  is switched to the closed condition. 
         [0059]    Conversely, in case the clock signal OSC is at the high value—i.e., in case the minimum value occurs when the high period Th is not expired yet—the driving signal Vdr is kept at the low value, so that the MOSFET  150  remains in the open condition. In this case, the control circuit  105  repeats the same operations for the next minimum value of the switching voltage Vsw in the sequence. 
         [0060]    In the example illustrated in the  FIG. 3A , the first minimum value of the switching voltage Vsw in the sequence occurs when the clock signal OSC is at the low value, so that the driving signal Vdr is switched to the high value immediately. Conversely, in the example illustrated in the  FIG. 3B  both the first two minimum values of the switching voltage Vsw in the sequence occur when the clock signal OSC is at the high value. In this case, the driving signal Vdr is switched to the high value at the third minimum value, which occurs when the clock signal OSC is at the low value. 
         [0061]    When the driving signal Vdr is driven to the high value, the clock signal OSC is reset to the high value. In this way, the clock signal OSC is always synchronized with the rising edges of the driving signal Vdr. 
         [0062]    Using this solution (also referred to as “valley skipping”, since each minimum value is a stationary point placed at the bottom of a “valley”), it is possible to establish the maximum switching frequency of the MOSFET  150  by setting the duration of the high period Th of the clock signal OSC. It has to be appreciated that this solution allows the MOSFET  150  to switch always in a ZVS condition, so that the capacitive losses are reduced. 
         [0063]    Referring now to  FIG. 4 , a schematic diagram of the main functional blocks included in the control circuit  105  is illustrated according to an embodiment of the present invention. 
         [0064]    The control circuit  105  includes an oscillator block  405  coupled with an internal clock generator block  407 . The oscillator block  405  has the purpose of generating the clock signal OSC based on the frequency of an internal clock signal ICK generated by the clock generator block  407 . According to an embodiment of the present invention, the frequency of the internal clock signal ICK, and thus the frequency of the clock signal OSC, can be set by means of the setting signal SET, which is fed to the internal clock generator block  407  from the outside. For example, the frequency of the internal clock signal ICK may be proportional to the amplitude of the setting signal SET. It has to be appreciated that, according to this solution, the operations carried out by the control circuit  105  in the CF mode and in the QR mode are based on a same signal, i.e., the internal clock signal ICK. Indeed, the clock signal OSC, whose behavior varies according to the selected mode, always depends on the frequency of the internal clock signal ICK. In this way the control circuit  105  has to include a single internal clock generator block  407 . 
         [0065]    The clock signal OSC is provided to a QR block  410  and to a first input of a selector  415 , for example a multiplexer. 
         [0066]    The QR block  410  is in charge of the management of the operations necessary for driving the flyback converter in the QR mode. The QR block  410  further receives the auxiliary voltage Vaux—from the voltage divider circuit of the flyback circuit—and accordingly generates a QR driving signal QRDR that is provided to a second input terminal of the selector  415 . 
         [0067]    Depending on a chosen mode between the CF and the QR modes, the clock signal OSC or the QR driving signal QRDR are selected by the selector  415  and are provided to a set input of a SR flip-flop  420 . Particularly, according to the value of a selection signal SEL provided to a selection terminal of the selector  415 , the set input of the SR flip-flop  420  is fed with the clock signal OSC (CF mode) or with the QR driving signal QRDR (QR mode). For this purpose, the control circuit  105  includes a buffer block  422  that is adapted to receive the selection signal SEL from the outside and to latch its value. According to an embodiment of the present invention, the selection signal SEL is fed to the buffer block  422  from the outside through the same terminal adapted to receive the auxiliary voltage Vaux. In this case, the values the selection signal SEL is expected to take must be different with respect to those of the voltage range defined by the auxiliary voltage Vaux. 
         [0068]    The SR flip-flop  420  further includes a reset terminal receiving a reset signal RES from an output of a comparator  423  and an output terminal coupled to a driver  425 . The driver  425 , for example, a non-inverting buffer circuit, is adapted to generate the driving voltage Vdr in response to the output of the SR flip-flop  420 . The comparator  423  has a first input for receiving (from the flyback converter) the open voltage Vop, and a second input receiving a predetermined reference voltage Vref. When the open voltage Vop is lower than the reference voltage Vref, the reset signal RES is at the low value. Conversely, as soon as the open voltage Vop becomes higher than the reference voltage Vref, the reset signal RES switches to the high value. 
         [0069]    The QR block  410  includes a zero detector block  430  receiving the auxiliary voltage Vaux, and outputting a zero detection indication ZD each time the auxiliary voltage Vaux exhibits a falling edge followed by a consecutive rising edge. Circuits of this type are known to those skilled in the art, and usually include a comparator with hysteresis. 
         [0070]    The zero detection indication ZD is provided to a valley skip block  435  and to an exceptions management block  440 . Both the valley skip block  435  and the exceptions management block  440  also receive the clock signal OSC from the oscillator block  405 . The exceptions management block  440  still further receives the internal clock signal ICK from the clock generator block  407 . 
         [0071]    Based on the value of the clock signal OSC, the valley skip block  435  may ignore the zero detection indication ZD or not, as it has been described in greater detail with reference to the  FIGS. 3A and 3B . If the zero detection indication ZD occurred when the clock signal OSC is at the low value (meaning that the corresponding ZVS condition respects the maximum frequency requirements), the valley skip block  435  provides a (positive) pulse to a first input of an OR logic port  445 , whose output generates the QR driving signal QRDR. 
         [0072]    The exceptions management block  440  has the purpose of generating a (positive) pulse to a second input of the OR logic port  445  in case the zero detector block  430  does not generate a zero detection indication ZD after a predetermined period—e.g., after a predetermined number of periods of the internal clock signal ICK—because the oscillations of the auxiliary voltage Vaux are too damped. Moreover, the exceptions management block  440  outputs a pulse to the second input of the OR logic port  445  at the power-on of the flyback converter. 
         [0073]    The output of the OR logic port  445  is further connected to a reset terminal of the oscillator block  405 , in such a way that when the QR driving signal QRDR goes to the high value (i.e., when the valley skip block  435  or the exceptions management block  440  output a positive pulse), the oscillator block  405  is reset, that is, the clock signal OSC is switched to the high value and a new high period Th is initiated. 
         [0074]    When the CF mode is selected the set terminal of the SR flip-flop  420  receives the clock signal OSC, and its reset terminal is fed with the reset signal RES. Thus, as it has been described with reference to  FIG. 2 , the driving signal Vdr is made to switch to the high value every time the clock signal OSC exhibits a rising edge, and to the low value every time the open voltage Vop reaches the value of the reference voltage Vref. In addition, the driving signal Vdr may be also made to switch to the low value after a predetermined maximum period—for example, proportional to the period of the internal clock signal ICK—is expired. 
         [0075]    When the QR mode is selected, the set terminal of the SR flip-flop  420  is fed with the QR driving signal QRDR, and its reset terminal is fed again with the reset signal RES. Thus, as it has been described with reference to  FIGS. 3A and 3B , the driving signal Vdr is made to switch to the high value every time a ZVS condition has been established (i.e., when a zero detection indication ZD is generated) under the maximum frequency requirements established by the high period Th of the clock signal OSC. In any case, the driving signal Vdr is made to switch to the high voltage after the maximum period without the detection of any ZVS condition. Moreover, the driving signal Vdr switches to the high value immediately after the power-on of the flyback converter (when no ZVS conditions are still established). As above, the driving signal Vdr is made to switch to the low value every time the open voltage Vop reaches the value of the reference voltage Vref. 
         [0076]    The control circuit  105  herein proposed requires a reduced number of input/output terminals for receiving/providing voltages from/to the flyback converter. Particularly, the control circuit  105  includes a terminal for providing the driving signal Vdr, a terminal for receiving the open voltage Vop, a terminal for receiving the auxiliary voltage Vaux and a terminal for receiving the setting signal SET. 
         [0077]    Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply to the solution described above many logical and/or physical modifications and alterations. More specifically, although the present invention has been described with a certain degree of particularity with reference to preferred embodiment(s) thereof, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible. Particularly, the proposed solution may even be practiced without the specific details (such as the numerical examples) set forth in the preceding description to provide a more thorough understanding thereof. Conversely, well-known features may have been omitted or simplified in order not to obscure the description with unnecessary particulars. Moreover, it is expressly intended that specific elements and/or method steps described in connection with any disclosed embodiment of the invention may be incorporated in any other embodiment as a matter of general design choice. 
         [0078]    Particularly, similar considerations apply if the control device has a different structure or includes equivalent elements. Moreover, the input voltage and/or the output voltage can take any value. In any case, the MOSFET may be replaced with a bipolar transistor, or more generally with any other equivalent switching element. 
         [0079]    Even if reference has been made to a control device for a flyback converter, similar considerations can be applied to different types of switching power supplies. 
         [0080]    In a very simplified implementation, it is also possible to provide two distinct oscillators for the CF mode and the QR mode. 
         [0081]    The concepts of the present invention are also applicable in case the zero current condition is detected in another way, for example, by directly analyzing the input voltage of the primary winding. 
         [0082]    In any case, concepts of the present invention are also applicable in case the means for detecting the stationary points (i.e., minimum or maximum values) in the auxiliary voltage are capable of detecting a stationary point different than the first stationary point in the auxiliary voltage after the minimum period defined by the maximum frequency. 
         [0083]    Similar considerations apply if the oscillator is replaced with any equivalent element capable of timing the CF mode and the QR mode (for example, based on a counter). 
         [0084]    Even if less efficient, the concepts of the present invention are also applicable without the presence of means for switching the switching element after a maximum period. 
         [0085]    Alternatively, it is possible to use the same circuit for switching the switching element after the maximum period also at the power-on of the flyback converter (even without a dedicated circuit for this purpose). 
         [0086]    Similar considerations apply if the control device has a different number of input and/or output terminals. 
         [0087]    It should be readily apparent that the proposed structure might be part of the design of an integrated circuit. Furthermore, the proposed solution may be integrated with other circuits in the same chip, or it may be mounted in intermediate products. In any case, the integrated circuit is suitable to be used in complex systems such as computer systems, control systems, and in portable electronic devices where Vin would typically be supplied by a battery and the rectifier block  110  omitted. 
         [0088]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention.