Abstract:
The present invention relates to a Switching Mode Power Supply (SMPS) and a driving method thereof. To this end, the present invention provides a SMPS, which is driven in either a Continuous Conduction Mode (CCM) or Discontinuous Conduction Mode (DCM) in accordance with a determination of a time when a voltage between a first stage and a second stage of the switching transistor is resonated. In accordance with the present invention, the SMPS of a quasi-resonant switching mode can be driven with high efficiency regardless of a signal level of an input voltage and the amount of an output terminal load.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2006-0119114 filed in the Korean Intellectual Property Office on Nov. 29, 2006, the entire content of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a Switching Mode Power Supply (hereinafter, referred to as “SMPS”). More particularly, the present invention relates to a SMPS of a quasi-resonant switching mode and a driving method thereof. 
         [0004]    2. Description of the Related Art 
         [0005]    The SMPS is a device for rectifying an input AC voltage to an input DC voltage (DC-Link voltage), and transforming the input DC voltage into a DC output voltage having a different level. SMPSs can be generally used in power electronic devices, for example, battery power supplies, such as mobile phones and laptop computers. 
         [0006]    In general, the SMPS shows great conduction loss when an input voltage is lower than a specific level and small conduction loss when an input voltage is higher than a specific level. However, in this latter case the SMPS has an increased switching loss, making it difficult to improve its efficiency. 
         [0007]    To solve the problem, a method of driving the SMPS in continuous conduction mode (hereinafter, referred to as “CCM”) or discontinuous conduction mode (hereinafter, referred to as “DCM”) has been proposed. 
         [0008]      FIG. 1  is a graph illustrating the relationship between an input voltage and efficiency when the SMPS is driven in the CCM or DCM of the prior art. When the SMPS is driven in the CCM, the conduction loss is advantageously small when an input voltage is low, but the switching loss disadvantageously increases and efficiency is degraded when the input voltage increases or an output terminal load decreases. In contrast, when the SMPS is driven in the DCM, the switching loss can be reduced when an input voltage increases or an output terminal load decreases, since a transistor is turned on using a quasi-resonant switching mode when a drain voltage of the main switch of the SMPS is the lowest. However, when the input voltage is low, the conduction loss is greater in the DCM than in the CCM. 
         [0009]    Therefore, there has been a need for a new driving method capable of improving driving efficiency of the conventional SMPS in the quasi-resonant switching mode. 
         [0010]    The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
       SUMMARY 
       [0011]    The present invention has been made in an effort to provide a SMPS and a driving method thereof. 
         [0012]    A SMPS according to an embodiment of the present invention includes a power supply unit comprising a switching transistor coupled to a first coil of a primary side of a transformer for transforming an input DC voltage, wherein the power supply unit supplies power to second and third coils of a secondary side of the transformer according to an operation of the switching transistor, and a Pulse Width Modulation (PWM) signal generator for receiving a feedback voltage corresponding to a first voltage generated form the second coil, a sense signal corresponding to current flowing through the switching transistor, and a third voltage corresponding to a second voltage generated from the third coil, and controlling turn on-off of the switching transistor. The PWM signal generator controls an on or off time of the switching transistor so that the SMPS is driven in either a Continuous Conduction Mode (CCM) or Discontinuous Conduction Mode (DCM) in accordance with a determination of a time when a voltage between a first stage and a second stage of the switching transistor is resonated. Furthermore, according to an embodiment of the present invention, there is provided a method of driving a SMPS that supplies power to a second coil and a third coil of a secondary side of a transformer according to an operation of a switching transistor coupled to a first coil of a primary side of the transformer for transforming an input DC voltage, the method including the steps of (a) sensing a turn-on time of the switching transistor, (b) generating a first pulse string signal to maintain a first level from the turn-on time to a first period, (c) determining whether a second voltage proportional to a first voltage generated in the third coil has a resonance waveform until a first time where the first pulse string signal shifts to a second level, (d) if, as a result of the determination, the second voltage does not have the resonance waveform until the first time, driving the SMPS in a CCM, and (e) if, as a result of the determination, the second voltage has the resonance waveform before the first time, driving the SMPS in a DCM. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a graph illustrating the relationship between an input voltage and an efficiency when a SMPS is driven in a CCM or DCM in the prior art. 
           [0014]      FIG. 2  is a circuit diagram showing an embodiment of a SMPS. 
           [0015]      FIG. 3  is a detailed circuit diagram of a PWM signal generator  500 . 
           [0016]      FIG. 4  illustrates output waveforms of constituent elements included in the PWM signal generator  500  and a waveform of a sync voltage Vsync when the SMPS is driven in the CCM. 
           [0017]      FIG. 5  illustrates output waveforms of constituent elements included in the PWM signal generator  500  and a waveform of a sync voltage Vsync when the SMPS is driven in the DCM. 
           [0018]      FIG. 6  is a view illustrating a variation of an operation mode of the SMPS depending on a variation of an input voltage and load. 
           [0019]      FIG. 7  is a view illustrating variation in a driving method in which the SMPS is driven in the CCM and then driven in the DCM as an input voltage Vin increases. 
           [0020]      FIG. 8  is a view illustrating variation in a driving method in which the SMPS is driven in the CCM and then driven in the DCM as an output terminal load of an output unit ( 200  in  FIG. 2 ) decreases. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
         [0022]    Throughout this specification and the claims that follow, when it is described that an element is “coupled” to another element, the element may be “directly coupled” to the other element or “coupled” to the other element through a third element. In addition, when it is described that a SMPS is driven in the DCM, the SMPS is operated in a quasi-resonant switching mode. 
         [0023]    To solve the above problems, the present invention provides a SMPS that is driven with high efficiency regardless of a signal level of an input voltage and the amount of an output terminal load, and a driving method thereof. 
         [0024]      FIG. 2  is a circuit diagram showing an embodiment of a SMPS. The SMPS may include a power supply unit  100 , an output unit  200 , a bias voltage supplier  300 , a sync voltage Vsync generator  400  and a Pulse Width Modulation (PWM) signal generator  500 . 
         [0025]    The power supply unit  100  may include a bridge diode BD for rectifying an AC input, a capacitor Cin for smoothing the rectified voltage, and a primary coil L 1  of a transformer having one terminal connected to the capacitor Cin. The power supply unit  100  may transform the AC voltage into a DC voltage Vin using the bridge diode BD and the capacitor Cin, and may supply power to a secondary side of the transformer according to the duty of a switching transistor Qsw. 
         [0026]    The output unit  200  may include a secondary coil L 2  of the transformer, a diode D 1  having an anode connected to one terminal of the secondary coil L 2  of the transformer, and a capacitor C 1  connected between a cathode of the diode D 1  and a ground terminal. The voltage across the capacitor C 1  is an output voltage V 0 . 
         [0027]    The bias voltage supplier  300  may include a secondary coil L 3  of the transformer, a diode D 2  having an anode connected to one terminal of the secondary coil L 3  of the transformer, and a capacitor C 2  connected between a cathode of the diode D 2  and a ground terminal. The PWM signal generator  500  can be generally implemented using an Integrated Circuit (IC). The bias voltage supplier  300  may supply a bias voltage Vcc for driving the IC of the PWM signal generator  500 . 
         [0028]    In operation, if the switching transistor Qsw is repeatedly turned on and off, a voltage Vw is induced in the secondary coil L 3  of the transformer. This induced voltage Vw charges the capacitor C 2  through the diode D 2 , generating a bias voltage Vcc. 
         [0029]    The sync voltage generator  400  may include a resistor R 1  having one terminal connected to one terminal of the secondary coil L 3  of the transformer, a resistor R 2  having one terminal connected to the other terminal of the resistor R 1  and the other terminal connected to a ground terminal, a diode D 3  having a cathode connected to a node between the resistors R 1  and R 2  and an anode connected to the ground terminal, and a capacitor C 3  having one terminal connected to the node between the resistors R 1  and R 2  and the other terminal connected to a ground terminal. The sync voltage generator  400  may serve to supply the sync voltage Vsync to the PWM signal generator  500 . The sync voltage Vsync results from the induced voltage Vw, which is generated in the secondary coil L 3  of the transformer, divided by the resistors R 1  and R 2 , and then charged across the capacitor C 3 . 
         [0030]    The PWM signal generator  500  can receive a feedback signal Vfb, and a signal Vsense, which is a sensed signal of a current Ids flowing through the switching transistor Qsw. The PWM signal generator  500  may compare the feedback signal Vfb and the signal Vsense, generate a pulse width modulation signal according to the comparison result, and output a gate control signal VGS to control the turn-on/off of the switching transistor Qsw. The feedback signal Vfb can be a signal having information corresponding to the output voltage V 0 , and can be used to decide a turn-off time of the switching transistor Qsw. 
         [0031]    The PWM signal generator  500  may determine that the SMPS will operate according to which one of the CCM and the DCM based on a type of the sync voltage Vsync supplied from the sync voltage generator  400 . This will be described in detail later on with reference to  FIGS. 3 to 8 . 
         [0032]      FIG. 3  is a detailed circuit diagram of the PWM signal generator  500 . Circuits related to a portion for receiving the feedback signal Vfb, and the signal Vsense, and producing the gate control signal VGS are omitted from  FIG. 3 . 
         [0033]    The PWM signal generator  500  may include a falling edge pulse generator  510 , a comparator  520 , an SR NAND latch  530 , a comparator  540 , a pulse string generator  550 , a D flip-flop  560  and a gate driver  570 . 
         [0034]    The falling edge pulse generator  510  can include an OR gate  512  and a signal delayer  514 . The falling edge pulse generator  510  can sense the gate control signal VGS input to the OR gate  512  as a high level signal when the gate control signal VGS is higher than a predetermined voltage, and the gate control signal VGS as a low level signal when the gate control signal VGS is lower than a predetermined voltage. 
         [0035]    The gate control signal VGS can be input to the A input terminal of the OR gate  512 , and input with a delay and inverted through the signal delayer  514  to the B input terminal of the OR gate  512 . 
         [0036]    The signal delayer  514  may include an odd number of inverters and thus inverts a received signal and outputs the result to the OR gate  512 . Inverters, having a number greater or smaller than three, can be also used based on the speed of elements included in the PWM signal generator  500  reacting to an input signal. 
         [0037]    If the gate control signal VGS is a high level, then a high level signal is input to the A input terminal of the OR gate  512 , and a low level signal is input to the B input terminal. If the gate control signal VGS is a low level, then a low level signal is input to the A input terminal of the OR gate  512 , and a high level signal is input to the B input terminal. 
         [0038]    If the gate control signal VGS shifts from a low level to a high level, a signal input to the A input terminal of the OR gate  512  shifts from a low level to a high level, but a signal input to the B input terminal remains at a high level for a signal delay time proportional to the number of inverters included in the signal delayer  514 , and only then shifts to a low level. Through this time, the output signal of the OR gate  512  continues to maintain a high level. 
         [0039]    If the gate control signal VGS shifts from a high level to a low level, a signal input to the A input terminal of the OR gate  512  shifts from a high level to a low level, but a signal input to the B input terminal remains at a low level as long for a signal delay time proportional to the number of inverters included in the signal delayer  514 , and then shifts to a high level. During this signal delay time, the output signal of the OR gate  512  shifts from a high level to a low level and then shifts back to a high level again. In other words, the output signal of the OR gate  512  shifts from a high level to a low level when a signal input to the A input terminal of the OR gate  512  shifts from a high level to a low level, and then shifts back to a high level again when a signal input to the B input terminal of the OR gate  512  shifts from a low level to a high level. This operation is different from when the gate control signal VGS shifts from a low level to a high level. Due to this difference, the output signal of the falling edge pulse generator  510  generates a short pulse, consisting of the output signal shifting to a low level only when the gate control signal VGS shifts from a high level to a low level, and then returning to a high level after a predetermined signal delay time. The output signal of the falling edge pulse generator  510  can be input to a set terminal S of the SR NAND latch  530 . 
         [0040]    The comparator  520  can compare the sync voltage Vsync input to a non-inverting input terminal and two reference voltages Vref 1  and Vref 2 , input to an inverting input terminal. The comparator  520  can generate a high level signal or a low level signal according to the comparison result and output a generated signal to a reset terminal R of the SR NAND latch  530  and the gate driver  570 . The comparator  520  can output a high level signal when the sync voltage Vsync is higher than the reference voltage Vref 2 , and output a low level signal when the sync voltage Vsync is lower than the reference voltage Vref 1 . The reference voltages Vref 1  and Vref 2  may be voltages that are previously set by a circuit designer, and the reference voltage Vref 2  is set higher than the reference voltage Vref 1 . If the sync voltage Vsync is higher than the reference voltage Vref 1  but lower than the reference voltage Vref 2 , the comparator  520  continues to output a previous output signal. The comparator  520  can be implemented using a Schmidt Trigger. 
         [0041]    The SR NAND latch  530  can output a high level or low level signal through a non-inverting output terminal Q, which is generated in response to the output signal of the falling edge pulse generator  510 , input to the set terminal S, and in response to the output signal of the comparator  520 , input to the reset terminal R. The output signal of the SR NAND latch  530  can be output to the “clear” signal input terminal CLR of the D flip-flop  560 . 
         [0042]    The comparator  540  can compare the sync voltage Vsync, input to a non-inverting input terminal and a reference voltage Vref 3 , input to an inverting input terminal, and output a high level signal when the sync voltage Vsync is higher than the reference voltage Vref 3  and output a low level signal when the sync voltage Vsync is lower than the reference voltage Vref 3 , as a result of the comparison. 
         [0043]    The reference voltage Vref 3  can be set higher than the reference voltages Vref 1  and Vref 2 . The reference voltages Vref 1 , Vref 2  and Vref 3  are described in detail below. 
         [0044]    The voltage Vw, induced in the secondary coil L 3  of the transformer is proportional to a turn ratio of the primary coil L 1 , the secondary coil L 2  and the secondary coil L 3  of the transformer, and is lower than a voltage between the drain and source of the switching transistor Qsw (hereinafter, referred to as “Vds voltage”) by the DC voltage Vin applied across the capacitor Cin included in the power supply unit  100 . 
         [0045]    The sync voltage Vsync can be proportional to the voltage Vw induced in the secondary coil L 3  of the transformer and the PWM signal generator  500  can compare the sync voltage Vsync with the reference voltages Vref 1 , Vref 2  and Vref 3 . 
         [0046]    The sync voltage Vsync can be a voltage that is divided from the voltage Vw induced in the secondary coil L 3  of the transformer and charged in the capacitor C 3 . Therefore, the sync voltage Vsync remains at a voltage that is less than 0V when the voltage Vw at the secondary coil L 3  of the transformer becomes negative due to a resonance. 
         [0047]    The resistance of the resistors R 1  and R 2  and the capacitance of the capacitor C 3  in the sync voltage generator  400  can determine the zero-crossing point where the sync voltage Vsync changes from a positive value to a negative value according to a resonance waveform of the voltage Vw by an RC time delay. 
         [0048]    The resistance of the resistors R 1  and R 2  and the capacitance of the capacitor C 3  can be set to values that enable the zero-crossing point to become the time where the drain voltage of the switching transistor Qsw becomes lowermost. 
         [0049]    The reference voltage Vref 1  can be used to determine whether the sync voltage Vsync has reached a zero-crossing point when the SMPS is driven in the DCM. In general, the reference voltage Vref 1  is set somewhat higher than 0V. The reference voltage Vref 2  can be used to determine whether the resonance waveform of the sync voltage Vsync rose past the zero-crossing point, and is set somewhat higher than the reference voltage Vref 1 . The reference voltage Vref 3  can be used to determine whether the sync voltage Vsync resonates, and set somewhat lower than the sync voltage Vsync before its resonance. 
         [0050]    The pulse string generator  550  can generate signals TB and TSmax, and supply the signal TB to a clock signal input terminal of the D flip-flop  560  and the signal TSmax to the gate driver  570 . The signal TB can be a pulse string signal, which assumes a high level for a predetermined time from a time at which the gate control signal VGS shifts to a high level, and then has a low level for a short time. The signal TB can serve to prevent the gate control signal VGS from shifting a low level to a high level while the signal TB is maintained a high level, so that the switching transistor Qsw is prevented from turning on. The signal TSmax can be a pulse string signal, which assumes a high level during a time in which a predetermined time TW is added to an interval where the signal TB is a high level, and then has a low level for a short time. The time TW can be time used to determine whether the sync voltage Vsync has reached the zero-crossing point after the signal TB shifted from a high level to a low level. In this case, the pulse string refers to a group of pulses that are continuously toggled at a constant duty. 
         [0051]    The D flip-flop  560  can output a signal received from the comparator  540  through a non-inverting output terminal Q when the TB signal falls from a high level to a low level (falling edge). 
         [0052]    The inverse of the output signal of the non-inverting output terminal Q of the SR NAND latch  530  can be input to the clear signal input terminal CLR of the D flip-flop  560 . Due to this, if the output signal of the non-inverting output terminal Q of the SR NAND latch  530  becomes a low level, a signal output through the non-inverting output terminal Q of the D flip-flop  560  becomes a low level regardless of the output signal of the comparator  540  and the signal TB. 
         [0053]    The gate driver  570  can prevent the gate control signal VGS from changing to a high level until the signal TB toggles from a high level to a low level. If the signal output through the non-inverting output terminal Q of the D flip-flop  560  has a high level until the signal TB toggles from a high level to a low level, the gate driver  570  drives the SMPS in the CCM by changing the gate control signal VGS to a high level, and drives the SMPS in the DCM in other cases. 
         [0054]    If it is determined that the sync voltage Vsync has reached the zero-crossing point while the signal TSmax remains a high level after the signal TB shifts to a low level, the gate driver  570  can change the gate control signal VGS to a high level, thus turning on the switching transistor Qsw. In this case, the gate driver  570  can employ the reference voltages Vref 1  and Vref 2  in order to determine whether the sync voltage Vsync has reached the zero-crossing point. The gate driver  570  may determine that the sync voltage Vsync rises if the output signal of the comparator  520  changes from a low level to a high level and consider that a time at which the output signal of the comparator  520  changes from a high level to a low level as a point where the sync voltage Vsync has reached the zero-crossing point, and thus changes the gate control signal VGS to a high level. 
         [0055]    If it was not determined that the sync voltage Vsync reached the zero-crossing point while the signal TSmax remained a high level after the signal TB signal shifted to a low level, that is, if the output signal of the comparator  520  does not change from a high level to a low level, the gate driver  570  may turn on the switching transistor Qsw by changing the gate control signal VGS to a high level simultaneously when the signal TSmax becomes a low level. 
         [0056]    The operation of the SMPS implemented through the above-mentioned PWM signal generator  500  is described in detail below with reference to  FIGS. 4 and 5 . 
         [0057]      FIG. 4  illustrates output waveforms of the elements included in the PWM signal generator  500  and the corresponding waveform of the sync voltage Vsync when the SMPS is driven in the CCM. 
         [0058]    In a period T 1 , the gate control signal VGS can be in a high level state, and the switching transistor Qsw is turned on accordingly. Since the gate control signal VGS is in a high level state, the output signal of the falling edge pulse generator  510  is a high level. Further, since the sync voltage Vsync is lower than the reference voltage Vref 1 , the output signal of the comparator  520  has a low level. Accordingly, the output signal of the non-inverting output terminal Q of the SR NAND latch  530  becomes a low level, and the non-inverting output terminal Q of the D flip-flop  560  outputs a low level signal. 
         [0059]    In a period T 2 , the gate control signal VGS can shift to a low level, and therefore the switching transistor Qsw is turned off. The period T 2  can be a short period before the output signal of the falling edge pulse generator  510  changes from a high level to a low level after the gate control signal VGS has changed to a low level. At this time, the voltage Vds rises. Correspondingly, the sync voltage Vsync rises and becomes higher than the reference voltage Vref 2 , so that the comparator  520  outputs a high level signal. Accordingly, the output signal of the non-inverting output terminal Q of the SR NAND latch (refer to  530  in  FIG. 3 ) becomes a low level, and the non-inverting output terminal Q of the D flip-flop  560  outputs a low level signal. 
         [0060]    In a period T 3 , the output signal of the falling edge pulse generator  510  can change from a high level to a low level. At this time, the sync voltage Vsync is still higher than the reference voltage Vref 2 , and the comparator  520  outputs a high level signal. Therefore, the output signal of the non-inverting output terminal Q of the SR NAND latch  530  changes from a low level to a high level. Meanwhile, since the sync voltage Vsync is higher than the reference voltage Vref 3 , the comparator  540  outputs a high level signal. At this time, the signal TB is not toggled, but remains a high level. Thus, the non-inverting output terminal Q of the D flip-flop  560  keeps outputting a low level signal. 
         [0061]    In a period T 4 , the output signal of the falling edge pulse generator  510  can shift from a low level to a high level. At this time, since the sync voltage Vsync is still higher than the reference voltage Vref 2 , the comparator  520  outputs a high level signal, and the output signal of the non-inverting output terminal Q of the SR NAND latch  530  remains a high level. Meanwhile, since the sync voltage Vsync is higher than the reference voltage Vref, the comparator  540  outputs a high level signal. At this time, the signal TB is not toggled, but remains a high level. Therefore, the non-inverting output terminal Q of the D flip-flop  560  keeps outputting a low level signal. 
         [0062]    In a period T 5 , the signal TB can change from a high level to a low level. Accordingly, the D flip-flop  560  outputs a high level signal, which is received from the comparator  540  through the non-inverting output terminal Q when the signal TB toggles from a high level to a low level. If it is determined that a signal received from the D flip-flop  560  shifts to a high level, the gate driver  570  changes the gate control signal VGS to a high level, thus turning on the switching transistor Qsw. In  FIG. 4 , the time at which the gate control signal VGS shifts to a high level is shown as the time at which the period T 6  begins because of a signal delay time is taken into consideration. 
         [0063]    In a period T 6 , as the switching transistor Qsw turns on, the voltage Vds can fall. Accordingly, the sync voltage Vsync falls and becomes lower than the reference voltage Vref 1 , so that the output signal of the comparator  520  changes from a high level to a low level. At this time, the output signal of the falling edge pulse generator  510  keeps a high level. Accordingly, the output signal of the non-inverting output terminal Q of the SR NAND latch  530  changes from a high level to a low level, and the output signal of the non-inverting output terminal Q of the D flip-flop  560  also changes from a high level to a low level signal regardless of the level of a signal received from the comparator  540 . 
         [0064]    Meanwhile, periods subsequent to the period T 6  are the same as those subsequent to the period T 2  and will not be described. 
         [0065]      FIG. 5  illustrates output waveforms of the elements included in the PWM signal generator  500  and a corresponding waveform of the sync voltage Vsync when the SMPS is driven in the DCM. 
         [0066]    In a period T 1 , since the gate control signal VGS is in a low level state, the switching transistor Qsw can be turned off. Since the gate control signal VGS is in a low level state, the output signal of the falling edge pulse generator  510  keeps a high level. Since the sync voltage Vsync is lower than the reference voltage Vref 1 , the output signal of the comparator  520  has a low level, and the output signal of the non-inverting output terminal Q of the SR NAND latch  530  become also a low level. At this time, the signal TB is not toggled, but remains a high level. Thus, the non-inverting output terminal Q of the D flip-flop  560  continues to output a low level signal. 
         [0067]    In a period T 2 , since the gate control signal VGS is in a low level state, the output signal of the falling edge pulse generator  510  can remain a high level, and since the sync voltage Vsync is higher than the reference voltage Vref 2 , the output signal of the comparator  520  has a high level. Accordingly, the output signal of the non-inverting output terminal Q of the SR NAND latch  530  keeps a low level, and the signal TB is not toggled and keeps a high level. Thus, the non-inverting output terminal Q of the D flip-flop  560  continues to output the low level signal. 
         [0068]    Periods from a period T 3  to a period T 6  are the same as the periods T 1  and T 2 , and will not be described in order to avoid redundancy. 
         [0069]    Thereafter, in a period T 7 , the signal TB can change from a high level to a low level. At this time, since the output signal of the non-inverting output terminal Q of the SR NAND latch  530  keeps a low level, the D flip-flop  560  outputs a signal, which is received from the comparator  540  through the non-inverting output terminal Q when the signal TB toggles from a high level to a low level. In the period T 7 , since the output signal of the comparator  540  is a low level signal, the non-inverting output terminal Q of the D flip-flop  560  outputs a low level signal. At this time, the sync voltage Vsync becomes lower than the reference voltage Vref 1  and, therefore, the gate driver  570  determines that the sync voltage Vsync has reached the zero-crossing point and changes the gate control signal VGS to a high level, thus turning on the switching transistor Qsw. In  FIG. 5 , the time at which the gate control signal VGS shifts to a high level is shown as the time at which the period T 8  begins because a signal delay time is taken into consideration. 
         [0070]    In the period T 8 , as the switching transistor Qsw is turned on, the voltage Vds can fall. Correspondingly, the sync voltage Vsync remains lower than the reference voltage Vref 1 , and the output signal of the comparator  520  remains a low level. At this time, the output signal of the falling edge pulse generator  510  remains a high level, and the output signal of the non-inverting output terminal Q of the SR NAND latch  530  remains a low level. In a period T 9 , the signal TB can be toggled from a low level to a high level, and the non-inverting output terminal Q of the D flip-flop  560  outputs the low level signal that is previously output. 
         [0071]    In the period T 9 , as the gate control signal VGS changes to a low level and the switching transistor Qsw turns off, the sync voltage Vsync can rise and thus become higher than the reference voltage Vref 2 . Accordingly, the output signal of the comparator  520  changes from a low level to a high level. The period T 9  can be a short period before the output signal of the falling edge pulse generator  510  changes from a high level to a low level after the gate control signal VGS has changed to a low level. At this time, as the output signal of the falling edge pulse generator  510  keeps a high level, the output signal of the non-inverting output terminal Q of the SR NAND latch  530  keeps a low level, and the non-inverting output terminal Q of the D flip-flop  560  continues outputting the low level signal. 
         [0072]    In a period T  10 , the output signal of the falling edge pulse generator  510  can change from a high level to a low level. At this time, since the sync voltage Vsync still keeps higher than the reference voltage Vref 2 , the comparator  520  outputs a high level signal. Accordingly, the output signal of the non-inverting output terminal Q of the SR NAND latch  530  changes from a low level to a high level. Meanwhile, since the sync voltage Vsync is higher than the reference voltage Vref 3 , the comparator  540  outputs a high level signal. At this time, the signal TB is not toggled, but keeps a high level. Thus, the non-inverting output terminal Q of the D flip-flop  560  continues outputting the low level signal. 
         [0073]    In a period T 11 , the output signal of the falling edge pulse generator  510  can change from a low level to a high level. At this time, since the sync voltage Vsync still remains higher than the reference voltage Vref 2 , the comparator  520  outputs a high level signal and the output signal of the non-inverting output terminal Q of the SR NAND latch  530  remains a high level. Meanwhile, since the sync voltage Vsync is higher than the reference voltage Vref 3 , the comparator  540  outputs a high level signal. At this time, the signal TB is not toggled, but remains a high level. Therefore, the non-inverting output terminal Q of the D flip-flop  560  continues outputting the low level signal that is previously output. The sync voltage Vsync may start resonance in the second half of the period T 11 . 
         [0074]    In a period T 12 , the sync voltage Vsync can resonate and fall below the reference voltage Vref 1 . Accordingly, the output signal of the comparator  520  changes from a high level to a low level. At this time, since the output signal of the falling edge pulse generator  510  keeps a high level, the output signal of the non-inverting output terminal Q of the SR NAND latch  530  changes from a high level to a low level. Meanwhile, as the sync voltage Vsync is lower than the reference voltage Vref 3 , the comparator  540  outputs a low level signal. At this time, the signal TB is not toggled, but remains a high level. Thus, the non-inverting output terminal Q of the D flip-flop  560  continues outputting the low level signal. Periods after the period T 12  are essentially the same as those following the period T 1 , and will not be described. 
         [0075]    Switching of a driving method of another SMPS, as illustrated in  FIGS. 3 to 5 , is described in short below. 
         [0076]    If the gate control signal VGS changes to a low level and the switching transistor Qsw is turned off, the falling edge pulse generator  510  outputs a low level signal for a short period of time. When the signal level of the sync voltage Vsync continues to remain higher than the reference voltage Vref 2  from a point of time at which the output signal of the falling edge pulse generator  510  becomes a low level (hereinafter, referred to as a “time A”) to a point of time at which the signal TB is toggled from a high level to a low level (hereinafter, referred to as a “time B”), the output signal of the non-inverting output terminal of the SR NAND latch  530  can remain a high level signal from time A to time B. If the sync voltage Vsync even temporarily falls below the reference voltage Vref 1  from time A to time B, the output signal of the SR NAND latch  530  changes to a low level and the output signal of the non-inverting output terminal Q of the D flip-flop  560  remains a low level. The sync voltage Vsync being lower than the reference voltage Vref 1  indicates that the sync voltage Vsync resonates. Furthermore, whether the sync voltage Vsync resonates within the period TB is dependent on the signal level of the input voltage Vin or the amount of the output terminal load. That is, the sync voltage Vsync resonates when the input voltage Vin is high or when the amount of the output terminal load is low. Meanwhile, if the sync voltage Vsync does not resonate, the sync voltage Vsync remains higher than the reference voltage Vref 3  from when the switching transistor Qsw is turned off to when the switching transistor Qsw is turned on next time. Accordingly, at time B, a high level signal is input to the gate driver  570  through the non-inverting output terminal Q of the D flip-flop  560  and the gate driver  570  drives the switching transistor Qsw in the CCM. 
         [0077]    In sum, the PWM signal generator  500  drives the switching transistor Qsw in the CCM or DCM according to the signal level of the sync voltage Vsync from time A to time B. 
         [0078]    Hereinafter, an embodiment of the driving principle of the SMPS is described with reference to  FIGS. 6 to 8 . 
         [0079]      FIG. 6  illustrates a variation of an operation mode of the SMPS depending on a variation of an input voltage and load. 
         [0080]    As mentioned above with reference to  FIGS. 2 to 5 , the SMPS of the quasi-resonant switching mode is selectively driven in the CCM or the DCM according to the signal level of the sync voltage Vsync. 
         [0081]    In  FIG. 6 , a region in which the SMPS is driven in the CCM varies according to an inductance value of the primary coil L 1  of the transformer included in the power supply unit  100 . In other words, if the inductance of the primary coil L 1  of the transformer is higher, the region in which the SMPS is driven in the CCM is enlarged even if the input voltage Vin or the amount of the output terminal load is the same. 
         [0082]      FIG. 7  illustrates an embodiment of the driving method in which the SMPS is driven in the CCM and then driven in the DCM as the input voltage Vin increases. 
         [0083]    From  FIGS. 7(   a ) to  7 ( c ), it can be seen that the slope of the current Ids flowing from the drain to the source of the switching transistor Qsw is proportional to the input voltage Vin. Visibly, the input voltage Vin increases from  FIG. 7(   a ) to  FIG. 7(   c ), and therefore the slope of the current Ids increases as well. 
         [0084]    As mentioned above, the SMPS is driven in the CCM when the input voltage Vin is low, and is driven in the DCM when the input voltage Vin rises. Accordingly, in  FIG. 7(   a ) in which the input voltage Vin is the lowest, the SMPS is driven in the CCM, and in  FIGS. 7(   b ) and  7 ( c ), the SMPS is driven in the DCM as the input voltage Vin rises. 
         [0085]      FIG. 8  illustrates an embodiment of the driving method in which the SMPS is driven in the CCM and then driven in the DCM as the output terminal load of the output unit  200  decreases. 
         [0086]    First, in  FIGS. 8(   a ) to  8 ( c ), the input voltage Vin is essentially the same and, therefore, the slope of the current Ids flowing from the drain to the source of the switching transistor Qsw is essentially the same. If the output terminal load of the output unit  200  decreases, the amount of the current Ids flowing from the drain to the source of the switching transistor Qsw decreases. Indeed, it can be seen that the output terminal load of the output unit  200  decreases from  FIG. 8(   a ) toward  FIG. 8(   c ). 
         [0087]    As mentioned above, the SMPS is driven in the CCM when the output terminal load of the output unit  200  is high, and is driven in the DCM if the output terminal load of the output unit  200  in  FIG. 2 . Accordingly, in  FIG. 8(   a ) in which the output terminal load of the output unit  200  is the highest, the SMPS is driven in the CCM. In  FIGS. 8(   b ) and  8 ( c ), the SMPS is driven in the DCM as the load is decreasing. 
         [0088]    The SMPS as described above is driven in the CCM or the DCM according to the input voltage Vin and the amount of the output terminal load, and therefore can be driven with high efficiency regardless of the input voltage Vin and the output terminal load. 
         [0089]    The exemplary embodiments of the present invention as described above are not implemented through only an apparatus and method, but can be implemented through a software, program, or data structure for realizing functions corresponding to the above embodiments on a recording medium in which the program is recorded. Such implementation will be evident to those having ordinary skill in the art from the description of the above embodiments. 
         [0090]    While this invention has been described in connection with particular embodiments, it is understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.