JP2007288987A - Dc-dc converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a DC-DC converter which does not employ a resistor for load detection to detect the magnitude of a load connected to an output line 108, and does not cause useless power consumption by the resistor for the load detection. <P>SOLUTION: An integrating circuit 10 which inputs a fly-back voltage generated in a second inductor 105 is connected in parallel to the second inductor 105. When the load connected to the output line 108 is light or zero, an output of the integrating circuit 10 becomes a predetermined value or less, the fly-back voltage is not generated in an entire off-time T2. Consequently, the load is determined to be light or zero from the output of the integrating circuit 10 to operate a switching control circuit 110 in an intermittent oscillation operating mode. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a DC-DC converter that converts an unstable DC power supply voltage on the input side and outputs a DC voltage subjected to constant voltage control.

  In the DC-DC converter, a direct current power source, a first inductance, and a main switching element are connected in series, and the main switching element is turned on and off at a high frequency, whereby an inductor current appearing in a second inductance coupled to the first inductor. Is rectified and smoothed, and a DC voltage boosted to a desired voltage is output between the output terminals.

  In such a DC-DC converter, the main switching element is controlled to be turned on and off at a fixed cycle regardless of the size of the load connected between the output terminals. Even when there is no load or light load at the time, the main switching circuit operates in the same manner and the switching operation is performed wastefully, so that power loss due to switching loss or the like has occurred.

  Therefore, in the conventional DC-DC converter, the magnitude of the load connected between the output terminals is determined, and when it is determined that the load is light or no load, the on / off control of the main switching element is temporarily suspended. The mode is shifted to the intermittent operation mode (see Patent Document 1).

Japanese Unexamined Patent Publication No. 2004-56981 (Specification, fifth page item 0027 to sixth page item 0030, FIGS. 2 to 4)

  This conventional DC-DC converter will be described using a SEPIC (single-end primary inductance converter) type DC-DC converter 100 shown in FIG.

  As shown in FIG. 7, reference numeral 101 denotes a battery as a DC input power supply, and a first inductor 102 and an FET 103 serving as a main switching element are connected in series between a high potential side terminal 101a and a low potential side terminal 101b. ing. The FET 103 is ON / OFF controlled at a fixed period T by a control IC 110 as a switching control circuit, and the current flowing through the first inductor 102 is ON / OFF controlled by this switching operation.

  A coupling capacitor 104 and a second inductor 105 constituting a CL coupling circuit are connected in series with the FET 103 in parallel. Further, a rectifying / smoothing circuit including a rectifying diode 106 and a smoothing capacitor 107 is connected in parallel with the second inductor 105. The circuit is connected, and the output voltage appearing at both ends of the second inductor 105 is rectified and smoothed between the high-voltage side output line 108A and the low-voltage side output line 108B at both ends of the smoothing capacitor 107 and output.

  A first voltage dividing resistor 111, 112 is connected in series between the high voltage side output line 108A and the low voltage side output line 108B, and the output voltage is divided by the resistance ratio of the first voltage dividing resistors 111, 112. The voltage is output from the intermediate connection point J1 to the control IC 110. During the continuous operation mode, the control IC 110 compares the divided voltage of the output voltage with a predetermined output setting voltage, changes the ON time within the fixed period T, that is, changes the ON duty ratio, and controls the switching of the FET 103. Constant voltage control is performed to match the voltage.

  Furthermore, between the high-voltage side output line 108A and the low-voltage side output line 108B, there are second voltage dividing resistors 113 and 114 for monitoring the output voltage in the intermittent operation mode when the load is light load or no load. Each is connected in series. The intermediate connection point J2 of the second voltage dividing resistors 113 and 114 is connected to the voltage detection terminal VS of the load detection IC 120, and the output input from the intermediate connection point J2 in the load detection IC 120 during the intermittent operation mode. The divided voltage is compared with a predetermined start voltage and stop voltage.

  Reference numeral 115 denotes a load detection resistor connected in series to the low-voltage side output line 108B, and both ends thereof are connected to the current detection terminal IS and the low-voltage side reference terminal GND of the load detection IC 120. The potential across the load detection resistor 115 is proportional to the size of the load connected between the high-voltage side output line 108A and the low-voltage side output line 108B during constant voltage control of the output voltage. When the potential between the current detection terminal IS and the low-voltage side reference terminal GND becomes equal to or higher than a predetermined value, the IC 120 determines that the load is a normal magnitude and that the load is light or no load when the potential is lower than the predetermined value. is doing.

  The output terminal OUT of the load detection IC 120 is connected to the high potential side terminal 101a via the pull-down resistor 116, and the connection point with the pull-down resistor 116 is connected to the operation control terminal ON / OFF of the control IC 110. Yes.

  The DC-DC converter 100 configured as described above is switched from the output terminal OUT to the operation control terminal ON / OFF of the control IC 110 while the load detection IC 120 determines that a normal load is connected. The ON signal is continuously output, and the control IC 110 is set to the continuous operation mode to enter a continuous operation state. As a result, the FET 103 is controlled to be turned on / off at a fixed period T, and the entire DC-DC converter 100 repeats the oscillation operation, whereby an output current corresponding to the load is generated between the high-voltage side output line 108A and the low-voltage side output line 108B. Is output. In this continuous operation mode, the on-duty ratio of the FET 103 is controlled, and constant voltage control is performed so that the output voltage becomes, for example, 5 V set from the output set voltage.

  When the load connected between the high-voltage side output line 108A and the low-voltage side output line 108B becomes light load or no load, the output current flowing through the load detection resistor 115 decreases. It is determined that there is no load, and the control IC 110 is controlled to operate in the intermittent operation mode. That is, in the intermittent operation mode, the control IC 110 itself is intermittently driven and controlled so that the output voltage between the high-voltage side output line 108A and the low-voltage side output line 108B is maintained at, for example, 2V set from the start voltage. When the output voltage drops below 2 V, an ON signal is output from the output terminal OUT of the load detection IC 120 to the operation control terminal ON / OFF of the control IC 110, and the control IC 110 enters an operating state.

  As a result, the control IC 110 controls the FET 103 to be turned on / off at a fixed period T. On the other hand, the load connected between the high-voltage side output line 108A and the low-voltage side output line 108B is a light load or no load. Rises quickly. When the output voltage exceeds, for example, 2.2 V set from the stop voltage, an OFF signal is output from the output terminal OUT of the load detection IC 120 to the operation control terminal ON / OFF of the control IC 110, and the operation of the control IC 110 itself operates. It becomes a dormant state to pause. As a result, the switching operation of the FET 103 is stopped and no power is supplied to the second inductor 105. Therefore, when the output voltage gradually decreases and falls below 2V set from the start voltage, the load detection IC 120 becomes the control IC 110. Is set to the operating state again. Thereafter, as long as the load detection IC 120 determines that the load is light or no load, the above operation is repeated, and the control IC 110 performs an intermittent operation.

  In the intermittent operation mode, when a normal load is connected between the high-voltage side output line 108A and the low-voltage side output line 108B, the current flowing through the output line 108B increases. Therefore, the load detection IC 120 again returns from the output terminal OUT. An ON signal is continuously output to the operation control terminal ON / OFF of the control IC 110, and the control IC 110 is shifted to the continuous operation mode.

  According to the DC-DC converter 100, when the load is light or no load, the control IC 110 is suspended, and during this time, the switching operation of the FET 103 is stopped. Can be prevented.

  However, in this conventional DC-DC converter 100, a load detection resistor 115 is connected in series to one of the high-voltage side output line 108A or the low-voltage side output line 108B, and the size of the load is determined from the current flowing through the output line. Therefore, even in the continuous operation mode in which a normal load is connected, the current of the output line flows through the load detection resistor 115, and wasteful power is consumed.

  If the resistance value of the load detection resistor 115 is lowered, the power consumption can be reduced. However, the current flowing at the boundary value for determining a light load from a normal load is weak, and the load detection IC 120 In order to generate a predetermined potential that can be determined as a light load, the lower limit is limited. For example, if a current flowing at a boundary value for determining a light load from a normal load is 10 mA, and the potential between the current detection terminal IS and the low-voltage side reference terminal GND is 50 mV or more, the load detection IC 120 can determine. The resistance value of the load detection resistor 115 cannot be 5 Ω or less. If the current of 1 A flows through the output line by switching to the continuous operation mode, at least 5 W of power is wasted in the load detection resistor 115. It will be consumed.

  Also, load detection is performed by determining the size of the load connected between the output lines from the potential between the current detection terminal IS and the low-voltage side reference terminal GND, and controlling the operation of the control IC 110 in the continuous operation mode and the intermittent operation mode. Therefore, there is a problem that the entire circuit is complicated and expensive.

  The present invention has been made in consideration of such conventional problems, and does not use a load detection resistor for detecting the magnitude of the load connected to the output line. It is an object of the present invention to provide a DC-DC converter that does not cause useless power consumption by the device.

  There is also provided a DC-DC converter that controls the operation of a switching control circuit according to the size of a load connected between output lines with a simple circuit without using a load detection IC that controls the operation of the control IC 110. For the purpose.

In order to achieve the above object, a DC-DC converter according to claim 1 includes a first inductor having one end connected to a high potential side terminal of a DC power source, and the other end of the first inductor connected to a low potential side of the DC power source. A main switching element connected to the terminal, a switching control circuit for controlling the main switching element on and off at a fixed period, a coupling capacitor connected to the other end of the first inductor, and the other end of the first inductor, A second inductor connected to the low potential side terminal of the DC power supply via the coupling capacitor, a rectifying / smoothing circuit for rectifying and smoothing the output appearing at both ends of the second inductor, and outputting between the pair of output terminals; A load determination circuit that outputs a light load determination signal when the load connected between the output terminals is determined to be light load or no load, and in a continuous operation mode The switching control circuit for controlling on / off of the main switching element so as to control the output voltage between the output terminals at a constant voltage. While the light load determination signal is output, the main switching element is turned on / off. A DC-DC converter that is in an intermittent operation mode in which control is temporarily suspended,
The load determination circuit includes an integrating circuit that is connected in parallel to the second inductor and that inputs a voltage generated in the second inductor during an OFF time in which the main switching element is controlled to be off. Outputting a light load determination signal from the integration circuit when the output voltage of the integration circuit that has input the voltage generated in the second inductor is lower than a predetermined set voltage during the on / off control. Features.

  When a normal load is connected between the output terminals while the output voltage is under constant voltage control, the energy release time is long, and the output current discharge ends before the OFF time ends. do not do. Therefore, the inductor current continuously flows through the second inductor during the OFF time, and a voltage substantially the same as the output voltage is generated during the entire OFF time. Since the capacitor of the integrating circuit connected in parallel with the second inductor is charged with the voltage across the second inductor during the entire OFF time, the output of the integrating circuit is set by repeating the ON / OFF operation of the main switching element. More than voltage. As a result, the light control signal is not output, and the switching control circuit continuously repeats the ON / OFF control of the main switching element in the continuous operation mode.

  When the load connected between the output terminals is light or no load, the energy release time is shortened, and the output current is interrupted before the OFF time ends. Therefore, the inductor current flowing through the second inductor is interrupted when the output current is interrupted, and the voltage generated at the second inductor during the OFF time is 0V. The charging time for charging the capacitor of the integration circuit with the voltage generated in the second inductor during the OFF time is shortened, and the output of the integration circuit does not reach the set voltage even when the main switching element is repeatedly turned on and off. A light load determination signal is output. As a result, the switching control circuit shifts to an intermittent operation mode in which on / off control of the main switching element is temporarily suspended.

  When a normal load is connected between the output terminals while the switching control circuit is operating in the intermittent operation mode, the main switching element is repeatedly controlled on and off within a fixed period. The discharge of the output current does not end, and the output of the integration circuit becomes equal to or higher than the set voltage. As a result, the output of the light load determination signal stops, and the switching control circuit returns to the continuous operation mode.

The DC-DC converter according to claim 2 includes a first inductor having one end connected to a high potential side terminal of a DC power supply, and a main switching element connecting the other end of the first inductor to a low potential side terminal of the DC power supply. A switching control circuit for controlling on / off of the main switching element at a fixed cycle, a coupling capacitor connected to the other end of the first inductor, and the other end of the first inductor via the coupling capacitor. A second inductor connected to the low potential side terminal of the power supply, a rectifying / smoothing circuit for rectifying and smoothing the output appearing at both ends of the second inductor and outputting the output between a pair of output terminals, and a load connected between the output terminals And a load determination circuit that outputs a light load determination signal when determining that the load is light or no load, and determines the output voltage between the output terminals in the continuous operation mode. The switching control circuit for controlling on / off of the main switching element so as to control the pressure is intermittently paused during on / off control of the main switching element while the light load determination signal is output. A DC-DC converter for operating mode,
The load determination circuit includes a current detection circuit that is connected in series to the first inductor or the second inductor and detects an inductor current flowing through the first inductor or the second inductor, and turns on the main switching element. A light load determination signal is output from the current detection circuit when an inductor current flowing through the first inductor or the second inductor becomes discontinuous within the fixed period during the off-control. .

  When a normal load is connected between the output terminals while the output voltage is under constant voltage control, the energy release time is long, and the output current discharge ends before the OFF time ends. do not do. Accordingly, the inductor current continuously flows through the first inductor or the second inductor during the OFF time, and the discontinuous state does not occur. The current detection circuit does not output a light load determination signal, and the switching control circuit continuously repeats on / off control of the main switching element in the continuous operation mode.

  When the load connected between the output terminals is light or no load, the energy release time is shortened, and the output current is interrupted before the OFF time ends. Accordingly, the inductor current flowing through the first inductor and the second inductor is interrupted during the OFF time and becomes discontinuous within a fixed period. Since the inductor current flowing through the first inductor or the second inductor becomes discontinuous within a fixed period, the current detection circuit outputs a light load determination signal, and the switching control circuit temporarily controls on / off of the main switching element. Transition to the intermittent operation mode in which it pauses.

  When a normal load is connected between the output terminals while the switching control circuit is operating in the intermittent operation mode, the main switching element is repeatedly controlled on and off within a fixed period. The discharge of the output current does not end, and the inductor current flowing through the first inductor and the second inductor is continuous. As a result, the output of the light load determination signal stops, and the switching control circuit returns to the continuous operation mode.

The DC-DC converter according to claim 3, wherein the fixed period is T, the inductance of the second inductor is L2, the set current flowing between the output terminals set to a predetermined value is Iset, the power supply voltage of the DC power supply is Vin, When the voltage between the output terminals for constant voltage control is Vout,
T = 2 * L2 * Iset * (1 / Vin + 1 / Vout) (1) From the equation (1), the inductance L2 of the second inductor is adjusted, and the current flowing between the output terminals is less than the set current Iset. The DC-DC converter according to claim 2, wherein a light load determination signal is output from the current detection circuit.

  If an inductor having an inductance L2 that satisfies the equation (1) is used as the second inductor with respect to the set current Iset that is arbitrarily set, when the current flowing between the output terminals is the set current Iset, , The energy release time, that is, the period until the output current ceases.

  Since the output voltage is controlled at a constant voltage and the load size is proportional to the output current, the boundary value of the load, which is the boundary between the normal load and the light load, can be set arbitrarily, and the boundary value load is connected between the output terminals. If the inductor L2 that satisfies the equation (1) is used as the second inductor, the current that flows when the current is applied is set as the second inductor. When a load having an arbitrarily set boundary value is connected, the continuous state of the inductor current The critical state is a discontinuous state, and the magnitude of the load can be determined from the continuity of the inductor current.

  According to the first to third aspects of the invention, since the load size can be detected without connecting the load detection resistor in series with the output line, it is useless due to the output current flowing through the load detection resistor. There is no power consumption.

  According to the first aspect of the present invention, the load connected between the output lines can be determined by a simple integration circuit without using a load detection IC. -A DC converter can be manufactured.

  According to the invention of claim 2, the load connected between the output lines with a simple current detection circuit that detects the presence / absence of a current without detecting a current value without using a load detection IC. Therefore, a DC-DC converter can be manufactured at a low cost without a complicated circuit.

  According to the invention of claim 3, since the magnitude of the actually connected load with respect to the arbitrarily set boundary value can be easily compared from the continuity of the inductor current, it is easy to determine light load or no load. Become.

  A SEPIC (single-end primary inductance converter) type DC-DC converter 1 according to an embodiment of the present invention will be described below with reference to FIGS. The SEPIC DC-DC converter is a step-down / step-up converter that can perform both step-down and step-up converters because constant voltage output control is possible even when the voltage of a battery cell serving as a DC power supply drops due to discharge. It is widely used as a DC-DC converter used in portable equipment.

  FIG. 1 is a circuit diagram of a DC-DC converter 1. Components that operate substantially the same as or similar to those of the conventional DC-DC converter 100 described above are denoted by the same reference numerals and detailed description thereof will be given. Omitted. As is clear from the DC-DC converter 100 shown in FIG. 7, the DC-DC converter 1 according to the present embodiment includes a load detection resistor 115 provided in the DC-DC converter 100, and a load detection resistor. In this circuit, the IC 120 is omitted.

  The DC input power supply 101 is an unstable power supply with a voltage fluctuation of about 10 to 20%. Between the high potential side terminal 101a and the low potential side terminal 101b, a first inductor 102 and an FET 103 serving as a main switching element are connected in series. It is connected to the. The gate of the FET 103 is connected to a control terminal DRAIVE 110 composed of a control IC as a switching control circuit. When the control IC 110 is in an operation state described later, the FET 103 is fixed to, for example, 800 nsec by the control IC 110. On / off control is performed at period T.

  In parallel with the FET 103, a second inductor 105 is connected via a coupling capacitor 104 connected to the first inductor 102, and a rectifying diode 106 having an anode connected to a connection point between the coupling capacitor 104 and the second inductor 105 is provided. It is connected. A smoothing capacitor 107 is connected between the cathode of the rectifying diode 106 and the low-potential side terminal 101b, and the output voltage appearing at both ends of the second inductor 105 is rectified and smoothed. Output between the low voltage side output lines 108B.

  The first voltage dividing resistors 111 and 112 for monitoring the output voltage in a continuous operation mode to be described later when a normal load is connected between the high voltage side output line 108A and the low voltage side output line 108B. However, in the intermittent operation mode described later when the load is light or no load, second voltage dividing resistors 113 and 114 for monitoring the output voltage are respectively connected in series.

  The intermediate connection point J1 between the first voltage dividing resistors 111 and 112 is connected to the input terminal SENCE of the control IC 110, so that the control IC 110 outputs from the intermediate connection point J1 during the continuous operation mode. The voltage divided voltage is compared with a predetermined output set voltage, and the ON time within the fixed period T, that is, the on-duty ratio is changed to control the switching of the FET 103, and the constant voltage control is performed so as to match the set voltage.

  The intermediate connection point J2 of the second voltage dividing resistors 113 and 114 is connected to the base of the drive control transistor 2 composed of an NPN transistor, and the collector of the drive control transistor 2 is connected via the pull-down resistor 116. The emitter is connected to the high potential side terminal 101a and the emitter is connected to the low potential side terminal 101b. The collector of the drive control transistor 2 is further connected to the operation control terminal ON / OFF of the control IC 110, so that the control IC 110 changes the pause state and the operation state in conjunction with the ON / OFF operation of the drive control transistor 2. The operation is shifted to the intermittent operation mode that repeats alternately.

  That is, during the intermittent operation mode, when the divided voltage of the output voltage input from the intermediate connection point J2 exceeds the operation voltage of the drive control transistor 2, the drive control transistor 2 is turned on, and the operation control terminal ON / OFF is turned on. The control IC 110 is put into a resting state by being pulled down to the potential of the low potential side terminal 101b. Further, when the divided voltage of the output voltage is lower than the operating voltage of the drive control transistor 2, the drive control transistor 2 is turned off to raise the operation control terminal ON / OFF to the potential of the high potential side terminal 101a, and control The IC 110 shifts to an operating state.

  As shown in the figure, the DC-DC converter 1 is connected in parallel to the first integrating resistor 4, the second integrating resistor 5, and the second integrating resistor 5 that are connected in series. An integrating circuit 10 including the integrating capacitor 6 is provided. The integrating circuit 10 is connected to the backflow blocking diode 3 having an anode connected to the connection point between the coupling capacitor 104 and the second inductor 105 so as to be in parallel with the second inductor 105, and a flyback generated in the second inductor 105. The integrating capacitor 6 is charged with voltage.

  The connection point J3 between the first integrating resistor 4 and the second integrating resistor 5, that is, the high-voltage side electrode of the integrating capacitor 6 is connected to the base of the load detecting transistor 7 constituted by an NPN transistor, thereby detecting the load. The collector of the transistor 7 is connected to the intermediate connection point J2 of the second voltage dividing resistors 113 and 114 via the resistor 8, and the emitter is connected to the low potential side terminal 101b. The resistance value of the resistor 8 is the resistance value of the second voltage dividing resistors 113 and 114 so that the base voltage of the drive control transistor 2 does not reach its operating potential when the load detection transistor 7 is turned on. Is sufficiently small.

  The SEPIC-type DC-DC converter 1 configured as described above is configured so that the FET 103 has a fixed period of 800 nsec by the control IC 110 operating in the continuous operation mode while a normal load is connected. On / off control is performed at T, and the entire DC-DC converter 1 repeats the oscillation operation, and an output current corresponding to the load is output between the high-voltage side output line 108A and the low-voltage side output line 108B. In this continuous operation mode, as in the above-described DC-DC converter 100, the divided voltage of the output voltage obtained from the first voltage dividing resistors 111 and 112 is compared with a predetermined output set voltage by the control IC 110, and is controlled. The IC 110 performs switching control of the FET 103 by changing the on-duty ratio within the fixed period T, and performs constant voltage control of the output voltage to, for example, 5.2 V set from the output setting voltage.

  FIG. 2 shows the DC-DC converter 1 shown in FIG. 1, in which the power supply voltage Vin of the DC input power supply 101 is set to 5 V and the output voltage Vout is controlled to a constant voltage of 5.2 V, while the FET 103 has a fixed period T of about 800 nsec. The drain-source voltage Vds of the FET 103 when ON / OFF control is performed, the current Ids flowing from the drain to the source, the first inductance current IL1 flowing through the first inductor 102, the second inductance current IL2 flowing through the second inductor 105, and rectification It is a wave form diagram which shows each waveform of the forward direction current ID of the diode 106 for an object.

In a state in which the DC-DC converter 1 continuously oscillates at a fixed period T of about 800 nsec, the first inductance current IL1 flowing through the first inductor 102 during the ON time T1 in the fixed period T is the power supply of the DC input power supply 101. If the voltage is Vin, the inductance of the first inductor 102 is L1, and the elapsed time after the FET 103 is turned on is t,
IL1 = Vin / L1 * t (2)
As shown in FIG. 2, the first inductor 102 is excited by flowing a first inductance current IL1 having a slope determined by the inductance L1 and the power supply voltage Vin through the FET 103.

At the same time, since one side of the coupling capacitor 104 is connected to the low potential side terminal 101b via the FET 103 that is turned ON, a negative voltage of the power supply voltage Vin is applied to the second inductor 105 by the charging voltage of the coupling capacitor 104. The second inductor 105 is also excited by flowing the second inductance current IL2 through the FET 103. If the inductance of the second inductor 105 is L2, the second inductance current IL2 flowing through the second inductor 105 during the ON time T1 is
IL2 = Vin / L2 * t (3)
The second inductance current IL2 reaches a peak when the ON time T1 elapses with a slope that is inversely proportional to the inductance L2 and proportional to the power supply voltage Vin.

  That is, during the ON time T1, the exciting currents IL1 and IL2 that increase with the passage of time flow to the first inductor 102 and the second inductor 105, respectively, and energy is accumulated.

  When the FET 103 is turned off and the OFF time T2 is reached, the flyback voltage generated at the other end of the first inductor 102 on the side connected to the coupling capacitor 104 becomes higher than the power supply voltage Vin as shown by the Vds voltage in FIG. In the state separated from the power supply voltage Vin by the coupling capacitor 104, power is supplied to the load connected between the output lines 108A and 108B through the rectifying diode 106 during the energy discharge time Tout. At the same time, a flyback voltage that is a reverse voltage is also generated in the second inductor 105, and as shown in FIGS. 3A and 3B, during the energy release time Tout, the first inductance current IL1 that decreases with time and the second inductance current IL1 The sum of the two inductance currents IL2 flows through the rectifying diode 106, and a part thereof becomes the output current Iout flowing through the output line 108, which is supplied to the load.

  When the load connected between the output lines 108A and 108B becomes heavy under the constant voltage control of the output voltage Vout, the first inductance current IL1 and the second inductance current IL2 also increase. As a result, when the load exceeds a predetermined magnitude, as shown in FIG. 3A, the first inductance current IL1 and the second inductance current IL2 continue to flow even after the fixed period T has elapsed, When the transition to the 0N time T1 occurs again, the first inductance current IL1 and the second inductance current IL2 that newly flow at the above-described 0N time T1 are superimposed on the inductance currents IL1 and IL2, and the output current Iout is transferred to the load. Supplied.

  In this way, in a state where a load having a predetermined magnitude is connected between the output lines 108A and 108B, energy release is not completed even when the OFF time T2 has elapsed, and therefore the second inductor L2 is always maintained throughout the OFF time T2. A flyback voltage is generated. The integrating capacitor 6 of the integrating circuit 10 is charged by this flyback voltage generated in the second inductor L2 during the OFF time T2 via the backflow blocking diode 3 and the first integrating resistor 4, and during the ON time T1. Although it is discharged through the second integrating resistor 5, as described above, it is charged by the flyback voltage throughout the OFF time T2, so that the charging voltage VJ3 rises by repeating oscillation at a fixed period T of about 800 nsec. As shown by A in FIG. 4, the load detection transistor 7 is stabilized beyond the operating voltage of 0.5V.

  As a result, the load detection transistor 7 is turned on, and, as shown in FIG. 4, in the drive control transistor 2, the potential VJ2 of the intermediate connection point J2 connected to the base is lowered to the vicinity of the potential of the low potential side terminal 101b. Operates off. Then, when the drive control transistor 2 is turned off, the operation control terminal ON / OFF is pulled up to the potential of the high potential side terminal 101a, and the control IC 110 enters an operating state. As long as a load of a predetermined size is connected to the output line 108, the charging voltage VJ3 of the integrating capacitor 6 is stable exceeding the operating voltage of the load detecting transistor 7, so that the control IC 110 operates. The device operates in a continuous operation mode in which the states are continuous, and the FET 103 is repeatedly turned on and off in a fixed period T.

  On the other hand, when the load connected between the output lines 108A and 108B becomes lighter, the output current Iout decreases, and the first inductance current IL1 and the second inductance current IL2 also decrease. As a result, as shown in FIG. 3B, the energy release time Tout becomes shorter than the OFF time T2, and a time during which the first inductance current IL1 and the second inductance current IL2 do not flow occurs during the fixed period T. The inductance current IL1 and the second inductance current IL2 are discontinuous.

  Here, when the energy release ends and the second inductance current IL2 is interrupted, the flyback voltage generated in the second inductor L2 also disappears. Accordingly, the integrating capacitor 6 of the integrating circuit 10 is charged by this flyback voltage generated in the second inductor L2 only during the energy discharge time Tout shorter than the OFF time T2, and therefore the charging voltage VJ3 is set by the control IC 110. Even when operating in the continuous operation mode, the operating voltage of the load detecting transistor 7 is not reached.

  When the control IC 110 is operating in the intermittent operation mode, the output voltage is set in the intermittent operation mode without the charging voltage VJ3 of the integrating capacitor 6 reaching the operating voltage of the load detection transistor 7. When the control voltage is exceeded, as shown by B in FIG. 5, the control IC 110 enters a halt state, the oscillation stops, and the charging voltage VJ3 returns to the potential of the low potential side terminal 101b.

  As a result, the load detection transistor 7 is turned off, and the base potential VJ2 of the drive control transistor 2 is changed only by the divided voltage of the output voltage divided by the second voltage dividing resistors 113 and 114. . That is, the drive control transistor 2 is turned on and off by the divided voltage of the output voltage input from the intermediate connection point J2 of the second voltage dividing resistors 113 and 114, and the control IC 110 is turned on and off of the drive control transistor 2. A transition is made to an intermittent operation mode in which a pause state and an operation state are alternately repeated in conjunction with the off operation.

  While the control IC 110 is operating in the intermittent operation mode, when the output voltage Vout exceeds 0.8 V, for example, the potential at the intermediate connection point J2 divided by the second voltage dividing resistors 113 and 114 is driven. The resistance ratio of the second voltage dividing resistors 113 and 114 is set so as to exceed the operating voltage of the control transistor 2, and when the output voltage Vout becomes less than 0.8 V, the control IC 110 becomes an operating state and is fixed. Continuous oscillation starts at period T. Immediately after the output voltage Vout exceeds 0.8 V, the oscillation state is stopped and the oscillation stops. However, even if the oscillation is suspended, the load connected to the output line 108 is light or no load. The voltage Vout slowly decreases to less than 0.8 V, and repeats the operation state after a pause of several seconds.

  Therefore, according to the DC-DC converter 1 according to the present embodiment, when the load is light or no load, the control IC 110 is suspended, and during this time, the switching operation of the FET 103 is stopped. In addition, power loss due to switching loss or the like can be prevented. Further, since the weight of the load connected between the output lines 108A and 108B can be determined from the charging voltage of the integrating capacitor 6 of the integrating circuit 10, a resistor for detecting the output current is connected to the output lines 108A and 108B. It can be configured with a simple circuit without using an IC circuit element.

  In the above-described embodiment, paying attention to the fact that the generation time of the flyback voltage generated in the second inductor L2 varies depending on the size of the load connected between the output lines 108A and 108B. The load magnitude is determined from the charging voltage of the integrating capacitor 6 to be detected, but the inductor currents IL1 and IL2 flowing through the first inductor 102 or the second inductor 105 are detected, and any one of the inductor currents is not detected during the OFF time T2. You may determine the magnitude | size of the load connected between output line 108A, 108B by whether it becomes continuous.

  FIG. 6 is a circuit diagram of the DC-DC converter 20 according to the second embodiment that detects the inductor current IL2 flowing through the second inductor 105 and determines the magnitude of the load. The components that operate substantially the same as or similar to each other are denoted by the same reference numerals, and detailed description thereof is omitted.

  As described above, the SEPIC DC-DC converter 20 performs the ON / OFF control of the FET 103 with the fixed period T under the constant voltage control of the output voltage Vout, as shown in FIG. When the load connected between the output lines 108A and 108B becomes heavy, the first inductance current IL1 and the second inductance current IL2 continue to flow even after the fixed period T has elapsed. On the other hand, when the load becomes lighter, as shown in FIG. 3B, the energy release time Tout becomes shorter than the OFF time T2, and the first inductance current IL1 and the second inductance current IL2 become discontinuous during the fixed period T. .

  In the DC-DC converter 20, the primary winding 21a of the current transformer 21 having one end connected to the low potential side terminal 101b is connected in series to the second inductor 105, and the continuity of the second inductance current IL2 is detected to detect the load. The size of is judged. As shown in FIG. 6, a backflow prevention diode 22 and a terminating resistor 23 are connected in series to both ends of the secondary winding 21b of the current transformer 21 whose one end is connected to the low potential side terminal 101b, and the OFF time T2 An integral input voltage is generated across the terminating resistor 23 in proportion to the second inductance current IL2 flowing through the second inductor 105. The second integration circuit 30 connected in parallel to the termination resistor 23 includes a third integration resistor 24, a fourth integration resistor 25, and a second integration capacitor 26, and the integration appearing at both ends of the termination resistor 23. An integrated output voltage obtained by integrating the input voltage is generated across the second integrating capacitor 26.

  Both ends of the second integration capacitor 26 are connected to the base and the emitter of the load detection transistor 7. Therefore, when the integrated output voltage exceeds the operating voltage of the load detection transistor 7, the load detection transistor 7 is turned on, The control IC 110 enters an operating state in conjunction with the drive control transistor 2 that is turned off.

  The DC-DC converter 20 configured as described above is in a state in which continuous oscillation is repeated at a fixed period T, and in a state where a load having a normal size is connected, even if the fixed period T elapses, The inductance current IL2 continues to flow. Since an integrated input voltage is generated at both ends of the termination resistor 23 during the entire OFF time T2, the integrated output voltage rises by repeating continuous oscillation at a fixed period T, and the operating voltage of the load detection transistor 7 is increased. Stable beyond.

  As a result, as in the first embodiment, the control IC 110 is in an operating state. As long as a load of a predetermined size is connected to the output line 108, the integrated output voltage is stable exceeding the operating voltage of the load detecting transistor 7, and therefore the control IC 110 is continuously connected in the operating state. It operates in the operation mode, and the FET 103 is repeatedly turned on and off at a fixed period T.

  On the other hand, when the load connected between the output lines 108A and 108B becomes lighter, the energy release time Tout becomes shorter than the OFF time T2, and the second inductance current IL2 becomes discontinuous during the fixed period T. Since the integrated input voltage that is input to the second integrating circuit 30 is generated only during the energy release time Tout that is shorter than the OFF time T2, the control IC 110 is continuously used as the integrated output voltage that is output from the second integrating circuit 30. Even when operating in the operation mode, the operating voltage of the load detection transistor 7 is not reached.

  Even when the control IC 110 operates in the intermittent operation mode, the output voltage is set in the intermittent operation mode by continuous oscillation without the charging voltage of the integrating capacitor 26 reaching the operation voltage of the load detection transistor 7. Exceeds the control voltage, and the control IC 110 is put into a halt state to stop oscillation.

  As described above, whether the control IC 110 is in the continuous operation mode or the intermittent operation mode, the integrated output voltage is detected when the control IC 110 does not reach the operation voltage of the load detection transistor 7. The control transistor 110 shifts to the intermittent operation mode in which the resting state and the operation state are alternately repeated only by the divided voltage of the output voltage divided by the second voltage dividing resistors 113 and 114 by the transistor for transistor 7.

  In the above-described embodiment, for example, as a DC-DC converter in which the output voltage Vout is controlled to a constant voltage of 5.2 V and the rated output is 1.5 W, when the load becomes less than 5 mW, The intermittent operation mode is determined, but the boundary of whether the control IC 110 is set to the continuous operation mode or the intermittent operation mode, that is, the boundary value of the load for determining light load or no load is a boundary value for load detection. It can be arbitrarily set by adjusting each circuit constant so that the base voltage of the transistor 7 becomes the operating voltage.

  Further, when a load having an arbitrary boundary value is connected by adjusting the inductance L2 of the second inductor 105 according to a load having a size arbitrarily set as a boundary value between a normal load and a light load. In addition, the critical state where the second inductance current IL2 shifts from continuous to discontinuous can be set, and the load weight can be more easily determined from the continuity of the second inductance current IL2.

In the critical state where the second inductance current IL2 shifts from continuous to discontinuous, the energy release time Tout during the OFF time T2 matches the OFF time T2, and the sum of the ON time T1 and the energy release time Tout matches the fixed period T. To do. That is,
T = T1 + Tout (4)

Here, ignoring the voltage drop caused by the FET 103 and the rectifying diode 106 and assuming that the efficiency of the input power and the output power is 100%, the average value of the second inductance current IL2 is equal to the output current Iout. If a set current flowing through the output line 108 is connected when a load having a size arbitrarily set as a boundary value between the load and the light load is connected, the set current Iset flows in the fixed period T. Since the average value of the second inductance current IL2 is ½ of the maximum output current that flows when the ON time T1 elapses, the inductance of the second inductor 105 is L2, and the power supply voltage of the DC input power supply 101 is Vin. Using the expression (3), the ON time T1 is
T1 = 2 * Iset * L2 / Vin (5)

The expression (3) can also be applied to the second inductance current IL2 that decreases at the same slope at the OFF time T2, and the sign can be applied. The output voltage for constant voltage control is Vout, and the expression (3) is similarly applied. If used, the OFF time T2, that is, the energy release time Tout is
Tout = 2 * Iset * L2 / Vout (6)

Therefore, the expression (4) is obtained by using these expressions (5) and (6).
T = 2 * L2 * Iset * (1 / Vin + 1 / Vout) (1) Since the inductance of the second inductor 105 other than L2 is obtained as a known constant, if an inductor having an inductance satisfying this equation (1) is used for the second inductor 105, a load less than the set boundary value is connected. Further, the second inductance current IL2 becomes discontinuous, and the control IC 110 can be shifted to the intermittent operation mode.

Further, the set current Iset can be expressed as Vout, where the output voltage for constant voltage control is Vout if the magnitude of the load set as the boundary value is Wset.
Since Iset = Wset / Vout (7),
Equation (1) is
T = 2 * L2 * Wset / Vout * (1 / Vin + 1 / Vout)... (8) is replaced with the value of the load arbitrarily set to the boundary value using the expression (8). The inductance of the inductor 105 may be obtained.

  In the present embodiment, since the second inductance current IL2 of the second inductor 105 is discontinuous, it is determined that the load is light or no load. However, the load similar to that of the above-described embodiment is applied to the first inductor 102. Since a determination circuit is provided and the first inductor current IL1 flowing through the first inductor 102 is discontinuous, it may be determined that the load is light or no load.

  The invention of the present application is not limited to the SEPIC system according to the above-described embodiment, but is a step-up, step-down, output inversion type DC-DC converter, or a switching power supply circuit that obtains an unstable DC power supply from an AC power supply. Can also be adapted.

  The present invention is suitable for a DC-DC converter to which loads of different sizes are connected.

1 is a circuit diagram of a DC-DC converter 1 according to an embodiment of the present invention. It is a wave form diagram of each part while the DC-DC converter 1 is oscillating. Regarding the DC-DC converter 1 oscillating at a fixed period T, (a) shows a case where a normal load is connected to the output line 108, and (b) shows a case where a light load is connected to the output line 108. 6 is a waveform diagram of a first inductance current IL1 flowing through the first inductor 102 and a second inductance current IL2 flowing through the second inductor 105 in the case. FIG. 6 is a waveform diagram showing a base voltage of the load detection transistor 7 and a base voltage of the drive control transistor 2 in a state where a light load to a normal load is connected to the output line 108. With the light load connected to the output line 108, the base voltage of the load detection transistor 7 when the control IC 110 operating in the intermittent operation mode changes from the sleep state to the temporary operation state, and the drive control transistor 2 It is a wave form diagram which shows the base voltage. It is a circuit diagram of the DC-DC converter 20 which concerns on 2nd Embodiment of this invention. 1 is a circuit diagram of a conventional DC-DC converter 100. FIG.

Explanation of symbols

1, 20 DC-DC converter 10 Integration circuit (load determination circuit)
21 Current transformer (load judgment circuit)
30 Second integration circuit (load determination circuit)
110 Control IC (switching control circuit)
101 DC input power supply 101a High voltage side terminal 101b Low voltage side terminal 102 First inductor 103 FET (main switching element)
104 coupling capacitor 105 second inductor 106 rectifier diode (rectifier smoothing circuit)
107 Smoothing capacitor (rectifying and smoothing circuit)
T fixed period T1 ON time T2 OFF time Tout Energy release period L1 Inductance L2 of first inductor Inductance IL1 of second inductor Inductor current IL2 of first inductor Indirect current Vin of second inductor DC input power supply voltage Vout Constant voltage controlled Output voltage Iout Output current Iset Set output current

Claims (3)

A first inductor having one end connected to the high potential side terminal of the DC power supply;
A main switching element for connecting the other end of the first inductor to a low potential side terminal of the DC power supply;
A switching control circuit for controlling on and off of the main switching element at a fixed period;
A coupling capacitor connected to the other end of the first inductor;
A second inductor that connects the other end of the first inductor to a low potential side terminal of the DC power supply via the coupling capacitor;
A rectifying / smoothing circuit for rectifying and smoothing the output appearing at both ends of the second inductor and outputting between the pair of output terminals;
A load determination circuit that outputs a light load determination signal when the load connected between the output terminals is determined to be a light load or no load;
The switching control circuit that controls on and off of the main switching element so that the output voltage between the output terminals is controlled at a constant voltage in a continuous operation mode. A DC-DC converter that is in an intermittent operation mode in which on / off control of an element is temporarily suspended,
The load determination circuit includes an integration circuit that is connected in parallel to the second inductor and inputs a voltage generated in the second inductor during an OFF time in which the main switching element is controlled to be off.
When the output voltage of the integration circuit that receives the voltage generated in the second inductor during the ON / OFF control of the main switching element is less than a predetermined set voltage, the light load determination signal is output from the integration circuit. DC-DC converter characterized by outputting. A first inductor having one end connected to the high potential side terminal of the DC power supply;
A main switching element for connecting the other end of the first inductor to a low potential side terminal of the DC power supply;
A switching control circuit for controlling on and off of the main switching element at a fixed period;
A coupling capacitor connected to the other end of the first inductor;
A second inductor that connects the other end of the first inductor to a low potential side terminal of the DC power supply via the coupling capacitor;
A rectifying / smoothing circuit for rectifying and smoothing the output appearing at both ends of the second inductor and outputting between the pair of output terminals;
A load determination circuit that outputs a light load determination signal when the load connected between the output terminals is determined to be a light load or no load;
The switching control circuit that controls on and off of the main switching element so that the output voltage between the output terminals is controlled at a constant voltage in a continuous operation mode. A DC-DC converter that is in an intermittent operation mode in which on / off control of an element is temporarily suspended,
The load determination circuit includes a current detection circuit that is connected in series to the first inductor or the second inductor and detects an inductor current flowing through the first inductor or the second inductor.
When the inductor current flowing through the first inductor or the second inductor becomes discontinuous within the fixed period while the main switching element is turned on / off, a light load determination signal is output from the current detection circuit. A DC-DC converter characterized by output. The fixed period is T, the inductance of the second inductor is L2, the set current flowing between the output terminals set to a predetermined value is Iset, the power supply voltage of the DC power supply is Vin, and the voltage between the output terminals for constant voltage control Is Vout,
T = 2 * L2 * Iset * (1 / Vin + 1 / Vout) (1) From the equation (1), the inductance L2 of the second inductor is adjusted,
The DC-DC converter according to claim 2, wherein a light load determination signal is output from the current detection circuit when a current flowing between the output terminals is less than the set current Iset.

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