JP6487743B2 - Switching power supply - Google Patents

Description

  The present invention relates to a switching power supply device using a power transformer.

  FIG. 7 shows a conventional self-excited switching power supply device (Patent Document 1). The operation of this self-excited switching power supply is as follows.

  When a circuit is activated when a DC voltage is applied between the high-voltage terminal 100a and the low-voltage terminal 100b by the power supply 100, the activation capacitor C12 is charged through the resistors R11 and R13 and the auxiliary winding L13 of the power transformer T10. . The starting capacitor C12 is connected in series with the resistors R11 and R13. For this reason, the voltage of the node N12 which is one terminal of the starting capacitor C12 gradually increases based on the time constant of the RC circuit.

  When the start-up capacitor C12 is charged to increase the voltage at the node N12 and the gate voltage of the NMOS power transistor MP2 reaches the threshold value, the power transistor MP2 is turned on. When the power transistor MP2 is turned on in this way, a current starts to flow through the primary winding L11 of the power transformer T10. Thereby, an induced electromotive force is generated in each of the windings L11, L12, and L13 of the power transformer T10. At this time, the induced voltage generated in the auxiliary winding L13 charges the control capacitor C11 via the resistor R12.

  The induced voltage generated in the auxiliary winding L13 at this time has a positive side on the side in the figure, so that a charging current flows from the auxiliary winding L13 to the resistor R15 and the diode D13, and the auxiliary capacitor C13 is also charged. The induced voltage generated in the auxiliary winding L13 is superimposed on the charging voltage of the starting capacitor C12 to increase the voltage at the node N12 and maintain the gate voltage of the power transistor MP2 at a voltage equal to or higher than the threshold voltage.

  As a result, the potential of the node N12 becomes higher than the potential of the auxiliary capacitor C13 to be charged, but current does not flow from the node N12 to the auxiliary capacitor C13 due to the diode D12 and the resistor R14, and the starting capacitor C12 is charged. The generated charge does not discharge. For this reason, the ON voltage on the gate side of the power transistor MP2 is maintained. During this ON operation, an excessive input to the gate of the power transistor MP2 is blocked by the Zener diode D14.

  Thereafter, when the charging voltage of the control capacitor C11 increases and reaches the threshold voltage of the NPN transistor Q11, the transistor Q11 becomes conductive. As a result, the power transistor MP2 is substantially short-circuited between the gate and the source by the transistor Q11 and turned off.

  Thus, when the power transistor MP2 is turned off and the current flowing through the primary winding L11 of the power transformer T10 is substantially cut off, a flyback voltage is generated in each winding of the power transformer T10. The flyback voltage generated in the secondary winding L12 is rectified and smoothed by a rectifying / smoothing circuit including a diode D11 and a capacitor C10, and is output as power supplied to a load connected between output terminals.

  On the other hand, the flyback voltage generated in the auxiliary winding L13 is proportional to the flyback voltage generated in the secondary winding L12 by the load connected to the output side, and the flyback voltage generated in the auxiliary winding L13. Thus, the starting capacitor C12 is charged.

  At this time, the Zener diode D14 becomes a charging path for charging the starting capacitor C12 from the low-voltage side terminal 100b side via the resistor R13, and reverse bias is applied to the gate of the power transistor MP2 to turn off the power transistor MP2. Acts to keep.

  At the same time, the charging voltage of the auxiliary capacitor C13 charged during the ON operation of the power transistor MP2 causes a discharge current to flow from the auxiliary capacitor C13 to the starting capacitor C12 via the diode D12 and the resistor R14. Charging speeds up. At this time, the discharge from the auxiliary capacitor C13 to the node N11 is blocked by the resistor R15 and the diode D13.

  When the discharge of the electrical energy accumulated in the secondary winding L12 is terminated by the induced counter electromotive force and the flyback voltage generated in the auxiliary winding L13 is also lowered, it has been held in the starting capacitor C12 until then. The charging voltage is applied to the gate of the power transistor MP2, turning on the power transistor MP2. The self-excited switching power supply device oscillates by repeating the series of operations as described above.

  Here, when the output side of the secondary winding L12 is short-circuited or the load is at a low voltage, the flyback voltage of the auxiliary winding L13, which is proportional to the flyback voltage of the secondary winding L12, is also obtained. Or does not contribute to charging the starting capacitor C12.

  Accordingly, the starting capacitor C12 is charged from the DC power supply 100 via the resistors R11 and R13 in the same manner as at the time of starting. However, during this off operation, the auxiliary capacitor C13 also receives the resistor R14 and the resistor R14. A charging current flows to the starting capacitor C12 via the diode D12.

  Thereby, in order to reduce the power consumption in the resistor R11, even if a resistor R11 having a large resistance value is used, it is shorter than the charging time determined by the capacitance of the starting capacitor C12 and the resistance values of the resistor R11 and the resistor R13. The starting capacitor C12 can be charged in the charging time. Accordingly, even when the starting capacitor C12 is charged again from the power supply 100, an ON voltage equal to or higher than the threshold can be quickly applied to the gate of the power transistor MP2, and the OFF period can be shortened.

JP 2001-327164 A

  In the self-excited switching power supply device of FIG. 7, when the power transistor MP2 is on, the node N11 which is one terminal of the auxiliary winding L13 becomes a positive voltage, and the capacitor C11 is charged by the resistor R12. A waiting time is formed until the transistor Q11 is turned on. When the transistor Q11 is turned on, the power transistor MP2 is turned off, and the node N11 becomes a negative voltage to charge the capacitor C12. Thereafter, when the conduction of the diode D11 of the secondary winding L12 ends and the voltage at the node N11 rises, the gate of the power transistor MP2 also rises and the power transistor MP2 is turned on. That is, the power transistor MP2 is turned on / off by detecting the polarity of the voltage of the auxiliary winding L13 (the voltage of the node N11).

  However, when the control circuit shown in FIG. 7 for controlling the self-excited switching power supply device that generates the output voltage by controlling the on / off of the power transistor MP2 is realized by a semiconductor integrated circuit (IC), the auxiliary winding L13 is used. If an attempt is made to detect the on / off state of the power transistor MP2 by using the positive or negative voltage generated in the circuit as it is, there is the following problem. That is, since a state in which a negative voltage is applied to the input terminal (node N11) of the semiconductor integrated circuit occurs, a complicated and expensive triple well or SOI process corresponding to the negative voltage operation is formed when the semiconductor integrated circuit is configured. There is a problem that the semiconductor process must be used.

  An object of the present invention is to provide a switching power supply device that can realize a control circuit with a semiconductor integrated circuit using a conventional inexpensive process without using a complicated and expensive triple well or SOI process.

  In order to achieve the above object, the invention according to claim 1 includes a primary winding having one end connected to the first input terminal and the other end connected to the second input terminal via the power transistor; A power transformer having a secondary winding to which a rectifying and smoothing circuit using a rectifier diode and an output capacitor is connected, and an auxiliary winding having one end connected to the control circuit and the other end connected to the second input terminal In the switching power supply device that extracts the output voltage from the secondary winding by controlling the power transistor on / off by the control circuit, the control circuit is generated in the auxiliary winding when the power transistor is turned on. A voltage / current conversion circuit that converts the positive voltage to be converted into a positive current and outputs the negative voltage generated in the auxiliary winding when the power transistor is turned off, and outputs the negative voltage. A voltage clamp circuit that generates a first clamp voltage based on the positive current output from the voltage / current conversion circuit and generates a second clamp voltage based on the negative current output from the voltage / current conversion circuit One of charging and discharging is performed when the first clamping voltage is generated in the voltage clamping circuit, and the charging is performed when the second clamping voltage is generated in the voltage clamping circuit. And a drive circuit for turning on / off the power transistor in accordance with a value of a voltage charged in the first capacitor.

  According to the second aspect of the present invention, a primary winding having one end connected to the first input terminal and the other end connected to the second input terminal via the power transistor, rectification smoothing by a rectifier diode and an output capacitor A power transformer having a secondary winding to which a circuit is connected and an auxiliary winding having one end connected to the control circuit and the other end connected to the second input terminal; In the switching power supply device that extracts the output voltage from the secondary winding by controlling on / off of the power supply, the control circuit converts the positive voltage generated in the auxiliary winding to a positive current when the power transistor is turned on. A voltage / current conversion circuit that converts a negative voltage generated in the auxiliary winding into a negative current when the power transistor is turned off, and outputs the negative current, and the voltage / current conversion circuit A voltage clamp circuit that generates a first clamp voltage based on the positive current applied and generates a second clamp voltage based on the negative current output from the voltage / current conversion circuit; and the voltage clamp circuit A first capacitor that is charged when the first clamp voltage is generated and discharged when the second clamp voltage is generated by the voltage clamp circuit, and the first capacitor is charged. Drive that turns off the power transistor when the applied voltage exceeds the first voltage, and turns on the power transistor when the voltage charged in the first capacitor falls below a second voltage lower than the first voltage And a circuit.

  According to a third aspect of the present invention, there is provided a primary winding having one end connected to the first input terminal and the other end connected to the second input terminal via the power transistor, and rectification smoothing by a rectifier diode and an output capacitor. A power transformer having a secondary winding to which a circuit is connected and an auxiliary winding having one end connected to the control circuit and the other end connected to the second input terminal; In the switching power supply device that extracts the output voltage from the secondary winding by controlling on / off of the power supply, the control circuit converts the positive voltage generated in the auxiliary winding to a positive current when the power transistor is turned on. A voltage / current conversion circuit that converts a negative voltage generated in the auxiliary winding into a negative current when the power transistor is turned off, and outputs the negative current, and the voltage / current conversion circuit A voltage clamp circuit that generates a first clamp voltage based on the positive current applied and generates a second clamp voltage based on the negative current output from the voltage / current conversion circuit; and the voltage clamp circuit A first capacitor that is charged when the first clamp voltage is generated and discharged when the second clamp voltage is generated in the voltage clamp circuit; and 1 is charged independently of the first capacitor when the clamp voltage is generated, and independent of the first capacitor when the second clamp voltage is generated in the voltage clamp circuit. A second capacitor to be discharged; a timing current generating circuit that charges the second capacitor with a larger current as the output voltage of the rectifying and smoothing circuit is lower; When the voltage charged in the capacitor exceeds the first voltage, the power transistor is turned off and the second capacitor is discharged. When the voltage charged in the second capacitor exceeds the third voltage, the power transistor is turned off. And a drive circuit for turning on the power transistor.

According to a fourth aspect of the present invention, there is provided a primary winding having one end connected to the first input terminal and the other end connected to the second input terminal via the power transistor, and rectification smoothing by a rectifier diode and an output capacitor. A power transformer having a secondary winding to which a circuit is connected and an auxiliary winding having one end connected to the control circuit and the other end connected to the second input terminal; In the switching power supply device that extracts the output voltage from the secondary winding by controlling on / off of the power supply, the control circuit converts the positive voltage generated in the auxiliary winding to a positive current when the power transistor is turned on. A voltage / current conversion circuit that converts a negative voltage generated in the auxiliary winding into a negative current when the power transistor is turned off, and outputs the negative current, and the voltage / current conversion circuit A voltage clamp circuit that generates a first clamp voltage based on the positive current applied and generates a second clamp voltage based on the negative current output from the voltage / current conversion circuit; and the voltage clamp circuit A first capacitor that is charged when the first clamp voltage is generated and discharged when the second clamp voltage is generated in the voltage clamp circuit, and an output voltage of the rectifying and smoothing circuit a timing current generating circuit that charges a large current is as low as the second capacitor, the second capacitor together with the voltage charged in the first capacitor turns off the said power transistor exceeds a first voltage A drive circuit that discharges and turns on the power transistor when a voltage charged in the second capacitor exceeds a third voltage; Characterized in that it obtain.

  According to a fifth aspect of the present invention, in the switching power supply device according to the first, second, or fourth aspect, the voltage clamp circuit generates a base-emitter voltage when the positive current is output from the voltage / current conversion circuit. A first transistor that generates as the first clamp voltage; and a second transistor that generates a base-emitter voltage as the second clamp voltage when the negative current is output from the voltage / current conversion circuit; A first current mirror circuit group that outputs a current corresponding to the collector current of the first transistor as a charging current for the first capacitor; and a current corresponding to the collector current of the second transistor. And a second current mirror circuit group that outputs a discharge current to the capacitor.

  According to a sixth aspect of the present invention, in the switching power supply device according to the third aspect, the voltage clamp circuit generates a base-emitter voltage when the positive current is output from the voltage / current conversion circuit. A first transistor that generates a clamp voltage; a second transistor that generates a base-emitter voltage as the second clamp voltage when the negative current is output from the voltage / current conversion circuit; A first current mirror circuit group for outputting a current corresponding to the collector current of the first transistor as a charging current for the first capacitor; and a current corresponding to the collector current of the second transistor for discharging to the first capacitor. A second current mirror circuit group that outputs current and a collector current of the first transistor; A third current mirror circuit group that outputs the current that has been output as a charging current for the second capacitor, and a fourth current that outputs a current corresponding to the collector current of the second transistor as a discharge current for the second capacitor. And a current mirror circuit group.

  According to a seventh aspect of the present invention, in the switching power supply device according to the second aspect, the voltage / current conversion circuit turns on / off the power transistor by making the positive current value different from the negative current value. The duty is set.

  According to an eighth aspect of the present invention, in the switching power supply device according to the second aspect, a constant current source for charging the first capacitor with a constant current is provided.

  According to the present invention, a positive voltage or a negative voltage generated in the auxiliary winding is converted into a positive current or a negative current by the voltage / current conversion circuit, and then taken into the voltage clamp circuit. Since the first clamp voltage or the second clamp voltage corresponding to the negative current is generated to control the on / off of the power transistor, it is not necessary to incorporate the negative voltage into the voltage clamp circuit. Therefore, it is not necessary to use a complicated and expensive triple well or SOI process when the control circuit constituting this voltage clamp and the subsequent circuit is constituted by a semiconductor integrated circuit, and the semiconductor integrated circuit can be obtained by a conventional inexpensive process. Can be realized.

1 is a circuit diagram of a switching power supply device according to a first embodiment of the present invention. It is an operation | movement waveform diagram of the switching power supply device of FIG. It is a circuit diagram of the switching power supply device of the 2nd Example of this invention. It is a circuit diagram of the switching power supply device of the 3rd Example of this invention. FIG. 5 is an operation waveform diagram of the switching power supply device of FIG. 4. It is a circuit diagram of the switching power supply device of the 4th Example of this invention. It is a circuit diagram of a conventional self-excited switching power supply device.

<First embodiment>
FIG. 1 shows a self-excited switching power supply of the first embodiment. T1 is a power transformer, and includes a primary winding L1, a secondary winding L2, and an auxiliary winding L3. The winding start (● mark) of the primary winding L1 is connected to the positive input terminal IN +, and the winding end is connected to the drain of the NMOS power transistor MP1. The source of the power transistor MP1 is connected to the negative input terminal IN−. An input voltage VIN is input between both input terminals IN + and IN−, and an input capacitor C1 is connected thereto. The anode of the rectifier diode D1 is connected to the end of the secondary winding L2 of the power transformer T1, and one end of the output capacitor C2 is connected to the cathode of the rectifier diode D1. The other end of the output capacitor C2 is connected to the winding start (marked with ●) of the secondary winding L2. The output capacitor C2 is connected between the positive output terminal OUT + and the negative output terminal OUT−, and performs a smoothing operation on the output voltage VOUT. These parts are the main components of the switching power supply, and are the same as the ringing choke converter (RCC) and the flyback power supply.

  In this embodiment, a voltage / current conversion circuit 10, a voltage clamp circuit 20, a capacitor C3, and a drive circuit 30 are provided as control circuits for controlling on / off of the power transistor MP1. The voltage / current conversion circuit 10 is connected to the winding start (● mark) of the auxiliary winding L3 of the power transformer T1. The output side of the voltage / current conversion circuit 10 is connected to the input side of the voltage clamp circuit 20, and one end of a capacitor C3 is connected to the output side of the voltage clamp circuit 20. The other end of the capacitor C3 is connected to the ground. The output side of the voltage clamp circuit 20 is connected to the input side of the drive circuit 30, and the output side of the drive circuit 30 is connected to the gate of the power transistor MP1.

Next, the operation will be described. The output voltage on the winding start side of the auxiliary winding L3 is VL3, and the clamp voltage on the input side of the voltage clamp circuit 20 is VFB. When the conductance inside the voltage / current conversion circuit 10 is G1, the current IFB input from the voltage / current conversion circuit 10 to the voltage clamp circuit 20 is expressed as follows.

  As shown in FIG. 2, the waveform of the output voltage VL3 of the auxiliary winding L3 changes depending on on / off of the power transistor MP1. FIG. 2 shows waveforms assuming that the operation is in the critical mode.

となる。Ｎ１は１次巻線Ｌ１の巻数、Ｎ３は補助巻線Ｌ３の巻数である。よって式（１）で表された電流ＩＦＢ（ＯＮ）は次のように表される。

When the power transistor MP1 is on, the output voltage VL3 (ON) of the auxiliary winding L3 is

It becomes. N1 is the number of turns of the primary winding L1, and N3 is the number of turns of the auxiliary winding L3. Therefore, the current IFB (ON) expressed by the equation (1) is expressed as follows.

と負電圧になる。Ｎ２は２次巻線Ｌ２の巻数、ＶＦはダイオードＤ１の順方向電圧である。よって式（１）で表された電流ＩＦＢ（ＯＦＦ）は次のように表される。

この電流ＩＦＢは負になり、パワートランジスタＭＰ１がオンしているときとは逆方向に流れる電流となる。 On the other hand, the output voltage VL3 (OFF) of the auxiliary winding L3 when the power transistor MP1 is off and the rectifier diode D1 on the secondary winding L2 side is conductive is:

And negative voltage. N2 is the number of turns of the secondary winding L2, and VF is the forward voltage of the diode D1. Therefore, the current IFB (OFF) expressed by the equation (1) is expressed as follows.

This current IFB becomes negative and becomes a current that flows in the opposite direction to that when the power transistor MP1 is on.

  In the voltage clamp circuit 20, these current IFBs are caused to flow from the voltage / current conversion circuit 10 into the internal circuit, or from the inside toward the voltage / current conversion circuit 10, and the current IFB flows in the direction (polarity). A corresponding first and second clamp voltage is generated. Then, one of the two types of currents corresponding to the two types of clamp voltages is output using a current mirror circuit or the like. A capacitor C3 is connected to the output side of the voltage clamp circuit 20. When the capacitor C3 is charged or discharged according to the clamp voltage, the voltage VC3 of the capacitor C3 has a waveform shown in FIG. In this way, the capacitor C3 is charged when the power transistor MP1 is on and discharged when it is off.

  Then, according to the voltage VC3 of the capacitor C3, the drive circuit 30 generates a voltage for turning on / off the power transistor MP1, and drives the gate of the power transistor MP1. In this operation, as shown in FIG. 2, the power transistor MP1 is turned off when the voltage VC3 of the capacitor C3 is charged to the voltage V1, and is turned on when discharged to the voltage V2, and this is repeated.

  This control assumes a switching power supply that operates in a critical mode such as a ringing choke converter (RCC), but is not limited thereto. For example, the drive circuit 30 generates a PWM (pulse width modulation) signal whose duty is controlled according to the voltage VC3 of the capacitor C3, or a PFM (pulse frequency modulation) whose frequency is controlled according to the voltage VC3 of the capacitor C3. ) Signal may be generated, and the power transistor MP1 may be controlled by these PWM signal and PFM signal. Furthermore, a hysteresis control system in which the threshold value is controlled according to the voltage VC3 of the capacitor C3 can be adopted as the drive circuit 30.

  As described above, in the first embodiment, the positive and negative voltages VL3 generated in the auxiliary winding L3 are converted into the current IFB having polarity, and the voltage clamp circuit is changed according to the polarity of the current IFB. In FIG. 20, two types of clamp voltages are generated, and the capacitor C3 is charged and discharged with two types of currents corresponding to the two types of clamp voltages, so that the power transistor MP1 is turned on / off. For this reason, no negative voltage is applied to the voltage clamp circuit 20, and when the voltage clamp circuit 20 as a control circuit or the drive circuit 30 in the subsequent stage is configured by a semiconductor integrated circuit, a complicated and expensive triple well or SOI There is no need to use a semiconductor process such as a process.

<Second embodiment>
FIG. 3 shows the configuration of the switching power supply device of the second embodiment. This embodiment is an example in which the circuit of FIG. 1 is embodied. In this embodiment, the voltage / current conversion circuit 10 includes a resistor R1 and a series circuit of a resistor R2 and a diode D2 connected in parallel to the resistor R1. Further, the voltage clamp circuit 20 and the drive circuit 30 are constituted by semiconductor integrated circuits.

  In the voltage clamp circuit 20, Q1, Q2, Q3, and Q4 are NPN transistors, M1, M2, M3, and M4 are PMOS transistors, and M5 and M6 are NMOS transistors. Ia and Ib are constant current sources.

  The collector of the transistor Q1 and the emitter of the transistor Q2 are connected to the input node N1. The collector and base of the transistor Q1 are connected in common, and further connected to the base of the transistor Q3. Transistors Q1 and Q3 have a current mirror configuration with their emitters connected to ground.

  Thereby, when the current IFB flows from the input node N1, the node N1 is clamped to the base-emitter voltage of the transistor Q1. This voltage is the first clamp voltage, and a current corresponding thereto is taken out from the collector of the transistor Q3.

  The base of the transistor Q2 is commonly connected to the base and collector of the transistor Q4. The emitter of transistor Q4 is connected to the collector and base of transistor Q5, and the emitter of transistor Q5 is connected to ground. The constant current Ia flows from the constant current source Ia to the collector of the transistor Q4, so that the base-collector voltages of the transistors Q4 and Q5 are kept constant.

  Thus, when current IFB flows out from input node N1, the emitter voltage of transistor Q2 is clamped at a voltage that is lower than the collector voltage of transistor Q4 by the voltage between the base and emitter of transistor Q2. This voltage is the second clamp voltage, and a current corresponding thereto is taken out from the collector of the transistor Q2.

  The current flowing through the collector of the transistor Q3 is converted into a charging current flowing from the node N2 to the capacitor C3 by the current mirror circuit including the transistors M3 and M4. On the other hand, the current flowing through the collector of the transistor Q2 is converted into a discharge current flowing through the capacitor C3 by the current mirror circuit including the transistors M1 and M2 and the current mirror circuit including the transistors M5 and M6. The capacitor C3 is constantly supplied with the current Ib from the constant current source Ib.

  Here, the first current mirror circuit group described in the claims includes transistors Q1, Q3, M3, and M4. The second current mirror circuit group includes transistors Q2, M1, M2, M5, and M6. Moreover, the 1st capacitor | condenser described in the claim is comprised by the capacitor | condenser C3.

  In the drive circuit 30, 31 is a comparator in which the reference voltage V1 is set, 32 is a comparator in which the reference voltage V2 (<V1) is set, 33 is an RS flip-flop circuit, and 34 is a driver that drives the power transistor MP1. The comparator 31 sets the output to “H” when the voltage VC3 of the capacitor C3 exceeds the voltage V1. The comparator 32 sets the output to “H” when the voltage VC3 of the capacitor C3 falls below the voltage V2. The RS flip-flop circuit is reset when the output of the comparator 31 becomes “H”, the Q terminal is set to “L”, and the QX terminal is set to “H”, whereby the driver 34 sets the output voltage VG to “L” and power The transistor MP1 is turned off. Further, the RS flip-flop circuit is set when the output of the comparator 32 becomes “H”, the Q terminal is set to “H”, the QX terminal is set to “L”, and thereby the driver 34 sets the output voltage VG to “H”. To turn on the power transistor MP1.

となる。なお、Ｒ１//Ｒ２は抵抗Ｒ１，Ｒ２の並列抵抗値を示す。また、ダイオードＤ２の順方向電圧は０Ｖとしている。 Next, the operation will be described. When the power transistor MP1 is on, the current IFB (ON) flowing into the node N1 is

It becomes. R1 // R2 indicates the parallel resistance value of the resistors R1 and R2. The forward voltage of the diode D2 is 0V.

となる。 When the power transistor MP1 is off, the current IFB (OFF) flowing out from the node N1 is

It becomes.

となる。ＶＢＥ（Ｑ１）はトランジスタＱ１のベース・エミッタ間電圧である。 A current corresponding to these currents IFB (ON) and IFB (OFF) flows through the node N1 of the voltage clamp circuit 20. When the power transistor MP1 is on, the current IFB (ON) flows into the collector of the transistor Q1, so the first clamp voltage VFB (ON) is

It becomes. VBE (Q1) is a base-emitter voltage of the transistor Q1.

  The current flowing through the transistor Q1 is output as the collector current of the transistor Q3 by the operation of the current mirror, and further, the current is output from the drain of the transistor M4 by the operation of the current mirror of the transistors M3 and M4.

At this time, the capacitor C3 connected to the node N2 on the output side of the voltage clamp circuit 20 is charged by the drain current of the transistor M4, so that the charging current IC3 (ON) is as follows. a is the total current ratio set by the current mirror circuit.

となる。ＶＢＥ（Ｑ４）はトランジスタＱ４のベース・エミッタ間電圧、ＶＢＥ（Ｑ５）はトランジスタＱ５のベース・エミッタ間電圧、ＶＢＥ（Ｑ２）はトランジスタＱ２のベース・エミッタ間電圧である。 Next, when the power transistor MP1 is turned off and the secondary side rectifier diode D1 is conducting, the current IFB (OFF) flows from the emitter of the transistor Q2 via the node N1, so that the second clamp voltage VFB. (OFF)

It becomes. VBE (Q4) is the base-emitter voltage of the transistor Q4, VBE (Q5) is the base-emitter voltage of the transistor Q5, and VBE (Q2) is the base-emitter voltage of the transistor Q2.

  The collector current of the transistor Q2 flows to the drain of the transistor M1, the current flows to the drain of the transistor M2 due to the current mirror operation in the transistors M1 and M2, and the drain current of the transistor M6 due to the current mirror operation of the transistors M5 and M6. Flowing.

At this time, the capacitor C3 connected to the node N2 on the output side of the voltage clamp circuit 20 is discharged by the drain current of the transistor M6, so the discharge wire IC3 (OFF) is as follows. b is the total current ratio set by the current mirror circuit.

  In the drive circuit 30, when the power transistor MP1 is on, the capacitor C3 is being charged by the current IC3 (ON) shown in the equation (9), and the period during which the charging voltage VC3 does not reach the reference voltage V1 is The output is “L”. Thereafter, when the charging voltage VC3 reaches the reference voltage V1, the output of the comparator 31 changes to “H”, the RS flip-flop circuit 33 is reset, and the output voltage VG of the driver 34 becomes “L”. The power transistor MP1 is turned off.

  When the power transistor MP1 is turned off, discharging of the capacitor C3 is started by the current IC3 (OFF) shown in Expression (11), the charging voltage VC3 becomes equal to or lower than the reference voltage V1, and the output of the comparator 31 returns to “L”. Thereafter, when the voltage VC3 of the capacitor C3 further decreases and falls below the reference voltage V2, the output of the comparator 32 changes to “H”, the RS flip-flop circuit 33 is set, and the output voltage VG of the driver 34 becomes “H”. The power transistor MP1 is turned on.

  Thus, the power transistor MP1 is on / off controlled, and the on / off duty can be set by adjusting the charge / discharge current value of the capacitor C3 according to the values of the resistors R1 and R2. For example, when R1 = R2 is set, the combined resistance value of the voltage / current conversion circuit 10 increases (= R1) when the current IFB flows into the voltage clamp circuit 20, so the first voltage clamped by the transistor Q1 The period of the clamp voltage of 1 becomes longer. Further, since the combined resistance value becomes small (= R1 // R2) when flowing out, the period of the second clamp voltage clamped by the transistors Q2, Q4, Q5 is shortened, so that the off period of the power transistor MP1 Becomes shorter. That is, the duty can be controlled in a large direction.

  In addition, by providing the constant current source Ib, when the output voltage VOUT decreases due to an increase in the load current, the discharge current increases from the equation (11), and the off period of the power transistor MP1 is shortened. For this reason, control is performed to increase the duty and the output voltage VOUT is increased, and control for stabilizing the output voltage VOUT is performed.

  When the output voltage VOUT of the secondary winding L2 changes according to the equation (11), IC3 (OFF) changes.

  For example, when the output voltage VOUT decreases, the effect of the constant current Ib increases and the off period of the power transistor MP1 decreases. The charging current IC3 (ON) that determines the ON period becomes constant depending on the input voltage VIN according to the equation (9). For this reason, the duty is increased to increase the output voltage VOUT.

  Conversely, when the output voltage VOUT rises, the discharge current of the capacitor C3 decreases, the off period of the power transistor MP1 becomes longer, the duty becomes smaller, and the output voltage VOUT decreases.

  As described above, the polarity of the voltage VL3 of the auxiliary winding L3 is converted into the direction in which the current IFB flows, and the current IFB is used to control on / off of the power transistor MP1.

<Third embodiment>
FIG. 4 shows a switching power supply device according to a third embodiment. In this embodiment, the output voltage VOUT of the secondary winding L2 is detected, and the output voltage VOUT is feedback controlled to be stabilized. Here, the voltage / current conversion circuit 10A includes only the resistor R1. In the voltage clamp circuit 20A, a PMOS transistor M7 and an NMOS transistor M8 are newly added, the transistor M7 is current-mirror connected to the transistor M3, and the transistor M8 is current-mirror connected to the transistor M5. A capacitor C4 is connected between the node N3 at the common drain connection point of the transistors M7 and M8 and the ground.

  Note that the third current mirror circuit group described in the claims includes transistors Q1, Q3, M3, M4, and M7. The fourth current mirror circuit group includes transistors Q2, M1, M2, M5, M6, and M8. Moreover, the 2nd capacitor | condenser described in the claim is comprised by the capacitor | condenser C4.

  In the drive circuit 30A, the voltage VC4 of the capacitor C4 is input to the non-inverting input terminal of the comparator 32A. The reference voltage V <b> 2 is input to the inverting input terminal of the comparator 32. An NMOS transistor M9 controlled by the output voltage of the comparator 31 is connected between the non-inverting input terminal of the comparator 32 and the ground.

  Further, an output voltage feedback circuit 40 that detects the output voltage VOUT of the secondary winding L2 is provided, and a timing current that outputs a current Ic that decreases as the detected output voltage VOUT increases as a charging current to the capacitor C4. A generation circuit 50 is provided.

  Next, the operation will be described with reference to FIG. The operation of turning off the power transistor MP1 when the capacitor C3 is charged and the voltage VC3 exceeds the reference voltage V1 while the power transistor MP1 is on is the same as the circuit of FIG.

  When the capacitor C3 is charged and the voltage VC3 exceeds the reference voltage V1 and the output of the comparator 31 becomes “H”, the transistor M9 is turned on to discharge the charge of the capacitor C4.

  When the voltage VC3 of the capacitor C3 exceeds the reference voltage V1, the power transistor MP1 is turned off. Thereafter, when the discharge of the capacitor C3 is started and the voltage VC3 falls below the reference voltage V1, the output of the comparator 31 becomes “L”. The transistor M9 is turned off and the transistor M9 is disconnected from the capacitor C4. At this time, the transistor M8 is on, and the capacitor C4 is charged by the difference between the output current Ic of the timing current generation circuit 50 and the pull-in current of the transistor M8.

  When the output voltage VOUT of the secondary winding L2 is high with the power transistor MP1 turned off, the current Ic from the timing current generation circuit 50 is reduced by the detection signal obtained by the output voltage feedback circuit 40, or It becomes almost 0A. At that time, the capacitor C4 is not charged.

  Thereafter, when the output voltage VOUT decreases or the secondary-side diode D1 becomes non-conductive, the output current Ic of the timing current generating circuit 50 increases and the capacitor C4 is charged. As shown in FIG. The voltage VC4 of C4 increases. When the voltage VC4 of the capacitor C4 reaches the reference voltage V3, the output of the comparator 32A becomes “H”, the Q terminal of the RS flip-flop circuit 33 becomes “H”, and the output voltage VG of the driver 34 becomes “H”. Thus, the power transistor MP1 is turned on.

  When the power transistor MP1 is turned on, the capacitor C4 is charged by the current from the transistor M7, and the voltage VC4 further rises. Therefore, the output of the comparator 32A maintains the “H” state until the power transistor MP1 is turned off.

  As described above, if the output voltage VOUT is high, the charging current of the capacitor C4 is reduced by the timing current generation circuit 50 to lengthen the off period of the power transistor MP1, and if the output voltage VOUT is low, the timing current generation circuit 50 sets the capacitor. By increasing the charging current of C4 and shortening the off period of the power transistor MP1, the output voltage VOUT is stabilized.

<Fourth embodiment>
FIG. 6 shows a switching power supply device according to a fourth embodiment. In this embodiment, the voltage clamp circuit 20A of the third embodiment described with reference to FIG. 4 is replaced with the voltage clamp circuit 20 of the second embodiment, and the charging of the capacitor C4 outputs the current Ic output from the timing current generation circuit 50. It is intended to be done only by

  In operation, the power transistor MP1 keeps the output voltage VOUT constant, except that the voltage clamp circuit 20 is not involved in creating the on-timing of the power transistor MP1. In this way, the output voltage VOUT is stabilized.

10, 10A: Voltage / current conversion circuit 20, 20A: Clamp circuit 30, 30A: Drive circuit, 31, 32, 32A: Comparator, 33: RS flip-flop circuit, 34: Driver 40: Output voltage feedback circuit 50: Timing current Generator circuit

Claims (8)

A primary winding having one end connected to the first input terminal and the other end connected to the second input terminal via the power transistor, and a secondary winding to which a rectifying and smoothing circuit including a rectifier diode and an output capacitor is connected. A power transformer having a wire and an auxiliary winding having one end connected to the control circuit and the other end connected to the second input terminal, and the power transistor is turned on / off by the control circuit. In the switching power supply device for extracting the output voltage from the secondary winding,
The control circuit includes:
When the power transistor is turned on, the positive voltage generated in the auxiliary winding is converted into a positive current and output, and when the power transistor is turned off, the negative voltage generated in the auxiliary winding is converted into a negative current. Output voltage / current conversion circuit,
A voltage clamp circuit that generates a first clamp voltage based on the positive current output from the voltage / current conversion circuit and generates a second clamp voltage based on the negative current output from the voltage / current conversion circuit When,
One of charging and discharging is performed when the first clamping voltage is generated in the voltage clamping circuit, and the charging and discharging are performed when the second clamping voltage is generated in the voltage clamping circuit. A first capacitor in which the other of
A drive circuit for turning on / off the power transistor in accordance with a value of a voltage charged in the first capacitor;
A switching power supply device comprising: A primary winding having one end connected to the first input terminal and the other end connected to the second input terminal via the power transistor, and a secondary winding to which a rectifying and smoothing circuit including a rectifier diode and an output capacitor is connected. A power transformer having a wire and an auxiliary winding having one end connected to the control circuit and the other end connected to the second input terminal, and the power transistor is turned on / off by the control circuit. In the switching power supply device for extracting the output voltage from the secondary winding,
The control circuit includes:
When the power transistor is turned on, the positive voltage generated in the auxiliary winding is converted into a positive current and output, and when the power transistor is turned off, the negative voltage generated in the auxiliary winding is converted into a negative current. Output voltage / current conversion circuit,
A voltage clamp circuit that generates a first clamp voltage based on the positive current output from the voltage / current conversion circuit and generates a second clamp voltage based on the negative current output from the voltage / current conversion circuit When,
A first capacitor that is charged when the first clamp voltage is generated in the voltage clamp circuit and discharged when the second clamp voltage is generated in the voltage clamp circuit;
When the voltage charged in the first capacitor exceeds the first voltage, the power transistor is turned off, and when the voltage charged in the first capacitor falls below a second voltage lower than the first voltage. A drive circuit for turning on the power transistor;
A switching power supply device comprising: A primary winding having one end connected to the first input terminal and the other end connected to the second input terminal via the power transistor, and a secondary winding to which a rectifying and smoothing circuit including a rectifier diode and an output capacitor is connected. A power transformer having a wire and an auxiliary winding having one end connected to the control circuit and the other end connected to the second input terminal, and the power transistor is turned on / off by the control circuit. In the switching power supply device for extracting the output voltage from the secondary winding,
The control circuit includes:
When the power transistor is turned on, the positive voltage generated in the auxiliary winding is converted into a positive current and output, and when the power transistor is turned off, the negative voltage generated in the auxiliary winding is converted into a negative current. Output voltage / current conversion circuit,
A voltage clamp circuit that generates a first clamp voltage based on the positive current output from the voltage / current conversion circuit and generates a second clamp voltage based on the negative current output from the voltage / current conversion circuit When,
A first capacitor that is charged when the first clamp voltage is generated in the voltage clamp circuit and discharged when the second clamp voltage is generated in the voltage clamp circuit;
Charging is performed independently from the first capacitor when the first clamp voltage is generated in the voltage clamp circuit, and the second clamp voltage is generated when the second clamp voltage is generated in the voltage clamp circuit. A second capacitor that is discharged independently of the first capacitor;
A timing current generating circuit that charges the second capacitor with a larger current as the output voltage of the rectifying and smoothing circuit is lower;
When the voltage charged in the first capacitor exceeds the first voltage, the power transistor is turned off and the second capacitor is discharged, and the voltage charged in the second capacitor becomes the third voltage. A drive circuit that turns on the power transistor when exceeded,
A switching power supply device comprising: A primary winding having one end connected to the first input terminal and the other end connected to the second input terminal via the power transistor, and a secondary winding to which a rectifying and smoothing circuit including a rectifier diode and an output capacitor is connected. A power transformer having a wire and an auxiliary winding having one end connected to the control circuit and the other end connected to the second input terminal, and the power transistor is turned on / off by the control circuit. In the switching power supply device for extracting the output voltage from the secondary winding,
The control circuit includes:
When the power transistor is turned on, the positive voltage generated in the auxiliary winding is converted into a positive current and output, and when the power transistor is turned off, the negative voltage generated in the auxiliary winding is converted into a negative current. Output voltage / current conversion circuit,
A voltage clamp circuit that generates a first clamp voltage based on the positive current output from the voltage / current conversion circuit and generates a second clamp voltage based on the negative current output from the voltage / current conversion circuit When,
A first capacitor that is charged when the first clamp voltage is generated in the voltage clamp circuit and discharged when the second clamp voltage is generated in the voltage clamp circuit;
A timing current generating circuit that charges a large current the lower the output voltage of the rectifying smoothing circuit to the second capacitor,
When the voltage charged in the first capacitor exceeds the first voltage, the power transistor is turned off and the second capacitor is discharged, and the voltage charged in the second capacitor becomes the third voltage. A drive circuit that turns on the power transistor when exceeded,
A switching power supply device comprising:
In the switching power supply device according to claim 1, 2, or 4,
The voltage clamp circuit includes a first transistor that generates a base-emitter voltage as the first clamp voltage when the positive current is output from the voltage / current conversion circuit, and the voltage / current conversion circuit A second transistor that generates a base-emitter voltage as the second clamp voltage when a negative current is output, and a current corresponding to the collector current of the first transistor as a charging current for the first capacitor A first current mirror circuit group for outputting, and a second current mirror circuit group for outputting a current corresponding to the collector current of the second transistor as a discharge current for the first capacitor. Switching power supply. In the switching power supply device according to claim 3,
The voltage clamp circuit includes a first transistor that generates a base-emitter voltage as the first clamp voltage when the positive current is output from the voltage / current conversion circuit, and the voltage / current conversion circuit A second transistor that generates a base-emitter voltage as the second clamp voltage when a negative current is output, and a current corresponding to the collector current of the first transistor as a charging current for the first capacitor A first current mirror circuit group for outputting, a second current mirror circuit group for outputting a current corresponding to the collector current of the second transistor as a discharge current for the first capacitor, and the first transistor A third capacitor that outputs a current corresponding to the collector current as a charging current for the second capacitor. And Ntomira circuits, switching power supply unit, characterized in that it comprises a fourth current mirror circuit group for outputting the discharge current to said second capacitor a current corresponding to the collector current of the second transistor. The switching power supply device according to claim 2,
The switching power supply device, wherein the voltage / current conversion circuit sets an on / off duty of the power transistor by making a value of the positive current different from a value of the negative current. The switching power supply device according to claim 2,
A switching power supply comprising a constant current source for charging the first capacitor with a constant current.

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