JP4774932B2 - Semiconductor device for switching power supply control and circuit for switching power supply control - Google Patents

Description

  The present invention relates to a semiconductor device for switching power supply control and a circuit for switching power supply control, and more particularly to a secondary side by turning on / off a switch element connected between a primary winding of a transformer and a ground. The present invention relates to a switching power supply control semiconductor device and a switching power supply control circuit for supplying power to a load connected to the power supply.

  2. Description of the Related Art Conventionally, in a flyback type switching power supply device, there is a switching power supply control IC that induces a voltage on a secondary side by turning on / off a switch element connected to a primary winding of a transformer. This switching power supply control IC includes a starting circuit, and supplies power from the starting circuit until the power supply voltage supplied from the auxiliary winding on the primary side of the transformer is stabilized.

  FIG. 7 is a block diagram showing an example of a conventional switching power supply device. The switching power supply device includes a bridge diode BD101, capacitors C101 to C103, diodes D101 and 102, a transformer T101, a power transistor PwT101, a resistor R101, a photocoupler PC101, a switching power supply control IC100, and a voltage detection circuit 111. The switching power supply control IC 100 includes a high voltage input terminal VH, a power supply terminal Vcc, an output terminal OUT, a current detection terminal IS, a ground terminal GND, and a feedback terminal FB, and includes a starter circuit 101 and a pulse control unit 102. Yes. The illustrated switching power supply device shows an example of an insulated AC-DC converter that rectifies a commercial power supply of AC 100 V and supplies power to a load 121 via a transformer T101.

  The bridge diode BD101 rectifies the commercial power supply. The rectified DC voltage is applied to a series circuit in which a primary coil N101 of the transformer T101 and a power transistor PwT101 that is a switching element are connected in series.

  The feedback terminal FB is connected to the phototransistor PT101 of the photocoupler PC101. The current detection terminal IS is connected to a current detection resistor R101 that detects the current of the power transistor PwT101, and the voltage of the resistor R101 is input thereto. The ground terminal GND is grounded, and the output terminal OUT is connected to the gate of the power transistor PwT101. The power supply terminal Vcc is connected to the primary auxiliary winding (coil) N103 of the transformer T101 via the diode D101. The switching power supply control IC 100 is operated by a voltage induced in the coil N103.

  A capacitor C102 for stabilizing the voltage supplied from the coil N103 is externally attached to the power supply terminal Vcc. At startup, a startup current flows from the bridge diode BD101 to the capacitor C102 via the startup circuit 101. When the voltage of the power supply terminal Vcc rises to a voltage necessary for the switching power supply control IC 100 to operate, the starting current is stopped from the starting circuit 101. As a result, the power supply voltage supplied to the power supply terminal Vcc becomes a voltage at which the switching power supply control IC 100 can operate.

  The pulse control unit 102 of the switching power supply control IC 100 includes an oscillation circuit therein, and the output voltage of the triangular wave oscillated thereby, the load level received at the feedback terminal FB, and the voltage input to the current detection terminal IS. In response, a pulse-modulated pulse signal is output from the output terminal OUT, and the power transistor PwT101 is controlled to be turned on / off.

  A load 121 is connected to the secondary coil N102 of the transformer T101 through a rectifier circuit including a diode D102 and a capacitor C103. A voltage detection circuit 111 that detects a voltage supplied to the load 121 is connected between the rectifier circuit and the load 121. The voltage detected by the voltage detection circuit 111 is fed back to the feedback terminal FB of the switching power supply control IC 100 via the photodiode PD101 of the photocoupler PC101 and the phototransistor PT101. Since the feedback signal is transmitted to the primary side by the photocoupler PC101, the primary side and the secondary side are electrically insulated.

  FIG. 8 is a circuit diagram of the activation circuit of FIG. As shown in the figure, the startup circuit includes a junction transistor J111, bipolar transistors Q111 and Q112, NMOS transistors MN111 and MN112, resistors R111 and R112, Zener diodes ZD111 and ZD112, and a current source I111.

  The drain of the junction transistor J111 is connected to the high voltage input terminal VH, and the source is connected to the bipolar transistors Q111 and Q112 connected in Darlington connection. The gate is connected to ground. The drain current of the junction transistor J111 decreases as the potential difference between the source and gate increases.

FIG. 9 is a diagram showing voltage-current characteristics of a general junction transistor. A curve I _JFET shown in the figure shows the relationship between the source-gate voltage and the drain current of the junction transistor. In the figure, the horizontal axis (Vsg) represents the potential difference between the source and gate of the junction transistor J111, and the vertical axis (Idr) represents the drain current. As shown in the figure, in the junction transistor J111, the drain current decreases exponentially as the potential difference between the source and gate increases. Therefore, immediately after the start-up, the voltage at the power supply terminal Vcc is around 0 V, so that the potential difference between the gate and the source is small and a large current flows. The straight line I _CM will be described in the section of the best mode for carrying out the invention.

  When the switching power supply control IC 100 is activated, an H state control signal cont is input to the NMOS transistor MN112 from the low voltage malfunction prevention circuit (not shown), and the NMOS transistor MN111 is turned off. The Darlington-connected bipolar transistors Q111 and Q112 cause a current to flow from the high voltage input terminal VH to the power supply terminal Vcc by the base current flowing through the resistor R111.

  When the voltage of the power supply terminal Vcc rises to the threshold level of the low voltage malfunction prevention circuit, the low voltage malfunction prevention circuit outputs the control signal cont in the L state and turns on the NMOS transistor MN111. Then, a voltage is applied to the Zener diode ZD111 via the resistor R111, and an increase in the base voltage of the Darlington-connected bipolar transistor Q111 is suppressed. As a result, the base current does not flow through the Darlington-connected bipolar transistors Q111 and Q112, and the supply of current from the high voltage input terminal VH to the power supply terminal Vcc is stopped. At this time, the current flowing from the junction transistor J111 is a minute current flowing to the ground via the resistor R111 (about 2 MΩ), the Zener diode ZD111, and the NMOS transistor MN111.

  FIG. 10 is a diagram showing voltage-current characteristics of the starter circuit of FIG. In the figure, the horizontal axis indicates the voltage of the power supply terminal Vcc, and the vertical axis indicates the current flowing through the power supply terminal Vcc. A solid line graph shown in the figure shows voltage-current characteristics at room temperature, a dotted line shows voltage-current characteristics at a temperature higher than room temperature, and a dashed-dotted line shows voltage-current characteristics at a temperature lower than room temperature. Further, 15V shown in the figure indicates a threshold level of the low-voltage malfunction prevention circuit, and is a voltage at which the switching power supply control IC 100 starts operating. As shown in the figure, the current value of the power supply terminal Vcc varies with temperature, and the current decreases as the temperature increases. This is caused by the temperature effect of the junction transistor J111 (the drain current decreases as the temperature increases).

Note that there is a switching power supply circuit that cuts off the current flowing in the capacitor by turning off the field effect transistor when the capacitor connected to the power supply terminal of the switching power supply control IC is charged to some extent (see, for example, Patent Document 1). ). According to this, while suppressing power loss, the resistance value of the resistor connected to the capacitor can be set small, and the charging time of the capacitor can be shortened.
JP 2000-175449 A

  However, due to the characteristics of the junction transistor J111, when the voltage of the power supply terminal Vcc is low, the charging current to the external voltage stabilizing capacitor C102 is large, and when the voltage of the power supply terminal Vcc is high, the charging current is Get smaller. For this reason, when the voltage at the power supply terminal Vcc is low, a large amount of current flows and a considerable amount of heat is generated.

  Further, when the power supply terminal Vcc and the ground are short-circuited due to some trouble, a large amount of current flows through the junction transistor J111 or the bipolar transistors Q111 and Q112, and the switching power supply control IC 100 may be broken or broken. was there.

  Also, the junction transistor J111 is strongly affected by temperature changes, and the drain current decreases as the temperature increases. Therefore, at a high temperature, the supply current from the high voltage input terminal (starting terminal) VH to the power supply terminal Vcc decreases, and the power supply startup time becomes longer. In the worst case, the power supply does not start. The fact that the start-up time of the power supply is strongly influenced by temperature has a problem that it makes the power supply design difficult. The above-mentioned Patent Document 1 does not take any measures for these problems.

  Note that if the capacitor C102 of the power supply terminal Vcc is reduced in order to prevent the power supply start time at a high temperature from becoming long, the power supply terminal Vcc is supplied before power is supplied from the coil N103 of the transformer T101 to the power supply terminal Vcc after the power supply operation starts. There is a possibility that the terminal voltage of Vcc drops and the low voltage malfunction prevention circuit works. Therefore, the capacitor C102 of the power supply terminal Vcc cannot be made extremely small, and the power supply design becomes difficult.

  The present invention has been made in view of such points, and suppresses heat generation when the voltage of the power supply terminal is low, prevents failure when the power supply terminal is shorted to ground, and further facilitates power supply design. An object is to provide a semiconductor device for switching power supply control and a circuit for switching power supply control.

In the present invention, in order to solve the above-described problem, a switching power supply that supplies power to a load connected to the secondary side by turning on / off a switch element connected between the primary winding of the transformer and the ground. In a control semiconductor device, a capacitor is externally attached, a power supply terminal to which a power supply voltage is input from an auxiliary winding on the primary side of the transformer, and a startup voltage to which a voltage supplied to the primary side of the transformer is input A starting element for charging the capacitor by flowing a starting current from the starting terminal to the power supply terminal, a control circuit for controlling the starting current to flow at the time of starting, and constant the starting current has a starting current constant circuit so as to, a starter circuit having, a, the charging activation element has a drain connected to the start terminal, a gate connected to the ground And a constant starting current constant circuit between a first transistor and a resistor connected in series between the source of the junction transistor and the ground, and between the source of the junction transistor and the power supply terminal. And a second transistor for supplying a current mirror current with respect to a current flowing through the first transistor. A switching power supply control semiconductor device is provided.
Further, in the present invention, in order to solve the above-described problem, the switch element connected between the primary winding of the transformer and the ground is turned on / off to supply power to the load connected to the secondary side. In a switching power supply control semiconductor device, a capacitor is externally attached, and a power supply terminal to which a power supply voltage is input from an auxiliary winding on the primary side of the transformer and a voltage supplied to the primary side of the transformer are input. A start-up terminal, a charge start-up element that charges the capacitor by passing a start-up current from the start-up terminal to the power supply terminal, a control circuit that controls the start-up current to flow during start-up, and the start-up current A startup circuit having a startup current constant circuit configured to be constant, and the control circuit includes a transistor circuit connected between the startup element for charging and the power supply terminal. The a switching power supply control semiconductor device and supplying there is provided a current mirror current to a current flowing through the first transistor as a control current to the transistor circuit.

  According to such a switching power supply control semiconductor device, at the time of start-up, the start-up current flowing from the start-up terminal to the power supply terminal is made to flow constantly by the start-up current constant circuit. As a result, the capacitor externally attached to the power supply terminal is charged with a constant current.

  In the semiconductor device for controlling a switching power supply according to the present invention, at the time of start-up, the start-up current flowing from the start-up terminal to the power supply terminal is made to flow constantly by the start-up current constant circuit. As a result, since the capacitor externally connected to the power supply terminal is charged with a constant current, it suppresses heat generation when the voltage of the power supply terminal is low, prevents a failure when the power supply terminal is shorted to ground, Power supply design can be facilitated.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 2 is a circuit diagram of a switching power supply device to which a switching power supply control IC is applied. The switching power supply device includes a bridge diode BD1, capacitors C1 to C3, diodes D1 and 2, a transformer T1, a power transistor PwT1, a resistor R1, a photocoupler PC1, a switching power supply control IC 10, and a voltage detection circuit 21. The switching power supply control IC 10 includes a high voltage input terminal VH, a power supply terminal Vcc, an output terminal OUT, a current detection terminal IS, a ground terminal GND, and a feedback terminal FB. It has various circuits for turning on / off PwT1. The illustrated switching power supply device is an example of an insulated AC-DC converter that rectifies a commercial power supply of AC 100V and supplies power to a load 31 via a transformer T1.

  The bridge diode BD1 rectifies the commercial power supply. The rectified DC voltage is applied to a series circuit in which a primary coil N1 of the transformer T1 and a power transistor PwT1 that is a switching element are connected in series.

  The feedback terminal FB of the switching power supply control IC 10 is connected to the phototransistor PT1 of the photocoupler PC1. The current detection terminal IS is connected to a current detection resistor R1 for detecting the current of the power transistor PwT1, and the voltage of the resistor R1 is input thereto. The ground terminal GND is grounded, and the output terminal OUT is connected to the gate of the power transistor PwT1. The power supply terminal Vcc is connected to the primary auxiliary winding (coil) N3 of the transformer T1 via the diode D1. The switching power supply control IC 10 operates by a voltage induced in the coil N3.

  A capacitor C2 for stabilizing the voltage supplied from the coil N3 is externally attached to the power supply terminal Vcc. At the time of start-up, a start-up current is supplied from the bridge diode BD1 to the capacitor C2 via the start-up circuit 11. At this time, a high breakdown voltage junction type transistor (JFET) not shown in FIG. 2 in the starter circuit 11 connected to the high voltage input terminal VH supplies a current to the power supply terminal Vcc. When the voltage at the power supply terminal Vcc rises to a voltage necessary for the switching power supply control IC 10 to operate, an off signal is input to the starter circuit 11, and the starter current from the starter circuit 11, that is, from the high voltage input terminal VH. The supply of current to the capacitor C2 at the power supply terminal Vcc is stopped. As a result, the power supply voltage supplied to the power supply terminal Vcc becomes a voltage at which the switching power supply control IC 10 can operate.

  As will be described later, the switching power supply control IC 10 includes an oscillator 13 therein, and the triangular wave output voltage oscillated thereby, the load level received at the feedback terminal FB, and the voltage input to the current detection terminal IS. Then, a pulse-modulated pulse signal is output from the output terminal OUT, and the power transistor PwT1 is controlled to be turned on / off.

  A load 31 is connected to the secondary coil N2 of the transformer T1 through a rectifier circuit including a diode D2 and a capacitor C3. A voltage detection circuit 21 that detects a voltage supplied to the load 31 is connected between the rectifier circuit and the load 31. The voltage detected by the voltage detection circuit 21 is fed back to the feedback terminal FB of the switching power supply control IC 10 via the photodiode PD1 of the photocoupler PC1 and the phototransistor PT1. Since the feedback signal is transmitted to the primary side by the photocoupler PC1, the primary side and the secondary side are electrically insulated.

  The switching power supply control IC 10 includes a start circuit 11, a reference voltage circuit (5V REF) 12, an oscillator 13, a slope compensation circuit 14, a drive circuit 15, a PWM comparator 16, a flip-flop 17, a pulse blanking circuit (Blanking) 18, and a resistor. R2 to R4, a diode D3, and a NAND circuit Z1 are included.

As will be described in detail later, the startup circuit 11 supplies the capacitor C2 with the charging current from the commercial power source input to the high voltage input terminal VH during startup as described above.
The reference voltage circuit 12 generates a voltage of 5 V from the power supply voltage input to the power supply terminal Vcc and supplies it to each circuit of the switching power supply control IC 10.

  The oscillator 13 outputs a triangular wave and an on trigger signal ON_TRG synchronized with the triangular wave. The oscillator 13 defines the switch-on start timing of the power transistor PwT1 with the on trigger signal ON_TRG, and regulates the upper limit value of the duty of the PWM signal output from the PWM comparator 16 with the on signal ON_SIG.

  The slope compensation circuit 14 is a circuit that suppresses subharmonic oscillation of the pulse signal. The resistor R1 detects a current flowing through the power transistor PwT1, and outputs a voltage Vis proportional to the amount of current to the current detection terminal IS. The slope compensation circuit 14 connected to the current detection terminal IS supplies an output voltage signal Vsl_out obtained by adding a slope compensation signal to the detected voltage Vis to the non-inverting input terminal of the PWM comparator 16.

  The drive circuit 15 includes a series circuit of a PMOS transistor M1 and an NMOS transistor M2. The source of the PMOS transistor M1 is connected to the power supply terminal Vcc, and the voltage from the coil N3 of the transformer T1 is input. The source of the NMOS transistor M2 is connected to the ground. In the PMOS transistor M1 and the NMOS transistor M2, the output signal of the NAND circuit Z1 is input to the gate, and the voltage of the power supply terminal Vcc and the voltage of the ground are output from the drain to the output terminal OUT.

  The diode D3 subtracts the forward voltage from the feedback signal input to the feedback terminal FB. The resistors R2 and R3 divide the feedback signal obtained by subtracting the forward voltage. The divided feedback signal is input to the inverting input terminal of the PWM comparator 16. The output terminal of the PWM comparator 16 is connected to the reset input terminal R of the flip-flop 17 and determines the duty of the switch-on period in the PWM signal based on the output voltage signal Vsl_out to the inverting input terminal that determines the threshold level. .

  The pulse blanking circuit 18 is a kind of monostable multivibrator, and its output terminal is connected to the set input terminal S of the flip-flop 17 and outputs an output signal BLK_OUT. As a result, the flip-flop 17 ignores the output of the PWM comparator 16 only for several hundreds [ns] immediately after the PWM signal is turned on. That is, the pulse blanking circuit 18 functions so that the output of the drive circuit 15 cannot be stopped by switching noise generated immediately after the power transistor PwT1 is turned on.

The operation of FIG. 2 will be described.
The primary side and the secondary side of the transformer T1 are electrically insulated, and the feedback signal of the DC output voltage supplied to the load 31 is transmitted to the primary side of the transformer T1 by the photocoupler PC1. The secondary coil N2 of the transformer T1 has a predetermined secondary voltage based on the DC input voltage value on the primary side by the on / off operation of the power transistor PwT1 connected to the primary coil N1. Is induced. The secondary voltage induced in the secondary coil N2 is rectified and smoothed by the secondary diode D2 and the capacitor C3 and output to the load 31.

  The ON signal ON_SIG of the oscillator 13 is output via the NAND circuit Z1, and if the flip-flop 17 is in the set state, the PMOS transistor M1 of the drive circuit 15 is turned on and the NMOS transistor M2 is turned off. Accordingly, the drive voltage is applied from the power supply terminal Vcc to the gate of the power transistor PwT1 through the PMOS transistor M1, and the power transistor PwT1 is switched on.

  The current detection resistor R1 detects a current flowing through the power transistor PwT1, and outputs a voltage Vis proportional to the amount of current to the current detection terminal IS. The slope compensation circuit 14 supplies an output voltage signal Vsl_out obtained by adding a slope compensation signal to the voltage Vis to the non-inverting input terminal of the PWM comparator 16 to suppress subharmonic oscillation of the pulse signal.

  If the output voltage to the load 31 is lower than the set voltage, the current flowing through the photodiode PD1 of the photocoupler PC1 decreases and the current flowing through the phototransistor PT1 decreases. As a result, the terminal voltage of the feedback terminal FB increases, and the threshold level of the PWM comparator 16 increases. However, the current flowing through the power transistor PwT1 is determined by the input voltage to the coil N1 on the primary side of the transformer T1 and its inductance, and its rate of increase (slope) is constant. For this reason, the PWM signal output from the PWM comparator 16 has a long ON period.

  On the other hand, when the output voltage to the load 31 is higher than the set voltage, the current flowing through the photodiode PD1 of the photocoupler PC1 increases, and the current flowing through the phototransistor PT1 increases. Therefore, the terminal voltage of the feedback terminal FB becomes low, and the threshold level of the PWM comparator 16 becomes low. As a result, the ON period of the PWM signal output from the PWM comparator 16 is shortened. In this way, the output voltage for the load 31 can be output stably.

Next, details of the activation circuit 11 to which an on / off signal is input as the control signal cont will be described.
FIG. 1 is a circuit diagram of the activation circuit of FIG. 2 according to the first embodiment. As shown in the figure, the startup circuit 11 includes a junction transistor J11, bipolar transistors Q11 and Q12, PMOS transistors MP11 to MP13, NMOS transistors MN11 and MN12, resistors R11 and R12, zener diodes ZD11 and ZD12, and a current source I11. is doing.

  The junction transistor J11 is an N-channel high breakdown voltage junction transistor. The drain of the junction transistor J11 is connected to the high voltage input terminal VH. The gate is connected to the ground. The source is connected to PMOS transistors MP11 to MP13 which are current mirror connected.

  The source of the PMOS transistor MP11 is connected to the source of the junction transistor J11. The gate and drain are connected, and one end is connected to a resistor R11 connected to the ground. The resistor R11 is a resistor having no temperature dependency.

  The gate of the PMOS transistor MP12 is connected to the gate of the PMOS transistor MP11. The source is connected to the source of the junction transistor J11. The drain is connected to the base of the bipolar transistor Q11.

  The gate of the PMOS transistor MP13 is connected to the gate of the PMOS transistor MP11. The source is connected to the source of the junction transistor J11. The drain is connected to the collectors of the bipolar transistors Q11 and Q12.

  The bipolar transistors Q11 and Q12 are Darlington connected. A resistor R12 is connected between the base and emitter of the bipolar transistor Q12. Zener diode ZD12 is connected between power supply terminal Vcc and the emitter of bipolar transistor Q12.

  The current source I11 is connected between the power supply terminal Vcc and the drain of the NMOS transistor MN12. A control signal cont from a low voltage malfunction prevention circuit (not shown) is input to the gate of the NMOS transistor MN12. The source is connected to ground.

  The gate of the NMOS transistor MN11 is connected to the drain of the NMOS transistor MN12. The source is connected to ground. Zener diode ZD11 is connected between the base of bipolar transistor Q11 and the drain of NMOS transistor MN11.

  When the switching power supply control IC 10 is activated, the H state control signal cont is input to the NMOS transistor MN12, and the NMOS transistor MN11 is turned off. As a result, the Darlington-connected bipolar transistors Q11 and Q12 can pass a current, and the junction transistor J11 supplies a constant current for charging the external capacitor C2 from the high voltage input terminal VH to the power supply terminal Vcc. Shed.

  When the voltage of the power supply terminal Vcc rises to the threshold level of the low-voltage malfunction prevention circuit, the control signal cont becomes L state by the low-voltage malfunction prevention circuit, and the NMOS transistor MN11 is turned on. Then, a voltage is applied to the Zener diode ZD11 via the PMOS transistor MP12, and an increase in the base voltage of the Darlington-connected bipolar transistor Q11 is suppressed. As a result, the base current does not flow through the Darlington-connected bipolar transistors Q11 and Q12, and the supply of a constant current from the high voltage input terminal VH to the power supply terminal Vcc of the junction transistor J11 is stopped.

  Next, the principle of a constant current flowing from the high voltage input terminal VH to the power supply terminal Vcc will be described. First, it is assumed that the voltage input to the high voltage input terminal VH is completely raised and fixed. The PMOS transistor MP11 is diode-connected, and the source-drain voltage drops at a substantially constant voltage. The drain of the PMOS transistor MP11 is connected to a resistor R11 having one end connected to the ground, and the current flowing through the PMOS transistor MP11 is determined by the source voltage of the junction transistor J11 and the resistance value of the resistor R11. Therefore, when the source voltage of the junction transistor J11 decreases, the current flowing through the PMOS transistor MP11 decreases, and when the source voltage increases, the current flowing through the PMOS transistor MP11 increases.

  The PMOS transistors MP11 to MP13 constitute a current mirror circuit. Therefore, since the current flowing through the PMOS transistor MP11 is current-mirrored to the PMOS transistor MP13, the current flowing through the PMOS transistor MP13 decreases as the source voltage of the junction transistor J11 decreases, as in the PMOS transistor MP11. Increasing as you go up.

  In the junction transistor J11, as described with reference to FIG. 9, the drain current increases when the source voltage decreases, and the drain current decreases when the source voltage increases. Therefore, since the drain current flowing through the junction transistor J11 and the current flowing through the PMOS transistor MP13 move in opposite directions with respect to the movement of the source voltage of the junction transistor J11, the source voltage is stabilized when they are balanced.

A method for obtaining a balanced stable point will be described with reference to FIG. In the above-mentioned background art section, FIG. 9 has been described as the characteristic of the junction transistor J111 shown in FIG. 8, but here it is treated as the characteristic of the junction transistor J11 shown in FIG. A straight line I _CM shown in FIG. 9 is a current characteristic obtained by current mirroring the current flowing through the resistor R11 from the PMOS transistor MP11 to the PMOS transistor MP13. The voltage V _sg applied between the gate and source of the junction transistor J11 is also a voltage applied to the resistor R11 via the PMOS transistor MP11. If the voltage drop by the PMOS transistor MP11 is ignored, the current flowing through the resistor R11 Is proportional to the voltage V _sg . Therefore, the current defined by the current mirror operation and flowing through the PMOS transistor MP13 has a characteristic indicated by a straight line I _CM . Since the current flowing through the junction transistor J11 is equal to the current flowing through the PMOS transistor MP13 (the current flowing through the resistor R11 is negligible because the mirror ratio is large), the stable point obtained by the intersection of the curve I _JFET and the straight line I _CM is Become. Since the voltage V _sg cannot be smaller than the voltage of the power supply terminal Vcc, when the voltage of the power supply terminal Vcc is larger than the voltage V _const of the intersection, the voltage at the intersection is the zener diode ZD12, bipolar transistor Q12 and the drop voltage of the PMOS transistor MP13 are added, and the PMOS transistor MP13 operates in the non-saturated region. This stable point is determined by the source-gate voltage / current characteristics of the junction transistor J11 and the threshold values of the resistor R11 and the PMOS transistor MP11. The source voltage of the junction transistor J11 is related to the voltage value of the power supply terminal Vcc. The voltage V _{const of the} constant voltage value that does not. As a result, the current flowing through the resistor R11 and the PMOS transistor MP11 becomes a constant current, and the current flowing through the PMOS transistors MP12 and MP13 is also a constant current.

Next, the operation of FIG. 1 will be described using specific numerical values.
First, it is assumed that the voltage input to the high voltage input terminal VH has risen completely and is fixed at 100V. The resistance value of the resistor R11 is 2 MΩ. Further, the ratio of the gate sizes W / L of the PMOS transistors MP11 to MP13 is set to 1: 1: 100 in order.

  As described above, the current flowing through the PMOS transistor MP11 is determined by the source-gate voltage / current characteristics of the junction transistor J11, the resistor R11, and the PMOS transistor MP11. Here, for example, when the source voltage of the junction transistor J11 is balanced at about 21 V, the current flowing through the PMOS transistor MP11 is about 10 μA.

  The starter circuit 11 has a capability of flowing a large amount of current from the high voltage input terminal VH to the power supply terminal Vcc. However, since the current flowing from the high voltage input terminal VH to the power supply terminal Vcc is determined by the PMOS transistor MP12 connected to the PMOS transistor MP11 and current mirror, it is about 1 mA regardless of the voltage of the power supply terminal Vcc.

FIG. 3 is a diagram showing voltage-current characteristics of the starter circuit of FIG. In the figure, the horizontal axis indicates the voltage of the power supply terminal Vcc, and the vertical axis indicates the current flowing through the power supply terminal Vcc. A solid line graph shown in the figure shows voltage-current characteristics at room temperature, a dotted line shows voltage-current characteristics at a temperature higher than room temperature, and a dashed-dotted line shows voltage-current characteristics at a temperature lower than room temperature. Note that the straight line I _CM is the characteristic of the current obtained by current mirroring the current flowing through the resistor R11 from the PMOS transistor MP11 to the PMOS transistor MP13, as in FIG. The straight line I _CM is drawn as having no temperature characteristic, but may have a temperature characteristic in which the resistance value decreases as the temperature increases, as will be described later. Further, 15V shown in the figure indicates a threshold level of the low-voltage malfunction prevention circuit, and is a voltage at which the switching power supply control IC 10 starts to operate. If the resistor R11 does not have temperature dependence (if the resistance value does not change with temperature), as shown in the figure, the current value of the power supply terminal Vcc is increased until the voltage of the power supply terminal Vcc reaches at least 15 V (control signal). Constant current characteristics are maintained (until cont transitions from the H state to the L state). However, as shown in FIG. 3, the constant current value has a slight temperature characteristic. That is, the capacitor C2 externally attached to the power supply terminal Vcc of the switching power supply control IC 10 is charged with a constant current determined by the temperature.

  The constant current value has the temperature characteristic due to the temperature characteristic of the junction transistor J11, which can be canceled by applying the resistor R11 having the temperature characteristic that the resistance value decreases as the temperature increases. .

  As described above, in the start-up circuit 11 of the switching power supply control IC 10, the start-up current that flows from the high-voltage input terminal VH to the power-supply terminal Vcc to which the voltage stabilizing capacitor C2 is externally connected at the time of start-up is current-mirror connected PMOS transistor MP11. -Mixed with MP13. Thus, the voltage stabilizing capacitor C2 externally attached to the power supply terminal Vcc is charged with a constant current, so that heat generation when the voltage at the power supply terminal Vcc is low can be suppressed.

Even if the power supply terminal Vcc is shorted to the ground, a constant current flows, so that an excessive current does not flow, and the switching power supply control IC 10 can be prevented from being broken.
Furthermore, by canceling the temperature characteristic of the junction transistor J11 (current decreases as the temperature rises) with the temperature characteristic of the resistor R11 (resistance value decreases as the temperature rises), the current of the power supply terminal Vcc at the time of startup becomes Therefore, the current can be kept almost constant without being affected by temperature changes, the supply current from the high voltage input terminal VH to the power supply terminal Vcc is not reduced even at high temperatures, and the start-up time of the power supply is not prolonged. For this reason, it is not necessary to design the capacitor C2 to be small in order to shorten the start-up time, and the power supply design can be facilitated.

  FIG. 4 is a circuit diagram of the activation circuit of FIG. 2 according to the second embodiment. In the present embodiment, a diode D11 that cancels the temperature characteristic of the junction transistor J11 is connected as a temperature compensation element between the PMOS transistor MP11 and the resistor R11 of the starting circuit of FIG. In the first embodiment, the current mirror configuration eliminates the influence of the terminal voltage of the power supply terminal Vcc, reduces the influence of the temperature change, and reduces the influence of the temperature characteristics of the junction transistor J11. . However, as shown in FIG. 3, in the junction transistor J11, the source voltage slightly changes depending on the temperature characteristics, and the temperature characteristics cannot be completely canceled. Therefore, in the present embodiment, a diode D11 is connected between the PMOS transistor MP11, which is a current source that is connected to the source of the junction transistor J11 and determines the starting current, and the resistor R11, and the temperature characteristics of the junction transistor J11 are expressed as a diode. This is canceled by the temperature characteristic of the forward voltage of D11. Thereby, it is possible to realize a circuit that supplies a constant current from the high voltage input terminal VH to the power supply terminal Vcc without being affected by temperature and charges the capacitor C2 of the power supply terminal Vcc with a constant current.

  The resistor R11 is a resistor having no temperature dependency of about 1.4 MΩ. The current flowing through the PMOS transistor MP11 is determined by the source voltage of the junction transistor J11, the threshold value of the PMOS transistor MP11, the forward voltage of the diode D11, and the resistor R11 having no temperature dependency.

  Here, the temperature coefficient of the junction transistor J11 is about −0.02 V / ° C. (actual value obtained from dVsg / dT (T is temperature) with the current flowing through the junction transistor J11 being a fixed value of 1 mA), the PMOS transistor MP11. The temperature coefficient of the diode is about -0.002 V / ° C, and the temperature coefficient of the forward voltage of the diode is about -0.002 V / ° C. Therefore, if the diode D11 is configured by connecting nine diodes in series, the temperature characteristic can be canceled.

  For example, the source voltage of the junction transistor J11 is room temperature Tj = about 25 ° C. and Vsg = about 21V. At this time, the threshold voltage of the PMOS transistor MP11 is about 0.7V, and the forward voltage of the diode is about 0.7V. Therefore, the voltage drop by one PMOS transistor MP11 and nine diodes D11 connected in series is about 7V. It becomes. Therefore, since the potential difference between both ends of the resistor R11 is 14V, the flowing current is about 10 μA. On the other hand, at a high temperature Tj = 125 ° C., the source voltage Vsg of the junction transistor J11 is about 19V, the threshold value of the PMOS transistor MP11 is about 0.5V, and the forward voltage of the diode is about 0.5V. The voltage drop due to the nine PMOS transistors MP11 and the nine diodes D11 connected in series is about 5V. Therefore, since the potential difference between both ends of the resistor R11 is 14V, the flowing current is about 10 μA as in the case of room temperature Tj = about 25 ° C.

  The PMOS transistors MP11 to MP13 have a current mirror configuration. Based on the ratio of the gate width W / L, the current of the PMOS transistor MP12 is 1 time and the current of the PMOS transistor MP13 is 100 times the current 1 of the PMOS transistor MP11. .

  FIG. 5 is a diagram showing voltage-current characteristics of the starting circuit of FIG. The starting element has a capability of flowing a considerably large amount of current, but since the peak current is determined by the PMOS transistor MP13 connected to the current mirror, it becomes about 1 mA regardless of the terminal voltage of the power supply terminal Vcc. Even when the temperature changes, the current of the PMOS transistor MP11 becomes about 10 μA, so that the current of the PMOS transistor MP13 connected to the current mirror becomes about 1 mA. Therefore, even when the temperature changes, a current of about 1 mA flows from the high voltage input terminal VH to the power supply terminal Vcc. In this way, by configuring the activation circuit with a constant current circuit connected to the current mirror, the capacitor C2 of the power supply terminal Vcc can be charged with a constant current without being affected by the terminal voltage or temperature of the power supply terminal Vcc. It becomes possible.

  FIG. 6 is a circuit diagram of the activation circuit of FIG. 2 according to the third embodiment. In the present embodiment, a PMOS transistor MP14 is connected as a temperature compensation element between the PMOS transistor MP11 and the resistor R11 instead of the diode D11 of the starting circuit of FIG. This PMOS transistor MP14 is also composed of nine PMOS transistors connected in series, like the diode D11 of the second embodiment.

  Also in this embodiment, it becomes possible to cancel the temperature characteristic of the source voltage of the junction transistor J11, and the switching power supply control IC when the terminal voltage of the power supply terminal Vcc is low as in the second embodiment. The power supply terminal Vcc can be prevented from being destroyed even when the power supply terminal Vcc is shorted to the ground, and even when there is a change in the terminal voltage or temperature of the power supply terminal Vcc, it is affected by the temperature. In addition, the capacitor C2 of the power supply terminal Vcc can be charged with a constant current, which facilitates power supply design.

FIG. 3 is a circuit diagram of the activation circuit of FIG. 2 according to the first embodiment. It is a circuit diagram of a switching power supply device to which a switching power supply control IC is applied. It is the figure which showed the voltage-current characteristic of the starting circuit of FIG. FIG. 3 is a circuit diagram of the activation circuit of FIG. 2 according to a second embodiment. FIG. 5 is a diagram showing voltage-current characteristics of the starting circuit of FIG. 4. FIG. 6 is a circuit diagram of the activation circuit of FIG. 2 according to a third embodiment. It is a block diagram which shows an example of the conventional switching power supply device. It is a circuit diagram of the starting circuit of FIG. It is the figure which showed the voltage-current characteristic of the common junction type transistor. It is the figure which showed the voltage-current characteristic of the starting circuit of FIG.

Explanation of symbols

10 Switching power supply control IC
11 Start-up circuit 21 Voltage detection circuit 31 Load BD1 Bridge diode C1 to C3 Capacitor D1, D2, D11 Diode T1 Transformer PwT1 Power transistor R1, R11, R12 Resistor PC1 Photocoupler VH High voltage input terminal Vcc Power supply terminal OUT Output terminal IS Current detection Terminal GND Ground terminal FB Feedback terminal J11 Junction transistor Q11, Q12 Bipolar transistor MP11-MP14 PMOS transistor MN11, MN12 NMOS transistor ZD11, ZD12 Zener diode I11 Current source

Claims (10)

In a switching power supply control semiconductor device for supplying power to a load connected to a secondary side by turning on / off a switch element connected between a primary winding of a transformer and a ground,
A power supply terminal to which a power supply voltage is input from an auxiliary winding on the primary side of the transformer;
A starting terminal to which a voltage supplied to the primary side of the transformer is input;
A starting element for charging that charges the capacitor by flowing a starting current from the starting terminal to the power supply terminal, a control circuit that controls the starting current to flow at the time of starting, and a constant starting current. A startup circuit having a constant startup current; and
Have
The charging activation element is a junction transistor having a drain connected to the activation terminal and a gate connected to the ground,
The starting current constant circuit is connected between a first transistor and a resistor connected in series between the source of the junction transistor and the ground, and between the source of the junction transistor and the power supply terminal, and A second transistor for flowing a current mirror current with respect to a current flowing in the first transistor;
A semiconductor device for controlling a switching power supply.   2. The semiconductor device for controlling a switching power supply according to claim 1, wherein the resistor has a temperature characteristic that the resistance value decreases or keeps a constant value when the temperature rises.   2. The semiconductor device for controlling a switching power supply according to claim 1, wherein a temperature compensation element for canceling a temperature characteristic of the junction transistor is connected between the first transistor and the resistor.   4. The semiconductor device for controlling a switching power supply according to claim 3, wherein the temperature compensating element is a diode.   4. The semiconductor device for controlling a switching power supply according to claim 3, wherein the temperature compensation element is a transistor.   In a switching power supply control semiconductor device for supplying power to a load connected to a secondary side by turning on / off a switch element connected between a primary winding of a transformer and a ground,
  A power supply terminal to which a power supply voltage is input from an auxiliary winding on the primary side of the transformer;
  A starting terminal to which a voltage supplied to the primary side of the transformer is input;
  A starting element for charging that charges the capacitor by flowing a starting current from the starting terminal to the power supply terminal, a control circuit that controls the starting current to flow at the time of starting, and a constant starting current. A startup circuit having a constant startup current; and
  Have
  The control circuit includes a transistor circuit connected between the charging activation element and the power supply terminal, and supplies a current mirror current corresponding to a current flowing through the first transistor as a control current for the transistor circuit. A semiconductor device for controlling a switching power supply.   The charging activation element is a junction transistor having a drain connected to the activation terminal and a gate connected to the ground,
  The constant starting current circuit is connected between the first transistor and the resistor connected in series between the source of the junction transistor and the ground, and between the source of the junction transistor and the power supply terminal, A second transistor for passing a current mirror current with respect to a current flowing in the first transistor;
  The semiconductor device for controlling a switching power supply according to claim 6.   The transistor circuit is a Darlington-connected bipolar transistor, and the base terminal of the Darlington-connected bipolar transistor and the ground are connected via a third transistor, and the third transistor is off. 8. The control current is supplied to a base terminal of the Darlington-connected bipolar transistor, and the control current is bypassed to the ground when the third transistor is on. Switching power supply control semiconductor device.   In a switching power supply control circuit for supplying power to a load connected to the secondary side by turning on / off a switch element connected between the primary winding of the transformer and the ground,
  A power supply terminal to which a capacitor is connected and a power supply voltage is input from the auxiliary winding on the primary side of the transformer;
  A starting terminal to which a voltage supplied to the primary side of the transformer is input;
  A starting element for charging that charges the capacitor by flowing a starting current from the starting terminal to the power supply terminal, a control circuit that controls the starting current to flow at the time of starting, and a constant starting current. A startup circuit having a constant startup current; and
  Have
  The charging activation element is a junction transistor having a drain connected to the activation terminal and a gate connected to the ground,
  The starting current constant circuit is connected between a first transistor and a resistor connected in series between the source of the junction transistor and the ground, and between the source of the junction transistor and the power supply terminal, and A second transistor for flowing a current mirror current with respect to a current flowing in the first transistor;
  A switching power supply control circuit comprising:   In a switching power supply control circuit for supplying power to a load connected to the secondary side by turning on / off a switch element connected between the primary winding of the transformer and the ground,
  A power supply terminal to which a capacitor is connected and a power supply voltage is input from the auxiliary winding on the primary side of the transformer;
  A starting terminal to which a voltage supplied to the primary side of the transformer is input;
  A starting element for charging that charges the capacitor by flowing a starting current from the starting terminal to the power supply terminal, a control circuit that controls the starting current to flow at the time of starting, and a constant starting current. A startup circuit having a constant startup current; and
  Have
  The control circuit includes a transistor circuit connected between the charging activation element and the power supply terminal, and supplies a current mirror current corresponding to a current flowing through the first transistor as a control current for the transistor circuit. A circuit for controlling a switching power supply.

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