JP2006014465A - Switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize a switching power supply which can safely shut down an operation without malfunction, fracture or damage of/to other component at AC input power off time. <P>SOLUTION: A control circuit 12 controls an FETQ1 in a separate excitation, stabilizes DC input to a desired voltage and outputs as DC output. Starting resistors R1, R2 which generate starting power for starting the control circuit 12 by the DC input are connected in series with one another and provided. A voltage detector 2 which outputs a first control signal when the voltage becomes a predetermined value or less by monitoring the voltage of the DC input is provided. The control circuit 12 has a mute part for temporarily stopping the control of the FETQ1 by the input of the first control signal. The voltage detector 2 monitors the DC of the connecting point between the starting resistors R1, R2. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a switching power supply device such as a separately excited AC-DC converter provided with a control circuit having a start circuit.

  2. Description of the Related Art Conventionally, switching power supply devices are known as devices that can increase the voltage conversion efficiency and can be reduced in size and weight. This is because in a switching power supply, the current flowing through the primary winding of the transformer is switched (connected) at a high frequency, and the alternating current flowing through the secondary side of the transformer is controlled and converted to the desired voltage. Therefore, the conversion efficiency of the transformer can be improved, and the transformer can be reduced in size and weight.

  As the switching power supply device, there are known a separately-excited switching control using a control IC for controlling the switching and a self-excited switching control not using this control IC.

  FIG. 6 shows a first conventional example of a switching power supply using such separately excited switching control. In the switching power supply, an alternating current obtained by switching a direct current rectified by an rectifier circuit (D1 to D4) at an arbitrary frequency by the main switching element Q1 is applied to the primary main winding N1 of the transformer T1, and the transformer A desired voltage alternating current is converted from the secondary winding N2 of T1 into a direct current via a rectifying diode D6 and a smoothing capacitor C5 and output.

  At this time, for example, PWM (Pulse Width Modulation) control is performed on the main switching element Q1 in accordance with output voltage information fed back from the output voltage detection unit 21 that detects the DC voltage on the secondary side via the photocoupler PC. Since the circuit (control means) 22 is provided as a control IC, the DC output voltage can be stabilized to a desired value.

  In the switching power supply device, as a power supply (auxiliary power supply) for the control circuit 22, a direct current rectified by the rectifier circuits (D1 to D4) and the smoothing capacitor C3 at the initial stage of input, a direct current of a predetermined voltage by the resistors R1 and R2, and an output When stable, a direct current having a predetermined voltage obtained by rectifying the alternating current from the primary side sub-winding N3 of the transformer T1 by the rectifying diode D6 and the smoothing capacitor C5 is used. A plurality of resistors R1 and R2 are used in consideration of withstand voltage.

  However, in the switching power supply device of the first conventional example, when the input voltage is lowered, the auxiliary power supply voltage is also lowered. Therefore, the switching control of the main switching element Q1 by the control circuit 22 becomes unstable, and the switching loss is higher than the steady state. In addition, there is a disadvantage that there is a risk that the main switching element Q1 is destroyed.

  Therefore, in order to avoid the above inconvenience, a switching power supply circuit to which a low voltage shutdown function is added is known as a second conventional example. In the switching power supply circuit of the second conventional example, for example, as shown in FIG. 7, a voltage detection circuit 41 for detecting a DC voltage rectified by a rectifier circuit (D1 to D4) and a smoothing capacitor C3, and a voltage detection circuit 41 Is provided with a switch circuit 31 for turning off the direct current of the auxiliary power supply to the control circuit 22 by a short circuit.

  The voltage detection circuit 41 includes Zener diodes ZD1, ZD2 and resistors R3, R4 connected in series. The other terminal of the Zener diode ZD1 is connected between the smoothing capacitor C3 and the primary main winding N1 of the transformer T1. The other terminal of the resistor R4 is grounded. A diode D5 having one terminal connected to the midpoint between the Zener diode ZD2 and the resistor R3 is provided.

  The switch circuit 31 includes a resistor R5 and a transistor Q2 as a switch. The resistor R5 is connected between the auxiliary power supply of the control circuit 22 and the emitter of the transistor Q2. The other terminal of the diode D5 is connected to the base of the transistor Q2. The collector of the transistor Q2 is grounded.

  Next, the operation of the switching power supply circuit of the second conventional example will be described. The voltage detection circuit 41 is connected after rectification by a bridge diode, and when the AC input voltage is turned off from the AC 100 V input in a normal operation state, each Zener is lowered while the voltage is lowered from the power supply input voltage to zero volts. The voltage becomes lower than the Zener voltage of the diodes ZD1, ZD2, the current of each Zener diode ZD1, ZD2 becomes zero, the voltage on the cathode side of the switching diode D5 becomes lower than the anode side, and the diode D5 passes through the resistors R3, R4. Current flows to ground.

  Then, the base voltage of the transistor Q2 becomes lower than the emitter voltage, the transistor Q2 is turned on, a current flows to the ground through the resistor R5 and the emitter / collector of the transistor Q2, and the voltage of the auxiliary power supply (Vcc) of the control circuit 22 is changed to the transistor Q1. The saturation voltage is reduced to about 0.2 V, and the control circuit 22 stops operating. Thereby, the switching power supply circuit of the second conventional example has a low voltage shutdown function.

As a third conventional example, there is a switching power supply described in JP-A-5-83933. The switching power supply of the third conventional example includes a first resistor R ₁ having one end connected to the output terminal of the rectifying and smoothing circuit, and _{second to fourth} resistors R _{2 to} R ₄ having one end connected to the other end of the resistor R _1. A first Zener diode Z ₁ having a cathode connected to the other end of the resistor R ₂ , and a first NPN transistor Q ₁ having a base connected to the anode of the Zener diode Z ₁ and a collector connected to the other end of the resistor R _3. A second NPN transistor Q ₂ having a base connected to the collector of the transistor Q _{1 and} a collector connected to the other end of the resistor R ₄ , a fifth resistor R ₅ having one end connected to the collector of the transistor Q ₂ , a base the first and PNP transistor Q _3, the other end of the switching control turn the one end to the collector of the transistor Q ₃ which connects the emitter to the reference voltage terminal of the switching control circuit to the other end of the resistor R ₅ It is provided with a AC voltage detection circuit comprising a sixth resistor R ₆ Metropolitan connected to the dead time control pin.
JP-A-5-83933 (publication date: April 2, 1993)

  The problems of the switching power supply circuit of the second conventional example will be described with reference to FIG. In the switching power supply circuit of the second conventional example, when the input power is off (when the voltage of the smoothing capacitor C3 starts to drop), the voltage at the output terminal P3 is also turned off, but a pulse appears in the output voltage after about 3 seconds. As a result, there is a problem that a device connected to the subsequent stage malfunctions (for example, in the case of an audio device, a noise is generated from a speaker).

  The operation mechanism of this problem is that, after the input power is turned off, the output voltage drops and the voltage generated in the primary side auxiliary winding N3 of the transformer T1 in the switching power supply circuit of the second conventional example also drops at the same time. The Vcc voltage also drops and the control circuit 22 temporarily stops operating. Next, electricity remaining in the smoothing capacitor C3 flows in through the resistors R1 and R2, the Vcc voltage of the control circuit 22 rises as shown in FIG. 8 of the operation diagram, and the control circuit 22 is activated after about 3 seconds. Voltage is generated at the output. At this time, since the input power source remains off, the output voltage drops immediately.

  Next, when the input voltage is off, the control circuit 22 stops operating when the rectified voltage of the smoothing capacitor C3 falls below the Zener voltage of each Zener diode ZD1, ZD2 when the input voltage is off. In general, the Zener voltage of the Zener diode ZD1 and the Zener diode ZD2 is about 36V at the upper limit, so that it operates when the voltage of the smoothing capacitor C3 becomes 72V or less (input power supply). Converted to voltage, about 50V).

  However, in this case, when the input power supply voltage is 230V, it does not operate unless the input voltage drops to 50V. In this case, the current flowing through the transistor Q1 and the fuse during 50 V operation increases in inverse proportion to the voltage drop, so the fuse may be blown or the transistor Q1 may be destroyed.

  Furthermore, since the output impedance of the voltage generated at the terminal of the primary side auxiliary winding N3 of the transformer T1 is low, the current flowing into the transistor Q2 may be increased and the transistor Q2 may be destroyed. Such a problem also occurs in the switching power supply of the third conventional example.

  In order to solve the above-described problem, the switching power supply according to the present invention has a control means for controlling the main switching element by a separately excited type, stabilizing the DC input to a desired voltage and outputting it as a DC output. In the switching power supply device, the start-up power supply circuit having a plurality of start-up resistors connected to each other in series to generate a start-up power supply for starting up the control means by the direct-current input, and the direct-current input circuit A voltage detection switch circuit for detecting a voltage with a Zener diode and outputting a first control signal when the voltage value is equal to or lower than a predetermined value; and the control means receives the first control signal to input the main switching element A mute unit for temporarily stopping control of the power supply circuit, and the voltage detection switch circuit includes a plurality of activation resistors in the activation power supply circuit. It is characterized by being adapted to monitor the one of the DC connection point the Zener diode.

  According to the above configuration, the voltage detection switch circuit monitors a direct current at a connection point between a plurality of resistors of the activation circuit, thereby reducing a direct current voltage input to the voltage detection switch circuit and detected. Can do. As a result, the above-described configuration can use a Zener diode or a switching element with a low breakdown voltage, and there is no need to use a plurality of Zener diodes in series in order to ensure a breakdown voltage as in the prior art. It can be simplified.

  Further, in the above configuration, at least one starting resistor among the plurality of starting resistors is interposed between the capacitor used in the rectifier circuit for the auxiliary power supply to the control means and the voltage detection switch circuit. Since the control to the main switching element is stopped by inputting the first control signal to the mute terminal of the mute unit of the control means, the influence of the capacitor used in the rectifier circuit for the auxiliary power supply to the control means is affected. This can prevent the destruction of the main transistor element and the adverse effects on other electronic devices, which have occurred in the past.

  In the switching power supply device, the voltage detection switch circuit may include a switch element that operates according to a detection result of the Zener diode and outputs the first control signal.

  In the switching power supply device, the switch element may be a transistor, and the voltage detection switch circuit may include a diode for protecting the transistor between the Zener diode and the transistor.

  According to the above configuration, since the protective diode is provided, the withstand voltage of the transistor of the switch element to be used can be reduced, and the cost and size can be reduced.

  In the switching power supply device, the voltage detection switch circuit may include a first bias resistor for the Zener diode.

  According to the above configuration, since the first bias resistor is provided, a Zener diode having a small withstand voltage can be used, and the cost and size can be reduced.

  The switching power supply device may have a second bias resistor for reducing the applied voltage by voltage division.

  According to the above configuration, since the second bias resistor is further provided, a Zener diode having a smaller withstand voltage can be used, and the cost and size can be reduced.

  In the switching power supply device, the voltage detection switch circuit may include a current limiting resistor for preventing overcurrent.

  The switching power supply device may further include an input-side rectifier circuit for generating the DC input from an AC input.

In the switching power supply device, when Vin indicates an AC input voltage, Vzd1 indicates a Zener voltage of a Zener diode, and k indicates a set value of a rate of decrease of the AC input voltage at which the control is temporarily stopped, the activation is performed. The ratio (R1 / R2) of the first starting resistance and the second starting resistance before and after the connection point at which the voltage is detected in the power supply circuit for power is based on the following formula (1):
R1 / R2 = ((Vin × √2 × k) / Vzd1) −1 Formula (1)
It may be set.

  In the switching power supply device according to the present invention, as described above, the starting circuit has a plurality of resistors connected in series with each other, and the control means temporarily stops the control of the main switching element. The voltage detection switch circuit is configured to monitor a direct current at a connection point between a plurality of resistors of the start circuit with a Zener diode, and the stop control signal is a mute of the mute unit. In this configuration, the signal is input to the terminal.

  Therefore, in the above configuration, the voltage detection switch circuit monitors the direct current at the connection point between the plurality of resistors of the start-up circuit, thereby reducing the DC voltage input to the voltage detection switch circuit and detected. Thus, it is possible to use a Zener diode or a switching element having a low withstand voltage, and it is not necessary to use a plurality of Zener diodes in series in order to ensure a withstand voltage as in the prior art, and the circuit configuration can be simplified.

  In addition, the above configuration is used in a branch circuit for auxiliary power to the control means because a stop control signal is input to the mute terminal of the mute section of the control means to stop the control of the main switching element. The effect of the capacitor can be prevented by monitoring the direct current at the connection point between the plurality of resistors of the start-up circuit, and the damage to the main transistor element and other adverse effects on other electronic devices, which have occurred in the past, can be avoided. Play.

  Embodiments of the switching power supply device according to the present invention will be described with reference to FIGS. 1 to 5 as follows.

(First embodiment)
In the switching power supply according to the first embodiment of the present invention, as shown in FIG. 1, for example, input terminals P1 and P2 to which commercial AC power of 100 V and 60 Hz is input and input terminals P1 and P2 are input. Are provided with a line filter L1 to which an alternating current is input through a fuse, and noise removing line capacitors C1 and C2 provided before and after the line filter L1, respectively. In addition, examples of the voltage of the commercial AC power supply include 117V, 200V, and 220V.

  The switching power supply device includes the diodes D1 to D4 combined in a Wheatstone bridge type to which an AC input from which noise from the line filter L1 is removed is input, and ripple components from the diodes D1 to D4. A smoothing capacitor C3 for further rectifying the direct current and outputting the direct current input from the alternating current input is provided.

  In the switching power supply device, the primary side main winding N1 to which the DC input from the smoothing capacitor C3 is input and the primary side main winding N1 have a secondary side winding N2 having a reverse polarity, A transformer T1 having a primary side auxiliary winding N3 for auxiliary power is provided. In the transformer T1, the primary side auxiliary winding N3 is formed in the forward direction with the primary side main winding N1.

  Further, a FET (main switching element) Q1, which is a field effect transistor that is connected to the primary main winding N1 of the transformer T1 and switches (disconnects) the current flowing through the primary main winding N1, is connected to the DC input. In order to set the load voltage applied to the load to a desired value, it is provided so as to convert it into alternating current of a predetermined frequency. In the present embodiment, the FET Q1 is an N type, but a P type can be used if necessary. Further, other types of transistors such as bipolar transistors can be used as long as they have a switching function. is there.

  Therefore, the drain of the FET Q1 is connected to the ground side of the primary main winding N1, the source of the FET Q1 is connected to the ground side of each of the diodes D1 to D4, and the gate (control terminal) of the FET Q1 is the control circuit 12 described later. It is connected to the output terminal OUT.

  On the other hand, the secondary side winding N2 of the transformer T1 rectifies the output alternating current output from the secondary side winding N2 and whose voltage is adjusted to a desired value by being stepped up or down and outputs it as a direct current output. There are provided a rectifying diode D6 and a smoothing capacitor C5, and output terminals P3 and P4 for taking out the DC output to the outside. Further, an output voltage detector 11 connected in parallel to the output terminals P3 and P4 is provided for detecting the voltage of the DC output. Further, in the secondary side winding N2 of the transformer T1, the light emitting part PCa of the photocoupler PC for transmitting the output voltage value detected by the output voltage detecting part 11 to the primary side in a non-contact optical manner is provided. It is attached.

  The control circuit 12 receives a detection signal indicating an output voltage value from the light receiving unit PCb of the photocoupler PC, and inputs a control signal (switching pulse signal) to the gate of the FET Q1 based on the detection signal. This is a control IC that can change the switching operation of the FET Q1, that is, the switching frequency and the duty ratio of the switching pulse signal, for example, PWM control.

  In addition to the output terminal OUT and the feedback terminal FB, the control circuit 12 is provided with at least a power supply terminal Vcc and a mute terminal M. The power supply terminal Vcc is for applying a power supply voltage (12V to 25V) for driving the control circuit 12.

  The primary side sub-winding N3 of the transformer T1 is provided with a rectifying diode D6 and a smoothing capacitor C4 so that a direct current of a predetermined voltage can be supplied to the power supply terminal Vcc of the control circuit 12 as an auxiliary power supply. The smoothing capacitor C4 is a starting current storage capacitor from the starting resistors R1 and R2 for rectification and startup.

  Further, the power supply terminal Vcc of the control circuit 12 also has a DC of a predetermined voltage generated from the DC input from each of the diodes D1 to D4 via each of the starting resistors R1 and R2 for starting the control circuit 12. It is designed to be entered.

  In the switching power supply device of the present invention, the control circuit 12 is provided with a mute function (mute unit) for temporarily stopping the switching operation of the FET Q1 by inputting a preset stop control signal (first control signal). And a voltage detection circuit 2 for monitoring a DC voltage from each of the diodes D1 to D4 and a switch circuit 1 for outputting a stop control signal according to a detection signal from the voltage detection circuit 2 are provided. It has been.

  The operation impedance of the mute terminal M for the mute function is set to several kilohms. The mute terminal M is used to stop (mute) the control signal from the output terminal OUT to the FET Q1 when the voltage applied to the mute terminal M becomes a predetermined value, for example, 0.4 V to 1.0 V or less. As an example for exhibiting such a mute function, the control circuit 12 is provided with a comparator (not shown) having an inverting input terminal connected to the mute terminal M. A power supply voltage is applied to the inverting input terminal of the comparator via a constant current source (3 μA to 10 μA). On the other hand, a predetermined voltage value of 0.4 V to 1.0 V, for example, 0.8 V is constantly applied to the non-inverting input terminal of the comparator.

  Therefore, by short-circuiting the mute terminal M through a resistor, the voltage of the inverting input terminal is set to a predetermined voltage value, for example, 0.8 V or less, so that the output from the comparator is changed, and the output This makes it possible to stop (mute) the control signal from the output terminal OUT of the control circuit 12 to the FET Q1.

  The voltage detection circuit 2 has the anode terminal of the Zener diode ZD1 for detecting the voltage and the first bias resistor R3 connected in series with each other, and the Zener diode ZD1 and the first bias resistor R3 are connected to each other. A diode D5 having a cathode terminal connected to the connection point is provided. The diode D5 is for withstand voltage protection of a transistor Q2 described later.

  In the voltage detection circuit 2, the cathode terminal of the Zener diode ZD1 is connected to a connection point between the activation resistors R1 and R2. That is, a direct current obtained by dividing the direct current input from each of the diodes D1 to D4 by the respective starting resistors R1 and R2 is applied to the Zener diode ZD1. The other terminal of the first bias resistor R3 is connected to the negative side of the primary side auxiliary winding N3 of the transformer T1.

  The switch circuit 1 includes a transistor Q2 having an anode terminal of a diode D5 connected to the base, and a resistor R5 connected to the emitter of the transistor Q2. The resistor R5 is a current limiting resistor for the transistor Q2. The collector of the transistor Q2 is connected to the negative side of the primary side auxiliary winding N3 of the transformer T1, and the other terminal of the resistor R5 is connected to the mute terminal M of the control circuit 12.

  In addition, it is preferable to set the resistance values of the starting resistors R1 and R2 by the following equation (1). Such setting is also preferable in other embodiments described later. Vin represents an AC input voltage applied to each of the input terminals P1 and P2, and Vzd1 represents a Zener voltage of the Zener diode ZD1.

R1 / R2 = ((Vin × √2 × k) / Vzd1) −1 Formula (1)
The voltage (Vr2) at the connection point between R1 and R2 is ((R2 / R1 + R2) * (Vin * 0.7 * √2)), and the voltage of Vr2 when the transistor Q2 is turned on and the control circuit 12 is stopped is Vzd1. Therefore, the expression (1) can be derived.

  k is a coefficient by which the control circuit 12 activates the mute function and stops the operation of the FET Q1 when the AC input voltage drops to a predetermined value, for example, 70% from when the AC input voltage is turned off. The voltage to stop can be set to any value. When the predetermined value is 70%, k = 0.7.

  Next, the operation and effect of the first embodiment will be described. In the present embodiment, a switch circuit 1 and a voltage detection circuit 2 that connect and input a stop control signal, which is a voltage detection signal of an input power supply, to a mute terminal M of a control circuit 12 having a mute function, It is connected to the voltage detection circuit 1 from the connection point of the resistors R1 and R2, and more preferably, the ratios of the starting resistors R1 and R2 are set and combined as shown in the equation (1).

  In the present embodiment, by setting as described above, the AC input voltage is about 70% (the value of about 70% can be adjusted to an arbitrary value by changing the ratio of R1 and R2). Then, the operation of the FET Q1 can be forcibly stopped, and the conventional problems can be avoided. In other words, in the present embodiment, by adjusting the ratio of R1 and R2 regardless of the input power supply voltage, a circuit configuration that stops the operation of the control circuit 12 when the input power supply voltage is reduced is possible. The operation can be stopped without destroying parts such as the FET Q1.

  In addition, the voltage input for detecting the voltage value of the DC input to the voltage detection circuit 2 is also used as a starting power source for the control circuit 12, thereby simplifying the circuit configuration and the voltage detection circuit. Since the input voltage to 2 can be made lower than in the prior art, the configuration of the voltage detection circuit 2 can be simplified.

  Further, since the mute terminal of the control circuit 12 has a high operating impedance of generally several kilo ohms, the transistor Q2 is a small product, and an element with a low withstand voltage and current can be used. Also, the transistor Q2 can be safely operated without worrying about the destruction of the transistor Q2.

  The circuit of the voltage detection circuit 2 can omit the Zener diode ZD2 and the resistor R4 from the circuit of the voltage detection circuit 41 of the conventional switching power supply circuit. This is because the input voltage to the voltage detection circuit 2 can be set lower than that of the conventional switching power supply circuit. As a result, in this embodiment, the power consumption of a general ½ watt resistor is reduced and can be realized by one. Further, a single Zener diode can be used as shown in Equation (1).

(Second embodiment)
In the second embodiment, as shown in FIG. 2, the second bias resistor is connected between the Zener diode ZD1, the first bias resistor R3, and the diode D5 with respect to the voltage detection circuit 2 shown in FIG. A voltage detection circuit 3 in which R7 is added to the series is provided. This is because the cathode voltage of the diode D5 is divided by the first bias resistor R3 and the second bias resistor R7 and can be set low, and the diode D5 having a low withstand voltage can be used, resulting in low cost. Can be realized. For example, a 500V withstand voltage diode D5 has been used in the past, but in this embodiment, a general 50V withstand voltage diode D5 can be used. As a result, inexpensive and small parts can be used. .

(Third embodiment)
In the third embodiment, as shown in FIG. 3, a voltage detection circuit 4 in which the diode D5 is omitted is provided for the voltage detection circuit 2 shown in FIG. By the way, in the conventional switching power supply circuit, when the diode D5 is omitted, the base voltage of the transistor Q2 is too high, and there is no general component that can withstand the breakdown voltage. In this embodiment, since the input voltage to the voltage detection circuit 4 is set low even if the diode D5 is omitted, it is necessary to adopt a transistor Q2 having a high base voltage withstand voltage. Can be used.

  The present embodiment is a switching power supply device in combination with a control circuit 12 with a mute function, in which the diode D5 is omitted from the first embodiment, and the base voltage of the transistor Q2 is higher than that of the conventional switching power supply circuit. Since it becomes low, it became possible by adopting a high withstand voltage (about 100 V). However, in the conventional switching power supply circuit, since the base voltage of the transistor Q2 is about 330V, it is too high, and there is no general component that can withstand such a withstand voltage. Although there are special parts, the part price is about 10 times higher and the shape dimensions are larger.

(Fourth embodiment)
In the fourth embodiment, as shown in FIG. 4, with respect to the voltage detection circuit 4 shown in FIG. 3, the second point is between the connection point to the transistor Q2, the first bias resistor R3, and the diode D5. A voltage detection circuit 5 to which a second bias resistor R7 is added is provided. In the present embodiment, the base voltage of the transistor Q2 is divided and lowered by the second bias resistor R7 and the first bias resistor R3, so that low-breakdown and inexpensive parts can be employed for the transistor Q2. became.

  That is, in the present embodiment, the diode D5 is omitted from the voltage detection circuit 3 in combination with the control circuit 12 with the mute function, and the second bias resistor R7 is added, and the withstand voltage of the base voltage of the transistor Q2 is increased. It has become possible to employ a relatively low transistor of about 80V.

  In any of the circuits described in the first to fourth embodiments, spike noise is prevented from occurring in the DC output voltage after the AC input power is turned off, as shown in FIG. Has been. In this way, when the AC input power supply voltage drops arbitrarily (for example, when it drops to 70%), it is possible to forcibly stop the operation of the control circuit 12 while preventing the occurrence of the spike noise. It is possible to safely stop the operation of the control circuit 12 and the FET Q1 by avoiding other parts from being damaged or damaged.

  The switching power supply device of the present invention can be safely stopped with a simple circuit configuration without causing malfunction or damage or damage to other components, so that an electrical device and an electronic device such as an audio device, a lighting device, and a personal computer can be used. It can be suitably used as a power source.

1 is a circuit diagram showing a first embodiment of a switching power supply device according to the present invention. It is a circuit diagram which shows 2nd Embodiment of the switching power supply device which concerns on this invention. It is a circuit diagram which shows the 3rd embodiment of the switching power supply device which concerns on this invention. It is a circuit diagram which shows the 4th form of implementation of the switching power supply device which concerns on this invention. It is a graph which shows that spike noise does not generate | occur | produce in the voltage of the DC output after AC input power supply OFF in each said 1st thru | or 4th form of implementation. It is a circuit diagram of the switching power supply device of the first conventional example. It is a circuit diagram of the switching power supply device of a 2nd prior art example. In the said 2nd prior art example, it is a graph which shows that spike noise generate | occur | produces in the voltage of the DC output after AC input power supply OFF.

Explanation of symbols

1: Switch circuit 2: Voltage detection circuit 11: Output voltage detection unit 12: Control circuit (control means)
P1 & P2: AC input terminal of commercial power supply HS1: Fuse L1: Line filter C1 & C2: Line capacitors D1, D2, D3, D4: Rectifier diode C3: Block capacitor (rectifier capacitor)
R1 & R2: Start-up resistor ZD1: Zener diode R3 & R4 for voltage detection: Bias resistor D5 of Zener diode ZD1: Diode withstand voltage protection for transistor Q2 Q2: Switch transistor R5: Current limit resistor for transistor Q2 PC: Photocoupler C4 : Capacitor D6 for starting current accumulation from starting resistor for rectification and startup D6: Auxiliary power supply rectifier diode N3: Primary side secondary winding N1: Primary side main winding Q1: SW element (FET)
N2: Secondary winding D6: Diode for output voltage rectification C5: Capacitors for output voltage rectification P3 & P4: DC output terminal

Claims (8)

In the separately-excited type, in the switching power supply device having the control means for controlling the main switching element, stabilizing the DC input to a desired voltage and outputting it as a DC output,
A starting power supply circuit having a plurality of starting resistors connected in series with each other to generate a starting power supply for starting the control means by the DC input;
A voltage detection switch circuit that detects a voltage of the DC input with a Zener diode and outputs a first control signal when the voltage value is a predetermined value or less;
The control means includes a mute unit for temporarily stopping control of the main switching element by the input of the first control signal,
The switching power supply device characterized in that the voltage detection switch circuit monitors a direct current at any of connection points between a plurality of starting resistors in the starting power supply circuit by the Zener diode.   2. The switching power supply device according to claim 1, wherein the voltage detection switch circuit includes a switch element that operates according to a detection result of the Zener diode and outputs the first control signal. . The switch element is a transistor,
3. The switching power supply device according to claim 2, wherein the voltage detection switch circuit includes a diode for protecting the transistor between the Zener diode and the transistor.   4. The switching power supply device according to claim 1, wherein the voltage detection switch circuit includes a first bias resistor for the Zener diode. 5.   5. The switching power supply device according to claim 4, further comprising a second bias resistor for reducing a voltage applied to the voltage detection switch circuit by dividing the voltage.   6. The switching power supply device according to claim 1, wherein the voltage detection switch circuit includes a current limiting resistor for preventing overcurrent.   The switching power supply device according to claim 1, further comprising an input side rectifier circuit for generating the DC input from an AC input. When Vin indicates the voltage of the AC input, Vzd1 indicates the Zener voltage of the Zener diode, and k indicates the set value of the rate of decrease of the voltage of the AC input that temporarily stops the control, the voltage in the starting power supply circuit The ratio (R1 / R2) of the first starting resistance and the second starting resistance before and after the connection point at which is detected is based on the following equation (1):
R1 / R2 = ((Vin × √2 × k) / Vzd1) −1 Formula (1)
8. The switching power supply device according to claim 7, wherein the switching power supply device is set.

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