JP2006109598A - Switching power supply device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a switching power supply device that can completely be stopped in operation while avoiding an erroneous operation and the breakage of the other component by a simple circuit constitution when the voltage of an AC input is lowered. <P>SOLUTION: There is arranged a line commutation type control circuit 12 that converts the AC input to the DC output of a desired voltage by switching, and stably outputs it. A first starting resistor R1 and a second starting resistor R2 as starting circuits that feed power necessary for starting the control circuit 12 to a power supply terminal VCC of the control circuit 12 are connected to the AC input. A diode D6 and a smoothing capacitor C4 that are sub-rectifying circuits are arranged that feed the power necessary for starting the control circuit 12 to the power supply terminal VCC by rectifying an AC on the basis of the AC input. There is arranged an input voltage detection circuit 2 that detects whether or not the voltage of the AC input is lowered to a prescribed value or lower. There is also arranged a switch circuit 1 that stops the supply of power to the control circuit 12 by a detection signal from the input voltage detection circuit 2. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a switching power supply device that is an AC-DC converter and is separately excited and has a startup circuit.

  2. Description of the Related Art Conventionally, switching power supply devices are known as devices that can increase voltage conversion efficiency and can be reduced in size and weight. This is because in a switching power supply device, the current flowing through the winding on the primary side of the transformer is switched (connected / disconnected) at a high frequency so that the alternating current flowing on the secondary side of the transformer is controlled and converted to a desired voltage. This is because 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 type of separately-excited switching control using a control IC (integrated circuit) for controlling the switching and a type of self-excited switching control not using this control IC. Yes.

  FIG. 6 shows a first conventional example of a switching power supply using such separately excited switching control. In the switching power supply apparatus, an alternating current obtained by switching a direct current obtained by rectifying an alternating current input from each of the input terminals P1 and P2 by a rectifier circuit (D1 to D4) at an arbitrary frequency by the main switching element Q1 is a primary side main of the transformer T1. What is applied to the winding N1, converts the alternating current of the desired voltage from the secondary winding N2 of the transformer T1 into direct current through the rectifying diode D6 and the smoothing capacitor C5, and outputs it from the output terminals P3 and P4 It is.

  At this time, for example, PWM (Pulse Width Modulation) control is performed on the main switching element Q1 according to the output voltage information fed back from the output voltage detection circuit 21 that detects the DC voltage on the secondary side via the photocoupler PC. Since the circuit 22 is provided as a control IC, the output voltage of the DC output 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, an output When stable, a direct current of a predetermined voltage obtained by rectifying the alternating current from the primary side auxiliary winding N3 of the transformer T1 by the auxiliary rectifier circuit including the rectifying diode D6 and the smoothing capacitor C5 is input to the power supply terminal of the control circuit 22 and used. Yes. 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 AC input voltage decreases, the auxiliary power supply voltage also decreases. Therefore, the switching control of the main switching element Q1 by the control circuit 22 becomes unstable, and switching is performed from the steady state. There is a disadvantage that the loss increases and the main switching element Q1 may be 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, as shown in FIG. 7, for example, an input voltage detection circuit 41 for detecting a DC voltage rectified by a rectifier circuit (D1 to D4) and a smoothing capacitor C3, and an input voltage detection A switch circuit 31 is provided for turning off the direct current of the auxiliary power supply to the control circuit 22 by a short circuit according to an output signal from the circuit 41.

The input voltage detection circuit 41 has Zener diodes ZD1, ZD2 and resistors R3, R4 connected in series with each other. 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 input voltage detection circuit 41 is connected to the direct current after the alternating current input is rectified by the bridge diode of the rectifier circuit (D1 to D4). When the voltage of the AC input is in a normal operating state, for example, from an AC 100V input, the detected voltage in the input voltage detection circuit 41 is reduced from the power supply input voltage toward zero volts, and each Zener diode ZD1, It becomes less than the Zener voltage of 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 is connected to the ground via the resistors R3, R4. Current flows.

  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 R1 having one end connected to the output terminal of the rectifying and smoothing circuit, second to fourth resistors R2 to R4 having one end connected to the other end of the resistor R1, and a resistor R2. A first Zener diode Z1 having a cathode connected to the other end, a first NPN transistor Q1 having a base connected to the anode of the Zener diode Z1 and a collector connected to the other end of the resistor R3, and a base to the collector of the transistor Q1. A second NPN transistor Q2 having a collector connected to the other end of the resistor R4, a fifth resistor R5 having one end connected to the collector of the transistor Q2, a base being the other end of the resistor R5, and an emitter being a reference of the switching control circuit The first PNP transistor Q3 connected to the voltage terminal, one end of the collector of the transistor Q3 and the other end of the dead terminal of the switching control circuit It is provided with a AC voltage detection circuit comprising a sixth resistor R6 Metropolitan connected to arm control terminal.
JP-A-5-83933 (publication date: April 2, 1993)

  The problem 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 secondary 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, the electricity remaining in the smoothing capacitor C3 flows through the resistors R1 and R2, and the control circuit 2
The VCC voltage of 2 rises as shown in FIG. 8 of the operation diagram, and after about 3 seconds, the control circuit 22 is activated and a voltage is generated at the output. At this time, since the AC input as the power source remains off, the output voltage immediately drops.

  Next, in the switching power supply circuit of the second conventional example, when the rectified voltage of the smoothing capacitor C3 falls below the Zener voltage of each Zener diode ZD1, ZD2 when the AC input voltage is off, the control circuit 22 performs its operation. When the output load is small, the voltage discharge time of the smoothing capacitor C3 becomes long, the holding time becomes long, and the control circuit 22 may be delayed from being turned off.

  The following two points can be cited as problems caused by such a delay. In general, the upper limit of the Zener voltage of the Zener diode ZD1 and the Zener diode ZD2 is about 36V. Therefore, when the above-described operation occurs when the voltage of the smoothing capacitor C3 becomes 72V or less (AC input as a power source). When the rated voltage of the AC input is 100V, the AC input voltage does not drop to 50V, so that the operation does not work. 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 that there is a problem that the fuse is blown or the transistor Q1 is 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 connected to the VCC terminal of the control circuit 22 increases, and the transistor Q2 is destroyed. May cause problems.

  The problem that the transistors Q1 and Q2 may be destroyed due to the delay of the operation stop of the control circuit 22 by the smoothing capacitor C3 when the voltage of the AC input is lowered (off) is a problem of the third conventional example. This also occurs in a switching power supply.

  In order to solve the above problems, a switching power supply according to the present invention converts an AC input into a DC output of a desired voltage by switching using a separately excited control circuit, and stably outputs the DC voltage. A startup circuit that supplies power necessary for startup of the control circuit to a power supply terminal of the control circuit, a sub-rectifier circuit that supplies power required for operation of the control circuit to the power supply terminal by AC rectification based on the AC input, An input voltage detection circuit that detects whether or not the voltage of the AC input has dropped below a predetermined value; and a switch circuit that stops the supply of power to the control circuit by a detection signal from the input voltage detection circuit. The activation circuit is connected to the AC input.

  According to the above configuration, since the start circuit and the sub rectifier circuit are connected to the power supply terminal of the control circuit, even if an AC input is connected to the start circuit and input, the AC is rectified to some extent by the sub rectifier circuit. The control circuit can be activated by the AC input and can be activated by the AC input, and when the normal control circuit with stable switching is operated, the control is performed by the power supplied from the sub-rectifier circuit to the control circuit. The circuit can be operated.

  For this reason, in the above configuration, since the starting circuit is connected to the AC input, the smoothing capacitor for rectification when the direct current obtained by rectifying the AC input is connected to the starting circuit as in the prior art is directly connected to the starting circuit. Therefore, even when the voltage of the AC input is lowered, for example, when shut off, the input voltage detection circuit and the switch circuit are used to prevent the above-described problem caused by the smoothing capacitor. Stopping the operation of the control circuit can be executed quickly while suppressing the delay, and the occurrence of malfunctions in the electrical equipment connected to the subsequent stage can be reduced.

  As a result, when the voltage of the AC input is lowered, for example, when shut off, the above configuration can quickly suppress the delay and stop the operation of the control circuit, and the switching can be caused by the delay of the stop. For example, destruction of a transistor as a main switching element and destruction of a transistor as a switching element in a switch circuit can be avoided, and malfunction in an electrical device connected to a subsequent stage can be prevented.

  In the switching power supply device, the starting circuit is connected to the AC input via a plurality of resistors, and the input voltage detection circuit detects a voltage divided by the plurality of resistors. Also good.

  In the switching power supply device, the input voltage detection circuit may include at least one Zener diode for detecting whether or not the voltage of the AC input is equal to or higher than a predetermined value.

  In the switching power supply device, the input voltage detection circuit may include at least one bias resistor of the Zener diode.

  In the switching power supply device, the switch circuit may include a transistor as a switch element that short-circuits a power supply terminal of the control circuit to the ground.

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

  In the switching power supply device, the input voltage detection circuit may have a plurality of bias resistors so as to reduce the voltage applied to the control terminal of the transistor by voltage division.

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

In the switching power supply device, the plurality of resistors of the starting circuit have a first starting resistor and a second starting resistor connected in series with each other, Vin indicates an AC input voltage, and Vzd1 indicates a Zener diode. Z indicates the set value of the rate of decrease of the AC input voltage at which the operation of the control circuit is stopped, and the ratio (R1 / R2) of the first starting resistance to the second starting resistance is In the following formula (1), R1 / R2 = ((Vin × √2 × k) / Vzd1) −1 Formula (1)
It may be set.

  As described above, the switching power supply device according to the present invention is connected to the AC input and supplies power necessary for starting the control circuit to the power supply terminal of the control circuit, and is necessary for the operation of the control circuit. A sub-rectifier circuit that supplies power to the power supply terminal by AC rectification based on the AC input, an input voltage detection circuit configured to detect a voltage value of the AC input, and the input voltage detection circuit. And a switch circuit that stops the supply of power to the control circuit that controls the switching according to the detection signal.

  Therefore, in the above configuration, since the startup circuit that supplies power at the time of startup to the control circuit is connected to the AC input as the power source of the power, the DC circuit after the startup circuit is rectified from the AC input as in the prior art It is possible to eliminate the influence of the smoothing capacitor for rectification due to the connection to.

Therefore, the above configuration can prevent the conventional problems caused by the smoothing capacitor, and even when the AC input voltage is lowered, for example, shut off, the operation of the control circuit is stopped by the input voltage detection circuit and the switch circuit. Can be executed quickly while suppressing the delay, and the occurrence of malfunctions in the electrical equipment connected to the subsequent stage can be reduced.

  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)
As shown in FIG. 1, the switching power supply device according to the first embodiment of the present invention has input terminals P1 and P2 to which an AC input, which is a commercial AC power supply of, for example, 100 V and 60 Hz, is input. A line filter L1 to which alternating current from the terminals P1 and P2 is input via a fuse, and line capacitors C1 and C2 for noise removal provided respectively before and after the line filter L1 are provided. Other examples of the voltage of the commercial AC power supply include 117V, 200V, 220V, and 230V.

  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 is provided for further rectifying the direct current and outputting a direct current input to an FET Q1 and a transformer T1, which will be described later, from the alternating current input. A first main rectifier circuit is formed by the diodes D1 to D4 and the smoothing capacitor C3.

  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 AC output, which is 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 DC 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. The rectifying diode D6 and the smoothing capacitor C5 form a second main rectifier circuit.

Further, an output voltage detection circuit 11 connected in parallel to the output terminals P3 and P4 is provided for detecting the voltage of the DC output and feeding it back to the primary side of the transformer T1. Further, in the secondary side winding N2 of the transformer T1, a photocoupler PC for optically transmitting the output voltage value (output voltage information) detected by the output voltage detection circuit 11 to the primary side in a non-contact manner. The light emitting part PCa 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.

  The control circuit 12 is provided with at least a power supply terminal VCC in addition to the output terminal OUT and the feedback terminal FB. The power supply terminal VCC is for applying a power supply voltage (12 V to 25 V, set to 20 V in the present embodiment) 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 capacitor for accumulating starting current from the starting resistors R1 and R2 for rectification and starting. A sub-rectifier circuit is formed by the rectifier diode D6 and the smoothing capacitor C4.

  The capacity of the smoothing capacitor C4 is smaller than the capacity of the smoothing capacitor C3, for example, 1/10 or less, in order to rectify high-frequency alternating current having a frequency significantly higher than the commercial alternating-current input frequency. The conventional problem caused by the capacitor C3 does not occur with the capacity of the smoothing capacitor C4.

  In the switching power supply device of the present invention, the AC input to each of the diodes D1 to D4 is connected to each of the starting resistors R1 and R2 at the power supply terminal VCC of the control circuit 12 in order to supply power when the control circuit 12 is started. Is entered through. The AC input is slightly rectified by the sub-rectifier circuit at the power supply terminal VCC of the control circuit 12 and becomes pseudo DC. For this reason, a pseudo direct current that includes ripples but is inconvenient for starting the control circuit 12 is input to the power supply terminal VCC when the control circuit 12 is started.

  Further, in the above switching power supply device, the control circuit 12 is connected to the input voltage detection circuit 2 for monitoring the voltage of the AC input, and the power supply terminal VCC is grounded by the detection signal from the input voltage detection circuit 2. There is provided a switch circuit 1 for stopping the power supply to the power supply terminal VCC at a zero potential due to a short circuit.

  Therefore, the output of the control signal from the output terminal OUT of the control circuit 12 to the FET Q1 can be quickly stopped by shorting the power supply terminal VCC to the ground by the switch circuit 1.

  The input 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. 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 input voltage detection circuit 2, the cathode terminal of the Zener diode ZD1 is connected to a connection point between the starting resistors R1 and R2. That is, to the Zener diode ZD1, the pseudo DC that is rectified by the sub-rectifier circuit after the AC input is divided by the starting resistors R1 and R2 is applied. The other terminal of the first bias resistor R3 is a transformer T1.
Is connected to the negative side of the primary side auxiliary winding N3.

  The switch circuit 1 includes a transistor Q2 in which an anode terminal of the diode D5 is connected to a base (control terminal), 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 power supply terminal VCC of the control circuit 12. In this embodiment, an example in which an N-type transistor is used as the transistor Q2 has been described. However, a P-type transistor, an N-type, or a P-type FET can be used as long as it has a switching function. .

  Moreover, 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 the voltage of the AC input applied to the input terminals P1 and P2, and Vzd1 represents the 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 *
Since the voltage of Vr2 when the transistor Q2 is turned on and the control circuit 12 is stopped is equal to the Zener voltage of Vzd1, Equation (1) can be derived.

  k is a coefficient for stopping the power supply to the control circuit 12 and stopping the operation of the FET Q1 when it drops to a predetermined value, for example, 70% from the time when the AC input voltage is off, and the operation is stopped by changing this value k. 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, the switch circuit 1 and the input voltage detection circuit for short-circuiting the power supply terminal VCC of the control circuit 12 by the input voltage detection signal of the AC input power supply obtained by bypassing the smoothing capacitor C3 2 is connected to the input voltage detection circuit 2 from the connection point of each of the starting resistors R1 and R2, and more preferably, the ratio of the starting resistors R1 and R2 is set and combined as shown in Equation (1).

  In the present embodiment, by setting the starting resistors R1 and R2 as described above, the AC input voltage is about 70% (by changing the ratio of R1 and R2, the value of about 70% is an arbitrary value. Therefore, the operation of the FET Q1 can be forcibly stopped, and the conventional problem caused by the smoothing capacitor C3 can be avoided. In other words, in the present embodiment, a circuit configuration that quickly stops the operation of the control circuit 12 without delay when the power supply voltage of an arbitrary AC input is lowered by adjusting the ratio of R1 and R2 regardless of the AC input power supply voltage. It is possible to stop the operation without destroying the parts such as the FET Q1.

  In addition, by using the voltage input to detect the voltage value to the input voltage detection circuit 2 also as the power supply for starting to the control circuit 12, each starting resistor R1, By using the divided voltage by R2, the input voltage to the input voltage detection circuit 2 can be lowered as compared with the conventional case, so that the configuration of the input voltage detection circuit 2 can be simplified.

  In addition, since the input voltage to the input voltage detection circuit 2 can be made lower than before, the transistor Q2 is a small product, and an element having a rated value with 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 input voltage detection circuit 2 can omit the Zener diode ZD2 and the resistor R4 from the circuit of the input voltage detection circuit 41 of the conventional switching power supply circuit. This is because the input voltage to the input voltage detection circuit 2 can be set lower than that of the conventional switching power supply circuit. As a result, in the present 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, a second bias is provided between the Zener diode ZD1, the first bias resistor R3, and the diode D5 with respect to the input voltage detection circuit 2 shown in FIG. An input voltage detection circuit 3 in which a resistor 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, an input voltage detection circuit 4 in which the diode D5 is omitted is provided for the input 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 input voltage detection circuit 4 is set low even if the diode D5 is omitted, it is necessary to employ a transistor Q2 having a high base voltage withstand voltage. It became possible to use parts.

  In the present embodiment, the switching power supply in combination with the control circuit 12 is a circuit in which the diode D5 is omitted from the first embodiment, and the base voltage of the transistor Q2 is lower than that in the circuit of the conventional switching power supply device. It became possible by adopting a high one (about 100V). 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 input voltage detection circuit 4 shown in FIG. 3, between the connection point to the transistor Q2, the first bias resistor R3 and the diode D5, An input 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 a low withstand voltage and inexpensive component can be adopted for the transistor Q2. Became.

  That is, this embodiment is a combination of the input voltage detection circuit 5 with the second bias resistor R7 added to the control circuit 12 in the input voltage detection circuit 3 shown in FIG. A relatively low transistor having a base voltage withstand voltage of about 80 V can be employed.

In any of the circuits described in the first to fourth embodiments, spike noise is generated in the DC output voltage after the power is turned off, which is an AC input, as shown in FIG. Is prevented. 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 includes a first rectifier circuit that rectifies an AC input and outputs it as a DC input, a main switching element that outputs a switching AC by switching the rectified DC input from the first rectifier circuit, A transformer that outputs an AC output in which the voltage of the switching AC is adjusted, a second rectifier circuit that outputs a DC output obtained by rectifying a voltage-adjusted AC output from the secondary side of the transformer, and a voltage of the DC output. An output voltage detection circuit to detect, and a control signal for controlling the switching according to output voltage information fed back from the output voltage detection circuit is output, and the DC output voltage is stabilized and output to a desired value. A separate excitation type control circuit and a start-up circuit for supplying power necessary for starting the control circuit to the power supply terminal of the control circuit at the time of start-up And a third rectifier circuit for rectifying the alternating current from the tertiary winding from the transformer to which the switching alternating current is input and supplying the electric power necessary for the operation of the control circuit to the power supply terminal of the control circuit during operation (Sub-rectifier circuit), an input voltage detection circuit that detects whether or not the voltage of the AC input has dropped below a predetermined value, and the supply of power to the control circuit is stopped by a detection signal from the input voltage detection circuit Stop means (switch circuit) for causing the start circuit to be connected to the AC input via a plurality of resistors, and the input voltage detection circuit detects a voltage divided by the plurality of resistors. It may be a thing.

  In each of the above embodiments, the switching power supply device using the transformer T1 is shown. However, in the switching power supply device of the present invention, the transformer T1 can be omitted. Even in this case, the auxiliary power to the power supply terminal VCC of the control circuit 12 may be used by dividing the alternating current obtained by switching as necessary and rectifying it by the sub-rectifier circuit.

  The switching power supply device of the present invention safely stops operation without malfunction or damage to other parts or damage due to a simple circuit configuration even when the voltage value of the AC input decreases (for example, at shut-off). Therefore, it can be suitably used as a power source for audio devices, lighting devices, electric devices such as personal computers, and electronic devices.

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

P1, P2: commercial power supply input terminal HS1: fuse L1: line filter C1, C2: line capacitors D1, D2, D3, D4: rectifier diode C3: smoothing capacitor (rectifier capacitor)
R1, R2: Starting resistors ZD1, ZD2: Voltage detecting Zener diode R3, R4: ZD1, ZD2 bias resistor D5: Q2 withstand voltage protection diode Q2: Switch transistor R5: Q2 current limiting resistor PC: Photocoupler C4 : Capacitor D6 for starting current accumulation from starting resistor for rectification and startup D3: Diode for auxiliary power supply rectification N3: Coil for auxiliary power supply N1: Primary coil Q1: Element for main switching (FET)
N2: Secondary winding D6: Output voltage rectifier diode C5: Output voltage rectifier capacitor P3, P4: DC output voltage terminal

Claims (9)

In a switching power supply device that converts an AC input into a DC output of a desired voltage by switching by a separately excited control circuit and outputs it stably,
An activation circuit for supplying power necessary for activation of the control circuit to a power supply terminal of the control circuit;
A sub-rectifier circuit that supplies power necessary for operation of the control circuit to the power supply terminal by AC rectification based on the AC input;
An input voltage detection circuit for detecting whether or not the voltage of the AC input has dropped below a predetermined value;
A switch circuit that stops the supply of power to the control circuit by a detection signal from the input voltage detection circuit,
The switching power supply device, wherein the starting circuit is connected to the AC input. The activation circuit is connected to the AC input via a plurality of resistors,
The switching power supply device according to claim 1, wherein the input voltage detection circuit detects a voltage divided by the plurality of resistors.   The switching power supply device according to claim 1, wherein the input voltage detection circuit has at least one Zener diode for detecting whether or not the voltage of the AC input is equal to or higher than a predetermined value.   The switching power supply device according to claim 3, wherein the input voltage detection circuit includes at least one bias resistor of the Zener diode.   The switching power supply device according to any one of claims 1 to 4, wherein the switch circuit includes a transistor as a switch element for short-circuiting a power supply terminal of the control circuit to ground.   The switching power supply device according to claim 5, wherein the input voltage detection circuit includes a diode for protecting the transistor between the transistor and the transistor.   The switching power supply device according to claim 5 or 6, wherein the input voltage detection circuit has a plurality of bias resistors so as to reduce an applied voltage to the control terminal of the transistor by voltage division.   The switching power supply device according to any one of claims 5 to 7, wherein the switch circuit includes a current limiting resistor for preventing an overcurrent to the transistor. The starting circuit has a first starting resistor and a second starting resistor connected in series with each other, connected between the AC input,
Vin represents the voltage of the AC input, Vzd1 represents the Zener voltage of the Zener diode, k represents the setting value of the rate of decrease of the voltage of the AC input, which stops the operation of the control circuit, The ratio (R1 / R2) to the second starting resistance is expressed by the following equation (1): R1 / R2 = ((Vin × √2 × k) / Vzd1) −1 Equation (1)
The switching power supply device according to claim 1, wherein the switching power supply device is set.

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