JP2019176612A - Switching power supply - Google Patents

Abstract

To provide a switching power supply capable of protecting overvoltage by appropriately changing overvoltage detection level, without using a light load detection circuit.SOLUTION: A switching power supply 1 includes: a switching control portion 21 that performs switching control in which an output voltage on a secondary side of a transformer 11 becomes a target voltage using a duty ratio of a pulse inputted to a switching element 13; a voltage generating circuit 22 that rectifies and smooths an AC voltage induced in an auxiliary winding L3 and outputs as a DC voltage; and an overvoltage protection circuit 23 that inputs a signal indicating an overvoltage to the switching control portion 21, when the DC voltage exceeds a predetermined voltage. When the signal indicating the overvoltage is not inputted when the predetermined voltage of the overvoltage protection circuit 23 is the first voltage and the duty ratio of the pulse is displaced first during the first predetermined period, the switching control portion 21 switches the predetermined voltage of the overvoltage protection circuit 23 from the first voltage to the second voltage.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a switching power supply that performs switching control on a switching element connected to a primary coil of a transformer.

Conventionally, a switching power supply that includes a switching element connected to a primary winding of a transformer, converts an AC voltage induced in the secondary winding of the transformer by a switching operation of the switching element into a DC voltage, and maintains a constant output voltage. is there.
In the switching power supply, when the voltage of the primary side auxiliary winding corresponding to the output voltage on the secondary side of the transformer exceeds the overvoltage detection level, switching control to the switching element is stopped, and protection against the secondary side overvoltage is performed. Figured.

  Here, when the switching power supply outputs a constant output voltage to the secondary side, a difference in load occurs depending on a load of an output destination, for example, whether or not an image forming unit is used. The coupling coefficient between the secondary coil of the transformer and the auxiliary winding varies depending on whether the load is constant or light. As a result, the voltage of the auxiliary winding at a light load is lower than the voltage of the auxiliary winding at a constant load.

  In the switching power supply described in Patent Document 1, after detecting whether or not the load is light using a light load detection circuit, if the load is light, the switching element is switched to control for intermittent oscillation operation and then overvoltage is applied. A technique for changing the detection level is disclosed.

JP 2006-109581 A

  However, the switching power supply disclosed in Patent Document 1 has a problem that a light load detection circuit is separately required in order to change the overvoltage detection level.

  An object of the present invention is to provide a switching power supply that can appropriately change an overvoltage detection level and protect an overvoltage of the switching power supply without using a light load detection circuit.

To achieve the above object, the present invention relates to a load unit, a transformer that outputs a secondary output voltage to the load unit, a switching element connected to a primary coil of the transformer, and an input to the switching element. A switching control unit that performs switching control in which the output voltage on the secondary side of the transformer becomes a target voltage by using a duty ratio of the generated pulse, and an AC voltage induced in an auxiliary winding provided on the primary side of the transformer A voltage generation circuit unit that rectifies and smoothes and outputs as a DC voltage, and an overvoltage protection circuit unit that inputs a signal indicating an overvoltage to the switching control unit when the DC voltage exceeds a predetermined voltage. The switching control unit can switch the predetermined voltage of the overvoltage protection circuit unit to the first voltage or the second voltage, and the overvoltage is the overvoltage. The switching control is stopped when a signal indicating that the duty ratio of the pulse is within the first predetermined range from the first predetermined range when the predetermined voltage of the overvoltage protection circuit unit is the first voltage. If a signal indicating the overvoltage is not input during a first predetermined period after the first displacement when a first displacement that is a displacement to a second predetermined range that is outside the range is performed, the first After a predetermined period, the predetermined voltage of the overvoltage protection circuit unit is switched from the first voltage to the second voltage.

The switching control unit stops switching control for the switching element when a signal indicating an overvoltage is input. The switching control unit can switch the predetermined voltage of the overvoltage protection circuit unit to the first voltage or the second voltage.
The switching control unit can determine that the load unit is a light load using the pulse duty ratio, for example, a light load if the pulse duty ratio is in the first predetermined range. it can.

  However, if the pulse duty ratio is displaced to the second predetermined range which is outside the first predetermined range, whether the output voltage on the secondary side of the transformer is in an overvoltage state when the load is lightly loaded, or the load is The switching control unit cannot determine whether or not it is only switched to a constant load only by the duty ratio of the pulse.

  Therefore, in the configuration of the present invention, when the duty ratio of the pulse is displaced from the first predetermined range to the second predetermined range that is outside the first predetermined range, the predetermined voltage of the overvoltage protection circuit unit is immediately changed from the first voltage to the first voltage. Instead of switching to the two voltages, a signal indicating that the switching control unit is overvoltage during the first predetermined period from the time when the switching control unit is displaced from the first predetermined range to the second predetermined range outside the first predetermined range. Detect whether it was entered.

When the switching control unit receives a signal indicating that the voltage is overvoltage during the first predetermined period from the time of the displacement, the output voltage on the secondary side of the transformer is in the overvoltage state when the load unit is in a light load state. It can be seen that this is a variation in the duty ratio of the pulse due to the above.
On the other hand, the switching control unit knows that the load unit has changed from a light load state to a constant load when a signal indicating an overvoltage is not input even after the first predetermined period from the time of the displacement, Thereafter, the predetermined voltage of the overvoltage protection circuit unit is switched from the first voltage to the second voltage.

  Unlike the conventional switching power supply, the switching power supply of this configuration can appropriately change the overvoltage detection level to enable overvoltage protection of the switching power supply.

  According to the present invention, the overvoltage protection of the switching power supply can be performed by appropriately changing the overvoltage detection level.

It is a circuit diagram of a switching power supply concerning one embodiment of the present invention. It is a graph showing an example of the behavior of the voltage induced in the secondary side output voltage of the transformer and the auxiliary winding at the time of constant load and light load. It is a graph showing an example of the behavior of the duty ratio of the switching operation of the switching element when the load fluctuates. It is a graph for demonstrating the switching aspect of the Zener voltage by a Zener diode in the time of a constant load and a light load. It is a graph showing an example of the behavior of the duty ratio of the switching operation of the switching element after a steady state and after occurrence of an overvoltage at light load. It is a flowchart showing the first half part of the control procedure performed by the switching control part. It is a flowchart showing the second half part of the control procedure performed by the switching control part.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

<Circuit Overview of Embodiment>
FIG. 1 shows a circuit configuration of a switching power supply according to an embodiment of the present invention. In FIG. 1, this switching power supply 1 converts an AC voltage supplied from a commercial AC power supply AC via a pair of power supply lines PS1 and PS2 into a DC voltage, and is mounted on an electronic device such as a printer. The The switching power supply 1 includes a transformer 11, and the transformer 11 is divided into a primary side on the left side where the primary winding L1 is provided and a secondary side on the right side where the secondary winding L2 is provided. Yes. The transformer 11 is provided with an auxiliary winding L3 on the primary side. The primary winding L1 is an example of a primary coil.

<Primary side configuration 1>
The switching power supply 1 includes a primary side rectifying / smoothing circuit 12 and a switching element 13 on the primary side of the transformer 11.

<Primary rectification smoothing circuit>
The primary side rectifying / smoothing circuit 12 includes a diode bridge 14 and a smoothing capacitor C1.

  The diode bridge 14 is a circuit having a configuration in which four diodes D1, D2, D3, and D4 are bridge-connected. Specifically, in the diode bridge 14, a series circuit of two diodes D1 and D2 and a series circuit of the remaining two diodes D3 and D4 are connected in parallel. Feed lines PS1 and PS2 are connected to the connection point of the diodes D1 and D2 in one series circuit and the connection point of the diodes D3 and D4 in the other series circuit, respectively. One connection point of the series circuit of the diodes D1 and D2 and the series circuit of the diodes D3 and D4 is connected to one end which is the upper end of the primary winding L1 of the transformer 11 via the wiring W1, and the other connection point is Are connected to the ground line G3 on the primary side.

  One electrode of the smoothing capacitor C1 is connected to the wiring W1, and the other electrode is connected to the ground line G3.

  The AC voltage supplied from the commercial AC power supply AC is full-wave rectified by the diode bridge 14, and the voltage after the full-wave rectification is smoothed by the smoothing capacitor C1, so that the voltage between the wiring W1 and the ground line G3 is obtained. DC voltage is generated.

<Switching element>
The switching element 13 is composed of an N-channel MOSFET (NMOS), and its drain terminal is connected to the other end, which is the lower end in the figure of the primary winding L1, via the wiring W2. The source terminal of the switching element 13 is connected to the ground line G3 via the resistor R1. The switching element 13 performs a switching operation (ON / OFF) by the voltage input to the gate terminal.

  When the switching element 13 is turned on, a current flows through the primary winding L1 of the transformer 11, and energy is stored in the primary winding L1. Thereafter, when the switching element 13 is turned off, the energy accumulated in the primary winding L1 is released, an electromotive force is generated in the primary winding L1, and the primary winding L1 and the secondary winding L2 of the transformer 11 A secondary voltage corresponding to the turn ratio with the secondary winding L2 is generated. By repeatedly turning on / off the switching element 13, a secondary voltage is generated in a pulse manner.

<Secondary configuration>
The switching power supply 1 includes a secondary side rectifying / smoothing circuit 15 and a constant voltage circuit 16 on the secondary side of the transformer 11.

<Secondary rectification smoothing circuit>
The secondary side rectifying / smoothing circuit 15 includes a rectifying diode D5 and a smoothing capacitor C2. One end, which is the upper end of the secondary winding L2 of the transformer 11, shown in the figure is connected to the output terminal via the wiring W3, and the other end is connected to the secondary-side ground line G2 indicated by "0V" shown in the figure. . The rectifier diode D5 is interposed in the middle of the wiring W3 with the anode facing the secondary winding L2. One electrode of the smoothing capacitor C2 is connected to the wiring W3 on the cathode side of the rectifier diode D5, and the other electrode is connected to the ground line G2. The secondary voltage generated in a pulsed manner in the secondary winding L2 is rectified and smoothed by the secondary side rectifying and smoothing circuit 15 and converted into a DC voltage.

<Constant voltage circuit>
The constant voltage circuit 16 includes a shunt regulator IC (Integrated Circuit) 17, resistors R2, R3, R4 and a photocoupler PC. The resistors R2 and R3 are connected in series. One end of the series circuit of the resistors R2 and R3 is connected to the wiring W3, and the other end is connected to the ground line G2. A reference voltage is set in advance in the shunt regulator IC 17, and a voltage that is a voltage obtained by dividing the voltage output from the output terminal and is a connection point between the resistor R2 and the resistor R3 is input as a feedback voltage. The light-emitting diode PD of the photocoupler PC has an anode connected to the wiring W3 via the resistor R4 and a cathode connected to the shunt regulator IC17. With this configuration, a constant voltage (Output) is output from the output terminal and output to the load unit 90 connected to the output terminal. When the feedback voltage exceeds the reference voltage, the light emitting diode PD of the photocoupler PC emits light.

<Primary side configuration 2>
The switching power supply 1 includes a switching control unit 21 on the primary side of the transformer 11. The switching control unit 21 includes an IC incorporating a CPU and the like, and includes a first terminal FB, a second terminal OUT, a third terminal VCC, and a fourth terminal SW.

  The phototransistor PT on the light receiving side of the photocoupler PC is connected to the first terminal FB via the resistor R5. Thus, when the secondary-side feedback voltage exceeds the reference voltage and the light emitting diode PD of the photocoupler PC emits light, the phototransistor PT is turned on, and the voltage is input to the first terminal FB. Since the current flowing through the light emitting diode PD increases or decreases according to the potential difference between the feedback voltage on the secondary side and the reference voltage, the light emission amount of the light emitting diode PD increases or decreases. Since the current flowing through the phototransistor PT changes according to the increase or decrease in the amount of light emission, the voltage input to the first terminal FB changes.

  The second terminal OUT of the switching control unit 21 is connected to the gate terminal of the switching element 13 via the resistor R6. The switching control unit 21 controls the duty ratio of the switching operation of the switching element 13 based on the input voltage of the first terminal FB that changes according to the secondary side feedback voltage. As a result, the feedback voltage on the secondary side is kept at a constant value as the target voltage.

<Primary side configuration 3>
Further, the switching power supply 1 includes a voltage generation circuit 22 and an overvoltage protection circuit 23 on the primary side of the transformer 11. The voltage generation circuit 22 is an example of a voltage generation circuit unit, and the overvoltage protection circuit 23 is an example of an overvoltage protection circuit unit.

<Voltage generation circuit>
The voltage generation circuit 22 includes a rectifier diode D6, a resistor R7, and a smoothing capacitor C3. A wiring W4 as an example of an output line is connected to one end which is the upper end of the auxiliary winding L3 in the figure. The rectifier diode D6 is interposed in the middle of the wiring W4 with the anode directed toward the auxiliary winding L3. The resistor R7 is interposed between the anode of the rectifier diode D6 and one end of the auxiliary winding L3. One electrode of the smoothing capacitor C3 is connected to the wiring W4 on the cathode side of the rectifier diode D6, and the other electrode is connected to the ground line G1 on the primary side. When the secondary voltage is generated in a pulse manner in the secondary winding L2, an AC voltage is generated in a pulse manner in the auxiliary winding L3. As for the AC voltage generated in the auxiliary winding L3, one voltage that is either positive or negative of the AC voltage is output by the rectifier diode D6, and the one voltage that is either positive or negative is the smoothing capacitor. Smoothed by C3 and output as a DC voltage. The rectifier diode D6 corresponds to an example of a rectifier circuit, and the smoothing capacitor C3 corresponds to an example of a smoothing circuit.

<Overvoltage protection circuit>
The overvoltage protection circuit 23 includes an NPN transistor Tr1 and three Zener diodes ZD1, ZD2, and ZD3. The collector of the NPN transistor Tr1 is connected to the wiring W4. The emitter of the NPN transistor Tr1 is connected to the third terminal VCC of the switching control unit 21. Zener diodes ZD1 and ZD2 are connected in series, the cathode of one Zener diode ZD2 is connected to the collector of NPN transistor Tr1, and the anode of the other Zener diode ZD1 is connected to the emitter of NPN transistor Tr1. The cathode of the Zener diode ZD3 is connected to the base of the NPN transistor Tr1, and the anode of the Zener diode ZD3 is connected to the ground line G1. A resistor Rc is interposed between the collector of the NPN transistor Tr1 and the cathode of the Zener diode ZD3.

<Basic operation of switching power supply>
In the above configuration, when the voltage output from the output terminal of the transformer 11 is a normal voltage in a steady state, a current flows through the Zener diode ZD3 and the resistor Rc by the DC voltage of the wiring W4, and the third terminal VCC of the switching control unit 21 In addition, a constant voltage that is lower than the breakdown voltage of the Zener diode ZD3 by the base-emitter voltage of the NPN transistor Tr1 is input as a drive voltage.

  For example, an abnormality such as a circuit failure including a failure of the photocoupler PC occurs in the switching power supply 1 and normal control of the switching operation of the switching element 13 by the switching control unit 21 becomes impossible, so that the switching element 13 has a maximum duty ratio. When the switching operation is performed, the voltage output from the output terminal starts to increase, and accordingly, the voltage induced in the auxiliary winding L3 increases and the DC voltage of the wiring W4 starts to increase. When the Zener diodes ZD1 and ZD2 become conductive due to the rise of the DC voltage of the wiring W4, a voltage obtained by subtracting the breakdown voltage of the Zener diodes ZD1 and ZD2 from the DC voltage of the wiring W4 is applied to the third terminal VCC of the switching control unit 21. Entered. When the voltage of the signal input to the third terminal VCC exceeds a predetermined voltage value identified as overvoltage, the switching control unit 21 activates the overvoltage protection function and stops the switching operation of the switching element 13. Thus, the voltage output from the output terminal is reduced. As a result, when the voltage induced in the auxiliary winding L3 exceeds the overvoltage detection level, the switching operation of the switching element 13 is stopped, and protection against the overvoltage on the secondary side is achieved.

<Protection delay due to lower coupling ratio between secondary coil and auxiliary winding>
By the way, when the switching power supply 1 outputs a constant output voltage to the secondary side as described above, a difference in load in the load unit 90 as an output destination, for example, whether or not to use an image forming unit in the printer, etc. The magnitude of the load is caused by. The coupling rate between the secondary winding L2 and the auxiliary winding L3 of the transformer 11 varies depending on whether the load is constant or light. That is, generally, the coupling rate between the secondary winding L2 and the auxiliary winding L3 of the transformer 11 is low when the load applied to the secondary side of the transformer 11 is small. For this reason, in the case of a light load, as described above, even if the voltage output from the output terminal of the transformer 11 increases, the AC voltage induced in the auxiliary winding L3 becomes low. It takes time for the voltage input to the third terminal VCC of the unit 21 to reach a predetermined voltage value identified as an overvoltage. For this reason, compared to the case of a constant load, the switching operation of the switching element 13 described above cannot be easily stopped, and the secondary output voltage may reach an overvoltage, and execution of the protection function may be delayed.

  For example, in the example shown in FIG. 2A, at a constant load indicated by a one-dot chain line, an abnormality such as a circuit failure occurs at time t0, and the voltage output from the secondary output terminal is about 25 [V]. As the voltage starts to rise, the voltage induced in the auxiliary winding L3 starts to rise from about 80 [V]. Then, when the voltage induced in the auxiliary winding L3 at the subsequent time t2 reaches the overvoltage protection operation voltage Vp set to about 100 [V], the switching operation of the switching element 13 is stopped, so that the secondary voltage is reduced. The voltage output from the output terminal on the side decreases without reaching the dangerous voltage Vd set to about 35 [V]. As a result, protection against overvoltage on the secondary side is achieved.

  On the other hand, at the time of the load indicated by the solid line, when an abnormality such as a circuit failure occurs at time t0 and the voltage output from the secondary output terminal starts to rise from about 25 [V], the auxiliary winding The voltage induced in the line L3 starts to rise from about 70 [V], which is lower than that at the time of constant load. The voltage induced in the auxiliary winding L3 finally reaches the overvoltage protection operation voltage Vp at time t1 later than time t2, and the switching operation of the switching element 13 is stopped. Therefore, at the timing of time t1, the voltage output from the secondary output terminal exceeds the dangerous voltage Vd, and sufficient protection against the secondary overvoltage cannot be achieved.

<Early timing of protection function activation at light load>
Accordingly, in response to the above, in the switching power supply 1 of the present embodiment, a Zener diode that is turned on by increasing the DC voltage of the wiring W4 is replaced with two Zener diodes ZD1 and ZD2 at light load. By setting only ZD2 and inputting a voltage obtained by subtracting the breakdown voltage of the Zener diode ZD2 from the DC voltage of the wiring W4 to the third terminal VCC, the operation timing of the overvoltage protection function is advanced.

  In this case, the duty ratio of the switching operation of the switching element 13 can be used to determine whether the load is constant or light. That is, for example, as shown in FIG. 3, when the load is large, the duty ratio of the switching operation of the switching element 13 by the control signal from the second terminal OUT of the switching control unit 21 is increased and the output current on the secondary side is increased. When the load is small, the duty ratio of the switching operation of the switching element 13 by the control signal from the second terminal OUT of the switching control unit 21 is reduced and the output current value on the secondary side is also reduced. Become.

Therefore, in the present embodiment, as shown in FIG. 4A, when the duty ratio of the switching operation of the switching element 13 is large, the zener diode ZD1 and ZD2 is a Zener that is the breakdown voltage of the whole. By maintaining the voltage at a high value, the operation timing of the overvoltage protection function is made normal. On the other hand, as shown in FIG. 4B, when the duty ratio of the switching operation of the switching element 13 is small, the anode side and the cathode side of the Zener diode ZD1 are made conductive to make the remaining one The Zener voltage, which is the breakdown voltage of only the Zener diode ZD2, can be lowered to a low value, and the operation timing of the overvoltage protection function can be advanced.

<Additional NPN transistor>
In this embodiment, an NPN transistor Tr2 is further provided on the primary side of the transformer 11 in order to accelerate the operation timing of the overvoltage protection function, as shown in FIG. The collector of the NPN transistor Tr2 is connected to the connection point of the two Zener diodes ZD1 and ZD2, that is, the cathode side of the Zener diode ZD1 and the anode side of the Zener diode ZD2. The emitter of the NPN transistor Tr2 is connected to the third terminal VCC of the switching control unit 21. The base of the NPN transistor Tr2 is connected to the fourth terminal SW of the switching control unit 21. Note that the NPN transistor Tr2 corresponds to an example of a conduction part.

  The switching control unit 21 outputs a signal indicating whether or not to make the anode side and the cathode side of the Zener diode ZD1 conductive from the fourth terminal SW to the base of the NPN transistor Tr2.

  When the signal from the fourth terminal SW indicates that the anode side and the cathode side of the Zener diode ZD1 are conducted, the NPN transistor Tr2 is turned on. When the NPN transistor Tr2 is turned on, the current from the wiring W4 via the Zener diode ZD2 is short-circuited without passing through the Zener diode ZD1, flows from the collector of the NPN transistor Tr2, and flows from the emitter of the NPN transistor Tr2 to the switching control unit 21. Output to the third terminal VCC. In this case, the Zener voltage can be lowered to a low value. That is, when the increased DC voltage of the wiring W4 exceeds the breakdown voltage of one Zener diode ZD2, a corresponding signal is input to the third terminal VCC of the switching control unit 21, and the switching control unit 21 has an overvoltage protection function. Operate. Note that the breakdown voltage of the Zener diode ZD2 at this time corresponds to an example of the first voltage.

  When the signal from the fourth terminal SW does not indicate that the anode side and the cathode side of the Zener diode ZD1 are conducted, the NPN transistor Tr2 remains in the OFF state. When the NPN transistor Tr2 is off, the current from the wiring W4 via the Zener diode ZD2 is output to the third terminal VCC of the switching control unit 21 via the Zener diodes ZD1 and ZD2. In this case, the Zener voltage is maintained at a high value. That is, when the increased DC voltage of the wiring W4 exceeds the breakdown voltage of the two Zener diodes ZD1 and ZD2, a corresponding signal is input to the third terminal VCC of the switching control unit 21, and the switching control unit 21 is overvoltage protected. Activate the function. Note that the breakdown voltage of the Zener diode ZD2 at this time corresponds to an example of the second voltage.

  Note that three or more Zener diodes may be provided in addition to the two Zener diodes ZD1 and ZD2. In that case, the anode side and the cathode side of only the Zener diode ZD1 are not conducted as described above, but the anode side and the cathode of the entire at least one Zener diode excluding a part of the three or more Zener diodes. The same operation as described above can be performed by connecting the side.

<Speciality of duty ratio behavior when overvoltage occurs>
However, when an overvoltage occurs, the duty ratio of the switching operation increases regardless of the size of the load. For this reason, it is impossible to accurately identify whether the load is a constant load or a light load only by the magnitude of the duty ratio. For example, FIG. 5 shows the case of a light load. In the steady state on the left side of the figure, the duty ratio of the switching operation of the switching element 13 is small, and when an overvoltage occurs due to an abnormality such as a circuit failure, as shown on the right side of the figure. The duty ratio of the switching operation of the switching element 13 is increased. Therefore, even if the duty ratio of the switching operation is increased, it is because the load unit 90 is in a constant load state or the secondary side is in an overvoltage state while the load unit 90 is lightly loaded. It cannot be determined by itself.

<Outline of the method of the present embodiment>
Therefore, in the present embodiment, first, the NPN transistor Tr2 is turned on so that the anode side and the cathode side of the Zener diode ZD1 are electrically connected to reduce the Zener voltage. As shown in FIG. 2B corresponding to FIG. 2A, the light load overvoltage protection operating voltage Vl corresponding to the lowered Zener voltage is set to about 90 [V].

  For example, when the duty ratio of the switching operation becomes large at time t0, the NPN transistor Tr2 is not immediately turned off at that timing to increase the Zener voltage to a high value, but at time t3 after time t0. In the meantime, it is detected whether the DC voltage of the wiring W4 exceeds the breakdown voltage of the Zener diode ZD2 and is input to the third terminal VCC of the switching control unit 21. In other words, this detects whether or not the voltage induced in the auxiliary winding L3 has reached the overload protection operating voltage Vl for light load. The time t3 at this time is determined in advance to be longer than the time necessary for the voltage of the auxiliary winding L3 to reach the overvoltage protection operating voltage Vl when an overvoltage occurs in a light load state. Yes. As a result, when a signal input indicating an overvoltage to the third terminal VCC is made between the time t0 and the time t3, the switching control unit 21 causes the load unit to It can be seen that the output voltage on the secondary side of the transformer 11 is in an overvoltage state when 90 is a light load. Therefore, the Zener voltage is maintained at a low value by keeping the NPN transistor Tr2 on. As a result, since the voltage induced in the auxiliary winding L3 reaches the overvoltage protection operating voltage Vl for light load, the switching operation of the switching element 13 is stopped. As shown by the solid line in FIG. The voltage output from the output terminal on the secondary side decreases without reaching the dangerous voltage Vd. As a result, protection against overvoltage on the secondary side is achieved.

  On the other hand, when the switching control unit 21 does not input a signal to the third terminal VCC indicating an overvoltage between the time t0 and the time t3, the cause of the increased duty ratio is the load unit 90. It can be seen that is changed from a light load state to a constant load. Therefore, thereafter, the NPN transistor Tr2 is turned off to increase the Zener voltage, and the Zener voltage is maintained at a high value. Then, similarly to the above-described FIG. 2A, at time t2, as indicated by the one-dot chain line in FIG. 2B, the DC voltage of the wiring W4 exceeds the breakdown voltage of the Zener diodes ZD1, ZD2, and the switching control unit. 21 is input to the third terminal VCC. In other words, the voltage induced in the auxiliary winding L3 reaches the overvoltage protection operating voltage Vc for a constant load. The overvoltage protection operation voltage Vc is set to, for example, about 100 [V], similarly to the overvoltage protection operation voltage Vp shown in FIG. Since the switching operation of the switching element 13 is stopped when the voltage induced in the auxiliary winding L3 reaches the overvoltage protection operation voltage Vc for a constant load, the voltage is output from the secondary output terminal as shown in the figure. The voltage drops without reaching the dangerous voltage Vd, and protection against the overvoltage on the secondary side is achieved.

<Control procedure>
A control procedure executed by the switching control unit 21 to realize the above-described method will be described with reference to the flowcharts of FIGS. 6 and 7.

  In FIG. 6, first, in step S5, the switching control unit 21 performs a predetermined initial operation.

  Thereafter, in step S10, the switching control unit 21 starts measuring the duty ratio. In this example, as the duty ratio, for example, an on-duty ratio that is a ratio between a state in which the pulse of the signal is high and a state in which it is low is measured.

  In step S15, the switching control unit 21 outputs a signal for turning on the base of the NPN transistor Tr2. As a result, the aforementioned Zener voltage is set to a low voltage for light load only by the Zener diode ZD2. Thereafter, the process proceeds to step S20.

  In step S20, it is determined whether the duty ratio measured after the start of measurement in step S10 is greater than a predetermined value that is predetermined as a threshold value. If it is larger than the predetermined value, the condition is satisfied (S20: Yes), and the process proceeds to step S30 described later. If it is equal to or smaller than the predetermined value, the condition is not satisfied (S20: No), and the process proceeds to step S25. Note that a range equal to or smaller than the predetermined value corresponds to an example of a first predetermined range, and a range larger than the predetermined value corresponds to an example of a second predetermined range. Moreover, the state where the condition of step S20 is satisfied corresponds to an example in which the duty ratio has made the first displacement.

  In step S25, the switching control unit 21 receives the voltage corresponding to the overload protection operating voltage Vl for light load shown in FIG. 2B, in other words, the overvoltage detection value at light load, with respect to the third terminal VCC. Determine whether or not. The overvoltage detection value at the time of light load is hereinafter referred to as “first overvoltage detection value” as appropriate, and is also shown in FIG. If the first overvoltage detection value is input, the condition is satisfied (S25: Yes), and the process proceeds to Step S50 described later. If the first overvoltage detection value is not input, the condition is not satisfied (S25: No), and the same procedure is repeated by returning to step S20.

  On the other hand, in step S30, the switching control unit 21 starts measuring the elapsed time thereafter using, for example, a timer (not shown). Thereafter, the process proceeds to step S35.

  In step S35, the switching control unit 21 determines whether or not the first overvoltage detection value is input to the third terminal VCC, as in step S25. If the first overvoltage detection value is not input, the condition is not satisfied (S35: No), and the process proceeds to Step S40 described later. If the first overvoltage detection value is input, the condition is satisfied (S35: Yes), and the process proceeds to step S50.

  In step S50, the switching control unit 21 instructs the switching element 13 to stop oscillation. Thereafter, this flow is terminated.

  On the other hand, in step S40, the switching control unit 21 determines whether or not the measured duty ratio is greater than a predetermined value, as in step S20. If it is larger than the predetermined value, the condition is satisfied (S40: Yes), and the process proceeds to step S45 described later. If it is equal to or smaller than the predetermined value, the condition is not satisfied (S40: No), and the same procedure is returned to the above-described step S20. repeat.

  In step S45, the switching control unit 21 determines whether or not the elapsed time measured after the start of measurement in step S30 has passed a predetermined period. This predetermined period is a time corresponding to the period from time t0 to time t3 shown in FIG. 2B, and corresponds to an example of a first predetermined period. This predetermined period is hereinafter simply referred to as “first predetermined period” as appropriate, and is also shown in FIG. If the first predetermined period has not elapsed, the condition is not satisfied (S45: No), and the same procedure is repeated by returning to step S35 described above. If the first predetermined period has elapsed, the condition is satisfied, and the routine goes to Step S55 in FIG.

  In FIG. 7, in step S55, the switching control unit 21 outputs a signal for turning off the base of the NPN transistor Tr2. As a result, the aforementioned Zener voltage is set to a high voltage for a constant load by the two Zener diodes ZD1 and ZD2. Thereafter, the process proceeds to step S60.

  In step S60, the switching control unit 21 determines whether or not the duty ratio measured after the start of measurement in step S10 is equal to or less than the predetermined value set in advance as a threshold value. If it is equal to or smaller than the predetermined value, the condition is satisfied (S60: Yes), and the process proceeds to step S70 described later. If it is greater than the predetermined value, the condition is not satisfied (S60: No), and the process proceeds to step S65. Note that the state in which the condition of step S60 is satisfied corresponds to an example in which the duty ratio has changed to the second.

  In step S65, the switching control unit 21 receives the voltage corresponding to the overvoltage protection operating voltage Vc for a constant load shown in FIG. 2B, in other words, the overvoltage detection value at a constant load, with respect to the third terminal VCC. Determine whether or not. In addition, the overvoltage detection value at the time of the constant load is hereinafter referred to as “second overvoltage detection value” as appropriate, and is also shown in FIG. If the second overvoltage detection value is input, the condition is satisfied (S65: Yes), and the process proceeds to step S50 described above with reference to FIG. If the second overvoltage detection value is not input, the condition is not satisfied (S65: No), and the same procedure is repeated by returning to step S60 described above.

  On the other hand, in step S70, like the above-described step S30, the switching control unit 21 starts measuring the elapsed time thereafter using, for example, a timer. Thereafter, the process proceeds to step S75.

  In step S75, the switching control unit 21 determines whether or not the second overvoltage detection value is input to the third terminal VCC, as in step S65. If the second overvoltage detection value is not input, the condition is not satisfied (S75: No), and the process proceeds to step S80 described later. If the second overvoltage detection value is input, the condition is satisfied (S75: Yes), and the process proceeds to step S50 described above with reference to FIG.

  In step S80, the switching control unit 21 determines whether or not the measured duty ratio is equal to or less than a predetermined value, as in step S60. If the value is equal to or smaller than the predetermined value, the condition is satisfied (S80: Yes), and the process proceeds to step S85 to be described later. If the value is larger than the predetermined value, the condition is not satisfied (S80: No). repeat.

  In step S85, the switching control unit 21 determines whether or not the elapsed time measured after the start of measurement in step S70 has passed a predetermined period. This predetermined period may be a time corresponding to, for example, the period from time t0 to time t3 shown in FIG. 2B, and corresponds to an example of a second predetermined period. This predetermined period is hereinafter simply referred to as “second predetermined period” as appropriate, and is also shown in FIG. If the second predetermined period has not elapsed, the condition is not satisfied (S85: No), and the same procedure is repeated by returning to step S75 described above. If the second predetermined period has elapsed, the condition is satisfied, and the process returns to step S15 in FIG. 6 to repeat the same procedure.

<Effect of embodiment>
As described above, in the switching power supply 1 of the present embodiment, when the duty ratio of the switching operation of the switching control unit 21 is larger than a predetermined value, the NPN transistor Tr2 is immediately turned off to turn the Zener voltage to a high value. Instead of switching, it is detected whether or not a signal indicating an overvoltage is input to the third terminal VCC of the switching control unit 21 during a first predetermined period from time t0 to t3.

  When a signal indicating an overvoltage is input during the first predetermined period, the duty ratio is increased because the secondary side of the transformer 11 is in an overvoltage state in a light load state of the load unit 90. I understand that. If a signal indicating an overvoltage is not input even after the first predetermined period has elapsed, it can be seen that the load unit 90 has changed from a light load state to a constant load state, and thereafter, as shown in step S55. Then, the NPN transistor Tr2 is turned off to switch the Zener voltage to a high value. Thereby, according to the switching power supply 1 of this embodiment, unlike the conventional switching power supply, the overvoltage protection of the switching power supply 1 can be performed by appropriately changing the overvoltage detection level.

In the present embodiment, in particular, when the duty ratio is larger than a predetermined value without receiving a signal indicating an overvoltage during the first predetermined period, the NPN transistor Tr2 is turned on after the first predetermined period elapses. To turn the Zener voltage to a higher value. On the contrary, when the duty ratio is shifted from the state larger than the predetermined value to the predetermined value or less before the first predetermined period elapses, the Zener voltage is not switched to a high value as shown in Step S40.
Thereby, when the duty ratio is temporarily reduced, the overvoltage of the switching power supply 1 can be protected in response to the temporary fluctuation of the duty ratio by not switching the Zener voltage to a high value.

  In the present embodiment, in particular, when the duty ratio of the switching operation of the switching control unit 21 is less than the predetermined value from the state larger than the predetermined value, a signal indicating an overvoltage is not input during the second predetermined period. In this case, it is detected whether or not a signal indicating an overvoltage is input during the second predetermined period, instead of immediately switching the NPN transistor Tr2 from OFF to ON and switching the Zener voltage to a low value. .

  When a signal indicating an overvoltage is input during the second predetermined period, the duty ratio is increased because the secondary side of the transformer 11 is in an overvoltage state while the load unit 90 is in a constant load state. I understand that. If a signal indicating an overvoltage is not input even after the second predetermined period has elapsed, it can be seen that the load unit 90 has changed from a constant load state to a light load state, and thereafter, from step S85 to step S15. As indicated by the transition, the NPN transistor Tr2 is turned on from off to switch the Zener voltage to a low value. According to the switching power supply 1 of the present embodiment, the overvoltage protection level of the switching power supply 1 can be protected by appropriately changing the overvoltage detection level.

  In the above, the flowcharts shown in FIGS. 6 and 7 are not intended to limit the present invention to the procedures shown in the above-described flow, and the addition / deletion of the procedures or the order of the procedures are within the scope not departing from the spirit and technical idea of the invention. Changes may be made.

  In addition to those already described above, the methods according to the above-described embodiments and modifications may be used in appropriate combination.

  In addition, although not illustrated one by one, the present invention is implemented with various modifications within a range not departing from the gist thereof.

DESCRIPTION OF SYMBOLS 1 Switching power supply 11 Transformer 13 Switching element 21 Switching control part 22 Voltage generation circuit (voltage generation circuit part)
23 Overvoltage protection circuit (overvoltage protection circuit)
90 Load section C3 Smoothing capacitor (smoothing circuit)
D6 Rectifier diode (rectifier circuit)
L1 Primary winding (primary coil)
L3 Auxiliary winding Tr2 NPN transistor (conduction part)
W4 wiring (output line)

Claims (6)

A load section;
A transformer that outputs a secondary side output voltage to the load section;
A switching element connected to a primary coil of the transformer;
A switching control unit that performs switching control in which the output voltage on the secondary side of the transformer becomes a target voltage using a duty ratio of a pulse input to the switching element;
A voltage generating circuit unit that rectifies and smoothes an AC voltage induced in an auxiliary winding provided on the primary side of the transformer, and outputs the DC voltage as a DC voltage;
When the DC voltage exceeds a predetermined voltage, an overvoltage protection circuit unit that inputs a signal indicating an overvoltage to the switching control unit;
With
The switching controller is
The predetermined voltage of the overvoltage protection circuit unit can be switched to the first voltage or the second voltage, and the switching control is stopped when a signal indicating the overvoltage is input,
A first displacement in which the duty ratio of the pulse is a displacement from a first predetermined range to a second predetermined range outside the first predetermined range when the predetermined voltage of the overvoltage protection circuit unit is the first voltage. When a signal indicating the overvoltage is not input during the first predetermined period after the first displacement, the predetermined voltage of the overvoltage protection circuit unit is set to the predetermined voltage after the first predetermined period. Switching from the first voltage to the second voltage;
A switching power supply characterized by that. The switching power supply according to claim 1,
When the duty ratio of the pulse is within the second predetermined range without inputting a signal indicating the overvoltage during the first predetermined period after the first displacement, the first predetermined period After the elapse, the predetermined voltage of the overvoltage protection circuit unit is switched from the first voltage to the second voltage,
If the duty ratio of the pulse is displaced from the second predetermined range to the first predetermined range after the first displacement and before reaching the first predetermined period, the predetermined voltage of the overvoltage protection circuit unit is set to Do not switch from the first voltage to the second voltage;
A switching power supply characterized by that. The switching power supply according to claim 1 or 2,
The switching controller is
When the predetermined voltage of the overvoltage protection circuit unit is the second voltage, the duty ratio of the pulse is a second displacement that is displaced from the second predetermined range to the first predetermined range. When a signal indicating the overvoltage is not input during the second predetermined period after the displacement, the predetermined voltage of the overvoltage protection circuit unit is changed from the second voltage to the first voltage after the second predetermined period has elapsed. Switch to voltage,
A switching power supply characterized by that. The switching power supply according to claim 3,
The switching controller is
When the duty ratio of the pulse is within the first predetermined range without inputting a signal indicating the overvoltage during the second predetermined period after the second displacement, the second predetermined period After the elapse, the predetermined voltage of the overvoltage protection circuit unit is switched from the first voltage to the second voltage,
If the duty ratio of the pulse is displaced from the first predetermined range to the second predetermined range after reaching the second predetermined period after the second displacement, the predetermined voltage of the overvoltage protection circuit unit is Do not switch from the first voltage to the second voltage;
A switching power supply characterized by that. The switching power supply according to claim 4,
The first predetermined range is a predetermined range of the duty ratio of the pulse when the load portion is light load, and the second predetermined range is a predetermined of the duty ratio of the pulse when the load portion is a constant load. Range,
The first voltage is a voltage at which the DC voltage reaches the output voltage on the secondary side of the transformer from a normal voltage to an overvoltage when the load unit is the light load,
The second voltage is a voltage at which the DC voltage reaches the secondary output voltage of the transformer from a normal voltage to an overvoltage when the load unit is the constant load.
A switching power supply characterized by that. The switching power supply according to claim 5,
The voltage generation circuit unit includes:
A rectifier circuit that is connected to one end of the auxiliary winding via an output line and outputs one voltage that is either positive or negative of the AC voltage with respect to the AC voltage output from one end of the auxiliary winding. When,
A smoothing circuit connected to the rectifier circuit, smoothing a positive or negative voltage output from the rectifier circuit, and outputting as the DC voltage;
The overvoltage protection circuit unit is
A plurality of zener diodes connected to the smoothing circuit and through which a current flows when the DC voltage output by the smoothing circuit exceeds a breakdown voltage;
Furthermore,
Among the plurality of Zener diodes, having a conduction portion for conducting the anode side and the cathode side of at least one Zener diode,
The switching control unit transmits a signal indicating whether or not to conduct the anode side and the cathode side to the conduction unit,
The overvoltage protection circuit unit is
When the at least one Zener diode is conducted by the conducting part, when the first voltage that is the breakdown voltage of the Zener diode excluding the at least one Zener diode among the plurality of Zener diodes is exceeded. , A signal indicating that the overvoltage is input to the switching control unit,
When the at least one Zener diode is not conducted by the conducting unit, a signal indicating the overvoltage is generated when the second voltage that is the breakdown voltage of the plurality of Zener diodes is exceeded. Input to the switching controller,
A switching power supply characterized by that.

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