JP4847932B2 - Switching power supply - Google Patents

Description

  The present invention relates to a switching power supply device including a current transformer in a current-voltage conversion circuit for controlling a switching current of the switching power supply device.

  In a switching power supply, a current-voltage conversion circuit composed of a current transformer is used as a method for detecting a switching current. For example, in the case of a current transformer in which the primary side winding is 1 turn and the secondary side winding is 150 turns, when switching current flows to the primary side winding, A method of converting a voltage signal into a voltage signal with a voltage drop generated in a current detection resistor provided in a path through which a current that is 1 / 150th of that current is generated from the winding and flows out of the current is common.

  This method is characterized by less power loss and less heat generation than the method of converting a voltage signal by directly inserting a current detection resistor in the switching current (large current) path. That is, in the case of a current-voltage conversion circuit composed of a current transformer, the primary side winding of the current transformer occupies most of the power loss as the primary side winding through which the switching current (large current) flows. For example, since a small number of turns such as one turn is sufficient, the resistance value can be suppressed relatively easily by a design device such as using a thick electric wire or a copper plate, and a low-loss conversion circuit as a whole has been realized.

  In particular, in recent years, there has been a growing need for miniaturization, thinning, and large currents in switching power supply devices, and current transformers used inside are required to be even smaller, thinner, and have lower loss. I came. However, even if the resistance value of the primary winding is kept low, there is a limit in structural constraints only by the above-described design ingenuity, so two small and thin current transformers are used, and the primary Some circuits use side windings connected in parallel.

  An example of a switching power supply device having such a current-voltage conversion circuit constituted by two current transformers will be described with reference to the circuit diagram of FIG. The switching power supply device 30 is a circuit configured in a general single forward system, and is connected in parallel with primary windings CT1a and CT2a of current transformers CT1 and CT2 between both ends of a DC power supply Ein that supplies an input voltage. A series circuit of a circuit, a switching element Q1 made of, for example, a MOS FET, and a primary winding T1a of the output transformer T1 is connected. An output rectifying / smoothing circuit 10 is connected to the secondary winding T1b of the output transformer T1, which is constituted by a rectifying diode D11, a flywheel diode D12, a choke coil L11, and a smoothing capacitor C11 as is well known. The voltage induced in the primary side winding T1b is rectified and smoothed. A load 12 is connected between both ends of the smoothing capacitor C11 to supply the output voltage Vo.

  Further, a signal corresponding to the fluctuation of the output voltage Vo is fed back to a pulse width control circuit (not shown), the switching element Q1 is driven by a drive pulse signal whose pulse conduction width is variably controlled, and the output voltage Vo is controlled to be stabilized. ing.

  The current transformers CT1 and CT2 are the same, and both the primary windings CT1a and CT2a are formed in one turn, and the secondary windings CT1b and CT2b are formed in a predetermined plurality of N turns. A rectifier circuit 14 including a rectifier diode D1 that rectifies and outputs a current that flows when the switching element Q1 is ON is connected between both ends of the secondary winding CT1b of the current transformer CT1. A current detection resistor RS1 is connected in parallel. Similarly, a rectifier circuit 16 including a rectifier diode D2 that rectifies and outputs a current that flows when the switching element Q1 is turned on is connected between both ends of the secondary winding CT2b of the current transformer CT2. A current detection resistor RS2 is connected in parallel to the output. The current detection resistors RS1 and RS2 constitute a series circuit, and both ends thereof are connected to the current control unit 18.

  The current control unit 18 compares a predetermined reference voltage Vr1, which will be described later, with a total value Vs of generated voltages of the current detection resistors RS1 and RS2, and generates a drive pulse according to the output of the comparator CP. A drive circuit 18a is provided, and the output of the drive circuit 18a is connected to the drive terminal of the switching element Q1.

  Next, an operation for detecting the switching current and controlling the switching element Q1 will be described. Since the switching element Q1 operates to repeatedly turn on and off intermittently, and current flows only during the on period, the switching current I has a pulse waveform that repeats a substantially trapezoidal shape. The switching current I is shunted to the primary windings CT1a and CT2a connected in parallel. However, since the current transformers CT1 and CT2 use the same components, the current of I / 2 flows equally to each other. .

  The current flowing into the primary winding CT1a is converted to I / (2 · N) by the turns ratio of the current transformer CT1, flows out of the secondary winding CT1b, and flows into the current detection resistor RS1 via the rectifier diode D1. Flows in. Similarly, the current flowing into the primary winding CT2a is converted to I / (2 · N) by the turn ratio of the current transformer CT2, flows out of the secondary winding CT2b, and is detected through the rectifier diode D2. It flows into the resistor RS2. Here, since the current detection resistors RS1 and RS2 have the same resistance value RS, the voltage drops of the current detection resistors RS1 and RS2 are both RS · I / (2 · N), and the current detection resistors RS1 and RS2 A voltage Vs = RS · I / N is generated at both ends of the series circuit. Thus, the switching current I is converted into a voltage signal Vs that is substantially proportional to the switching current I.

  The current control unit 18 is set not to operate when the switching current I is normal, but to operate when the switching current I increases more than expected and its peak value exceeds a certain value. For example, when the switching current I increases due to a failure of the load 12 or the like, the voltage signal Vs increased in proportion thereto is input to the comparator CP of the current control unit 18 and compared with the reference voltage Vr1. The reference voltage Vr1 is defined in consideration of the safety of the switching power supply device and the load, and the comparator CP determines that an abnormality has occurred when the peak value of the input voltage signal Vs exceeds the reference voltage Vr1. Then, the output is inverted, and the drive circuit 18a receiving the inverted signal starts an operation of narrowing the duty of the switch element Q1. As a result, the power supply capability of the switching power supply 30 is limited, and accidents such as smoke and fire of internal components of the load 12 and the switching power supply circuit itself can be prevented. Therefore, when the switching current I shows a normal value, the peak value of the voltage signal Vs does not exceed the reference voltage Vr1, and the current control unit 18 does not operate.

  Another example of the switching power supply device using a current-voltage conversion circuit constituted by two current transformers will be described with reference to the circuit diagram of FIG. Here, the same components as those of the switching power supply device 30 in FIG. The switching power supply device 40 is configured by connecting the outputs of the rectifier circuits 14 and 16 of the switching power supply device 30 in parallel and providing a current detection resistor RS3 instead of the series circuit of the current detection resistors RS1 and RS2.

  Next, the operation of detecting the switching current I and controlling the switching element Q1 in the switching power supply device 40 will be described. Since the switching element Q1 operates to repeatedly turn on and off intermittently, and current flows only during the on period, the switching current I has a pulse waveform that repeats a substantially trapezoidal shape. The switching current I is shunted to the primary windings CT1a and CT2a connected in parallel. However, since the current transformers CT1 and CT2 use the same components, the current of I / 2 flows equally to each other. .

  The current flowing into the primary winding CT1a is converted to I / (2 · N) by the turn ratio of the current transformer CT1, flows out of the secondary winding CT1b, and flows into the current detection resistor RS3 via the rectifier diode D1. Flows in. Similarly, the current flowing into the primary winding CT2a is converted to I / (2 · N) by the turn ratio of the current transformer CT2, flows out of the secondary winding CT2b, and is detected through the rectifier diode D2. It flows into the resistor RS3. That is, an I / N current flows through the current detection resistor RS3, and a voltage Vs = RS3 · I / N is generated. Thus, the switching current I is converted into a voltage signal Vs that is substantially proportional to the switching current I.

  In addition, the operation of the current control unit 18 of the switching power supply 40 is the same as that of the switching power supply 30 described above.

  In the switching power supply devices 30 and 40 described above, two small and thin current transformers CT1 and CT2 can be used. As for the resistance loss of the primary side windings CT1a and CT2a that occupy most of the loss of the current transformer, only one half of the switching current is required to flow through one of the primary side windings connected in parallel. Loss per piece is reduced. At the same time, a small component can be used due to the small current, and if two current transformers are mounted side by side, the low-profile mounting of the component is realized as a result. On the other hand, when trying to realize a single current transformer with the same component height and the same resistance loss, the magnetic core and insulating bobbin, which are constituent materials, inevitably have a flat floor area and a flat shape. Considering the problem of yield in the manufacturing process such as pressing, firing and molding by the mold, it is difficult to mass-produce.

Further, as disclosed in Patent Document 1, primary windings of a plurality of current transformers are respectively inserted in series into a plurality of output transformer windings, and current detection resistors connected to secondary winding outputs A method for detecting a power supply abnormality by detecting a difference between generated voltage signals has been proposed.
JP 2002-51544 A

  However, in the conventional switching power supply devices 30 and 40, when the windings of the current transformers CT1 and CT2 themselves are broken, the current transformer CT1 or CT2 is soldered and mounted on the printed circuit board. In some cases, a problem may arise in the safety of the switching power supply device when a fault occurs in the assembly of the switching power supply device or when a mounting failure occurs in which no terminal is connected to the current transformer CT1 or CT2.

  For example, when one end of the primary winding CT1a is disconnected in the switching power supply 30, the I / 2 of the switching current I that should flow through the primary winding CT1a is also added to the primary winding CT2a. Flows in. Therefore, twice the normal current flows through the primary winding CT2a, and its resistance loss is four times the expected.

  At this time, since no current flows through the disconnected primary winding CT1a, no current flows out from the secondary winding CT1b, and a voltage drop occurs in the current detection resistor RS1 connected via the rectifier diode D1. do not do. On the other hand, all the switching current I flows into the primary winding CT2a, is converted to I / N by the turn ratio of the current transformer CT2, flows out of the secondary winding CT2b, and flows to the current detection resistor RS2 via the rectifier diode D2. . Here, since the current detection resistors RS1 and RS2 have the same resistance value RS, the voltage Vs = RS · the sum of the respective voltage drops is added to both ends of the series circuit of the current detection resistors RS1 and RS2. I / N occurs. That is, even when one end of the primary winding CT1a breaks down, the same voltage signal Vs is generated as when there is no failure, so that the current control unit 18 is determined to be operating normally, and the switch The operation for reducing the duty of the element Q1 is not performed.

  Accordingly, the current transformer CT2a generates heat abnormally due to a resistance loss four times larger than expected, and does not perform the operation of limiting the power supply capability by narrowing the duty of the switching element Q1, so that the heat generation state continues, As a result, there was a risk of accidents such as smoke and fire due to the heat generated by the current transformer CT2.

  Even when the connection of one end of the secondary side winding CT1b is broken, the switching current I is greatly biased to the primary side winding CT2a, and the primary side winding CT2a is 2 to 3 times larger than expected. Since the heat is abnormally generated due to a certain resistance loss, and the operation of limiting the power supply capacity by narrowing the duty of the switching element Q1 is not performed, the heat generation state continues. As a result, the current transformer CT2 generates heat and generates smoke and ignition. There was a risk of accidents.

  Further, even when a disconnection occurs in the current transformer CT2 that is in a symmetrical relationship with the current transformer CT1, the operation is the same as that when the current transformer CT1 is disconnected. As a result, the current transformer CT1 generates heat and generates smoke and ignition. There was a risk of accidents. Further, in the switching power supply device 40 as well as the switching power supply device 30, there is a risk of causing an accident such as smoke or fire due to heat generation of the current transformer that is not disconnected.

  In the circuit disclosed in Patent Document 1, the switching current is divided into a plurality of output transformer windings, and each transformer winding current is converted into an independent voltage signal through the plurality of current transformers. Since only a signal having a high level is input to the overcurrent detection circuit, the entire switching current is not accurately detected. Further, a technique in which the unbalance of the independent voltage signals generated when an output circuit abnormality occurs in one of the DC-DC converters is converted into a single voltage signal by a complicated circuit means called a difference detection circuit. It is difficult to adopt the complicated circuit means especially in a small-sized and low-cost switching power supply device only by being disclosed.

  The present invention has been made in view of the above-described background art, and provides a switching power supply apparatus that can reduce the size and thickness of the apparatus with a simple configuration and can prevent the occurrence of abnormal heat generation or the like. For the purpose.

  In the present invention, a series circuit of a switching element connected to an input power source and a primary side winding of an output transformer, and a voltage signal corresponding to a current flowing through the series circuit are input, and a peak value of the voltage signal is a specified value. Exceeding the current control unit that reduces the duty of the switching element, the same two current transformers in which the primary side windings are connected in parallel and the primary side winding is inserted in series with the switching element, Two rectifier circuits connected in parallel to each of the secondary windings of the two current transformers and rectifying the current during the on-period of the switching element, and connected in parallel to each of the outputs of the two rectifier circuits, and their resistances A switching power supply device having two current detection resistors having the same value, wherein each rectifier circuit connected to the secondary side of the two current transformers is connected in series The two current detection resistors are also connected in series, and both ends of the series circuit of the current detection resistors are connected to the input of the current control unit, and the midpoints of the outputs of the two rectifier circuits connected in series And a switching power supply device provided with an abnormal current detection resistor having both ends connected to the middle point of the two current detection resistors.

  An abnormal-time control unit that monitors the voltage generated in the abnormal current detection resistor and performs control to narrow the duty of the switching element when the voltage exceeds a specified value is provided.

  An abnormality control unit is provided that monitors the voltage generated in the abnormal current detection resistor and latches the switching operation of the switching element when the voltage exceeds a specified value.

  Further, commutation diodes having a direction to block the rectified current are connected in parallel to the outputs of the two rectifier circuits, and the abnormal voltage detection resistor includes the forward impedance of the commutation diode and the current detection resistor. This is a switching power supply device set to a value larger than the total impedance value.

  According to the switching power supply device of the present invention, it is possible to detect a disconnection failure occurring in any of the windings of two current transformers having primary side windings connected in parallel with a simple circuit. It is possible to easily find and exclude defective products even at the shipping inspection stage of manufacturing plants. Further, even when a disconnection failure occurs during operation after shipment, it is possible to prevent accidents such as smoke and fire due to heat generation of the current transformer by the power limiting operation. In addition, since two thin current transformers are used, a current-voltage conversion circuit that is small, thin, low loss, and high in safety can be obtained, which contributes to miniaturization and thinning of the power supply device. To do. In addition, the switching currents shunted to the primary windings of the two current transformers are converted into voltage signals via the current transformers, and then summed in the series circuit of the voltage detection resistors. The obtained voltage signal can be obtained with high accuracy.

  Hereinafter, a first embodiment of the present invention switching power supply will be described with reference to FIG. The switching power supply device 50 is a circuit configured in a general single forward system, and primary windings CT1a and CT2a of current transformers CT1 and CT2 are connected in parallel between both ends of a DC power supply Ein that supplies an input voltage. A series circuit of a circuit, a switching element Q1 made of, for example, a MOS FET, and a primary winding T1a of the output transformer T1 is connected. An output rectifying / smoothing circuit 10 is connected to the secondary winding T1b of the output transformer T1, which is constituted by a rectifying diode D11, a flywheel diode D12, a choke coil L11, and a smoothing capacitor C11 as is well known. The voltage induced in the secondary winding T1b is rectified and smoothed. A load 12 is connected between both ends of the smoothing capacitor C11 to supply the output voltage Vo.

  Further, a signal corresponding to the fluctuation of the output voltage Vo is fed back to a pulse width control circuit (not shown), the switching element Q1 is driven by a drive pulse signal whose pulse conduction width is variably controlled, and the output voltage Vo is controlled to be stabilized. ing.

  The current transformers CT1 and CT2 are the same, and both the primary winding is formed in one turn and the secondary winding is formed in a predetermined plurality of N turns. A rectifier circuit 14 that rectifies and outputs a current that flows when the switching element Q1 is ON is connected between both ends of the secondary winding CT1b of the current transformer CT1, and a current detection resistor RS1 is connected to the output of the rectifier circuit 14. Connected in parallel. Similarly, a rectifier circuit 16 that rectifies and outputs a current that flows when the switching element Q1 is turned on is connected between both ends of the secondary winding CT2b of the current transformer CT2. The detection resistor RS2 is connected in parallel. The current detection resistors RS1 and RS2 constitute a series circuit, and both ends thereof are connected to the current control unit 18. The current control unit 18 compares a predetermined reference voltage Vr1, which will be described later, with a total value Vs of generated voltages of the current detection resistors RS1 and RS2, and generates a drive pulse according to the output of the comparator CP. A drive circuit 18a is provided, and the output of the drive circuit 18a is connected to the drive terminal of the switching element Q1.

  The above configuration is substantially the same as that of the conventional switching power supply device 30, but in this embodiment, the midpoint of the series circuit of the rectifier circuits 14 and 16 and the midpoint of the series circuit of the current detection resistors RS1 and RS2 The short circuit connection is disconnected, and the abnormality detection resistor Rx is inserted there. Further, both ends of the abnormality detection resistor Rx are input to an abnormality control unit 20 described later, and a control signal output by the abnormality control unit 20 is input to an input 18b of the current control unit 18.

  Next, an operation for detecting the switching current I and controlling the switching element Q1 will be described. Since the switching element Q1 operates to repeatedly turn on and off intermittently, and current flows only during the on period, the switching current I has a pulse waveform that repeats a substantially trapezoidal shape. This switching current I is shunted to the primary windings CT1a and CT2a of the current transformers CT1 and CT2 connected in parallel. However, since the current transformers CT1 and CT2 use the same components, the current of I / 2 is respectively Evenly.

  The current that flows into the primary winding CT1a is converted into I / (2 · N) by the turn ratio of the current transformer CT1, flows out of the secondary winding CT1b, and passes through the rectifier diode D1 and the current detection resistor RS1. It flows into the abnormality detection resistor Rx. Similarly, the current flowing into the secondary winding CT2a is converted to I / (2 · N) by the turn ratio of the current transformer CT2, flows out of the secondary winding CT2b, and is detected through the rectifier diode D2. It flows into the resistor RS2 and the abnormality detection resistor Rx. Here, since the current detection resistors RS1 and RS2 have the same resistance value RS, the voltage drops of the current detection resistors RS1 and RS2 are both RS · I / (2 · N), and the current detection resistors RS1 and RS2 A voltage Vs = RS · I / N is generated at both ends of the series circuit. As a result, the switching current I is converted into a voltage signal Vs that is substantially proportional thereto.

  On the other hand, the currents flowing out from the secondary windings CT1b and CT2b and flowing through the abnormality detection resistor Rx are equal in value I / (2 · N) and the direction thereof is reversed, resulting in a result. And no voltage drop occurs across the abnormality detection resistor Rx. That is, substantially the same operation as that of the conventional switching power supply device 30 in which the abnormality detection resistor Rx is removed by a short circuit is performed. Further, since a voltage drop does not occur across the abnormality detection resistor Rx, the abnormality control circuit 20 described later does not operate.

  The current control unit 18 is set not to operate when the switching current I is normal, but to operate when the switching current I increases more than expected and its peak value exceeds a certain value. For example, when the switching current I increases due to a failure of the load 12 or the like, the voltage signal Vs increased in proportion thereto is input to the comparator CP of the current control unit 18 and compared with the reference voltage Vr1. The reference voltage Vr1 is defined in consideration of the safety of the switching power supply device and the load, and the comparator CP determines that an abnormality has occurred when the peak value of the input voltage signal Vs exceeds the reference voltage Vr1. Then, the output is inverted, and the drive circuit 18a receiving the inverted signal starts an operation of narrowing the duty of the switch element Q1. Therefore, when the switching current I shows a normal value, the peak value of the voltage signal Vs does not exceed the reference voltage Vr1, and the current control unit 18 does not operate.

  Next, an operation when one end of the primary winding CT1a of the current transformer CT1 is disconnected will be described. When the primary winding CT1a is disconnected, I / 2 of the switching current I that should flow through the primary winding CT1a also flows into the current transformer CT2a. Therefore, twice the normal current flows through the primary winding CT2a, and its resistance loss is four times the expected.

  At this time, since no current flows through the disconnected primary winding CT1a, no current flows out from the primary winding CT1b, and no voltage drop occurs in the current detection resistor RS1 connected via the rectifier diode D1. Does not occur. On the other hand, all of the switching current I flows into the primary winding CT2a, is converted into I / N by the turn ratio of the current transformer CT2, flows out of the secondary winding CT2b, and flows into the current detection resistor RS2 and the abnormality via the rectifier diode D2. It flows to the detection resistor Rx. Here, since the current detection resistors RS1 and RS2 have the same resistance value RS, the voltage Vs = RS · the sum of the respective voltage drops is added to both ends of the series circuit of the current detection resistors RS1 and RS2. The same voltage signal as when I / N primary winding CT1a is normal is generated. However, since only the current I / N flowing out from the secondary winding CT2b flows through the abnormality detection resistor Rx, and no current that cancels the current I / N is supplied, a constant voltage drop occurs.

  Similarly, when one end of the secondary winding CT1b is disconnected, a constant voltage drop is generated in the abnormality detection resistor Rx in the same direction as when the primary winding CT1a is disconnected. In addition, when one end of the primary side winding CT2a or the secondary side winding CT2b is disconnected, a constant voltage drop occurs in the opposite direction to the above in the abnormality detection resistor Rx. Therefore, it is possible to detect that one end of the current transformer CT1 or the current transformer CT2 is broken by monitoring the voltage generated at both ends of the abnormality detection resistor Rx.

  Next, the abnormality control unit 20 to which the voltage across the abnormality detection resistor Rx is input will be described with reference to FIG. The abnormality control unit 20 includes an abnormal current detection circuit 22 and a bias circuit 24. The abnormality detection circuit 22 includes a comparator CP1 that compares the voltage across the abnormality detection resistor Rx with the positive reference voltage Vr2, a comparator CP2 that compares the voltage across the abnormality detection resistor Rx with the negative reference voltage Vr3, An OR circuit 26 to which the outputs of the comparators CP1 and CP2 are input is provided. The bias circuit 24 includes a switch SW that is turned on / off according to an output signal of the OR circuit 26, an internal power supply E1, and a bias resistor R.

  Next, the operation of the abnormality control unit 20 will be described. When either of the windings of the current transformer CT1 or the current transformer CT2 is disconnected, a voltage is generated in the positive direction or the negative direction at both ends of the abnormality detection resistor Rx, and the generated voltage is compared with the reference voltages Vr2 and Vr3. . The reference voltages Vr2 and Vr3 are higher than the voltage generated at both ends of the abnormality detection resistor Rx due to variations in characteristics of the two current transformers CT1 and CT2, the influence of external noise, and the like, and the reference voltages Vr2 and Vr3 It is set to a low value that can reliably detect disconnection. Unless there is a special reason, it is appropriate that the reference voltages Vr2 and Vr3 have the same value. When the voltage across the abnormality detection resistor Rx exceeds the reference voltage Vr2 or Vr3, one output of the comparator CP1 or CP2 is inverted, and at the same time, the output of the OR circuit 26 is inverted. When the OR circuit 26 is inverted, the switch SW is turned on, and a bias current can be supplied from the internal power supply E1 via the bias resistor R10.

  The output of the abnormality control unit 20 is connected to the input unit 18a of the current control unit 18, and the bias current flows through the series circuit including the current detection resistors RS1 and RS2 to forcibly increase the voltage signal Vs, and the reference voltage A voltage exceeding Vr1 is generated. As a result, the comparator CP determines that an abnormality has occurred and its output is inverted. Upon receiving the inverted signal, the drive circuit 18a starts an operation for narrowing the duty of the switch element Q1. As a result, the power supply capability of the switching power supply device is limited, the current flowing through the current transformers CT1 and CT2 is limited, and accidents such as smoke and fire can be prevented.

  According to the switching power supply device 50 of this embodiment, by including the abnormality control unit 20, it is possible to accurately detect the abnormality of the two current transformers CT1 and CT2, and easily at the shipping inspection stage of the power supply device. It is possible to find and exclude defective products that are disconnected. Furthermore, even when a disconnection failure occurs during operation after shipment, accidents such as smoke and fire due to heat generation of the current transformers CT1 and CT2 can be reliably prevented by the power limiting operation. In addition, since two thin current transformers are used, a small and thin current-voltage conversion circuit can be obtained, which contributes to miniaturization and thinning of the power supply device.

  Next, a second embodiment of the switching power supply device of the present invention will be described with reference to FIG. Here, the same components as those of the switching power supply device 50 of the first embodiment are denoted by the same reference numerals, and description thereof is omitted. In the present embodiment, the rectifier circuits 14 and 16 of the switching power supply device 50 are configured by rectifier diodes D1 and D2, and commutation diodes D3 and D4 are provided at the outputs of the rectifier circuits 14 and 16 instead of the abnormal time control unit 20. The rectifier diodes D1 and D2 are connected in parallel so as to block the current.

  The operation of detecting the switching current and controlling the switching element Q1 is the same as that of the first embodiment when the current transformers CT1 and CT2 are normal without failure, and the voltage drop of the current detection resistors RS1 and RS2 Are both RS · I / (2 · N), and a voltage Vs = RS · I / N, in which the respective voltage drops are added, is generated at both ends of the series circuit of the current detection resistors RS1 and RS2. At this time, since the switching current I is a normal value, the peak value of the voltage signal Vs does not exceed the reference voltage Vr1, and the current control unit 18 does not operate. Further, a reverse voltage is applied to the commutation diodes D3 and D4, so that no current flows and the commutation diodes D3 and D4 do not operate.

  Next, the operation when one end of the primary winding CT1a is disconnected will be described. When the primary winding CT1a is disconnected, I / 2 of the switching current I that should flow through the primary winding CT1a also flows into the primary winding CT2a. Therefore, twice the normal current flows through the primary winding CT2a, and its resistance loss is four times the expected.

  At this time, since no current flows through the disconnected primary winding CT1a, no current flows out from the secondary winding CT1b. On the other hand, all of the switching current I flows into the primary winding CT2a, is converted into I / N by the turn ratio of the current transformer CT2, flows out of the secondary winding CT2b, and is rectified by the rectifier diode D2. The current I / N rectified by the rectifier diode D2 flows to the current detection resistor RS2 via a parallel circuit including a series circuit of the commutation diode D3 and the current detection resistor RS1 and the abnormality detection resistor Rx. Here, when the forward impedance of the commutation diode D3 is Z3, the resistance value of the abnormality detection resistor Rx is set so as to have a relationship of Rx> Z3 + RS1, and the current I / N flowing out from the rectifier diode D2 is Only a small amount flows through the abnormality detection resistor Rx, most of which flows through the commutation diode D3 and the current detection resistor RS1, and then flows into the current detection resistor RS2, causing a voltage drop across the current detection resistor RS1. For example, if the abnormality detection resistor Rx is set so as to have a relationship of Rx >> Z3 + RS1, most of the current I / N flowing out from the rectifier diode D2 is passed through the commutation diode D3 and the current detection resistor RS1. The voltage Vs = 2 · RS · I is obtained by adding the respective voltage drops to both ends of the series circuit of the current detection resistors RS1 and RS2 having the same resistance value RS. / N is generated. This corresponds to a voltage twice as large as the generated voltage Vs = RS · I / N when the primary winding CT1a is normal without being disconnected.

  The reference voltage Vr1 is set to have a relationship of RS · I / N <Vr1 <2 · RS · I / N. At this time, a voltage exceeding the reference voltage Vr1 is generated in the voltage signal Vs. Therefore, the comparator CP1 determines that an abnormality has occurred and its output is inverted, and the drive circuit 18a receiving the inverted signal starts an operation for narrowing the duty of the switch element Q1. As a result, the power supply capability of the switching power supply device is limited, the current flowing through the current transformer is reduced, and accidents such as smoke and fire due to heat generation of the current transformer can be prevented.

  Note that if Rx is selected to be a large value that has a relationship of Rx >> Z3 + RS1, the detection sensitivity of the abnormality is increased, but malfunction may easily occur due to variations in the component characteristics of the current transformers CT1, CT2, external noise, and the like. As for the abnormality detection resistor Rx, preferable operating characteristics can be obtained when Rx = (5 to 20) · (Z4 + RS2).

  Even when one end of the secondary winding CT1b is disconnected, accidents such as smoke and ignition can be prevented by the same operation as when the primary winding CT1a is disconnected.

  Next, the operation when one end of the primary winding CT2a is disconnected will be described. When the primary winding CT2a is disconnected, I / 2 of the switching current I that should flow through the primary winding CT2a also flows into the primary winding CT1a. Therefore, twice the normal current flows through the primary winding CT1a, and the resistance loss is four times as expected.

  At this time, since no current flows through the disconnected primary winding CT2a, no current flows out from the secondary winding CT2b. On the other hand, all of the switching current I flows into the primary winding CT1a, is converted into I / N by the turn ratio of the current transformer CT1, flows out of the primary winding CT1b, and is rectified by the rectifier diode D1. The current I / N rectified by the rectifier diode D1 flows to the current detection resistor RS1 via a parallel circuit including a series circuit of the commutation diode D4 and the current detection resistor RS2 and the abnormality detection resistor Rx. Here, when the forward impedance of the commutation diode D4 is Z4, Rx is set to have a relationship of Rx> Z4 + RS2, and the current I / N flowing out from the rectifier diode D1 is supplied to the abnormality detection resistor Rx. Little flows, most of which flows through the commutation diode D4 and the current detection resistor RS2 to the current detection resistor RS1, and a voltage drop occurs across the current detection resistor RS2. For example, if Rx is set so as to have a relationship of Rx >> Z4 + RS2, most of the current I / N flowing out from the rectifier diode D2 is passed through the commutation diode D4 and the current detection resistor RS2, and the current detection resistor RS1. At both ends of the series circuit of the current detection resistors RS1 and RS2 having the same resistance value RS, the voltage Vs = 2 · RS · I / N obtained by adding up the respective voltage drops is appear. This corresponds to a voltage twice as high as the generated voltage Vs = RS · I / N when the secondary winding CT2a is normal without being disconnected.

  Here, the reference voltage Vr1 is set to have a relationship of RS · I / N <Vr1 <2 · RS · I / N. At this time, a voltage exceeding the reference voltage Vr1 is generated in the voltage signal Vs. Therefore, the comparator CP1 determines that an abnormality has occurred and its output is inverted, and the drive circuit 18a receiving the inverted signal starts an operation to narrow the duty of the switch element Q1. As a result, the power supply capability of the switching power supply device is limited, the current flowing through the current transformer is reduced, and accidents such as smoke and fire due to heat generation of the current transformer can be prevented.

  Note that if Rx is selected to be a large value having a relationship of Rx >> Z4 + RS2, abnormality detection sensitivity increases, but malfunction may easily occur due to variations in component characteristics of the current transformers CT1 and CT2 and external noise. When the abnormality detection resistance Rx is set to about Rx = (5 to 20) · (Z4 + RS2), preferable circuit characteristics can be obtained.

  Even when one end of the secondary winding CT2b is disconnected, accidents such as smoke and fire can be prevented by the same operation as when the primary winding CT2a is disconnected.

  In addition, this invention is not limited to the said embodiment, The rectifier circuits 14 and 16 should just be comprised, for example with the diode and the other semiconductor element. Further, the abnormal current detection circuit 22 only needs to output a logic-inverted signal when the voltage generated by the abnormality detection resistor Rx exceeds a predetermined specified value, and is not limited to the circuit configuration of the present embodiment. Absent. The bias circuit 24 may be any circuit that has a function of supplying a bias current to the input unit 18a and generating a voltage exceeding the reference voltage Vr when a logic signal indicating abnormality is input. It is not limited to. Further, if the abnormal time control unit 20 controls the switching element Q1 to limit the power supply capability of the switching power supply device, for example, the control unit 20 latches the switching operation itself of the switching element Q1 or outputs the output voltage. A signal may be input to the pulse width modulation circuit to be controlled to reduce the duty, and the circuit configuration and function shown in this embodiment are not limited.

  Further, the power conversion method of the switching power supply device is not limited to the single forward method, and any known flyback method, half-bridge method, push-pull method, or chopper method may be used as long as the power conversion is performed by switching. Further, if the circuit operation intended by the present invention is to be performed reliably, for example, a surge absorbing capacitor is connected to the abnormality detection resistor Rx and the current detection resistors Rs1 and Rs2, or the secondary side winding. An auxiliary capacitor or resistor for exciting reset operation of the current transformer may be connected to the lines CT1b and CT2b, or a corresponding malfunction prevention part may be added.

1 is a schematic circuit diagram showing a first embodiment of a switching power supply device of the present invention. It is a circuit block diagram which shows an example of the abnormal time control part with which this embodiment is provided. It is a schematic circuit diagram which shows 2nd embodiment of the switching power supply device of this invention. It is a schematic circuit diagram which shows a general switching power supply device. It is a schematic circuit diagram which shows another general switching power supply device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Output rectification smoothing circuit 12 Load 14,16 Rectification circuit 18 Current control part 18a Drive circuit 20 Abnormal time control part 22 Abnormal current detection circuit 24 Bias circuit 30,40,50,60 Switching power supply Ein DC power supply T1 Output transformer Q1 Switching Element D1, D2, D11 Rectifier diode D12 Flywheel diode L11 Choke coil C11 Smoothing capacitor CT1, CT2 Current transformer CT1a, CT2a Primary winding CT1b, CT2b Secondary winding RS1, RS2, RS3 Current detection resistor Rx Abnormal current Detection resistors CP, CP1, CP2 Comparator Vr1, Vr2 Reference voltage D3, D4 Commutation diode

Claims (4)

When a voltage signal corresponding to the current flowing through the series circuit of the switching element connected to the input power source and the primary winding of the output transformer and the current flowing through the series circuit is input, and the peak value of the voltage signal exceeds a specified value, A current control unit for reducing the duty of the switching element, two identical current transformers in which primary windings are connected in parallel and the primary winding is inserted in series with the switching element, and the two current transformers Two rectifier circuits connected in parallel to each of the secondary windings of the switching element and rectifying the current during the ON period of the switching element, and connected in parallel to each of the outputs of the two rectifier circuits, and having the same resistance value In a switching power supply device having two current detection resistors,
Each rectifier circuit connected to the secondary side of the two current transformers is connected in series, and the two current detection resistors are also connected in series, and both ends of the series circuit of the current detection resistors are connected to the current control circuit. An abnormal current detection resistor having both ends connected to the midpoint of the two rectifier circuit outputs connected in series and connected in series to the midpoint of the two current detection resistors is provided. Switching power supply.   2. The abnormality control unit according to claim 1, further comprising: an abnormality control unit that monitors a voltage generated in the abnormal current detection resistor and performs control to reduce a duty of the switching element when the voltage exceeds a specified value. Switching power supply.   2. The abnormality control unit according to claim 1, further comprising: an abnormality control unit that monitors a voltage generated in the abnormal current detection resistor and latches the switching operation of the switching element when the voltage exceeds a specified value. Switching power supply. The outputs of the two rectifier circuits are each connected in parallel with a commutation diode in a direction to block the rectified current, and the abnormal voltage detection resistor has a forward impedance of the commutation diode and a current detection resistance impedance. 2. The switching power supply device according to claim 1, wherein the switching power supply device is set to a value larger than the total value.

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