Isolated power conversion circuit and power supply unit

Abstract

In a configuration that suppresses the voltage applied to the switching element, the voltage withstand capability required for the components constituting the surge protection circuit is reduced, and the circuit can be realized with a simpler configuration. [Solution] The flyback converter 10 comprises an isolation transformer 50 having a first primary winding L11, a second primary winding L12, and a secondary winding L2, a capacitor C1 connected to the input terminal, a switching element TR1 disposed between the capacitor C1 and the first primary winding L11, a switching element TR2 disposed between the capacitor C1 and the second primary winding, a surge protection circuit, and a rectifier and smoothing circuit. The surge protection circuit comprises a clamp circuit having diodes D1 to D4. Diodes D1 and D2 are arranged in series between one end of the capacitor C1 and the other end of the first primary winding L11, and diodes D3 and D4 are arranged in series between the other end of the capacitor C1 and one end of the second primary winding L12.

Description

【Technical Field】 【0001】 The present invention relates to an isolated power conversion circuit that converts a predetermined input DC voltage into a desired DC voltage and outputs it, and a power supply device including the isolated power conversion circuit. 【Background Art】 【0002】 Conventionally, there are power supply devices equipped with isolated power conversion circuits using isolation transformers that adopt various methods. For example, there are flyback converters that adopt the flyback method, forward converters that adopt the forward method, and so on. 【0003】 Here, an isolated power conversion circuit using a basic isolation transformer will be described taking the flyback converter as an example. 【0004】 As shown in FIG. 5, the flyback converter 100 includes an isolation transformer 50, and the isolation transformer 50 insulates the input side and the output side of the flyback converter 100. The isolation transformer 50 has a primary winding L1 on the input side and a secondary winding L2 on the output side. As shown by dots in FIG. 5, the polarity of the secondary winding L2 is opposite to that of the primary winding L1. 【0005】 One terminal (one end) on the side with dots of the primary winding L1 of the isolation transformer 50 is connected to the plus terminal of the input terminal and one end of the capacitor C1 in FIG. 5, and the other terminal (the other end) on the side without dots in FIG. 5 is connected to the drain terminal of the switching element TR1. Also, the source terminal of the switching element TR1 is connected to the minus terminal of the input terminal (primary ground) and the other end of the capacitor C1. Further, the switching element TR1 is controlled to turn on and off by connecting its gate terminal to the switching control unit. 【0006】 In Figure 5, the terminal without a dot (the other end) of the isolation transformer 50's secondary winding L2 is connected to the anode of diode D5, and the terminal with a dot (the other end) is connected to the negative terminal (secondary ground) of the output terminal and the other end of capacitor C4. The cathode of diode D5 is connected to the positive terminal of the output terminal and one end of capacitor C4. Diode D5 and capacitor C4 form a rectifier and smoothing circuit, which rectifies and smooths the desired voltage, supplying it as the output voltage Vout to the load connected to the output terminal. 【0007】 In the flyback converter 100, when the switching element TR1 is turned off, a surge voltage is generated on the input side due to the leakage inductance of the isolation transformer 50, and this surge voltage is applied between the drain and source of the switching element TR1, which leads to a problem where the voltage withstand design of the switching element TR1 becomes stringent. 【0008】 Furthermore, as shown in Figure 6, there is a flyback converter 110 (two-switch flyback converter) that uses two switching elements TR1 and TR2. In the flyback converter 110, the drain terminal of switching element TR2 is connected to the positive terminal of the input terminal and one end of capacitor C1, the source terminal of switching element TR2 is connected to one end of the primary winding L1, the other end of the primary winding L1 is connected to the drain terminal of the first switching element TR1, and the source terminal of switching element TR1 is connected to the negative terminal of the input terminal and the other end of capacitor C1. 【0009】 Furthermore, in order to address the voltage withstand capability issues of the switching elements, there is a flyback converter 120 that adds a clamp circuit equipped with two diodes D2 and D3 as a surge protection circuit to the flyback converter 110 shown in Figure 6, as shown in Figure 7. 【0010】 Diode D2 has its anode connected to the other end of the primary winding L1 and the drain terminal of the switching element TR1, and its cathode connected to the positive terminal of the input terminal, the drain terminal of the switching element TR2 and one end of the capacitor C1. Diode D3 has its anode connected to the negative terminal of the input terminal, the source terminal of the switching element TR1 and the other end of the capacitor C1, and its cathode connected to one end of the primary winding L1 and the source terminal of the switching element TR2. 【0011】 In the flyback converter 120, when switching elements TR1 and TR2 are turned off, if a surge voltage is generated on the input side due to the leakage inductance of the isolation transformer 50, diodes D2 and D3 conduct, thereby recirculating the energy corresponding to the surge voltage (hereinafter referred to as surge energy) to capacitor C1. Furthermore, when diode D2 conducts, the drain-source voltage Vds1 of switching element TR1 is clamped by the voltage Vc1 (≈input voltage Vin) of capacitor C1, and when diode D3 conducts, the drain-source voltage Vds2 of switching element TR2 is clamped by the voltage Vc1 (≈input voltage Vin) of capacitor C1, thus suppressing the effects of the surge voltage. 【0012】 Furthermore, as a further countermeasure, there is a flyback converter 130 that, for example as shown in Figure 8, adds a snubber circuit consisting of a resistor R4, a capacitor C5, and a diode D7 as a surge protection circuit in addition to the clamp circuit. 【0013】 The snubber circuit is arranged in parallel with the primary winding L1. One end of the primary winding L1 is connected to the cathode of the diode D7, and the other ends of the resistor R4 and capacitor C5 are connected to the cathode of the diode D7, while the anode of the diode D7 is connected to the other end of the primary winding L1. 【0014】 In the flyback converter 130, when a surge voltage occurs on the input side due to the leakage inductance of the isolation transformer 50, the clamp circuit recirculates the surge energy to capacitor C1, clamping the drain-source voltages Vds1 and Vds2 of the switching elements TR1 and TR2 with the voltage Vc1 of capacitor C1. At the same time, the surge energy is temporarily stored in capacitor C5 of the snubber circuit, and the energy stored in capacitor C5 is discharged through resistor R4, thereby further suppressing the effects of the surge voltage. [Prior art documents] [Patent Documents] 【0015】 [Patent Document 1] Japanese Patent Publication No. 2016-149920 [Overview of the Initiative] [Problems that the invention aims to solve] 【0016】 However, in the case of countermeasures using surge protection circuits, even when switching elements TR1 and TR2 are turned on, a high voltage equivalent to the input voltage Vin is applied to diodes D2 and D3 (clamp circuit), resistor R4, capacitor C5, and diode D7 (snubber circuit). Therefore, during the steady operation of the flyback converter, regardless of the presence or absence of surge voltage, a high voltage at least equivalent to the input voltage Vin is constantly applied to each component of the surge protection circuit. This increases the electrical stress on the components of the surge protection circuit and may lead to a decrease in the reliability of the components. 【0017】 Therefore, it is necessary to select components with high voltage resistance for use in surge protection circuits, which currently leads to constraints on component selection and increased costs. 【0018】 Furthermore, in surge protection circuits that include both clamping and snubber circuits, the number of components naturally increases and the circuit configuration becomes more complex; therefore, it is desirable to implement them with the simplest possible configuration. 【0019】 As described above by taking the flyback converter as an example, this is not limited to the flyback converter, and the same applies to other power supply devices such as forward converters that have basically the same input-side configuration. 【0020】 An object of the present invention is to provide an isolated power conversion circuit that can reduce the withstand voltage required for components constituting a surge countermeasure circuit in a configuration that suppresses the voltage applied to a switching element, and can further realize a circuit with a simple configuration, and a power supply device including the isolated power conversion circuit. 【Means for Solving the Problem】 【0021】 (Isolated Power Conversion Circuit) The present invention is an isolated power conversion circuit that converts a predetermined input DC voltage into a desired DC voltage and outputs it, having an isolation transformer having a first primary winding and a second secondary winding arranged in series on the input side and having a secondary winding on the output side, an input smoothing capacitor connected to an input portion of the input DC voltage, a first switching element arranged between the other end of the input smoothing capacitor and the other end of the first primary winding on the side not connected to the second primary winding, a second switching element arranged between one end of the input smoothing capacitor and one end of the second primary winding on the side not connected to the first primary winding, a surge countermeasure circuit arranged on the input side, a rectifying and smoothing circuit connected to the secondary winding of the isolation transformer, and comprising, The surge countermeasure circuit comprises a clamp circuit having a first diode, a second diode, a third diode and a fourth diode, The first diode and the second diode are arranged in series between one end of the input smoothing capacitor and the other end of the first primary winding, The third diode and the fourth diode are arranged in series between the other end of the input smoothing capacitor and one end of the second primary winding, When the first diode and the second diode conduct, the voltage of the first switching element is clamped by the voltage of the input smoothing capacitor, and when the third diode and the fourth diode conduct, the voltage of the second switching element is clamped by the voltage of the input smoothing capacitor. 【0022】 (Surge protection circuit with a snubber circuit) The surge protection circuit further includes a snubber circuit having a capacitor and a resistor. The snubber circuit discharges the energy stored in the capacitor through the resistor. 【0023】 (First snubber circuit and second snubber circuit) The snubber circuit includes a first snubber circuit provided for the first primary winding and a second snubber circuit provided for the second primary winding. 【0024】 (Details of the first snubber circuit and the second snubber circuit) The first diode is arranged closer to the first primary winding side than the second diode. The fourth diode is arranged closer to the second primary winding side than the third diode. The first snubber circuit is composed of a first resistor, a first capacitor, and a first diode. The second snubber circuit is composed of a second resistor, a second capacitor, and a fourth diode. 【0025】 (Connection 1 of the first snubber circuit and the second snubber circuit) One end of the first resistor and the first capacitor is connected to the node between the first primary winding and the second primary winding, and the other end is connected to the first diode. One end of the second resistor and the second capacitor is connected to the node between the first primary winding and the second primary winding, and the other end is connected to the fourth diode. 【0026】 (Connection 2 of the first snubber circuit and the second snubber circuit) The input smoothing capacitor is composed of multiple capacitors. The first resistor has one end connected to the midpoint of the input smoothing capacitor and the other end connected to the first diode. The first capacitor has one end connected to the node between the first primary winding and the second primary winding, and the other end connected to the first diode. The second resistor has one end connected to the midpoint of the input smoothing capacitor and the other end connected to the fourth diode. The second capacitor has one end connected to the node between the first and second primary windings, and the other end connected to the fourth diode. 【0027】 (Semiconductor switching element) The first and second switching elements are semiconductor switching elements. 【0028】 (power supply) Another embodiment of the present invention is a power supply device characterized by comprising the aforementioned isolated power conversion circuit. [Effects of the Invention] 【0029】 (Effects of isolated power conversion circuits) The present invention is an isolated power conversion circuit that converts a predetermined input DC voltage into a desired DC voltage and outputs it, comprising: an isolation transformer having a first primary winding and a second primary winding arranged in series on the input side and a secondary winding on the output side; an input smoothing capacitor connected to the input portion of the input DC voltage; a first switching element disposed between the other end of the input smoothing capacitor and the other end of the first primary winding that is not connected to the second primary winding; a second switching element disposed between one end of the input smoothing capacitor and the other end of the second primary winding that is not connected to the first primary winding; a surge protection circuit disposed on the input side; and a rectifier-smoothing circuit connected to the secondary winding of the isolation transformer, wherein the surge protection circuit comprises a first diode, a second diode, a third diode and The clamp circuit includes a fourth diode. The first and second diodes are arranged in series between one end of the input smoothing capacitor and the other end of the first primary winding, and the third and fourth diodes are arranged in series between the other end of the input smoothing capacitor and one end of the second primary winding. When the first and second diodes conduct, the voltage of the first switching element is clamped by the voltage of the input smoothing capacitor, and when the third and fourth diodes conduct, the voltage of the second switching element is clamped by the voltage of the input smoothing capacitor. Thus, similar to conventional power supplies, the clamp circuit makes it possible to suppress the effects of surge voltages generated on the input side due to the leakage inductance of the isolation transformer. 【0030】 Furthermore, by arranging the first and second diodes in series, and the third and fourth diodes in series, the voltage can be shared between the first and second diodes, and between the third and fourth diodes, thereby reducing the voltage applied to each diode in the clamp circuit. In addition, since the voltage applied to each diode in the clamp circuit can be reduced, the voltage withstand capability required of the diodes is lowered, expanding the range of usable components and making component selection easier, as well as reducing component costs. 【0031】 (Effect of surge protection circuit with snubber circuit) Furthermore, the surge protection circuit includes a snubber circuit with a capacitor and a resistor. The snubber circuit discharges the energy stored in the capacitor through the resistor, thereby enabling more efficient suppression of the effects of surge voltage. 【0032】 (Effects of the first and second snubber circuits) Furthermore, the snubber circuit comprises a first snubber circuit provided for the first primary winding and a second snubber circuit provided for the second primary winding. The first diode is positioned closer to the first primary winding than the second diode, and the fourth diode is positioned closer to the second primary winding than the third diode. The first snubber circuit consists of a first resistor, a first capacitor, and a first diode, while the second snubber circuit consists of a second resistor, a second capacitor, and a fourth diode. As a result, the first and fourth diodes act as common diodes in both the clamp circuit and the snubber circuit, forming a surge protection circuit. This reduces the number of components in the surge protection circuit and simplifies the circuit. 【0033】 (Effect of connection 1 between the first snubber circuit and the second snubber circuit) Furthermore, since the first resistor and first capacitor are connected at one end to a node between the first and second primary windings and at the other end to the first diode, and the second resistor and second capacitor are connected at one end to a node between the first and second primary windings and at the other end to the fourth diode, the voltage can be shared between the first and second snubber circuits, reducing the voltage applied to each component of the snubber circuit. This lowers the voltage rating required for the snubber circuit components, expands the range of usable components, simplifies component selection, and reduces component costs. 【0034】 (Effect of connection 2 between the first and second snubber circuits) Furthermore, the input smoothing capacitor is composed of multiple capacitors, with one end of the first resistor connected to the midpoint of the input smoothing capacitor and the other end connected to the first diode, one end of the first capacitor connected to the node between the first and second primary windings and the other end connected to the first diode, one end of the second resistor connected to the midpoint of the input smoothing capacitor and the other end connected to the fourth diode, and one end of the second capacitor connected to the node between the first and second primary windings and the other end connected to the fourth diode. As a result, similar to "Connection 1 of the First and Second Snubber Circuits," the required voltage withstand voltage for the snubber circuit components is reduced, the range of usable components is expanded, making component selection easier and reducing component costs. 【0035】 Furthermore, in a clamp circuit, the voltage can be shared between the first and second diodes arranged in series, and also between the third and fourth diodes arranged in series. However, due to individual differences in components (for example, the output capacitance of the switching element or the leakage current of the diodes), the voltage is not always evenly distributed. In "Connection 2 of the First and Second Snubber Circuits," the voltage at the node between the first and second diodes and the voltage at the node between the third and fourth diodes become equal to the voltage at the midpoint of the input smoothing capacitor (= half of the input voltage), thereby making it possible to equalize the voltages across the first and second diodes and the third and fourth diodes. 【0036】 Furthermore, even a power supply equipped with an isolated power conversion circuit will have the same effect as the isolated power conversion circuit described above. [Brief explanation of the drawing] 【0037】 [Figure 1] This is an explanatory diagram showing the circuit diagram of a flyback converter equipped with an isolated power conversion circuit according to the first embodiment. [Figure 2]This is an explanatory diagram showing the circuit diagram of a flyback converter equipped with an isolated power conversion circuit according to the second embodiment. [Figure 3] This is an explanatory diagram showing the circuit diagram of a forward converter equipped with an isolated power conversion circuit according to the first embodiment. [Figure 4] This is an explanatory diagram showing the circuit diagram of a forward converter equipped with an isolated power conversion circuit according to the second embodiment. [Figure 5] This is an explanatory diagram showing the circuit diagram of a conventional flyback converter. [Figure 6] This is an explanatory diagram showing the circuit diagram of a conventional two-switch flyback converter. [Figure 7] This is an explanatory diagram showing the circuit diagram of a conventional 2-switch flyback converter equipped with a clamp circuit. [Figure 8] This is an explanatory diagram showing the circuit diagram of a conventional 2-switch flyback converter equipped with a clamp circuit and a snubber circuit. [Figure 9] This is an explanatory diagram showing the circuit diagram of a two-switch flyback converter with two clamp circuits and two snubber circuits in series. [Modes for carrying out the invention] 【0038】 The following describes embodiments of the isolated power conversion circuit and the power supply device equipped with the isolated power conversion circuit according to the present invention. However, the present invention is not limited to the following embodiments. 【0039】 [Basic Concepts of the Embodiment] First, the basic concepts of the embodiment will be explained. The embodiment generally relates to an isolated power conversion circuit that converts a predetermined input DC voltage into a desired DC voltage and outputs it. The scope of the invention extends to a power supply device equipped with the isolated power conversion circuit, and the power supply device includes a "flyback converter," which is a switching power supply device employing a flyback method, a "forward converter," which is a switching power supply device employing a forward method, and so on. Furthermore, the DC voltage input to the isolated power conversion circuit includes various DC voltages, such as those obtained by smoothing an AC voltage and converting it to a DC voltage, and pulsating DC voltages output from a power factor correction circuit. 【0040】 The "isolated power conversion circuit" of the embodiment comprises an "isolation transformer," an "input smoothing capacitor," a "first switching element," a "second switching element," a "surge protection circuit," and a "rectifier and smoothing circuit." 【0041】 An "isolation transformer" is a transformer that has a "first primary winding" and a "second primary winding" arranged in series on the input side, and a "secondary winding" on the output side. 【0042】 An "input smoothing capacitor" is a capacitor located on the input side of an isolated power conversion circuit, connected to the input section of the input DC voltage, and serves as a return point for surge energy from the surge protection circuit. 【0043】 The "first switching element" and the "second switching element" are provided on the input side of an isolated power conversion circuit, and semiconductor switching elements such as MOSFETs are used, with their on / off operation controlled by a predetermined switching control unit. The "switching control unit" includes those using a control IC and a gate driver IC, and those using a gate drive circuit equipped with a control IC and a drive transformer, etc. 【0044】 Furthermore, the "first switching element" is provided on the input side of the isolated power conversion circuit and is positioned between the other end of the input smoothing capacitor and the other end of the first primary winding that is not connected to the second primary winding, while the "second switching element" is positioned between one end of the input smoothing capacitor and the other end of the second primary winding that is not connected to the first primary winding. 【0045】 A "rectifier-smoothing circuit" is provided on the output side of an isolated power conversion circuit and connected to the secondary winding. Its configuration is arbitrary, but examples include a rectifier-smoothing circuit consisting of a diode and a capacitor for a flyback converter, and a rectifier-smoothing circuit consisting of two diodes, a capacitor, and a coil for a forward converter. 【0046】 The "surge protection circuit" includes at least a clamping circuit, and the "clamping circuit" has four diodes, from the first diode to the fourth diode. 【0047】 The "first diode" and the "second diode" are arranged in series between one end of the input smoothing capacitor and the other end of the first primary winding (the side not connected to the second primary winding), and the "third diode" and the "fourth diode" are arranged in series between the other end of the input smoothing capacitor and one end of the second primary winding (the side not connected to the first primary winding). 【0048】 Furthermore, due to the presence of the clamp circuit, when the first and second switching elements are turned off and a surge voltage is generated on the input side due to the leakage inductance of the isolation transformer, the first to fourth diodes conduct, returning the surge energy to the input smoothing capacitor. The conduction of the first and second diodes causes the voltage of the first switching element to be clamped by the voltage of the input smoothing capacitor, and the conduction of the third and fourth diodes causes the voltage of the second switching element to be clamped by the voltage of the input smoothing capacitor. Note that "clamped by the voltage of the input smoothing capacitor" means that the voltage is fixed at the voltage of that input smoothing capacitor. 【0049】 Therefore, similar to conventional power supply units, the clamp circuit makes it possible to suppress the effects of surge voltages generated on the input side due to the leakage inductance of the isolation transformer. 【0050】 Furthermore, by arranging the first and second diodes in series, and the third and fourth diodes in series, the voltage can be shared between the first and second diodes, and between the third and fourth diodes, thereby reducing the voltage applied to each diode in the clamp circuit. In addition, since the voltage applied to each diode in the clamp circuit can be reduced, the voltage withstand capability required of the diodes is lowered, expanding the range of usable components and making component selection easier, as well as reducing component costs. 【0051】 Furthermore, the "surge protection circuit" may also include a "snubber circuit" having a capacitor and a resistor, and the "snubber circuit" discharges the energy stored in the capacitor through the resistor. 【0052】 The "snubber circuit" comprises a "first snubber circuit" provided for the first primary winding and a "second snubber circuit" provided for the second primary winding, and the "first snubber circuit" and the "second snubber circuit" are RCD circuits composed of, for example, resistors, capacitors, and diodes. 【0053】 Here, assuming that the first diode in the clamp circuit is positioned closer to the first primary winding than the second diode, and the fourth diode is positioned closer to the second primary winding than the third diode, the "first snubber circuit" can be composed of the "first resistor," the "first capacitor," and the first diode of the clamp circuit, and the second snubber circuit can be composed of the "second resistor," the "second capacitor," and the fourth diode of the clamp circuit. 【0054】 Therefore, the first and fourth diodes are used as common diodes in the clamp circuit and snubber circuit to form a surge protection circuit, which reduces the number of components in the surge protection circuit and simplifies the circuit. 【0055】 Furthermore, there are mainly two connection methods for the first and second snubber circuits. 【0056】 One connection method (connection 1 of the first and second snubber circuits) involves the "first resistor" and "first capacitor" having one end connected to a node between the first and second primary windings and the other end connected to the first diode, while the "second resistor" and "second capacitor" have one end connected to a node between the first and second primary windings and the other end connected to the fourth diode. 【0057】 Furthermore, as a second connection method (connection 2 of the first and second snubber circuits), it is assumed that the input smoothing capacitor is composed of multiple capacitors to create a midpoint (intermediate potential), and the "first resistor" has one end connected to the midpoint of the input smoothing capacitor and the other end connected to the first diode, the "first capacitor" has one end connected to the node between the first primary winding and the second primary winding and the other end connected to the first diode, the "second resistor" has one end connected to the midpoint of the input smoothing capacitor and the other end connected to the fourth diode, and the "second capacitor" has one end connected to the node between the first primary winding and the second primary winding and the other end connected to the fourth diode. 【0058】 In this clamp circuit, the voltage can be shared between the first and second diodes arranged in series, as well as between the third and fourth diodes also arranged in series. However, due to individual differences in components (for example, the output capacitance of the switching element or the leakage current of the diodes), the voltage is not always evenly distributed. 【0059】 Therefore, by adopting the second connection method, the voltage at the node between the first and second diodes and the voltage at the node between the third and fourth diodes become equal to the voltage at the midpoint of the input smoothing capacitor (= half of the input voltage), making it possible to equalize the voltages across the first and second diodes and the voltages across the third and fourth diodes. 【0060】 Furthermore, the designations "No. ○" attached to the primary winding, switching element, diode, snubber circuit, resistor, and capacitor are merely used to distinguish between multiple components with the same name. 【0061】 Hereinafter, the first and second switching elements are MOSFETs, and the surge protection circuit is a circuit comprising a clamp circuit and a snubber circuit. The connection method of the first and second snubber circuits is described as follows: the first connection method (connection 1 of the first and second snubber circuits) is referred to as the isolated power conversion circuit of the first embodiment, and the second connection method (connection 2 of the first and second snubber circuits) is referred to as the isolated power conversion circuit of the second embodiment. A flyback converter will be used as an example for explanation. 【0062】 [Specific details of the embodiment] The specific details of the embodiments shown below will be explained in the following sections. a. Flyback converter with isolated power conversion circuit of the first embodiment b. Flyback converter with isolated power conversion circuit of the second embodiment c. Modified form of the present invention 【0063】 [a. Flyback converter with isolated power conversion circuit of the first embodiment] First, a flyback converter equipped with an isolated power conversion circuit according to the first embodiment will be described. In this description, we will refer to Figure 1, which shows a circuit diagram of the flyback converter equipped with an isolated power conversion circuit according to the first embodiment. 【0064】 As shown in Figure 1, the flyback converter 10 of the first embodiment includes an isolation transformer 50, a switching element TR1 (first switching element), a switching element TR2 (second switching element), a capacitor C1 (input smoothing capacitor), a surge protection circuit, and a rectifier / smoothing circuit, and the input side and output side of the flyback converter 10 are isolated by the isolation transformer 50. 【0065】 The isolation transformer 50 has a first primary winding L11 and a second primary winding L12 on the input side, and a secondary winding L2 on the output side. As shown by the dots in Figure 1, the polarity of the secondary winding L2 is reversed with respect to the first primary winding L11 and the second primary winding L12. 【0066】 The second primary winding L12 of the isolation transformer 50 has one terminal (one end) with a dot in Figure 1 connected to the source terminal of the switching element TR2, and the other terminal (one end) without a dot in Figure 1 connected to the terminal (one end) with a dot in Figure 1 of the first primary winding L11. 【0067】 As mentioned above, one end of the first primary winding L11 of the isolation transformer 50 is connected to the other end of the second primary winding L12, and the terminal on the side without a dot in Figure 1 (the other end) is connected to the drain terminal of the switching element TR1. 【0068】 In the switching element TR2, the drain terminal is connected to the positive terminal of the input terminal (input section) and one end of the capacitor C1, as described above, the source terminal is connected to one end of the second primary winding L12, and the gate terminal is connected to a switching control unit (not shown). 【0069】 As mentioned above, in the switching element TR1, the drain terminal is connected to the other end of the first primary winding L11, the source terminal is connected to the negative terminal (primary ground) of the input terminal and the other end of the input smoothing capacitor C1, and the gate terminal is connected to a switching control unit (not shown). 【0070】 In Figure 1, the secondary winding L2 of the isolation transformer 50 has the terminal without a dot (the other end) connected to the anode of diode D5 (rectifier diode), and the terminal with a dot (the other end) connected to the other end of capacitor C4 (output smoothing capacitor) which is connected to the negative terminal (secondary ground) of the output terminal (output section). Also, the cathode of diode D2 is connected to the positive terminal of the output terminal and one end of capacitor C4. 【0071】 The rectifier and smoothing circuit is implemented using diode D5 and capacitor C4, and the desired voltage, rectified and smoothed by the rectifier and smoothing circuit, is supplied as the output voltage Vout to the load connected to the output terminal. 【0072】 As described above, the circuit configuration is the same as the flyback converter 110 shown in Figure 6 as an example of a conventional two-switch flyback converter, except that the primary winding of the isolation transformer 50 is composed of a first primary winding L11 and a second primary winding L12. The flyback converter 10 of the embodiment shown in Figure 1 is characterized by having a surge protection circuit different from the conventional flyback converters 120 and 130 shown in Figures 7 and 8. 【0073】 Furthermore, in the flyback converter 10, when the switching elements TR1 and TR2 are turned on, primary current flows through the first primary winding L11 and the second primary winding L12, and energy is stored in the first primary winding L11 and the second primary winding L12. 【0074】 Next, when the switching elements TR1 and TR2 are turned off, the primary current is interrupted, and the energy stored in the first primary winding L11 and the second primary winding L12 is output from the secondary winding L2 to the rectifier and smoothing circuit, which then rectifies and smooths the desired output voltage. 【0075】 However, since leakage inductance is present in the isolation transformer 50, when the switching elements TR1 and TR2 are turned on, primary current flows through this leakage inductance and energy is stored. Since the leakage inductance is not coupled to any of the windings of the isolation transformer 50, when the switching elements TR1 and TR2 are turned off, a surge voltage is generated on the input side without power being transferred to the output side. 【0076】 The resulting surge voltage is then applied between the drain and source of switching elements TR1 and TR2. If the voltage applied between the drain and source of switching elements TR1 and TR2 due to the generated surge voltage exceeds the withstand voltage of switching elements TR1 and TR2, there is a risk of damaging switching elements TR1 and TR2, thus necessitating a surge protection circuit. 【0077】 The surge protection circuit includes a clamping circuit, which consists of diodes D1 (first diode), D2 (second diode), D3 (third diode), and D4 (fourth diode). 【0078】 Diodes D1 and D2 are arranged in series between one end of capacitor C1 and the other end of the primary winding L11. Diode D1's anode is connected to the other end of the primary winding L11 and the drain terminal of switching element TR1, and its cathode is connected to the anode of diode D2. Diode D2's anode is connected to the cathode of diode D1, as described above, and its cathode is connected to the positive terminal of the input terminal, one end of capacitor C1 and the drain terminal of switching element TR2. 【0079】 Diodes D3 and D4 are arranged in series between the other end of capacitor C1 and one end of the second primary winding L12. Diode D3 has its anode connected to the negative terminal of the input terminal, the other end of capacitor C1, and the source terminal of switching element TR2, and its cathode connected to the anode of diode D4. Diode D4, as described above, has its anode connected to the cathode of diode D3, and its cathode connected to one end of the second primary winding L12 and the source terminal of switching element TR1. 【0080】 The clamp circuit works by having diodes D1-D4 conduct when a surge voltage caused by leakage inductance occurs on the input side, thereby recirculating the surge energy to capacitor C1. 【0081】 Furthermore, because diodes D1 and D2 conduct, the potential Vd1 at the drain terminal of switching element TR1 becomes approximately the same as the potential at one end of input smoothing capacitor C1. As a result, the drain-source voltage Vds1 of switching element TR1 is clamped by the voltage Vc1 (approximately input voltage Vin) of input smoothing capacitor C1, thereby suppressing the effect of surge voltage on switching element TR1. 【0082】 Furthermore, because diodes D3 and D4 conduct, the potential Vd2 at the source terminal of switching element TR2 becomes approximately the same as the GND potential of the primary ground. As a result, the drain-source voltage Vds2 of switching element TR2 is clamped by the voltage Vc1 (approximately the input voltage Vin) of the input smoothing capacitor C1, thereby suppressing the effect of surge voltage on switching element TR2. 【0083】 Furthermore, when switching elements TR1 and TR2 are turned on, the drain-source connections of TR1 and TR2 conduct, and the input voltage Vin is applied to the clamp circuit. Therefore, during steady-state operation of the switching power supply, a high voltage at least equivalent to the input voltage Vin is constantly applied to the clamp circuit. However, since the clamp circuit has diodes D1 and D2 arranged in series, and diodes D3 and D4 arranged in series, the voltage applied to each of the diodes D1 to D4 can be shared, and the voltage withstand voltage required for the diodes D1 to D4 used in the clamp circuit can be reduced. 【0084】 Furthermore, the surge protection circuit is equipped with a snubber circuit, which comprises a first snubber circuit provided for the first primary winding L11 and a second snubber circuit provided for the second primary winding L12. 【0085】 The first snubber circuit consists of a resistor R2 (first resistor), a capacitor C2 (first capacitor), and a clamping diode D1. The resistor R2 and capacitor C2 have one end connected to a node between the first primary winding L11 and the second primary winding L12, and the other end connected to the cathode of diode D1. 【0086】 Therefore, when a surge voltage is generated due to the leakage inductance of the first primary winding L11, the first snubber circuit temporarily stores the generated surge energy in capacitor C2, and then discharges the surge energy stored in capacitor C2 through resistor R2, thereby suppressing the effect of the surge voltage on the switching elements TR1 and TR2. 【0087】 The second snubber circuit consists of a resistor R3 (second resistor), a capacitor C3 (second capacitor), and a diode D4 in the clamp circuit. One end of resistor R3 and capacitor C3 is connected to the node between the first primary winding L11 and the second primary winding L12, and the other end is connected to the atode of diode D4. 【0088】 Therefore, when a surge voltage is generated due to the leakage inductance of the second primary winding L12, the second snubber circuit temporarily stores the generated surge energy in capacitor C3, and then discharges the surge energy stored in capacitor C3 through resistor R3, thereby suppressing the effect of the surge voltage on the switching elements TR1 and TR2. 【0089】 Furthermore, since the first snubber circuit is arranged in parallel with the first primary winding L11, and the second snubber circuit is arranged in parallel with the second primary winding L12, the voltage resistance required for the components used in the snubber circuit can be reduced compared to the case where one snubber circuit is arranged in parallel with both the first and second primary windings L11 and L12. 【0090】 For example, in general, the first primary winding L11 and the second primary winding L12 of the isolation transformer 50 used in a flyback converter are designed to have the same inductance and are driven at the same time ratio by the switching elements TR1 and TR2. As a result, the potential at the nodes of the first primary winding L11 and the second primary winding L12 is about half of the input voltage Vin, and the voltage applied to both the first and second snubber circuits remains about half of the input voltage Vin. 【0091】 In other words, in the surge protection circuit of this embodiment, the required voltage withstand capability for all components can be reduced, expanding the range of usable components and making component selection easier, while also reducing component costs. 【0092】 Furthermore, in a conventional flyback converter 130 equipped with a clamp circuit and a snubber circuit as a surge protection circuit, as shown in Figure 8, for example, if two of each component of the surge protection circuit were placed in series, as in the flyback converter 140 shown in Figure 9, in order to lower the voltage rating required for the components used in the surge protection circuit, the number of components would increase, resulting in multiple circuit configurations. However, in the surge protection circuit of this embodiment, by providing diodes D1 and D4 as common components for both the clamp circuit and the snubber circuit, the number of components can be reduced, and the circuit configuration can be made as simple as possible. 【0093】 [b. Flyback converter with isolated power conversion circuit of the second embodiment] Next, a flyback converter equipped with an isolated power conversion circuit according to the second embodiment will be described. In this description, refer to Figure 2, which shows the circuit diagram of the flyback converter equipped with an isolated power conversion circuit according to the second embodiment. 【0094】 As shown in Figure 2, the flyback converter 20 of the second embodiment is similar to the first embodiment in that it includes an isolation transformer 50, a switching element TR1 (first switching element), a switching element TR2 (second switching element), an input smoothing capacitor and surge protection circuit, and a rectifier and smoothing circuit. However, the input smoothing capacitor is composed of capacitors C11 and C12, and the connection destination of the snubber circuit of the surge protection circuit differs from that of the first embodiment. 【0095】 Therefore, the second embodiment will mainly describe the connection of the snubber circuit of the surge protection circuit, which differs from that of the first embodiment, and will omit further explanation as other aspects are basically the same as those of the first embodiment. 【0096】 The snubber circuit of the surge protection circuit comprises a first snubber circuit and a second snubber circuit, similar to the first embodiment. The first snubber circuit consists of a resistor R2 (first resistor), a capacitor C2 (first capacitor), and a diode D1 of the clamp circuit. The second snubber circuit consists of a resistor R3 (second resistor), a capacitor C3 (second capacitor), and a diode D4 of the clamp circuit. 【0097】 Furthermore, similar to the first embodiment, the capacitor C2 of the first snubber circuit has one end connected to a node between the first primary winding L11 and the second primary winding L12, and the other end connected to the cathode of diode D1, and the capacitor C3 has one end connected to a node between the first primary winding L11 and the second primary winding L12, and the other end connected to the anode of diode D4. 【0098】 On the other hand, the resistor R2 in the first snubber circuit and the resistor R3 in the second snubber circuit differ from those in the first embodiment. Resistor R2 has one end connected to the node between capacitors C11 and C12 and the other end connected to the cathode of diode D1, while resistor R3 has one end connected to the node between capacitors C11 and C12 and the other end connected to the cathode of diode D4. 【0099】 Furthermore, capacitors C11 and C12 are of the same capacitance, and the voltage at the node between capacitors C11 and C12 is adjusted to be at the midpoint (= half of the input voltage). 【0100】 In the clamp circuit of the surge protection circuit, as described in the first embodiment, diodes D1 and D2 are arranged in series, and diodes D3 and D4 are also arranged in series. Therefore, the voltage applied to each of the diodes D1 to D4 can be shared. However, due to individual differences in components (for example, the output capacitance of the switching element and the leakage current of the diodes), the voltage is not always distributed evenly. 【0101】 However, in the second embodiment, the voltage at the node between diodes D1 and D2 and the voltage at the node between diodes D3 and D4 are equal to the voltage at the node between capacitors C11 and C12 (= half of the input voltage). This makes it possible to equalize the voltages across diodes D1 and D2 and diodes D3 and D4, thereby making component selection easier. 【0102】 [c. Variations of the present invention] Modified examples of the isolated power conversion circuit and the power supply device equipped with the isolated power conversion circuit according to the present invention will be described. In addition to the embodiments described above, the isolated power conversion circuit and the power supply device equipped with the isolated power conversion circuit of the present invention include the following modifications. 【0103】 (power supply) In the above embodiment, a flyback converter was used as an example, but the isolated power conversion circuit is also applicable to forward converters. Examples of forward converters equipped with an isolated power conversion circuit include the forward converter 30 equipped with the isolated power conversion circuit of the first embodiment shown in Figure 3, and the forward converter 40 equipped with the isolated power conversion circuit of the second embodiment shown in Figure 4. 【0104】 The forward converters 30 and 40 have basically the same input configuration as the flyback converters 10 and 20 shown in Figures 1 and 2. The rectifier and smoothing circuit connected to the secondary winding L2 of the isolation transformer 50 consists of diodes D5 (rectifier diode), D6 (freewheeling diode), capacitor C4 (output smoothing capacitor), and coil L3 (output smoothing coil). The polarity of the secondary winding L2 is the same as that of the first primary winding L11 and the second primary winding L12. 【0105】 Furthermore, the power supply unit may, of course, include configurations other than the isolated power conversion circuit. For example, it may include a conversion circuit that generates the DC voltage input to the isolated power conversion circuit from the AC voltage, or a power factor correction circuit, etc., in the preceding stage of the isolated power conversion circuit. 【0106】 (Surge protection circuit) In the above embodiment, the surge protection circuit included a clamp circuit and a snubber circuit, but it is not limited to this, and a surge protection circuit consisting only of a clamp circuit may also be used. 【0107】 (Input smoothing capacitor of the second embodiment) In the above embodiment, the midpoint of the input smoothing capacitor was generated by arranging two capacitors in series, but the invention is not limited to this, and the midpoint of the input smoothing capacitor may be generated using two or more capacitors. 【0108】 (others) Furthermore, the present invention includes appropriate modifications that do not impair its purpose and advantages, and is not limited by the numerical values ​​shown in the above embodiments. [Explanation of symbols] 【0109】 10, 20, 100, 110, 120, 130, 140: Flyback converter 30,40: Forward converter 50: Isolation transformer L1: Primary winding L11: Primary winding L12: Secondary winding L2: Secondary winding L3: Coil R2,R3,R4,R41,R42:Resistance C1, C11, C12, C2, C3, C4, C5, C51, C52: Capacitors D1, D2, D21, D22, D3, D31, D32, D4, D5, D6, D7, D71, D72: diode TR1, TR2: Switching elements

Claims

[Claim 1] An isolated power conversion circuit that converts a predetermined input DC voltage into a desired DC voltage and outputs it, An isolation transformer having a first primary winding and a second primary winding arranged in series on the input side, and a secondary winding on the output side, An input smoothing capacitor connected to the input section of the aforementioned DC voltage, A first switching element is positioned between the other end of the input smoothing capacitor and the other end of the first primary winding that is not connected to the second primary winding, A second switching element is disposed between one end of the input smoothing capacitor and the end of the second primary winding that is not connected to the first primary winding, A surge suppression circuit is located on the input side, A rectifier and smoothing circuit connected to the secondary winding of the isolation transformer, Equipped with, The surge protection circuit described above is: The clamp circuit comprises a first diode, a second diode, a third diode, and a fourth diode. The first diode and the second diode are arranged in series between one end of the input smoothing capacitor and the other end of the first primary winding. The third diode and the fourth diode are arranged in series between the other end of the input smoothing capacitor and one end of the second primary winding. When the first diode and the second diode conduct, the voltage of the first switching element is clamped by the voltage of the input smoothing capacitor. An isolated power conversion circuit characterized in that the voltage of the second switching element is clamped by the voltage of the input smoothing capacitor when the third diode and the fourth diode conduct. [Claim 2] An isolated power conversion circuit according to claim 1, The surge protection circuit further includes a snubber circuit having a capacitor and a resistor. The snubber circuit is an isolated power conversion circuit characterized by discharging the energy stored in the capacitor through the resistor. [Claim 3] The isolated power conversion circuit according to claim 2, The aforementioned snubber circuit is A first snubber circuit provided for the first primary winding, A second snubber circuit provided for the second primary winding, An isolated power conversion circuit characterized by having the following features. [Claim 4] An isolated power conversion circuit according to claim 3, The first diode is positioned closer to the first primary winding than the second diode. The fourth diode is positioned closer to the second primary winding than the third diode. The first snubber circuit is composed of a first resistor, a first capacitor, and a first diode. The isolated power conversion circuit is characterized in that the second snubber circuit is composed of a second resistor, a second capacitor, and the fourth diode. [Claim 5] An isolated power conversion circuit according to claim 4, The first resistor and the first capacitor have one end connected to a node between the first primary winding and the second primary winding, and the other end connected to the first diode. An isolated power conversion circuit characterized in that one end of the second resistor and the second capacitor is connected to a node between the first primary winding and the second primary winding, and the other end is connected to the fourth diode. [Claim 6] An isolated power conversion circuit according to claim 4, The input smoothing capacitor is composed of multiple capacitors, The first resistor has one end connected to the midpoint of the input smoothing capacitor and the other end connected to the first diode. The first capacitor has one end connected to a node between the first primary winding and the second primary winding, and the other end connected to the first diode. The second resistor has one end connected to the midpoint of the input smoothing capacitor and the other end connected to the fourth diode. The isolated power conversion circuit is characterized in that one end of the second capacitor is connected to a node between the first primary winding and the second primary winding, and the other end is connected to the fourth diode. [Claim 7] An isolated power conversion circuit according to claim 1, An isolated power conversion circuit characterized in that the first switching element and the second switching element are semiconductor switching elements. [Claim 8] A power supply device characterized by comprising an isolated power conversion circuit according to any one of claims 1 to 7.

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