PWM control device and power supply system

Abstract

This PWM control device controls a switching power supply circuit provided with a switching transformer and two or more primary-side switching elements including at least a pre-switch and a post-switch. The PWM control device controls the conduction states of the pre-switch and the post-switch in a clamp control mode in which the duty ratio of the pre-switch and the duty ratio of the post-switch are substantially the same.

Description

PWM control device and power supply system

[0001] The present invention relates to a PWM control device and a power supply system. This application claims priority to Japanese Patent Application No. 2024-153921 filed on September 6, 2024, and Japanese Patent Application No. 2024-196649 filed on November 11, 2024, the contents of which are incorporated herein by reference.

[0002] Conventionally, there has been known a switching power supply circuit in which the primary sides of two transformers are connected in series, and the secondary sides are connected in parallel or series with the polarity reversed, and power is supplied to the secondary sides from a half or full bridge circuit (see, for example, Patent Document 1).

[0003] In a switching power supply circuit having a half or full bridge circuit, any two switching elements of the switching power supply circuit operate at different timings and with different pulse widths. For example, if the duty ratio of one switching element, a leading switch, is (D), the other switching element, a trailing switch, operates at a duty ratio of (1-D). Note that the actual duty ratio of the trailing switch is smaller than (1-D) to provide a dead time.

[0004] Patent No. 6902673

[0005] The above-mentioned technology has the problem that the power output from each transformer is biased due to differences in duty ratios. When the current values ​​output from each transformer are different, a higher voltage is applied to the rectifying element on the transformer that outputs a larger current than to the rectifying element on the transformer that outputs a smaller current, making it necessary to select rectifying elements with high withstand voltage and current ratings.

[0006] During normal operation, operating the transformer at a duty ratio close to 0.5 can alleviate asymmetry and reduce the imbalance in the power handled by one side of the transformer, but during soft start, overload protection, and other operations, the duty ratio is reduced, increasing the asymmetry. In this case, a higher voltage is applied to the rectifier element on the transformer side that outputs a larger current than to the rectifier element on the transformer side that outputs a smaller current, and since the switching power supply circuit operates for a smaller proportion of the overall operating time by reducing the duty ratio, it is necessary to select a rectifier element with a high withstand voltage and high current rating, which poses the issues of higher manufacturing costs and increased losses due to the deterioration of forward characteristics in elements with a normally high withstand voltage.

[0007] SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned points, and has as its object to provide a highly efficient, low-cost switching power supply circuit and power supply system.

[0008] One aspect of the present invention is a PWM control device that controls a switching power supply circuit that includes a switching transformer and two or more primary-side switching elements including at least a leading switch and a trailing switch, and that controls the conduction states of the leading switch and the trailing switch in a clamp control mode in which the duty ratio of the leading switch and the duty ratio of the trailing switch are made substantially equal.

[0009] In one aspect of the present invention, the PWM control device controls the conduction states of the leading switch and the trailing switch by switching between a steady mode in which the duty ratio of the trailing switch is set to a value corresponding to the inverse of the duty ratio of the leading switch, and the clamp control mode.

[0010] In one aspect of the present invention, when the switching power supply circuit is started up, the PWM control device starts to control the conduction states of the leading switch and the trailing switch in the clamp control mode in which the duty ratio of the leading switch and the duty ratio of the trailing switch are small, and then gradually increases the duty ratio of the leading switch and the duty ratio of the trailing switch, and when the duty ratio of the leading switch or the duty ratio of the trailing switch becomes equal to or greater than a predetermined threshold, switches from the clamp control mode to the steady mode to control the conduction states of the leading switch and the trailing switch.

[0011] In addition, in one aspect of the present invention, during protection operation against overload of the switching power supply circuit, the PWM control device 20 switches from the steady mode to the clamp control mode in which the duty ratio of the leading switch and the duty ratio of the trailing switch are small, and controls the conduction states of the leading switch and the trailing switch.

[0012] In one aspect of the present invention, the switching power supply circuit includes a half-bridge circuit on the primary side that includes two of the switching elements, and one of the leading switch or the trailing switch is the switching element on the high potential side of the half-bridge circuit, and the other of the leading switch or the trailing switch is the switching element on the low potential side of the half-bridge circuit.

[0013] In one aspect of the present invention, the switching power supply circuit includes a full bridge circuit on the primary side that includes four of the switching elements, and one of the leading switch or the trailing switch is the switching element on the high potential side that is connected to a first end of the switching transformer, and the switching element on the low potential side that is connected to a second end of the switching transformer, and the other of the leading switch or the trailing switch is the switching element on the low potential side that is connected to the first end of the switching transformer, and the switching element on the high potential side that is connected to the second end of the switching transformer.

[0014] In one aspect of the present invention, the magnetic core of the switching transformer has a magnetic permeability of 15 or more and 120 or less when the magnetic flux density is 0.3 [T].

[0015] Another aspect of the present invention is a power supply system comprising the above-described PWM control device, the switching power supply circuit, and an OFF-DELAY circuit having one end connected to a switching element connected to a secondary side of the switching power supply circuit and delaying the OFF timing of the secondary-side switching element, wherein the PWM control device outputs a signal that is substantially the same as a signal that controls the conduction state of the leading switch and the trailing switch to the other end of the OFF-DELAY circuit.

[0016] Another aspect of the present invention is a PWM control device that controls a switching power supply circuit that includes a switching transformer and two or more primary-side switching elements that include at least a leading switch and a trailing switch, and that controls the conduction states of the leading switch and the trailing switch in a clamp control mode in which the duty ratio of the trailing switch is set to a value obtained by multiplying the duty ratio of the leading switch by a set value.

[0017] According to the present invention, it is possible to provide a highly efficient and low-cost switching power supply circuit and power supply system.

[0018] FIG. 1 is a diagram illustrating an example of the configuration of a power supply system according to an embodiment. FIG. 1 is a diagram illustrating a first example of a drive pulse according to an embodiment and the associated changes in current and voltage during soft start. FIG. 2 is a diagram illustrating an example of a voltage applied to a first diode and a voltage applied to a second diode in a first example of a control method by a PWM control device according to an embodiment. FIG. 3 is a diagram illustrating an example of a predetermined threshold value according to an embodiment. FIG. 4 is a diagram illustrating a first example of a control method by a PWM control device according to an embodiment. FIG. 5 is a diagram illustrating a third example of a control method by a PWM control device according to an embodiment. FIG. 6 is a diagram illustrating an example of an overload protection operation. FIG. 7 is a diagram illustrating an example of the configuration of a power supply system according to a first modified example. FIG. 8 is a diagram illustrating an example of the configuration of a power supply system according to a second modified example. FIG. 9 is a diagram illustrating an example of a simulation result of current values ​​of each part of a power supply system according to the second modified example. FIG. 10 is a diagram illustrating a fourth example of a control method by a PWM control device according to an embodiment. The relationship between the duty ratio of a switching element and the output voltage Vout in the fourth example of the control method is shown for each of the normal mode, the first example of the control method, and the fourth example of the control method. FIG. 11 is a diagram illustrating a conventional drive pulse and the associated changes in current and voltage during soft start. FIG. 10 is a diagram showing an example of a voltage applied to a first diode and a voltage applied to a second diode in a conventional control method.

[0019] A PWM control device and a power supply system according to preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the drawings, identical or similar parts are designated by identical or similar reference numerals. Note that the present invention is not limited to these embodiments and includes various modifications and improvements. In other words, the components described below include those that can be easily imagined by a person skilled in the art and those that are substantially identical, and the components described below can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the components of the present embodiment may be made without departing from the spirit and scope of the present invention.

[0020] [Configuration Example in Case of Half-Bridge Circuit] Fig. 1 is a diagram showing an example of the configuration of a power supply system 1 according to an embodiment. The power supply system 1 includes a switching power supply circuit 10 and a PWM control device 20. Note that the circuit diagram shown in Fig. 1 is merely an example, and the present embodiment is not limited to this example.

[0021] The switching power supply circuit 10 is a power supply circuit that outputs an AC voltage based on a DC input voltage by, for example, controlling the conduction state of a switching element. The switching power supply circuit 10 is supplied with a DC voltage from an input power supply Vin. The negative electrode of the input power supply Vin is connected to a reference voltage. The reference voltage may be, for example, 0 [V], a small voltage close to 0 [V], or any voltage set by the designer of the power supply system 1. An input capacitor Cin is connected in parallel to the input power supply Vin. The switching power supply circuit 10 may also output an AC voltage based on a DC voltage obtained by rectifying the AC input voltage.

[0022] The switching power supply circuit 10 includes, as its components, at least a switching transformer, two or more primary-side switching elements, and a resonant coil Lr. In Fig. 1, the switching power supply circuit 10 includes a half-bridge circuit 11 on the primary side. The half-bridge circuit 11 includes a leading switch SWLead and a trailing switch SWLag as switching elements. The switching elements switch between on and off conduction states in response to control by a PWM control device 20.

[0023] The leading switch SWLead is, for example, an n-type semiconductor having a drain connected to the input power supply Vin, a source connected to the trailing switch SWLag and the resonant coil Lr, and a gate connected to the PWM control device 20. The source of the leading switch SWLead is electrically connected to the high-potential side terminal of the first transformer T1 via the resonant coil Lr.

[0024] The trailing switch SWLag is, for example, an n-type semiconductor having a drain connected to the leading switch SWLead and the resonant coil Lr, a source connected to a reference voltage, and a gate connected to the PWM control device 20. The drain of the trailing switch SWLag is electrically connected to the high-potential side terminal of the first transformer T1 via the resonant coil Lr.

[0025] The resonant coil Lr has one end connected to the leading switch SWLead and the trailing switch SWLag, and the other end connected to the high-potential terminal of the first transformer T1. The resonant coil Lr is provided in the switching power supply circuit 10 to achieve soft switching.

[0026] In FIG. 1 , the switching transformers are a first transformer T1 and a second transformer T2. The primary windings of the first transformer T1 and the second transformer T2 are connected in series. The secondary windings of the first transformer T1 and the second transformer T2 may be connected in series or in parallel. In FIG. 1 , the first transformer T1 and the second transformer T2 are connected in parallel. Also, in FIG. 1 , the turns ratios (ratio of the number of primary windings to the number of secondary windings) of the first transformer T1 and the second transformer T2 are assumed to be substantially the same. The first transformer T1 and the second transformer T2 are magnetically independent. The magnetic core of the switching transformer has a permeability of 15 to 120 at a magnetic flux density of 0.3 T. The magnetic core may be, for example, a magnetic core made of a low-permeability material with an initial permeability of 15 to 120, or a gapped magnetic core made of a material with a high initial permeability (e.g., ferrite). It is particularly preferable that the magnetic core of the switching transformer have a magnetic permeability of 24 or more and 90 or less when the magnetic flux density is 0.3 T. It is also particularly preferable that the magnetic core of the switching transformer be made of a material with low initial magnetic permeability, that is, ...

[0027] 1, the secondary windings of the first transformer T1 and the second transformer T2 are wound with polarities opposite to those of the primary windings. Therefore, when a current flows from the high potential side to the low potential side of the primary winding of the first transformer T1, the secondary winding of the first transformer T1 also flows the current from the high potential side to the low potential side. Conversely, when a current flows from the low potential side to the high potential side of the primary winding of the first transformer T1, the secondary winding of the first transformer T1 also flows the current from the low potential side to the high potential side.

[0028] Similar to the first transformer T1, when a current flows from the high potential side to the low potential side of the primary winding of the second transformer T2, the second transformer T2 passes a current from the high potential side to the low potential side of the secondary winding of the second transformer T2. Also, when a current flows from the low potential side to the high potential side of the primary winding of the second transformer T2, the secondary winding of the second transformer T2 passes a current from the low potential side to the high potential side.

[0029] The high-potential side of the primary winding of the first transformer T1 is electrically connected to the leading switch SWLead and the trailing switch SWLag via the resonant coil Lr. The low-potential side of the primary winding of the first transformer T1 is connected to the high-potential side of the primary winding of the second transformer T2. In the following description, the high-potential side terminal of the primary winding of the first transformer T1 may be simply referred to as the first port P1 or the first port P1 of the switching transformer.

[0030] The high-potential side of the primary winding of the second transformer T2 is connected to the low-potential side of the primary winding of the first transformer T1. The low-potential side of the primary winding of the second transformer T2 is connected to the first capacitor C1 and the second capacitor C2. At this time, the low-potential side of the primary winding of the second transformer T2 is electrically connected to a reference voltage via the second capacitor C2. In the following description, the low-potential side of the primary winding of the second transformer T2 may be simply referred to as the second terminal P2 or the second terminal P2 of the switching transformer.

[0031] The first capacitor C1 has one end connected to the input power supply Vin, the input capacitor Cin, and the drain of the leading switch SWLead, and the other end connected to the second capacitor C2 and the second end P2. The second capacitor C2 has one end connected to the first capacitor C1 and the second end P2, and the other end connected to a reference voltage. The first capacitor C1 and the second capacitor C2 are half-bridge capacitors that cut the DC component of the current output from the input power supply Vin. Note that the switching power supply circuit 10 does not necessarily need to include both the first capacitor C1 and the second capacitor C2. The switching power supply circuit 10 may include either the first capacitor C1 or the second capacitor C2.

[0032] The high-potential side of the secondary winding of the first transformer T1 is connected to the first diode D1, and the low-potential side of the secondary winding of the first transformer T1 is connected to the high-potential side of the secondary winding of the second transformer T2, the smoothing capacitor Cout, and the load resistor LOAD.

[0033] The high-potential side of the secondary winding of the second transformer T2 is connected to the low-potential side of the secondary winding of the first transformer T1, the smoothing capacitor Cout, and the load resistor LOAD. The low-potential side of the secondary winding of the second transformer T2 is connected to the second diode D2.

[0034] The first diode D1 has a cathode connected to the high potential side of the secondary winding of the first transformer T1 and an anode connected to a reference voltage, and blocks current flowing from the low potential side to the high potential side of the secondary winding of the first transformer T1.

[0035] The second diode D2 has a cathode connected to the low-potential side of the secondary winding of the second transformer T2 and an anode connected to a reference voltage. The second diode D2 blocks current flowing from the high-potential side to the low-potential side of the secondary winding of the first transformer T1. The first diode D1 and the second diode D2 are examples of rectifying elements.

[0036] The smoothing capacitor Cout has one end connected to the low potential side of the secondary winding of the first transformer T1 and the high potential side of the secondary winding of the second transformer T2, and the other end connected to a reference voltage. The smoothing capacitor Cout smoothes the pulsating currents output from the secondary windings of the first transformer T1 and the second transformer T2.

[0037] The load resistor LOAD has one end connected to the low potential side of the secondary winding of the first transformer T1 and the high potential side of the secondary winding of the second transformer T2, and the other end connected to a reference voltage. The load resistor LOAD consumes the power it supplies.

[0038] The PWM control device 20 controls the on / off conduction states of the leading switch SWLead and the trailing switch SWLag by outputting drive pulses to each of the leading switch SWLead and the trailing switch SWLag. The PWM control device 20 controls the on / off conduction states of the leading switch SWLead and the trailing switch SWLag, thereby causing the switching power supply circuit 10 to perform a desired operation.

[0039] If the output voltage from the secondary side of the switching transformer is suddenly increased to its maximum value during startup of the switching power supply circuit 10, an inrush current (rush current) will be generated in the secondary side capacitance (e.g., smoothing capacitor Cout), which may destroy the transistor. To prevent this, a soft start operation is generally performed. Specifically, the soft start is a function that prevents the generation of an inrush current by gradually increasing the output voltage during startup of the switching power supply circuit 10. [Example of conventional control]

[0040] First, an example of conventional control of the on / off conduction state during soft start will be described. FIG. 13 is a diagram illustrating conventional drive pulses and the accompanying changes in current and voltage during soft start. Note that FIG. 13 is simplified for the purpose of explanation, and shows an example in which the output is increased to the maximum with a few pulses. In reality, the output is gradually increased to the maximum over hundreds or thousands of pulses. Also, dead time is not shown in FIG. 13.

[0041] 13A shows an example of a drive pulse in a conventional control method. The drive pulse in the conventional control method outputs a signal with a pulse width of (D) to the leading switch and (1-D) to the trailing switch. Therefore, at the beginning of the soft start, the duty ratio of the drive pulse input to the leading switch (hereinafter sometimes referred to as the leading pulse) is significantly different from the duty ratio of the drive pulse input to the trailing switch (hereinafter sometimes referred to as the trailing pulse).

[0042] Fig. 13(B) shows the input currents of the first transformer and the second transformer in the conventional control method. Fig. 13(B) shows that in the case of the conventional control method, the current flowing from the first transformer to the second transformer is larger than the current flowing from the second transformer to the first transformer in the early stage of soft start.

[0043] 13C shows an example of the voltages applied to the first diode on the first transformer side and the second diode on the second transformer side in a conventional control system. From FIG. 13C, it can be seen that in the case of the conventional control system, the voltage applied to the second diode is much larger than the voltage applied to the first diode in the early stage of soft start. Therefore, in the case of the conventional control system, a large voltage is applied to the second diode in the early stage of soft start, and therefore it is clear that the withstand voltage of the second diode needs to be increased.

[0044] FIG. 14 shows an example of the voltage across the first diode and the voltage across the second diode in a conventional control system. In FIG. 14, the horizontal axis represents the duty ratio of the lead switch, and the vertical axis represents the voltage across the first or second diode (%). The voltage across the diode is expressed as a percentage because it varies depending on the design of the switching power supply circuit. From FIG. 14, it can be seen that in the case of the conventional control system, the voltage across the second diode is much greater than the voltage across the first diode at the beginning of the soft start. It can also be seen that the voltage across the first diode and the voltage across the second diode become closer to each other from the beginning to the end of the soft start.

[0045] [First Example of Control Method of PWM Control Device According to Embodiment] Next, an example of control of the on / off conduction state according to the embodiment will be described. Fig. 2 is a diagram for explaining a first example of a drive pulse according to the embodiment and the accompanying changes in current and voltage during soft start. Fig. 2 is simplified for the purpose of explanation, as is Fig. 13 .

[0046] FIG. 2A shows a first example of drive pulses output from the PWM control device 20 according to the embodiment. In the drive pulses shown in FIG. 2, the leading pulse is (D) and the trailing pulse is (D). That is, the duty ratio of the leading switch SWLead and the duty ratio of the trailing switch SWLag are substantially the same. In the following description, a control method in which the duty ratios of the leading switch SWLead and the trailing switch SWLag are substantially the same may be simply referred to as a clamp control mode or clamp mode. The term "substantially the same" may include cases where the duty ratios are the same, or cases where the duty ratios are different to the extent that the clamp control mode function can be performed. For example, "substantially the same" may include fluctuations in the duty ratio during control, i.e., cases where the duty ratios are controlled to be the same but the resulting duty ratios of the drive pulses output are different. 2, the conduction state of the trailing switch SWLag is turned on immediately after the conduction state of the leading switch SWLead is turned off. In practice, the conduction state of the trailing switch SWLag may be turned on after a dead time is provided after the conduction state of the leading switch SWLead is turned off.

[0047] 2B shows a first example of the input currents of the first transformer T1 and the second transformer T2 in the drive system according to the embodiment. As can be seen from FIG. 2B, in the first example of the control system according to the embodiment, at the beginning of soft start, the current flowing from the first transformer T1 to the second transformer T2 is approximately the same as the current flowing from the second transformer T2 to the first transformer T1. Note that the first transformer T1 and the second transformer T2 are connected in series, and therefore have the same current value. Therefore, the first transformer T1 and the second transformer T2 can be simply interpreted as indicating the input current of the switching transformer, without distinguishing between them.

[0048] 2C shows an example of the voltage applied to the first diode D1 on the first transformer T1 side and the second diode D2 on the second transformer T2 side in the drive method according to the embodiment. In the first example of the control method according to the embodiment, at the beginning of the soft start, the currents input to the first transformer T1 and the second transformer T2 are approximately the same as shown in FIG. 2B, so the magnitude and duration of the voltage applied to the first diode D1 and the voltage applied to the second diode D2 are approximately the same as shown in FIG. 2C.

[0049] FIG. 3 is a diagram illustrating an example of the voltage applied to the first diode D1 and the voltage applied to the second diode D2 in a first example of a control method using the PWM control device 20 according to the embodiment. In FIG. 3, the horizontal axis represents the duty ratio of the leading switch, and the vertical axis represents the voltage [%] applied to the first diode D1 or the second diode D2. As can be seen from FIG. 3, in the first example of the control method, the voltage applied to the first diode D1 and the voltage applied to the second diode D2 are approximately the same value at the beginning of the soft start. Furthermore, as can be seen from FIG. 3, the voltage applied to the second diode D2 is generally smaller than when the leading switch SWLead operates at a duty ratio of (D) and the trailing switch SWLag operates at a duty ratio of (1-D). Therefore, in the clamp control mode, it is not necessary to increase the breakdown voltage of the second diode D2 simply for the general operation such as soft start, in which one duty ratio is small and the other duty ratio is large. In the following description, the case in which the duty ratio of the leading switch SWLead is (D) and the duty ratio of the trailing switch SWLag is (1-D) may be simply referred to as the steady mode or normal mode.

[0050] In a soft start in a first example of the control method according to the embodiment, the PWM control device 20 switches from the clamp control mode to the steady mode when the duty ratio reaches x. In the first example of the control method according to the embodiment, the PWM control device 20 starts operation in the clamp control mode, in which the duty ratios of the leading switch SWLead and the trailing switch SWLag are small, during soft start (step 1). After starting operation in the clamp control mode, the PWM control device 20 gradually increases the duty ratios of the leading switch SWLead and the trailing switch SWLag (step 2). The PWM control device 20 switches from the clamp control mode to the steady mode when the duty ratios of the leading switch SWLead and the trailing switch SWLag reach or exceed a predetermined threshold (step 3). The predetermined threshold is a value determined by the designer of the power supply system 1 or the PWM control device 20, such as x.

[0051] FIG. 4 is a diagram illustrating an example of a predetermined threshold value according to an embodiment. FIG. 4 illustrates the relationship between the duty ratio of the switching element and the output voltage Vout for each of a steady mode (also referred to as a normal mode) and a clamp control mode (also referred to as a clamp mode). The switching power supply circuit 10 outputs a predetermined output voltage by adjusting the duty ratio of the switching element when the magnitude of the input voltage fluctuates. This control is also referred to as operational compensation. Specifically, when the input voltage is low, the switching power supply circuit 10 increases the duty ratio of the switching element to output a predetermined output voltage. On the other hand, when the input voltage is high, the switching power supply circuit 10 decreases the duty ratio of the switching element to output a predetermined output voltage. The predetermined threshold value is, for example, a value close to the duty ratio when a maximum input voltage at which the switching power supply circuit 10 can perform operational compensation is input, and may be, for example, a value slightly smaller than the duty ratio. By operating in the clamp control mode during a period when the duty ratio is small until the duty ratio enters the range of operational compensation, and operating in the steady mode when the duty ratio enters the range where operational compensation is possible, the switching power supply circuit 10 does not need to select a rectifying element with a high rating just for the short period of time during which a slow start is performed until the range of operational compensation is entered. Note that the method of determining the predetermined threshold described above may also be used to determine a threshold for switching to the clamp control mode during an overload protection operation, i.e., during overcurrent protection or overvoltage protection.

[0052] [Second Example of Control Method of PWM Control Device According to Embodiment] FIG. 5 is a diagram illustrating a second example of a control method by the PWM control device 20 according to the embodiment. In the first example of the control method, the duty ratio of the leading switch SWLead and the duty ratio of the trailing switch SWLag are substantially the same. The second example of the control method differs from the first example of the control method in that the duty ratio of the trailing switch SWLag is greater than the duty ratio of the leading switch SWLead by a set value α. The set value is a value set by the designer of the power supply system 1 or the PWM control device 20. The set value may be, for example, a value equal to or less than the difference between the duty ratio of the leading switch SWLead or the trailing switch SWLag when the switching power supply circuit 10 is operating normally and the duty ratio in the first example of the control method.

[0053] 5 shows a diagram in which the horizontal axis represents the duty ratio of the leading switch and the vertical axis represents the voltage [%] applied to the first diode D1 or the second diode D2. As can be seen from Fig. 5, in the second example of the control method, the voltage applied to the second diode D2 is smaller overall than when the leading switch SWLead operates at a duty ratio of (D) and the trailing switch SWLag operates at a duty ratio of (1-D).

[0054] [Third Example of Control Method of PWM Control Device According to Embodiment] Figure 6 is a diagram showing a third example of a control method by the PWM control device 20 according to the embodiment. In the first example of the control method, an example was described in which the conductive state of the trailing switch SWLag is turned on immediately after the conductive state of the leading switch SWLead is turned off. The third example of the control method differs from the first example of the control method in that the conductive state of the trailing switch SWLag is turned on after a shift time has elapsed since the conductive state of the leading switch SWLead was turned off. Like Figure 2, Figure 6 is simplified for ease of explanation.

[0055] The shift time may be a time that is predetermined by the designer of the power supply system 1 or the PWM control device 20. The shift time is, for example, a time that is equal to or longer than the dead time. The shift time is also determined so that a time is provided during which the conductive state of the leading switch SWLead is turned on after the dead time has elapsed since the conductive state of the trailing switch SWLag was turned off. In other words, the shift time is a time that is equal to or shorter than the time during which the conductive state of the leading switch SWLead is turned off minus two times the dead time. The shift time does not need to be determined strictly, and may be a time of different lengths for each drive pulse.

[0056] The PWM control device 20 switches between the clamp control mode described in the first to third control method examples and the steady mode as necessary. Specifically, the PWM control device 20 controls the on / off conduction states of the leading switch SWLead and the trailing switch SWLag in the steady mode when power is normally supplied to the load resistor LOAD, and in the clamp control mode when the supplied power is to be reduced.

[0057] In the above description, an example has been described in which the PWM control device 20 performs control in the clamp control mode during soft start, i.e., when the switching power supply circuit 10 is started. However, this embodiment is not limited to this example. The PWM control device 20 according to the embodiment may also perform control in the clamp control mode, for example, when performing overload protection, i.e., when protecting against overcurrent or overvoltage. FIG. 7 is a diagram illustrating an example of overload protection. The PWM control device 20 may perform control in the clamp control mode when it receives a detection signal related to an overcurrent or overvoltage from a detection unit (not shown). Specifically, the PWM control device 20 detects the value of a current flowing through an element (e.g., a switching transformer or a rectifier element) in the switching power supply circuit 10. When the detected current value exceeds a predetermined current limit setting, the PWM control device 20 switches the conduction state of the switching element from on to off to suppress the output current or voltage. As a result, the PWM control device 20 reduces the duty ratio during overload protection to protect against the overload. When the duty ratio during overload protection falls below a threshold value shown in FIG. 4 (e.g., duty ratio x), the PWM control device 20 switches from steady mode to clamp control mode. For example, the PWM control device 20 may match the duty ratio of the trailing switch SW Lag, which has a higher duty ratio than the leading switch SWLead, to the duty ratio of the leading switch SWLead. Alternatively, the PWM control device 20 may control the duty ratios of the leading switch SWLead and the trailing switch SW Lag to be smaller than the duty ratios in the steady state. This eliminates the need for the switching power supply circuit 10 to select a rectifying element with a high rating just for the short period of time during which overload protection is performed.

[0058] In the above description, an example in which the first diode D1 and the second diode D2 are used as rectifying elements is shown. However, this embodiment is not limited to this example, and switching elements such as transistors may also be used. By employing switching elements as rectifying elements to perform synchronous rectification, the power supply system 1 can operate with higher efficiency.

[0059] [Configuration Example in Case of Full Bridge Circuit] Fig. 8 is a diagram showing an example of the configuration of a power supply system 1A according to a first modification. The power supply system 1A includes a switching power supply circuit 10A and a PWM control device 20A as components. Note that the description of matters already explained above may be omitted.

[0060] The switching power supply circuit 10A differs from the switching power supply circuit 10 in that it is a full-bridge circuit having four switching elements. The switching power supply circuit 10A also differs from the switching power supply circuit 10 in that it has two switching elements instead of the first diode D1 and the second diode D2.

[0061] The leading switch SWLead is, for example, a high-potential side switching element connected to a high-potential side terminal of the first transformer T1 (first end P1 of the switching transformer) and a low-potential side switching element connected to a low-potential side terminal of the second transformer T2 (second end P2 of the switching transformer). In the following description, the high-potential side leading switch SWLead may be referred to as the first leading switch SWLead1, and the low-potential side leading switch SWLead may be referred to as the second leading switch SWLead2.

[0062] The first leading switch SWLead1 is an n-type semiconductor having a drain connected to the input power supply Vin, a source connected to the trailing switch SWLag and the resonant coil Lr, and a gate connected to the PWM control device 20. The source of the leading switch SWLead is electrically connected to the first end P1 via the resonant coil Lr. That is, the first leading switch SWLead1 is connected in the same manner as the leading switch SWLead in the switching power supply circuit 10, which is a half-bridge circuit.

[0063] The second leading switch SWLead2 is, for example, an n-type semiconductor having a drain connected to the second terminal P2 via the DC blocking capacitor CDCcut, a source connected to a reference voltage, and a gate connected to the PWM control device 20. That is, the second leading switch SWLead2 is provided in place of the second capacitor C2 in the switching power supply circuit 10.

[0064] The trailing switch SWLag is, for example, a high-potential side switching element connected to a high-potential side terminal of the first transformer T1 and a low-potential side switching element connected to a low-potential side terminal of the second transformer T2. In the following description, the high-potential side trailing switch SWLag may be referred to as the first trailing switch SWLag1, and the low-potential side trailing switch SWLag may be referred to as the second trailing switch SWLag2.

[0065] The first trailing switch SWLag1 is, for example, an n-type semiconductor having a drain connected to the input power supply Vin, a source connected to the second terminal P2 via the DC blocking capacitor CDCcut, and a gate connected to the PWM control device 20. The second leading switch SWLead2 is provided in place of the second capacitor C2 in the switching power supply circuit 10.

[0066] The second trailing switch SWLag2 is, for example, an n-type semiconductor having a drain connected to the leading switch SWLead and the resonant coil Lr, a source connected to a reference voltage, and a gate connected to the PWM control device 20. The drain of the trailing switch SWLag is electrically connected to the first terminal P1 via the resonant coil Lr. That is, the second trailing switch SWLag2 is connected in the same manner as the trailing switch SWLag in the switching power supply circuit 10, which is a half-bridge circuit.

[0067] When the first capacitor C1 is replaced with the first trailing switch SWLag1 and the second capacitor C2 is replaced with the second leading switch SWLead2, the switching power supply circuit 10A further includes a DC cut capacitor CDCcut, which prevents excessive current from flowing through the switching transformer.

[0068] The secondary-side switching element provided in the switching power supply circuit 10A in place of the first diode D1 is a first rectifier transistor SR1. The first rectifier transistor SR1 is an n-type semiconductor having a drain connected to the high-potential terminal of the secondary winding of the first transformer T1, a source connected to a reference voltage, and a gate connected to the PWM control device 20.

[0069] The secondary-side switching element provided in the switching power supply circuit 10A in place of the second diode D2 is a second rectifier transistor SR2. The second rectifier transistor SR2 is an n-type semiconductor having a drain connected to the low-potential terminal of the secondary winding of the second transformer T2, a source connected to a reference voltage, and a gate connected to the PWM control device 20.

[0070] The PWM control device 20A differs from the PWM control device 20 in that it controls the on / off conduction state by outputting drive pulses to secondary-side switching elements in addition to the leading switch SWLead and the trailing switch SWLag. The PWM control device 20A outputs a drive pulse (signal) to the first rectifier transistor SR1 that is substantially the same as that to the leading switch SWLead, and outputs a drive pulse to the second rectifier transistor SR2 that is substantially the same as that to the trailing switch SWLag. The power supply system 1A operates with higher efficiency by performing synchronous rectification using the PWM control device 20A. Note that the term "substantially the same" may include cases where the duty ratios are identical, or cases where the duty ratios are different enough to allow synchronous rectification. For example, "substantially the same" may include fluctuations in the duty ratio during control, i.e., cases where the duty ratios are controlled to be the same but the resulting duty ratios of the drive pulses are different.

[0071] Although the above description has been given of a case where synchronous rectification is used in the power supply system 1A including a full-bridge circuit, the present embodiment is not limited to this example. For example, the power supply system 1A including a full-bridge circuit may perform rectification using diodes without using synchronous rectification. Furthermore, the power supply system 1A including a half-bridge circuit may perform synchronous rectification using transistors or the like.

[0072] [Configuration Example When OFF Delay is Performed by a Full Bridge Circuit] Figure 9 is a diagram showing an example of the configuration of a power supply system 1B according to a second modification. The power supply system 1B has as its components a switching power supply circuit 10A, a PWM control device 20A, a first OFF Delay circuit 30-1, and a second OFF Delay circuit 30-2. The switching power supply circuit 10A and the PWM control device 20A, which have already been described above, will not be described again. When the first OFF Delay circuit 30-1 and the second OFF Delay circuit 30-2 are not to be distinguished from each other, they may be simply referred to as the OFF Delay circuit 30.

[0073] The OFF-delay circuit 30 has one end connected to the first rectifier transistor SR1 and the second rectifier transistor SR2, which are secondary-side switching elements, and the other end connected to the PWM control device 20A. The OFF-delay circuit 30 delays the OFF timing of the switching elements connected to it. The OFF-delay circuit 30 does not delay the ON timing of the switching elements connected to it.

[0074] Specifically, the first OFF-Delay circuit 30-1 has one end connected to the gate of the first rectifier transistor SR1 and the other end connected to a terminal that outputs a leading pulse among the terminals of the PWM control device 20 A. As a result, the first rectifier transistor SR1 turns on at approximately the same time as the leading switch SWLead, and turns off later than the leading switch SWLead.

[0075] The second OFF-Delay circuit 30-2 has one end connected to the gate of the second rectifier transistor SR2 and the other end connected to a terminal of the PWM control device 20 A that outputs a trailing pulse, so that the second rectifier transistor SR2 turns on substantially at the same time as the trailing switch SWLag and turns off later than the trailing switch SWLag.

[0076] The time by which the OFF-delay circuit 30 delays the turn-off timing is determined by the delay time caused by the resonant coil Lr and the leakage inductance of the switching transformer. Due to the inertia of the resonant coil Lr and the leakage inductance of the switching transformer, which are inserted to achieve soft switching, the current value of the output current from the switching transformer becomes zero with a delay after the drive pulse output from the PWM control device 20 is turned off. In order to output the delayed output current to the load resistor LOAD side, the OFF-delay circuit 30 delays the turn-off timing of the first rectifier transistor SR1 and the second rectifier transistor SR2 by the time the output current flows.

[0077] The PWM control device 20A outputs leading pulses to the first leading switch SWLead1 and the second leading switch SWLead2, and further outputs the leading pulses to the first rectifier transistor SR1 via a first OFF-Delay circuit 30-1. The PWM control device 20B outputs trailing pulses to the first trailing switch SWLag1 and the second trailing switch SWLag2, and further outputs the trailing pulses to the second rectifier transistor SR2 via a second OFF-Delay circuit 30-2.

[0078] FIG. 10 shows an example of a simulation result of the current values ​​of each part of the power supply system 1B according to the second modification. FIG. 10(A) shows the simulation result, with the horizontal axis representing time and the vertical axis representing the preceding pulse, the input current to the first transformer T1 and the second transformer T2, and the output current from the first transformer T1. FIG. 10(B) shows the simulation result, with the horizontal axis representing time and the vertical axis representing the following pulse, the input current to the first transformer T1 and the second transformer T2, and the output current from the second transformer T2. From FIGS. 10(A) and 10(B), it can be seen that the input and output currents of the switching transformers become zero with a delay from the on / off of the drive pulse. The PWM control device 20A and the OFF Delay circuit 30 can delay the off timing of the first rectifier transistor SR1 and the second rectifier transistor SR2 by a time corresponding to this delay, thereby optimizing synchronous rectification.

[0079] In the above, the connection of the leading switch SWLead and the trailing switch SWLag has been described. However, this embodiment is not limited to this example, and the leading switch SWLead and the trailing switch SWLag may refer to switching elements that are reversed. Specifically, the leading switch SWLead may be connected to the position of the trailing switch SWLag, and the trailing switch SWLag may be connected to the position of the leading switch SWLead.

[0080] Summary of the Embodiments According to the above-described embodiment, the PWM control device 20 controls a switching power supply circuit 10 including a switching transformer (e.g., a first transformer T1 and a second transformer T2) and two or more primary-side switching elements including at least a leading switch SWLead and a trailing switch SWLag. The PWM control device 20 controls the conduction states of the leading switch SWLead and the trailing switch SWLag in a clamp control mode in which the duty ratio of the leading switch SWLead and the duty ratio of the trailing switch SWLag are substantially equal. The PWM control device 20 according to the embodiment can solve the problem of asymmetry between leading pulses and trailing pulses. Therefore, the PWM control device 20 prevents the voltage applied to the secondary-side rectifying elements from becoming large, eliminating the need to select rectifying elements with high ratings. Therefore, even a switching power supply circuit 10 using inexpensive elements with a normally low breakdown voltage can be used as a highly efficient power supply. Furthermore, the ability to use rectifying elements with a normally low breakdown voltage reduces losses. Therefore, the PWM control device 20 can provide a highly efficient and low-cost power supply system 1.

[0081] Furthermore, the clamp control mode performed by the PWM control device 20 according to the above-described embodiment can reduce losses in the transformer and rectifier elements on the high-load side without requiring phase-shift control that outputs a large number of signals. This allows the PWM control device 20 to reduce the number of complex control circuits, such as control ICs and gate drivers, thereby reducing costs. While synchronous rectification is used to reduce losses in the rectifier circuit, the power supply system 1 according to the embodiment operates appropriately with only two signals: a leading pulse that controls the leading switch SWLead and a trailing pulse that controls the trailing switch SWLag. This allows the PWM control device 20 to be easily manufactured and at low cost.

[0082] According to the above-described embodiment, the PWM control device 20 controls the switching power supply circuit 10, which includes a switching transformer and two or more primary-side switching elements including at least the leading switch SWLead and the trailing switch SWLag. The PWM control device 20 controls the conduction states of the leading switch SWLead and the trailing switch SWLag in a clamp control mode in which the duty ratio of the trailing switch SWLag is set to a value obtained by adding a set value to the duty ratio of the leading switch SWLead. The PWM control device 20 according to the embodiment can suppress changes in the pulse width of the trailing switch SWLag when switching from the clamp control mode to the steady mode, thereby suppressing changes in the output current and output voltage output from the switching transformer. This allows the power supply system 1 to reduce the effects of switching between the clamp control mode and the steady mode.

[0083] Furthermore, according to the above-described embodiment, the PWM control device 20 controls the conduction states of the leading switch SWLead and the trailing switch SWLag by switching between a steady mode, in which the trailing switch SWLag has a value corresponding to the inverse of the duty ratio of the leading switch SWLead, and a clamp control mode. When the power supply system 1 operates in a steady state and at high output, the duty ratios of the leading switch SWLead and the trailing switch SWLag are both close to 50%, so the impact of asymmetry is relatively small. On the other hand, when the power supply system 1 operates at low output, the duty ratios of the leading switch SWLead and the trailing switch SWLag are both far from 50%, so the impact of asymmetry is significant. By switching to the clamp control mode when the power supply system 1 operates at low output, the power supply system 1 can resolve the asymmetry issue at low output while maintaining high output. Therefore, the PWM control device 20 according to the embodiment can provide a highly efficient and low-cost power supply system 1.

[0084] Furthermore, according to the above-described embodiment, when the switching power supply circuit 10 starts up, the PWM control device 20 starts controlling the conduction states of the leading switch SWLead and the trailing switch SWLag in the clamp control mode, in which the duty ratios of the leading switch SWLead and the trailing switch SWLag are small. Then, the PWM control device 20 gradually increases the duty ratios of the leading switch SWLead and the trailing switch SWLag. When the duty ratio of the leading switch SWLead or the trailing switch SWLag reaches or exceeds a predetermined threshold, the PWM control device 20 switches from the clamp control mode to the steady mode, thereby controlling the conduction states of the leading switch SWLead and the trailing switch SWLag. That is, the PWM control device 20 according to the embodiment controls the conduction states in the clamp control mode during soft start. This solves the problem of asymmetry, even when output needs to be throttled. Furthermore, since the switching power supply circuit 10 performs a special operation called soft start, which has a short operating time compared to the overall operation, it is not necessary to select a rectifying element with a high rating. Therefore, the PWM control device 20 according to the embodiment can provide a highly efficient and low-cost power supply system 1.

[0085] Furthermore, according to the above-described embodiment, when the PWM control device 20 is performing a protective operation against an overload of the switching power supply circuit 10, the PWM control device 20 switches from the steady mode to a clamp control mode in which the duty ratios of the leading switch SWLead and the trailing switch SWLag are small, thereby controlling the conduction states of the leading switch SWLead and the trailing switch SWLag. This solves the problem of asymmetry even in a state in which output throttling is required, and therefore the switching power supply circuit 10 does not need to select a rectifying element with a high rating for the protective operation, a special operation whose operating time is short compared to the overall operation. Therefore, the PWM control device 20 according to the embodiment can provide a highly efficient and low-cost power supply system 1.

[0086] Furthermore, according to the above-described embodiment, the switching power supply circuit 10 includes a half-bridge circuit on the primary side that includes two switching elements, and one of the leading switch SWLead and the trailing switch SWLag is a high-potential-side switching element of the half-bridge circuit, while the other of the leading switch SWLead and the trailing switch SWLag is a low-potential-side switching element of the half-bridge circuit. That is, the PWM control device 20 controls the switching power supply circuit 10 that includes the half-bridge circuit. This allows the power supply system 1 to have a reduced number of components, resulting in lower costs and a smaller size.

[0087] Furthermore, according to the above-described embodiment, the switching power supply circuit 10A includes a full-bridge circuit on the primary side that includes four switching elements, and one of the leading switch SWLead and the trailing switch SWLag is a high-potential side switching element connected to the first terminal P1 of the switching transformer and a low-potential side switching element connected to the second terminal P2 of the switching transformer, while the other of the leading switch SWLead and the trailing switch SWLag is a low-potential side switching element connected to the first terminal P1 of the switching transformer and a high-potential side switching element connected to the second terminal P2 of the switching transformer. In other words, the PWM control device 20 controls the switching power supply circuit 10A that includes the full-bridge circuit. This allows the power supply system 1 to reduce the load on each switching element on the primary side and support higher output.

[0088] Furthermore, according to the above-described embodiment, the magnetic core of the switching transformer has a magnetic permeability of 15 or more and 120 or less when the magnetic flux density is 0.3 T. This makes it difficult for the magnetization of the switching transformer to saturate, and energy loss is less likely to occur. This makes it possible to reduce the size and improve the efficiency of the above-described switching power supply circuit 10. The above-described DC voltage conversion circuit can meet the demand for miniaturization.

[0089] According to the above-described embodiment, the power supply system 1B includes the above-described PWM control device 20A, the above-described switching power supply circuit 10A, and an OFF-Delay circuit 30, one end of which is connected to a switching element connected to the secondary side of the switching power supply circuit 10 and which delays the turn-off timing of the secondary-side switching element. The PWM control device 20A outputs to the other end of the OFF-Delay circuit 30 a signal substantially identical to a signal controlling the conduction state of the leading switch SWLead and the trailing switch SWLag. Due to inertia caused by the magnetic field of the resonant coil Lr inserted to achieve soft switching, the current value of the output current from the switching transformer becomes zero with a delay after the drive pulse output from the PWM control device 20 is turned off. According to the power supply system 1B according to the embodiment, the turn-off timing of the first rectifier transistor SR1 and the second rectifier transistor SR2 is delayed by the time during which the output current flows, so that the delayed output current can be output to the load resistor LOAD. Therefore, the power supply system 1B according to this embodiment can achieve high efficiency by further optimizing synchronous rectification.

[0090] [Fourth Example of Control Method of PWM Control Device According to Embodiment] A fourth example of a control method by the PWM control device 20 according to the embodiment will be described with reference to FIGS. 11 and 12. FIG.

[0091] FIG. 11 is a diagram illustrating a fourth example of a control method by the PWM control device 20 according to the embodiment. The fourth example of the control method differs from the first and second examples of the control method in that the duty ratio of the trailing switch SW Lag is a value obtained by multiplying the duty ratio of the leading switch SWLead by a set value α. That is, when the duty ratio of the leading switch SWLead is (D), the duty ratio of the trailing switch SW Lag operates at (D × α). Note that the description of matters already described in the first and second examples of the control method may be omitted. Note that while FIGS. 3, 5, and 14 partially omit the relationship between the voltage applied to the rectifying element and the duty ratio in steady mode, the relationship is the same as that in FIG. 11.

[0092] 11 shows a diagram in which the horizontal axis represents the duty ratio of the leading switch and the vertical axis represents the voltage [%] applied to the first diode D1 or the second diode D2. As can be seen from Fig. 11, in the fourth example of the control method, the voltage applied to the second diode D2 is smaller overall than when the leading switch SWLead operates at a duty ratio of (D) and the trailing switch SWLag operates at a duty ratio of (1-D).

[0093] 12 shows the relationship between the duty ratio of the switching element and the output voltage Vout in the fourth example of the control method for each of the normal mode, the first example of the control method, and the fourth example of the control method. The fourth example of the control method changes the slope of the line showing the relationship between the duty ratio of the switching element and the output voltage Vout in the first example of the control method. This makes it possible to reduce the amount of change in the output voltage when switching from the clamp control mode to the steady mode.

[0094] The set value according to the fourth example of the control method is, for example, a value smaller than the value obtained by dividing the duty ratio of the trailing switch SWLag in steady mode by the duty ratio of the leading switch SWLead in steady mode. However, of the two switching elements, the switching element with the smaller duty ratio is designated as the leading switch SWLead. This allows the PWM control device 20 to reduce the amount of change in output voltage when switching from the clamp control mode to the steady mode while suppressing asymmetry. Note that when the set value is 1, the fourth example of the control method is the same as the first example of the control method.

[0095] Generally, a PWM control device determines the duty ratio of one of two switching elements included in a switching power supply circuit based on the duty ratio of the other switching element, and performs processing. Here, the "one switching element" is generally a switching element whose output voltage Vout of the switching power supply circuit increases as the duty ratio increases, in order to simplify calculations by the PWM control device. That is, the PWM control device mainly controls the duty ratio of the two switching elements included in the switching power supply circuit between 0% and 50% and performs various calculations based on the duty ratio of the switching element with the smaller duty ratio (without taking dead time into consideration). For the above reasons, the PWM control device 20 according to the embodiment performs processing such as determining the duty ratio ((1-D), D, (D×α), etc.) of the trailing switch SWLag based on the duty ratio (D) of the leading switch SWLead. However, if the set value is smaller than 1, the duty ratio (D×α) of the trailing switch SW Lag will be smaller than the duty ratio (D) of the leading switch SWLead. In this case, the PWM control device 20 performs calculations based on the duty ratio of the trailing switch SWLead in the clamp control mode according to the fourth example of the control method, and performs calculations based on the duty ratio of the leading switch SWLead in the steady mode, which makes the calculations more complicated. Therefore, it is more desirable that the set value be greater than 1.

[0096] [Summary of the Fourth Example of the Control Method] According to the above-described embodiment, the PWM control device 20 controls the switching power supply circuit 10 including a switching transformer and two or more primary-side switching elements including at least the leading switch SWLead and the trailing switch SWLag. The PWM control device 20 controls the conduction states of the leading switch SWLead and the trailing switch SWLag in a clamp control mode in which the duty ratio of the trailing switch SWLag is set to a value obtained by multiplying the duty ratio of the leading switch SWLead by a set value. The PWM control device 20 according to the embodiment can suppress changes in the pulse width of the trailing switch SWLag when switching from the clamp control mode to the steady mode, thereby suppressing changes in the output current and output voltage output from the switching transformer. This allows the power supply system 1 to reduce the impact of switching between the clamp control mode and the steady mode.

[0097] The functions of the PWM control device 20 and the OFF-delay circuit 30 may be implemented using electronic circuits as necessary. Furthermore, the functional units that implement these functions do not need to be included in a single device, and the PWM control device 20 may be configured from multiple devices. At least some of the functions of the PWM control device 20 may be implemented using an MCU (Micro Controller Unit) or the like. The MCU includes a central processing unit (CPU), RAM, and I / O (Input / Output). The MCU may be implemented using existing technology. The central processing unit writes data to RAM, reads data from RAM, and performs arithmetic and logical operations according to instructions. The RAM stores data and programs. Each element in the RAM has an address and can be accessed using the address. RAM is an abbreviation for "random access memory." The I / O (Input / Output) is a port through which the central processing unit exchanges data with external input / output devices. Furthermore, all or part of the functional units of the PWM control device 20 may be realized using hardware such as an ASIC, PLD, FPGA, or dedicated IC. Furthermore, all or part of each functional unit may be realized by a combination of software and hardware.

[0098] In addition, all or part of the functions of the components of the PWM control device 20 and the OFF-DELAY circuit 30 in the above-described embodiment may be realized by recording a program for realizing these functions on a computer-readable recording medium, and reading and executing the program recorded on the recording medium into a computer system. Note that the term "computer system" here includes hardware such as an OS and peripheral devices.

[0099] Furthermore, "computer-readable recording media" refers to portable media such as flexible disks, optical magnetic disks, ROMs, and CD-ROMs, as well as recording units such as hard disks built into computer systems. Furthermore, "computer-readable recording media" may also include devices that dynamically store programs for a short period of time, such as communication lines when transmitting programs via networks such as the Internet or communication lines such as telephone lines, and devices that store shift time programs, such as volatile memory within computer systems that serve as servers or clients in such cases. Furthermore, the above-mentioned programs may be programs that realize some of the aforementioned functions, or may be programs that can realize the aforementioned functions in combination with programs already stored in the computer system.

[0100] Although one embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to the above, and various design modifications can be made without departing from the spirit of the present invention. Furthermore, the configurations described in the above-described embodiments and examples can be combined.

[0101] According to the present invention, it is possible to provide a highly efficient and low-cost switching power supply circuit and power supply system.

[0102] REFERENCE SIGNS LIST 1...power supply system, 10...switching power supply circuit, 20...PWM control device, T1...first transformer, T2...second transformer, Vin...input power supply, Cin...input capacitor, SWLead...leading switch, SWLag...following switch, Lr...resonant coil, C1...first capacitor, C2...second capacitor, D1...first diode, D2...second diode, Cout...smoothing capacitor..., LOAD...load resistor, SR1...first rectifier transistor, SR2...second rectifier transistor, CDCcut...DC cut capacitor, 30...OFF Delay circuit

Claims

1. A PWM control device that controls a switching power supply circuit having a switching transformer and two or more primary-side switching elements including at least a leading switch and a trailing switch, wherein the PWM control device controls the conduction states of the leading switch and the trailing switch in a clamp control mode in which the duty ratio of the leading switch and the duty ratio of the trailing switch are substantially the same.

2. The PWM control device according to claim 1, wherein the conduction states of the leading switch and the trailing switch are controlled by switching between a steady mode in which the duty ratio of the trailing switch is set to a value corresponding to the inverse of the duty ratio of the leading switch, and the clamp control mode.

3. The PWM control device according to claim 2, wherein, at the start of the switching power supply circuit, control of the conduction states of the leading switch and the trailing switch is started in the clamp control mode in which the duty ratio of the leading switch and the duty ratio of the trailing switch are small, and then the duty ratio of the leading switch and the duty ratio of the trailing switch are gradually increased, and when the duty ratio of the leading switch or the duty ratio of the trailing switch becomes equal to or greater than a predetermined threshold, the PWM control device switches from the clamp control mode to the steady mode to control the conduction states of the leading switch and the trailing switch.

4. The PWM control device according to claim 2, wherein, during a protection operation against an overload of the switching power supply circuit, the steady mode is switched to the clamp control mode in which the duty ratio of the leading switch and the duty ratio of the trailing switch are small, thereby controlling the conduction states of the leading switch and the trailing switch.

5. The PWM control device according to claim 1, wherein the switching power supply circuit includes a half-bridge circuit on the primary side that includes two of the switching elements, one of the leading switch and the trailing switch being the switching element on the high potential side of the half-bridge circuit, and the other of the leading switch and the trailing switch being the switching element on the low potential side of the half-bridge circuit.

6. The PWM control device according to claim 1, wherein the switching power supply circuit comprises a full-bridge circuit on the primary side that includes four of the switching elements, one of the leading switch or the trailing switch being the switching element on the high potential side that is connected to a first end of the switching transformer and the switching element on the low potential side that is connected to a second end of the switching transformer, and the other of the leading switch or the trailing switch being the switching element on the low potential side that is connected to the first end of the switching transformer and the switching element on the high potential side that is connected to the second end of the switching transformer.

7. The PWM control device according to claim 1, wherein the magnetic core of the switching transformer has a magnetic permeability of 15 or more and 120 or less when the magnetic flux density is 0.3 T.

8. A power supply system comprising: the PWM control device according to claim 1; the switching power supply circuit; and an OFF-DELAY circuit, one end of which is connected to a switching element connected to the secondary side of the switching power supply circuit, for delaying the OFF timing of the switching element on the secondary side, wherein the PWM control device outputs a signal that is substantially the same as a signal that controls the conductive states of the leading switch and the trailing switch to the other end of the OFF-DELAY circuit.

9. A PWM control device that controls a switching power supply circuit including a switching transformer and two or more primary-side switching elements including at least a leading switch and a trailing switch, wherein the PWM control device controls the conduction states of the leading switch and the trailing switch in a clamp control mode in which the duty ratio of the trailing switch is set to a value obtained by multiplying the duty ratio of the leading switch by a set value.

Images