PWM control device and power supply system

Abstract

To provide highly efficient and low-cost switching power supply circuits and power supply systems. [Solution] The PWM control device controls a switching power supply circuit comprising a switching transformer and two or more primary-side switching elements, including at least a leading switch and a trailing switch, and controls the conduction state of the leading switch and the trailing switch in a clamp control mode in which the duty cycle of the leading switch and the duty cycle of the trailing switch are substantially the same.

Description

Technical Field

[0001] The present invention relates to a PWM control device and a power supply system.

Background Art

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

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

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the case of the above-described technology, there is a problem that the power values output from each transformer are biased due to the difference in the duty ratio. When the current values output from each transformer are different, a higher voltage is applied to the rectifying element on the transformer side with a larger output current than to the rectifying element on the transformer side with a smaller output current, and it is necessary to select a rectifying element with a high withstand voltage and current rating.

[0006] Under normal operation, setting the duty cycle close to 0.5 mitigates asymmetry and reduces the power imbalance handled by one transformer. However, in cases such as soft start or overload protection, the duty cycle is reduced, increasing the asymmetry. In this case, the rectifier element on the transformer with a high output current is subjected to a higher voltage than the rectifier element on the transformer with a low output current. This further exacerbates the bias in selecting rectifier elements with high voltage and current ratings. Switching power supply circuits require the selection of high-rated rectifier elements for operations that involve reducing the duty cycle, which accounts for a small proportion of the total operating time. This results in higher manufacturing costs and increased losses because high-voltage elements typically have reduced forward characteristics.

[0007] Therefore, the present invention has been made in view of the above points, and aims to provide a highly efficient and low-cost switching power supply circuit and power supply system. [Means for solving the problem]

[0008] One aspect of the present invention is a PWM control device for controlling a switching power supply circuit comprising 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 state of the leading switch and the trailing switch in a clamp control mode in which the duty cycle of the leading switch and the duty cycle of the trailing switch are substantially the same.

[0009] Furthermore, in one embodiment of the present invention, the PWM control device controls the conduction state of the preceding switch and the following switch by switching between a steady-state mode in which the duty cycle of the trailing switch is set to a value corresponding to the reciprocal of the duty cycle of the preceding switch, and the clamp control mode.

[0010] Furthermore, in one embodiment of the present invention, the PWM control device starts controlling the conduction state of the leading switch and the trailing switch in the clamp control mode when the switching power supply circuit is started, in which the duty cycle of the leading switch and the duty cycle of the trailing switch are small. Then, it gradually increases the duty cycle of the leading switch and the duty cycle of the trailing switch, and when the duty cycle of the leading switch or the duty cycle of the trailing switch exceeds a predetermined threshold, it switches from the clamp control mode to the steady-state mode and controls the conduction state of the leading switch and the trailing switch.

[0011] Furthermore, in one embodiment of the present invention, the PWM control device 20, during the protection operation against overload of the switching power supply circuit, switches from the steady-state mode to the clamp control mode in which the duty cycle of the leading switch and the duty cycle of the trailing switch are small, thereby controlling the conduction state of the leading switch and the trailing switch.

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

[0013] Furthermore, in one embodiment of the present invention, the switching power supply circuit includes a full bridge circuit on the primary side comprising four of the switching elements, wherein one of the leading switch or the trailing switch comprises a high-potential switching element connected to the first end of the switching transformer and a low-potential switching element connected to the second end of the switching transformer, and the other of the leading switch or the trailing switch comprises a low-potential switching element connected to the first end of the switching transformer and a high-potential switching element connected to the second end of the switching transformer.

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

[0015] Furthermore, one aspect of the present invention is a power supply system comprising the above-described PWM control device, the switching power supply circuit, and an OFFDelay circuit, one end of which is connected to a switching element connected to the secondary side of the switching power supply circuit, and which delays the off timing of the switching element on the secondary side, wherein the PWM control device outputs a signal substantially identical to the signal that controls the conduction state of the preceding switch and the following switch to the other end of the OFFDelay circuit.

[0016] Furthermore, one aspect of the present invention is a PWM control device for controlling a switching power supply circuit including a switching transformer and two or more primary-side switching elements, at least one of which include a leading switch and a trailing switch, wherein the PWM control device controls the conduction state of the leading switch and the trailing switch in a clamp control mode in which the duty cycle of the trailing switch is set to a value obtained by multiplying the duty cycle of the leading switch by a set value. [Effects of the Invention]

[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. [Brief explanation of the drawing]

[0018] [Figure 1] This figure shows an example of the configuration of a power supply system according to the embodiment. [Figure 2] This figure illustrates a first example of a drive pulse according to the embodiment, and the resulting changes in current and voltage during soft start. [Figure 3] This figure shows an example of the voltage applied to the first diode and the voltage applied to the second diode in a first example of a control method using a PWM control device according to the embodiment. [Figure 4] It is a diagram for explaining an example of a predetermined threshold value according to an embodiment. [Figure 5] It is a diagram showing a second example of a control method by a PWM control device according to an embodiment. [Figure 6] It is a diagram showing a third example of a control method by a PWM control device according to an embodiment. [Figure 7] It is a diagram for explaining an example of an overload protection operation. [Figure 8] It is a diagram showing an example of the configuration of a power supply system according to a first modification example. [[ID=1...]] [Figure 9] It is a diagram showing an example of the configuration of a power supply system according to a second modification example. [Figure 10] It is an example of a simulation result of current values of each part of a power supply system according to a second modification example. [Figure 11] It is a diagram showing a fourth example of a control method by a PWM control device according to an embodiment. [Figure 12] In the fourth example of the control method, the relationship between the duty ratio of the switching element and the output voltage Vout is shown for each of the normal mode, the first example of the control method, and the fourth example of the control method. [Figure 13] It is a diagram for explaining a conventional drive pulse and the accompanying changes in current and voltage during soft start. [Figure 14] It is a diagram showing an example of the voltage applied to the first diode and the voltage applied to the second diode in a conventional control method.

Embodiments for Carrying Out the Invention

[0019] The PWM control device and power supply system according to this embodiment will be described in detail below with reference to the attached drawings, with reference to preferred embodiments. In the drawings, identical or similar parts are denoted by the same or similar reference numerals. This embodiment is not limited to these embodiments and includes various modifications or 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 the same, and the components described below can be combined as appropriate. Furthermore, this embodiment may include various omissions, substitutions, or modifications of components without departing from the spirit of the present invention.

[0020] [Example configuration for a half-bridge circuit] Figure 1 shows an example of the configuration of a power supply system 1 according to an embodiment. The power supply system 1 comprises a switching power supply circuit 10 and a PWM control device 20. Note that the circuit diagram shown in Figure 1 is merely an example, and this 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, for example, by 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 side 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 comprises at least a switching transformer, two or more primary-side switching elements, and a resonant coil Lr as its components. In Figure 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 accordance with the control of the PWM control device 20.

[0023] The leading switch SWLead is an n-type semiconductor comprising, for example, 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 terminal of the first transformer T1 via the resonant coil Lr.

[0024] The trailing switch SWLag is an n-type semiconductor comprising, for example, 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 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 Figure 1, the switching transformers are the first transformer T1 and the second transformer T2. The primary windings of the first transformer T1 and the second transformer T2 are connected by DC. The secondary windings of the first transformer T1 and the second transformer T2 may be connected by DC or in parallel. In Figure 1, the first transformer T1 and the second transformer T2 are connected in parallel. Also in Figure 1, the turns ratio (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 approximately the same. Furthermore, 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]. This 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). Furthermore, it is particularly preferable that the magnetic permeability of the magnetic core of the switching transformer is 24 to 90 when the magnetic flux density is 0.3 [T]. In addition, it is particularly preferable to use a material with low initial permeability of 15 to 120 for the magnetic core of the switching transformer in order to suppress heat generation, i.e., loss, due to leakage magnetic flux in the gap. Note that permeability refers to relative permeability.

[0027] In Figure 1, the secondary windings of the first transformer T1 and the second transformer T2 are wound with opposite polarity to the primary windings. Therefore, when 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 will also flow from the high potential side to the low potential side. Conversely, when 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 will also flow from the low potential side to the high potential side.

[0028] Similar to the first transformer T1, the second transformer T2 allows current to flow from the high-potential side to the low-potential side of its secondary winding when current flows from the high-potential side to the low-potential side of its primary winding. Conversely, when current flows from the low-potential side to the high-potential side of the primary winding of the second transformer T2, current flows from the low-potential side to the high-potential side of its secondary winding.

[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 a 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 terminal on the high-potential side of the primary winding of the first transformer T1 may be simply referred to as the first terminal P1 or the first terminal 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 the 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 preceding switch SWLead, and the other end connected to the second capacitor C2 and the second terminal P2. The second capacitor C2 has one end connected to the first capacitor C1 and the second terminal P2, and the other end connected to the reference voltage. The first capacitor C1 and the second capacitor C2 are half-bridge capacitors, which are 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 have both the first capacitor C1 and the second capacitor C2. The switching power supply circuit 10 only needs to have 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. 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 cathode of the first diode D1 is connected to the high-potential side of the secondary winding of the first transformer T1, and the anode is connected to the reference voltage. The first diode D1 blocks the current flowing from the low-potential side to the high-potential side of the secondary winding of the first transformer T1.

[0035] The cathode of the second diode D2 is connected to the low-potential side of the secondary winding of the second transformer T2, and the anode is connected to the reference voltage. The second diode D2 blocks the 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 rectifier 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 smooths the pulsating current 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 the reference voltage. The load resistor LOAD consumes the power supplied.

[0038] The PWM control device 20 controls the on / off conduction state of the leading switch SWLead and the trailing switch SWLag by outputting drive pulses for each of them. By controlling the on / off conduction state of the leading switch SWLead and the trailing switch SWLag, the PWM control device 20 causes the switching power supply circuit 10 to perform the desired operation.

[0039] If the output voltage from the secondary side of the switching transformer is abruptly increased to its maximum during startup of the switching power supply circuit 10, an inrush current (rush current) will occur relative to the capacitance of the secondary side (for example, the smoothing capacitor Cout), which may damage the transistor. To prevent this, a soft start operation is commonly performed. Specifically, a soft start is a function that prevents inrush current from occurring by gradually increasing the output voltage when the switching power supply circuit 10 starts up. [Conventional control examples]

[0040] First, we will explain an example of controlling the conventional on / off conduction state during soft start. Figure 13 is a diagram illustrating the conventional drive pulses and the resulting changes in current and voltage during soft start. Note that Figure 13 is a simplified diagram for illustrative purposes and shows an example where the output is increased to maximum in a few pulses. In reality, the output is gradually increased to maximum over hundreds or thousands of pulses. Also, dead time is not shown in Figure 13.

[0041] Figure 13(A) shows an example of a drive pulse in a conventional control system. In the conventional control system, the drive pulse outputs a signal with a pulse width of (D) to the leading switch and (1-D) to the trailing switch. Therefore, in the early stages of a soft start, the duty cycle of the drive pulse input to the leading switch (hereinafter sometimes referred to as the leading pulse) and the duty cycle of the drive pulse input to the trailing switch (hereinafter sometimes referred to as the trailing pulse) are significantly different.

[0042] Figure 13(B) shows the input currents to the first and second transformers in a conventional control system. From Figure 13(B), it can be seen that in the conventional control system, 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 during the initial stages of soft start.

[0043] Figure 13(C) 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 Figure 13(C), it can be seen that in the conventional control system, the voltage applied to the second diode is significantly larger than the voltage applied to the first diode in the initial stages of soft start. Therefore, in the conventional control system, a large voltage is applied to the second diode in the initial stages of soft start, which necessitates increasing the voltage rating of the second diode.

[0044] Figure 14 shows an example of the voltages across the first and second diodes in a conventional control method. In Figure 14, the horizontal axis represents the duty cycle of the preceding switch, and the vertical axis represents the voltage across the first or second diode [%]. Note that the voltage across the diodes changes depending on the design of the switching power supply circuit, so it is expressed as a percentage. From Figure 14, it can be seen that in the conventional control method, the voltage across the second diode is significantly larger than the voltage across the first diode at the beginning of the soft start. It can also be seen that the voltages across the first and second diodes become closer from the beginning to the end of the soft start.

[0045] [First example of a control method for a PWM control device according to an embodiment] Next, an example of controlling the on / off conduction state according to the embodiment will be described. Figure 2 is a diagram illustrating a first example of a drive pulse according to the embodiment and the resulting changes in current and voltage during soft start. As with Figure 13, Figure 2 has been simplified for illustrative purposes.

[0046] Figure 2(A) shows a first example of a drive pulse output from the PWM control device 20 according to the embodiment. In the drive pulse shown in Figure 2, the leading pulse is (D) and the trailing pulse is (D). That is, the duty cycle of the leading switch SWLead and the duty cycle of the trailing switch SWLag are approximately the same. In the following description, a control method in which the duty cycle of the leading switch SWLead and the duty cycle of the trailing switch SWLag are approximately the same may be simply referred to as clamp control mode or clamp mode. The range of approximately the same may include cases where they are identical, or cases where they differ to the extent that the function of clamp control mode can be performed. For example, approximately the same may include fluctuations in the duty cycle during control, that is, cases where control is performed so that the duty cycle is the same, but the duty cycle of the resulting output drive pulses is different. In the clamp control mode shown in Figure 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, there may be a dead time after the leading switch SWLead turns off before the trailing switch SWLag turns on.

[0047] Figure 2(B) 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. From Figure 2(B), in the first example of the control system according to the embodiment, in the initial stages of soft start, the current flowing from the first transformer T1 to the second transformer T2 is approximately the same in magnitude as the current flowing from the second transformer T2 to the first transformer T1. Since the first transformer T1 and the second transformer T2 are connected in series, they have the same current value. Therefore, it can be understood that this simply represents the input current of the switching transformers without distinguishing between the first transformer T1 and the second transformer T2.

[0048] Figure 2(C) shows an example of the voltages 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 system according to the embodiment. In the first example of the control system according to the embodiment, in the initial stages of soft start, as shown in Figure 2(B), the currents input to the first transformer T1 and the second transformer T2 are approximately the same. Therefore, as shown in Figure 2(C), the magnitude and duration of the voltages applied to the first diode D1 and the second diode D2 are approximately the same.

[0049] Figure 3 shows an example of the voltage across the first diode D1 and the voltage across the second diode D2 in a first example of the control method by the PWM control device 20 according to the embodiment. In Figure 3, the horizontal axis is the duty cycle of the leading switch, and the vertical axis is the voltage across the first diode D1 or the second diode D2 [%]. From Figure 3, it can be seen that in the first example of the control method, in the initial stages of soft start, the voltage across the first diode D1 and the voltage across the second diode D2 are approximately the same value. Also, from Figure 3, the voltage across the second diode D2 is generally smaller compared to the case where the duty cycle of the leading switch SWLead is (D) and the duty cycle of the trailing switch SWLag is (1-D). Therefore, with clamp control mode, it is not necessary to increase the voltage rating of the second diode D2 simply because it is a common operation such as soft start where one duty cycle becomes smaller and the other duty cycle becomes larger. In the following explanation, the case where the duty cycle of the leading switch SWLead is (D) and the duty cycle of the trailing switch SWLag is (1-D) may simply be referred to as steady mode or normal mode.

[0050] In the soft start of the first example of the control method according to the embodiment, the PWM control device 20 switches from clamp control mode to steady mode when the duty cycle becomes s. In the soft start of the first example of the control method according to the embodiment, the PWM control device 20 starts operating in clamp control mode where the duty cycles of the leading switch SWLead and the trailing switch SWLag are small (first step). After starting to operate in clamp control mode, the PWM control device 20 gradually increases the duty cycles of the leading switch SWLead and the trailing switch SWLag (second step). The PWM control device 20 switches from clamp control mode to steady mode when the duty cycles of the leading switch SWLead and the trailing switch SWLag become greater than or equal to a predetermined threshold (third step). The predetermined threshold is a value determined by the designer of the power supply system 1 or the PWM control device 20, for example, x.

[0051] Figure 4 is a diagram illustrating an example of a predetermined threshold value according to the embodiment. Figure 4 shows the relationship between the duty cycle of the switching element and the output voltage Vout for both the steady-state mode (also called the normal mode) and the clamp control mode (also called the clamp mode). The switching power supply circuit 10 outputs an output voltage of a specified magnitude by adjusting the duty cycle of the switching element when the magnitude of the input voltage fluctuates. This type of control is also called operation compensation. Specifically, when the input voltage is low, the switching power supply circuit 10 increases the duty cycle of the switching element in order to output an output voltage of a specified magnitude. On the other hand, when the input voltage is high, the switching power supply circuit 10 decreases the duty cycle of the switching element in order to output an output voltage of a specified magnitude. The predetermined threshold value is, for example, a value close to the duty cycle when the maximum input voltage at which the switching power supply circuit 10 can perform operation compensation is input, and may be, for example, a value slightly smaller than that duty cycle. By operating in clamp control mode during periods of low duty cycles before entering the compensation range, and then operating in steady mode once the compensation range is reached, the switching power supply circuit 10 does not need to select a rectifier element with a high rating solely for the short period of slow start before entering the compensation range. The method for determining the predetermined threshold described above may also be used to determine the threshold for switching to clamp control mode during overload protection operations, i.e., during overcurrent protection or overvoltage protection.

[0052] [Second example of a control method for a PWM control device according to an embodiment] Figure 5 shows a second example of a control method using the PWM control device 20 according to the embodiment. In the first example of the control method, the duty cycle of the leading switch SWLead and the duty cycle of the trailing switch SWLag are approximately the same. In the second example of the control method, the duty cycle of the trailing switch SWLag is greater than the duty cycle of the leading switch SWLead by a set value α, which is different from the first example of the control method. 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 less than or equal to the difference between the duty cycle of the leading switch SWLead or trailing switch SWLag when the switching power supply circuit 10 is operating normally and the duty cycle in the first example of the control method.

[0053] Figure 5 shows a graph where the horizontal axis represents the duty cycle of the leading switch, and the vertical axis represents the voltage [%] across the first diode D1 or the second diode D2. From Figure 5, it can be seen that in the second example of the control method, the voltage across the second diode D2 is generally smaller compared to the case where the duty cycle of the leading switch SWLead is (D) and the duty cycle of the trailing switch SWLag is (1-D).

[0054] [Third example of a control method for a PWM control device according to an embodiment] Figure 6 shows a third example of the 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 conduction state of the trailing switch SWLag is turned on immediately after the conduction state of the leading switch SWLead is turned off. In the third example of the control method, the conduction state of the trailing switch SWLag is turned on after a shift time has elapsed since the conduction state of the leading switch SWLead was turned off. As with Figure 2, Figure 6 has been simplified for illustrative purposes.

[0055] The shift time may be a time predetermined by the designer of the power supply system 1 or the PWM control device 20. The shift time is, for example, a time longer than or equal to the dead time. Furthermore, it is determined that there is a time between the time the leading switch SWLag is turned off and the dead time has elapsed, and the leading switch SWLead is turned on. That is, the shift time is a time longer than or equal to the time the leading switch SWLead is turned off minus the dead time twice. Note that the shift time does not need to be precisely determined and may be a different length for each drive pulse.

[0056] The PWM control device 20 switches between the clamp control mode and the steady-state mode as needed, as described in the first to third examples of the control method. Specifically, the PWM control device 20 controls the on / off conduction state of the leading switch SWLead and the trailing switch SWLag in steady-state mode when supplying power to the load resistor LOAD as usual, and in clamp control mode when reducing the supplied power.

[0057] In the above description, an example was given in which the PWM control device 20 controls in clamp control mode during soft start, that is, 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 this embodiment may, for example, control in clamp control mode during overload protection operations, that is, during overcurrent protection or overvoltage protection. Figure 7 is a diagram illustrating an example of overload protection operation. The PWM control device 20 may control in clamp control mode when it obtains a detection signal related to overcurrent or overvoltage from a detection unit (not shown). Specifically, the PWM control device 20 detects the current value flowing through elements (e.g., switching transformers and rectifier elements) in the switching power supply circuit 10. If the detected current value exceeds a predetermined current limit setting value, the PWM control device 20 switches the conduction state of the switching element from on to off in order to suppress the output current and voltage. As a result, the PWM control device 20 reduces the duty cycle during overload to provide protection against overload. When the duty cycle during overload protection falls below the threshold shown in Figure 4 (for example, duty cycle x), the PWM control device 20 switches from steady-state mode to clamp control mode. For example, the PWM control device 20 may set the duty cycle of the trailing switch SWLag, which has a higher duty cycle than the leading switch SWLead, to match the duty cycle of the leading switch SWLead. Alternatively, the PWM control device 20 may control the duty cycles of both the leading switch SWLead and the trailing switch SWLag to a smaller duty cycle compared to the steady state. This eliminates the need for the switching power supply circuit 10 to select a rectifier element with a high rating solely for the short period of time required for overload protection.

[0058] In the above description, an example of using a first diode D1 and a second diode D2 as rectifier 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 rectifier elements and performing synchronous rectification, the power supply system 1 can operate with higher efficiency.

[0059] [Example configuration for a full-bridge circuit] Figure 8 shows an example of the configuration of the power supply system 1A according to the first modified example. The power supply system 1A has a switching power supply circuit 10A and a PWM control device 20A as its components. Note that some matters already explained above may be omitted from further explanation.

[0060] Switching power supply circuit 10A differs from switching power supply circuit 10 in that it is a full-bridge circuit equipped with four switching elements. Furthermore, switching power supply circuit 10A differs from switching power supply circuit 10 in that it is equipped with two switching transformers instead of the first diode D1 and the second diode D2.

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

[0062] The first leading switch SWLead1 is an n-type semiconductor comprising 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 terminal P1 via the resonant coil Lr. In other words, the first leading switch SWLead1 is connected in the same way 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 an n-type semiconductor comprising, for example, a drain connected to the second terminal P2 via a DC cut capacitor CDCcut, a source connected to a reference voltage, and a drain connected to the PWM control device 20. In other words, 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 switching element connected to the high-potential terminal of the first transformer T1, and a low-potential switching element connected to the low-potential terminal of the second transformer T2. In the following description, the high-potential trailing switch SWLag may be referred to as the first trailing switch SWLag1, and the low-potential trailing switch SWLag may be referred to as the second trailing switch SWLag2.

[0065] The first trailing switch SWLag1 is an n-type semiconductor comprising, for example, a drain connected to the input power supply Vin via a DC blocking capacitor CDCcut, a source connected to the second terminal P2, and a drain connected to the PWM control device 20. The second leading switch SWLead2 is provided in place of the first capacitor C1 in the switching power supply circuit 10.

[0066] The second trailing switch SWLag2 is an n-type semiconductor comprising, for example, 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. In other words, the second trailing switch SWLag2 is connected in the same way as the trailing switch SWLag in the switching power supply circuit 10, which is a half-bridge circuit.

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

[0068] The secondary switching element provided in the switching power supply circuit 10A in place of the first diode D1 is the first rectifier transistor SR1. The first rectifier transistor SR1 is an n-type semiconductor comprising 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 drain connected to the PWM control device 20.

[0069] The secondary switching element provided in the switching power supply circuit 10A in place of the second diode D2 is the second rectifier transistor SR2. The second rectifier transistor SR2 is an n-type semiconductor comprising 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 drain connected to the PWM control device 20.

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

[0071] Although the above description described the case in which synchronous rectification is used in a power supply system 1A equipped with a full-bridge circuit, this embodiment is not limited to this example. For example, a power supply system 1A equipped with a full-bridge circuit may perform rectification using diodes instead of synchronous rectification. Also, a power supply system 1A equipped with a half-bridge circuit may perform synchronous rectification using transistors or the like.

[0072] [Example configuration for OFF-Delay using a full-bridge circuit] Figure 9 shows an example of the configuration of the power supply system 1B according to the second modified example. The power supply system 1B has a switching power supply circuit 10A, a PWM control device 20A, a first OFFDelay circuit 30-1, and a second OFFDelay circuit 30-2 as its components. The switching power supply circuit 10A and the PWM control device 20A, which have already been described above, may be omitted from further explanation. In addition, when the first OFFDelay circuit 30-1 and the second OFFDelay circuit 30-2 are not distinguished, they may simply be referred to as the OFFDelay circuit 30.

[0073] The OFFDelay 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 OFFDelay circuit 30 delays the off timing of the connected switching elements. The OFFDelay circuit 30 does not delay the on timing of the connected switching elements.

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

[0075] Furthermore, one end of the second OFFDelay circuit 30-2 is connected to the gate of the second rectifier transistor SR2, and the other end is connected to the terminal of the PWM control device 20A that outputs the trailing pulse. As a result, the second rectifier transistor SR2 turns on in approximately the same time as the trailing switch SWLag, and turns off in a delayed manner than the trailing switch SWLag.

[0076] The time by which the OFFDelay circuit 30 delays the off-timing is determined by the delay time due to the leakage inductance of the resonant coil Lr and the switching transformer. Due to the inertia of the leakage inductance of the resonant coil Lr and 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 OFFDelay circuit 30 delays the 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 a leading pulse to the first leading switch SWLead1 and the second leading switch SWLead2, and further outputs it to the first rectifier transistor SR1 via the first OFFDelay circuit 30-1. The PWM control device 20B outputs a trailing pulse to the first trailing switch SWLag1 and the second trailing switch SWLag2, and further outputs it to the second rectifier transistor SR2 via the second OFFDelay circuit 30-2.

[0078] Figure 10 shows an example of simulation results for the current values ​​of each part of the power supply system 1B according to the second modification. Figure 10(A) shows the simulation results where the horizontal axis is time, and the vertical axis is the leading pulse, the input current to the first transformer T1 and the second transformer T2, and the output current from the first transformer T1. Figure 10(B) shows the simulation results where the horizontal axis is time, and the vertical axis is the trailing pulse, the input current to the first transformer T1 and the second transformer T2, and the output current from the second transformer T2. From Figures 10(A) and 10(B), it can be seen that the input current and output current values ​​of the switching transformer become 0 with a delay from the on / off of the drive pulse. The PWM control device 20A and the OFFDelay circuit 30 can optimize synchronous rectification by delaying the off timing of the first rectifier transistor SR1 and the second rectifier transistor SR2 by a time corresponding to this delay.

[0079] In the above description, the connection of the leading switch SWLead and the trailing switch SWLag was explained. However, this embodiment is not limited to this example, and the leading switch SWLead and the trailing switch SWLag may refer to opposite switching elements. 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 Embodiments] According to the embodiment described above, the PWM control device 20 controls a switching power supply circuit 10 comprising a switching transformer (for example, a first transformer T1 and a second transformer T2) and two or more primary-side switching elements including a leading switch SWLead and a trailing switch SWLag. The PWM control device 20 controls the conduction state of the leading switch SWLead and the trailing switch SWLag in a clamp control mode in which the duty cycle of the leading switch SWLead and the duty cycle of the trailing switch SWLag are substantially the same. The PWM control device 20 according to this embodiment can solve the problem of asymmetry between leading and trailing pulses. Therefore, with the PWM control device 20, the voltage applied to the secondary-side rectifier element does not become large, so it is not necessary to select a rectifier element with a high rating. Therefore, even a switching power supply circuit 10 using elements that normally have a low voltage rating and are less expensive can be used as a highly efficient power supply. In addition, by being able to use rectifier elements that normally have a low voltage rating, losses can be reduced. 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 embodiment reduces the losses of the transformer and rectifier elements on the high-load side without performing phase shift control which involves a large number of output signals. As a result, the PWM control device 20 can reduce the number of complex control circuits such as control ICs and gate drivers, thereby reducing costs. In addition, although a synchronous rectification method is used to reduce the losses of the rectifier circuit, the power supply system 1 according to the embodiment operates appropriately with only two signals: a leading pulse to control the leading switch SWLead and a trailing pulse to control the trailing switch SWLag. Therefore, the PWM control device 20 can be easily manufactured, and costs can be reduced.

[0082] Furthermore, according to the embodiment described above, the PWM control device 20 controls a switching power supply circuit 10 which includes a switching transformer and at least two primary-side switching elements, including a leading switch SWLead and a trailing switch SWLag. In a clamp control mode, the duty cycle of the trailing switch SWLag is set to the value obtained by adding a set value to the duty cycle of the leading switch SWLead. The PWM control device 20 according to this embodiment can suppress the change in pulse width of the trailing switch SWLag when switching from clamp control mode to steady mode, and consequently, the change in output current and output voltage output from the switching transformer. As a result, the power supply system 1 can reduce the impact when switching between clamp control mode and steady mode.

[0083] Furthermore, according to the embodiment described above, the PWM control device 20 controls the conduction state of the leading switch SWLead and the trailing switch SWLag by switching between a steady-state mode, in which the value of the trailing switch SWLag is the reciprocal of the duty cycle of the leading switch SWLead, and a clamp control mode. When the power supply system 1 is operating in a steady state and at high output, the duty cycles of the leading switch SWLead and the trailing switch SWLag are close to 50%, so the effect of asymmetry is relatively small. In contrast, when the power supply system 1 is operating at low output, the duty cycles of the leading switch SWLead and the trailing switch SWLag are far from 50%, so the effect of asymmetry is large. With the PWM control device 20 that switches to clamp control mode when the power supply system 1 is operating at low output, the power supply system 1 can solve the problem of asymmetry at low output while ensuring 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 embodiment described above, when the switching power supply circuit 10 is started, the PWM control device 20 starts controlling the conduction state of the leading switch SWLead and the trailing switch SWLag in a clamp control mode where the duty cycle of the leading switch SWLead and the duty cycle of the trailing switch SWLag are small. Then, it gradually increases the duty cycle of the leading switch SWLead and the trailing switch SWLag, and when the duty cycle of the leading switch SWLead or the duty cycle of the trailing switch SWLag exceeds a predetermined threshold, it switches from the clamp control mode to a steady mode and controls the conduction state of the leading switch SWLead and the trailing switch SWLag. In other words, the PWM control device 20 according to the embodiment controls the conduction state in clamp control mode during soft start. This solves the problem of asymmetry even in situations where it is necessary to reduce the output. In addition, the switching power supply circuit 10 does not need to select a rectifier element with a high rating for soft start, which is a special operation with a short operating time in relation 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.

[0085] Furthermore, according to the embodiment described above, the PWM control device 20 switches from steady mode to a clamp control mode in which the duty cycle of the leading switch SWLead and the trailing switch SWLag are small when the switching power supply circuit 10 is performing an overload protection operation, thereby controlling the conduction state of the leading switch SWLead and the trailing switch SWLag. This solves the problem of asymmetry even when it is necessary to reduce the output, so the switching power supply circuit 10 does not need to select a rectifier element with a high rating for a special operation such as protection operation, which has a short operating time in relation to the overall operation. Accordingly, 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 embodiment described above, the switching power supply circuit 10 includes a half-bridge circuit with two switching elements on the primary side, where one of the leading switch SWLead or trailing switch SWLag is the high-potential switching element of the half-bridge circuit, and the other of the leading switch SWLead or trailing switch SWLag is the low-potential switching element of the half-bridge circuit. In other words, the PWM control device 20 controls the switching power supply circuit 10 with the half-bridge circuit. With this, the power supply system 1 can be made smaller and less expensive by reducing the number of components.

[0087] Furthermore, according to the embodiment described above, the switching power supply circuit 10A has a full-bridge circuit with four switching elements on the primary side, where one of the leading switch SWLead or trailing switch SWLag is a high-potential switching element connected to the first terminal P1 of the switching transformer and a low-potential switching element connected to the second terminal P2 of the switching transformer, and the other of the leading switch SWLead or trailing switch SWLag is a low-potential switching element connected to the first terminal P1 of the switching transformer and a high-potential 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 with the full-bridge circuit. With this, the power supply system 1 can reduce the load on each of the primary-side switching elements and handle higher output.

[0088] Furthermore, according to the embodiment described above, the magnetic core of the switching transformer has a permeability of 15 to 120 when the magnetic flux density is 0.3 [T]. This makes it less likely for the magnetization of the switching transformer to saturate, thus reducing energy loss. Therefore, the switching power supply circuit 10 can be miniaturized and made more efficient. The DC voltage conversion circuit described above can meet the demand for miniaturization.

[0089] Furthermore, according to the embodiment described above, the power supply system 1B includes the PWM control device 20A described above, the switching power supply circuit 10A described above, and an OFFDelay circuit 30, one end of which is connected to a switching element connected to the secondary side of the switching power supply circuit 10, which delays the off timing of the switching element on the secondary side. The PWM control device 20A outputs a signal to the other end of the OFFDelay circuit 30 that is substantially the same as the signal that controls the conduction state of the leading switch SWLead and the trailing switch SWLag. Due to the inertia caused by the magnetic field of the resonant coil Lr inserted to realize soft switching, the current value of the output current from the switching transformer becomes 0 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, by delaying the off timing of the first rectifier transistor SR1 and the second rectifier transistor SR2 by the amount of time the output current flows, the delayed output current can be output to the load resistor LOAD side. Therefore, the power supply system 1B according to the embodiment can be made more efficient by further optimizing synchronous rectification.

[0090] [Fourth example of a control method for a PWM control device according to an embodiment] A fourth example of a control method using the PWM control device 20 according to the embodiment will be described with reference to Figures 11 and 12.

[0091] Figure 11 shows a fourth example of the 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 cycle of the trailing switch SWLag is the duty cycle of the leading switch SWLead multiplied by a set value α. That is, when the duty cycle of the leading switch SWLead is (D), the duty cycle of the trailing switch SWLag operates at (D × α). Note that explanations of matters already described in the first and second examples of the control method may be omitted. Note that in Figures 3, 5, and 14, the relationship between the voltage applied to the rectifier element and the duty cycle in steady mode is partially omitted, but this relationship is the same as in Figure 11.

[0092] Figure 11 shows a graph where the horizontal axis represents the duty cycle of the leading switch and the vertical axis represents the voltage [%] across the first diode D1 or the second diode D2. From Figure 11, it can be seen that in the fourth example of the control method, the voltage across the second diode D2 is generally smaller compared to the case where the duty cycle of the leading switch SWLead is (D) and the duty cycle of the trailing switch SWLag is (1-D).

[0093] Figure 12 shows the relationship between the switching element's duty cycle and the output voltage Vout in the fourth example of the control method, for 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 straight line showing the relationship between the switching element's duty cycle 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 clamp control mode to steady mode.

[0094] The setting value for the fourth example of the control method is, for example, a value smaller than the value obtained by dividing the duty cycle of the trailing switch SWLag in steady mode by the duty cycle of the leading switch SWLead in steady mode. However, the leading switch SWLead is defined as the switching element with the smaller duty cycle of the two switching elements. With this, the PWM control device 20 can reduce the amount of change in output voltage when switching from clamp control mode to steady mode while suppressing asymmetry. Note that when the setting 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 cycle of one of the two switching elements in a switching power supply circuit based on the duty cycle of the other switching element and performs processing accordingly. Here, the "one of the switching elements" is generally the switching element whose output voltage Vout increases as its duty cycle increases, from the viewpoint of simplifying calculations by the PWM control device. In other words, the PWM control device mainly controls the duty cycle of the two switching elements in the switching power supply circuit between 0% and 50%, and performs various calculations based on the duty cycle of the switching element with the smaller duty cycle (dead time is not considered). For the reasons stated above, the PWM control device 20 according to this embodiment performs processing such as determining the duty cycle of the trailing switch SWLag ((1-D), D, or (D×α), etc.) based on the duty cycle (D) of the leading switch SWLead. However, if the set value is less than 1, the duty cycle of the trailing switch SWLag (D × α) becomes smaller than the duty cycle of the leading switch SWLead (D). In this case, the PWM control device 20 performs calculations based on the duty cycle of the trailing switch SWLag in clamp control mode according to the fourth example of the control method, and performs calculations based on the duty cycle of the leading switch SWLead in steady mode, which makes the calculations more complex. Therefore, it is more desirable for the set value to be greater than 1.

[0096] [Summary of the fourth example of the control method] Furthermore, according to the embodiment described above, the PWM control device 20 controls a switching power supply circuit 10 which includes a switching transformer and at least two primary-side switching elements, including a leading switch SWLead and a trailing switch SWLag. In a clamp control mode, the duty cycle of the trailing switch SWLag is set to a value obtained by multiplying the duty cycle of the leading switch SWLead by a set value, and the conduction state of the leading switch SWLead and the trailing switch SWLag is controlled. According to the PWM control device 20 of the embodiment, the change in the pulse width of the trailing switch SWLag when switching from the clamp control mode to the steady-state mode, and consequently the change in the output current and output voltage output from the switching transformer, can be suppressed. As a result, the power supply system 1 can reduce the impact when switching between the clamp control mode and the steady-state mode.

[0097] The functions of the PWM control device 20 and the OFFDelay circuit 30 may be implemented using electronic circuits as needed. Furthermore, the functional units that implement these functions do not necessarily have to be included in a single device; the PWM control device 20 may be composed of multiple devices. At least some of the functions of the PWM control device 20 can be implemented using an MCU (Microcontroller Unit), etc. An MCU consists of 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 each instruction. RAM stores data and programs. Each element in RAM has an address and can be accessed using that address. RAM stands for "Random Access Memory." I / O (Input / Output) is a port for the Central Processing Unit to exchange data with external input / output devices, etc. Furthermore, all or part of the functional units of the PWM control device 20 may be implemented using hardware such as an ASIC, PLD, FPGA, or dedicated IC. Also, all or part of each functional unit may be implemented through a combination of software and hardware.

[0098] Furthermore, the entirety or a part thereof of the functions of the PWM control device 20 and OFFDelay circuit 30 in the above-described embodiment may be realized by recording a program for realizing these functions on a computer-readable recording medium, having a computer system read the program recorded on this recording medium, and executing it. The term "computer system" here includes hardware such as the operating system and peripheral devices.

[0099] Furthermore, "computer-readable recording media" refers to portable media such as flexible disks, magneto-optical disks, ROMs, and CD-ROMs, as well as recording units such as hard disks built into computer systems. In addition, "computer-readable recording media" may also include those that dynamically hold programs for a short period of time, such as communication lines used when transmitting programs over networks such as the Internet or communication lines such as telephone lines, and those that hold shift-time programs, such as volatile memory inside computer systems that act as servers or clients in such cases. Moreover, the above-mentioned programs may be for the purpose of realizing some of the functions described above, and may also be able to realize the above-mentioned functions in combination with programs already recorded in the computer system.

[0100] Although one embodiment of this invention has been described in detail above with reference to the drawings, the specific configuration is not limited to that described above, and various design changes can be made without departing from the spirit of this invention. Furthermore, the configurations described in each embodiment and example above may be combined. [Explanation of Symbols]

[0101] 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…Leading 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…OFFDelay circuit

Claims

1. A PWM control device for controlling a switching power supply circuit comprising a switching transformer and two or more primary-side switching elements including at least a leading switch and a trailing switch, In a clamp control mode in which the duty cycle of the preceding switch and the duty cycle of the following switch are approximately the same, the conduction state of the preceding switch and the following switch is controlled. PWM control device.

2. The conduction state of the leading switch and the trailing switch is controlled by switching between a steady-state mode, in which the duty cycle of the trailing switch is set to a value corresponding to the reciprocal of the duty cycle of the leading switch, and the clamp control mode. The PWM control device according to claim 1.

3. When the switching power supply circuit is started, the control of the conduction state of the leading switch and the trailing switch is initiated in the clamp control mode, where the duty cycle of the leading switch and the duty cycle of the trailing switch are small. Then, the duty cycle of the leading switch and the duty cycle of the trailing switch are gradually increased, and when the duty cycle of the leading switch or the duty cycle of the trailing switch exceeds a predetermined threshold, the circuit switches from the clamp control mode to the steady-state mode and controls the conduction state of the leading switch and the trailing switch. The PWM control device according to claim 2.

4. During the protection operation against overload of the switching power supply circuit, the circuit switches from the steady-state mode to the clamp control mode in which the duty cycle of the leading switch and the duty cycle of the trailing switch are small, thereby controlling the conduction state of the leading switch and the trailing switch. The PWM control device according to claim 2.

5. The switching power supply circuit includes a half-bridge circuit with two of the switching elements on the primary side. The leading switch or the trailing switch is the high-potential switching element of the half-bridge circuit. The other of the preceding switch or the following switch is the low-potential switching element of the half-bridge circuit. The PWM control device according to claim 1.

6. The switching power supply circuit includes a full bridge circuit with four of the switching elements on the primary side. The preceding switch or the following switch is a high-potential switching element connected to the first end of the switching transformer and a low-potential switching element connected to the second end of the switching transformer. The other of the preceding switch or the following switch is a low-potential switching element connected to the first end of the switching transformer and a high-potential switching element connected to the second end of the switching transformer. The PWM control device according to claim 1.

7. The magnetic core of the switching transformer has a permeability of 15 to 120 at a magnetic flux density of 0.3 [T]. The PWM control device according to claim 1.

8. The PWM control device according to claim 1, The aforementioned switching power supply circuit, An OFFDelay circuit is provided, with one end connected to a switching element connected to the secondary side of the switching power supply circuit, which delays the off-timing of the switching element on the secondary side. Equipped with, The PWM control device outputs a signal substantially identical to the signal controlling the conduction state of the preceding switch and the following switch to the other end of the OFFDelay circuit. Power supply system.

9. A PWM control device for controlling 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, In a clamp control mode where the duty cycle of the trailing switch is set to a value obtained by multiplying the duty cycle of the preceding switch by a set value, the conduction state of the preceding switch and the trailing switch is controlled. PWM control device.

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