Power conversion device

Abstract

To obtain a power conversion device capable of effectively performing a drooping operation and recovery from the drooping operation.SOLUTION: A power conversion device according to one embodiment of the present disclosure includes a switching circuit, a transformer, a current sensor, a rectifier circuit, a smoothing circuit, a first voltage sensor and a control circuit. The control circuit can generate a first value, can generate a second value based on the first value, can generate the second value based on an upper limit if the first value is higher than the upper limit, can generate a slope signal having a signal value that decreases from the second value, and can control a switching operation at a timing based on a result of comparison between the slope signal and a signal according to a peak current. When load current increases, the control circuit can decrease a switching frequency of the switching circuit from a predetermined frequency and can increase the upper limit, if a detected voltage becomes lower than a first threshold voltage.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a power conversion device that converts electric power. [Background technology]

[0002] In general, a power conversion device is designed to reduce the output voltage when the load current increases above the rated value. For example, Patent Document 1 discloses a power conversion device that first reduces the switching frequency in response to a reduction in the output voltage, and then reduces the current threshold value to protect against the load current. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-341791 Summary of the Invention [Problem to be solved by the invention]

[0004] It is desirable for a power conversion device to sufficiently reduce its output voltage when the load becomes heavy, and then to return to its original operating state when the load becomes lighter and returns to the rated range. In this way, a power conversion device is expected to effectively perform drooping and recovery from drooping.

[0005] It is desirable to provide a power converter that can effectively droop and recover from droop. [Means for solving the problem]

[0006] The power conversion device of the present invention includes an input power terminal, a switching circuit, a transformer, a current sensor, a rectifier circuit, a smoothing circuit, a first voltage sensor, an output power terminal, and a control circuit. The switching circuit is connected to the input power terminal and has one or more switching elements. The transformer has a first winding and a second winding led to the switching circuit. The current sensor is capable of detecting a current flowing through one of the first winding and the second winding. The rectifier circuit is connected to the second winding and is capable of rectifying a signal supplied from the second winding. The smoothing circuit is connected to the rectifier circuit and is capable of smoothing the voltage rectified by the rectifier circuit. The first voltage sensor is capable of detecting a voltage in the smoothing circuit. The output power terminal is connected to the smoothing circuit. The control circuit is capable of controlling the switching operation of the switching circuit based on the peak current of the current detected by the current sensor and the voltage detected by the first voltage sensor. The control circuit is capable of generating a first value based on the voltage detected by the first voltage sensor, generating a second value based on the first value if the first value is lower than an upper limit value, and generating the second value based on the upper limit value if the first value is higher than the upper limit value, generating a slope signal whose signal value decreases over time from the second value with the same period as the switching period of the switching circuit, comparing the slope signal with a signal corresponding to the peak current, and controlling the switching operation of the switching circuit at a timing corresponding to the comparison result. When the load current at the output power terminal increases and the voltage detected by the first voltage sensor becomes lower than a first threshold voltage, the control circuit is capable of reducing the switching frequency of the switching circuit from a predetermined frequency and increasing the upper limit value. [Effects of the Invention]

[0007] According to the power conversion device of the present invention, the drooping operation and recovery from the drooping operation can be performed effectively. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a circuit diagram showing an example of the configuration of a power conversion device according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a block diagram showing a specific example of the control circuit shown in FIG. [Figure 3] FIG. 3 is an explanatory diagram illustrating an example of the operation of the upper limit generating unit illustrated in FIG. [Figure 4] FIG. 4 is a characteristic diagram illustrating an example of the drooping operation in the power conversion device shown in FIG. [Figure 5] FIG. 5 is a timing waveform diagram illustrating an example of the operation of the power conversion device shown in FIG. [Figure 6] FIG. 6 is a timing waveform diagram showing an example of the operation of peak current control in the power conversion device shown in FIG. [Figure 7] FIG. 7 is a timing waveform diagram showing an example of the drooping operation in the power conversion device shown in FIG. [Figure 8A] FIG. 8A is an explanatory diagram illustrating an example of a drooping operation according to a reference example. [Figure 8B] FIG. 8B is an explanatory diagram illustrating an example of a drooping operation according to another reference example. [Figure 9] FIG. 9 is a circuit diagram illustrating an example of a configuration of a power conversion device according to a modified example. [Figure 10] FIG. 10 is a circuit diagram illustrating a configuration example of a power conversion device according to the second embodiment. [Figure 11] FIG. 11 is a block diagram showing a specific example of the control circuit shown in FIG. [Figure 12] FIG. 12 is a timing waveform diagram showing an example of the drooping operation in the power conversion device shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0009] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The description will be made in the following order: 1. First embodiment 2. Second embodiment

[0010] <1. First embodiment> [Configuration example] 1 shows an example of the configuration of a power conversion system 1 including a power conversion device according to a first embodiment of the present invention. The power conversion system 1 includes a high-voltage battery BH, a power conversion device 10, and a low-voltage battery BL. The voltage of the high-voltage battery BH is, for example, 400 V, and the voltage of the low-voltage battery BL is, for example, 12 V. This power conversion system 1 is configured to convert power supplied from the high-voltage battery BH and supply the converted power to the low-voltage battery BL.

[0011] The power conversion device 10 is configured to convert power by stepping down the voltage supplied from the high-voltage battery BH and supply the converted power to the low-voltage battery BL. The power conversion device 10 has terminals T11 and T12, a capacitor 11, a voltage sensor 12, a current sensor 13, a switching circuit 14, an inductor 15, a transformer 16, a rectifier circuit 17, a smoothing circuit 20, a voltage sensor 18, a control circuit 30, and terminals T21 and T22. The high-voltage battery BH, the capacitor 11, the voltage sensor 12, the current sensor 13, the switching circuit 14, and the inductor 15 form a primary circuit of the power conversion system 1, and the rectifier circuit 17, the smoothing circuit 20, the voltage sensor 18, and the low-voltage battery BL form a secondary circuit of the power conversion system 1.

[0012] Terminals T11 and T12 are configured to receive a voltage from a high-voltage battery BH. Terminal T11 is connected to the positive terminal of the high-voltage battery BH, and terminal T12 is connected to the negative terminal of the high-voltage battery BH. Furthermore, within the power conversion device 10, terminal T11 is connected to a voltage line L11A, and terminal T12 is connected to a reference voltage line L12.

[0013] One end of the capacitor 11 is connected to the voltage line L11A, and the other end is connected to the reference voltage line L12.

[0014] The voltage sensor 12 is configured to detect a voltage VH on the voltage line L11A. One end of the voltage sensor 12 is connected to the voltage line L11A, and the other end is connected to a reference voltage line L12. The voltage VH is the voltage on the voltage line L11A relative to the voltage on the reference voltage line L12. The voltage sensor 12 detects this voltage VH and generates a detection signal SVH corresponding to this voltage VH.

[0015] The current sensor 13 is configured to detect a current Ipri that flows from the terminal T11 to the transformer 16 via the switching circuit 14. One end of the current sensor 13 is connected to the voltage line L11A, and the other end is connected to the voltage line L11B. The current sensor 13 generates a detection signal SIPri that corresponds to the current Ipri.

[0016] The switching circuit 14 is configured to convert the DC voltage supplied from the high-voltage battery BH into an AC voltage by performing a switching operation. The switching circuit 14 is a full-bridge circuit and includes transistors SA, SB, SC, and SD. The transistors SA to SD are switching elements that perform switching operations based on control signals GA to GD, respectively. The transistors SA to SD are configured using, for example, N-type field effect transistors (FETs). Each of the transistors SA to SD has a body diode. For example, the anode of the body diode of the transistor SA is connected to the source of the body of the transistor SA, and the cathode is connected to the drain of the body of the transistor SA. The same is true for the transistors SB to SD. Note that, although an N-type field effect transistor is used in this example, any switching element may be used.

[0017] The transistor SA is provided in a path connecting the voltage line L11B and the node N1, and is configured to connect the node N1 to the voltage line L11B when it is turned on. The drain of the transistor SA is connected to the voltage line L11B, the gate is supplied with a control signal GA, and the source is connected to the node N1. The transistor SB is provided in a path connecting the node N1 and the reference voltage line L12, and is configured to connect the node N1 to the reference voltage line L12 when it is turned on. The drain of the transistor SB is connected to the node N1, the gate is supplied with a control signal GB, and the source is connected to the reference voltage line L12.

[0018] The transistor SC is provided in a path connecting the voltage line L11B and the node N2, and is configured to connect the node N2 to the voltage line L11B when it is turned on. The drain of the transistor SC is connected to the voltage line L11B, the gate is supplied with a control signal GC, and the source is connected to the node N2. The transistor SD is provided in a path connecting the node N2 and the reference voltage line L12, and is configured to connect the node N2 to the reference voltage line L12 when it is turned on. The drain of the transistor SD is connected to the node N2, the gate is supplied with a control signal GD, and the source is connected to the reference voltage line L12.

[0019] One end of the inductor 15 is connected to the node N2, and the other end is connected to the winding 16A of the transformer 16.

[0020] Transformer 16 is configured to insulate the primary circuit from the secondary circuit in terms of DC current and connect them in terms of AC current, convert the AC voltage supplied from the primary circuit using the transformation ratio of transformer 16, and supply the converted AC voltage to the secondary circuit. Transformer 16 has windings 16A, 16B, and 16C. Winding 16A is a primary winding, and one end is connected to node N1 of switching circuit 14 and the other end is connected to the other end of inductor 15. Windings 16B and 16C are secondary windings. One end of winding 16B is connected to the cathode of diode D2 (described later) in rectifier circuit 17, and the other end is connected to voltage line L21A. Winding 16C is connected to voltage line L21A and the other end is connected to the cathode of diode D1 (described later) in rectifier circuit 17.

[0021] The rectifier circuit 17 is configured to rectify the AC voltage output from the windings 16B and 16C of the transformer 16. The rectifier circuit 17 has diodes D1 and D2. The anode of the diode D1 is connected to the reference voltage line L22, and the cathode is connected to the other end of the winding 16C of the transformer 16. The anode of the diode D2 is connected to the reference voltage line L22, and the cathode is connected to one end of the winding 16B of the transformer 16.

[0022] The smoothing circuit 20 is configured to smooth the voltage rectified by the rectifier circuit 17. The smoothing circuit 20 has an inductor 21 and a capacitor 22. One end of the inductor 21 is connected to the voltage line L21A, and the other end is connected to the voltage line L21B. One end of the capacitor 22 is connected to the voltage line L21B, and the other end is connected to the reference voltage line L22.

[0023] The voltage sensor 18 is configured to detect a voltage VL on the voltage line L21B. One end of the voltage sensor 18 is connected to the voltage line L21B, and the other end is connected to a reference voltage line L22. The voltage VL is the voltage on the voltage line L21B relative to the voltage on the reference voltage line L22. The voltage sensor 18 detects this voltage VL and generates a detection signal SVL corresponding to this voltage VL.

[0024] The control circuit 30 is configured to control the operation of the power conversion device 10 by controlling the switching operation of the switching circuit 14 based on the detection signal SVH supplied from the voltage sensor 12, the detection signal SIPri supplied from the current sensor 13, and the detection signal SVL supplied from the voltage sensor 18. The control circuit 30 is configured using, for example, a microcontroller.

[0025] 2 shows an example of the configuration of the control circuit 30. The control circuit 30 has an AD (Analog to Digital) conversion unit 31, a target voltage value setting unit 32, a subtraction unit 33, a PI (Proportional Integral) control unit 34, an upper limit value generation unit 35, a limiting processing unit 36, a slope compensation unit 37, a DA (Digital to Analog) conversion unit 38, a comparator 39, and a control signal generation unit 41.

[0026] The AD conversion unit 31 is configured to perform AD conversion at a predetermined sampling period based on the detection signal SVL, which is an analog signal supplied from the voltage sensor 18, to generate a signal S31, which is a digital signal.

[0027] The target voltage value setting unit 32 is configured to generate a target value Stgt corresponding to a target voltage Vtgt of the voltage VL.

[0028] The subtraction unit 33 is configured to generate a signal S33 by subtracting the signal S31 supplied from the AD conversion unit 31 from the target value Stgt supplied from the target voltage value setting unit 32. In the control by the power conversion device 10, negative feedback control is performed so that the signal value of the signal S33, which is the subtraction result of the subtraction unit 33, becomes sufficiently small.

[0029] The PI control unit 34 is configured to perform PI control based on the signal S33 supplied from the subtraction unit 33, thereby generating a reference value Sref1 for the peak current of the current Ipri.

[0030] The upper limit value generating unit 35 is configured to generate an upper limit value Slim of the reference value Sref1 generated by the PI control unit 34 based on the detection signals SVL and SVH.

[0031] Specifically, as will be described later, when the load current Iload of the power conversion device 10 increases and the voltage VL becomes lower than the threshold voltage Vth1, the upper limit value generation unit 35 gradually decreases the upper limit value Slim in accordance with the voltage VL. Furthermore, when the load current Iload is high and the voltage VL increases as the load current Iload decreases, the upper limit value generation unit 35 gradually increases the upper limit value Slim in accordance with the voltage VL. When the voltage VL becomes higher than the threshold voltage Vth1, the upper limit value generation unit 35 returns the upper limit value Slim to its original predetermined value. When comparing the voltage VL with the threshold voltage Vth1, a hysteresis characteristic is used to prevent hunting.

[0032] The upper limit value generating unit 35 has, for example, table data TBL indicating the relationship between the detection signal SVL indicating the voltage VL and the upper limit value Slim, and uses this table data TBL as a look-up table to generate the upper limit value Slim based on the detection signal SVL.

[0033] FIG. 3 shows an example of table data TBL. Data W1 indicated by a solid line represents table data TBL. In FIG. 3, the horizontal axis represents the value of the detection signal SVL, and the vertical axis represents the upper limit value Slim. Note that, for convenience of explanation, data W1 is shown as a straight line in FIG. 3, but table data TBL is actually composed of a plurality of discrete values. The upper limit value generation unit 35 generates the upper limit value Slim based on the detection signal SVL using such table data TBL. The upper limit value generation unit 35 may output the value of the upper limit value Slim included in this table data TBL as is. Alternatively, the upper limit value generation unit 35 may generate the upper limit value Slim by, for example, performing an interpolation operation based on the detection signal SVL using table data TBL. In other words, since table data TBL is composed of a plurality of discrete values, the upper limit value generation unit 35 can generate the upper limit value Slim by performing an interpolation operation using these discrete values. In this case, the upper limit value generating unit 35 can precisely set the upper limit value Slim according to the voltage VL, regardless of the step size in the table data TBL, for example. However, this is not limiting, and instead, the upper limit value generating unit 35 may generate the upper limit value Slim based on the detection signal SVL using, for example, a function.

[0034] In this example, the upper limit value generating unit 35 generates the upper limit value Slim of the reference value Sref1 generated by the PI control unit 34 based on the detection signal SVL. However, instead, for example, the upper limit value Slim of the reference value Sref1 may be generated based on both the detection signal SVL and the detection signal SVH. In this case, the upper limit value generating unit 35 has table data TBLB indicating the relationship between the detection signal SVL indicating the voltage VL, the detection signal SVH indicating the voltage VH, and the upper limit value Slim. Using this table data TBLB as a look-up table, the upper limit value generating unit 35 generates the upper limit value Slim based on the detection signals SVL and SVH. For example, the upper limit value generating unit 35 lowers the upper limit value Slim as the voltage VH increases. That is, in this case, the duty ratio is small and the time it takes for the slope signal SLP (described later) to fall is short, so the upper limit value generating unit 35 lowers the upper limit value Slim.

[0035] Furthermore, as will be described later, when the load current Iload of the power conversion device 10 increases and the voltage VL becomes lower than the threshold voltage Vth2, the upper limit value generation unit 35 increases the upper limit value Slim based on the voltage VH. Here, the threshold voltage Vth2 is lower than the threshold voltage Vth1. Specifically, as shown by data W2 in FIG. 3 , the upper limit value generation unit 35 can correct the upper limit value Slim by multiplying the upper limit value Slim obtained using the table data TBL (data W1) by a coefficient K. The value of this coefficient K is 1 or greater, and the upper limit value generation unit 35 calculates this coefficient K based on the voltage VH. However, this is not limiting, and the upper limit value generation unit 35 may correct the upper limit value Slim by adding a value corresponding to the voltage VH to the upper limit value Slim obtained using the table data TBL (data W1). Furthermore, when the load current Iload decreases and the voltage VL increases, the upper limit value generation unit 35 decreases the upper limit value Slim if the voltage VL becomes higher than the threshold voltage Vth2. Specifically, the upper limit generating unit 35 generates the upper limit Slim without using the coefficient K, using the table data TBL (data W1) shown in FIG.

[0036] The limiting processor 36 is configured to generate a reference value Sref2 by limiting the reference value Sref1 using an upper limit Slim. Specifically, when the reference value Sref1 is equal to or less than the upper limit Slim, the limiting processor 36 outputs the reference value Sref1 as is as the reference value Sref2. When the reference value Sref1 is greater than the upper limit Slim, the limiting processor 36 outputs the upper limit Slim as the reference value Sref2.

[0037] The slope compensation unit 37 is configured to generate a slope signal Sslp, whose signal value gradually decreases from the reference value Sref2 supplied from the limiting processing unit 36, at the same period as the switching period of the switching circuit 14, based on the reference value Sref2. The slope of the slope signal Sslp as the signal value gradually decreases may be a fixed value or a variable value. When the slope is a variable value, the slope is set based on, for example, the input voltage of the power conversion device 10 or the output voltage of the power conversion device 10.

[0038] The DA conversion unit 38 is configured to perform DA conversion based on the slope signal Sslp, which is a digital signal, to generate the slope signal SLP, which is an analog signal.

[0039] The comparator 39 is configured to compare the detection signal SIPri and the slope signal SLP supplied from the current sensor 13. The detection signal SIPri is supplied to a positive input terminal of the comparator 39, and the slope signal SLP is supplied to a negative input terminal of the comparator 39. The comparator 39 compares these signals and generates a signal S39 indicating the comparison result.

[0040] The control signal generating unit 41 is configured to generate the control signals GA to GD based on the signal S39 supplied from the comparator 39 and the detection signal SVL. The duty ratio of the four control signals GA to GD is approximately 50%. The control signal generating unit 41 performs phase shift control based on the signal S39 to generate the control signals GA to GD.

[0041] Furthermore, as will be described later, when the load current Iload of the power conversion device 10 increases and the voltage VL becomes lower than the threshold voltage Vth2, the control signal generating unit 41 reduces the frequency of the control signals GA to GD, thereby reducing the switching frequency of the switching circuit 14. Furthermore, when the load current Iload is large in this manner, and the load current Iload decreases and the voltage VL becomes higher than the threshold voltage Vth2, the control signal generating unit 41 returns the switching frequency to the original predetermined frequency.

[0042] With this configuration, the control circuit 30 controls the switching operation of the switching circuit 14 based on the detection signal SVL so that the voltage VL becomes the same as the target voltage Vtgt. The control circuit 30 also controls the switching operation of the switching circuit 14 based on, for example, the detection signal SIPri so as to respond to changes in the load.

[0043] FIG. 4 shows an example of the drooping behavior of the power conversion device 10 when the load is changed. The horizontal axis represents the load current Iload, and the vertical axis represents the voltage VL. When the load current Iload is smaller than the current I1, the voltage VL maintains the target voltage Vtgt. When the load current Iload exceeds the current I1, the voltage VL begins to decrease from the target voltage Vtgt. When the load current Iload further increases and the voltage VL falls below the threshold voltage Vth1, the voltage VL decreases at a steeper slope. Specifically, when the voltage VL is lower than the threshold voltage Vth1, the upper limit value generation unit 35 of the control circuit 30 gradually decreases the upper limit value Slim in accordance with the voltage VL, thereby decreasing the voltage VL. When the load current Iload further increases and the voltage VL falls below the threshold voltage Vth2, the voltage VL decreases at a steeper slope. Specifically, when the voltage VL is lower than the threshold voltage Vth2, the upper limit value generating unit 35 of the control circuit 30 increases the upper limit value Slim, and the control signal generating unit 41 of the control circuit 30 decreases the switching frequency of the switching circuit 14, thereby decreasing the voltage VL. This allows the power conversion device 10 to protect against the load current.

[0044] Terminals T21 and T22 (FIG. 1) are configured to supply the voltage generated by the power conversion device 10 to the low-voltage battery BL. Within the power conversion device 10, terminal T21 is connected to the voltage line L21B, and terminal T22 is connected to the reference voltage line L22. Furthermore, terminal T21 is connected to the positive terminal of the low-voltage battery BL, and terminal T22 is connected to the negative terminal of the low-voltage battery BL.

[0045] Here, terminals T11 and T12 correspond to a specific example of an "input power terminal" in an embodiment of the present disclosure. Switching circuit 14 corresponds to a specific example of a "switching circuit" in an embodiment of the present disclosure. Transistors SA to SD correspond to a specific example of a "plurality of switching elements" in an embodiment of the present disclosure. Transformer 16 corresponds to a specific example of a "transformer" in an embodiment of the present disclosure. Winding 16A corresponds to a specific example of a "first winding" in an embodiment of the present disclosure. Windings 16B and 16C correspond to a specific example of a "second winding" in an embodiment of the present disclosure. Current sensor 13 corresponds to a specific example of a "current sensor" in an embodiment of the present disclosure. Rectifier circuit 17 corresponds to a specific example of a "rectifier circuit" in an embodiment of the present disclosure. Smoothing circuit 20 corresponds to a specific example of a "smoothing circuit" in an embodiment of the present disclosure. Voltage sensor 18 corresponds to a specific example of a "first voltage sensor" in an embodiment of the present disclosure. Terminals T21 and T22 correspond to a specific example of an "output power terminal" in an embodiment of the present disclosure. Voltage sensor 12 corresponds to a specific example of a "second voltage sensor" in an embodiment of the present disclosure. Control circuit 30 corresponds to a specific example of a "control circuit" in an embodiment of the present disclosure.

[0046] The reference value Sref1 corresponds to a specific example of a "first value" in an embodiment of the present disclosure. The upper limit value Slim corresponds to a specific example of an "upper limit value" in an embodiment of the present disclosure. The reference value Sref2 corresponds to a specific example of a "second value" in an embodiment of the present disclosure. The slope signal SLP corresponds to a specific example of a "slope signal" in an embodiment of the present disclosure. The threshold voltage Vth1 corresponds to a specific example of a "second threshold voltage" in an embodiment of the present disclosure. The threshold voltage Vth2 corresponds to a specific example of a "first threshold voltage" in an embodiment of the present disclosure.

[0047] [Actions and Actions] Next, the operation and function of the power conversion device 10 of this embodiment will be described.

[0048] (Overview of overall operation) First, referring to FIG. 1, an overview of the overall operation of the power conversion device 10 will be described. The voltage sensor 12 generates a detection signal SVH by detecting a voltage VH on the voltage line L11A. The current sensor 13 generates a detection signal SIPri by detecting a current Ipri flowing from the terminal T11 to the transformer 16 via the switching circuit 14. The switching circuit 14 converts the DC voltage supplied from the high-voltage battery BH into an AC voltage by performing a switching operation. The transformer 16 insulates the primary side circuit from the secondary side circuit in terms of DC and connects them in terms of AC. It converts the AC voltage supplied from the primary side circuit using the transformation ratio of the transformer 16 and supplies the converted AC voltage to the secondary side circuit. The rectifier circuit 17 rectifies the AC voltage output from windings 16B and 16C of the transformer 16. The smoothing circuit 20 smoothes the voltage rectified by the rectifier circuit 17. The voltage sensor 18 generates a detection signal SVL by detecting a voltage VL on the voltage line L21A. The control circuit 30 controls the operation of the power conversion device 10 by controlling the switching operation of the switching circuit 14 based on the detection signal SVH supplied from the voltage sensor 12, the detection signal SIPri supplied from the current sensor 13, and the detection signal SVL supplied from the voltage sensor 18.

[0049] (Detailed operation) FIG. 5 shows an example of operation of power conversion device 10, with (A) to (D) showing the waveforms of control signals GA to GD, respectively, (E) showing the waveform of the voltage (transformer voltage Vtr) applied to winding 16A of transformer 16, and (F) showing the waveform of the current (transformer current Itr) flowing through winding 16A of transformer 16. Control signal GA is the waveform of the gate voltage relative to the source voltage of transistor SA. The same is true for control signals GB to GD. Transformer voltage Vtr is the voltage at node N1 relative to node N2. Transformer current Itr is depicted as a positive current flowing through winding 16A from node N1 to node N2.

[0050] In this example, the frequency of the four control signals GA to GD is 200 kHz. The duty ratio of the control signals GA to GD is approximately 50%. While the control signal GA is at a high level, the control signal GB is maintained at a low level, and while the control signal GB is at a high level, the control signal GA is maintained at a low level. The control signals GA and GB are provided with a period during which both signals are at a low level (so-called dead time Td). Similarly, while the control signal GC is at a high level, the control signal GD is maintained at a low level, and while the control signal GD is at a high level, the control signal GC is maintained at a low level. The control signals GC and GD are provided with a period during which both signals are at a low level (dead time Td).

[0051] As shown in FIGS. 5A and 5D, the control circuit 30 sets the control signal GD to a high level during the period from timing t11 to t13. This causes the transistor SD to be turned on during this period from timing t11 to t13. The control circuit 30 also sets the control signal GA to a high level during the period from timing t12 to t15. This causes the transistor SA to be turned on during this period from timing t12 to t15. As a result, as shown in FIG. 5E, during the period from timing t12 to t13 when both the control signals GA and GD are at a high level, the transformer voltage Vtr becomes a positive voltage (400 V in this example). Specifically, the transformer voltage Vtr changes from 0 V to a positive voltage at timing t12, and then changes from the positive voltage to 0 V at timing t13. During this period from timing t12 to t13, the transformer 16 transmits power from the primary circuit to the secondary circuit.

[0052] Similarly, as shown in FIGS. 5B and 5C, the control circuit 30 sets the control signal GC to a high level during the period from timing t14 to t17. As a result, the transistor SC is turned on during this period from timing t14 to t17. The control circuit 30 also sets the control signal GB to a high level during the period from timing t16 to t19. As a result, the transistor SB is turned on during this period from timing t16 to t19. As a result, as shown in FIG. 5E, during the period from timing t16 to t17 when the control signals GB and GC are both at a high level, the transformer voltage Vtr becomes a negative voltage (in this example, "-400 V"). Specifically, the transformer voltage Vtr changes from 0 V to a negative voltage at timing t16, and then changes from the negative voltage to 0 V at timing t17. During this period from timing t16 to t17, the transformer 16 transmits power from the primary side circuit to the secondary side circuit.

[0053] As shown in FIG. 5(E), the transformer voltage Vtr changes in the order 0V, a positive voltage, 0V, a negative voltage, and again 0V. In response, the transformer current Itr flows through the winding 16A of the transformer 16 as shown in FIG. 5(F). Specifically, the transformer current Itr is positive from timing t12 to t13 and reaches a positive peak at timing t13. The transformer current Itr is negative from timing t16 to t17 and reaches a negative peak at timing t17.

[0054] The current Ipri varies in accordance with the transformer current Itr. The current sensor 13 detects the current Ipri and generates a detection signal SIPri. The control circuit 30 performs peak current control based on the detection signal SIPri.

[0055] FIG. 6 shows an example of the operation of peak current control, where (A) shows the waveform of the transformer current Itr, and (B) shows the waveform of the slope signal SLP and the waveform of the detection signal SIPri.

[0056] As shown in FIG. 6A, the transformer current Itr is a positive current from timing t20 to t21 and reaches a positive peak at timing t21. Furthermore, the transformer current Itr is a negative current from timing t22 to t23 and reaches a negative peak at timing t23. In response to this transformer current Itr, the detection signal SIPri rises at timing t20, gradually rises from timing t20 to t21, and falls at timing t21, as shown in FIG. 6B. Furthermore, the detection signal SIPri rises at timing t22, gradually rises from timing t22 to t23, and falls at timing t23. Thus, the detection signal SIPri reaches its peaks at timings t21 and t23. The peak of the detection signal SIPri at timing t21 corresponds to the positive peak of the transformer current Itr at this timing t21, and the peak of the detection signal SIPri at timing t23 corresponds to the negative peak of the transformer current Itr at this timing t23.

[0057] That is, when transistors SA and SD are on, current flows in the primary circuit in the following order: voltage line L11A, current sensor 13, voltage line L11B, transistor SA, winding 16A of transformer 16, inductor 15, transistor SD, and reference voltage line L12. When transistors SB and SC are on, current flows in the primary circuit in the following order: voltage line L11A, current sensor 13, voltage line L11B, transistor SC, inductor 15, winding 16A of transformer 16, transistor SB, and reference voltage line L12. Thus, regardless of the polarity of transformer current Itr, current Ipri flows in current sensor 13 from voltage line L11A to voltage line L11B. Therefore, the waveform of detection signal SIPri becomes as shown in FIG. 6B, which corresponds to the waveform of transformer current Itr (FIG. 6A).

[0058] The slope compensation unit 37 of the control circuit 30 generates a slope signal Sslp whose signal value gradually decreases from a reference value Sref2. The DA conversion unit 38 then generates an analog slope signal SLP based on the slope signal Sslp supplied from the slope compensation unit 37. As shown in FIG. 6B, the slope signal SLP has a waveform whose signal level gradually decreases from a reference level REF2 corresponding to the reference value Sref2. This slope signal SLP is a signal synchronized with the control signals GA and GB.

[0059] The comparator 39 compares the detection signal SIPri with the slope signal SLP.

[0060] For example, from timing t20 to t22, the slope signal SLP gradually decreases from the reference level REF2 (FIG. 6B). From timing t20 to t21, the detection signal SIPri gradually increases, and at timing t21, the detection signal SIPri reaches the same signal level as the slope signal SLP. The comparator 39 outputs the comparison result, and the control signal generator 41 transitions the control signals GC and GD based on the comparison result of the comparator 39. Specifically, as shown in FIGS. 5C and 5D, the control signal generator 41 transitions the control signal GD from high to low at timing t13, and transitions the control signal GC from low to high at timing t14. With this transition of the control signal GD, the transistor SD is turned off, and therefore the transformer current Itr changes toward 0 A at timing t13 (FIG. 5F). In response to this, the detection signal SIPri decreases, as shown in FIG. 6.

[0061] Similarly, for example, during the period from timing t22 to t24, the slope signal SLP gradually decreases from the reference level REF2 (FIG. 6(B)). During the period from timing t22 to t23, the detection signal SIPri gradually increases, and at timing t23, the detection signal SIPri reaches the same signal level as the slope signal SLP. The comparator 39 outputs this comparison result, and the control signal generator 41 transitions the control signals GC and GD based on the comparison result of the comparator 39. Specifically, as shown in FIGS. 5(C) and 5(D), the control signal generator 41 transitions the control signal GC from a high level to a low level at timing t17, and transitions the control signal GD from a low level to a high level at timing t18. With this transition of the control signal GC, the transistor SC enters an off state, and therefore, at timing t17, the transformer current Itr changes toward 0 A (FIG. 5(F)). In response to this, the detection signal SIPri decreases, as shown in FIG. 6.

[0062] In this way, in the power conversion device 10, phase shift control is performed based on the peak current of the current Ipri, and the change timing of the control signals GC and GD is set. As shown in Fig. 5, the phase of the control signal GD leads the phase of the control signal GA by a time Tsft. In this way, the phase shift control is performed in the power conversion device 10.

[0063] (About drooping operation) As shown in Fig. 4, when the load current Iload is smaller than the current I1, the voltage VL maintains the target voltage Vtgt. When the load current Iload exceeds the current I1, the voltage VL drops from the target voltage Vtgt. This drooping operation allows the power conversion device 10 to protect against the load current. This drooping operation will be described in detail below.

[0064] FIG. 7 shows an example of the drooping operation of the power conversion device 10, where (A) shows the waveform of the slope signal SLP, (B) shows the waveform of the detection signal SIPri, (C) shows the upper limit level LIM corresponding to the upper limit value Slim, (D) to (G) show the control signals GA to GD, respectively, and (H) shows the voltage VL. In FIG. 6, the horizontal axis represents time. In this example, the load gradually becomes heavier with the passage of time, and the load current Iload gradually increases. Note that in FIG. 6, for example, the length of the period from timing t36 to t37 and the length of the period from timing t37 to t38 are shortened for ease of explanation, but in reality, these periods are longer periods that include more switching cycles.

[0065] During the period from timing t31 to t33, the power conversion device 10 operates in the same manner as in the examples shown in FIGS. 5 and 6. Specifically, during the period from timing t31 to t33, the slope signal SLP gradually decreases from the reference level REF2 (FIG. 7(A)). During the period from timing t31 to t32, the detection signal SIPri gradually increases (FIG. 7(B)), and at timing t32, the detection signal SIPri reaches the same signal level as the slope signal SLP. The comparator 39 outputs this comparison result, and the control signal generator 41 transitions the control signals GC and GD based on the comparison result of the comparator 39 (FIGS. 7(F) and (G)). As a result, the detection signal SIPri decreases at timing t32 (FIG. 7(B)).

[0066] In this example, the load current Iload gradually increases over time. As a result, the signal level of the slope signal SLP during the period from timing t33 ​​to t34 becomes higher than the signal level of the slope signal SLP during the period from timing t32 to t33 (FIG. 7A). That is, in this case, the PI control unit 34 of the control circuit 30 increases the reference value Sref1 as the load current Iload increases. In this example, the reference value Sref1 is lower than the upper limit value Slim, so the limiting processing unit 36 ​​outputs this reference value Sref1 as the reference value Sref2. As a result, the reference level REF2 increases, and the signal level of the slope signal SLP increases. As the load current Iload continues to increase, the reference level REF2 continues to increase, and the signal level of the slope signal SLP also continues to increase.

[0067] Then, at timing t35, when the reference level REF2 of the slope signal SLP reaches the upper limit level LIM corresponding to the upper limit value Slim, the increase in the signal level of the slope signal SLP stops (FIG. 7A). That is, although the load current Iload continues to increase gradually thereafter, the reference value Sref1 generated by the PI control unit 34 becomes higher than the upper limit value Slim, and the limiting processing unit 36 ​​outputs this upper limit value Slim as the reference value Sref2. That is, the reference level REF2 becomes the same as the upper limit level LIM. Therefore, the signal level of the slope signal SLP does not increase but is maintained. In this case, the power conversion device 10 cannot transmit sufficient power from the primary-side circuit to the secondary-side circuit, and therefore, at timing t36, the voltage VL starts to decrease from the target voltage Vtgt.

[0068] Then, at timing t37, when the voltage VL becomes lower than the threshold voltage Vth1, the power conversion device 10 lowers the upper limit level LIM (FIG. 7C). Specifically, because the voltage VL is lower than the threshold voltage Vth1, the upper limit value generation unit 35 of the control circuit 30 lowers the upper limit value Slim in accordance with the voltage VL. Because the reference value Sref1 generated by the PI control unit 34 is higher than the upper limit value Slim, the limiting processing unit 36 ​​outputs this upper limit value Slim as the reference value Sref2. Therefore, the upper limit level LIM corresponding to this upper limit value Slim lowers. Because the upper limit level LIM lowers, the reference level REF2 lowers, and the signal level of the slope signal SLP also lowers. As a result, the voltage VL lowers at a slope greater than that during the period from timing t36 to t37 after timing t37. Since the load current Iload continues to increase, the voltage VL continues to decrease, the reference level REF2 continues to decrease, and the signal level of the slope signal SLP also continues to decrease.

[0069] Then, at timing t38, when the voltage VL becomes lower than the threshold voltage Vth2, the power conversion device 10 reduces the switching frequency of the switching circuit 14 (FIGS. 7D-7G) and increases the upper limit level LIM (FIG. 7C). Specifically, because the voltage VL is lower than the threshold voltage Vth2, the control signal generation unit 41 of the control circuit 30 reduces the frequency of the control signals GA-GD. This frequency can be, for example, approximately 70% of the frequency in the period before timing t38. The control signal generation unit 41 can gradually reduce this frequency at a rate of, for example, −10 kHz / msec. Furthermore, because the voltage VL is lower than the threshold voltage Vth2, the upper limit value generation unit 35 of the control circuit 30 increases the upper limit value Slim. That is, reducing the switching frequency of the switching circuit 14 can cause discontinuity in the drooping characteristic shown in FIG. 4, as will be described later. Therefore, the power conversion device 10 improves this discontinuity by increasing the upper limit value Slim. As a result, from timing t38 onwards, voltage VL decreases at a steeper slope than during the period from timing t37 to t38. As load current Iload continues to increase, voltage VL continues to decrease.

[0070] After timing t38, the power conversion device 10 lowers the switching frequency of the switching circuit 14 and continues to lower the upper limit level LIM, so that the voltage VL continues to decrease.

[0071] In this way, the power conversion device 10 reduces the voltage VL in response to an increase in the load current Iload, thereby enabling the power conversion device 10 to provide protection against the load current.

[0072] When the load current Iload is large as described above, and the voltage VL increases as a result of a decrease in the load current Iload, the power conversion device 10 performs an operation opposite to that shown in Fig. 7. That is, when the voltage VL increases, the power conversion device 10 gradually increases the upper limit level LIM in accordance with the voltage VL. When the voltage VL becomes higher than the threshold voltage Vth2, the power conversion device 10 returns the switching frequency of the switching circuit 14 to the original frequency in the period before timing t38 and reduces the upper limit value Slim. When the voltage VL becomes higher than the threshold voltage Vth1, the power conversion device 10 returns the upper limit level LIM to the original level in the period before timing t37.

[0073] 7, when the voltage VL becomes lower than the threshold voltage Vth2, the power conversion device 10 reduces the switching frequency of the switching circuit 14 and increases the upper limit level LIM (FIGS. 7(C) to 7(G)). This makes it possible to improve discontinuities that may occur in the drooping operation, as will be described below with some examples.

[0074] 8A and 8B show examples of drooping operation according to the reference example. In these examples, when the load becomes heavier and the voltage VL becomes lower than the threshold voltage Vth2, the power conversion device 10 reduces the switching frequency of the switching circuit 14 without increasing the upper limit level LIM, unlike the present embodiment.

[0075] In the example of FIG. 8A , as in the case of the present embodiment ( FIGS. 4 and 7 ), when the load current Iload exceeds the current I1 due to an increase in load, the voltage VL decreases from the target voltage Vtgt, as indicated by the arrow in FIG. 8A . When the voltage VL falls below the threshold voltage Vth1, the voltage VL decreases at a steeper rate. When the load further increases and the voltage VL falls below the threshold voltage Vth2, in this example, the voltage VL decreases and the load current Iload decreases. That is, in this example, when the voltage VL falls below the threshold voltage Vth2, the power conversion device 10 reduces the switching frequency of the switching circuit 14 without increasing the upper limit level LIM. Therefore, in the power conversion device 10, the amount of power supplied from the primary-side circuit to the secondary-side circuit decreases, making it impossible to supply current to the load, and the load current Iload decreases. Then, when the load further increases, the load current Iload increases and the voltage VL decreases. Therefore, in this drooping characteristic, a discontinuity occurs in the portion W3.

[0076] In this case, for example, if the load becomes heavy and then becomes lighter, it is difficult to predict how the power conversion device 10 will operate. That is, for example, the power conversion device 10 may not return to its original operating state via the portion indicated by the portion W3, but may operate without passing through the portion indicated by the portion W3, as indicated by the dashed arrow. In this case, the power conversion device 10 may not be able to return to its original operating state.

[0077] In the example of FIG. 8B, as in the example of FIG. 8A, a discontinuity occurs as shown in portion W4. Therefore, in this example, the power conversion device 10 may not be able to return to its original operating state. Also, in this example, the load current Iload decreases significantly in portion W4, and the load current Iload falls below the current I1. In a power conversion device, when the load current Iload is equal to or less than the current I1, it is often required that the voltage VL be close to the target voltage Vtgt. The power conversion device 10 having the drooping characteristic shown in FIG. 8B cannot meet this requirement.

[0078] On the other hand, in the power conversion device 10 according to the present embodiment, when the voltage VL becomes lower than the threshold voltage Vth2, the switching frequency of the switching circuit 14 is reduced and the upper limit level LIM is increased (FIGS. 7(C) to 7(G)). In this power conversion device 10, since the upper limit level LIM is increased, even if the switching frequency is reduced, it is possible to prevent a decrease in the amount of power supplied from the primary side circuit to the secondary side circuit. This makes it possible to reduce the possibility of discontinuity occurring in the drooping characteristic in the power conversion device 10, as shown in FIG. 4. As a result, the power conversion device 10 can return to the original operating state after the drooping operation.

[0079] Thus, power conversion device 10 includes input power terminals (terminals T11, T12), a switching circuit 14 connected to input power terminals (terminals T11, T12) and having a plurality of switching elements (transistors SA to SD), transformer 16 having a first winding (winding 16A) and a second winding (windings 16B, 16C) led to switching circuit 14, current sensor 13 capable of detecting a current flowing in one of the first winding (winding 16A) and the second winding (windings 16B, 16C), and a current sensor 13 connected to the second winding (windings 16B, 16C) and supplied from the second winding. The power supply circuit is provided with a rectifier circuit 17 capable of rectifying the received signal, a smoothing circuit 20 connected to the rectifier circuit 17 and capable of smoothing the voltage rectified by the rectifier circuit 17, a first voltage sensor (voltage sensor 18) capable of detecting the voltage in the smoothing circuit 20, output power terminals (terminals T21, T22) connected to the smoothing circuit 20, and a control circuit 30 capable of controlling the switching operation of a plurality of switching elements (transistors SA to SD) based on the peak current of the current detected by the current sensor 13 and the voltage detected by the first voltage sensor (voltage sensor 18). The control circuit 30 is capable of generating a first value (reference value Sref1) based on the voltage VL detected by the first voltage sensor (voltage sensor 18), generating a second value (reference value Sref2) based on the first value (reference value Sref1) if the first value (reference value Sref1) is lower than the upper limit value Slim, and generating a second value (reference value Sref2) based on the upper limit value Slim if the first value (reference value Sref1) is higher than the upper limit value Slim, generating a slope signal SLP whose signal value decreases over time from the second value (reference value Sref2) with the same period as the switching period of the switching circuit 14, comparing the slope signal SLP with a signal corresponding to the peak current (detection signal SIPri), and controlling the switching operation of the switching circuit 14 at a timing corresponding to the comparison result.When the load current Iload at the output power terminals (terminals T21, T22) increases and the voltage VL detected by the first voltage sensor (voltage sensor 18) becomes lower than the first threshold voltage (threshold voltage Vth2), the control circuit 30 is configured to reduce the switching frequency of the switching circuit 14 from a predetermined frequency and increase the upper limit value Slim. In this manner, in the power conversion device 10, when the voltage VL becomes lower than the threshold voltage Vth2, the switching frequency of the switching circuit 14 is reduced. This allows the voltage VL to be sufficiently reduced, thereby providing protection against the load current. Furthermore, in the power conversion device 10, the switching frequency is reduced and the upper limit value Slim is increased. This reduces the possibility of discontinuity occurring in the drooping characteristic, allowing the device to return to its original operating state after the drooping operation. As a result, the power conversion device 10 can effectively perform the drooping operation and return from the drooping operation.

[0080] Furthermore, the power conversion device 10 is further provided with a second voltage sensor (voltage sensor 12) capable of detecting the voltage VH at the input power terminals (terminals T11 and T12). When the voltage VL detected by the first voltage sensor (voltage sensor 18) is lower than the first threshold voltage (threshold voltage Vth2), the control circuit 30 is configured to set the upper limit value Slim based on the voltage VH detected by the second voltage sensor (voltage sensor 12). This allows the power conversion device 10 to set the upper limit value Slim based on the voltage VH, thereby reducing the possibility of discontinuity occurring in the drooping characteristic. Specifically, the upper limit value Slim is determined by the voltages VH and VL and the switching frequency. The voltage VL is known because it is the threshold voltage Vth2, and the switching frequency is known because it is set by the power conversion device 10. Therefore, the power conversion device 10 sets the upper limit value Slim based on the voltage VH, thereby reducing the possibility of discontinuity occurring in the drooping characteristic. This reduces the possibility of discontinuity occurring in the drooping characteristics in the power conversion device 10, allowing the power conversion device 10 to return to its original operating state after the drooping operation. As a result, the power conversion device 10 can effectively return from the drooping operation.

[0081] Furthermore, in the power conversion device 10, the control circuit 30 is configured to reduce the upper limit value Slim from a predetermined value when the voltage VL detected by the first voltage sensor (voltage sensor 18) falls below a second threshold voltage (threshold voltage Vth1) that is higher than the first threshold voltage (threshold voltage Vth2). As a result, in the power conversion device 10, when the voltage VL falls below the threshold voltage Vth1, as shown in FIG. 7 , the upper limit value Slim is reduced from a predetermined value to reduce the voltage VL. Then, in the power conversion device 10, when the voltage VL further decreases and becomes lower than the threshold voltage Vth2, the switching frequency of the switching circuit 14 is reduced from the predetermined frequency and the upper limit value Slim is increased to reduce the voltage VL. This allows the power conversion device 10 to sufficiently reduce the voltage VL, thereby protecting the load current. As a result, the power conversion device 10 can effectively perform a drooping operation.

[0082] In particular, in the power conversion device 10, after reducing the voltage VL by reducing the upper limit value Slim from a predetermined value, the switching frequency of the switching circuit 14 is reduced from the predetermined frequency and the upper limit value Slim is increased. This reduces the possibility of magnetic saturation occurring in the transformer 16. That is, for example, if the power conversion device 10 reduces the switching frequency of the switching circuit 14 from a predetermined frequency and increases the upper limit value Slim without sufficiently reducing the voltage VL, increasing the upper limit value Slim may cause magnetic saturation in the transformer 16. In the power conversion device 10 according to the present embodiment, after reducing the voltage VL by reducing the upper limit value Slim from a predetermined value, the switching frequency of the switching circuit 14 is reduced from the predetermined frequency and the upper limit value Slim is increased. This reduces the possibility of magnetic saturation occurring in the transformer 16.

[0083] Furthermore, in the power conversion device 10, when the load current Iload decreases after the voltage VL detected by the first voltage sensor (voltage sensor 18) becomes lower than the first threshold voltage (threshold voltage Vth2), the control circuit 30 can return the switching frequency to a predetermined frequency and lower the upper limit value Slim when the voltage VL detected by the first voltage sensor (voltage sensor 18) becomes higher than the first threshold voltage (threshold voltage Vth2), and can return the upper limit value Slim to a predetermined value when the voltage VL detected by the first voltage sensor (voltage sensor 18) becomes higher than the second threshold voltage (threshold voltage Vth1). This allows the power conversion device 10 to return to its original operating state after a drooping operation, and to effectively recover from the drooping operation.

[0084] [effect] As described above, this embodiment includes an input power terminal, a switching circuit connected to the input power terminal and having a plurality of switching elements, a transformer having a first winding and a second winding led to the switching circuit, a current sensor capable of detecting a current flowing through one of the first winding and the second winding, a rectifier circuit connected to the second winding and capable of rectifying a signal supplied from the second winding, a smoothing circuit connected to the rectifier circuit and capable of smoothing the voltage rectified by the rectifier circuit, a first voltage sensor capable of detecting a voltage in the smoothing circuit, an output power terminal connected to the smoothing circuit, and a control circuit capable of controlling the switching operation of the plurality of switching elements based on the peak current of the current detected by the current sensor and the voltage detected by the first voltage sensor. The control circuit generates a first value based on the voltage detected by the first voltage sensor, generates a second value based on the first value if the first value is lower than an upper limit value, and generates the second value based on the upper limit value if the first value is higher than the upper limit value. It also generates a slope signal whose signal value decreases over time from the second value with the same period as the switching period of the switching circuit, compares the slope signal with a signal corresponding to the peak current, and controls the switching operation of the switching circuit at a timing corresponding to the comparison result. When the load current Iload at the output power terminal increases and the voltage detected by the first voltage sensor falls below a first threshold voltage, the control circuit reduces the switching frequency of the switching circuit from a predetermined frequency and raises the upper limit value. This allows for effective drooping and recovery from drooping.

[0085] In this embodiment, a second voltage sensor capable of detecting the voltage at the input power terminal is further provided. When the voltage detected by the first voltage sensor is lower than the first threshold voltage, the control circuit is capable of setting an upper limit value based on the voltage detected by the second voltage sensor. This allows for effective recovery from drooping.

[0086] In this embodiment, the control circuit is configured to lower the upper limit from a predetermined value when the voltage detected by the first voltage sensor becomes lower than a second threshold voltage that is higher than the first threshold voltage, thereby enabling the drooping operation to be performed effectively.

[0087] In this embodiment, when the load current decreases after the voltage detected by the first voltage sensor becomes lower than the first threshold voltage, the control circuit can return the switching frequency to a predetermined frequency and lower the upper limit when the voltage detected by the first voltage sensor becomes higher than the first threshold voltage, and can return the upper limit to a predetermined value when the voltage detected by the first voltage sensor becomes higher than the second threshold voltage, thereby enabling an effective recovery from drooping.

[0088] [Variation 1-1] In the above embodiment, the switching operation of the switching circuit 14 is controlled using phase shift control, but this is not limiting. Instead, the switching operation of the switching circuit 14 may be controlled using, for example, PWM (Pulse Width Modulation) control. In this case, the control circuit 30 controls the switching operation by controlling the switching duty ratio of the transistors SA to SD.

[0089] [Variation 1-2] In the above embodiment, the current sensor 13 is provided in a stage preceding the switching circuit 14, but the present invention is not limited to this and the current sensor may be provided at any position as long as it can detect the current flowing through the transformer 16. For example, as in a power conversion system 1A shown in FIG. 9, the current sensor may be provided in a secondary side circuit. This power conversion system 1A includes a power conversion device 10A. The power conversion device 10A includes a current sensor 13A and a control circuit 30A.

[0090] Current sensor 13A is configured to detect current Isec flowing from transformer 16 toward smoothing circuit 20. One end of current sensor 13A is connected to voltage line L21A1, and the other end is connected to voltage line L21A2. Voltage line L21A1 is connected to the other end of winding 16B and one end of winding 16C of transformer 16. Voltage line L21A2 is connected to one end of inductor 21 of smoothing circuit 20. Current sensor 13A generates a detection signal SIsec corresponding to current Isec.

[0091] Similar to the control circuit 30 (FIG. 1) according to the above embodiment, the control circuit 30A is configured to control the operation of the power conversion device 10A by controlling the switching operation of the switching circuit 14 based on the detection signal SVH supplied from the voltage sensor 12, the detection signal SIsec supplied from the current sensor 13A, and the detection signal SVL supplied from the voltage sensor 18.

[0092] [Variation 1-3] In the above embodiment, the rectifier circuit 17 is configured using the diodes D1 and D2, but this is not limiting, and instead, the rectifier circuit 17 may be configured using two transistors. In this case, so-called synchronous rectification can be performed.

[0093] [Other variations] Two or more of these variations may also be combined.

[0094] <2. Second embodiment> Next, a power conversion system 2 according to a second embodiment will be described. This embodiment differs from the first embodiment in the drooping operation. That is, in the first embodiment, as shown in FIG. 7, an operation is performed when the voltage VL is lower than the threshold voltage Vth1, and an operation is performed when the voltage VL is lower than the threshold voltage Vth2. Instead, in the second embodiment, of these two operations, the operation when the voltage VL is lower than the threshold voltage Vth2 is performed. Note that components that are substantially the same as those in the power conversion system 1 according to the first embodiment are denoted by the same reference numerals, and descriptions thereof will be omitted as appropriate.

[0095] 10 shows an example of the configuration of the power conversion system 2. The power conversion system 2 includes a power conversion device 50. The power conversion device 50 includes a control circuit 60.

[0096] 11 shows an example of the configuration of the control circuit 60. The control circuit 60 has an upper limit generating unit 65.

[0097] The upper limit value generating unit 65 is configured to generate an upper limit value Slim of the reference value Sref1 generated by the PI control unit 34 based on the detection signals SVL and SVH. Specifically, as will be described later, when the load current Iload of the power conversion device 50 increases and the voltage VL becomes lower than the threshold voltage Vth2, the upper limit value generating unit 65 increases the upper limit value Slim based on the voltage VH. Specifically, the upper limit value generating unit 65 can correct the upper limit value Slim by multiplying a predetermined upper limit value Slim by a coefficient K. The coefficient K is equal to or greater than 1, and the upper limit value generating unit 65 calculates the coefficient K based on the voltage VH. Furthermore, when the load current Iload decreases and the voltage VL increases, the upper limit value generating unit 65 decreases the upper limit value Slim when the voltage VL becomes higher than the threshold voltage Vth2.

[0098] That is, the upper limit value generation unit 35 according to the first embodiment performs an operation when the voltage VL becomes lower than the threshold voltage Vth1 and an operation when the voltage VL becomes lower than the threshold voltage Vth2, whereas the upper limit value generation unit 65 according to the present embodiment performs the operation when the voltage VL becomes lower than the threshold voltage Vth2 out of these two operations.

[0099] 12 shows an example of the drooping operation of the power conversion device 50, where (A) shows the waveform of the slope signal SLP, (B) shows the waveform of the detection signal SIPri, (C) shows the upper limit level LIM corresponding to the upper limit value Slim, (D) to (G) show the control signals GA to GD, respectively, and (H) shows the voltage VL. In this FIG. 12, the horizontal axis represents time. In this example, the load gradually becomes heavier and the load current Iload gradually increases with the passage of time.

[0100] During the period from timing t41 to t43, the power conversion device 50 operates in the same manner as in the first embodiment (FIGS. 5 and 6). Specifically, during the period from timing t41 to t43, the slope signal SLP gradually decreases from the reference level REF2 (FIG. 12(A)). During the period from timing t41 to t42, the detection signal SIPri gradually increases (FIG. 12(B)), and at timing t42, the detection signal SIPri reaches the same signal level as the slope signal SLP. The comparator 39 outputs this comparison result, and the control signal generator 41 transitions the control signals GC and GD based on the comparison result of the comparator 39 (FIGS. 12(F) and (G)). As a result, at timing t42, the detection signal SIPri decreases (FIG. 12(B)).

[0101] In this example, the load current Iload gradually increases over time. As a result, the signal level of the slope signal SLP during the period from timing t43 to t44 becomes higher than the signal level of the slope signal SLP during the period from timing t32 to t33 (FIG. 12A). That is, in this case, the PI control unit 34 of the control circuit 60 increases the reference value Sref1 as the load current Iload increases. In this example, the reference value Sref1 is lower than the upper limit value Slim, so the limiting processing unit 36 ​​outputs this reference value Sref1 as the reference value Sref2. As a result, the reference level REF2 increases, and the signal level of the slope signal SLP increases. As the load current Iload continues to increase, the reference level REF2 continues to increase, and the signal level of the slope signal SLP also continues to increase.

[0102] Then, at timing t45, when the reference level REF2 of the slope signal SLP reaches the upper limit level LIM corresponding to the upper limit value Slim, the increase in the signal level of the slope signal SLP stops ( FIG. 12(A)). That is, although the load current Iload continues to increase gradually thereafter, the reference value Sref1 generated by the PI control unit 34 becomes higher than the upper limit value Slim, and the limiting processing unit 36 ​​outputs this upper limit value Slim as the reference value Sref2. That is, the reference level REF2 becomes the same as the upper limit level LIM. Therefore, the signal level of the slope signal SLP does not increase but is maintained. In this case, the power conversion device 10 cannot transmit sufficient power from the primary-side circuit to the secondary-side circuit, and therefore, at timing t46, the voltage VL starts to decrease from the target voltage Vtgt.

[0103] Then, at timing t47, when the voltage VL becomes lower than the threshold voltage Vth2, the power conversion device 50 reduces the switching frequency of the switching circuit 14 (FIGS. 12D-12G) and increases the upper limit level LIM (FIG. 12C). Specifically, because the voltage VL is lower than the threshold voltage Vth2, the control signal generation unit 41 of the control circuit 60 reduces the frequency of the control signals GA-GD. This frequency can be, for example, approximately 70% of the frequency in the period before timing t47. The control signal generation unit 41 can gradually reduce this frequency, for example, at a rate of -10 kHz / msec. Furthermore, because the voltage VL is lower than the threshold voltage Vth2, the upper limit value generation unit 65 of the control circuit 60 increases the upper limit value Slim. That is, reducing the switching frequency of the switching circuit 14 can cause a discontinuity in the drooping characteristic, so the power conversion device 50 improves this discontinuity by increasing the upper limit value Slim. As a result, from timing t47 onwards, voltage VL decreases at a steeper slope than during the period from timing t46 to t47. As load current Iload continues to increase, voltage VL continues to decrease.

[0104] In this way, the power conversion device 50 reduces the voltage VL in response to an increase in the load current Iload, thereby enabling the power conversion device 50 to provide protection against the load current.

[0105] In this manner, when the load current Iload is large and the voltage VL increases as a result of a decrease in the load current Iload, the power conversion device 50 performs an operation opposite to the operation shown in Fig. 12. That is, when the voltage VL increases and becomes higher than the threshold voltage Vth2, the power conversion device 50 returns the switching frequency of the switching circuit 14 to the original frequency in the period before timing t47, and also reduces the upper limit value Slim.

[0106] In the power conversion device 50, as in the power conversion device 10 according to the first embodiment, when the voltage VL falls below the threshold voltage Vth2, as shown in FIG. 12, the switching frequency of the switching circuit 14 is reduced and the upper limit level LIM is increased (FIGS. 12(C) to 12(G)). As a result, as in the first embodiment, the voltage VL can be sufficiently reduced and discontinuities that may occur in the drooping characteristics can be improved. As a result, the drooping operation and recovery from the drooping operation can be performed effectively.

[0107] [Variation 2] The power conversion device 50 according to the above embodiment may be applied to each of the modifications of the first embodiment.

[0108] Although the present invention has been described above with reference to several embodiments and modifications, the present invention is not limited to these embodiments and can be modified in various ways.

[0109] For example, although the above embodiment has the circuit configuration shown in Fig. 1, the present invention is not limited to this. For example, the switching circuit 14 of the primary side circuit may be a half-bridge type circuit. Also, for example, the rectifier circuit 17 of the secondary side circuit may be a full-bridge type circuit.

[0110] The effects described in this specification are merely examples, and the effects of the present disclosure are not limited to the effects described in this specification. Therefore, other effects may be obtained with respect to the present disclosure.

[0111] Furthermore, the present disclosure may take the following aspects.

[0112] (1) an input power terminal; a switching circuit connected to the input power terminal and having a plurality of switching elements; a transformer having a first winding and a second winding connected to the switching circuit; a current sensor capable of detecting a current flowing through one of the first winding and the second winding; a rectifier circuit connected to the second winding and capable of rectifying a signal supplied from the second winding; a smoothing circuit connected to the rectifier circuit and capable of smoothing the voltage rectified by the rectifier circuit; a first voltage sensor capable of detecting a voltage in the smoothing circuit; an output power terminal connected to the smoothing circuit; a control circuit capable of controlling the switching operations of the plurality of switching elements based on a peak current of the current detected by the current sensor and a voltage detected by the first voltage sensor; Equipped with The control circuit A first value can be generated based on the voltage detected by the first voltage sensor; If the first value is lower than an upper limit value, a second value can be generated based on the first value, and if the first value is higher than the upper limit value, the second value can be generated based on the upper limit value; a slope signal whose signal value decreases over time from the second value at the same period as the switching period of the switching circuit; The slope signal can be compared with a signal corresponding to the peak current, and the switching operation of the switching circuit can be controlled at a timing corresponding to the comparison result. When the load current at the output power terminal increases, When the voltage detected by the first voltage sensor becomes lower than a first threshold voltage, the switching frequency of the switching circuit can be reduced from a predetermined frequency and the upper limit can be increased. Power conversion device. (2) further comprising a second voltage sensor capable of detecting a voltage at the input power terminal; The control circuit is capable of setting the upper limit value based on the voltage detected by the second voltage sensor when the voltage detected by the first voltage sensor is lower than the first threshold voltage. The power conversion device according to (1) above. (3) The control circuit is capable of reducing the upper limit value from a predetermined value when the voltage detected by the first voltage sensor becomes lower than a second threshold voltage that is higher than the first threshold voltage. The power conversion device according to (1) or (2) above. (4) When the voltage detected by the first voltage sensor is lower than the second threshold voltage, the control circuit is capable of gradually lowering the upper limit value in accordance with the voltage detected by the first voltage sensor. The power conversion device according to (3) above. (5) The control circuit has table data indicating a relationship between the voltage detected by the first voltage sensor and the upper limit value, and is capable of setting the upper limit value based on the voltage detected by the first voltage sensor using the table data. The power conversion device according to (4) above. (6) The control circuit is capable of setting the upper limit value by performing an interpolation calculation based on the voltage detected by the first voltage sensor using the table data. The power conversion device according to (5) above. (7) further comprising a second voltage sensor capable of detecting a voltage at the input power terminal; The control circuit is capable of setting the upper limit value based on the voltage detected by the second voltage sensor. The power conversion device according to (4) above. (8) When the load current decreases after the voltage detected by the first voltage sensor becomes lower than the first threshold voltage, the control circuit When the voltage detected by the first voltage sensor becomes higher than the first threshold voltage, the switching frequency is returned to the predetermined frequency and the upper limit value is reduced. The power conversion device according to any one of (1) to (7) above. (9) When the load current decreases after the voltage detected by the first voltage sensor becomes lower than the first threshold voltage, the control circuit When the voltage detected by the first voltage sensor becomes higher than the first threshold voltage, the switching frequency is returned to the predetermined frequency and the upper limit value is reduced; When the voltage detected by the first voltage sensor becomes higher than the second threshold voltage, the upper limit value can be returned to the predetermined value. The power conversion device according to any one of (3) to (7) above. [Explanation of symbols]

[0113] 1,1A,2...power conversion system, 10,10A,50...power conversion device, 11...capacitor, 12...voltage sensor, 13,13A...current sensor, 14...switching circuit, 15...inductor, 16...transformer, 16A,16B,16C...winding, 17...rectifier circuit, 18...voltage sensor, 20...smoothing circuit, 21...inductor, 22...capacitor, 30,30A,60...control circuit, 31...AD conversion unit, 32...target voltage value setting unit, 33...subtraction unit, 34...PI control unit, 35,65...upper limit value generation unit, 36...limitation processing unit, 37...slope compensation unit, 38...DA conversion unit, 39...comparator, 41...control signal generation unit, BH...high voltage battery, BL...low voltage battery, D1,D2...diode, GA,GB,GC,GD...control signal, Iload... Load current, Ipri...current, Isec...current, Itr...transformer current, K...coefficient, L11A, L11B...voltage line, L12...reference voltage line, L21A, L21A1, L21A2, L21B...voltage line, L22...reference voltage line, LIM...upper limit level, REF2...reference level, SA, SB, SC, SD...transistor, SIPri...detection signal, SIsec...detection signal, SLP...slope signal, SVH...detection signal, SVL...detection signal, Slim...upper limit value, Sref1...reference value, Sref2...reference value, Sslp...slope signal, Stgt...target value, TBL...table data, T11, T12...terminal, T21, T22...terminal, VH...voltage, VL...voltage, Vtgt...target voltage, Vth1, Vth2...threshold voltage, Vtr...transformer voltage.

Claims

1. an input power terminal; a switching circuit connected to the input power terminal and having a plurality of switching elements; a transformer having a first winding and a second winding connected to the switching circuit; a current sensor capable of detecting a current flowing through one of the first winding and the second winding; a rectifier circuit connected to the second winding and capable of rectifying a signal supplied from the second winding; a smoothing circuit connected to the rectifier circuit and capable of smoothing the voltage rectified by the rectifier circuit; a first voltage sensor capable of detecting a voltage in the smoothing circuit; an output power terminal connected to the smoothing circuit; a control circuit capable of controlling the switching operations of the plurality of switching elements based on a peak current of the current detected by the current sensor and a voltage detected by the first voltage sensor; Equipped with The control circuit A first value can be generated based on the voltage detected by the first voltage sensor; When the first value is lower than an upper limit value, a second value can be generated based on the first value, and when the first value is higher than the upper limit value, the second value can be generated based on the upper limit value; a slope signal whose signal value decreases over time from the second value at the same period as the switching period of the switching circuit; The slope signal can be compared with a signal corresponding to the peak current, and the switching operation of the switching circuit can be controlled at a timing corresponding to the comparison result. When the load current at the output power terminal increases, When the voltage detected by the first voltage sensor becomes lower than a first threshold voltage, the switching frequency of the switching circuit can be reduced from a predetermined frequency and the upper limit can be increased. Power conversion device.

2. a second voltage sensor capable of detecting a voltage at the input power terminal; The control circuit is capable of setting the upper limit value based on the voltage detected by the second voltage sensor when the voltage detected by the first voltage sensor is lower than the first threshold voltage. The power conversion device according to claim 1 .

3. The control circuit is capable of reducing the upper limit value from a predetermined value when the voltage detected by the first voltage sensor becomes lower than a second threshold voltage that is higher than the first threshold voltage. The power conversion device according to claim 1 .

4. When the voltage detected by the first voltage sensor is lower than the second threshold voltage, the control circuit is capable of gradually lowering the upper limit value in accordance with the voltage detected by the first voltage sensor. The power conversion device according to claim 3 .

5. The control circuit has table data indicating a relationship between the voltage detected by the first voltage sensor and the upper limit value, and is capable of setting the upper limit value based on the voltage detected by the first voltage sensor using the table data. The power conversion device according to claim 4.

6. The control circuit is capable of setting the upper limit value by performing an interpolation calculation based on the voltage detected by the first voltage sensor using the table data. The power conversion device according to claim 5 .

7. a second voltage sensor capable of detecting a voltage at the input power terminal; The control circuit is capable of setting the upper limit value based on the voltage detected by the second voltage sensor. The power conversion device according to claim 4.

8. When the load current decreases after the voltage detected by the first voltage sensor becomes lower than the first threshold voltage, the control circuit When the voltage detected by the first voltage sensor becomes higher than the first threshold voltage, the switching frequency is returned to the predetermined frequency and the upper limit value is reduced. The power conversion device according to claim 1 .

9. When the load current decreases after the voltage detected by the first voltage sensor becomes lower than the first threshold voltage, the control circuit When the voltage detected by the first voltage sensor becomes higher than the first threshold voltage, the switching frequency is returned to the predetermined frequency and the upper limit value is reduced, When the voltage detected by the first voltage sensor becomes higher than the second threshold voltage, the upper limit value can be returned to the predetermined value. The power conversion device according to claim 3 .

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