Switching power supply device and control method thereof

Abstract

To suppress an increase of an output voltage at low cost by appropriately determining deterioration of a load current.SOLUTION: A switching power supply device 1 comprises: a voltage conversion unit 2 that outputs a certain target voltage Vtarget via a first switch SW1, a second switch SW2, and a resonance circuit, which are subjected to PFM control; a frequency control unit 4 that converts an output voltage Vout of the voltage conversion unit 2 to a modulation frequency Fmod of the PFM control; and a driving circuit 5 that drives the first switch SW1 and the second switch SW2 on the basis of the modulation frequency Fmod through PFM control. The switching power supply device 1 also includes an intermittent control circuit 8 that performs intermittent control of the PFM control of the driving circuit 5 when both the modulation frequency Fmod and the output voltage Vout have increased.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a switching power supply device and a control method thereof. [Background technology]

[0002] LLC resonant circuit switching power supplies have been widely used because they can meet a variety of market needs, from small to large capacities, with high efficiency and low noise. When controlled by pulse frequency modulation (PFM), for example, such switching power supplies maintain a constant output voltage by increasing the switching frequency under light loads where the output current is small, and decreasing the switching frequency under heavy loads, thereby supplying stable power to the load device.

[0003] Here, in the switching power supply device described above, an upper limit frequency is set to ensure stable operation of the switching element, so when the load current is extremely low or in a no-load state, the increase in output voltage cannot be suppressed, and the overvoltage protection (OVP) may function, leading to latch-off.

[0004] To address this issue, for example, Patent Document 1 discloses a switching power supply that removes a dummy load by activating an active resistor when the load current is equal to or less than a predetermined threshold. Patent Document 1 also discloses a control mode that maintains the output voltage within a predetermined range by intermittent operation that repeatedly activates and stops switching when the output voltage cannot be controlled even with the dummy load. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-107996 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the above-mentioned conventional technology, the resistance value of the current detection resistor used to measure the load current must be high in order to measure minute currents, but the power conversion efficiency decreases due to increased loss when the load current is large. Furthermore, measuring minute load currents using a current detection resistor with a relatively small resistance requires a high-performance detection IC and a negative power supply to ensure operation near zero voltage, which increases costs.

[0007] Furthermore, in the switching power supply devices described above, an increase in output voltage is not necessarily caused by a decrease in load current, and performing intermittent switching during a temporary increase in output voltage can lead to unstable operation. Furthermore, if the increase in output voltage is caused by an internal fault, it may be more appropriate to stop control by appropriately latching off rather than forcibly continuing control through the intermittent operation. Therefore, the above-described conventional technology, which controls intermittent switching based on the output voltage, may not always be able to provide appropriate control in response to an increase in output voltage.

[0008] The present disclosure has been made in consideration of these problems, and its purpose is to provide a switching power supply device and a control method thereof that can appropriately determine a decrease in load current and suppress an increase in output voltage at low cost. [Means for solving the problem]

[0009] In order to achieve the above object, the switching power supply device of the present disclosure is a switching power supply device comprising: a voltage conversion unit that outputs a constant target voltage using a PFM-controlled switching element and a resonant circuit; a frequency control unit that converts the output voltage of the voltage conversion unit to a modulation frequency for the PFM control; and a drive circuit that drives the switching element with the PFM control based on the modulation frequency, and an intermittent control circuit that intermittently controls the PFM control of the drive circuit when both the modulation frequency and the output voltage increase.

[0010] Furthermore, in order to achieve the above object, the control method for a switching power supply device disclosed herein is a control method for a switching power supply device that outputs a constant target voltage using a PFM-controlled switching element and a resonant circuit, converting an output voltage output as the target voltage to a modulation frequency for the PFM control, driving the switching element with the PFM control based on the modulation frequency, and intermittently controlling the PFM control when both the modulation frequency and the output voltage increase. [Effects of the Invention]

[0011] According to the switching power supply device and the control method thereof according to the present disclosure, it is possible to appropriately determine a drop in load current and suppress an increase in output voltage at low cost. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is an overall configuration diagram of a switching power supply device; [Figure 2] FIG. 2 is a circuit diagram illustrating an internal configuration of a feedback circuit. [Figure 3] 4 is a waveform showing voltage changes of the modulation frequency and the output voltage during normal operation and abnormal operation. [Figure 4] 4 is a waveform showing changes in the modulation frequency and output voltage of a conventional power supply device when the load current decreases. [Figure 5]6 is a waveform showing changes in the modulation frequency and the output voltage of the switching power supply device of the present embodiment when the load current decreases. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, the embodiments will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the contents described below, and can be implemented with any modifications within the scope that does not change the gist of the disclosure. Furthermore, all drawings used to explain the embodiments are schematic representations of components, and may be partially emphasized, enlarged, reduced, or omitted to facilitate understanding, and may not accurately represent the scale, shape, etc. of the components.

[0014] FIG. 1 is an overall configuration diagram of a switching power supply 1. The switching power supply 1 of this embodiment is an isolated DC-DC converter using a so-called LLC resonant circuit, and includes a voltage conversion unit 2 and a feedback circuit 3. The switching power supply 1 of this embodiment converts an input voltage Vin input to a pair of input terminals, consisting of a high-side input terminal Tin(+) and a low-side input terminal Tin(-), provided on the primary side of the voltage conversion unit 2, and outputs the converted voltage as an output voltage Vout from a pair of output terminals, consisting of a high-side output terminal Tout(+) and a low-side output terminal Tout(-), provided on the secondary side of the voltage conversion unit 2. In this manner, the switching power supply 1 controls the output voltage Vout to a constant target voltage Vtarget, and supplies stabilized power to a load device (not shown) as an output destination. Here, in this embodiment, both the low-side input terminal Tin(-) and the low-side output terminal Tout(-) are assumed to be grounded.

[0015] The voltage conversion unit 2 includes an input capacitor Cin, a first switch SW1, a second switch SW2, a transformer Tr, a resonant coil Lr, a resonant capacitor Cr, a first diode D1, a second diode D2, and an output capacitor Cout.

[0016] The input capacitor Cin has one end connected to the high potential side input terminal Tin(+) and the other end connected to the low potential side input terminal Tin(-), and serves as an input filter that smoothes the input voltage Vin.

[0017] The first switch SW1 and the second switch SW2 are switching elements formed, for example, by MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), and are PFM controlled (Pulse Frequency Modulation) by a drive circuit 5 (described later) connected to their gates to convert the DC input voltage Vin into AC power. The drain of the first switch SW1 is connected to the high-potential input terminal Tin(+), and the source is connected to the drain of the second switch SW2. The second switch SW2 is connected to the low-potential input terminal Tin(-). Resistors for limiting current are connected to the terminals of each switch as appropriate.

[0018] The transformer Tr includes a primary coil L11 and secondary coils L21 and L22, and transmits AC power supplied from the primary circuit to the secondary circuit while providing DC insulation between the primary and secondary circuits of the voltage conversion unit 2. The winding start end of the primary coil L11 is connected to the connection point between the first switch SW1 and the second switch SW2 via a resonant coil Lr. The winding end end of the primary coil L11 is connected to the source of the second switch SW2 via a resonant capacitor Cr. In other words, the resonant coil Lr, the primary coil L11, and the resonant capacitor Cr form an LLC resonant circuit.

[0019] The first diode D1 and the second diode D2 are, for example, Schottky barrier diodes, and constitute a rectifier circuit that converts AC power output from the transformer Tr into DC power. The anode of the first diode D1 is connected to the winding start end of the secondary coil L21, and the cathode is connected to one end of the output capacitor Cout. The anode of the second diode D2 is connected to the winding end end of the secondary coil L22, and the cathode is connected to one end of the output capacitor Cout. The junction point of the secondary coils L21 and L22 is connected to the other end of the output capacitor Cout.

[0020] The output capacitor Cout has one end connected to the high potential side output terminal Tout(+) and the other end connected to the low potential side output terminal Tout(-), and serves as an output filter that smoothes the output voltage Vout.

[0021] As a result, the voltage conversion unit 2 outputs a stabilized output voltage Vout to a pair of output terminals. Note that the voltage conversion unit 2 is not limited to the configuration shown in Fig. 1, and various forms of power conversion mechanisms including switching elements and resonant circuits can be adopted.

[0022] Next, a specific description will be given of the feedback circuit 3 for controlling the voltage conversion unit 2. Fig. 2 is a circuit diagram showing the internal configuration of the feedback circuit 3. The feedback circuit 3 includes a frequency control unit 4, a drive circuit 5, an output voltage detection unit 6, an overvoltage protection circuit 7, and an intermittent control circuit 8.

[0023] The frequency control unit 4 is a circuit that takes the output voltage Vout output from the voltage conversion unit 2 as an input value and converts it into a modulation frequency Fmod for PFM control, and in this embodiment outputs it to the drive circuit 5 as a frequency control voltage corresponding to the modulation frequency Fmod.

[0024] More specifically, the frequency control unit 4 includes a first amplifier AMP1, as well as a plurality of resistors and capacitors for adjusting the gain and removing high-frequency noise. The first amplifier AMP1 receives a divided voltage of the output voltage Vout output to the high-potential output terminal Tout(+) as its non-inverting input, and a predetermined first reference voltage Ref1 as its inverting input, thereby outputting a frequency control voltage for PFM control according to the output voltage Vout. Note that the frequency control unit 4 sets an upper limit frequency Fmax for the PFM control frequency to prevent damage to the first switch SW1 and the second switch SW2.

[0025] The drive circuit 5 is a driver for driving the first switch SW1 and the second switch SW2 with PFM control based on the modulation frequency Fmod converted by the frequency control unit 4, and by alternately turning the gate voltages of the first switch SW1 and the second switch SW2 ON / OFF in accordance with the modulation frequency Fmod, it controls the output voltage Vout output by the voltage conversion unit 2 to be the target voltage Vtarget, regardless of fluctuations in the input voltage Vin of the voltage conversion unit 2 or the load.

[0026] The output voltage detection unit 6 includes an output amplifier AMPv and is appropriately equipped with a plurality of resistors for adjusting the amplification factor, and is a circuit that uses the voltages of the high-potential side output terminal Tout(+) and the low-potential side output terminal Tout(-) as input values and detects the output voltage Vout from the difference between them.

[0027] The overvoltage protection circuit 7 monitors the output voltage Vout detected by the output voltage detection unit 6, and when the output voltage Vout rises and reaches a predetermined overvoltage threshold Vovp, it puts the drive circuit 5 into a so-called latch-off state and stops the operation of the drive circuit 5. More specifically, the overvoltage protection circuit 7 includes a second comparator CP2 that receives the output value of the output voltage detection unit 6 as its non-inverting input and a predetermined second reference voltage Ref2 as its inverting input, and a latch-off diode Doff that has an anode connected to the first auxiliary power source SP1 via a resistor and a cathode connected to the output terminal of the second comparator CP2 and the drive circuit 5.

[0028] The intermittent control circuit 8 is a circuit that intermittently controls the PFM control of the drive circuit 5 when both the modulation frequency Fmod from the frequency control unit 4 and the output voltage Vout from the output voltage detection unit 6 increase. Here, the intermittent control circuit 8 according to this embodiment includes a voltage rise detection circuit 8a, a voltage determination diode Dv, a frequency rise detection circuit 8b, a frequency determination diode Df, and an oscillation stop circuit 8c.

[0029] The voltage rise detection circuit 8a mainly comprises a third comparator CP3, a first resistor R1, a second resistor R2, and a first hysteretic diode Da. The third comparator CP3 receives the output signal of the output voltage detection unit 6 at its non-inverting input via the first resistor R1, and receives a predetermined third reference voltage Ref3 at its inverting input. The first hysteretic diode Da has its anode connected to the inverting input of the third comparator CP3 via the second resistor R2, and its cathode connected to the output side of the third comparator CP3. This makes the third comparator CP3 a so-called hysteresis comparator.

[0030] With this configuration, when the voltage on the output side of the third comparator CP3 is at low level, a voltage obtained by dividing the output voltage Vout detected by the output voltage detection unit 6 using the first resistor R1 and the second resistor R2 is input to the non-inverting input. When this voltage rises to the third reference voltage Ref3, that is, when the output voltage Vout reaches the first output voltage threshold Vth1, the voltage on the output side of the third comparator CP3 switches from low level to high level.

[0031] Furthermore, when the voltage on the output side of the third comparator CP3 is at a high level, the output voltage Vout output from the output voltage detection unit 6 is input to the non-inverting input via only the first resistor R1. When this voltage drops to the third reference voltage Ref3, that is, when the output voltage Vout drops to the second output voltage threshold Vth2, the voltage on the output side of the third comparator CP3 switches from a high level to a low level.

[0032] Here, the voltage rise detection circuit 8a in this embodiment sets the voltage when the output voltage Vout rises to +2% of the target voltage Vtarget as the first output voltage threshold Vth1, and sets the voltage when the output voltage Vout falls to -2% of the target voltage Vtarget as the second output voltage threshold Vth2. That is, the voltage rise detection circuit 8a switches the output from low level to high level when the output voltage Vout rises 2% from the target voltage Vtarget, and switches the output from high level to low level when the output voltage Vout falls 2% from the target voltage Vtarget.

[0033] The voltage determination diode Dv has its cathode connected to the output side of the third comparator CP3 and is in a reverse bias state when the voltage rise detection circuit 8a outputs a high-level signal.

[0034] The frequency rise detection circuit 8b primarily comprises a fourth comparator CP4, a third resistor R3, a fourth resistor R4, and a second hysteretic diode Db. The fourth comparator CP4 receives the output signal of the frequency control unit 4 at its non-inverting input via the third resistor R3, and receives a predetermined fourth reference voltage Ref4 at its inverting input. The second hysteretic diode Db has its anode connected to the inverting input of the fourth comparator CP4 via the fourth resistor R4, and its cathode connected to the output of the fourth comparator CP4. This makes the fourth comparator CP4 a so-called hysteresis comparator.

[0035] With this configuration, when the voltage on the output side of the fourth comparator CP4 is at low level, a voltage obtained by dividing the frequency control voltage corresponding to the modulation frequency Fmod output by the frequency control unit 4 using the third resistor R3 and the fourth resistor R4 is input to the non-inverting input. Then, when this voltage rises to the fourth reference voltage Ref4, that is, when the modulation frequency Fmod reaches the first frequency threshold Fth1, the voltage on the output side of the fourth comparator CP4 switches from low level to high level.

[0036] Furthermore, when the voltage on the output side of the fourth comparator CP4 is at a high level, a frequency control voltage corresponding to the modulation frequency Fmod output from the frequency control unit 4 is input to the non-inverting input via only the third resistor R3. When this voltage drops to the fourth reference voltage Ref4, that is, when the modulation frequency Fmod drops to the second frequency threshold Fth2, the voltage on the output side of the fourth comparator CP4 switches from a high level to a low level.

[0037] The frequency determination diode Df has its cathode connected to the output side of the fourth comparator CP4 and is in a reverse bias state when the frequency increase detection circuit 8b outputs a high-level signal.

[0038] The oscillation stop circuit 8c mainly comprises a fifth comparator CP5, a fifth resistor R5, and a sixth resistor R6. The fifth comparator CP5 has a non-inverting input connected to the second auxiliary power supply SP2 via the fifth resistor R5 and the sixth resistor R6, an inverting input connected to a predetermined fifth reference voltage Ref5, and an output connected to the drive circuit 5. The node between the fifth resistor R5 and the sixth resistor R6 is connected to the anodes of a voltage determination diode Dv and a frequency determination diode Df.

[0039] The fifth comparator CP5 has a low-level non-inverting input when at least one of the voltage determination diode Dv and the frequency determination diode Df is forward-biased, and a high-level non-inverting input when both are reverse-biased. At this time, the fifth comparator CP5 outputs a control signal to the drive circuit 5 to temporarily suspend PFM control. When at least one of the voltage determination diode Dv and the frequency determination diode Df returns to a forward-biased state, the fifth comparator CP5 stops the control signal suspending PFM control, thereby resuming PFM control by the drive circuit 5. In other words, when both the modulation frequency Fmod and the output voltage Vout increase, the intermittent control circuit 8 intermittently controls the PFM control of the drive circuit 5 to suppress fluctuations in the output voltage Vout.

[0040] Next, we will explain the operation of each part of the switching power supply device 1 when the load current Iload is sufficiently large. Figure 3 shows waveforms showing the voltage changes of the modulation frequency Fmod and the output voltage Vout during normal operation and abnormal operation. Here, the horizontal axis represents the passage of time, and the vertical axis represents the magnitude of the frequency and voltage. The output level of the overvoltage protection circuit 7 is also represented as OVP.

[0041] As shown in the period from timing T0 to timing T1 in Figure 3, when the switching power supply device 1 is operating in a steady state, the output voltage Vout is 54 V, which is the target voltage Vtarget of this embodiment, and accordingly the modulation frequency Fmod is also maintained at a constant steady frequency Fs.

[0042] Here, as shown in the period from timing T1 to timing T2, the switching power supply 1 may experience fluctuations in, for example, the input voltage Vin to the voltage conversion unit 2 or the load current Iload. Even in such a case, the frequency control unit 4 adjusts the modulation frequency Fmod to P2 accordingly so that the output voltage Vout is not affected by the fluctuations indicated by the dashed line P1. As a result, the output voltage Vout can maintain the target voltage Vtarget of 54 V even during the period from timing T1 to timing T2. In other words, as shown in the period from timing T0 to timing T3, the switching power supply 1 can supply a constant, stable output voltage Vout to the load device during normal operation, even in the presence of input and load fluctuations.

[0043] On the other hand, during abnormal operation of the switching power supply device 1, as shown at timing T3, the modulation frequency Fmod drops even though the output voltage Vout has started to rise, resulting in a state in which fluctuations in the output voltage Vout cannot be suppressed. For example, in the frequency control unit 4, an internal failure occurs in the first amplifier AMP1, or the first reference voltage Ref1, which should be constant, becomes a voltage that differs from the default value, which can lead to an increase in the output voltage Vout. Note that even when the output voltage Vout rises and the modulation frequency Fmod is maintained at the steady frequency Fs, this also results in abnormal operation in which fluctuations in the output voltage Vout cannot be suppressed.

[0044] In such a case, at timing T4 when the output voltage Vout rises and reaches a predetermined overvoltage threshold Vovp, the overvoltage protection circuit 7 outputs a high-level signal as OVP to stop operation of the drive circuit 5 and appropriately latch off the switching power supply device 1. As a result, the switching power supply device 1 sets the output voltage Vout and modulation frequency Fmod to 0, as shown at timing T5, to ensure safety against overvoltage. In this embodiment, the overvoltage threshold Vovp is set to a voltage at which the output voltage Vout rises by 10% above the target voltage Vtarget.

[0045] Here, even during abnormal operation of the switching power supply device 1, if the first reference voltage Ref1 becomes a voltage different from the predetermined value as described above, the device may return to normal operation before latching off. For example, if the first reference voltage Ref1 gradually returns to the predetermined value during abnormal operation, the output voltage Vout and modulation frequency Fmod return to a steady state as indicated by the dashed lines P3 and P4 in FIG. 3. In this case, the switching power supply device 1 can continue to operate normally even after timing T4 without latching off. Note that this return to the steady state may be completed instantaneously if the first reference voltage Ref1 returns abruptly.

[0046] Next, the operation and control method of the switching power supply 1 under light or no load conditions will be described. In a typical power supply using a resonant circuit, when the load current Iload decreases and the load becomes light or no load, it becomes difficult to stabilize the output voltage Vout and the modulation frequency Fmod, and both increase. Below, the effects of the switching power supply 1 according to this embodiment will be described in comparison with a conventional power supply not equipped with the above-mentioned intermittent control circuit 8.

[0047] 4 shows waveforms illustrating the changes in the modulation frequency Fmod and the output voltage Vout of a conventional power supply device when the load current Iload is reduced, where the rated load current Iload is 0.1 A or more.

[0048] As shown in FIG. 4, in the conventional power supply device, while the load current Iload is equal to or greater than the rated current, the modulation frequency Fmod is at the steady-state frequency Fs and the output voltage Vout is at a constant output of 54 V. However, when the load current Iload falls below the rated current, as at timing T6, the increase in the modulation frequency Fmod and the output voltage Vout cannot be suppressed.

[0049] Furthermore, as the modulation frequency Fmod and output voltage Vout continue to rise, the modulation frequency Fmod reaches the upper limit frequency Fmax at timing T7 and plateaus, causing the output voltage Vout to rise further and reach the overvoltage threshold Vovp (i.e., +10%) at timing T8, at which point the output signal of the overvoltage protection circuit 7 switches from low to high, resulting in latch-off and causing the drive circuit 5 to stop PFM control.

[0050] Then, at timing T9, both the modulation frequency Fmod and the output voltage Vout become 0. As a result, the switching power supply device 1 is able to stop operation due to the overvoltage protection function that prevents an excessive increase in the output voltage Vout, but the continued operation is prevented due to a drop in the load current Iload that is not a fault.

[0051] To address this issue, one possible solution is to detect an increase in the output voltage Vout and temporarily stop the PWM control of the drive circuit 5 to operate intermittently. However, this approach would force the control to continue through intermittent operation even when the output voltage Vout increases due to an internal fault, as in the period from timing T3 to timing T4 in Figure 3, which could worsen the situation.

[0052] Therefore, the switching power supply device 1 of this embodiment is provided with an intermittent control circuit 8 that intermittently controls the PFM control of the drive circuit 5 when both the modulation frequency Fmod and the output voltage Vout increase, thereby determining whether the increase in the output voltage Vout is due to a decrease in the load current Iload and suppressing the increase in the output voltage Vout by appropriate means.

[0053] 5 is a waveform diagram showing changes in the modulation frequency Fmod and the output voltage Vout of the switching power supply 1 of this embodiment when the load current Iload drops. As shown in FIG. 5, in the switching power supply 1 of this embodiment as well, when the load current Iload drops to the rated current or less, as at timing T10, the modulation frequency Fmod and the output voltage Vout rise.

[0054] However, as shown at timing T11, when the modulation frequency Fmod increases to or above the first frequency threshold Fth1 and the output voltage Vout increases to or above the first output voltage threshold Vth1, both the voltage determination diode Dv and the frequency determination diode Df shown in FIG. 2 become reverse biased. Therefore, when both the modulation frequency Fmod and the output voltage Vout increase, the intermittent control circuit 8 switches the output of the fifth comparator CP5 from low to high, temporarily suspending the PFM control of the drive circuit 5. As a result, both the modulation frequency Fmod and the output voltage Vout decrease, as shown in the period from timing T11 to timing T12.

[0055] Here, as shown in Fig. 5, the first frequency threshold Fth1 is set to a frequency lower than the upper limit frequency Fmax of PFM control. The first output voltage threshold Vth1 is set to the voltage value of the output voltage Vout when the modulation frequency Fmod coincides with the upper limit frequency Fmax, i.e., a voltage lower than the +5% line in this embodiment. When the modulation frequency Fmod and the output voltage Vout reach their respective thresholds, it is determined that they have increased, and intermittent PFM control is initiated. This allows the switching power supply device 1 to reliably initiate intermittent control before the modulation frequency Fmod reaches the upper limit frequency Fmax.

[0056] 5, the timing at which the modulation frequency Fmod reaches the first frequency threshold Fth1 and the timing at which the output voltage Vout reaches the first voltage threshold Vth1 (+2%) are shown to be simultaneous. However, these timings do not necessarily have to be simultaneous. For example, the thresholds for both may be set so that the modulation frequency Fmod reaches the first frequency threshold Fth1 before the output voltage Vout reaches the first output voltage threshold Vth1 around timing T11 at which intermittent control is started. In this case, intermittent control is started when the output voltage Vout rises to the first voltage threshold Vth1 (+2%). Therefore, the upper limit of the output voltage Vout when the load current Iload decreases can be limited to an arbitrary set value (here, +2%).

[0057] Then, as shown at timing T12, when the modulation frequency Fmod drops to the second frequency threshold Fth2 or the output voltage Vout drops to the second output voltage threshold Vth2, which is −2%, the voltage determination diode Dv or the frequency determination diode Df shown in Fig. 2 becomes forward biased. Therefore, when at least one of the modulation frequency Fmod or the output voltage Vout drops, the intermittent control circuit 8 switches the output of the fifth comparator CP5 from high level to low level, and performs control to resume PFM control of the drive circuit 5.

[0058] As a result, both the modulation frequency Fmod and the output voltage Vout increase again, as shown in the period from timing T12 to timing T13. However, during periods of light load or no load, the switching power supply device 1 can limit the output voltage Vout to within a range of ±2% by using intermittent control that repeatedly stops and restarts PFM control, as shown at timings T13 and T14, for example.

[0059] Here, the second output voltage threshold Vth2 is not limited to -2%, but by setting it to at least a value lower than the target voltage Vtarget, a period during which PFM control is stopped can be secured until the output voltage Vout during intermittent operation crosses the target voltage Vtarget, thereby suppressing a decrease in operating efficiency during intermittent control.

[0060] Then, as shown at timing T15, when the load current Iload recovers to the rated value of 0.1 A or more, the modulation frequency Fmod and the output voltage Vout return to the steady-state frequency Fs and the target voltage Vtarget, respectively, and steady-state operation continues.

[0061] As described above, the switching power supply 1 according to this embodiment performs intermittent control by repeatedly stopping and restarting PFM control when both the output voltage Vout and the modulation frequency Fmod of the PFM control in the voltage conversion unit 2, which supplies a constant output voltage Vout via a resonant circuit, increase. This allows the switching power supply 1 to control the output voltage Vout within a predetermined voltage range, even when, for example, a light-load or no-load condition makes it impossible to suppress an increase in the output voltage Vout by adjusting the modulation frequency Fmod alone. By monitoring not only the increase in the output voltage Vout but also the increase in the modulation frequency Fmod, the switching power supply 1 can determine whether the increase in the output voltage Vout is due to a decrease in the load current Iload or an internal fault, etc. This allows the switching power supply 1 to appropriately identify the decrease in the load current Iload and suppress the increase in the output voltage Vout at low cost without interfering with proper latch-off in the event of an internal fault.

[0062] <Implementation> A first embodiment of the present disclosure is a switching power supply device comprising: a voltage conversion unit that outputs a constant target voltage using a PFM-controlled switching element and a resonant circuit; a frequency control unit that converts the output voltage of the voltage conversion unit to a modulation frequency of the PFM control; and a drive circuit that drives the switching element with the PFM control based on the modulation frequency, and an intermittent control circuit that intermittently controls the PFM control of the drive circuit when both the modulation frequency and the output voltage increase.

[0063] A switching power supply according to a first embodiment performs intermittent control by repeatedly stopping and restarting PFM control in a voltage conversion section that supplies a constant output voltage via a resonant circuit when both the output voltage and the modulation frequency of the PFM control increase. This allows the switching power supply to control the output voltage within a predetermined voltage range, even when, for example, a light-load or no-load condition makes it impossible to suppress an increase in output voltage by adjusting the modulation frequency alone. By monitoring not only the increase in output voltage but also the increase in modulation frequency, the switching power supply can determine whether the increase in output voltage is due to a decrease in load current or an internal fault, etc. This allows the switching power supply to appropriately identify a decrease in load current and suppress an increase in output voltage at low cost without interfering with appropriate latch-off in the event of an internal fault.

[0064] A second embodiment of the present disclosure is a switching power supply device in which, in the first embodiment described above, the modulation frequency is determined to have increased when it reaches a predetermined first frequency threshold that is set lower than the upper limit frequency of the PFM control, and the output voltage is determined to have increased when it reaches a predetermined first output voltage threshold that is set lower than the voltage when the modulation frequency is the upper limit frequency.

[0065] According to the second embodiment, when the load current decreases, thresholds for detecting an increase in the modulation frequency and the output voltage are set so that intermittent control is started before the modulation frequency reaches the upper limit frequency of PFM control, so that the output voltage can be reliably suppressed at a stage where control of the output increase is possible.

[0066] A third embodiment of the present disclosure is a switching power supply device according to the second embodiment described above, wherein, at the start of the intermittent control, the modulation frequency is set to reach the first frequency threshold before the output voltage reaches the first output voltage threshold.

[0067] According to the third embodiment, when the load current decreases, the modulation frequency first reaches the first frequency threshold, and then the intermittent control of PFM control is started when the output voltage reaches the first output voltage threshold, so that the output voltage during the intermittent control can be limited to an arbitrarily set voltage range.

[0068] A fourth embodiment of the present disclosure is a switching power supply device in which, in any of the first to third embodiments described above, the drive circuit resumes the PFM control when the output voltage drops to a predetermined second output voltage threshold that is lower than the target voltage.

[0069] According to the fourth embodiment, in the intermittent operation of PFM control, after the PFM control is temporarily stopped, the resumption of PFM control is suspended until the output voltage crosses the target voltage, thereby ensuring a stop period, thereby suppressing a decrease in the operating efficiency in the intermittent control.

[0070] A fifth embodiment of the present disclosure is a control method for a switching power supply device that outputs a constant target voltage using a PFM-controlled switching element and a resonant circuit, the control method converting an output voltage output as the target voltage to a modulation frequency of the PFM control, driving the switching element with the PFM control based on the modulation frequency, and intermittently controlling the PFM control when both the modulation frequency and the output voltage increase.

[0071] A control method for a switching power supply according to a fifth embodiment performs intermittent control by repeatedly stopping and restarting PFM control in a voltage conversion unit that supplies a constant output voltage via a resonant circuit when both the output voltage and the modulation frequency of the PFM control increase. This control method for a switching power supply allows the output voltage to be controlled within a predetermined voltage range, even when, for example, a light load or no load condition makes it impossible to suppress an increase in output voltage by adjusting the modulation frequency alone. By monitoring not only the increase in output voltage but also the increase in modulation frequency, the switching power supply can determine whether the increase in output voltage is due to a decrease in load current or an internal fault, etc. This allows the switching power supply to appropriately identify a decrease in load current and suppress an increase in output voltage at low cost without interfering with appropriate latch-off in the event of an internal fault.

[0072] A sixth embodiment of the present disclosure is a control method for a switching power supply device, in which, in the above-mentioned fifth embodiment, the modulation frequency is determined to have increased when it reaches a predetermined first frequency threshold that is set lower than the upper limit frequency of the PFM control, and the output voltage is determined to have increased when it reaches a predetermined first output voltage threshold that is set lower than the voltage when the modulation frequency is the upper limit frequency.

[0073] According to the sixth embodiment, when the load current decreases, thresholds for detecting an increase in the modulation frequency and the output voltage are set so that intermittent control is started before the modulation frequency reaches the upper limit frequency of PFM control, so that the output voltage can be reliably suppressed at a stage where control of the output increase is possible.

[0074] A seventh embodiment of the present disclosure is a control method for a switching power supply device according to the sixth embodiment, wherein, at the start of the intermittent control, the modulation frequency is set to reach the first frequency threshold before the output voltage reaches the first output voltage threshold.

[0075] According to the seventh embodiment, when the load current decreases, the modulation frequency first reaches the first frequency threshold, and then the intermittent control of PFM control is started when the output voltage reaches the first output voltage threshold, so that the output voltage during the intermittent control can be limited to an arbitrarily set voltage range.

[0076] An eighth embodiment of the present disclosure is a control method for a switching power supply device, in any of the fifth to seventh embodiments described above, in which the PFM control is resumed when the output voltage drops to a predetermined second output voltage threshold that is lower than the target voltage.

[0077] According to the eighth embodiment, in the intermittent operation of PFM control, after temporarily stopping the PFM control, the resumption of PFM control is postponed until the output voltage crosses the target voltage, thereby ensuring a stop period, thereby suppressing a decrease in operating efficiency in the intermittent control. [Explanation of symbols]

[0078] 1. Switching power supply 2 Voltage conversion section 3 Feedback circuit 4 Frequency control section 5. Drive circuit 6 Output voltage detection section 7 Overvoltage protection circuit 8 Intermittent control circuit 8a Voltage rise detection circuit 8b Frequency increase detection circuit 8c Oscillation stop circuit SW1 First switch SW2 Second switch Tr transformer Lr resonance coil Cr Resonant Capacitor Dv voltage judgment diode Df Frequency determination diode AMP1, AMPv 1st amplifier, output amplifier CP2~CP5 2nd comparator to 5th comparator Ref1~Ref5 1st reference voltage~5th reference voltage R1~R6 1st~6th resistors

Claims

1. a voltage conversion unit that outputs a constant target voltage using a PFM-controlled switching element and a resonant circuit; a frequency control unit that converts the output voltage of the voltage conversion unit into a modulation frequency for the PFM control; a drive circuit that drives the switching element by the PFM control based on the modulation frequency, an intermittent control circuit that intermittently controls the PFM control of the drive circuit when both the modulation frequency and the output voltage increase.

2. The modulation frequency is determined to be increased when it reaches a predetermined first frequency threshold that is set lower than an upper limit frequency of the PFM control, 2. The switching power supply device according to claim 1, wherein the output voltage is determined to be increased when it reaches a predetermined first output voltage threshold value that is set lower than the voltage when the modulation frequency is the upper limit frequency.

3. 3. The switching power supply device according to claim 2, wherein the modulation frequency is set to reach the first frequency threshold before the output voltage reaches the first output voltage threshold at the start of the intermittent control.

4. 4. The switching power supply device according to claim 1, wherein the drive circuit resumes the PFM control when the output voltage drops to a predetermined second output voltage threshold value that is lower than the target voltage.

5. A control method for a switching power supply device that outputs a constant target voltage using a PFM-controlled switching element and a resonant circuit, comprising: converting an output voltage output as the target voltage into a modulation frequency for the PFM control; driving the switching element under the PFM control based on the modulation frequency; A control method for a switching power supply device, wherein the PFM control is intermittently controlled when both the modulation frequency and the output voltage increase.

6. The modulation frequency is determined to be increased when it reaches a predetermined first frequency threshold that is set lower than an upper limit frequency of the PFM control, 6. The control method for a switching power supply according to claim 5, wherein the output voltage is determined to be increased when it reaches a predetermined first output voltage threshold value that is set lower than the voltage when the modulation frequency is the upper limit frequency.

7. 7. The control method for a switching power supply according to claim 6, wherein, at the start of the intermittent control, the modulation frequency is set to reach the first frequency threshold before the output voltage reaches the first output voltage threshold.

8. 8. The control method for a switching power supply device according to claim 5, wherein the PFM control is resumed when the output voltage drops to a predetermined second output voltage threshold value that is lower than the target voltage.

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