LLC resonant converter circuit

Abstract

An LLC resonant converter circuit (1) is provided with: a resonant capacitor (Cr); a resonance coil (Np) which is a primary winding of the transformer (3) and which constitutes a series resonance circuit (11) together with the resonance capacitor; an inverter circuit (12); and a frequency control circuit (15) that outputs, to the inverter circuit, a drive signal whose frequency is changed in accordance with the voltage of the feedback signal, and the LLC resonant converter circuit (1) is further provided with: a circuit (30) that detects the output power from the secondary-side circuit (20) and generates the feedback signal; a circuit (40) that detects a phase difference between a resonance signal of the series resonance circuit and the drive signal; and a circuit (50) that performs voltage control of the feedback signal on the basis of the phase difference so as to suppress a decrease in the frequency of the drive signal caused by the frequency control circuit (15).

Description

Technical Field

[0001] This invention relates to an LLC resonant converter circuit that uses a feedback signal corresponding to the output power from the secondary side circuit for frequency control. Background Technology

[0002] In LLC resonant converter circuits, frequency control is performed using a feedback signal corresponding to the output power from the secondary side circuit. For example, Patent Document 1 discloses a current resonant converter device comprising a DC power supply, a converter transformer, a series circuit of first and second switching elements, a series circuit of the primary winding of the converter transformer and a resonant capacitor, a rectifier smoothing circuit, a converter control circuit, an operational amplifier, and an optocoupler unit. The rectifier smoothing circuit is connected to the secondary winding of the converter transformer, rectifies and smooths the power obtained from the secondary winding, and provides it to the load. The converter control circuit controls the on / off frequency of the first and second switching elements. The operational amplifier detects and amplifies the load voltage supplied to the load. The optocoupler unit detects changes in the output voltage of the operational amplifier and feeds them back to the converter control circuit. Furthermore, this device adjusts the gain of the operational amplifier based on the detection output of a load current detection circuit that detects the load current flowing in the load. This stabilizes the gain characteristics from light load to heavy load, preventing abnormal oscillations caused by gain increases under heavy load conditions.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2005-39975 Summary of the Invention

[0006] The problem the invention aims to solve

[0007] It is known that when an LLC resonant converter circuit deviates from the intended resonance condition, the recovery current of the body diode of the switching element causes a follow-through current, leading to increased switching losses and damage to the switching element. This is called detuning. To prevent this detuning, the switching frequency is controlled within a specified range higher than the resonant frequency so that the switching frequency (operating frequency) of the switching element is not lower than the resonant frequency.

[0008] However, when the LLC resonant converter circuit is replaced with a load or transformer with different characteristics, the resonant characteristics may change unexpectedly, causing detuning.

[0009] The present invention was made in view of the following problem, and provides a circuit technology for an LLC resonant converter that can suppress detuning even when replaced with loads or transformers with different characteristics.

[0010] Solution for solving the problem

[0011] According to the present invention, an LLC resonant converter circuit is provided, comprising: a primary-side circuit connected to a DC power supply and including a series resonant circuit; a transformer; and a secondary-side circuit that rectifies and smooths the AC power obtained from the primary-side circuit via the transformer and outputs DC power, wherein the primary-side circuit includes: a resonant capacitor; a resonant coil, which is the primary winding of the transformer and together with the resonant capacitor constitutes the series resonant circuit; an inverter circuit including a plurality of switching elements connected in a half-bridge or full-bridge configuration, the inverter circuit being configured to switch the direction of current flowing in the resonant coil by switching the plurality of switching elements according to a drive signal; and a frequency control circuit, wherein... The inverter circuit outputs a drive signal whose frequency has changed according to the voltage of the feedback signal. The secondary-side circuit includes: a secondary-side coil, which is the secondary winding of the transformer; and a rectifier circuit, which rectifies and smooths the AC power generated in the secondary-side coil. The LLC resonant converter circuit further includes: a detection circuit, which detects the output power from the secondary-side circuit and generates the feedback signal; a phase comparison circuit, which detects the phase difference between the resonant signal of the series resonant circuit and the drive signal; and a feedback control circuit, which controls the voltage of the feedback signal according to the phase difference detected by the phase comparison circuit, so as to suppress the frequency reduction of the drive signal caused by the frequency control circuit.

[0012] The effects of the invention

[0013] Based on the above method, a circuit technology for an LLC resonant converter can be provided that can suppress detuning even when replaced with loads or transformers with different characteristics. Attached Figure Description

[0014] Figure 1 This is a circuit diagram of the LLC resonant converter circuit (converter circuit) involved in the implementation method.

[0015] Figure 2A It is a conceptual diagram showing the signals at points A, B, C, and D of the phase comparison circuit when the phase difference between the resonant signal and the driving signal is large.

[0016] Figure 2BIt is a conceptual diagram showing the signals at points A, B, C, and D of the phase comparison circuit when the phase difference between the resonant signal and the driving signal is small.

[0017] Figure 3 This is a circuit diagram showing the inverter circuit modified in the LLC resonant converter circuit (converter circuit) involved in the implementation method. Detailed Implementation

[0018] The embodiments of the present invention will now be described. Furthermore, the embodiments described below are merely illustrative, and the present invention is not limited to the structures of the following embodiments.

[0019] [Circuit Structure]

[0020] Figure 1 This is a circuit diagram of the LLC resonant converter circuit (hereinafter sometimes simply referred to as the converter circuit) 1 involved in the implementation method.

[0021] The converter circuit 1 includes at least: a primary side circuit 10 connected to a DC power supply and including a series resonant circuit 11; a transformer 3; a secondary side circuit 20 that rectifies and smooths the AC power obtained from the primary side circuit 10 via the transformer 3 and outputs DC power; a detection circuit 30; a phase comparison circuit 40; and a feedback control circuit 50.

[0022] The secondary-side circuit 20 includes output terminals (VOUT+) and (VOUT-) from which DC power is output. There are no particular restrictions on how the output DC power is utilized. Loads of various characteristics can be connected to these output terminals.

[0023] Furthermore, the secondary-side circuit 20 includes a secondary-side coil Ns, which serves as the secondary winding of the transformer 3, and a rectifier circuit 21 connected to the secondary-side coil Ns to rectify and smooth the AC power generated in the secondary-side coil Ns. The rectifier circuit 21 includes a bridge rectifier circuit composed of rectifier diodes D1, D2, D3, and D4, and a capacitor C20. The AC power generated in the secondary-side coil Ns is half-wave rectified by the bridge rectifier circuit and smoothed by the output capacitor C20, thereby being converted into DC power.

[0024] Transformer 3 consists of a primary winding Np and a secondary winding Ns that are electrically insulated from each other, as well as an iron core, which magnetically couples the primary winding Np and the secondary winding Ns. Figure 1 Parasitic elements such as the magnetizing inductance (Lm) and leakage inductance (Lr) of transformer 3, the output capacitance of transistors Q1 and Q2 (described later), and parasitic diodes are not shown in the figure.

[0025] The primary side circuit 10 includes a series resonant circuit 11, an inverter circuit 12, a VCO (Voltage-controlled oscillator) circuit 15, an input capacitor C10, and resistive elements R1 and R2, etc.

[0026] The primary-side circuit 10 is connected to an external DC power supply via an input terminal (DCVin). Additionally, the primary-side circuit 10 includes a DC voltage input terminal (Vcc).

[0027] The series resonant circuit 11 is formed by the primary coil Np of the transformer 3 and the resonant capacitor Cr connected in series. Therefore, the primary coil Np can also be called the resonant coil.

[0028] The inverter circuit 12 includes a plurality of switching elements connected in a half-bridge or full-bridge configuration. The inverter circuit 12 is configured to switch these switching elements according to a drive signal (VCOout) from the VCO circuit 15 described later, thereby switching the direction of the current flowing in the resonant coil (primary side coil Np) of the series resonant circuit 11.

[0029] Specifically, the inverter circuit 12 includes a drive circuit 13, two transistors Q1 and Q2 as switching elements, etc. For example, FETs (Field Effect Transistors) are used for transistors Q1 and Q2. Figure 1 In the example, transistors Q1 and Q2 are N-channel MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), forming a half-bridge circuit.

[0030] The drive circuit 13 is connected to transistors Q1 and Q2 in a manner that allows the application of gate-source voltages (hereinafter sometimes referred to as VGS voltages) to transistors Q1 and Q2. Furthermore, the drain of transistor Q1 is connected to the input terminal (DCVin) of the DC power supply.

[0031] The drive circuit 13 alternately switches the on / off states of transistors Q1 and Q2 by applying a VGS voltage exceeding the threshold voltage to them. During this switching operation, the drive circuit 13 switches the on / off states of transistors Q1 and Q2 according to the pulse period of the drive signal (VCOout) from the VCO circuit 15.

[0032] The series resonant circuit 11 is connected to the source of transistor Q1 and positioned between the drain and source of transistor Q2. Thus, when transistor Q1 is on and transistor Q2 is off, the load current can flow to the drain-source of transistor Q1, the primary coil Np, and the resonant capacitor Cr. Conversely, when transistor Q1 is off and transistor Q2 is on, the load current can flow to the resonant capacitor Cr, the primary coil Np, and the drain-source of transistor Q2 through the power stored in the resonant capacitor Cr.

[0033] In this way, the direction of the current flowing in the primary side coil Np is switched by the switching action of transistors Q11 and Q12 in inverter circuit 12.

[0034] The input capacitor C10 is connected in parallel with transistors Q1 and Q2. Specifically, the input capacitor C10 is connected to the drain of transistor Q1 and the source of transistor Q2 to smooth the input voltage.

[0035] VCO circuit 15 outputs a drive signal to drive circuit 13, the frequency of which is changed according to the voltage of the feedback signal. VCO circuit 15 can be referred to as a frequency control circuit. VCO circuit 15 has a feedback signal input terminal (VCOin terminal), which is connected to the DC voltage input terminal (Vcc) via resistor element R1, and connected to the collector terminal of the phototransistor of the optocoupler PC of the detection circuit 30 described later via resistor element R2.

[0036] Therefore, "the voltage of the feedback signal" refers to the voltage applied to the VCOin terminal from the collector terminal of the phototransistor of the optocoupler PC of the detection circuit 30 through the line connected to the VCOin terminal of the VCO circuit 15.

[0037] exist Figure 1 In the example, the VCO circuit 15 is configured such that the higher the voltage of the feedback signal (VCOin terminal) within a specified range, the lower the frequency of the output drive signal; conversely, the lower the voltage of the feedback signal (VCOin terminal) within the specified range, the higher the frequency of the output drive signal. Furthermore, there are no limitations on the specific structure of the VCO circuit 15.

[0038] The detection circuit 30 detects the output power from the secondary-side circuit 20 and generates a feedback signal (output feedback voltage). Figure 1 In the example, the detection circuit 30 includes capacitors Cd and Cf, resistive elements R5, R6, R7, R8 and R9, variable resistor VR1, shunt regulator IC, optocoupler PC, etc.

[0039] A capacitor Cd is disposed between the output terminals (VOUT+) and (VOUT-) of the secondary side circuit 20. Resistors R8 and R9, and a variable resistor VR1, which are connected in series, as well as resistors R5 and R6, and a shunt regulator IC, which are also connected in series, are connected in parallel with the capacitor Cd. Furthermore, a wiring branching from the cathode of resistor R6 and the shunt regulator IC is connected to the slider terminal (the terminal connected to the slider) of the variable resistor VR1. A resistor R7 and a capacitor Cf are disposed in series on this wiring. A wiring extending from the reference terminal of the shunt regulator IC is connected between resistor R7 and the slider terminal of the variable resistor VR1.

[0040] The primary and secondary sides of an optocoupler PC are electrically isolated. An optocoupler PC consists of a photodiode on the primary side and a phototransistor on the secondary side. The optocoupler PC becomes conductive (the photodiode on the primary side emits light) by applying an input voltage exceeding the non-emitting forward voltage. In the conductive state, the output impedance of the phototransistor on the secondary side increases or decreases according to the increase or decrease of the current flowing through the photodiode on the primary side. In the following description, the current flowing through the photodiode on the primary side of the optocoupler PC is sometimes referred to as the diode current of the optocoupler PC.

[0041] The photodiode on the primary side of the optocoupler PC is connected in parallel with resistor R6. That is, the cathode of the photodiode in the optocoupler PC is connected between resistors R5 and R6, and the anode is connected between resistor R6 and the cathode of the shunt regulator IC and to one end of capacitor Cf.

[0042] In the phototransistor on the secondary side of the optocoupler PC, the collector terminal is connected to the input terminal (VCOin terminal) of the feedback signal of the VCO circuit 15 via the resistor element R2, and the emitter terminal is connected to ground (GND).

[0043] Thus, in the detection circuit 30, a DC voltage is obtained from the output power from the secondary side circuit 20 through the capacitor Cd. This DC voltage is divided by the resistors R8 and R9 and the variable resistor VR1 corresponding to the position of the slider and applied to the reference terminal of the shunt regulator IC. The divided voltage is controlled to be a reference voltage (e.g., 2.5V).

[0044] As a result, if the voltage of the slider of the variable resistor VR1 is to be higher than the reference voltage, the diode current of the optocoupler PC will increase; if the voltage of the slider of the variable resistor VR1 is to be lower than the reference voltage, the diode current of the optocoupler PC will decrease.

[0045] When the diode current of the optocoupler PC increases, the output impedance of the optocoupler PC decreases, and as a result, the voltage at the input terminal (VCOin terminal) of the feedback signal in the VCO circuit 15 decreases. Conversely, when the diode current of the optocoupler PC decreases, the output impedance of the optocoupler PC increases, and as a result, the voltage at the VCOin terminal of the VCO circuit 15 increases.

[0046] Furthermore, in this embodiment, when the voltage of the VCOin terminal of the VCO circuit 15 decreases, the frequency of the drive signal output from the VCO circuit 15 increases, and when the voltage of the VCOin terminal of the VCO circuit 15 increases, the frequency of the drive signal output from the VCO circuit 15 decreases.

[0047] The variable resistor VR1 is configured to variably set the output voltage (the voltage between the output terminals (VOUT+) and (VOUT-)) from the secondary side circuit 20. Since the voltage applied to the slider of the variable resistor VR1 is controlled by the shunt regulator IC to become a reference voltage, the output voltage is set according to the resistance ratio of the resistor elements R8 and R9 and the variable resistor VR1 corresponding to the position of the slider.

[0048] Therefore, the output voltage of converter circuit 1 can be variably set.

[0049] Phase comparison circuit 40 detects the phase difference between the resonant signal and the drive signal of series resonant circuit 11. The phase of the resonant current changes in the same way as the voltage phase of the resonant capacitor, therefore... Figure 1 In the example, the voltage signal applied to the resonant capacitor Cr is obtained as the resonant signal of the series resonant circuit 11.

[0050] The phase comparison circuit 40 includes a half-wave rectifier circuit 41, a phase comparator 42, a filter circuit 43, etc.

[0051] The half-wave rectifier circuit 41 obtains the voltage signal applied to the resonant capacitor Cr between the resonant capacitor Cr and the primary side coil Np (resonant coil), and performs half-wave rectification on the voltage signal.

[0052] Phase comparator 42 compares the phase of the voltage signal after half-wave rectification by half-wave rectifier circuit 41 with the phase of the drive signal output from VCO circuit 15, and outputs a phase difference signal representing the phase difference.

[0053] The filter circuit 43 smooths the phase difference signal output from the phase comparator 42 and outputs a DC signal. The DC signal output from the filter circuit 43 is a DC voltage signal representing the phase difference; therefore, the filter circuit 43 is referred to as a circuit that smooths the signal representing the phase difference and outputs a phase difference voltage. There are no specific limitations on the structure of the filter circuit 43; for example, the filter circuit 43 can be formed as a CR filter using resistive elements and capacitors.

[0054] The feedback control circuit 50 includes a differential amplifier circuit 60, a diode D5, etc. It performs voltage control of the feedback signal based on the phase difference detected by the phase comparison circuit 40 to suppress the frequency reduction of the drive signal caused by the VCO circuit 15.

[0055] Here, the "voltage control of the feedback signal" performed by the feedback control circuit 50 refers to controlling the voltage applied to the VCOin terminal of the VCO circuit 15.

[0056] The cathode of diode D5 is connected to the output terminal of differential amplifier circuit 60, and the anode is connected to a branch line branching from the collector terminal of the phototransistor of optocoupler PC to the VCOin terminal of VCO circuit 15. Diode D5 can be in either a conducting state (current flowing) or a cut-off state (current not flowing) depending on the differential voltage output from differential amplifier circuit 60. As a result, when diode D5 is in the conducting state, the voltage of the feedback signal output from detection circuit 30 is reduced by the feedback control circuit 50 and then input to the VCOin terminal of VCO circuit 15. When diode D5 is in the cut-off state, the feedback control circuit 50 does not affect the voltage of the feedback signal output from detection circuit 30, and detection circuit 30 dominates the voltage control of the feedback signal.

[0057] The differential amplifier circuit 60 is located between the filter circuit 43 and the cathode of the diode D5, and outputs the voltage difference between the phase difference voltage output from the filter circuit 43 and the reference voltage.

[0058] exist Figure 1 In the example, the differential amplifier circuit 60 has resistive elements R11, R12, R13 and R14, a variable resistor VR2, an operational amplifier OP, etc.

[0059] Resistors R11 and R12 are connected in series between the output terminal of the filter circuit 43 and the output terminal of the operational amplifier OP. The inverting input terminal of the operational amplifier OP is connected between resistors R11 and R12. The output terminal of the operational amplifier OP is connected to the cathode of diode D5. Additionally, the non-inverting input terminal of the operational amplifier OP is connected to one end of resistor R13, and the other end of resistor R13 is connected to the slider terminal (the terminal connected to the slider) of variable resistor VR2. One end of the resistive element of variable resistor VR2 is connected to one end of resistor R14, and the other end of resistor R14 is connected to the input terminal (Vcc) of the DC power supplied to the VCO circuit 15. The other end of the resistive element of variable resistor VR2 is connected to ground (GND).

[0060] According to this structure, the differential amplifier circuit 60 amplifies and outputs the differential voltage obtained by subtracting the phase difference voltage from the reference voltage based on the resistance ratio of resistors R11 and R12. The output differential voltage is applied to the cathode of diode D5. Furthermore, the reference voltage of the differential amplifier circuit 60 is the voltage obtained by dividing the DC voltage supplied from the input terminal (Vcc) through resistor R14 and the resistance of the variable resistor VR2 corresponding to the position of the slider of the variable resistor VR2, and then stepping it down through resistor R13.

[0061] Furthermore, when the phase difference voltage is higher than the reference voltage, the differential amplifier circuit 60 outputs a differential voltage that prevents current from flowing through diode D5, and when the phase difference voltage is lower than the reference voltage, it outputs a differential voltage that allows current to flow through diode D5.

[0062] Thus, the feedback control circuit 50 is configured to switch between a state where the voltage of the feedback signal output from the detection circuit 30 is reduced and input to the VCOin terminal of the VCO circuit 15, and a state where the voltage of the feedback signal output from the detection circuit 30 is directly input to the VCOin terminal, through the rectification effect of the diode D5 corresponding to the differential voltage output from the differential amplifier circuit 60. Furthermore, when the phase difference voltage is higher than the reference voltage, the voltage control of the feedback control circuit 50 does not involve the feedback signal; therefore, based on the output power from the secondary side circuit 20, the frequency of the drive signal is controlled using the feedback signal output from the detection circuit 30. When the phase difference voltage is lower than the reference voltage, the decrease in the frequency of the drive signal is suppressed by using the feedback control circuit 50 to control the voltage of the feedback signal output from the detection circuit 30.

[0063] Here, the decision on whether to limit the reduction of the drive signal frequency, i.e., the reduction of the switching frequency (operating frequency) of the inverter circuit 12, is based on the differential voltage output from the differential amplifier circuit 60. This differential voltage corresponds to the difference between the phase difference voltage output from the filter circuit 43 and the reference voltage of the differential amplifier circuit 60. Therefore, the reference voltage of the differential amplifier circuit 60 becomes the threshold for determining whether to limit the reduction of the operating frequency, and is set to be changeable according to the position of the slider of the variable resistor VR2.

[0064] Therefore, the variable resistor VR2 can be described as a circuit element capable of variably setting a threshold value for the phase difference of the feedback signal, which determines whether the voltage control based on the feedback control circuit 50 applies to the feedback signal. In other words, it can be described as a circuit element capable of variably setting the degree to which the operating frequency is limited when it approaches the resonant frequency in a frequency region higher than the resonant frequency of the series resonant circuit 11. Thus, the feedback control circuit 50 can be described as a circuit element that includes a threshold value for the phase difference of the feedback signal, which can be variably set to determine whether to perform voltage control of the feedback signal to suppress the frequency reduction of the drive signal caused by the VCO circuit 15. Furthermore, this circuit element is not limited to a variable resistor.

[0065] 〔action〕

[0066] The converter circuit 1 with such a circuit structure operates as follows.

[0067] Inverter circuit 12 switches the on / off states of transistors Q1 and Q2 according to the pulse period of the drive signal from VCO circuit 15. This reverses the direction of the current flowing through the primary winding Np of transformer 3. When transistor Q1 is on and transistor Q2 is off, load current flows through the path between the drain and source of transistor Q1, through the primary winding Np, and through the resonant capacitor Cr, charging the resonant capacitor Cr. When transistor Q1 is off and transistor Q2 is on, the power charged to the resonant capacitor Cr causes load current to flow in the opposite direction through the path between the drain and source of transistor Q2, through the resonant capacitor Cr, and through the primary winding Np.

[0068] In the primary side circuit 10, the switching action of the inverter circuit 12 induces a resonant current in the series resonant circuit 11, generating AC power in the secondary side coil Ns of the transformer 3. In the secondary side circuit 20, the AC power generated in the secondary side coil Ns is rectified and smoothed by the rectifier circuit 21, thereby converting it into DC power and outputting it.

[0069] The detection circuit 30 detects the output power from the secondary-side circuit 20 and generates a feedback signal. Specifically, based on the DC power output from the secondary-side circuit 20, a DC voltage is obtained through capacitor Cd. The shunt regulator IC controls this DC voltage, which is then divided by resistors R8 and R9 and the resistance of the variable resistor VR1 corresponding to the position of the slider, to become a reference voltage (e.g., 2.5V). That is, if the voltage of the slider of the variable resistor VR1 is to be higher than the reference voltage, the diode current of the optocoupler PC is increased; if the voltage of the slider of the variable resistor VR1 is to be lower than the reference voltage, the diode current of the optocoupler PC is decreased.

[0070] According to the optocoupler PC, the output impedance increases or decreases depending on the magnitude of the diode current on the primary side, which in turn causes the voltage at the VCOin terminal (feedback signal) of the VCO circuit 15 to rise or fall.

[0071] By operating the detection circuit 30, when the DC power output from the secondary side circuit 20 decreases, the voltage of the feedback signal input to the VCO circuit 15 will increase; when the DC power output from the secondary side circuit 20 increases, the voltage of the feedback signal will decrease.

[0072] The higher the voltage of the feedback signal within the specified range, the lower the frequency of the drive signal output by the VCO circuit 15; conversely, the lower the voltage of the feedback signal within the specified range, the higher the frequency of the drive signal output by the VCO circuit 15.

[0073] By controlling the frequency of the VCO circuit 15, the switching frequency is controlled within a specified range higher than the resonant frequency, so that the switching frequency (operating frequency) of the inverter circuit 12 is not lower than the resonant frequency of the series resonant circuit 11.

[0074] Here, when the load connected to the output terminals (VOUT+) and (VOUT-) of the transformer 3 and secondary circuit 20 is replaced with a load of different characteristics, the output power from the secondary circuit 20 is affected, and the resonant frequency of the series resonant circuit 11 changes. Since the output power of the secondary circuit 20 decreases due to this replacement, the voltage of the feedback signal increases under the control of the detection circuit 30, and the frequency of the drive signal output from the VCO circuit 15 decreases. At this time, since the resonant frequency has changed due to this replacement, if the control range of the switching frequency remains unchanged, the switching frequency becomes lower than the changed resonant frequency, which may cause a so-called detuning phenomenon.

[0075] Therefore, in this embodiment, the feedback control circuit 50 performs voltage control of the feedback signal based on the phase difference detected by the phase comparison circuit 40, so as to suppress the frequency reduction of the drive signal caused by the VCO circuit 15.

[0076] Specifically, the phase comparator circuit 40 outputs a phase difference voltage representing the phase difference between the voltage signal obtained by half-wave rectification of the voltage signal applied to the resonant capacitor Cr and the drive signal output from the VCO circuit 15. In the feedback control circuit 50, the differential amplifier circuit 60 amplifies and outputs the difference voltage obtained by subtracting this phase difference voltage from the reference voltage and applies it to the cathode of diode D5. When the phase difference voltage is higher than the reference voltage, the output is a difference voltage that prevents current from flowing through diode D5; when the phase difference voltage is lower than the reference voltage, the output is a difference voltage that allows current to flow through diode D5.

[0077] Therefore, when the phase difference voltage is higher than the reference voltage, the detection circuit 30 dominates the voltage control of the feedback signal. When the phase difference voltage is lower than the reference voltage, the feedback control circuit 50 suppresses the decrease in the frequency of the drive signal (maintains or increases the frequency of the drive signal) by acting on the feedback signal output from the detection circuit 30.

[0078] Figure 2A and Figure 2B This is a conceptual diagram showing the signal curves at points A, B, C, and D in the phase comparator circuit 40. Figure 2A This illustrates the case where the phase difference between the resonant signal and the driving signal is large. Figure 2B This illustrates the case where the phase difference is small.

[0079] The signal at point A is the half-wave rectified voltage signal output from the half-wave rectifier circuit 41; the signal at point B is the drive signal output from the VCO circuit 15; the signal at point C is the phase difference signal output from the phase comparator 42; and the signal at point D is the phase difference voltage signal output from the filter circuit 43.

[0080] In addition, Figure 2A and Figure 2B The diagram illustrates how the phase comparator 42 is constructed using an XOR circuit. However, the phase comparator 42 is not limited to this structure.

[0081] according to Figure 2A and Figure 2B The greater the phase difference between the driving signal and the resonant signal (the voltage signal applied to the resonant capacitor Cr), the higher the phase difference voltage output from the filter circuit 43; the smaller the phase difference, the lower the phase difference voltage.

[0082] Moreover, according to Figure 2A and Figure 2B For example, when the switching frequency (operating frequency) is close to the resonant frequency of the series resonant circuit 11, the phase difference voltage becomes low. Thus, the decrease in the frequency of the drive signal is suppressed by the voltage control of the feedback signal by the feedback control circuit 50. When the switching frequency (operating frequency) is far from the resonant frequency, the phase difference voltage becomes high. Thus, the voltage control of the feedback signal by the detection circuit 30 becomes dominant.

[0083] Therefore, even if the switching frequency decreases due to the decrease in output power when the load or transformer 3 is replaced with a different load or transformer 3, the decrease in switching frequency can be suppressed when the switching frequency is close to the resonant frequency, thus preventing detuning.

[0084] [Variation Example]

[0085] The above-described embodiments can be modified as appropriate.

[0086] Figure 3 This is a circuit diagram showing the inverter circuit 12 modified in the converter circuit 1 according to the above-described embodiment.

[0087] It can also make Figure 1 The inverter circuit 12 involved in the above-described embodiment is structurally modified as follows: Figure 3 The structure shown is a full-bridge connection of switching elements.

[0088] exist Figure 3 In this example, the inverter circuit 12 includes four transistors Q1, Q2, Q3, and Q4 connected in a full-bridge configuration as switching elements. For transistors Q3 and Q4, similarly to transistors Q1 and Q2, FETs (Field-Effect Transistors) or N-channel MOSFETs can be used, for example.

[0089] The source of transistor Q3 is connected to the drain of transistor Q4, the drain of transistor Q3 is connected to the drain of transistor Q1, and the source of transistor Q4 is connected to the source of transistor Q2. Furthermore, transistors Q3 and Q4 are connected to the drive circuit 13 so that a VGS voltage can be applied to them respectively. The drain of transistor Q3 is connected to the input terminal (DCVin) of the DC power supply, and the source of transistor Q4 is connected to ground (GND). Moreover, a series resonant circuit 11 is provided along the path from the source of transistor Q1 to the drain of transistor Q2, and then to the source of transistor Q3 and the drain of transistor Q4. The connection between transistors Q1 and Q2 is as described above.

[0090] The drive circuit 13 switches the on / off states of transistors Q1, Q2, Q3 and Q4 according to the pulse period of the drive signal from the VCO circuit 15, so as to switch the direction of the current flowing in the primary side coil Np.

[0091] In this way, even if the structure of the inverter circuit 12 is changed from a half-bridge mode to a full-bridge mode, the same effect as the above-described implementation method can be obtained.

[0092] Alternatively, the above-described embodiment can be modified so that the VCO circuit 15 is configured such that the higher the voltage of the feedback signal (VCOin terminal of the VCO circuit 15) within a specified range, the higher the frequency of the output drive signal; and the lower the voltage of the feedback signal within the specified range, the lower the frequency of the output drive signal. In this case, the detection circuit 30 is configured such that when the output power from the secondary side circuit 20 is high, the voltage of the feedback signal input to the VCO circuit 15 increases, and when the output power is low, the voltage of the feedback signal decreases. Furthermore, the feedback control circuit 50 can also be configured such that when the phase difference voltage is higher than the reference voltage, no processing is performed on the feedback signal input to the VCO circuit 15, and when the phase difference voltage is lower than the reference voltage, the voltage of the feedback signal is increased.

[0093] Alternatively, the above embodiment can be modified so that the phase comparison circuit 40 is configured such that the larger the phase difference between the resonant signal of the series resonant circuit 11 and the driving signal, the lower the phase difference voltage output from the filter circuit 43; and the smaller the phase difference, the higher the phase difference voltage. In this case, the configuration is such that when the phase difference voltage is higher than the reference voltage, current flows through diode D5, and the voltage of the feedback signal decreases; when the phase difference voltage is lower than the reference voltage, current does not flow through diode D5, and the feedback signal is not processed.

[0094] In summary, the feedback control circuit 50 can be configured as follows: the closer the resonant signal of the series resonant circuit 11 is to the resonant frequency, the more voltage control of the feedback signal is performed to suppress the frequency reduction of the drive signal. When the resonant signal is far from the resonant frequency, the detection circuit 30 dominates the voltage control of the feedback signal.

[0095] Some or all of the above-described embodiments and modifications can also be determined as follows. However, the above-described embodiments and modifications are not limited to the following description.

[0096] <1>

[0097] An LLC resonant converter circuit includes: a primary-side circuit connected to a DC power supply and including a series resonant circuit; a transformer; and a secondary-side circuit that rectifies and smooths the AC power obtained from the primary-side circuit via the transformer and outputs DC power.

[0098] The primary-side circuit includes:

[0099] Resonant capacitor;

[0100] The resonant coil is the primary winding of the transformer and together with the resonant capacitor forms the series resonant circuit.

[0101] An inverter circuit includes multiple switching elements connected in a half-bridge or full-bridge configuration, the inverter circuit being configured to switch the direction of current flowing in the resonant coil by switching the multiple switching elements according to a drive signal; and

[0102] A frequency control circuit outputs a drive signal to the inverter circuit, the frequency of which is changed according to the voltage of the feedback signal.

[0103] The secondary side circuit includes:

[0104] The secondary winding is the secondary winding of the transformer; and

[0105] A rectifier circuit rectifies and smooths the alternating current generated in the secondary coil.

[0106] The LLC resonant converter circuit also features:

[0107] A detection circuit that detects the output power from the secondary side circuit and generates the feedback signal;

[0108] A phase comparison circuit that detects the phase difference between the resonant signal of the series resonant circuit and the driving signal; and

[0109] A feedback control circuit controls the voltage of the feedback signal based on the phase difference detected by the phase comparison circuit, so as to suppress the frequency reduction of the drive signal caused by the frequency control circuit.

[0110] <2>

[0111] according to <1> The described LLC resonant converter circuit, wherein,

[0112] The feedback control circuit includes circuit elements that can be variably set to a threshold value for the phase difference, which is used to suppress voltage control of the feedback signal for suppressing frequency reduction of the drive signal caused by the frequency control circuit.

[0113] <3>

[0114] according to <1> The described LLC resonant converter circuit, wherein,

[0115] The phase comparison circuit includes a filtering circuit that smooths the signal representing the phase difference and outputs a phase difference voltage.

[0116] The feedback control circuit includes:

[0117] A differential amplifier circuit, whose output is the voltage difference between the phase difference voltage from the filter circuit and the reference voltage; and

[0118] A diode is disposed between the input terminal of the feedback signal in the frequency control circuit and the output terminal of the differential amplifier circuit.

[0119] The LLC resonant converter circuit is configured to switch between a state in which the voltage of the feedback signal input to the frequency control circuit is reduced and a state in which the voltage of the feedback signal is not reduced, by means of the rectification effect of the diode corresponding to the differential voltage output from the differential amplifier circuit.

[0120] <4>

[0121] according to <3> The described LLC resonant converter circuit, wherein,

[0122] The frequency control circuit is configured such that: the higher the voltage of the feedback signal within a specified range, the lower the frequency of the output drive signal; conversely, the lower the voltage of the feedback signal within the specified range, the higher the frequency of the output drive signal.

[0123] The diode is connected to the output terminal of the differential amplifier circuit on the cathode side and to the input terminal of the feedback signal in the frequency control circuit on the anode side.

[0124] The differential amplifier circuit is configured such that when the phase difference voltage is higher than the reference voltage, it outputs a differential voltage that prevents current from flowing through the diode, and when the phase difference voltage is lower than the reference voltage, it outputs a differential voltage that allows current to flow through the diode.

[0125] <5>

[0126] according to <3> or <4> The described LLC resonant converter circuit, wherein,

[0127] The differential amplifier circuit includes a variable resistor that can variably set the reference voltage.

[0128] Explanation of reference numerals in the attached figures

[0129] 1: LLC resonant converter circuit (converter circuit); 3: Transformer; 10: Primary side circuit; 11: Series resonant circuit; 12: Inverter circuit; 13: Drive circuit; 15: VCO circuit; 20: Secondary side circuit; 21: Rectifier circuit; 30: Detection circuit; 40: Phase comparator circuit; 41: Half-wave rectifier circuit; 42: Phase comparator; 43: Filter circuit; 50: Feedback control circuit; 60: Differential amplifier circuit; Np: Primary side coil; Ns: Secondary side coil; Cr: Resonant capacitor; Q1, Q2, Q3, Q4: Transistors; PC: Optocoupler; OP: Operational amplifier; D1, D2, D3, D4, D5: Diodes; IC: Shunt regulator; VR1, VR2: Variable resistors.

Claims

1. An LLC resonant converter circuit, comprising: a primary-side circuit connected to a DC power supply and including a series resonant circuit; a transformer; and a secondary-side circuit that rectifies and smooths the AC power obtained from the primary-side circuit via the transformer and outputs DC power, wherein... The primary-side circuit includes: Resonant capacitor; The resonant coil is the primary winding of the transformer and together with the resonant capacitor forms the series resonant circuit. An inverter circuit includes multiple switching elements connected in a half-bridge or full-bridge configuration, the inverter circuit being configured to switch the direction of current flowing in the resonant coil by switching the multiple switching elements according to a drive signal; and A frequency control circuit outputs a drive signal to the inverter circuit, the frequency of which is changed according to the voltage of the feedback signal. The secondary side circuit includes: The secondary winding is the secondary winding of the transformer; and A rectifier circuit rectifies and smooths the alternating current generated in the secondary coil. The LLC resonant converter circuit also features: A detection circuit that detects the output power from the secondary side circuit and generates the feedback signal; A phase comparison circuit that detects the phase difference between the resonant signal of the series resonant circuit and the driving signal; and A feedback control circuit controls the voltage of the feedback signal based on the phase difference detected by the phase comparison circuit, so as to suppress the frequency reduction of the drive signal caused by the frequency control circuit.

2. The LLC resonant converter circuit according to claim 1, wherein, The feedback control circuit includes circuit elements that can be variably set to a threshold value for the phase difference, which is used to suppress voltage control of the feedback signal for suppressing frequency reduction of the drive signal caused by the frequency control circuit.

3. The LLC resonant converter circuit according to claim 1, wherein, The phase comparison circuit includes a filtering circuit that smooths the signal representing the phase difference and outputs a phase difference voltage. The feedback control circuit includes: A differential amplifier circuit, whose output is the voltage difference between the phase difference voltage from the filter circuit and the reference voltage; and A diode is disposed between the input terminal of the feedback signal in the frequency control circuit and the output terminal of the differential amplifier circuit. The LLC resonant converter circuit is configured to switch between a state in which the voltage of the feedback signal input to the frequency control circuit is reduced and a state in which the voltage of the feedback signal is not reduced, by means of the rectification effect of the diode corresponding to the differential voltage output from the differential amplifier circuit.

4. The LLC resonant converter circuit according to claim 3, wherein, The frequency control circuit is configured such that: the higher the voltage of the feedback signal within a specified range, the lower the frequency of the output drive signal; conversely, the lower the voltage of the feedback signal within the specified range, the higher the frequency of the output drive signal. The diode is connected to the output terminal of the differential amplifier circuit on the cathode side and to the input terminal of the feedback signal in the frequency control circuit on the anode side. The differential amplifier circuit is configured such that when the phase difference voltage is higher than the reference voltage, it outputs a differential voltage that prevents current from flowing through the diode, and when the phase difference voltage is lower than the reference voltage, it outputs a differential voltage that allows current to flow through the diode.

5. The LLC resonant converter circuit according to claim 3 or 4, wherein, The differential amplifier circuit includes a variable resistor that can variably set the reference voltage.

Images