Single-phase direct ac-ac converter and power control method thereof

Abstract

To provide a single-phase direct AC-AC converter that has a wide soft switching range combined with pulse width modulation and variable inductance control suitable for fixed frequency operation, and capable of effectively reducing switching losses and optimizing circuit parameters.SOLUTION: In a high-frequency power conversion circuit equipped with a resonant tank consisting of a coil Lp and a capacitor Cr, and a bidirectional switch 1b, a variable inductor Lv, whose inductance changes depending on the conduction current, is connected in series with the coil Lp of the resonant tank. The bidirectional switch 1b drives the switch so as to be always in reverse conduction mode, detects the effective value of the conduction current, compares it with a reference value, changes the inductance of the variable inductor Lv, and changes the rate of rise of the current flowing through the coil Lp connected in series to adjust the effective current of the bidirectional switch 1b.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention is a single-phase direct AC-AC converter capable of directly generating single-phase high-frequency alternating current from a commercial power supply, and particularly relates to a fixed-frequency single-phase direct AC-AC converter integrated with a variable inductor for applications such as induction heating systems and wireless power transmission (WPT).

Background Art

[0002] Induction Heating (IH) systems are high-efficiency, energy-saving, and accurate heating technologies, and are used in various applications such as many industrial processing, metal melting, welding, and household cooking applications. Its principle is based on electromagnetic induction, using an alternating current to generate a changing magnetic field through a conductor coil, thereby generating a current in an adjacent conductor object, and this current converts electrical energy into thermal energy through the resistance of the material, thereby heating the object. The IH system is more efficient than conventional heating methods because it directly generates thermal energy inside the object that needs to be heated, reducing energy waste.

[0003] In an IH system, using a high-frequency inverter, commercial power is once rectified by a rectifier circuit, converted into DC pulse power, and then converted into high-frequency current by an inverter circuit. As shown in FIG. 12, a high-frequency inverter usually consists of a rectifier stage, a boost PFC (power factor correction) stage, and a DC-HFAC (DC-high-frequency AC) inverter, and these components are the core components of the IH system. A high-frequency inverter usually features a three-stage or two-stage configuration. Therefore, under multi-stage conversion, a large number of components are required for the entire IH system, making it easy for efficiency to decline, the design to become very complex, and power to be unable to be controlled efficiently. That is, at the stage of transmitting power from the power source to the load, there are many power losses in power semiconductor devices and passive components. In addition, there is a manufacturing error of about 10 to 30% in passive components such as inductors and capacitors in the circuit, and when the error becomes large, the reliability of the power transmission system decreases. Also, conventionally, control by frequency conversion has been mainstream. However, when two or more devices operate simultaneously, mechanical noise audible to the human ear is likely to occur due to frequency differences. In addition, since the harmonic removal effect in the smoothing filter also greatly depends on the load power, the filter capacitance for the operating frequency cannot always be optimally designed. As described above, there are practical problems in power control based on pulse frequency modulation in high-frequency IH application devices.

[0004] There is a need for a single-phase direct AC-AC converter with fixed-frequency control that can expand the controllable power range compared to conventional heating methods, simultaneously solve mechanical noise, noise due to frequency modulation, and electromagnetic interference problems, and directly generate single-phase high-frequency AC from a commercial power source. The inventors of the present invention have already proposed a single-phase direct AC-AC converter with fixed-frequency control (see Patent Document 2). In the proposed direct AC-AC converter, as shown in FIG. 13, a single-phase commercial power source and an IH load are connected, and a capacitor C is connected in parallel to the IH load connected via bidirectional switches (Q 1 , Q 2 ). ris connected, and when switched off, it forms a resonant circuit with the equivalent inductance of the IH load including the self-inductance of the work coil and the equivalent inductance of the heating load. Input power supply V in is in the positive half cycle, then Q 1 is the main switch, Q 2 is the auxiliary switch, and in the negative half cycle, their roles are swapped. Due to the high-speed operation of the main switch, the AC voltage from input V in is directly converted into high-frequency AC while including an envelope for the IH load as shown in FIG. 14. This direct AC-AC converter has two operating patterns: the RB mode (conducting mode) in which the auxiliary switch is always off and the RC mode (reverse conducting mode) in which the auxiliary switch is always on. The single-phase direct AC-AC converter of the present invention focuses on the fact that by driving the switches so that it is always in the reverse conducting mode only and changing the inductance of the variable inductor, the effective current of the bidirectional switch can be adjusted.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In view of such a situation, an object of the present invention is to provide a single-phase direct AC-AC converter that has a wide soft-switching range by combining pulse width modulation and variable inductance control suitable for fixed-frequency operation, effectively reduces switching losses, and can optimize circuit parameters.

Means for Solving the Problems

[0007] To solve the above problems, in the single-phase direct AC-AC converter of the present invention, in a high-frequency power conversion circuit including a resonant tank composed of a coil and a capacitor and a bidirectional switch, a variable inductor whose inductance changes with the conduction current is connected in series to the coil of the resonant tank. Then, the drive circuit of the bidirectional switch drives the switch so that it is always in the reverse conduction mode only, detects the effective value of the conduction current, compares it with a reference value, changes the inductance of the variable inductor, and changes the rising rate of the current flowing through the coil connected in series to adjust the effective current of the bidirectional switch. In the present invention, by the pulse width modulation (PWM) technology combined with the variable inductance technology, the effective power of each pulse is adjusted by the inductance value, and the output power is controlled according to the load conditions. The drive circuit of the bidirectional switch drives it fixed at a predetermined switching frequency. Here, the reverse conduction mode simply refers to the conduction operation of both switches with a section where the load current (a load resonant tank composed of a working coil and a parallel capacitor) instantaneously regenerates to the input power source before the main switch turns on.

[0008] The drive circuit of the bidirectional switch in the single-phase direct AC-AC converter of the present invention switches the main switch and the auxiliary switch in the positive half-cycle (positive half-period cycle) and negative half-cycle (negative half-period cycle) of the input power source, and keeps the auxiliary switch on all the time. When the main switch turns off, the capacitor voltage of the resonant tank drops, resonates with the coil and the variable inductor, the voltage of the main switch rises from zero to achieve zero voltage soft switching (ZVS) turn-on, and when the voltage of the bidirectional switch reaches the peak value and then drops to zero, at the same time, the current flows backward through the anti-parallel diode of the main switch, and the main switch is turned on during this period to achieve zero voltage soft switching turn-on. Then, after the main switch turns on, the current changes from backward flow to forward flow and rises, and based on the difference between the effective value of the current and the reference value, the inductance of the variable inductor is changed to adjust the amount of current flowing through the coil.

[0009] In addition, the switching period of the main switch in the drive circuit of the bidirectional switch is fixed at 20 kHz or more, and the low-frequency pulsation in the current of the input power supply is reduced. Since the effective value of the conduction current can be adjusted while maintaining the high-frequency switching period, problems such as low-frequency pulsation in the current of the input power supply are avoided.

[0010] The power control method of the single-phase direct AC-AC converter of the present invention includes a resonant tank composed of a coil and a capacitor and a bidirectional switch, and is a power control method of a high-frequency power conversion circuit in which a variable inductor whose inductance changes according to the conduction current is connected in series to the coil of the resonant tank. The drive circuit of the bidirectional switch includes the following steps 1) to 3). 1) A step of driving the switch so that it is always in the reverse conduction mode only. 2) A step of detecting the effective value of the conduction current and comparing it with a reference value. 3) Based on the difference between the effective value of the current and the reference value, change the inductance of the variable inductor, and adjust the current amount by changing the rising rate of the current flowing through the coil connected in series to the variable inductor.

[0011] More specifically, the drive circuit of the bidirectional switch includes the following a) to h). a) In the positive half cycle of the input power supply, one of the bidirectional switches is the main switch and the other is the auxiliary switch. In the negative half cycle, the correspondence between the main switch and the auxiliary switch is reversed, and the auxiliary switch is always turned on to drive the switch so that it is always in the reverse conduction mode only. b) By turning off the main switch, the capacitor voltage of the resonant tank is decreased and resonated with the coil and the variable inductor. c) The voltage of the main switch rises from zero to achieve zero-voltage soft-switching turn-off. d) After the voltage of the bidirectional switch reaches the peak value and then decreases to zero, at the same time, the current flows backward through the anti-parallel diode of the main switch, and during that time, the main switch is turned on in the zero-voltage soft-switching turn-on step. e) Step of the current rising from reverse flow to forward flow after the main switch is turned on. f) Step of detecting the effective value of the conduction current and comparing it with a reference value. g) Step of changing the inductance of the variable inductor based on the difference between the effective value of the current and the reference value, and adjusting the current amount by changing the rising rate of the current flowing through the coil connected in series with the variable inductor. h) Step of the main switch turning off again and returning to the above a), and repeating each step.

Advantages of the Invention

[0012] According to the single-phase direct AC-AC converter of the present invention, since the effective value of the conduction current can be adjusted while maintaining the high-frequency switching period, problems of the prior art such as low-frequency pulsation and reduction of control resolution can be avoided, and the selection range of the operating switching frequency is also widened. As a result, there is an effect that the application range is widened as a high-frequency energy conversion system.

Brief Description of the Drawings

[0013]

Figure 1

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Figure 13

Figure 14

Best Mode for Carrying Out the Invention

[0014] Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings. Note that the scope of the present invention is not limited to the following examples and illustrated examples, and numerous modifications and variations are possible.

Example 1

[0015] The configuration of the main circuit of the single-phase direct AC-AC converter of the present invention is shown in FIG. 1. The main circuit 1 of the single-phase direct AC-AC converter includes a bidirectional switch 1b connected in series to a commercial power supply V in (UFAC)1a, and a variable inductor L whose inductance changes with the conduction current, V a coil L P and a capacitor C r which together form a resonant tank 1c, and a drive circuit (not shown) for the bidirectional switch 1b. The two switches (Q 1 , Q 2 ) of the bidirectional switch 1b in the figure are composed of power transistors of SiC-MOSFETs connected in series at the drain connection, and the gates are driven by a drive circuit (hereinafter also referred to as a control circuit). Parallel diodes (D 1 , D 2 ) are connected to each switch (Q 1 , D 2) is connected. Also, the resonant tank 1c is represented by a transformer model, where the primary side of the transformer is coil L p and capacitor C r and the secondary side of the transformer is coil L s and resistor R 0 . A high-frequency voltage V 0 is generated across resistor R o .

[0016] In a high-frequency induction heating system (IH system), a single-phase direct AC-AC converter is connected in parallel with a capacitor C r across an IH load connected via a bidirectional switch, and forms a resonance circuit with the equivalent inductance of the IH load including the self-inductance of the work coil and the equivalent inductance of the heating load when the main switch is off. Here, when the input power supply V in is in the positive half-cycle, Q 1 is the main switch and Q 2 is the auxiliary switch, and their roles are swapped in the negative half-cycle. Due to the high-speed operation of the main switch, the AC voltage from the input power supply V in is directly converted into a high-frequency AC with an envelope included in the IH load as shown in FIG. 14. The single-phase direct AC-AC converter of the present invention has only an operating pattern in a reverse conduction mode where the auxiliary switch is always on.

[0017] FIG. 2 shows the main circuit and control block diagram of the single-phase direct AC-AC converter. The control circuit 20 detects the current value i in flowing through the main circuit by the current sensor 11 and detects the voltage polarity by the voltage sensor 12. Also, the current flowing through coil L P is detected by the current sensor 13. This control can be performed using, for example, a microcomputer or FPGA, and all signal detections are performed by an ADC (A / D converter). Note that analog circuits such as sampling are omitted. In the control circuit 20, the voltage polarity of the input power supply is detected by the voltage sensor 12 to discriminate the main switch and the auxiliary switch. When the input power supply V in is in the positive half-cycle, Q 1is the main switch, Q 2 is the auxiliary switch, and in the negative half cycle, their correspondence is swapped, so the switch (Q 1 , Q 2 ) is selected to drive the gate of the power transistor of the corresponding switch. Q m Signal 21 is a clock signal within digital control, and uses the zero-crossing voltage signal of the power supply (the comparator output between the power supply voltage and the zero point) as a trigger signal to determine the on / off timing of the main switch and the auxiliary switch. As shown in FIG. 2, the comparator (Comp1) 22 determines whether the input power supply V in is in the positive half cycle or the negative half cycle, and selects the clock signal of Q m and the HIGH signal (denoted as the input signal "1" of the MUX in the figure) by the multiplexers (MUX) 23a, 23b. For example, in the case of the positive half cycle, the signal output from the comparator 22 becomes "1", and the multiplexer 23a selects the Q m signal, and the multiplexer 23b selects the HIGH signal, and drives the switches (Q 1 , Q 2 ) through the gate drivers respectively. In the case of the positive half cycle, the main switch Q 1 is turned on and off at high speed by the Q m signal, and the auxiliary switch Q 2 is always turned on. In the case of the negative half cycle, it is the reverse of the positive half cycle, the main switch Q 2 is turned on and off at high speed by the Q m signal, and the auxiliary switch Q 1 is always turned on.

[0018] Also, in the control circuit 20, the current value i in flowing through the main circuit and the current flowing through the coil L P are detected, and the current i v flowing through the second winding of the variable inductor L cControl it. The variable inductor is generally also called a saturable inductor. By applying a control winding separate from the winding through which the main current flows to the iron core (magnetic material) and changing the timing of magnetic flux density saturation / non-saturation / saturation in the magnetic body according to the power command value, the inductance can be dynamically adjusted. Variable inductor L v The current i flowing through the second winding of c is a control signal for adjusting the magnetic flux amount of the saturable inductor. This current i c is generated based on the input current feed-forward control as shown in FIG. 2. That is, the effective value of the input current i in is combined with the current command value i ref through RMS (Root Mean Square value) 24, and after passing through the error amplifier (PI) 26, the current value of the current i c is determined. The current command value i ref is the command value of the input current. The signal obtained from the error amplifier 26 is selected and determined by the multiplexer (MUX) 29 based on the initial value L int of the saturable inductor and the timing S Lv . This timing S Lv detects the timing (zero-crossing point) at which the load current reverses from negative to positive by the comparator (Comp2) 27, and the command value of the saturable inductor is determined by the MUX 29 using it as a trigger signal. The determined command value of the saturable inductor controls the current i v flowing through the second winding of the variable inductor L c .

[0019] Next, the circuit state transition diagram and theoretical operation waveforms of the single-phase direct AC-AC converter are shown in FIGS. 3 and 4 respectively, and the respective circuit operations will be described below. Note that FIG. 3 shows the positive half-cycle, and although not shown, there is a negative half-cycle as well. FIG. 3 shows the flow of the current i, and one cycle of each of Mode1 to 4 is illustrated in the theoretical operation waveform of FIG. 4.

[0020] (1) Mode1 Main switch Q 1After turning off (the auxiliary switch is always on), current i p flows from coil L r to capacitor C SW , and resonance occurs between L p and C r so that the magnetic energy of the coil returns to zero. At this time, when capacitor voltage V cr and switch current i sw are used as state variables, the circuit equation can be expressed as follows.

[0021] [Equation]

[0022] (2) Mode2 When the resonance voltage V SW starts to increase and the resonance returns to zero, switch Q 1 naturally conducts through its anti-parallel diode (or parasitic diode), and current i SW increases linearly according to the following equation.

[0023] [Equation]

[0024] (3) Mode3 Before the current reaches zero, main switch Q 1 turns on to achieve zero-voltage switching (ZVS), and current i SW continues to increase linearly.

[0025] (4) Mode4 When the current i SW of the switch changes in the positive direction, the circuit operation returns to the non-resonant state, and i SW continues to increase linearly.

[0026] Next, the automatic control of the inductance of the variable inductor will be described with reference to Fig. 5. The operation of the switch of the single-phase direct AC-AC converter is based on the input current i inDetermined by detecting the effective value of , the variable inductor L is modulated via a control circuit to ultimately achieve the desired output power. When the switch conduction current decreases from the command value, the value of the variable inductor is made smaller than that by PI control to increase the conduction current, i.e., the load current (input current). On the other hand, when the conduction current increases from the command value, the value of the variable inductor is made larger than that by PI control to decrease the conduction current, i.e., the load current (input current). During the period, from the timing when the conduction current is 0 (zero) or near it until the main switch turns off, the value of the variable inductor is set as the target value, and in other sections, it is set as the initial value (see Fig. 4). That is, the effective power of each switching pulse is adjusted using the change in the inductance of the variable inductor. v The inductance of is modulated. When the switch conduction current decreases from the command value, the value of the variable inductor is made smaller than that by PI control to increase the conduction current, i.e., the load current (input current). On the other hand, when the conduction current increases from the command value, the value of the variable inductor is made larger than that by PI control to decrease the conduction current, i.e., the load current (input current). During the period, from the timing when the conduction current is 0 (zero) or near it until the main switch turns off, the value of the variable inductor is set as the target value, and in other sections, it is set as the initial value (see Fig. 4). That is, the effective power of each switching pulse is adjusted using the change in the inductance of the variable inductor.

[0027] Fig. 6 is a waveform showing the conduction current of the variable inductor in the time domain. After the start of the simulation (t = 0 s), by changing the control winding current of the variable inductor (the current i flowing through the above-mentioned second winding c ), the difference in the transient time until saturation and the conduction current becomes constant is shown. By adjusting the control current until it enters the magnetic flux saturation region, it can be seen that the inductance of the variable inductor changes dynamically, and the time constant of the current flowing through the induction heating coil (work coil) changes.

[0028] Fig. 7 is a waveform showing the monotonic increase of the conduction current during one switching period (T sw ) of oscillation and power injection in the LC circuit, together with the variable inductor. During the power injection period, by adjusting the control current until it enters the magnetic flux saturation region and dynamically changing the inductance value of the variable inductor, the change in the switch conduction current is shown (see the circular region in the figure). The figure shows the relative change in the inductance value of the variable inductor and the current gradient. When the inductance value increases, the current gradient decreases, and the switch conduction current decreases. Thus, the principle of being able to adjust the effective value of the switch conduction current by using the converter of the present invention becomes clear.

[0029] FIG. 8 shows the control current I of the variable inductor c with respect to the variable inductor L v The value of. By inserting the variable inductor in series with the coil, the inductance of the LC circuit changes and the switch current changes. Table 1 below shows the plot of the graph of FIG. 8, and the current is the control current I of the variable inductor c .

[0030]

Table 1

[0031] FIG. 9 shows the switching simulation waveforms, where (1) is the control signal of the main switch Q 1 in the positive half cycle, (2) is the control signal of the saturable inductor L v , (3) is the bidirectional switch voltage, and (4) shows the waveform of the bidirectional switch current. When the input voltage is positive, the switch Q 1 becomes the main switch, and the auxiliary switch Q 2 does not operate constantly on. From FIGS. 9(1) and (3), it can be seen that zero voltage soft switching (ZVS) operation is achieved in the main switch of the bidirectional switch. As shown in FIG. 9(4), when the inductance value increases, the current gradient begins to decrease. When the main switch turns off, the current becomes zero, and the variable inductor returns to the low inductance of the initial value.

[0032] When the value of the variable inductor (saturable inductor) decreases (the magnetic flux in the magnetic path increases), the current flowing through the coil increases. That is, when the control signal of the saturable inductor is LOW (see FIG. 9(2)), the current of the control current source of the saturable inductor becomes the current corresponding to the target value of the saturable inductor, the saturable inductor becomes the target value (increases), the slope of the current decreases (see FIG. 9(4)), and the current flowing through the coil decreases. On the one hand, when the control signal of the saturable inductor is HIGH (refer to Fig. 9(2)), the current of the control current source of the saturable inductor reaches the maximum value, the saturable inductor becomes the initial value (which becomes small because it is saturated), the slope of the current increases, and the current flowing through the coil increases. The control signal of the saturable inductor and the value of the saturable inductor are shown in Fig. 4 and Fig. 9 respectively.

[0033] Also, for the variable inductor L v the turn-off, that is, the optimal moment for returning the magnetic flux of the variable inductor (returning from saturation to unsaturation) is the turn-off timing of the main switch. At that timing, the resonance frequency of the resonant tank remains unchanged, and the control circuit becomes simple.

[0034] Fig. 10(1) shows the input / output voltage and input / output current waveforms of the simulation results. Fig. 10(2) shows the output voltage and output current waveforms of the single-phase direct AC-AC converter. This circuit can convert a low-frequency input into a high-frequency output, and thus can be applied to an induction heating system.

[0035] Fig. 11 shows the power control curve of the single-phase direct AC-AC converter of the present invention. This is a characteristic curve showing that the effective value of the current flowing through the coil changes due to the addition of the variable inductor (saturable inductor), and the high-frequency power can be adjusted. The inductance of the variable inductor (saturable inductor) changes in synchronization with the main switch (however, the duty ratio is different). That is, the initial value and the target value are constantly switched, and the target value is updated every period of the main switch by automatic control. Fig. 11 shows the power corresponding to the inductance target value. While using the bidirectional switch only in the reverse conduction mode and driving at a fixed switching frequency, a variable inductor is installed in series with the high-frequency induction heating (IH) work coil to continuously adjust the rising rate of the current flowing through the coil and the effective value of the current.

[0036] From the characteristic curve of Fig. 11, it was found that by circuit simulation of the single-phase direct AC-AC converter of the present invention, power control of 50 to 500 W was achieved with the operation of a variable inductor of 30 to 100% (normalized value: 110 μH). In addition, high-frequency direct conversion from single-phase 60 Hz to 60 kHz was achieved.

Industrial Applicability

[0037] The present invention is useful in high-frequency induction heating systems (industrial, household and consumer applications).

Explanation of Reference Numerals

[0038] 1 Single-phase direct AC-AC converter 1a Commercial power supply 1b Bidirectional switch 1c LC resonance tank 20 Control circuit 26 Error amplifier 30 Control current source

Claims

1. In a high-frequency power conversion circuit including a resonant tank composed of a coil and a capacitor and a bidirectional switch, a variable inductor whose inductance changes due to a conduction current is connected in series to the coil of the resonant tank, a drive circuit of the bidirectional switch drives the switch so as to always be in a reverse conduction mode only, detects an effective value of the conduction current, compares it with a reference value, changes the inductance of the variable inductor, and changes a rising rate of a current flowing through the coil connected in series to adjust an effective current of the bidirectional switch. A single-phase direct AC-AC converter characterized by this.

2. The drive circuit of the bidirectional switch drives the switch fixed at a predetermined switching frequency. The single-phase direct AC-AC converter according to claim 1, characterized by this.

3. The drive circuit of the bidirectional switch swaps a main switch and a sub-switch in a positive half-cycle and a negative half-cycle of an input power supply, and keeps the sub-switch always on, both the positive half-cycle and the negative half-cycle perform the following operations 1) and 2) to adjust an effective current of the bidirectional switch flowing through the coil. The single-phase direct AC-AC converter according to claim 1, characterized by this: 1) When the main switch turns off, the capacitor voltage of the resonant tank drops from the power supply voltage, resonates with the coil and the variable inductor, the voltage of the main switch rises from zero to zero voltage soft switching (ZVS) turn-off, and after the voltage of the bidirectional switch reaches a peak value and then drops to zero, current flows backward through the anti-parallel diode of the main switch. During that time, the switch is driven to achieve zero voltage soft switching (ZVS) turn-on. 2) After the main switch turns on, the current changes from backward flow to forward flow and rises. Based on the difference between the effective value of the current and the reference value, the value of the variable inductor is changed. The timing of the change is performed when the current is zero or near zero, which brings a change to the gradient of the conduction current, and the value of the variable inductor is returned to the initial value at the turn-off timing of the main switch.

4. The switching period of the main switch in the drive circuit of the bidirectional switch is fixed at 20 kHz or more, and the low-frequency pulsation in the current of the input power supply is reduced. The single-phase direct AC-AC converter according to claim 3, characterized by this.

5. A power control method for a high-frequency power conversion circuit including a resonant tank composed of a coil and a capacitor and a bidirectional switch, wherein a variable inductor whose inductance changes with a conduction current is connected in series to the coil of the resonant tank, comprising: a drive circuit of the bidirectional switch; driving the switch so that it is always in the reverse conduction mode only; detecting an effective value of the conduction current and comparing it with a reference value; changing the inductance of the variable inductor based on the difference between the effective value of the current and the reference value, and adjusting the current amount by changing the rising rate of the current flowing through the coil connected in series to the variable inductor; A power control method for a single-phase direct AC-AC converter, characterized by comprising the above steps.

6. The drive circuit of the bidirectional switch; a) In the positive half cycle of the input power supply, one of the bidirectional switches is the main switch and the other is the auxiliary switch. In the negative half cycle, the correspondence between the main switch and the auxiliary switch is reversed, and the auxiliary switch is always turned on to drive the switch so that it is always in the reverse conduction mode only; b) By turning off the main switch, reducing the capacitor voltage of the resonant tank and resonating it with the coil and the variable inductor; c) The voltage of the main switch rises from zero to achieve zero-voltage soft-switching turn-off; d) After the voltage of the bidirectional switch reaches the peak value and then drops to zero, while the current flows backward through the anti-parallel diode of the main switch, driving the main switch to achieve zero-voltage soft-switching (ZVS) turn-on; e) After the main switch is turned on, the current changes from backward flow to forward flow and rises; f) Detecting an effective value of the conduction current and comparing it with a reference value; g) Changing the inductance of the variable inductor based on the difference between the effective value of the current and the reference value, and adjusting the current amount by changing the rising rate of the current flowing through the coil connected in series to the variable inductor; h) The main switch is turned off again and returns to a) above to repeat each step; A power control method for a single-phase direct AC-AC converter according to claim 5, characterized by comprising the above steps.

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