Converters and their control methods applicable to a wide range of output voltages

Abstract

This invention discloses a converter suitable for a wide range of output voltages and its control method. The converter includes a PWM half-bridge circuit. The control method includes the following steps: adjusting the switching frequency to put the PWM half-bridge circuit into discontinuous mode; when the PWM half-bridge circuit operates in discontinuous mode, the output inductor, the magnetizing inductance of the transformer, and the parasitic capacitance of the PWM half-bridge circuit oscillate; when the midpoint voltage of the primary-side switching bridge arm reaches its valley or peak, the corresponding power switch is triggered to turn on. This invention reduces switching losses by controlling the corresponding power switch in the PWM half-bridge circuit to turn on when the voltage across it oscillates to its valley.

Description

Technical Field

[0001] This invention relates to the field of power electronic converters, and in particular to a converter suitable for a wide range of output voltages and its control method. Background Technology

[0002] Currently, there are many types of consumer electronics products on the market, and they require a wide range of charging voltages. For example, mobile phones charge at 5V, routers at 12V, and laptops at 20V. To facilitate consumer use and avoid unnecessary waste, the industry has been working to promote adapters with wide-range output voltages to meet the needs of one-to-many applications.

[0003] Currently, most commercially available adapters with wide-range output voltage have a rated power of around 65W. The most commonly used topology is the flyback circuit, which offers a wide output voltage regulation capability and can ensure that the output efficiency meets energy efficiency requirements under light loads. However, with the development of client devices, power supply power levels are increasing, and the market demand for high-power, wide-range output power supplies has emerged, with power ratings rising to over 200W. Due to its inductive energy storage characteristics, the flyback circuit is not suitable for high-power, miniaturized, and high-power-density designs.

[0004] When the power supply exceeds 75W, a PFC circuit is required to meet harmonic requirements. Therefore, the input voltage range of this DC-DC stage is very narrow. Similar scenarios include home DC microgrids and vehicle charging, where the input voltage range is also small. For such narrow input and wide output applications, the industry has made many attempts, such as using a two-stage architecture of LLC plus Buck, which can meet the needs of high power and wide output range. Furthermore, due to the high efficiency of LLC, miniaturization is possible. However, since both stages operate, efficiency is low at low voltage output, failing to meet energy efficiency requirements. Additionally, the circuit structure is complex and costly.

[0005] Therefore, it is essential to find a converter with a wide range of output voltages and high efficiency under light loads, as well as its control method, to solve one or more of the aforementioned technical problems. Summary of the Invention

[0006] To achieve the above objectives, the present invention provides a control method for a converter suitable for a wide range of output voltages, the converter including a PWM half-bridge circuit, the PWM half-bridge circuit comprising:

[0007] The primary-side circuit includes a primary-side switching bridge arm composed of a first power switch and a second power switch connected in series.

[0008] A transformer includes a primary winding and a secondary winding, wherein the primary winding is coupled to the primary circuit;

[0009] A secondary-side rectifier circuit includes at least two synchronous rectifier diodes, the input terminal of which is coupled to the secondary-side coil; and

[0010] An output filter circuit, including an output inductor and an output capacitor, is coupled between the output terminal of the secondary rectifier circuit and a load.

[0011] The control method includes:

[0012] The PWM half-bridge circuit is brought into discontinuous mode by adjusting the switching frequency.

[0013] When the PWM half-bridge circuit operates in the discontinuous mode, the output inductor, the magnetizing inductance of the transformer, and the parasitic capacitance of the PWM half-bridge circuit oscillate. When the midpoint voltage of the primary-side switching bridge arm reaches the valley or the valley top, the corresponding first power switch or second power switch is triggered to turn on.

[0014] The present invention also provides a converter suitable for a wide range of output voltages, the converter including a PWM half-bridge circuit and a control unit, the PWM half-bridge circuit including:

[0015] The primary-side circuit includes a primary-side switching bridge arm composed of a first power switch and a second power switch connected in series.

[0016] A transformer includes a primary winding and a secondary winding, wherein the primary winding is coupled to the primary circuit;

[0017] The secondary-side rectifier circuit includes at least two synchronous rectifier diodes, and the input terminal of the secondary-side rectifier circuit is coupled to the secondary-side coil.

[0018] An output filter circuit, including an output inductor and an output capacitor, is coupled between the output terminal of the secondary rectifier circuit and a load.

[0019] The control unit is used to adjust the switching frequency to put the PWM half-bridge circuit into discontinuous mode. When the PWM half-bridge circuit is working in discontinuous mode, the output inductor, the magnetizing inductance of the transformer and the parasitic capacitance of the PWM half-bridge circuit oscillate. When the midpoint voltage of the primary-side switching bridge arm reaches the bottom or top of the valley, the corresponding first power switch or second power switch is triggered to turn on.

[0020] This invention enables the PWM half-bridge circuit to enter discontinuous mode (DCM) more quickly by rapidly adjusting the switching frequency when the output switches from high to low voltage. In DCM, the output inductor, magnetizing inductor, and parasitic capacitance of the PWM half-bridge circuit oscillate continuously. When the midpoint voltage of the primary-side switching arm reaches its lowest or highest point, the lower or upper transistor of the primary-side switching arm is turned on accordingly, thereby reducing switching losses. Furthermore, controlling the PWM half-bridge circuit to enter BURST mode from DCM when the load further decreases can further reduce switching losses. The following detailed description of the above embodiments will provide a further explanation of the technical solution of this invention. Attached Figure Description

[0021] To make the above and other objects, features, advantages and embodiments of the present invention more apparent and understandable, the accompanying drawings are described below:

[0022] Figure 1 The circuit diagram is for a PWM half-bridge circuit in the prior art.

[0023] Figure 2 for Figure 1 The waveform diagram of the PWM half-bridge circuit in continuous mode;

[0024] Figure 3 for Figure 1 The waveform diagram of the PWM half-bridge circuit in discontinuous mode;

[0025] Figure 4 This is a PWM half-bridge circuit diagram in a converter suitable for a wide range of output voltages according to a first preferred embodiment of the present invention.

[0026] Figure 5 This is a PWM half-bridge circuit diagram in a converter suitable for a wide range of output voltages according to a second preferred embodiment of the present invention.

[0027] Figure 6 This is a PWM half-bridge circuit diagram in a converter suitable for a wide range of output voltages according to a third preferred embodiment of the present invention.

[0028] Figure 7 This is a PWM half-bridge circuit diagram in a converter suitable for a wide range of output voltages according to a fourth preferred embodiment of the present invention.

[0029] Figure 8 The waveform diagram is shown below for the operation of the PWM half-bridge circuit according to the first preferred embodiment of the present invention.

[0030] Figure 9 The waveform diagram shows the PWM half-bridge circuit operating in BURST mode according to the first preferred embodiment of the present invention. Detailed Implementation

[0031] To provide a more detailed and complete description of the invention, reference can be made to the accompanying drawings and the various embodiments described below, in which the same reference numerals represent the same or similar components. Furthermore, well-known components and steps are not described in the embodiments to avoid unnecessarily limiting the invention. Additionally, for the sake of simplicity, some known and conventional structures and elements are illustrated in a simplified schematic manner in the drawings.

[0032] In view of the aforementioned technical problems in the prior art, and based on the requirements for converter power level and efficiency at low voltage output in current applications, this application provides a converter and its control method applicable to a wide range of output voltages. It adopts a PWM half-bridge circuit topology. On the one hand, it utilizes the wide-range voltage regulation capability of the PWM half-bridge circuit to realize the voltage switching function. On the other hand, during low voltage output, it enables the PWM half-bridge circuit to enter discontinuous mode, thereby improving the efficiency of the converter at low voltage and light load output.

[0033] Before describing the technical solution of this invention in detail, let's first introduce the existing PWM half-bridge circuit. See [link to relevant documentation]. Figure 1-3 The diagram shows the circuit diagram of a prior art PWM half-bridge circuit, as well as its operating waveforms in continuous mode and discontinuous mode. The control method for the PWM half-bridge circuit is single-voltage loop fixed-frequency control, such as... Figure 1 As shown, depending on the output voltage and given frequency, the PWM half-bridge circuit will operate in either continuous mode (CCM) or discontinuous mode (DCM) under different loads, as described below. Figure 2 and Figure 3 As shown.

[0034] When the switching transistors in the primary circuit of a PWM half-bridge circuit operate in hard-switching mode, the resulting losses mainly include two aspects: 1. Turn-on losses of the primary-side switching transistors. (where C) oss V is the parasitic capacitance of the primary-side switching transistor. turn_on f is the turn-on voltage of the primary-side switch. sw 1. (Switching frequency); 2. Hard-turning of the primary-side switch will produce a large step response, causing oscillations between the transformer's leakage inductance, the transformer's parasitic capacitance, and the parasitic capacitance of the secondary-side synchronous rectifier. This oscillation will rapidly decay to zero, generating losses in the circuit. The loss value can be expressed as... (where C) eq This refers to the equivalent capacitance of the transformer's parasitic capacitance and the synchronous rectifier tube's parasitic capacitance referred to the primary side of the transformer.

[0035] When the PWM half-bridge circuit operates in CCM mode, such as Figure 2As shown, the primary-side switch operates in hard-switching mode, and the turn-on voltage of the primary-side switch is V. in / 2, thus generating significant losses; when the PWM half-bridge circuit operates in DCM mode, the voltage across the primary-side switch S1 will oscillate before the primary-side switch is turned on, such as Figure 3 As shown, the existing fixed-frequency control results in the uncontrollable turn-on timing of the primary-side switch. When the primary-side switch S1 turns on exactly at the valley peak, the turn-on voltage is greater than V. in / 2, the losses will be greater.

[0036] The technical solution of the present invention will be described below with reference to specific embodiments. Specifically, the converter includes a PWM half-bridge circuit and a control unit.

[0037] See also Figure 4 The PWM half-bridge circuit includes: a primary-side circuit, comprising a primary-side switching bridge arm consisting of a first power switch S1 and a second power switch S2 connected in series; a transformer, comprising a primary-side coil and a secondary-side coil, the two ends of the primary-side coil being coupled to the output terminal of the primary-side circuit; a secondary-side rectifier circuit, comprising at least two synchronous rectifier diodes, namely synchronous rectifier diode SR1 and synchronous rectifier diode SR2, the input terminal of the secondary-side rectifier circuit being coupled to the secondary-side coil; and an output filter circuit, comprising an output inductor L. o and output capacitor C o The output filter circuit is coupled between the output terminal of the secondary rectifier circuit and a load.

[0038] Understandably, the output inductor L o The transformer can be integrated with the PWM half-bridge circuit, i.e., integrated into the same magnetic element, to reduce the overall volume of the magnetic element and improve the power density of the converter; of course, it can also be discrete, and this application is not limited thereto.

[0039] Furthermore, the control method includes the following steps: adjusting the switching frequency f sw To put the PWM half-bridge circuit into discontinuous mode, for example by reducing the switching frequency f. sw This causes the PWM half-bridge circuit to enter discontinuous mode; after the PWM half-bridge circuit enters discontinuous mode, the output inductor L... o The magnetizing inductance L of the transformer mThe parasitic capacitance of the PWM half-bridge circuit oscillates, and when the midpoint voltage of the primary-side switching arm reaches its lowest or highest point, it triggers the corresponding first power switch S1 or second power switch S2 to turn on. In this embodiment, the midpoint of the primary-side switching arm is the node connecting the first power switch S1 and the second power switch S2. The parasitic capacitance of the PWM half-bridge circuit is equivalent to the common parasitic capacitance of the first power switch S1, the second power switch S2, the synchronous rectifier, and the transformer, and can be considered equivalent to the parasitic capacitance of the midpoint of the primary-side switching arm to ground.

[0040] Understandably, the switching frequency f sw The lower the voltage, the easier it is for the PWM half-bridge circuit to enter DCM mode. Therefore, when the output switches from high voltage to low voltage, the switching frequency f is adjusted quickly. sw To enable the PWM half-bridge circuit to enter DCM mode faster, for example by rapidly reducing the switching frequency f. sw This allows the PWM half-bridge circuit to enter DCM mode more quickly. Once in DCM mode, during the switching state transition between the first power switch S1 and the second power switch S2 (i.e., the dead time between the first power switch S1 and the second power switch S2), the output inductor L... o The magnetizing inductance L of the transformer m This will cause oscillation with the parasitic capacitance of the PWM half-bridge circuit. Specifically, during the dead time from when the second power switch S2 is turned off to when the first power switch S1 is turned on, when the midpoint voltage of the primary-side switching bridge arm oscillates to the bottom, the lower switch of the primary-side switching bridge arm, i.e., the first power switch S1, is turned on; during the dead time from when the first power switch S1 is turned off to when the second power switch S2 is turned on, when the midpoint voltage of the primary-side switching bridge arm oscillates to the top, the upper switch of the primary-side switching bridge arm, i.e., the second power switch S2, is turned on, thereby reducing switching losses.

[0041] Preferably, see Figure 4 The primary-side circuit also includes a capacitor bridge arm composed of a first capacitor and a second capacitor connected in series. One end of the primary-side coil of the transformer is coupled to the midpoint of the capacitor bridge arm, wherein the midpoint of the capacitor bridge arm is the connection node between the first capacitor and the second capacitor. The other end of the primary-side coil is coupled to the midpoint of the primary-side switching bridge arm. The PWM half-bridge circuit of the present invention is not limited to... Figure 4 The structure within. For example, see [link to example]. Figures 5-6The primary-side circuit may further include a capacitor connected to either the positive or negative input terminal of the primary-side circuit. Specifically, one end of the capacitor is electrically coupled to either the positive or negative input terminal of the primary-side circuit, and the other end of the capacitor is electrically coupled to one end of the primary-side coil of the transformer. The other end of the primary-side coil is electrically coupled to the midpoint of the primary-side switching bridge arm. It is understood that the capacitor can also be connected in series at other locations in the primary-side circuit, such as between the other end of the primary-side coil and the midpoint of the primary-side switching bridge arm; however, the invention is not limited to this.

[0042] Preferably, the secondary-side rectifier circuit can be a full-wave rectifier circuit or a full-bridge rectifier circuit. For example... Figures 4-6 As shown, when the secondary winding of the transformer adopts a center-tapped structure, that is, the secondary winding of the transformer includes a first terminal, a second terminal, and a common terminal, the secondary rectifier circuit includes a first synchronous rectifier diode SR1 and a second synchronous rectifier diode SR2. One end of the first synchronous rectifier diode SR1 and one end of the second synchronous rectifier diode SR2 are respectively connected to the first terminal and the second terminal of the secondary winding. The other ends of the first synchronous rectifier diode SR1 and the second synchronous rectifier diode SR2 are simultaneously connected to the output capacitor C. o The output inductor L is connected to the negative terminal. o The two ends are respectively connected to the common terminal of the secondary coil and the output capacitor C. o Connect the positive ends.

[0043] like Figure 7 As shown, when the secondary rectifier circuit adopts a full-bridge structure, the secondary coil of the transformer includes a first terminal and a second terminal. The secondary rectifier circuit includes first to fourth synchronous rectifier diodes SR1-SR4. The first synchronous rectifier diode SR1 and the second synchronous rectifier diode SR2 are connected in series to form the first secondary bridge arm, and the third synchronous rectifier diode SR3 and the fourth synchronous rectifier diode SR4 are connected in series to form the second secondary bridge arm. The first terminal and the second terminal of the secondary coil are respectively connected to the midpoint of the first secondary bridge arm and the midpoint of the second secondary bridge arm. The output capacitor C... o Through the output inductor L o It is connected in parallel to both ends of the first and second arms of the secondary side.

[0044] According to another preferred embodiment of the present invention, the first power switch S1 is connected to the negative input terminal of the primary circuit, and the second power switch S2 is connected to the positive input terminal of the primary circuit. During the dead time from when the second power switch S2 is turned off to when the first power switch S1 is turned on, the first power switch S1 is triggered to turn on when the midpoint voltage of the primary switch arm reaches the valley. During the dead time from when the first power switch S1 is turned off to when the second power switch S2 is turned on, the second power switch S2 is triggered to turn on when the midpoint voltage of the primary switch arm reaches the valley.

[0045] To further reduce the switching frequency, the first power switch S1 is triggered to turn on when the midpoint voltage of the primary-side switching bridge arm is at the m-th valley; the second power switch S2 is triggered to turn on when the midpoint voltage of the primary-side switching bridge arm is at the m-th valley peak, where m is an integer greater than or equal to 1. In this embodiment, the value of m can vary according to the load size; generally, the smaller the load, the larger the value of m, and the larger the load, the smaller the value of m.

[0046] Preferably, in Figure 4-6 In the illustrated embodiment, during the switching state transition interval of the first power switch S1 and the second power switch S2, i.e., the dead time of the two power switches, the current on the first synchronous rectifier SR1 and the current on the second synchronous rectifier SR2 are detected, and the corresponding synchronous rectifier is controlled to be turned off or kept on based on the current on the synchronous rectifier. In this embodiment, during the dead time from when the first power switch S1 is turned off to when the second power switch S2 is turned on, the current on the first synchronous rectifier SR1 decreases linearly. When the current drops to 0, the first synchronous rectifier SR1 is turned off, and the second synchronous rectifier SR2 is kept on, so that the output inductor L o The magnetizing inductance L of the transformer m The parasitic capacitance of the PWM half-bridge circuit creates a continuous oscillation. During the dead time between the turn-off of the second power switch S2 and the turn-on of the first power switch S1, the current on the second synchronous rectifier SR2 decreases linearly. When the current drops to 0, the second synchronous rectifier SR2 is turned off, while the first synchronous rectifier SR1 remains on, causing the output inductor L to... o The magnetizing inductance L of the transformer m The parasitic capacitance of the PWM half-bridge circuit causes continuous oscillation.

[0047] Specifically, see Figure 8 The left side shows the operating waveform of the PWM half-bridge circuit under high voltage heavy load. At this time, the PWM half-bridge circuit is operating in CCM mode, and a relatively high switching frequency f is set. swThis reduces output current ripple, thereby reducing the size and losses of the output inductor. Figure 8 The right half of the waveform is the working waveform of the PWM half-bridge circuit under low voltage and light load output. When the output voltage is low, the current required by the system is also small, so the proportion of switching loss increases. At this time, it is necessary to quickly reduce the frequency to enter the discontinuous mode in order to realize the valley conduction of the primary side switching transistor and reduce the switching loss.

[0048] The following is combined with Figure 8 Valley control of the PWM half-bridge circuit of the present invention in discontinuous mode is further described in detail with examples.

[0049] [t0-t1] phase:

[0050] At time t0, the first power switch S1 is turned off, and the voltage V across S1 is... ds_S1 From 0V to V in / 2(where V) in (Input voltage); Primary current i p From peak current i p_pk It becomes 0A; the magnetizing inductance L m current i Lm Maintain peak current i Lm_pk Unchanged; the voltage V across the second synchronous rectifier diode SR2 remains unchanged. ds_SR2 By V in When the turns ratio of the transformer's primary and secondary sides (n) becomes 0, SR2 turns on. At this time, the first synchronous rectifier diode SR1 is also on, and the output inductor L... o Current i on Lo The current i on the first synchronous rectifier diode SR1 SR1 The current i on the second synchronous rectifier tube SR2 SR2 The current i decreases linearly until time t1, at which point the current i on the first synchronous rectifier diode SR1 decreases linearly. SR1 Reduced to 0, i SR2 and i Lo Reduced to n*i Lm_pk .

[0051] [t1-t2] phase:

[0052] At time t1, the current i on the first synchronous rectifier diode SR1 is SR1 The current i on the second synchronous rectifier diode SR2 drops to 0. SR2 and output inductor L o Current i on Lo Reduced to n*i Lm_pk The first synchronous rectifier SR1 is turned off, while the second synchronous rectifier SR2 remains on, and the output inductor L... o The magnetizing inductance L of the transformer mThe parasitic capacitance of the PWM half-bridge circuit causes oscillation; the voltage V across the first power switch S1 ds_S1 by As the equilibrium point, The amplitude is oscillated; the voltage V that the first synchronous rectifier diode SR1 withstands is... ds_SR1 by As the equilibrium point, The amplitude is oscillated; the current i of the output inductor is... Lo The equilibrium point is set at a linearly decreasing current, and the oscillations are based on a specific amplitude; all three oscillations have the same period, which is equal to... (where C) EQ (This refers to the parasitic capacitance of the PWM half-bridge circuit). Until time t2, the voltage V across the first power switch S1 is... ds_S1 At the peak of the oscillation, the voltage across the second power switch S2 is correspondingly at the trough of the oscillation. It is understandable that, in order to further reduce the switching frequency f... sw To improve efficiency under light loads, time t2 can be selected at the m-th valley peak, where the value of m depends on the load size and the switching frequency f. sw related.

[0053] [t2-t3] phase:

[0054] At time t2, the voltage V across the first power switch S1 is... ds_S1 At the peak of the oscillation, turning on the second power switch S2 at this time can achieve the peak-level turn-on of the second power switch S2; simultaneously, the primary current i p and excitation inductor current i Lm Linear decrease, output inductor current i Lo The primary current i increases linearly until time t3. p Reaching negative peak current -i p_pk .

[0055] [t3-t4] phase:

[0056] At time t3, the primary current i p When the negative peak value is reached, the second power switch S2 is turned off, and the voltage V across the first power switch S1 is... ds_S1 By V in Change to V in / 2; Primary current i p From negative peak current -i p_pk It becomes 0A; the magnetizing inductance L m current i Lm Maintain negative peak current -i Lm_pk The voltage V across the first synchronous rectifier diode SR1 remains unchanged. ds_SR1 By V inWhen / n becomes 0, the first synchronous rectifier diode SR1 turns on. At this time, the second synchronous rectifier diode SR2 is continuously conducting, and the output inductor L... o Current i on Lo The current i on the first synchronous rectifier diode SR1 SR1 The current i on the second synchronous rectifier tube SR2 SR2 The linear decrease continues until time t4, when i SR2 Reduced to 0, i SR1 and i Lo Reduced to n*i Lm_pk .

[0057] [t4-t5] phase:

[0058] At time t4, the current i on the second synchronous rectifier diode SR2 is SR2 When the current i drops to 0, the current i on the first synchronous rectifier diode SR1 SR1 and output inductor L o Current i on Lo Reduced to n*i Lm_pk The second synchronous rectifier SR2 is turned off, while the first synchronous rectifier SR1 remains on, and the output inductor L... o and transformer magnetizing inductance L m The parasitic capacitance of the PWM half-bridge circuit causes oscillation; the voltage V across the first power switch S1 ds_S1 by As the equilibrium point, The second synchronous rectifier diode SR2 oscillates to produce an amplitude; the voltage V it withstands is... ds_SR2 by As the equilibrium point, Oscillation is achieved for amplitude; output inductance L o current i Lo With a linearly decreasing current as the equilibrium point, and oscillations with another specific amplitude; and the oscillation periods of all three are equal to... Until time t5, the voltage V that the first power switch S1 withstands is... ds_S1 It is at the bottom of the oscillation. Similarly, when the load decreases, the frequency can be rapidly reduced by increasing the number of valleys during the t4-t5 period, thereby reducing switching losses and improving light-load efficiency. [t5-t6] stage:

[0059] At time t5, the voltage V across the first power switch S1 is... ds_S1 At the bottom of the oscillation, turning on the first power switch S1 at this time can achieve the bottom-out turn-on of the first power switch S1; at the same time, the primary current i p The current i of the magnetizing inductor Lm and the current i of the output inductor LoThe primary current i increases linearly until time t6. p Once the peak value is reached, the first power switch S1 is turned off, and the process is repeated thereafter.

[0060] According to another preferred embodiment of the present invention, a control method for a converter applicable to a wide range of output voltages is further provided, wherein when the load is further reduced, the PWM half-bridge circuit will enter the BURST mode from the discontinuous mode. Each BURST cycle includes a pulse enable interval (Burst ON) and a pulse disable interval (Burst OFF). In the pulse enable interval, the PWM half-bridge circuit operates in the aforementioned discontinuous mode; in the pulse disable interval, all pulse signals are turned off, that is, the drive signals of the primary circuit and the secondary rectifier circuit are turned off, so that the PWM half-bridge circuit stops working.

[0061] Furthermore, within each of these pulse enable intervals (Burst ON), the first switching cycle is processed, for example, by shortening the first switching cycle, to increase the current i in the magnetizing inductor. Lm and the current i of the output inductor Lo The predetermined trajectory is accessed, and the last switching cycle is processed, for example, by shortening the last switching cycle, so that the current i of the magnetizing inductor is reduced. Lm The pulse signal to the primary circuit is turned off when the signal reaches zero, thus avoiding losses and oscillations. For the remaining switching cycles during Burst ON, a constant switching frequency and duty cycle can be maintained. It can be understood that this predetermined trajectory is the trajectory of the current in the magnetizing inductor and the current in the output inductor in the aforementioned discontinuous mode.

[0062] Preferably, the number of switching cycles within the pulse enable interval is fixed, and the frequency of alternation between the pulse enable interval and the pulse disable interval is adjusted according to the load size. That is, the Burst frequency is adjusted according to the load size; the larger the load, the higher the frequency, and the smaller the load, the lower the frequency. Alternatively, the frequency of alternation between the pulse enable interval and the pulse disable interval is kept fixed, i.e., the Burst frequency is fixed, and the number of switching cycles within the pulse enable interval is adjusted according to the load size; the larger the load, the more cycles, and the smaller the load, the fewer cycles.

[0063] The following example uses a transformer secondary coil with a center tap; the specific control process is as follows: Figure 9 As shown, where:

[0064] [t0-t1] phase:

[0065] At time t0, the Burst ON state is entered, the first power switch S1 is turned on, and the voltage V across the first power switch S1 is... ds_S1 By V in / 2 becomes 0, and the voltage V that the first synchronous rectifier diode SR1 withstands... ds_SR1 By V o When the voltage drops to 0, the first synchronous rectifier diode SR1 turns on. At the same time, the voltage V across the second synchronous rectifier diode SR2... ds_SR2 By V o Change to V in / n, primary current i p The current i of the magnetizing inductor Lm and the current i of the output inductor Lo The current i in the magnetizing inductor increases linearly from zero until time t1. Lm Reaching its peak value, due to the current i in the magnetizing inductor Lm Since it cannot be detected, this time period can be calculated through the control chip.

[0066] [t1-t6] phase:

[0067] At time t1, the first power switch S1 is turned off. The subsequent operation during the period t1-t6 is similar to... Figure 8 The operation process from t0 to t5 is the same, so it will not be elaborated further here. During this period, the switching frequency and conduction time remain constant until t6, when the current Burst ON state enters its last switching cycle. [t6-t7] stage:

[0068] At time t6, the last switching cycle of the current Burst ON state is entered, and the first power switch S1 is turned on, with the primary current i p The current i of the magnetizing inductor Lm and the current i of the output inductor Lo The current i in the magnetizing inductor increases linearly until time t7. Lm Similarly, t6-t7 can be calculated from the negative value to 0.

[0069] [t7-t8] stage:

[0070] At time t7, the first power switch S1 is turned off, and the voltage V across S1 is... ds_S1 From 0 to V in / 2, the voltage V that the second synchronous rectifier diode SR2 withstands. ds_SR2 By V in When / n becomes 0, the second synchronous rectifier SR2 turns on. At this time, the first synchronous rectifier SR1 is in a continuously conducting state, and the primary current i p and the current i of the magnetizing inductor Lm Keeping it constant at 0, the current i of the output inductor Lo The current i decreases linearly until time t8, at which point the current in the output inductor decreases. Lo It dropped to 0.

[0071] [t8-t9] stage:

[0072] At time t8, the current i in the output inductor Lo When the voltage drops to 0, the first power switch S1, the second power switch S2, and the synchronous rectifiers SR1 and SR2 are all turned off. The voltage across the first power switch S1 and the second power switch S2 is V. in / 2, the voltage across synchronous rectifier diodes SR1 and SR2 changes from 0 to V. o It enters the Burst OFF state and re-enters the Burst ON state at time t9, after which the previous process is repeated.

[0073] It is understandable that the first and last switching cycles in the Burst ON process do not necessarily correspond to the first power switch S1; they can also be the second power switch S2, and the operating mode is the same.

[0074] According to another preferred embodiment of the present invention, Figure 9 The control methods in [the text] can also be used in [the text]. Figure 7 The secondary side shown is a PWM half-bridge circuit with a full-bridge rectifier circuit. In this circuit, the first synchronous rectifier SR1 and the third synchronous rectifier SR3 operate synchronously, and the second synchronous rectifier SR2 and the fourth synchronous rectifier SR4 operate synchronously. After the first power switch S1 is turned off, it controls the first synchronous rectifier SR1 and the third synchronous rectifier SR3 to turn on, while keeping the second synchronous rectifier SR2 and the fourth synchronous rectifier SR4 on. The current i on the first synchronous rectifier SR1... SR1 The current i on the second synchronous rectifier diode SR2 SR2 The current i on the third synchronous rectifier diode SR3 SR3 and the current i on the fourth synchronous rectifier diode SR4 SR4 The average decreases linearly until i SR2 and i SR4 When the voltage drops to 0, the second synchronous rectifier SR2 and the fourth synchronous rectifier SR4 are turned off, while the first synchronous rectifier SR1 and the third synchronous rectifier SR3 remain on, causing the output inductor L to... o and transformer magnetizing inductance L m The parasitic capacitance of the PWM half-bridge circuit creates oscillations when the voltage V across the first power switch S1... ds_S1 When the oscillation reaches its m-th trough, the second power switch S2 is turned on; similarly, after the second power switch S2 is turned off, the second synchronous rectifier SR2 and the fourth synchronous rectifier SR4 are turned on, while the first synchronous rectifier SR1 and the third synchronous rectifier SR3 remain on. The current i on the first synchronous rectifier SR1... SR1 The current i on the second synchronous rectifier diode SR2SR2 The current i on the third synchronous rectifier diode SR3 SR3 and the current i on the fourth synchronous rectifier diode SR4 SR4 Linear decrease, until i SR1 and i SR3 When the voltage drops to 0, the first synchronous rectifier diode SR1 and the third synchronous rectifier diode SR3 are turned off, while the second synchronous rectifier diode SR2 and the fourth synchronous rectifier diode SR4 remain on, causing the output inductor L to... o and transformer magnetizing inductance L m The parasitic capacitance of the PWM half-bridge circuit creates oscillations when the voltage V across the first power switch S1... ds_S1 When the m-th trough of the oscillation is reached, the first power switch S1 is turned on.

[0075] According to another preferred embodiment of the present invention, the present invention also provides a converter suitable for a wide range of output voltages. The converter includes a PWM half-bridge circuit and a control unit. The PWM half-bridge circuit includes: a primary-side circuit, including a primary-side switching bridge arm composed of a first power switch S1 and a second power switch S2 connected in series; a transformer, including a primary-side coil and a secondary-side coil, the primary-side coil being coupled to the output terminal of the primary-side circuit; a secondary-side rectifier circuit, including at least two synchronous rectifier diodes, the input terminal of the secondary-side rectifier circuit being coupled to the secondary-side coil; and an output filter circuit, including an output inductor and an output capacitor, the output filter circuit being coupled between the output terminal of the secondary-side rectifier circuit and a load. The control unit is coupled to the PWM half-bridge circuit (e.g., via wired or wireless communication). The control unit adjusts the switching frequency (e.g., by reducing the switching frequency) to put the PWM half-bridge circuit into discontinuous mode. In discontinuous mode, the output inductor, the magnetizing inductance of the transformer, and the parasitic capacitance of the PWM half-bridge circuit oscillate. When the midpoint voltage of the primary-side switching arm reaches its lowest or highest point, the corresponding first power switch S1 or second power switch S2 is triggered to turn on. The parasitic capacitance is the equivalent parasitic capacitance shared by the synchronous rectifier, the first power switch S1, the second power switch S2, and the transformer, and can be equivalently represented as the parasitic capacitance of the midpoint of the primary-side switching arm to ground.

[0076] Preferably, such as Figure 4As shown, the first power switch S1 is connected to the negative input terminal of the primary circuit, and the second power switch S2 is connected to the positive input terminal of the primary circuit. During the dead time from when the second power switch S2 is turned off to when the first power switch S1 is turned on, when the midpoint voltage of the primary switch arm reaches its lowest point, the control unit triggers the first power switch S1 to turn on. During the dead time from when the first power switch S1 is turned off to when the second power switch S2 is turned on, when the midpoint voltage of the primary switch arm reaches its highest point, the control unit triggers the second power switch S2 to turn on.

[0077] To further reduce the switching frequency and improve efficiency under light load, during the dead time from when the second power switch S2 is turned off to when the first power switch S1 is turned on, the control unit triggers the first power switch S1 to turn on when the midpoint voltage of the primary-side switch arm is at the m-th valley; during the dead time from when the first power switch S1 is turned off to when the second power switch S2 is turned on, the control unit triggers the second power switch S2 to turn on when the midpoint voltage of the primary-side switch arm is at the m-th valley peak, where m is an integer greater than or equal to 1.

[0078] Preferably, the control unit determines the value of m based on the size of the load; when the load decreases, the value of m is increased, and when the load increases, the value of m is decreased.

[0079] Preferably, when the secondary rectifier circuit is a full-wave rectifier circuit, such as Figures 4-6 As shown, the secondary winding of the transformer has a center-tapped structure, including a first terminal, a second terminal, and a common terminal. The secondary rectifier circuit includes a first synchronous rectifier diode SR1 and a second synchronous rectifier diode SR2. One end of the first synchronous rectifier diode SR1 and one end of the second synchronous rectifier diode SR2 are respectively connected to the first terminal and the second terminal of the secondary winding. The other ends of the first synchronous rectifier diode SR1 and the other ends of the second synchronous rectifier diode SR2 are simultaneously connected to the output capacitor C. o The output inductor L is connected to the negative terminal. o The two ends are respectively connected to the common terminal of the secondary coil and the output capacitor C. o Connect the positive ends.

[0080] Furthermore, in this embodiment, the converter also includes a current detection unit, which detects the current on the first synchronous rectifier SR1 and the current on the second synchronous rectifier SR2, and sends the detection result to the control unit. During the dead time between the first power switch S1 being turned off and the second power switch S2 being turned on, the current on the first synchronous rectifier SR1 decreases linearly. When the current drops to 0, the control unit controls the first synchronous rectifier SR1 to turn off and keeps the second synchronous rectifier SR2 continuously conducting, so that the output inductor L... o The magnetizing inductance L of the transformer m The parasitic capacitance of the PWM half-bridge circuit creates a continuous oscillation. During the dead time between the turn-off of the second power switch S2 and the turn-on of the first power switch S1, the current on the second synchronous rectifier SR2 decreases linearly. When the current drops to 0, the control unit controls the second synchronous rectifier SR2 to turn off, while keeping the first synchronous rectifier SR1 continuously conducting, so that the output inductor L... o The magnetizing inductance L of the transformer m The parasitic capacitance of the PWM half-bridge circuit causes continuous oscillation.

[0081] Preferably, when the secondary rectifier circuit is a full-bridge rectifier circuit, such as Figure 7 As shown, the secondary winding of the transformer includes a first terminal and a second terminal. The secondary rectifier circuit includes first to fourth synchronous rectifier diodes SR1-SR4. The first synchronous rectifier diode SR1 and the second synchronous rectifier diode SR2 are connected in series to form the first bridge arm of the secondary winding. The third synchronous rectifier diode SR3 and the fourth synchronous rectifier diode SR4 are connected in series to form the second bridge arm of the secondary winding. The first terminal and the second terminal of the secondary winding are respectively connected to the midpoint of the first bridge arm and the midpoint of the second bridge arm of the secondary winding. The output capacitor C... o Through the output inductor L o It is connected in parallel to both ends of the first and second arms of the secondary side.

[0082] Furthermore, in this embodiment, the converter also includes a current detection unit. This current detection unit detects the current on the first synchronous rectifier SR1, the second synchronous rectifier SR2, the third synchronous rectifier SR3, and the fourth synchronous rectifier SR4, and sends the detection result to the control unit. The control unit controls the first synchronous rectifier SR1 and the third synchronous rectifier SR3 to operate synchronously, and controls the second synchronous rectifier SR2 and the fourth synchronous rectifier SR4 to operate synchronously. Furthermore, after the first power switch S1 is turned off, the control unit turns on the first synchronous rectifier SR1 and the third synchronous rectifier SR3, while keeping the second synchronous rectifier SR2 and the fourth synchronous rectifier SR4 conducting. The current i on the first synchronous rectifier SR1...SR1 The current i on the second synchronous rectifier diode SR2 SR2 The current i on the third synchronous rectifier diode SR3 SR3 and the current i on the fourth synchronous rectifier diode SR4 SR4 Linear decrease, until i SR2 and i SR4 When the voltage drops to 0, the second synchronous rectifier SR2 and the fourth synchronous rectifier SR4 are turned off, while the first synchronous rectifier SR1 and the third synchronous rectifier SR3 remain on, causing the output inductor L to... o and transformer magnetizing inductance L m The parasitic capacitance of the PWM half-bridge circuit creates oscillations when the voltage V across the first power switch S1... ds_S1 When the oscillation reaches its m-th trough, the second power switch S2 is turned on. After the second power switch S2 is turned off, the second synchronous rectifier SR2 and the fourth synchronous rectifier SR4 are turned on, while the first synchronous rectifier SR1 and the third synchronous rectifier SR3 remain on. The current i on the first synchronous rectifier SR1... SR1 The current i on the second synchronous rectifier diode SR2 SR2 The current i on the third synchronous rectifier diode SR3 SR3 and the current i on the fourth synchronous rectifier diode SR4 SR4 Linear decrease, until i SR1 and i SR3 When the voltage drops to 0, the first synchronous rectifier diode SR1 and the third synchronous rectifier diode SR3 are turned off, while the second synchronous rectifier diode SR2 and the fourth synchronous rectifier diode SR4 remain on, causing the output inductor L to... o and transformer magnetizing inductance L m The parasitic capacitance of the PWM half-bridge circuit creates oscillations when the voltage V across the first power switch S1... ds_S1 When the device is at the m-th trough of the oscillation, the first power switch S1 is turned on. Here, m is an integer greater than or equal to 1.

[0083] According to another preferred embodiment of the present invention, the control unit is used to control the PWM half-bridge circuit to enter the BURST mode from the discontinuous mode when the load is further reduced. Each BURST cycle includes a pulse enable interval and a pulse disable interval. During the pulse enable interval, the PWM half-bridge circuit operates in the discontinuous mode; during the pulse disable interval, all pulse signals are turned off, so that the PWM half-bridge circuit stops working.

[0084] Preferably, in each of the pulse enable intervals (Burst ON), the control unit processes the first switching cycle to connect the current of the magnetizing inductor and the current of the output inductor to a predetermined trajectory; and processes the last switching cycle to turn off the pulse signal after the current of the magnetizing inductor is zero, thereby avoiding losses and oscillations. It is understood that the predetermined trajectory is the trajectory of the current of the magnetizing inductor and the current of the output inductor in the discontinuous mode described above.

[0085] Preferably, the control unit is used to fix the number of switching cycles in the pulse enable interval, and adjust the frequency of alternation between the pulse enable interval and the pulse disable interval according to the load size; the larger the load, the higher the frequency.

[0086] Preferably, the control unit is used to maintain a fixed frequency for alternating between the pulse enable interval and the pulse disable interval, and to adjust the number of switching cycles in the pulse enable interval according to the load size; the larger the load, the greater the number of cycles.

[0087] Preferably, the primary-side circuit further includes a capacitor bridge arm composed of a first capacitor and a second capacitor connected in series, such as... Figure 4 As shown, one end of the primary winding of the transformer is coupled to the midpoint of the capacitor bridge arm, and the other end of the primary winding is coupled to the midpoint of the primary switching bridge arm. Furthermore, the specific structure of the primary circuit can also be as follows: Figure 5 , Figure 6 The embodiments shown are not limited to these embodiments.

[0088] This invention enables the PWM half-bridge circuit to enter discontinuous mode (DCM) more quickly when the converter output switches from high voltage to low voltage by rapidly adjusting the switching frequency (e.g., reducing the switching frequency). Once in DCM, the output inductor, the magnetizing inductance of the transformer, and the parasitic capacitance of the PWM half-bridge circuit oscillate continuously. When the midpoint voltage of the primary-side switching arm reaches its lowest or highest point, the lower or upper transistor of the primary-side switching arm is switched on accordingly, thereby reducing switching losses. Furthermore, when the load decreases further, the PWM half-bridge circuit is controlled to transition from DCM to BURST mode.

[0089] Although the present invention has been disclosed above by way of embodiments, it is not intended to limit the present invention. Any person skilled in the art may make various modifications and refinements without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the scope defined in the appended claims.

Claims

1. A control method for a converter suitable for a wide range of output voltages, characterized by, The transformer comprises a PWM half-bridge circuit, the PWM half-bridge circuit comprising: a primary circuit comprising a primary switch bridge arm composed of a first power switch tube and a second power switch tube in series; a transformer comprising a primary coil and a secondary coil, the primary coil being coupled with the primary circuit; a secondary rectifier circuit comprising at least two synchronous rectifier tubes, an input end of the secondary rectifier circuit being coupled with the secondary coil; and an output filter circuit comprising an output inductor and an output capacitor, the output filter circuit being coupled between an output end of the secondary rectifier circuit and a load; the control method comprising: adjusting a switching frequency to make the PWM half-bridge circuit enter a discontinuous mode, when the PWM half-bridge circuit works in the discontinuous mode, the output inductor, an excitation inductance of the transformer and a parasitic capacitance of the PWM half-bridge circuit oscillate, and when a midpoint voltage of the primary switch bridge arm reaches a valley bottom or a valley top, a corresponding first power switch tube or second power switch tube is triggered to turn on.

2. The control method according to claim 1, characterized by, The first power switch tube is connected with a negative input end of the primary circuit, and the second power switch tube is connected with a positive input end of the primary circuit; when a midpoint voltage of the primary switch bridge arm reaches a valley bottom within a dead time from when the second power switch tube is turned off to when the first power switch tube is turned on, the first power switch tube is triggered to turn on; when the midpoint voltage of the primary switch bridge arm reaches a valley top within a dead time from when the first power switch tube is turned off to when the second power switch tube is turned on, the second power switch tube is triggered to turn on.

3. The control method according to claim 2, characterized by, When the midpoint voltage of the primary switch bridge arm is at an mth valley bottom, the first power switch tube is triggered to turn on; when the midpoint voltage of the primary switch bridge arm is at an mth valley top, the second power switch tube is triggered to turn on, where m is an integer greater than or equal to 1.

4. The control method according to claim 3, characterized by The value of m is determined according to the size of the load; when the load decreases, the value of m is increased; when the load increases, the value of m is decreased.

5. The control method according to claim 1, characterized by, The secondary coil of the transformer is in a center-tapped structure and comprises a first end, a second end and a common end; the secondary rectifier circuit comprises a first synchronous rectifier tube and a second synchronous rectifier tube; one end of the first synchronous rectifier tube and one end of the second synchronous rectifier tube are connected with the first end and the second end of the secondary coil respectively; the other end of the first synchronous rectifier tube and the other end of the second synchronous rectifier tube are simultaneously connected with one end of the output capacitor; the output inductor has two ends respectively connected with the common end of the secondary coil and the other end of the output capacitor.

6. The control method according to claim 5, characterized by The method further comprises: detecting a current on the first synchronous rectifier tube and a current on the second synchronous rectifier tube within a switching state switching interval of the first power switch tube and the second power switch tube. In the dead time from the first power switch tube being off to the second power switch tube being on, when the current on the first synchronous rectifier is reduced to 0, the first synchronous rectifier is turned off, the second synchronous rectifier is kept in the on state, so that the output inductor, the excitation inductor of the transformer and the parasitic capacitor form continuous oscillation. In the dead time from the second power switch tube being off to the first power switch tube being on, when the current on the second synchronous rectifier is reduced to 0, the second synchronous rectifier is turned off, the first synchronous rectifier is kept in the on state, so that the output inductor, the excitation inductor of the transformer and the parasitic capacitor form continuous oscillation.

7. The control method according to claim 1, characterized by, The secondary coil of the transformer comprises a first end and a second end, the secondary rectifier circuit comprises first to fourth synchronous rectifiers, the first synchronous rectifier and the second synchronous rectifier are connected in series to form a first bridge arm of the secondary side, the third synchronous rectifier and the fourth synchronous rectifier are connected in series to form a second bridge arm of the secondary side, the first end and the second end of the secondary coil are connected to the midpoints of the first bridge arm and the second bridge arm of the secondary side respectively, and the output capacitor is connected in parallel to the two ends of the first bridge arm and the second bridge arm of the secondary side through the output inductor.

8. The control method according to claim 7, characterized by, Further comprising: The first synchronous rectifier and the third synchronous rectifier are controlled to work synchronously, and the second synchronous rectifier and the fourth synchronous rectifier are controlled to work synchronously; In the dead time from the first power switch tube being off to the second power switch tube being on, when the current on the second synchronous rectifier and the fourth synchronous rectifier is reduced to 0, the second synchronous rectifier and the fourth synchronous rectifier are turned off, the first synchronous rectifier and the third synchronous rectifier are kept in the on state, so that the output inductor, the excitation inductor of the transformer and the parasitic capacitor form continuous oscillation; In the dead time from the second power switch tube being off to the first power switch tube being on, when the current on the first synchronous rectifier and the third synchronous rectifier is reduced to 0, the first synchronous rectifier and the third synchronous rectifier are turned off, the second synchronous rectifier and the fourth synchronous rectifier are kept in the on state, so that the output inductor, the excitation inductor of the transformer and the parasitic capacitor form continuous oscillation.

9. The control method according to claim 1, characterized by, Further comprising: When the load is further reduced, the PWM half-bridge circuit is controlled to enter a BURST mode from the discontinuous mode, each BURST period comprises a pulse enable interval and a pulse off interval, in the pulse enable interval, the PWM half-bridge circuit works in the discontinuous mode; In the pulse off interval, all pulse signals are turned off, and the PWM half-bridge circuit stops working.

10. The control method according to claim 9, characterized by, In each pulse enable interval, a first switching period is processed to make the current of the excitation inductor and the current of the output inductor access a predetermined trajectory, and a last switching period is processed to make the pulse signal of the primary side circuit be turned off after the current of the excitation inductor is 0.

11. The control method according to claim 10, characterized by, The predetermined trajectory is a trajectory of the current of the excitation inductance and the current of the output inductance in the discontinuous mode.

12. The control method according to claim 9, characterized by, The number of switching periods in the pulse enable interval is fixed, and the frequency of the alternation of the pulse enable interval and the pulse off interval is adjusted according to the size of the load, and the greater the load, the higher the frequency.

13. The control method according to claim 9, characterized by, The frequency of the alternation of the pulse enable interval and the pulse off interval is fixed, and the number of switching periods in the pulse enable interval is adjusted according to the size of the load, and the greater the load, the greater the number.

14. The control method according to claim 1, characterized by, The primary side circuit further comprises a capacitor bridge arm composed of a first capacitor and a second capacitor in series, one end of a primary side coil of the transformer is coupled to a midpoint of the capacitor bridge arm, and the other end of the primary side coil is coupled to a midpoint of the primary side switch bridge arm.

15. The control method according to claim 1, characterized by, The primary side circuit further comprises a capacitor, one end of the capacitor is coupled to a positive input terminal or a negative input terminal of the primary side circuit, the other end of the capacitor is coupled to one end of the primary side coil, and the other end of the primary side coil is coupled to the midpoint of the primary side switch bridge arm.

16. A converter suitable for a wide range of output voltages, characterized in that, The converter comprises a PWM half-bridge circuit and a control unit, the PWM half-bridge circuit comprises: The primary side circuit comprises a primary side switch bridge arm composed of a first power switch tube and a second power switch tube in series; The transformer comprises a primary side coil and a secondary side coil, the primary side coil is coupled to the primary side circuit; The secondary side rectifier circuit comprises at least two synchronous rectifier tubes, an input terminal of the secondary side rectifier circuit is coupled to the secondary side coil; The output filter circuit comprises an output inductance and an output capacitance, the output filter circuit is coupled between an output terminal of the secondary side rectifier circuit and a load; The control unit is configured to adjust a switching frequency to make the PWM half-bridge circuit enter a discontinuous mode, when the PWM half-bridge circuit works in the discontinuous mode, the output inductance, an excitation inductance of the transformer and a parasitic capacitance of the PWM half-bridge circuit oscillate, and when a midpoint voltage of the primary side switch bridge arm reaches a valley bottom or a valley top, a corresponding first power switch tube or second power switch tube is triggered to be turned on.

17. The variator of claim 16, wherein, The first power switch tube is connected to a negative input terminal of the primary side circuit, and the second power switch tube is connected to a positive input terminal of the primary side circuit; during a dead time from when the second power switch tube is turned off to when the first power switch tube is turned on, when the midpoint voltage of the primary side switch bridge arm reaches the valley bottom, the control unit triggers the first power switch tube to be turned on; during a dead time from when the first power switch tube is turned off to when the second power switch tube is turned on, when the midpoint voltage of the primary side switch bridge arm reaches the valley top, the control unit triggers the second power switch tube to be turned on.

18. The variator of claim 17, wherein, When the midpoint voltage of the primary side switch bridge arm is at an mth valley bottom, the control unit triggers the first power switch tube to be turned on; when the midpoint voltage of the primary side switch bridge arm is at an mth valley top, the control unit triggers the second power switch tube to be turned on, where m is an integer greater than or equal to 1.

19. The variator of claim 18, wherein, The control unit determines the value of m according to the size of the load, and increases the value of m when the load decreases, and decreases the value of m when the load increases.

20. The variator of claim 16, wherein, The secondary coil of the transformer is a center-tapped structure, including a first end, a second end and a common end, the secondary rectifier circuit includes a first synchronous rectifier and a second synchronous rectifier, one end of the first synchronous rectifier and one end of the second synchronous rectifier are connected with the first end and the second end of the secondary coil respectively, the other end of the first synchronous rectifier and the other end of the second synchronous rectifier are connected with one end of the output capacitor simultaneously, and the output inductor is connected with the common end of the secondary coil and the other end of the output capacitor respectively.

21. The variator of claim 20, wherein, The current detection unit is further included for detecting the current on the first synchronous rectifier and the current on the second synchronous rectifier and sending the detection results to the control unit; During the dead time from the turn-off of the first power switch tube to the turn-on of the second power switch tube, when the current on the first synchronous rectifier decreases to 0, the control unit controls the first synchronous rectifier to turn off and keeps the second synchronous rectifier to be continuously conductive, so that the output inductor, the excitation inductor and the parasitic capacitor form continuous oscillation; During the dead time from the turn-off of the second power switch tube to the turn-on of the first power switch tube, when the current on the second synchronous rectifier decreases to 0, the control unit controls the second synchronous rectifier to turn off and keeps the first synchronous rectifier to be continuously conductive, so that the output inductor, the excitation inductor and the parasitic capacitor form continuous oscillation.

22. The variator of claim 16, wherein, The secondary coil of the transformer includes a first end and a second end, the secondary rectifier circuit includes first to fourth synchronous rectifiers, the first synchronous rectifier and the second synchronous rectifier are connected in series to form a first bridge arm, the third synchronous rectifier and the fourth synchronous rectifier are connected in series to form a second bridge arm, the first end and the second end of the secondary coil are connected with the midpoint of the first bridge arm and the midpoint of the second bridge arm respectively, and the output capacitor is connected in parallel to the first bridge arm and the second bridge arm through the output inductor.

23. The variator of claim 22, wherein, The control unit is used for: controlling the first synchronous rectifier and the third synchronous rectifier to work synchronously, and controlling the second synchronous rectifier and the fourth synchronous rectifier to work synchronously; and during the dead time from the turn-off of the first power switch tube to the turn-on of the second power switch tube, when the currents on the second synchronous rectifier and the fourth synchronous rectifier decrease to 0, the second synchronous rectifier and the fourth synchronous rectifier are turned off, the first synchronous rectifier and the third synchronous rectifier are kept in the conductive state, so that the output inductor, the excitation inductor of the transformer and the parasitic capacitor form continuous oscillation; In the dead time from the second power switch tube being off to the first power switch tube being on, when the current on the first and third synchronous rectifier tubes is reduced to 0, the first and third synchronous rectifier tubes are turned off, the second and fourth synchronous rectifier tubes are kept on, so that the output inductor, the magnetizing inductor of the transformer and the parasitic capacitor form continuous oscillation.

24. The variator of claim 16, wherein, The control unit is configured to control the PWM half-bridge circuit to enter a BURST mode from the discontinuous mode when the load further decreases, each BURST period including a pulse enable interval and a pulse off interval, in the pulse enable interval, the PWM half-bridge circuit works in the discontinuous mode. In the pulse off interval, all pulse signals are turned off, and the PWM half-bridge circuit stops working.

25. The variator of claim 24, wherein, In each pulse enable interval, the control unit processes a first switching period to make the current of the magnetizing inductor and the current of the output inductor access a predetermined trajectory, and processes a last switching period to make the current of the magnetizing inductor be 0 and then turn off the pulse signal of the primary side circuit.

26. The variator of claim 16, wherein, The primary side circuit further includes a capacitor bridge arm composed of a first capacitor and a second capacitor in series, one end of a primary side coil of the transformer is coupled to a midpoint of the capacitor bridge arm, and the other end of the primary side coil is coupled to a midpoint of the primary side switch bridge arm.

27. The variator of claim 16, wherein, The primary side circuit further includes a capacitor, one end of the capacitor is coupled to a positive input terminal or a negative input terminal of the primary side circuit, the other end of the capacitor is coupled to one end of the primary side coil, and the other end of the primary side coil is coupled to the midpoint of the primary side switch bridge arm.

28. The variator of claim 16, wherein, The output inductor and the transformer are integrated.

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