An ultra-wide-range LC resonant converter and its gain extension method

Abstract

The application discloses an ultra-wide-range LC resonant converter and a gain expansion method thereof, and the converter comprises a high-frequency transformer T, the secondary side of the high-frequency transformer T is provided with two series windings with a number of turns n, and the number of turns ratio is 1:n:n; the first end of the first secondary side winding is connected with one end of a DC blocking capacitor C2, the other end of the DC blocking capacitor C2 is connected with the first bridge arm midpoint of a secondary side H-bridge circuit, and the first switch K1 and the second switch K2 are both connected with the second bridge arm midpoint of the secondary side H-bridge circuit. The application realizes the series connection of the two secondary side windings with the number of turns n by setting the first switch K1 and the second switch K2, the equivalent number of turns ratio of the secondary side winding is 1:n when K1 is disconnected and K2 is connected, and the equivalent number of turns ratio of the secondary side winding is 1:2n when K1 is connected and K2 is disconnected, so that the converter obtains the voltage gain ranges of 0.6n-1.4n and 1.2n-2.8n in the two number of turns modes, the ultra-wide-range continuous coverage of 0.6n-2.8n is realized, and the problem of the narrow gain range of the traditional LC resonant converter is effectively solved.

Description

Technical Field

[0001] This invention relates to the field of LC resonant converter technology, and more specifically, to an ultra-wide-range LC resonant converter and its gain extension method. Background Technology

[0002] LC resonant converters are widely used in power conversion applications such as electric vehicle on-board chargers, energy storage converters, and DC-DC converters due to their advantages of high efficiency, high power density, and electrical isolation. Among them, LC resonant converters, with their inherent bidirectional power transmission capability and wide output voltage regulation range, have become a research hotspot in these fields.

[0003] In traditional LC resonant converters, a single resonant control mode or phase-shift control mode is typically used to regulate the output voltage. Reference [Liu, Gang, Jang, Yungtaek, Jovanovic, Milan M., et al. Implementation of a 3.3-kW DC-DC Converter for EV On-Board Charger Employing the Series-Resonant Converter With Reduced-Frequency-Range Control[J].IEEE Transactions on Power Electronics, 2017, 32(6):4168-4184.] discloses an on-board charging DC-DC converter using a series resonant converter, such as... Figure 4 As shown, output voltage regulation is achieved through frequency modulation combined with phase-shift control. However, under the above control method, the output voltage gain range of the converter can generally only reach about 0.6 to 1.4 times, which is difficult to meet the operating conditions of large-range fluctuations in input / output voltage in applications such as electric vehicle charging and energy storage systems.

[0004] Furthermore, existing LC resonant converters typically use a fixed transformer turns ratio, significantly limiting the converter's voltage gain range. When adapting to loads of varying voltage levels or experiencing large input voltage fluctuations, the fixed turns ratio prevents the converter from maintaining high conversion efficiency across the entire voltage range. Some existing solutions attempt to extend the voltage range through frequency conversion control, but excessive frequency variations can cause resonant network parameter shifts, increased electromagnetic interference, and design difficulties for magnetic components. Other solutions propose adjusting the turns ratio through transformer tap switching or secondary winding series-parallel switching, but these often suffer from drawbacks such as high voltage stress on the switching switches, blind spots in gain at each tap, and complex control.

[0005] In addition, when traditional LC resonant converters employ phase-shifting or asymmetrical control, the volt-second product of the positive and negative half-cycles of the transformer is prone to imbalance, leading to magnetic core saturation and a sharp increase in excitation current. In severe cases, this may burn out power devices and affect system reliability.

[0006] In summary, achieving a wider output voltage gain range while simplifying circuit structure and control methods, and solving the transformer bias problem, are directions that urgently need to be improved in existing LC resonant converters. Summary of the Invention

[0007] To address the aforementioned technical problems in related technologies, this invention proposes an ultra-wide-range LC resonant converter and its gain extension method, which can overcome the above-mentioned shortcomings of the prior art.

[0008] To achieve the above-mentioned technical objectives, the technical solution of the present invention is implemented as follows: An ultra-wide range LC resonant converter includes a primary-side H-bridge circuit and a secondary-side H-bridge circuit. The two ends of the primary-side H-bridge circuit are respectively connected to a DC input power supply Vin. The midpoint of the first arm of the primary-side H-bridge circuit is connected to one end of a resonant inductor L1. The midpoint of the second arm of the primary-side H-bridge circuit is connected to one end of a resonant capacitor C1. The other ends of the resonant inductor L1 and the other ends of the resonant capacitor C1 are respectively connected to the two ends of the primary winding of a high-frequency transformer T. The high-frequency transformer T has two series windings, each with n turns, on its secondary side, with a turns ratio of 1:n:n. A second switch K2 is located at the end of the first secondary winding, and a first switch K1 is located at the end of the second secondary winding. The first end of the first secondary winding is connected to one end of a DC blocking capacitor C2, and the other end of the DC blocking capacitor C2 is connected to the midpoint of the first arm of the secondary H-bridge circuit. Both the first switch K1 and the second switch K2 are connected to the midpoint of the second arm of the secondary H-bridge circuit. The two ends of the secondary H-bridge circuit are connected in parallel to an output filter capacitor Co and a load Ro, respectively.

[0009] Furthermore, the primary-side H-bridge circuit is composed of a first switch Q1, a second switch Q2, a third switch Q3, and a fourth switch Q4. The first arm of the primary-side H-bridge circuit is formed by the first switch Q1 and the third switch Q3 connected in series, and the second arm of the primary-side H-bridge circuit is formed by the second switch Q2 and the fourth switch Q4 connected in series. The drains of the first switch Q1 and the second switch Q2 are connected to the positive terminal of the DC input power supply Vin, and the sources of the third switch Q3 and the fourth switch Q4 are connected to the negative terminal of the DC input power supply Vin. The connection between the source of the first switch Q1 and the drain of the third switch Q3 serves as the midpoint of the first arm of the primary-side H-bridge circuit, and the connection between the source of the second switch Q2 and the drain of the fourth switch Q4 serves as the midpoint of the second arm of the primary-side H-bridge circuit.

[0010] Furthermore, the secondary-side H-bridge circuit is composed of a fifth switch S1, a sixth switch S2, a seventh switch S3, and an eighth switch S4. The first arm of the secondary-side H-bridge circuit is formed by the fifth switch S1 and the seventh switch S3 connected in series, and the second arm of the secondary-side H-bridge circuit is formed by the sixth switch S2 and the eighth switch S4 connected in series. The drain of the fifth switch S1 and the drain of the sixth switch S2 are connected to one end of the filter capacitor Co and one end of the load Ro, respectively. The source of the seventh switch S3 and the source of the eighth switch S4 are connected to the other end of the filter capacitor Co and the other end of the load Ro, respectively. The source of the fifth switch S1 and the drain of the seventh switch S3 are connected as the midpoint of the first arm of the secondary-side H-bridge circuit, and the source of the sixth switch S2 and the drain of the eighth switch S4 are connected as the midpoint of the second arm of the secondary-side H-bridge circuit.

[0011] Furthermore, when the first switch K1 is open and the second switch K2 is closed, the equivalent turns ratio of the transformer is 1:n, and the converter operates in low-voltage mode; when the first switch K1 is closed and the second switch K2 is open, the two secondary windings are connected in series, the equivalent turns ratio of the transformer is 1:2n, and the converter operates in high-voltage mode.

[0012] Furthermore, the first to the eighth switching transistors are all N-channel MOSFETs, and the parasitic body diodes inside each switching transistor are oriented from the source to the drain.

[0013] A gain extension method for an ultra-wide-range LC resonant converter, applied to an ultra-wide-range LC resonant converter, characterized by comprising: The operating mode is switched by controlling the on / off states of the first switch K1 and the second switch K2 to change the equivalent turns ratio of the transformer. When low-voltage output is required, the first switch K1 is opened and the second switch K2 is closed, the transformer's equivalent turns ratio is 1:n, the converter operates in low-voltage mode, and obtains a voltage gain range of 0.6n to 1.4n. When high voltage output is required, the first switch K1 is closed and the second switch K2 is opened, so that the two secondary windings are connected in series. The equivalent turns ratio of the transformer is 1:2n, and the converter operates in high voltage mode to obtain a voltage gain range of 1.2n to 2.8n. The gain range of the low-voltage mode partially overlaps with that of the high-voltage mode, allowing the gain range of the entire converter to continuously cover 0.6n to 2.8n.

[0014] The beneficial effects of this invention are as follows: By setting a first switch K1 and a second switch K2 to connect two windings with the same number of turns n on the secondary side, when K1 is open and K2 is closed, the equivalent turns ratio of the secondary winding is 1:n. When K1 is closed and K2 is open, the secondary winding is connected in series and the equivalent turns ratio is 1:2n. This allows the converter to obtain voltage gain ranges of 0.6n~1.4n and 1.2n~2.8n in the two turns ratio modes, respectively. Moreover, the two gain ranges overlap, achieving a continuous coverage of an ultra-wide range of 0.6n~2.8n, effectively solving the problem of narrow gain range in traditional LC resonant converters. Meanwhile, by connecting a DC blocking capacitor C2 in series between the first end of the first secondary winding and the midpoint of the first arm of the secondary H-bridge, the DC blocking and AC passing characteristics are used to block the DC component from entering the resonant network, balancing the positive and negative half-cycle volt-second product, thereby effectively suppressing the transformer core bias saturation and improving system reliability. In addition, the secondary switching transistors conduct synchronously in resonant mode to achieve synchronous rectification, reducing conduction losses. In delayed turn-off mode, only the turn-off time of some secondary switching transistors needs to be controlled to achieve the boost effect, which is simple to control. This simplifies the control complexity and improves the conversion efficiency and adaptability under wide voltage conditions. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a structural diagram of the ultra-wide range LC resonant converter according to an embodiment of the present invention; Figure 2 This refers to the operating mode of the resonant mode switch of the ultra-wide range LC resonant converter according to an embodiment of the present invention; Figure 3This refers to the delayed turn-off mode switching mode of the ultra-wide range LC resonant converter according to an embodiment of the present invention. Figure 4 This is a structural diagram of a traditional LC resonant converter. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0018] like Figure 1 As shown in the embodiment of the present invention, an ultra-wide range LC resonant converter includes a primary-side H-bridge circuit and a secondary-side H-bridge circuit. The two ends of the primary-side H-bridge circuit are respectively connected to the DC input power supply Vin. The midpoint of the first arm of the primary-side H-bridge circuit is connected to one end of the resonant inductor L1. The midpoint of the second arm of the primary-side H-bridge circuit is connected to one end of the resonant capacitor C1. The other ends of the resonant inductor L1 and the other ends of the resonant capacitor C1 are respectively connected to the two ends of the primary winding of the high-frequency transformer T. The high-frequency transformer T has two series windings, each with n turns, on its secondary side, with a turns ratio of 1:n:n. A second switch K2 is located at the end of the first secondary winding, and a first switch K1 is located at the end of the second secondary winding. The first end of the first secondary winding is connected to one end of a DC blocking capacitor C2, and the other end of the DC blocking capacitor C2 is connected to the midpoint of the first arm of the secondary H-bridge circuit. Both the first switch K1 and the second switch K2 are connected to the midpoint of the second arm of the secondary H-bridge circuit. The two ends of the secondary H-bridge circuit are connected in parallel to an output filter capacitor Co and a load Ro, respectively.

[0019] Preferably, the primary-side H-bridge circuit is composed of a first switch Q1, a second switch Q2, a third switch Q3, and a fourth switch Q4. The first arm of the primary-side H-bridge circuit is formed by the first switch Q1 and the third switch Q3 connected in series, and the second arm of the primary-side H-bridge circuit is formed by the second switch Q2 and the fourth switch Q4 connected in series. The drains of the first switch Q1 and the second switch Q2 are connected to the positive terminal of the DC input power supply Vin, and the sources of the third switch Q3 and the fourth switch Q4 are connected to the negative terminal of the DC input power supply Vin. The connection between the source of the first switch Q1 and the drain of the third switch Q3 serves as the midpoint of the first arm of the primary-side H-bridge circuit, and the connection between the source of the second switch Q2 and the drain of the fourth switch Q4 serves as the midpoint of the second arm of the primary-side H-bridge circuit.

[0020] Preferably, the secondary-side H-bridge circuit is composed of a fifth switch S1, a sixth switch S2, a seventh switch S3, and an eighth switch S4. The first arm of the secondary-side H-bridge circuit is formed by the fifth switch S1 and the seventh switch S3 connected in series, and the second arm of the secondary-side H-bridge circuit is formed by the sixth switch S2 and the eighth switch S4 connected in series. The drain of the fifth switch S1 and the drain of the sixth switch S2 are connected to one end of the filter capacitor Co and one end of the load Ro, respectively. The source of the seventh switch S3 and the source of the eighth switch S4 are connected to the other end of the filter capacitor Co and the other end of the load Ro, respectively. The source of the fifth switch S1 and the drain of the seventh switch S3 are connected as the midpoint of the first arm of the secondary-side H-bridge circuit, and the source of the sixth switch S2 and the drain of the eighth switch S4 are connected as the midpoint of the second arm of the secondary-side H-bridge circuit.

[0021] Preferably, when the first switch K1 is open and the second switch K2 is closed, the equivalent turns ratio of the transformer is 1:n, and the converter operates in low-voltage mode; when the first switch K1 is closed and the second switch K2 is open, the two secondary windings are connected in series, the equivalent turns ratio of the transformer is 1:2n, and the converter operates in high-voltage mode.

[0022] Furthermore, the first to the eighth switching transistors are all N-channel MOSFETs, and the parasitic body diodes inside each switching transistor are oriented from the source to the drain.

[0023] A gain extension method for an ultra-wide-range LC resonant converter, applied to an ultra-wide-range LC resonant converter, includes: The operating mode is switched by controlling the on / off states of the first switch K1 and the second switch K2 to change the equivalent turns ratio of the transformer. When low-voltage output is required, the first switch K1 is opened and the second switch K2 is closed, the transformer's equivalent turns ratio is 1:n, the converter operates in low-voltage mode, and obtains a voltage gain range of 0.6n to 1.4n. When high voltage output is required, the first switch K1 is closed and the second switch K2 is opened, so that the two secondary windings are connected in series. The equivalent turns ratio of the transformer is 1:2n, and the converter operates in high voltage mode to obtain a voltage gain range of 1.2n to 2.8n. The gain range of the low-voltage mode partially overlaps with that of the high-voltage mode, allowing the gain range of the entire converter to continuously cover 0.6n to 2.8n.

[0024] To facilitate understanding of the above technical solutions of the present invention, the following detailed description of the above technical solutions of the present invention will be provided through specific usage methods.

[0025] In practical use, the ultra-wide range LC resonant converter according to the present invention includes the following components: Q1, Q2, Q3, and Q4 switching transistors form a primary-side H-bridge, resonant inductor L1, resonant capacitor C1, transformer T (transformer turns ratio is 1:n:n, and the two series windings on the secondary side are both n turns), C2 is a DC blocking capacitor, K2 is a low-voltage switch, K1 is a high-voltage switch, S1, S2, S3, and S4 switching transistors form a secondary-side H-bridge, Co is an output filter capacitor, and Ro is a load.

[0026] Compared to the traditional LC resonant circuit, a DC blocking capacitor C2 is added. Utilizing the capacitor's "DC blocking, AC passing" characteristic, it prevents DC voltage / current from entering the resonant network, balancing the positive and negative half-cycle volt-second product, suppressing transformer core magnetic saturation, and preventing a surge in excitation current that could burn out components. Two switches, K1 and K2, adjust the transformer turns ratio. With switch K2 closed and K1 open, the transformer turns ratio is 1:n; with switch K2 open and K1 closed, the transformer turns ratio is 1:2n.

[0027] The converter has a simple control method. In resonant mode, the secondary-side switch is turned on, which achieves the effect of synchronous rectification, reducing losses and improving efficiency. In delayed turn-off mode, only the secondary-side S3 and S4 switches are involved, making the control even simpler.

[0028] This converter has two operating modes: low-voltage mode and high-voltage mode. When switch K2 is closed and K1 is open, the transformer turns ratio is 1:n, and the converter operates in low-voltage mode, with a theoretical gain range of 0.6n-1.4n. When switch K2 is open and K1 is closed, the transformer turns ratio is 1:2n, and the converter operates in high-voltage mode, with a theoretical gain range of 1.2n-2.8n. After removing the overlap, the overall gain range of the converter can reach 0.6n-2.8n, which greatly improves the converter's operating range.

[0029] Taking the low-voltage mode as an example, the specific working mode of the converter is introduced. The high-voltage mode is similar. The specific control methods are divided into resonant mode and delayed turn-off mode.

[0030] 1. Resonance Mode like Figure 2 As shown, in this mode, the primary side switches Q1 and Q4 of the LC resonant converter operate at a 50% duty cycle simultaneously, while switches Q2 and Q3 operate at a 50% duty cycle complementaryly with Q1 and Q4 respectively, with a dead time of Ta. Secondary-side switches S1 and S4 turn on with a delay of Td relative to switches Q1 and Q4, and turn off synchronously; secondary-side switches S2 and S3 also turn on with a delay of Td relative to switches Q2 and Q3, and turn off synchronously. Figure 3As shown. The turn-on time delay Td ensures that the body diodes of the S1, S2, S3, and S4 switches are turned on first, realizing soft switching. The turn-on of the secondary switches S1, S2, S3, and S4 also plays a role in synchronous rectification, reducing the losses of the secondary switches and improving efficiency. The gain range of this mode converter is about 0.6-0.8.

[0031] 2. Delayed shutdown mode like Figure 3 As shown, in this mode, the primary side switches Q1 and Q4 of the LC resonant converter operate at a 50% duty cycle simultaneously, while switches Q2 and Q3 operate at a 50% duty cycle complementaryly with Q1 and Q4 respectively. The dead time is Ta, which is the same as in the resonant mode.

[0032] The secondary side switches S3 and S4 are turned on with a delay of Td relative to switches Q3 and Q4, and turned off with a delay of Tc. The turn-on time of secondary side S1 is delayed by a dead time Ta relative to the turn-off time of S3, and the turn-off time is synchronized with Q1. The turn-on time of S2 is delayed by a dead time Ta relative to the turn-off time of S4, and the turn-off time is synchronized with Q2.

[0033] In delayed shutdown mode, the delay time Tc is used. During this time period, the secondary winding is equivalent to a short circuit, which has the effect of energy storage and voltage boosting, thereby improving the gain. In this mode, the gain of the converter is about 0.8-1.4.

[0034] In summary, by utilizing the above-mentioned technical solution of the present invention, and by setting the first switch K1 and the second switch K2 to connect the two windings on the secondary side, each with n turns, when K1 is open and K2 is closed, the equivalent turns ratio of the secondary winding is 1:n; when K1 is closed and K2 is open, the secondary winding is connected in series, and the equivalent turns ratio is 1:2n. This allows the converter to obtain voltage gain ranges of 0.6n~1.4n and 1.2n~2.8n in the two turns ratio modes, respectively, with the two gain ranges overlapping, achieving a continuous ultra-wide range coverage of 0.6n~2.8n. This effectively solves the problem of narrow gain range in traditional LC resonant converters. The problem is that by connecting a DC blocking capacitor C2 in series between the first end of the first secondary winding and the midpoint of the first arm of the secondary H-bridge, the DC blocking and AC passing characteristics are used to block the DC component from entering the resonant network, balancing the positive and negative half-cycle volt-second product, thereby effectively suppressing the transformer core bias saturation and improving system reliability. In addition, the secondary switching transistors conduct synchronously in resonant mode to achieve synchronous rectification, reducing conduction losses. In delayed turn-off mode, only the turn-off time of some secondary switching transistors needs to be controlled to achieve the boost effect. The control method is simple, thus simplifying the control complexity while improving the conversion efficiency and adaptability under wide voltage conditions.

[0035] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An ultra-wide range LC resonant converter, characterized in that, It includes a primary-side H-bridge circuit and a secondary-side H-bridge circuit. The two ends of the primary-side H-bridge circuit are respectively connected to the DC input power supply Vin. The midpoint of the first bridge arm of the primary-side H-bridge circuit is connected to one end of the resonant inductor L1. The midpoint of the second bridge arm of the primary-side H-bridge circuit is connected to one end of the resonant capacitor C1. The other ends of the resonant inductor L1 and the other ends of the resonant capacitor C1 are respectively connected to the two ends of the primary winding of the high-frequency transformer T. The high-frequency transformer T has two series windings, each with n turns, on its secondary side, with a turns ratio of 1:n:n. A second switch K2 is located at the end of the first secondary winding, and a first switch K1 is located at the end of the second secondary winding. The first end of the first secondary winding is connected to one end of a DC blocking capacitor C2, and the other end of the DC blocking capacitor C2 is connected to the midpoint of the first arm of the secondary H-bridge circuit. Both the first switch K1 and the second switch K2 are connected to the midpoint of the second arm of the secondary H-bridge circuit. The two ends of the secondary H-bridge circuit are connected in parallel to an output filter capacitor Co and a load Ro, respectively.

2. The ultra-wide range LC resonant converter according to claim 1, characterized in that, The primary-side H-bridge circuit is composed of a first switch Q1, a second switch Q2, a third switch Q3, and a fourth switch Q4. The first arm of the primary-side H-bridge circuit is formed by the first switch Q1 and the third switch Q3 connected in series, and the second arm of the primary-side H-bridge circuit is formed by the second switch Q2 and the fourth switch Q4 connected in series. The drains of the first switch Q1 and the second switch Q2 are connected to the positive terminal of the DC input power supply Vin, and the sources of the third switch Q3 and the fourth switch Q4 are connected to the negative terminal of the DC input power supply Vin. The connection between the source of the first switch Q1 and the drain of the third switch Q3 serves as the midpoint of the first arm of the primary-side H-bridge circuit, and the connection between the source of the second switch Q2 and the drain of the fourth switch Q4 serves as the midpoint of the second arm of the primary-side H-bridge circuit.

3. The ultra-wide range LC resonant converter according to claim 1, characterized in that, The secondary-side H-bridge circuit is composed of a fifth switch S1, a sixth switch S2, a seventh switch S3, and an eighth switch S4. The first arm of the secondary-side H-bridge circuit is formed by the fifth switch S1 and the seventh switch S3 connected in series, and the second arm of the secondary-side H-bridge circuit is formed by the sixth switch S2 and the eighth switch S4 connected in series. The drain of the fifth switch S1 and the drain of the sixth switch S2 are connected to one end of the filter capacitor Co and one end of the load Ro, respectively. The source of the seventh switch S3 and the source of the eighth switch S4 are connected to the other end of the filter capacitor Co and the other end of the load Ro, respectively. The source of the fifth switch S1 and the drain of the seventh switch S3 are connected as the midpoint of the first arm of the secondary-side H-bridge circuit, and the source of the sixth switch S2 and the drain of the eighth switch S4 are connected as the midpoint of the second arm of the secondary-side H-bridge circuit.

4. The ultra-wide range LC resonant converter according to claim 1, characterized in that, When the first switch K1 is open and the second switch K2 is closed, the equivalent turns ratio of the transformer is 1:n, and the converter operates in low-voltage mode; when the first switch K1 is closed and the second switch K2 is open, the two secondary windings are connected in series, the equivalent turns ratio of the transformer is 1:2n, and the converter operates in high-voltage mode.

5. The ultra-wide range LC resonant converter according to claim 1, characterized in that, The first to the eighth switching transistors are all N-channel MOSFETs, and the parasitic body diodes inside each switching transistor are oriented from the source to the drain.

6. A gain extension method for an ultra-wide range LC resonant converter, applied to the ultra-wide range LC resonant converter as described in any one of claims 1 to 5, characterized in that, include: The operating mode is switched by controlling the on / off states of the first switch K1 and the second switch K2 to change the equivalent turns ratio of the transformer. When low-voltage output is required, the first switch K1 is opened and the second switch K2 is closed, the transformer's equivalent turns ratio is 1:n, the converter operates in low-voltage mode, and obtains a voltage gain range of 0.6n to 1.4n. When high voltage output is required, the first switch K1 is closed and the second switch K2 is opened, so that the two secondary windings are connected in series. The equivalent turns ratio of the transformer is 1:2n, and the converter operates in high voltage mode to obtain a voltage gain range of 1.2n to 2.8n. The gain range of the low-voltage mode partially overlaps with that of the high-voltage mode, allowing the gain range of the entire converter to continuously cover 0.6n to 2.8n.

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