Soft start method and circuit of fixed-frequency LLC resonant converter

Abstract

This invention relates to the field of resonant converter technology, specifically to a soft-start method and circuit for a fixed-frequency LLC resonant converter. The method includes the following steps: while keeping the primary-side switching frequency of the LLC resonant converter constant, the duty cycle of the PWM control signal of the LLC resonant converter is increased from 0 to 50%. A three-stage soft-start method is used during this increase: In the first stage, the pulse width of the PWM control signal increases from 0 to a first set pulse width according to a linear function; in the second stage, the pulse width of the PWM control signal remains unchanged at the first set pulse width until the output voltage of the LLC resonant converter reaches a preset voltage value; in the third stage, the pulse width of the PWM control signal increases from the first set pulse width according to a second exponential function until the pulse width corresponding to a 50% duty cycle is output. This method solves the problem of large primary-side starting current, which can easily damage devices, that exists in ordinary PWM mode soft-start.

Description

Technical Field

[0001] This invention relates to the field of resonant converter technology, specifically to a soft-start method and circuit for a fixed-frequency LLC resonant converter. Background Technology

[0002] LLC resonant converters are widely used due to their high efficiency and high power density, but their output filter capacitors have a large capacitance. In the initial startup phase, the capacitor voltage is zero, which is equivalent to a short circuit in the output. This causes excessively high voltage and current spikes in the resonant cavity, which can easily damage the power switching transistors, resonant inductors, or capacitors, and may even trigger short-circuit protection. Existing soft-start methods mainly include: (1) keeping the duty cycle at 50% and starting at a switching frequency much higher than the converter's resonant frequency, and then gradually reducing the switching frequency to achieve soft start. (2) keeping the switching frequency constant and gradually increasing the duty cycle from a small initial value to 50% to achieve soft start. (3) simultaneously adjusting the operating frequency and duty cycle, increasing the duty cycle from a small initial value while gradually decreasing the operating frequency from a maximum value to achieve soft start.

[0003] In the above methods, when using method (1), an excessively high starting frequency will lead to problems such as high requirements for the drive circuit, increased cost, and increased negative impact of transformer parasitic parameters on the converter's operating characteristics. When using methods (2) and (3), if the initial value of the duty cycle is designed too small, the switching transistors of the LLC resonant converter's power conversion circuit will be unable to achieve soft switching, reducing the converter's operating efficiency. In addition, although the above methods can effectively achieve soft starting of the output filter capacitor voltage, they fail to significantly reduce the peak value of the resonant current.

[0004] In patent literature, such as the invention patent with authorization announcement number CN115313836B entitled "A Soft-Start Control Method for LLC Resonant Converters," this method detects the resonant current in the resonant cavity. Based on the detected resonant current, a controller generates a pulse width modulation signal with a variable duty cycle and switching frequency according to a certain rule to switch the transistors of the half-bridge power conversion circuit. This keeps the resonant current of the LLC resonant converter within an acceptable range during startup, avoiding inrush current and damage to power devices, thus achieving soft startup of the LLC resonant converter. This method starts the LLC resonant converter at the highest switching frequency when it is powered on, and gradually increases the duty cycle to the maximum duty cycle. If the conditions for ending soft startup are not met at this stage, the system maintains the maximum duty cycle and switches to a gradual frequency reduction mode until soft startup is achieved. In the above method (1), an excessively high startup frequency will lead to problems such as high requirements for the drive circuit, increased cost, and increased negative impact of transformer parasitic parameters on the converter's operating characteristics. Summary of the Invention

[0005] To overcome the aforementioned shortcomings, this invention proposes a three-stage soft-start method for a fixed-frequency LLC resonant converter, under the premise that the converter operates at a fixed switching frequency. This method addresses the problem of high voltage and current spikes in existing LLC resonant converters during soft-start, which can easily lead to device damage, while also balancing cost and performance.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention provides a soft-start method for a fixed-frequency LLC resonant converter. Under the premise of keeping the primary-side switching frequency of the LLC resonant converter unchanged, the duty cycle of the PWM control signal is increased from 0 to 50%. A three-stage soft-start method is adopted in the process of increasing the duty cycle from 0 to 50%. The three-stage soft-start method includes: In the first stage, the pulse width of the PWM control signal increases from 0 to the first set pulse width according to a linear function or a first exponential function; In the second stage, the pulse width of the PWM control signal remains unchanged at the first set pulse width until the output voltage of the LLC resonant converter reaches the preset voltage value, which is less than the rated output voltage value. In the third stage, the pulse width of the PWM control signal increases from the first set pulse width according to the second exponential function until the pulse width corresponding to 50% duty cycle is output.

[0008] Furthermore, the duration of the first stage is the time it takes for the current of the switching transistor in the LLC resonant converter to increase from 0 to the design current, and the first set pulse width is determined based on the design current.

[0009] Furthermore, after the third stage, there is also a stabilization stage, which keeps the primary-side switching frequency unchanged and keeps the pulse width corresponding to the 50% duty cycle of the PWM control signal unchanged.

[0010] Furthermore, the first exponential function includes the exponential function of the first RC charge:

[0011] in, It is the input voltage of the first-stage PWM control IC. The voltage is constant, e is the natural constant, t1 is the elapsed time from the start of the charging process, R is the resistance, and C is the capacitance. It is a time constant.

[0012] Furthermore, the preset voltage value is set to V3, and the value range of V3 includes 80% of the rated output voltage of the LLC resonant converter. V3 The LLC resonant converter outputs 90% of its rated voltage.

[0013] Furthermore, the second exponential function includes the exponential function of the second RC charge:

[0014] in, V1 is the input voltage of the PWM control IC in the third stage, V2 is the pulse width adjustment voltage corresponding to the first set pulse width, e is the natural constant, t2 is the time elapsed from the start time of the third stage, and RC is the time constant.

[0015] Secondly, the present invention provides a startup circuit for a fixed-frequency LLC resonant converter, comprising: Transistors Q1 and Q2; resistors R1, R2, R3, and R4; capacitors C1 and C2; and Schmitt logic gate U1; The base connection enable voltage of transistor Q1, and the collector connection operating voltage V of transistor Q1. CC The emitter of transistor Q1 is connected to ground after being connected to a parallel branch consisting of resistor R1 and capacitor C1; the emitter of transistor Q1 is also connected to the input terminal of Schmitt logic gate U1. The output of Schmitt logic gate U1 is connected to the base of transistor Q2. The emitter of transistor Q2 is grounded. The collector of transistor Q2 is connected to the junction of resistors R2 and R3 in series through resistor R4. The end of the series branch formed by resistors R2 and R3 closest to resistor R2 is connected to the operating voltage V. CC The end closest to resistor R3 is grounded; the connection point between resistors R2 and R3 also serves as the output terminal Vss of the startup circuit, and capacitor C2 is connected between the output terminal Vss of the startup circuit and ground.

[0016] Furthermore, The soft-start circuit outputs a pulse width modulation voltage according to a soft-start method for a fixed-frequency LLC resonant converter. The PWM control IC receives the pulse width modulation voltage and outputs a PWM control signal with a corresponding pulse width. The soft-start method includes: The duty cycle of the PWM control signal increases from 0 to 50%, and a three-stage soft-start method is used in the process of increasing the duty cycle from 0 to 50%. The three-stage soft-start method includes: In the first stage, the pulse width of the PWM control signal increases from 0 to the first set pulse width according to a linear function or a first exponential function; In the second stage, the pulse width of the PWM control signal remains unchanged at the first set pulse width until the output voltage of the LLC resonant converter reaches the preset voltage value, which is less than the rated output voltage value. In the third stage, the pulse width of the PWM control signal increases from the first set pulse width according to the second exponential function until the pulse width corresponding to 50% duty cycle is output.

[0017] Compared to existing technologies, the advantages of this invention are as follows: This invention performs soft-start on an LLC resonant converter through a three-stage approach. In the first stage, the current of the switching transistor is increased from 0 to the design current. In the second stage, the design current is maintained to soft-start the output voltage until the output voltage reaches 80%-90% of the rated voltage. In the third stage, the soft-start ends by exponentially transitioning the PWM control signal to the pulse width signal corresponding to 50% duty cycle. In this invention, the current in the switching transistor is limited in stages, preventing excessive primary-side current from damaging the switching transistor. Attached Figure Description

[0018] Figure 1 The curve showing the relationship between the pulse width of the PWM control signal and the output voltage of the LLC resonant converter in Example 1 is shown. Figure 2 This is a schematic diagram of the main power circuit of the half-bridge LLC in Example 2; Figure 3 This is a circuit diagram of the soft-start circuit in Example 2. Detailed Implementation

[0019] The present invention will now be described in further detail with reference to specific embodiments. However, this should not be construed as limiting the scope of the present invention to the following embodiments; all technologies implemented based on the content of the present invention fall within the scope of the present invention.

[0020] In the PWM mode soft-start of LLC resonant converters, a fixed frequency is used, and the duty cycle is gradually increased from 0 to 50%. During this process, large current and voltage spikes may occur, which can easily lead to device damage. To solve this problem, a soft-start method for fixed-frequency LLC resonant converters is proposed. The main idea of ​​this method is to control the input voltage to the PWM control IC using software or hardware. By utilizing the proportionality between the input voltage and the pulse width of the PWM signal output by the PWM control IC, the pulse width of the drive is indirectly controlled. Then, through a three-stage transition, the duty cycle of the PWM signal gradually increases from 0 to 50% while meeting the requirements.

[0021] More specifically, a soft-start method for a fixed-frequency LLC resonant converter employs a fixed frequency and uses three-stage control to increase the duty cycle of the PWM signal from 0 to 50%. In the first stage, the voltage received by the PWM control IC increases linearly from 0, and the PWM signal output by the PWM control IC also increases linearly from 0 until the current of the switching transistor in the LLC resonant converter increases from 0 to the design current, at which point the first stage ends. In the second stage, the PWM control IC receives a constant voltage V1 and outputs a PWM signal with a first set pulse width until the output voltage of the LLC resonant converter reaches 80%-90% of the rated voltage, at which point the second stage ends. In the third stage, the PWM control receives a voltage that changes exponentially from V1 to V2 and outputs a PWM signal that changes exponentially from the first set pulse width to the second set pulse width, at which point the soft start ends.

[0022] In this invention, the first stage is mainly used to reduce the overshoot current during startup. The first stage corresponds to the process of the inverter current in the switching transistor reaching the design current from 0. In actual engineering, the first set pulse corresponding to the design current and the voltage V1 corresponding to the first set pulse width during the soft-start process can be determined by simulation software. The first stage uses a linear function to raise the voltage, mainly to avoid slow startup. The design current is used as the cutoff condition to clamp the primary resonant current of the LLC resonant converter within the design safety value, preventing overcurrent of power devices and transformer magnetic saturation.

[0023] The second stage primarily involves maintaining a constant design current and continuously transferring energy to soft-start the output voltage. During this process, no current overshoot occurs until the LLC resonant converter's output voltage reaches 80%–90% of its rated output voltage, at which point the second stage ends. In this stage, a fixed pulse width is used to allow the LLC resonant gain to build up slowly, resulting in a smooth output voltage rise. Simultaneously, the 80%–90% rated output voltage serves as a reserved voltage buffer, preventing both excessively low voltage leading to an overly long soft-start time and full-voltage oscillation in the third stage. Furthermore, in this stage, the output voltage of the main circuit is detected and compared to a threshold value to determine whether the second stage has ended.

[0024] In the third stage, the first set pulse width is exponentially transitioned to a pulse width with a 50% duty cycle to smoothly soft-start the output voltage to the rated voltage, prevent output voltage overshoot, and complete the soft start.

[0025] More specifically, the duty cycle of the PWM signal can be increased from 0 to 50% using either hardware control or pure software control. Pure software control means not using a soft-start circuit; the input voltage of the PWM control IC is controlled by software, causing the input voltage to change according to a corresponding function. Hardware control means using a soft-start circuit to control the input voltage of the PWM control IC, thus increasing the output pulse duty cycle of the PWM control IC from 0 to 50%. Furthermore, the PWM control IC and driver can be considered as a single unit, directly outputting the drive signal to the switching transistor. Alternatively, the PWM control IC and driver can be separated, with the driver receiving the PWM signal and then isolating and amplifying it to form the drive signal.

[0026] In the three stages of soft-start, in the first stage, the pulse width of the PWM control signal increases from 0 to a first set pulse width according to a linear function or a first exponential function. After isolation, amplification, and current amplification by the drive circuit, the PWM control signal forms a drive signal that acts on the switching transistor. In the second stage, the voltage value corresponding to the first set pulse width remains unchanged until the output voltage of the LLC resonant converter reaches a value close to the rated voltage threshold. Finally, in the third stage, the pulse width is increased according to a second exponential function until it increases to 50% of the pulse width corresponding to the rated operating voltage. This three-stage control process fully utilizes the design current to continuously transfer energy to the soft-start output voltage in the second stage. While ensuring the soft-start speed, it also controls the current and voltage in the switching transistor in stages, balancing soft-start speed and device safety.

[0027] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application will be described in further detail below with reference to the accompanying drawings.

[0028] Example 1 A soft-start method for a fixed-frequency LLC resonant converter includes the following steps: While keeping the primary-side switching frequency of the LLC resonant converter constant, the duty cycle of the PWM control signal is increased from 0 to 50%, and a three-stage soft-start method is adopted in the process of increasing the duty cycle from 0 to 50%. The three-stage soft-start method includes: In the first stage, the pulse width of the PWM control signal increases from 0 to the first set pulse width according to a linear function or a first exponential function; In the second stage, the pulse width of the PWM control signal remains unchanged at the first set pulse width until the output voltage of the LLC resonant converter reaches the preset voltage value, which is less than the rated output voltage value. In the third stage, the pulse width of the PWM control signal increases from the first set pulse width according to the second exponential function until the pulse width corresponding to 50% duty cycle is output.

[0029] The relationship between the pulse width of the PWM control signal and the output voltage of the LLC resonant converter is shown in the curve below. Figure 1 As shown, through Figure 1 The three-stage soft-start method will be further explained.

[0030] In the first stage, the input voltage V of the PWM control IC is controlled. SS Starting from 0 and increasing linearly, the PWM control IC outputs a PWM control signal with a corresponding pulse width. Since the pulse width of the PWM control signal is related to the input voltage V... SS The amplitude is directly proportional to V. SS When the pulse width of the PWM control signal linearly increases from 0 to V1, the pulse width of the PWM control signal increases linearly from 0 to the first set pulse width. At the same time, in the main circuit, the current of the switching transistor in the LLC resonant converter gradually increases from 0 to the design current. The duration of the first stage is the time it takes for the current in the switching transistor to increase from 0 to the design current. The first set pulse width is determined by the design current of the switching transistor in the LLC resonant converter and is related to the actual circuit parameters.

[0031] As a preferred option, the first pulse width is not fixed, but increases from 0 to a first set pulse width according to a linear function or a first exponential function.

[0032] Furthermore, the first exponential function includes the exponential function of the first RC charge:

[0033] in, It is the input voltage of the first-stage PWM control IC. RC is a constant operating voltage, e is a natural constant, t1 is the elapsed time from the start of the charging process, R is the resistance, C is the capacitance, and RC is the time constant.

[0034] When the input voltage V of the PWM control IC is controlled in the first stage SS When V1 is reached, the corresponding pulse width is the first set pulse width. The first set pulse width is maintained, and then the second stage is entered. That is, the second stage is a fixed pulse width, and the pulse width is the first set pulse width.

[0035] In the second stage, the pulse width of the PWM control signal remains unchanged at the first set pulse width until the output voltage of the LLC resonant converter reaches a preset voltage value, which is less than the rated voltage value of the LLC resonant converter output voltage. The main purpose of this second stage is to maintain a constant design current to prevent current overshoot, while continuously outputting energy to soft-start the output voltage.

[0036] In a specific embodiment, the preset voltage value V3 is 80% to 90% of the rated voltage; that is, the value range of V3 includes 80% of the rated output voltage. V3 The output voltage is 90% of the rated voltage. When the voltage is between 80% and 90% of the rated voltage, the operating voltage of the LLC resonant converter approaches the rated voltage. In the third stage, the duty cycle needs to be adjusted to 50%. If the output voltage at the second extreme is too low, such as below 80% of the rated voltage, the current in the output capacitor of the LLC resonant converter may generate a current spike in the third stage, resulting in current overshoot. If the voltage is above 90% of the rated voltage, the exponential function needs a certain adjustment time in the third stage to smoothly increase the duty cycle to 50%. If the output voltage is already very close to the rated value, even a slight deviation in the adjustment slope of the third stage will directly lead to output voltage overshoot.

[0037] In the third stage, the pulse width of the PWM control signal increases from the first set pulse width according to the second exponential function until the pulse width corresponding to 50% duty cycle is output.

[0038] Preferably, the second exponential function includes the exponential function of the second RC charging: ; in, V1 is the input voltage of the third-stage PWM control IC, V2 is the pulse width adjustment voltage corresponding to the first set pulse width, e is the natural constant, t2 is the time elapsed from the start time of the third stage, and RC is the time constant.

[0039] Furthermore, after the third stage, there is also a stabilization stage, which keeps the primary-side switching frequency unchanged and keeps the pulse width corresponding to the 50% duty cycle of the PWM control signal unchanged.

[0040] Example 2 Figure 2 This is a circuit diagram of a half-bridge LLC resonant converter. The circuit diagram shows a typical power stage topology of a half-bridge LLC resonant converter, mainly including the primary side circuit, transformer, secondary side circuit, and also includes the driver, PWM control IC, and soft-start circuit.

[0041] The primary circuit has two input buses: P+ (positive bus) and N- (negative bus), typically connected to the DC bus voltage output from the preceding PFC circuit. Two main power switches, Q1 and Q2, are connected across the positive bus P+ and the negative bus N-. Both Q1 and Q2 are N-channel power MOSFETs (or IGBTs), connected in a half-bridge configuration: the drain of Q1 is connected to P+, the source of Q1 is connected to the drain of Q2 to form the midpoint of the half-bridge, and the source of Q2 is connected to N-. The two switches alternately conduct under control signal, chopping the DC input voltage into a high-frequency square wave voltage, which is then applied to the series resonant cavity.

[0042] Transformer T1 provides electrical isolation and voltage transformation. Its primary winding is connected to the resonant cavity, and its secondary winding has two center-tapped windings, connected to the anodes of rectifier diodes D1 and D2 respectively. The center tap is typically connected to the negative output terminal OUT-. The cathodes of rectifier diodes D1 and D2 are connected in parallel to OUT+. from Figure 2 As can be seen, the gates of Q1 and Q2 are connected to the startup control circuit. The startup control circuit includes a drive circuit, a PWM control IC, and a soft-start circuit. The output voltage of the startup control circuit is used to control the conduction and turn-off of Q1 and Q2, thereby controlling the DC output voltages OUT+ and OUT-.

[0043] Vss represents the input voltage of the PWM control IC. The soft-start circuit generates a linearly related output voltage or an output voltage that increases exponentially, controlling the duty cycle of the PWM control signal output by the PWM control IC, thereby achieving a safe soft start for the LLC converter.

[0044] Furthermore, the circuit diagram of the soft start circuit is as follows: Figure 3 As shown, the soft start circuit includes: Switches Q3 and Q4; resistors R1, R2, R3, and R4; capacitors C1 and C2; and inverted Schmitt logic gate U1; The gate connection enable voltage of switch Q3, and the source connection operating voltage V of switch Q3. CC The drain of the switching transistor Q3 is connected to ground after being connected to a parallel branch consisting of resistor R1 and capacitor C1; the drain of the switching transistor Q3 is also connected to the input terminal of the inverted Schmitt logic gate U1. The output of the inverted Schmitt logic gate U1 is connected to the gate of the switching transistor Q4. The drain of the switching transistor Q4 is grounded. The source of the switching transistor Q4 is connected to the junction of the series resistors R2 and R3 through resistor R4. The end of the series branch formed by resistors R2 and R3 closest to resistor R2 is connected to the operating voltage V. CCThe end closest to resistor R3 is grounded; the connection point between resistors R2 and R3 also serves as the output terminal Vss of the driving voltage, and capacitor C2 is connected between the output terminal Vss of the driving voltage and ground.

[0045] The work process is as follows: exist Figure 3 In the first stage, a high-level enable signal is input, Q1 is turned on, and V... CC Charging C1 via Q1 clamps node S1 to a high level; U1 is an inverted Schmitt gate, when input S1 is high, output S2 is low, Q2 is turned off, and V... CC After voltage division by R2 and R3, the output V SS =V CC ×R2 / (R2+R3), constant voltage.

[0046] In the second stage, the enable signal switches to low level, Q1 is turned off, C1 slowly discharges through R1, and the voltage at node S1 gradually decreases. When it falls below the low threshold of U1, the output of U1 reverses and outputs a high level. At this time, Q2 turns on, and V... CC C2 is charged through R2, R4, and Q2, V SS The voltage across capacitor C2 increases exponentially. It's important to note that although the soft-start circuit has only two output stages, the first stage outputs a constant voltage. During the soft-start process, the input voltage to the PWM control IC cannot change abruptly; it gradually increases from 0 to a constant voltage, thus creating the first and second stages of the soft-start process. The second stage of the soft-start circuit corresponds to the third stage of the PWM control IC.

[0047] When the inputs P+ and N- are powered on, the switching frequencies Q1 and Q2 remain constant according to their operating frequencies, generating a square wave voltage at the midpoint of the half-bridge, which excites the resonant cavity Lr1-Cr1-Lm1. Because the initial Vss value provided by the soft-start circuit is relatively low, the drive duty cycle is limited, and the output voltage gradually increases. As Vss increases, the drive duty cycle rises to 50%, and the output voltage stabilizes.

[0048] This circuit has a compact structure, using only two primary-side switching transistors and two secondary-side rectifier diodes, along with resonant components, to achieve efficient power conversion. The introduction of a soft-start circuit, especially its combination with an exponential function control strategy, further enhances the system's reliability and cost advantages.

[0049] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A soft-start method for a fixed-frequency LLC resonant converter, characterized in that, While keeping the primary-side switching frequency of the LLC resonant converter constant, the duty cycle of the PWM control signal is increased from 0 to 50%, and a three-stage soft-start method is adopted in the process of increasing the duty cycle from 0 to 50%. The three-stage soft-start method includes: In the first stage, the pulse width of the PWM control signal increases from 0 to the first set pulse width according to a linear function or a first exponential function; In the second stage, the pulse width of the PWM control signal remains unchanged at the first set pulse width until the output voltage of the LLC resonant converter reaches the preset voltage value, which is less than the rated output voltage value. In the third stage, the pulse width of the PWM control signal increases from the first set pulse width according to the second exponential function until the pulse width corresponding to 50% duty cycle is output.

2. The soft-start method for a fixed-frequency LLC resonant converter as described in claim 1, characterized in that, The duration of the first stage is the time it takes for the current of the switching transistor in the LLC resonant converter to increase from 0 to the design current, and the first set pulse width is determined based on the design current.

3. The soft-start method for a fixed-frequency LLC resonant converter as described in claim 1, characterized in that, After the third stage, there is also a stabilization stage, which keeps the primary-side switching frequency unchanged and keeps the pulse width corresponding to the 50% duty cycle of the PWM control signal unchanged.

4. The soft-start method for a fixed-frequency LLC resonant converter as described in claim 1, characterized in that, The first exponential function includes the exponential function of the first RC charge: in, It is the input voltage of the first-stage PWM control IC. The voltage is constant, e is the natural constant, t1 is the elapsed time from the start of the charging process, R is the resistance, and C is the capacitance. It is a time constant.

5. The soft-start method for a fixed-frequency LLC resonant converter as described in claim 1, characterized in that, The preset voltage value is set to V3, and the value of V3 includes 80% of the rated output voltage of the LLC resonant converter. V3 The LLC resonant converter outputs 90% of its rated voltage.

6. The soft-start method for a fixed-frequency LLC resonant converter as described in claim 1, characterized in that, The second exponential function includes the exponential function of the second RC charging: in, V1 is the input voltage of the PWM control IC in the third stage, V2 is the pulse width adjustment voltage corresponding to the first set pulse width, e is the natural constant, t2 is the time elapsed from the start time of the third stage, and RC is the time constant.

7. A startup circuit for a fixed-frequency LLC resonant converter, characterized in that, include: Transistors Q1 and Q2; resistors R1, R2, R3, and R4; capacitors C1 and C2; and Schmitt logic gate U1; The base connection enable voltage of transistor Q1, and the collector connection operating voltage V of transistor Q1. CC The emitter of transistor Q1 is connected to ground after being connected to a parallel branch consisting of resistor R1 and capacitor C1; the emitter of transistor Q1 is also connected to the input terminal of Schmitt logic gate U1. The output of Schmitt logic gate U1 is connected to the base of transistor Q2. The emitter of transistor Q2 is grounded. The collector of transistor Q2 is connected to the junction of resistors R2 and R3 in series through resistor R4. The end of the series branch formed by resistors R2 and R3 closest to resistor R2 is connected to the operating voltage V. CC The end closest to resistor R3 is grounded; the connection point between resistors R2 and R3 also serves as the output terminal Vss of the startup circuit, and capacitor C2 is connected between the output terminal Vss of the startup circuit and ground.

8. The startup circuit of the fixed-frequency LLC resonant converter according to claim 7, characterized in that, The soft-start circuit outputs a pulse width modulation voltage according to a soft-start method for a fixed-frequency LLC resonant converter. The PWM control IC receives the pulse width modulation voltage and outputs a PWM control signal with a corresponding pulse width. The soft-start method includes: The duty cycle of the PWM control signal increases from 0 to 50%, and a three-stage soft-start method is used in the process of increasing the duty cycle from 0 to 50%. The three-stage soft-start method includes: In the first stage, the pulse width of the PWM control signal increases from 0 to the first set pulse width according to a linear function or a first exponential function; In the second stage, the pulse width of the PWM control signal remains unchanged at the first set pulse width until the output voltage of the LLC resonant converter reaches the preset voltage value, which is less than the rated output voltage value. In the third stage, the pulse width of the PWM control signal increases from the first set pulse width according to the second exponential function until the pulse width corresponding to 50% duty cycle is output.

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