A bidirectional LLC soft-start method

Abstract

The application discloses a bidirectional LLC soft starting method, comprising the following steps: detecting an LLC input voltage, and sending a soft starting preparation ready signal to a controller when reaching a preset range; setting an LLC initial wave quantity and an LLC initial switching frequency by the controller, starting wave sending from the initial switching frequency, and starting duty cycle from an initial duty cycle; performing superposition on a period value by LLC switch tube driving according to a preset slope, and gradually reducing LLC switching frequency to approach a resonance frequency point; when LLC switching frequency reaches a frequency hopping trigger value, LLC switching frequency occurs step, and is directly reduced to a frequency hopping target value, and then continues to perform period value step superposition; when LLC period value reaches a maximum value, LLC switching frequency reaches the resonance frequency point, and the duty cycle reaches a preset duty cycle threshold value; and LLC soft starting is completed. The method meets the demand of monotonous and smooth growth of the output voltage, and reduces the risk of switch tube failure during starting.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and more specifically to a bidirectional LLC soft-start method. Background Technology

[0002] Most common isolated DC-DC converters currently use LLC circuits, which are divided into closed-loop LLC and open-loop LLC. A common two-stage isolated AC / DC converter topology is shown below. Figure 1 As shown, during closed-loop LLC soft-start, the output port voltage is detected in real time, the setpoint is adjusted, and then the controller generates the corresponding drive to achieve a linear monotonic increase in output voltage until it stabilizes at the rated voltage. This method has two drawbacks: firstly, it requires additional cost for the isolation sampling circuit; secondly, it cannot avoid the problem of uneven output voltage rise caused by non-monotonic gain. During open-loop LLC soft-start, the duty cycle is typically fixed at 50%, and the frequency gradually decreases from a high frequency to the resonant frequency at a fixed rate. The frequency-voltage relationship during LLC soft-start is shown in [reference needed]. Figure 2 As shown. Because this method adjusts the control according to a single frequency change rate, it suffers from non-monotonic gain, leading to non-linear output voltage growth. This also results in an uneven output voltage rise and voltage dips, which can severely impact downstream equipment, damage downstream modules, and cause malfunctions in the downstream devices.

[0003] Therefore, how to solve the above-mentioned technical problems is the direction that those skilled in the art are dedicated to researching. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a bidirectional LLC soft-start method.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A bidirectional LLC soft-start method, applied to isolated DC-DC converters, includes the following steps: Step 1: Detect the LLC input voltage. When it reaches the preset range, the controller receives the LLC soft-start ready signal, and then prepares to generate a wave and output a drive signal to the power switch in the LLC circuit. Step 2: The controller sets the initial transmission quantity and initial switching frequency of LLC. Transmission starts from the initial switching frequency, and the duty cycle starts from the initial duty cycle, which is the minimum duty cycle and has a value greater than 0. Step 3: The LLC switching transistor driver superimposes the periodic values ​​according to a preset slope, and the LLC switching frequency gradually decreases and approaches the resonant frequency point. Step 4: When the LLC switching frequency reaches the frequency hopping trigger value, the LLC switching frequency undergoes a step change and directly decreases to the frequency hopping target value, and then the periodic value is continuously superimposed step by step. Step 5: When the LLC period value reaches its maximum value, the LLC switching frequency reaches the resonant frequency point, and the duty cycle reaches the preset duty cycle threshold. Step 6, LLC soft boot complete.

[0006] As a specific implementation method, the preset range of LLC input voltage in step 1 requires that there be no overvoltage or undervoltage faults in the LLC input.

[0007] In one specific implementation, in step 1), the controller generates a switching control signal based on the initial duty cycle and the initial switching frequency corresponding to the initial duty cycle, so as to control the power switching transistor of the LLC resonant converter.

[0008] As a specific implementation method, the preset duty cycle threshold in step 5) is the maximum duty cycle corresponding to the occurrence of hard switching phenomenon of power switch tube.

[0009] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art: This invention employs frequency modulation and duty cycle adjustment during LLC soft start, while using frequency hopping in frequency ranges where gain is not monotonic and in areas with severe hard switching, thus avoiding gain non-monotonicity points. In both open-loop and closed-loop LLC circuits, the switching transistors of the LLC section can be in a soft-switching state throughout the entire startup range, satisfying the requirement for monotonic and smooth output voltage growth while reducing the risk of switching transistor failure during startup. Attached Figure Description

[0010] Figure 1 This is a topology diagram of a commonly used two-stage isolated AC / DC converter; Figure 2 This is a frequency-voltage relationship diagram for a typical LLC soft start. Figure 3 Flowchart of the LLC soft start method described in Embodiment 1 of the present invention; Figure 4 This describes the relationship between the LLC soft-start frequency and output voltage in the sampling frequency hopping method described in this invention. Figure 5 This describes the relationship between the LLC soft-start frequency and output voltage as described in this invention. Detailed Implementation

[0011] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0012] This invention proposes a bidirectional LLC soft-start method for isolated DC-DC converters, comprising the following steps: Step 1: Detect the LLC input voltage. When it reaches the preset range, the controller receives the LLC soft-start ready signal and then prepares to generate a wave and output a drive signal to the power switch in the LLC circuit. Here, the preset range of the LLC input voltage requires that there is no overvoltage or undervoltage fault at the LLC input. The controller generates a switching control signal based on the initial duty cycle and the initial switching frequency corresponding to the initial duty cycle to control the power switch of the LLC resonant converter. Step 2: The controller sets the initial transmission quantity and initial switching frequency of LLC. Transmission starts from the initial switching frequency. The transmission quantity of LLC is the minimum value. The duty cycle starts from the initial duty cycle, which is the minimum duty cycle and is greater than 0. Step 3: The LLC switching transistor driver superimposes the periodic values ​​according to a preset slope, and the LLC switching frequency gradually decreases and approaches the resonant frequency point. Step 4: When the LLC switching frequency reaches the frequency hopping trigger value, the LLC switching frequency undergoes a step change and directly decreases to the frequency hopping target value, and then the periodic value is continuously superimposed step by step. Step 5: When the LLC period value reaches its maximum value, the LLC switching frequency reaches the resonant frequency point, and at the same time, the duty cycle reaches the preset duty cycle threshold. The preset duty cycle threshold is the maximum duty cycle corresponding to the hard switching phenomenon of the power switch, which is 50% here. Step 6, LLC soft boot complete. Example 1

[0013] See Figure 3 As shown, a bidirectional LLC soft-start method is described below: Soft-start the state machine, set the LLC waveform output to the minimum value, the switching frequency to the initial frequency (200kHz), the duty cycle to the initial duty cycle (3%), and start a 1ms countdown. The soft-start phase is set to LLC soft-start, and the controller enables LLC to generate waves. The switching frequency gradually decreases from a high frequency (200kHz) according to the set steps, and the duty cycle increases from the initial duty cycle (3%) according to the designed step size; When the switching frequency reaches the frequency hopping trigger value (135kHz), the controller initiates frequency hopping to cause a step change in the switching frequency. The switching frequency jumps from the frequency hopping trigger value (135kHz) to the frequency hopping target value (128kHz). After the frequency hopping is completed, the switching frequency continues to gradually decrease from the target frequency hopping value (128kHz) to the LLC resonant frequency point according to the set steps; When the switching frequency decreases to the resonant frequency and the duty cycle increases to the target value (50%), the LLC soft start is completed. The soft boot process is complete, and the LLC is set to running status.

[0014] See Figure 4 As shown, Figure 4 This invention presents the output voltage-frequency-time curve during LLC soft-start. By investigating the gain of existing resonant parameters at different frequencies, and adjusting the switching frequency strategy during soft-start based on gain changes, this invention employs a frequency-hopping approach. Based on measured data, it skips regions where the gain is not monotonic. Figure 4 As shown, the output voltage rises smoothly and linearly, which is friendly to the subsequent stages and reduces the product failure rate at startup.

[0015] See Figure 5 As shown, Figure 5 This is a solution for more complex situations, such as multiple non-monotonic gain ranges or situations where hard switching is still severe after frequency hopping. The blue curve is the frequency-time curve during LLC soft start, and the green curve is the duty cycle-time modulation curve during soft start. It mainly adds the dimension of duty cycle adjustment, and achieves monotonic and linear increase of LLC output voltage by adjusting the duty cycle while adjusting the frequency.

[0016] The soft-start method of this invention adopts frequency modulation and duty cycle adjustment during LLC soft-start, and adopts frequency hopping in the frequency range where the gain is not monotonic and in the region where hard switching is severe, thus avoiding the point where the gain is not monotonic. In both open-loop and closed-loop LLC circuits, the switching transistors of the LLC section can be in a soft-switching state throughout the entire startup range, which not only meets the requirement of monotonic and smooth growth of output voltage, but also reduces the risk of switching transistor failure during startup.

[0017] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A bidirectional LLC soft-start method, applied to an isolated DC-DC converter, characterized in that, Includes the following steps: Step 1: Detect the LLC input voltage. When it reaches the preset range, the controller receives the LLC soft-start ready signal, and then prepares to generate a wave and output a drive signal to the power switch in the LLC circuit. Step 2: The controller sets the initial transmission quantity and initial switching frequency of LLC. Transmission starts from the initial switching frequency, and the duty cycle starts from the initial duty cycle, which is the minimum duty cycle and has a value greater than 0. Step 3: The LLC switching transistor driver superimposes the periodic values ​​according to a preset slope, and the LLC switching frequency gradually decreases and approaches the resonant frequency point. Step 4: When the LLC switching frequency reaches the frequency hopping trigger value, the LLC switching frequency undergoes a step change and directly decreases to the frequency hopping target value, and then the periodic value is continuously superimposed step by step. Step 5: When the LLC period value reaches its maximum value, the LLC switching frequency reaches the resonant frequency point, and the duty cycle reaches the preset duty cycle threshold. Step 6, LLC soft boot complete.

2. The bidirectional LLC soft-start method according to claim 1, characterized in that, The preset range of LLC input voltage in step 1 requires that there be no overvoltage or undervoltage faults in the LLC input.

3. The bidirectional LLC soft-start method according to claim 1, characterized in that, In step 1), the controller generates a switching control signal based on the initial duty cycle and the initial switching frequency corresponding to the initial duty cycle, so as to control the power switching transistor of the LLC resonant converter.

4. The bidirectional LLC soft-start method according to claim 1, characterized in that, The preset duty cycle threshold in step 5) is the maximum duty cycle corresponding to the occurrence of hard switching phenomenon in the power switch.

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