Short-circuit protection control method applied to LLC-DCX resonant converter

Abstract

The invention discloses a short-circuit protection control method applied to an LLC-DCX resonant converter, and belongs to the field of electric energy conversion. According to the method, a Burst mode is utilized to convert the output voltage of the switching tube array from a continuous mode to a discontinuous mode, phase shift control is adopted to reduce the pulse width of the input voltage, and the two modulation modes are combined to limit the current increase caused by load switching of the LLC-DCX converter and the output overcurrent during short circuit, so that the output current of the LLC-DCX converter is reduced. And the function of load side short-circuit protection is realized. According to the method, a new control strategy is provided for the direct-current transformer working in an open-loop, fixed-frequency and fixed-transformation-ratio mode, and the short-circuit protection function can be provided for the LLC-DCX topology to achieve output under the severe working conditions of short circuit, load sudden change and the like.

Description

Technical Field

[0001] This invention belongs to the field of launch vehicle ground power supply design and power conversion technology, specifically relating to a short-circuit protection control method applied to LLC-DCX resonant converters. Background Technology

[0002] In power supply design, the most direct way to increase power density is to increase the operating frequency. Traditional pulse-width modulation (PWM) generates high switching losses at high frequencies. To address the problems caused by hard switching, circuit topologies with soft-switching characteristics have been extensively studied. LLC resonant converters, with their main control transistors achieving zero-voltage switching (ZVS) and rectifier diodes achieving zero-current switching (ZCS), offer numerous advantages such as high efficiency and high power density. In recent years, LLC resonant converters have been widely used in various scenarios. When operating over a wide gain range, a two-stage topology of LLC-DCX + non-isolated DC-DC converter is typically employed. The LLC resonant converter, operating at a fixed frequency near the resonant frequency, is used as a DC transformer (LLC-DCX). When the LLC converter operates at its resonant frequency, the voltage gain is constant and does not change with the load, exhibiting excellent DC transformer characteristics, making it suitable as the main circuit topology for DC transformers. It theoretically boasts the highest efficiency, extremely low control complexity, and low cost. After more than a decade of development, the design, control methods, and topology optimization of LLC resonant converters have been comprehensively studied and have achieved relatively mature applications.

[0003] LLC-DCX operates in an open-loop, fixed-frequency configuration in a two-stage topology. Due to its design requirements for high efficiency, constant turns ratio, and high power density, it typically employs a large magnetizing inductance to reduce turn-off losses in power devices. In practical applications, a large magnetizing inductance-to-leakage inductance ratio reduces its voltage regulation capability. Frequency modulation control alone is insufficient for voltage regulation of LLC-DCX. Therefore, in applications requiring current limiting protection and load shedding current limiting for the LLC-DCX output, a hysteresis-based comparison of the feedback signal or output voltage sampling signal is usually used to control the switching action. After a certain switching time, the switching signal is shielded for a period to achieve a match between the average output power and the load power. However, because the burst mode operates at a low frequency, it is likely to generate vibration noise within the human hearing range (20Hz-20kHz) when subjected to electromagnetic forces. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a short-circuit protection control method for LLC-DCX resonant converters, which solves the problems of limiting current increase caused by load switching and output overcurrent during short circuits in LLC-DCX converters, and realizes the function of short-circuit protection on the load side.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A short-circuit protection control method for LLC-DCX resonant converters, the method comprising:

[0007] The output current of the LLC-DCX resonant converter is monitored in real time using an isolation sampling chip. Monitoring thresholds Iref1 and Iref2 are set, where the overcurrent value < Iref1 < Iref2, and the overcurrent value represents the current value that exceeds the normal operating value of the LLC-DCX resonant converter.

[0008] When an overcurrent is detected in the output current of the LLC-DCX resonant converter, which is greater than Iref1 and less than Iref2, the output current is adjusted by adjusting the phase shift angle.

[0009] When the output current of the LLC-DCX resonant converter is detected to be greater than Iref2, the output current is adjusted through the Burst mode.

[0010] Furthermore, the Burst mode is used to convert the output voltage of the switching array from a continuous mode to a discontinuous mode; the adjustment of the phase shift angle reduces the pulse width of the input voltage through phase shift control.

[0011] Furthermore, the control method is based on an LLC-DCX resonant converter, the resonant converter topology of which consists of an LLC main circuit, a main transformer and a synchronous rectifier circuit topology.

[0012] Furthermore, the step of adjusting the output current by adjusting the phase shift angle includes gradually increasing the phase shift angle between the drive signals of the lagging bridge arm and the leading bridge arm, while simultaneously adjusting the drive timing of the synchronous rectifier. The drive timing of the synchronous rectifier is set to be turned on simultaneously with the main control transistor of the lagging bridge arm and turned off simultaneously with the main control transistor of the leading bridge arm. When the output current reaches below the normal operating current value and begins to gradually decrease, the phase shift angle is controlled to gradually decrease to zero, thereby achieving the switching from the short-circuit state to the normal operating mode.

[0013] Furthermore, the adjustment of the output current through the Burst mode includes: setting the comparator register to zero to realize the switching action of turning off the primary-side drive, entering the Burst-off mode, and the output capacitor supplies power to the load; when the output current is detected to be below the normal operating current value and begins to gradually decrease, the comparator register is reassigned to normally turn on the primary-side drive, and the energy is transferred from the primary-side drive to the secondary-side drive.

[0014] Furthermore, the phase shift angle adjustment range is 0° to 30°.

[0015] Secondly, the present invention provides a computer-readable storage medium having executable instructions stored thereon, which, when executed by a processor, enable the processor to implement the aforementioned short-circuit protection control method applied to an LLC-DCX resonant converter.

[0016] The beneficial effects of this invention are as follows:

[0017] By utilizing the Burst mode to convert the output voltage of the switching array from continuous to discontinuous mode and employing phase-shift control to reduce the pulse width of the input voltage, the combination of these two modulation methods limits the current increase caused by load switching and the output overcurrent during short circuits in the LLC-DCX converter, thereby achieving the function of short-circuit protection on the load side. This provides a new control strategy for DC transformers operating in open-loop, fixed-frequency, and fixed-ratio mode. Attached Figure Description

[0018] Figure 1 This is a waveform diagram of the Burst mode control of the present invention;

[0019] Figure 2 This is a control waveform diagram of the phase-shifting control mode of the present invention;

[0020] Figure 3 This is a waveform diagram of the input voltage of the resonant cavity after phase shifting according to the present invention;

[0021] Figure 4 This is a gain ratio diagram after phase shifting of the present invention;

[0022] Figure 5 This is a schematic diagram of the current detection circuit of the present invention;

[0023] Figure 6 This is the circuit schematic of the LLC-DCX converter of the present invention;

[0024] Figure 7 This is a block diagram of the control strategy of the present invention. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0026] The main methods of this invention include using Burst mode to convert the output voltage of the switching array from continuous to discontinuous mode, and employing phase-shift control to reduce the pulse width of the input voltage. By combining these two modulation methods, the increase in current caused by load switching and the output overcurrent during short circuits in the LLC-DCX converter are limited, thus achieving load-side short-circuit protection. Addressing the challenge of limiting the output current solely by adjusting the operating frequency due to the inherent parameter characteristics of the LLC-DCX DC transformer (a relatively large ratio of magnetizing inductance to leakage inductance), this method provides a new control strategy for open-loop, fixed-frequency, fixed-ratio DC transformers. It enables output short-circuit protection under adverse conditions such as short circuits and sudden load changes in the LLC-DCX topology. The technical highlights of this invention are described in detail below.

[0027] 1. Burst mode control characteristics;

[0028] When the output voltage is controlled in Burst mode, the switching transistor's on-time is the Burst-on period, and its off-time is the Burst-off period. The key waveforms are as follows: Figure 1 As shown.

[0029] Burst mode is activated by either a feedback signal or an output voltage. The sampling signal is compared with hysteresis to control the presence or absence of switching action. After a certain period of switching action, the switching signal is shielded for a period of time to achieve matching between the average output power and the load power. Figure 1 is a schematic diagram of the working principle of the hiccup mode. During the Burst-on working time, the switching transistor is driven. The switching devices operate normally, generating a resonant current. and Energy is transferred from the primary side to the secondary side, and the output voltage is... Rise. During the Burst-off operating period, turn off the switching transistor drive. When the switch is turned off, no power is transferred from the primary side to the secondary side; instead, energy is supplied by the output capacitor, and the output voltage... To reduce noise, the hiccup mode, which switches the output voltage from continuous to discontinuous operation, limits the output current and voltage. While simple and easy to implement, the low operating frequency of the hiccup mode can generate noise perceptible to the human ear. Magnetic components in the circuit generate magnetic fields when excited by alternating current, causing electromagnetic vibrations. Components such as inductors, transformer coils, and magnetic cores, which cannot be completely fixed to the printed circuit board, are particularly prone to generating vibration noise within the human hearing range (20Hz-20kHz) when subjected to electromagnetic forces. Therefore, in extreme short-circuit conditions, where the output current increases sharply, a burst-off mechanism immediately interrupts energy transfer on the primary side to protect the circuit. The increase in current caused by load shedding can be smoothly controlled through phase shifting, reducing the use of the hiccup mode and thus lowering noise.

[0030] 2. Control characteristics of phase-shift control mode;

[0031] When using frequency modulation control, in the two main control transistor arms of the full-bridge LLC resonant converter, the drive signals for transistor pairs (such as Q1 and Q4, Q2 and Q3) are the same, and the drive signals for the two transistors in the same arm are complementary. There is no phase difference between the drive signals of the two arms, and the no-load gain cannot be effectively adjusted by frequency modulation. Furthermore, the effect of limiting the output of the LLC-DCX by frequency modulation is limited. Therefore, to change the gain, the pulse width of the resonant cavity input voltage needs to be adjusted. Thus, phase-shift control is introduced. Phase shift means creating a phase shift angle θ between the drive signals of the lagging arm and the leading arm. Figure 2 The operating frequency is given. and resonant frequency The main waveforms of phase-shift control during equal phase shifting. After phase shifting, the resonant cavity input voltage waveform approximately changes from a symmetrical positive and negative square wave with a duty cycle of 0.5 to a three-level square wave with a duty cycle less than 0.5 (e.g., ...). Figure 3 The zero-level time is the phase shift time t. θ =θ / (2πfs), which is precisely the control phase shift angle range of 0~30° when the two control strategies are combined in the invention to avoid efficiency reduction.

[0032] It is important to note that the timing of the synchronous rectifier diodes differs between phase-shifting and frequency-modulation operations. Therefore, the driving timing of the synchronous rectifier diodes must be adjusted in real-time during phase-shifting. Based on the rectified current timing, the driving timing of the synchronous rectifier diodes is set to turn on simultaneously with the lagging bridge arm main control diode and turn off simultaneously with the leading bridge arm main control diode. This maintains a longer channel conduction time and prevents reverse current backflow at zero crossing.

[0033] For the resonant cavity input voltage u AB Performing Fourier decomposition on the waveform yields:

[0034] ,

[0035] Among them, coefficient The expression is:

[0036] ,

[0037] Where k is the harmonic order. Let k=1 be the amplitude of the input voltage to the resonant cavity, and we get u. AB The fundamental component is:

[0038] ,

[0039] Furthermore, due to the pre-phase shift u AB The fundamental component is:

[0040] ,

[0041] When the phase shift angle is θ, u AB The fundamental component is cos(θ / 2) times the value before phase shift, therefore the forward voltage gain at a phase shift angle of θ is:

[0042] ,

[0043] in, Let λ be the normalized frequency, λ be the resonant cavity inductance ratio, Q be the resonant cavity quality factor, and n be the transformer turns ratio. Figure 4 As shown, by phase-shifting the bridge arms within the range of 0° to 90°, the input square wave can be transformed into a three-level square wave, and its effective value can be changed within the range of Vin to 0. The drawback of phase-shifting is the circulating current loss when the resonant cavity input voltage is zero. To ensure high system efficiency, the phase shift angle of the module cannot be too large; the adjustment range is taken between 0° and 30°. When the load increase is not excessive, phase-shifting control can smoothly reduce the output voltage and current. When the load suddenly increases, the phase angle increases from 0° to 30° and remains at 30°. When the output current is limited to below 60A and begins to decrease, the phase shift angle θ between the drive signals of the lagging and leading bridge arms decreases from 30° to 0°, allowing the circuit to smoothly recover from current-limiting protection to normal operation.

[0044] Example

[0045] This invention provides a short-circuit protection control method for LLC-DCX resonant converters, specifically including:

[0046] a. Output current detection: such as Figure 5As shown, this invention uses the NSM2016 isolation sampling chip based on the Hall principle from Nanochip Microelectronics, connected in series in the main circuit of the LLC-DCX resonant converter to detect the output current in real time.

[0047] b. Current limiting protection technical solutions: such as Figure 6 , Figure 7 As shown, the primary side of the LLC-DCX consists of a full-bridge structure composed of four switching transistors Q1, Q2, Q3, and Q4. Under normal operating conditions, Q1 and Q4 are configured by two GPIO ports of the DSP28335 to maintain identical EPWM1A and EPWM2A square wave outputs, while Q2 and Q3 are configured by two GPIO ports of the DSP28335 to maintain identical EPWM1B and EPWM2B square wave outputs. Q5, Q6, Q7, and Q8 are synchronous rectifier transistors configured by EPWM3A and EPWM3B on the secondary side. The A and B channels of each EPWM pair provide complementary square wave outputs.

[0048] Burst mode disables the primary-side drive by setting the comparator register to zero. When the comparator register is set to zero, the PWM output is turned off, entering Burst-off mode, and the load is powered by the output capacitor. When the output sampling I / O detects that the output current has fallen below the normal operating current value and begins to gradually decrease, the comparator register is reassigned, and the primary-side drive is normally turned on, with energy transferred from the primary side to the secondary side.

[0049] The phase-shift control mode, configured through the phase register in the PWM configuration, establishes a phase shift angle θ between the drive signals of the lagging arms Q2 and Q3 and the leading arms Q1 and Q4. After the phase shift, the resonant cavity input voltage waveform approximately changes from a symmetrical positive and negative square wave with a duty cycle of 0.5 to a three-level square wave with a duty cycle less than 0.5. Figure 3 The effective value of the input voltage is controlled by the phase angle. After the sampling chip detects that the output current is normal, the phase angle is gradually reduced to zero, thereby achieving the switching from the short-circuit state to the normal operating mode.

[0050] Because the Burst mode operates at a low frequency, it generates noise that is perceptible to the human ear. Furthermore, phase-shift control can introduce circulating current losses and affect efficiency if the phase shift angle is too large. Therefore, the phase is generally adjusted within the range of 0° to 25° when the load change is not excessive. When the DSP detects that the output current is greater than Iref1 but not exceeding Iref2, it adjusts the output current by adjusting the phase shift angle. When the current is greater than Iref2, the Burst mode is used to regulate the output. Its control block diagram is as follows: Figure 7 As shown.

[0051] Sample the output current of the LLC-DCX and compare the sampled value of the output current with the pre-set Iref1 and Iref2 in the program. Since the module power supply needs to output 0~60A and 22~40V for normal operation, set Iref1 and Iref2 to 65A and 80A respectively.

[0052] When the output current is greater than 60A but less than 65A, considering the current increase caused by load changes, phase-shift control is used to increase the phase angle from 0° to 25° (by assigning a value to the phase register in each interrupt cycle to adjust the effective value of the primary input voltage). When the load current is detected to decrease to below 60A, the phase-shift angle is reduced from 25° to 0°. When the output current is greater than 80A, Burst mode control is used (by setting the comparator register to zero to achieve the switching action of closing the bridge arm).

[0053] Secondly, the present invention provides a computer-readable storage medium having executable instructions stored thereon, which, when executed by a processor, enable the processor to implement the aforementioned short-circuit protection control method applied to an LLC-DCX resonant converter.

[0054] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are 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. A short-circuit protection control method applied to an LLC-DCX resonant converter, characterized in that, The method comprises: The LLC-DCX resonant converter output current is monitored in real time by using an isolated sampling chip, and monitoring threshold Iref1 and Iref2 are set, wherein overcurrent value < Iref1 < Iref2, the overcurrent value represents a current value exceeding the normal working current of the LLC-DCX resonant converter; When it is monitored that the LLC-DCX resonant converter output current is overcurrent and greater than Iref1 and less than Iref2, the output current is adjusted by adjusting the phase shift angle; When it is monitored that the LLC-DCX resonant converter output current is greater than Iref2, the output current is adjusted by Burst mode.

2. The short-circuit protection control method for an LLC-DCX resonant converter according to claim 1, wherein The adjustment of the phase shift angle reduces the pulse width of the input voltage by phase shift control.

3. The short-circuit protection control method for an LLC-DCX resonant converter according to claim 1, wherein The Burst mode is used to convert the output voltage of the switch tube array from continuous mode to discontinuous mode.

4. The short-circuit protection control method for an LLC-DCX resonant converter according to claim 1, characterized by, The control method is based on the LLC-DCX resonant converter, and the resonant converter topology is composed of an LLC main circuit, a main transformer and a synchronous rectification circuit topology.

5. The short-circuit protection control method for an LLC-DCX resonant converter according to claim 2, wherein The adjustment of the output current by adjusting the phase shift angle comprises gradually increasing the phase shift angle existing between the driving signals of the lagging bridge arm and the leading bridge arm, and adjusting the driving timing of the synchronous rectification tube.

6. The short-circuit protection control method for an LLC-DCX resonant converter according to claim 5, wherein The driving timing of the synchronous rectification tube is set to be turned on at the same time as the lagging bridge arm main control tube and turned off at the same time as the leading bridge arm main control tube, when the output current reaches below the normal working current and starts to gradually decrease, the phase shift angle is gradually reduced to zero to realize the switching from the short circuit state to the normal working mode.

7. The short-circuit protection control method for an LLC-DCX resonant converter according to claim 3, wherein The adjustment of the output current by Burst mode comprises assigning zero to the comparison register to realize the closing of the switch action of the primary side driving, entering the Burst-off mode, and the output capacitor supplies power to the load.

8. The short-circuit protection control method for an LLC-DCX resonant converter according to claim 7, characterized by, When it is detected that the output current reaches below the normal working current and starts to gradually decrease, the comparison register is revalued, the primary side driving is normally started, and the energy is transmitted from the primary side driving to the secondary side driving.

9. The short-circuit protection control method for an LLC-DCX resonant converter according to claim 2, wherein The phase shift angle adjustment range is 0°-30°.

10. A computer-readable storage medium, characterized in that, The computer readable storage medium has stored executable instructions, which are executed by the processor to enable the processor to implement the short circuit protection control method applied to the LLC-DCX resonant converter according to any one of claims 1-9.

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