A short-circuit current cutoff control method and device for LLC converter

Abstract

The present invention discloses a short-circuit current cutoff control method and device for an LLC converter. When a short circuit is detected in the LLC converter, the primary and secondary switching tubes of the LLC converter are turned off. After a set duration, k pulse drive signals with set high-level durations and set low-level durations are generated to drive the primary switching tube, where k is a natural number not less than 2. After the k pulse drive signals with set high-level durations and set low-level durations have finished driving the primary switching tube, a pulse drive signal with a duty cycle of 50% is generated based on a pre-constructed output current small signal model to drive the primary switching tube until the output current of the LLC converter reaches a steady state. After the output current of the LLC converter reaches a steady state, the synchronous rectification of the LLC converter is turned on to complete the short-circuit current cutoff control of the LLC converter. The present invention has a fast control speed, a small dynamic resonant current, and a stable output current.

Description

Technical Field

[0001] The present invention belongs to the technical field of LLC converters in isolated DC-DC converters, and in particular relates to a short-circuit current cutoff control method and device for an LLC converter. Background Art

[0002] In the field of switching power supplies, LLC, as an efficient, high-power-density isolated DC-DC conversion topology, is widely used in various DC-to-DC isolation conversion scenarios. In airborne power supply applications, the converter is required to have a short-circuit current limiting capability of 2 to 3 times the rated current. Compared with the two-stage LLC+Buck topology, which uses the Buck stage to achieve short-circuit protection, the single-stage LLC can achieve higher power density, higher efficiency, and smaller mass, thereby improving aircraft reliability and increasing flight time. However, because the LLC converter is a frequency-controlled converter and the relationship between its output gain and frequency varies under different loads, short-circuit current blocking of the single-stage LLC is quite challenging.

[0003] There are three basic approaches to short-circuit protection for LLC converters. The first involves modifying the main power circuit topology. In the event of a short circuit, the protection circuitry bypasses most of the current, protecting the main power loop. However, due to the need for additional components to form the current limiting circuit, the converter increases in size and weight, hindering the high-density of airborne power supplies. The second approach involves PWM control of the converter, reducing the duty cycle to achieve low gain during a short circuit. However, this approach can result in a loss of soft switching, significantly increasing losses and placing higher demands on the LLC converter's thermal design. The heat sink can also increase in size, resulting in a decrease in overall power density. Furthermore, this approach is limited in its applicability, being applicable only to full-bridge LLC converters and not to half-bridge LLC converters. The third approach involves increasing the frequency. This allows the LLC converter's soft switching to be maintained. Furthermore, since the LLC converter's gain curve shifts downward at high currents, the frequency range decreases. This allows the LLC converter to reduce gain without increasing the frequency significantly during a short circuit, making it a natural approach for high-current applications.

[0004] However, when the LLC converter switches directly from normal mode to fixed high-frequency mode in the event of a short circuit, problems such as prolonged short-circuit overshoot current, uncontrollable resonant current during dynamic operation, and difficulty in accurately controlling the steady-state short-circuit current arise. The short-circuit overshoot current duration and accurate steady-state short-circuit current control are closely related to the safety of subsequent circuits. It is crucial to further shorten the short-circuit overshoot current duration and precisely control the steady-state short-circuit current. Furthermore, large transient resonant currents can pose a threat to circuit safety, making control of transient resonant current crucial. Summary of the Invention

[0005] In response to the problems existing in the prior art, the present invention provides a short-circuit current control method and device for an LLC converter, which has fast control speed, small dynamic resonant current and stable output current.

[0006] In order to solve the above technical problems, the present invention is implemented through the following technical solutions:

[0007] A short-circuit current cutoff control method for an LLC converter, comprising:

[0008] When a short circuit of the LLC converter is detected, turning off the primary side switch tube and the secondary side switch tube of the LLC converter;

[0009] After a set time, k pulse drive signals with set high-level durations and set low-level durations are generated to drive the primary switch tube, and the LLC converter is restarted, where k is a natural number not less than 2;

[0010] After the k pulse drive signals with set high-level durations and set low-level durations drive the primary switch tube, a pulse drive signal with a duty cycle of 50% is generated based on a pre-built output current small signal model to drive the primary switch tube until the output current of the LLC converter reaches a steady state;

[0011] When the output current of the LLC converter reaches a steady state, the synchronous rectification of the LLC converter is turned on to complete the short-circuit current cutoff control of the LLC converter.

[0012] Furthermore, the set duration is calculated using the following formula:

[0013] t0=4R L C f

[0014] Where: t0 is the set time; R L is the short-circuit resistance; C f is the output capacitor value.

[0015] Furthermore, in the k pulse driving signals with set high-level durations and set low-level durations driving the primary switch tube, the first set high-level duration and the first set low-level duration are calculated using the following formula:

[0016]

[0017]

[0018] The second set high level duration and set low level duration are calculated using the following formula:

[0019]

[0020]

[0021] The third to kth set high level durations and set low level durations are calculated using the following formula:

[0022]

[0023]

[0024] in:

[0025]

[0026]

[0027]

[0028]

[0029] Where, t 1on Set the high level duration for the first one; t 1off Set the low level duration for the first one; t 2on Set the high level duration for the second one; t 2off Set the low level duration for the second one; t 3～kon Set the high level duration for the 3rd to kth ones; t 3～koff Set the low level duration for the 3rd to kth ones; i rmax is the maximum allowable resonant current; v in is the input voltage; L r is the resonant inductance value; C r is the resonant capacitor value.

[0030] Furthermore, the pre-built output current small signal model is as follows:

[0031]

[0032]

[0033] in:

[0034]

[0035]

[0036]

[0037]

[0038]

[0039]

[0040]

[0041] Where: L m is the excitation inductance, ω s is the switching angular frequency, and n is the transformer ratio.

[0042] Furthermore, the frequency of the pulse driving signal with a duty cycle of 50% is adjusted by a digital compensator.

[0043] Furthermore, a digital compensator is used to perform open-loop gain compensation on the output current loop of the LLC converter.

[0044] Furthermore, the output current of the LLC converter reaches a steady state and satisfies the following conditions:

[0045] When it is detected that the output current of the LLC converter is 2 times or 3 times the rated current, the output current of the LLC converter reaches a steady state.

[0046] A short-circuit current cutoff control device for an LLC converter, comprising:

[0047] A first control module is configured to turn off the primary side switch tube and the secondary side switch tube of the LLC converter when a short circuit of the LLC converter is detected;

[0048] A second control module is configured to generate, after a set duration, a pulse drive signal with k high-level durations and k low-level durations to drive the primary switch tube, where k is a natural number not less than 2;

[0049] a third control module, configured to generate a pulse drive signal with a duty cycle of 50% based on a pre-built output current small signal model to drive the primary switch tube after the k pulse drive signals with high-level durations and low-level durations have driven the primary switch tube until the output current of the LLC converter reaches a steady state;

[0050] The fourth control module is used to start the synchronous rectification of the LLC converter after the output current of the LLC converter reaches a steady state, thereby completing the short-circuit current cutoff control of the LLC converter.

[0051] Compared with the prior art, the present invention has at least the following beneficial effects:

[0052] The present invention provides a short-circuit current cutoff control method for an LLC converter. When a short circuit occurs in the LLC converter, pulses are generated to drive the primary and secondary switches. Upon detecting a short circuit, the primary and secondary switches are shut down, and a set duration is allowed to elapse. During this time, the charge in the secondary output capacitor is rapidly discharged, reducing the peak duration of the LLC converter's output current. After the set duration, k pulse drive signals with set high and low level durations are generated to drive the primary switch. The set high and low level durations cause the resonant cavity current and voltage to follow specific trajectories within the state plane, limiting the maximum dynamic resonant current. After the k pulse drive signals with set high and low level durations have completed driving the primary switch, frequency modulation control of the primary switch is performed based on a small-signal model of the converter's output current. The compensated LLC converter's output current open-loop transfer function maintains amplitude and phase margins under varying loads and frequencies, ensuring precise control of the steady-state output current. Synchronous rectification is activated when the output current reaches steady-state, minimizing system losses and completing short-circuit current cutoff control. Compared with the traditional LLC short-circuit control method of directly switching from a normal state to a fixed high frequency, the present invention has the advantages of fast control speed, small dynamic resonant current and stable output current.

[0053] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] In order to more clearly illustrate the technical solutions in the specific embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the specific embodiments. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0055] Figure 1 This is a control timing diagram of a short-circuit current cutoff control method of an LLC converter according to the present invention;

[0056] Figure 2 This is a diagram showing the control strategy and resonant cavity state trajectory of a short-circuit current cutoff control method for an LLC converter before stage one, during stage one, and during stage two of the present invention;

[0057] Figure 3 A Bode diagram of an open-loop gain of a short-circuit current control method for an LLC converter in a third stage of the present invention;

[0058] Figure 4 A Bode diagram of the open-loop gain after steady-state short-circuit current control compensation in stage three of a short-circuit current cutoff control method of an LLC converter according to the present invention;

[0059] Figure 5 The waveforms of the LLC converter output voltage, output current and resonant current before and after short circuit are shown using a short circuit current cutoff control method of the LLC converter according to the present invention.

[0060] Figure 6 1 is a waveform diagram of the resonant current and output current of an LLC converter before and after a short circuit according to an embodiment of the present invention. DETAILED DESCRIPTION

[0061] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0062] As a specific embodiment of the present invention, Figure 1 As shown, a short-circuit current cutoff control method for an LLC converter specifically includes the following stages:

[0063] Phase 1: When a short circuit of the LLC converter is detected, the primary and secondary switching tubes of the LLC converter are turned off.

[0064] Preferably, a digital controller is used to control the shutdown of the primary side switch tube and the secondary side switch tube of the LLC converter. Specifically, when a short circuit of the LLC converter is detected, the Trip Zone function in the digital controller is used to shut down the PWM wave generation to achieve the shutdown of the primary side switch tube and the secondary side switch tube of the LLC converter.

[0065] Phase 2: After the set duration, k pulse drive signals with set high-level and low-level durations are generated to drive the primary-side switch, restarting the LLC converter. Here, k is a natural number not less than 2.

[0066] Preferably, in this second stage:

[0067] The set duration is calculated using the following formula:

[0068] t0=4R L C f

[0069] Where: t0 is the set time; R L is the short-circuit resistance; C f is the output capacitor value.

[0070] That is, after the primary and secondary switching tubes of the LLC converter are turned off, a delay t0 of a set duration is generated. In this embodiment, when the output capacitance of the LLC converter is equal to 100 μF, assuming that the short-circuit resistance is 10 mΩ to 20 mΩ, the delay time is 4 μs to 8 μs. In order to protect the subsequent circuit as much as possible and avoid a long short-circuit overshoot current, the delay is set to 8 μs.

[0071] Preferably, in this second stage:

[0072] The calculation of k set high level durations and set low level durations is as follows:

[0073] The first set high level duration and the first set low level duration are calculated using the following formula:

[0074]

[0075]

[0076] The second set high level duration and set low level duration are calculated using the following formula:

[0077]

[0078]

[0079] The third to kth set high level durations and set low level durations are calculated using the following formula:

[0080]

[0081]

[0082] in:

[0083]

[0084]

[0085]

[0086]

[0087] Where, t 1on Set the high level duration for the first one; t 1off Set the low level duration for the first one; t 2on Set the high level duration for the second one; t 2off Set the low level duration for the second one; t 3～non Set the high level duration for the 3rd to kth ones; t 3～n offSet the low level duration for the 3rd to kth ones; i rmax is the maximum allowable resonant current; v in is the input voltage; L r is the resonant inductance value; C r is the resonant capacitor value.

[0088] In this embodiment, after the set time, two pulse drive signals with set high level time and set low level time are generated to drive the primary switch tube. Figure 2 It can be seen that during the set duration of Phase 2, the resonant current and resonant capacitor voltage of the LLC converter gradually approach the origin from the normal operating trajectory and approach the origin after the delay t0. During the 1st and 2nd pulses of Phase 2, the resonant current and resonant capacitor voltage change along a specific trajectory, while the resonant current never exceeds the maximum allowable value. During the 3rd to kth pulses of Phase 2, the resonant current and resonant capacitor voltage trajectories overlap and always change within the maximum allowable resonant current range.

[0089] Phase 3: After k pulse drive signals with set high-level duration and set low-level duration drive the primary-side switch tube, a pulse drive signal with a duty cycle of 50% is generated based on the pre-built output current small signal model to drive the primary-side switch tube until the output current of the LLC converter reaches a steady state.

[0090] Preferably, in this stage three:

[0091] Combine Figure 3 As shown, the pre-built output current small signal model is as follows:

[0092]

[0093]

[0094] in:

[0095]

[0096]

[0097]

[0098]

[0099]

[0100]

[0101] Where: L m is the excitation inductance, ω s is the switching angular frequency, and n is the transformer ratio.

[0102] Specifically, a digital compensator is used to adjust the frequency of a pulse drive signal with a duty cycle of 50%, and a digital compensator is used to perform open-loop gain compensation on the output current loop of the LLC converter. The open-loop gain is obtained by using an output current small signal model or a frequency sweep method. Figure 3 It can be seen that when the load is 10mΩ, 100mΩ or 200mΩ and the frequency is 600kHz, 800kHz or 1MHz, the open-loop gain obtained by the small signal model and the swept frequency method is basically consistent. The digital compensator used is digital PI, digital PID or digital zero-pole. Figure 4 It can be seen that the open-loop transfer function after the digital compensator has a gain margin of more than 20dB and a phase margin of 70°.

[0103] Specifically, the output current of the LLC converter reaches a steady state and satisfies the following conditions:

[0104] When it is detected that the output current of the LLC converter is 2 times or 3 times the rated current, the output current of the LLC converter reaches a steady state.

[0105] Phase 4: When the output current of the LLC converter reaches a steady state, the synchronous rectification of the LLC converter is turned on to complete the short-circuit current cutoff control of the LLC converter.

[0106] Because the PWM waves of the pulse drive signals generated in stages 2 to 4 all have dead zones, the size of the dead zones must meet the soft switching conditions of the primary devices of the LLC converter, as follows:

[0107] Among them, the dead time of stage 2 meets the following requirements:

[0108]

[0109] Among them, the dead time of stage 3 and stage 4 meets the following requirements:

[0110]

[0111] Where: C oss is the output capacitance of the primary device of LLC converter; i rsteady is the peak value of the resonant current in steady state.

[0112] The present invention is described in more detail below in conjunction with the embodiments:

[0113] Figure 5 The short-circuit current cutoff control method of the LLC converter of the present invention is shown in the circuit simulation of the LLC converter short-circuit protection effect diagram. The simulated LLC converter working condition is 28V output, 1.5kW power under normal operation, and 2 times rated current cutoff in short circuit. Figure 5 In the figure, (a) simulates a short-circuit resistance of 10mΩ, (b) simulates a short-circuit resistance of 25mΩ, and (c) simulates a short-circuit resistance of 50mΩ. As can be seen from the figures, under different short-circuit resistance conditions, the short-circuit current cutoff control method of the LLC converter of the present invention can effectively limit the dynamic resonant current, achieve rapid short-circuit overshoot current limitation, and achieve steady-state short-circuit current control.

[0114] Figure 6 Figure 2 shows the resonant current and output current waveforms of an LLC converter before and after a short circuit, according to an embodiment of the present invention's short-circuit current cutoff control method for an LLC converter. This embodiment operates under the conditions of two LLC converters connected in parallel. Under normal operation, a single LLC converter operates at 28V output and 1.5kW power, and performs current cutoff at twice the rated current during a short circuit. As can be seen from the figure, the present invention's short-circuit current cutoff control method for an LLC converter effectively limits dynamic resonant current, achieves rapid short-circuit overshoot current limitation, and achieves steady-state short-circuit current control.

[0115] The present invention provides a short-circuit current cutoff control device for an LLC converter, comprising:

[0116] The first control module is configured to turn off the primary-side switch tube and the secondary-side switch tube of the LLC converter when a short circuit of the LLC converter is detected.

[0117] The second control module is used to generate a pulse drive signal with k high-level durations and low-level durations to drive the primary side switch tube after a set duration, where k is a natural number not less than 2.

[0118] The third control module is used to generate a pulse drive signal with a duty cycle of 50% based on a pre-built output current small signal model to drive the primary side switch tube after the pulse drive signal with k high-level durations and low-level durations drives the primary side switch tube until the output current of the LLC converter reaches a steady state.

[0119] The fourth control module is used to start the synchronous rectification of the LLC converter after the output current of the LLC converter reaches a steady state, thereby completing the short-circuit current cutoff control of the LLC converter.

[0120] Finally, it should be noted that the above-described embodiments are only specific implementations of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the above-described embodiments, those skilled in the art should understand that any person skilled in the art can modify or easily conceive of changes to the technical solutions described in the above-described embodiments within the technical scope disclosed by the present invention, or replace some of the technical features therein with equivalents. Such modifications, changes, or replacements do not deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A short-circuit current control method for an LLC converter, characterized in that: include: When a short circuit of the LLC converter is detected, turning off the primary side switch tube and the secondary side switch tube of the LLC converter; After a set time, k pulse drive signals with set high-level durations and set low-level durations are generated to drive the primary switch tube, and the LLC converter is restarted, where k is a natural number not less than 2; In the k pulse driving signals with set high-level durations and set low-level durations driving the primary switch tube, the first set high-level duration and the first set low-level duration are calculated using the following formula: The second set high level duration and set low level duration are calculated using the following formula: The third to kth set high level durations and set low level durations are calculated using the following formula: in: Where, t 1on Set the high level duration for the first one; t 1off Set the low level duration for the first one; t 2on Set the high level duration for the second one; t 2off Set the low level duration for the second one; t 3～kon Set the high level duration for the 3rd to kth ones; t 3～koff Set the low level duration for the 3rd to kth ones; i rmax is the maximum allowable resonant current; v in is the input voltage; L r is the resonant inductance value; C r is the resonant capacitance value; After the k pulse drive signals with set high-level durations and set low-level durations drive the primary switch tube, a pulse drive signal with a duty cycle of 50% is generated based on a pre-built output current small signal model to drive the primary switch tube until the output current of the LLC converter reaches a steady state; When the output current of the LLC converter reaches a steady state, the synchronous rectification of the LLC converter is turned on.

2. The short-circuit current control method of an LLC converter according to claim 1, characterized in that: The set duration is calculated using the following formula: t0=4R L C f Where: t0 is the set time; R L is the short-circuit resistance; C f is the output capacitor value.

3. The short-circuit current control method of an LLC converter according to claim 1, characterized in that: The pre-built output current small signal model is as follows: in: Where: L m is the excitation inductance, ω s is the switching angular frequency, and n is the transformer ratio.

4. The short-circuit current control method of an LLC converter according to claim 1, characterized in that: The frequency of the pulse driving signal with a duty cycle of 50% is adjusted by a digital compensator.

5. The short-circuit current control method of an LLC converter according to claim 1, characterized in that: A digital compensator is used to perform open-loop gain compensation on the output current loop of the LLC converter.

6. The short-circuit current control method of an LLC converter according to claim 1, characterized in that: The output current of the LLC converter reaches a steady state and satisfies the following conditions: When it is detected that the output current of the LLC converter is 2 times or 3 times the rated current, the output current of the LLC converter reaches a steady state.

7. A short-circuit current cutoff control device for an LLC converter, characterized in that: For implementing a short-circuit current cutoff control method of an LLC converter according to any one of claims 1 to 6, a short-circuit current cutoff control device of the LLC converter comprises: A first control module is configured to turn off the primary side switch tube and the secondary side switch tube of the LLC converter when a short circuit of the LLC converter is detected; A second control module is configured to generate, after a set duration, a pulse drive signal with k high-level durations and k low-level durations to drive the primary switch tube, where k is a natural number not less than 2; a third control module, configured to generate a pulse drive signal with a duty cycle of 50% based on a pre-built output current small signal model to drive the primary switch tube after the k pulse drive signals with high-level durations and low-level durations have driven the primary switch tube until the output current of the LLC converter reaches a steady state; The fourth control module is used to start the synchronous rectification of the LLC converter after the output current of the LLC converter reaches a steady state, thereby completing the short-circuit current cutoff control of the LLC converter.

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