CN116032108A - Composite modulation closed-loop control soft start method suitable for resonant converter - Google Patents
Composite modulation closed-loop control soft start method suitable for resonant converter Download PDFInfo
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- CN116032108A CN116032108A CN202310058921.2A CN202310058921A CN116032108A CN 116032108 A CN116032108 A CN 116032108A CN 202310058921 A CN202310058921 A CN 202310058921A CN 116032108 A CN116032108 A CN 116032108A
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Abstract
The invention discloses a soft start control method of an LLC resonant converter, and relates to the technical field of resonant converters. The invention adopts a PWM and PFM compound modulation method, takes the rising rate of the output voltage of the resonant converter as a control target, and performs closed-loop control on soft start, so that the rising of the output voltage is stable in the starting process, the charging current impact of the output capacitor is small, the surge current impact of the resonant cavity is effectively restrained, and the soft start process is more stable and reliable. The whole soft start process of the invention is controlled by the digital controller, and the starting requirements of different resonant converters can be met by flexibly setting the upper limit of the resonant current and the rising rate of the output voltage, thereby improving the applicability of the invention.
Description
Belonging to the field of
The invention relates to the technical field of resonant converters, in particular to a soft start control method of an LLC resonant converter.
Prior Art
The LLC resonant converter can realize zero-voltage conduction of a switching tube and zero-current turn-off of a rectifying diode in a wide load range, has the advantages of soft switching, high efficiency, wide output voltage range and the like, is a converter with excellent performance, and is widely applied. Usually, the LLC resonant converter is configured with a larger-capacity output capacitor at the output end for filtering switching current ripple and reducing output voltage ripple. When the LLC resonant converter is powered on and started, the voltage on the output capacitor is zero, which is equivalent to the short circuit of the secondary side of the transformer of the converter, the primary side voltage of the transformer is clamped to zero, and the impedance of the resonant cavity is small. The capacitor is charged when the converter is started, so that a resonant cavity of the converter can generate large surge current, the power device can bear large current stress and voltage stress, damage to the power device can cause the converter to malfunction, or overcurrent protection is triggered to influence the normal operation of the resonant converter. Soft start of the resonant converter is a common measure to solve the above problems.
The current LLC resonant converter soft start method mainly comprises the following steps:
(1) And (5) performing frequency-reducing soft start. The initial working frequency of the soft start is higher, so that the voltage and current stress of the resonant cavity can be reduced conveniently, and then the working frequency is reduced gradually until the output voltage is built up to finish the soft start. The method has the advantages of simple implementation, and the defect that the maximum switching frequency of the resonant converter is limited by components, so that the over-high starting frequency is difficult to realize, and the surge current during starting cannot be well restrained.
(2) Pulse width modulation controls soft start. The switching frequency is kept unchanged, the duty ratio is gradually increased from a small initial value to 50% until the establishment of the output voltage is completed, and soft start is realized. The soft start has the advantages of easy realization, obvious surge current inhibition effect and long soft start time.
(3) And (5) phase shifting soft start. The phase-shifting control can be essentially regarded as a pulse width modulation control mode, the duty ratio of all switching tubes is kept to be 0.5, and the starting current is limited by controlling the phase-shifting angle of two bridge arms to gradually increase from 0 degrees to 180 degrees. The phase-shifting start is only suitable for a full-bridge LLC resonant converter, but cannot be applied to a half-bridge LLC resonant converter, and the universality is poor.
(4) Hybrid control soft start. Hybrid control soft start including pulse width modulation and pulse frequency modulation, phase shift control, and pulse frequency modulation. The soft start of the pulse width modulation and pulse frequency modulation mixed control can inhibit surge current peak value in the starting to a certain extent, but the control process is too complex and the popularization and implementation are difficult. The phase-shifting control and pulse frequency modulation hybrid control soft start can limit current surge, but in the phase-shifting start process, the circuit works in a hard switching state, the loss is large, the phase-shifting control is only applicable to a full-bridge circuit, and the application range is limited.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a composite modulation closed-loop control starting method formed by pulse width modulation and pulse frequency modulation, and the method can effectively inhibit surge current impact when the LLC resonant converter is started, and the output voltage is stably and rapidly increased, so that the starting process of the resonant converter is stable and reliable, and the method has higher engineering use value.
The invention provides a soft start control method of an LLC resonant converter. The resonant converter is an LLC full-bridge resonant converter or an LLC half-bridge resonant converter, and comprises a full-bridge or half-bridge switching circuit, an LLC resonant circuit, a rectifying and filtering circuit, an output voltage and current detection circuit, a resonant circuit detection circuit, a control circuit (MCU) and a driving circuit. The control circuit (the controller can be a DSP, a singlechip, an ARM embedded processor or an FPGA programmable chip) detects the output voltage of the LLC resonant converter during soft start, generates a signal with a certain duty ratio or switching frequency according to the rising speed of the output voltage, and performs switching operation on a switching tube of the full-bridge (or half-bridge) power conversion circuit, so that the resonant current of the LLC resonant converter during start is kept within an acceptable range, the output voltage is quickly and stably established, and the soft start of the LLC resonant converter is realized.
The specific technical scheme is as follows: a composite modulation closed-loop control soft start method suitable for a resonant converter comprises the following steps:
(1) According to the output voltage rating U of the resonant converter o And soft start time T soft Index, calculating the output voltage rising rate reference value S rise ：S rise ＝U o /T soft 。
(2) The system starts up and the controller first operates in PWM (pulse width modulation) soft start mode. The control circuit outputs two paths of complementary driving signals to drive the switching tube of the full-bridge or half-bridge switching circuit to work. The driving signal starts the initial frequency f start Higher than the resonant frequency f of the resonant converter r . In general, the initial frequency f is started start The value is 1.5f r ～3f r Between, an initial duty cycle D start Take a value between 0 and 0.2.
(3) In the PWM soft start mode, the control circuit maintains the driving signal frequency at f start Unchanged, the duty cycle is initially D start And outputting a driving signal.
(4) Reading the output voltage value Uout of the control circuit, and calculating the current output voltage rising rate U rise And according to U rise And preset S rise Adjusting the duty ratio D of the driving signal by adopting a PID control algorithm, outputting a complementary driving signal for closed-loop control, and keeping the frequency of the driving signal at f start The resonance current value Ir is read at the same time without change;
(5) Judging whether the resonance current value Ir is larger than a preset maximum resonance current threshold Ir_th, if so, reducing the duty ratio D of the driving signal, and re-entering the step (4), otherwise, entering the step (6);
(6) Judging whether the duty ratio D is equal to the preset maximum duty ratio Dmax, if so, entering the step (7), otherwise, entering the step (12),
(7) Entering into PFM soft start mode, the control circuit keeps the duty ratio of the driving signal as D max Unchanged, the initial value of the frequency is f start And outputting a driving signal.
(8) Reading the output voltage value Uout of the control circuit, and calculating the current output voltage rising rate U rise And according to U rise And preset S rise Adjusting the frequency f of a driving signal by adopting a PID control algorithm, outputting a complementary driving signal for closed-loop control, keeping the duty ratio Dmax unchanged, and simultaneously reading a resonant current value Ir;
(9) Judging whether the resonance current value Ir is larger than a preset maximum resonance current threshold Ir_th, if so, increasing the driving signal frequency f, and re-entering the step (8); otherwise, go to step (10);
(10) Determining whether the output voltage Vout reaches the rated output voltage value Vo? If yes, entering a step (13), and ending the soft start; otherwise, entering the step (11);
(11) Judging whether the driving signal frequency f meets f less than or equal to fr, if yes, entering a step (13), and ending the soft start; otherwise, re-entering step (8);
(12) Judging whether the output voltage Vout reaches a rated output voltage value Vo, if so, entering a step (13); otherwise, re-entering step (8);
(13) And (5) ending the soft start.
The invention adopts a PWM and PFM composite modulation method, takes the rising rate of the output voltage of the resonant converter as a control target, and performs closed-loop control on soft start, so that the rising of the output voltage is stable in the starting process, the charging current impact of the output capacitor is small, the surge current impact of the resonant cavity is effectively restrained, and the soft start process is more stable and reliable. The whole soft start process of the invention is controlled by the digital controller, and the starting requirements of different resonant converters can be met by flexibly setting the upper limit of the resonant current and the rising rate of the output voltage, thereby improving the applicability of the invention.
Drawings
FIG. 1 is a block diagram of an LLC resonant converter;
fig. 2 is a schematic diagram of a half-bridge LLC resonant converter.
Fig. 3 is a soft start flow chart of an LLC resonant converter.
FIG. 4 is a waveform of a soft start resonant current with a start time of 30ms
FIG. 5 is a waveform of a soft start output voltage with a start time of 30ms
FIG. 6 is a waveform diagram of a soft start resonant current with a start time of 15ms
FIG. 7 is a waveform of a soft start output voltage with a start time of 15ms
Examples of the embodiments
Fig. 2 is a schematic diagram of an LLC resonant half-bridge converter, and a technical scheme in an embodiment of the invention will be described with reference to fig. 2.
The resonant converter parameters are: rated input voltage: 220VAC; an output voltage of 48V; rated output power 1000W. The resonant frequency was 65KHz. Setting the initial starting frequency of the resonant converter to be 2 times the resonant frequency, i.e. f start =130 KHz, setting the soft start initial duty cycle to D start =0.05, set the duty cycle maximum value D max =0.45, set the maximum allowable value I of the resonant current r_th =15a. Two different starting times are respectively set below to implement soft start for the resonant converter, and verification is carried out.
(1) Setting the starting time to be 30ms, namely the time for the output voltage of the resonant converter to rise from zero to 48V for 30ms, completing the establishment of the output voltage, ending the soft start, and setting the rising rate of the output voltage of the converter to be U rise ＝1.6V/ms。
Soft start is performed on a resonant LLC resonant half-bridge converter according to the LLC resonant converter soft start flowchart shown in fig. 3. Fig. 4 is a waveform diagram of the resonant current during soft start with a start time of 30ms, and it can be seen from the diagram that the time required for soft start is 30ms, the resonant current steadily rises during start, the maximum value of the resonant current is about 13.5A, and the maximum allowable value of the resonant current of the resonant converter is not exceeded by 15A. Fig. 5 is a waveform diagram of the output voltage in the soft start process with the start time of 30ms, and it can be seen that the output voltage of the resonant converter rises steadily during start, and the output voltage reaches 48V to complete soft start at 30ms, so as to enter a normal working state.
(2) Setting the starting time to be 15ms, namely the output voltage of the resonant converter rises from zero to 48V for 15ms, completing the establishment of the output voltage, ending the soft start, and setting the rising rate of the output voltage of the converter to be U rise ＝3.2V/ms。
Fig. 6 is a waveform diagram of the resonance current during soft start with a start time of 15ms, and it can be seen from the figure that the time required for soft start is 15ms, the resonance current steadily rises during start, the maximum value of the resonance current is close to 15A, and the resonance current is limited within the range of the maximum allowable value 15A of the resonance current. Fig. 7 is a waveform diagram of the output voltage in the soft start process with the start time of 15ms, and it can be seen that the output voltage of the resonant converter rises steadily during start, and the output voltage reaches 48V to complete soft start at 15ms, so as to enter a normal working state.
It can be seen that soft start can be completed by adopting different start time, and the shorter the start time is, the larger the peak value of the resonance current is during start. Therefore, when soft start is implemented, a proper start time can be set according to the requirement of the resonant current when the resonant converter starts.
When the invention is adopted for soft start, surge current impact can be limited to be within an allowable range, output voltage rises stably and rapidly, the starting process is stable and reliable, and the invention has higher engineering application value.
Claims (2)
1. A composite modulation closed-loop control soft start method suitable for a resonant converter is characterized by comprising the following steps:
(1) According to the output voltage rating U of the resonant converter o And soft start time T soft Index, calculating the output voltage rising rate reference value S rise ：S rise ＝U o /T soft ；
(2) The system starts up and the controller first operates in PWM (pulse width modulation) soft start mode. The control circuit outputs two paths of complementary driving signals to drive the switching tube of the full-bridge or half-bridge switching circuit to work; the driving signal starts the initial frequency f start Higher than the resonant frequency f of the resonant converter r ；
(3) In the PWM soft start mode, the control circuit maintains the driving signal frequency at f start Unchanged, the duty cycle is initially D start And outputting a driving signal.
(4) Reading the output voltage value Uout of the control circuit, and calculating the current output voltage rising rate U rise And according to U rise And preset S rise Adjusting the duty ratio D of the driving signal by adopting a PID control algorithm, outputting a complementary driving signal for closed-loop control, and keeping the frequency of the driving signal at f start The resonance current value Ir is read at the same time without change;
(5) Judging whether the resonance current value Ir is larger than a preset maximum resonance current threshold Ir_th, if so, reducing the duty ratio D of the driving signal, and re-entering the step (4); otherwise, entering step (6);
(6) Judging whether the duty ratio D is equal to a preset maximum duty ratio Dmax, if so, entering a step (7); otherwise, go to step (12);
(7) Entering into PFM soft start mode, the control circuit keeps the duty ratio of the driving signal as D max Unchanged, the initial value of the frequency is f start Outputting a driving signal;
(8) Reading the output voltage value Uout of the control circuit, and calculating the current output voltage rising rate U rise And according to U rise And preset S rise Adjusting the frequency f of a driving signal by adopting a PID control algorithm, outputting a complementary driving signal for closed-loop control, keeping the duty ratio Dmax unchanged, and simultaneously reading a resonant current value Ir;
(9) Judging whether the resonance current value Ir is larger than a preset maximum resonance current threshold Ir_th, if so, increasing the driving signal frequency f, and re-entering the step (8); otherwise, go to step (10);
(10) Judging whether the output voltage Vout reaches a rated output voltage value Vo, if so, entering a step (13), and ending the soft start; otherwise, entering the step (11);
(11) Judging whether the driving signal frequency f meets f less than or equal to fr, if yes, entering a step (13), and ending the soft start;
otherwise, re-entering step (8);
(12) Judging whether the output voltage Vout reaches a rated output voltage value Vo, if so, entering a step (13);
otherwise, re-entering step (8);
(13) And (5) ending the soft start.
2. A complex modulated closed loop control soft start method for a resonant converter as defined in claim 1 wherein said step (2) starts an initial frequency f start The value is 1.5f t ～3f t Between, an initial duty cycle D start Take a value between 0 and 0.2.
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN117277787A (en) * | 2023-09-25 | 2023-12-22 | 苏州博沃创新能源科技有限公司 | High-voltage wide-voltage-range input power DC-DC converter and control method |
| CN117713564A (en) * | 2024-02-06 | 2024-03-15 | 惠州市天宝创能科技有限公司 | LLC resonance wide-range voltage output control method and control circuit |
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN117277787A (en) * | 2023-09-25 | 2023-12-22 | 苏州博沃创新能源科技有限公司 | High-voltage wide-voltage-range input power DC-DC converter and control method |
| CN117713564A (en) * | 2024-02-06 | 2024-03-15 | 惠州市天宝创能科技有限公司 | LLC resonance wide-range voltage output control method and control circuit |
| CN117713564B (en) * | 2024-02-06 | 2024-04-23 | 惠州市天宝创能科技有限公司 | LLC resonance wide-range voltage output control method and control circuit |
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