A composite modulation closed-loop control soft start method suitable for resonant converters

Abstract

The present invention discloses a soft start control method for an LLC resonant converter, and relates to the technical field of resonant converters. The present invention adopts a PWM and PFM composite modulation method, takes the output voltage rise rate of the resonant converter as a control target, and performs closed-loop control on the soft start, so that the output voltage rises steadily during the start-up process, the output capacitor charging current impact is small, and the surge current impact of the resonant cavity is effectively suppressed, and the soft start process is more stable and reliable. The entire soft start process of the present invention is controlled by a digital controller, and the start-up requirements of different resonant converters can be met by flexibly setting the upper limit of the resonant current and the output voltage rise rate, thereby improving the applicability of the present invention.

Description

[0001] Field

[0002] The present invention relates to the technical field of resonant converters, and in particular to a soft start control method for an LLC resonant converter. Prior art

[0003] The LLC resonant converter can achieve zero voltage conduction of the switch tube and zero current shutdown of the rectifier diode within a wide load range. It has the advantages of soft switching, high efficiency and wide output voltage range. It is a converter with superior performance and is widely used. Usually, the LLC resonant converter is equipped with a large-capacity output capacitor at the output end to filter out the switch current ripple and reduce the 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 converter transformer. The primary voltage of the transformer is clamped to zero, and the impedance of the resonant cavity is very small at this time. When the converter is started, the capacitor must be charged, so the resonant cavity of the converter will generate a large surge current, which will cause the power device to bear a large current stress and voltage stress, causing damage to the power device and causing converter failure, or triggering overcurrent protection to affect the normal operation of the resonant converter. Soft starting the resonant converter is a common measure to solve the above problems.

[0004] At present, the main soft start methods of LLC resonant converter are:

[0005] (1) Frequency reduction soft start. The initial operating frequency of the soft start is relatively high, which is convenient for reducing the voltage and current stress of the resonant cavity. Then the operating frequency is gradually reduced until the output voltage is established to end the soft start. The advantage of this method is that it is simple to implement. The disadvantage is that since the maximum switching frequency of the resonant converter is limited by components, it is difficult to achieve a too high starting frequency and the inrush current at startup cannot be well suppressed.

[0006] (2) Pulse width modulation control soft start. Keep the switching frequency unchanged and gradually increase the duty cycle from a small initial value to 50% until the output voltage is established to achieve soft start. The advantages are that soft start is easy to implement and the inrush current suppression effect is more obvious. The disadvantage is that the soft start time is long.

[0007] (3) Phase-shift soft start. Phase-shift control can be regarded as a pulse width modulation control method. The duty cycle of all switches is kept at 0.5, and the starting current is limited by gradually increasing the phase shift angle of the two bridge arms from 0° to 180°. Phase-shift start is only applicable to full-bridge LLC resonant converters, and cannot be applied to half-bridge LLC resonant converters. It has poor versatility.

[0008] (4) Hybrid control soft start. This includes hybrid control soft start of pulse width modulation and pulse frequency modulation, and hybrid control soft start of phase shift control and pulse frequency modulation. Hybrid control soft start of pulse width modulation and pulse frequency modulation can suppress the surge current peak during startup to a certain extent, but the control process is too complicated and difficult to promote and implement. Hybrid control soft start of phase shift control and pulse frequency modulation can limit the current surge, but during the phase shift startup process, the circuit works in a hard switching state, resulting in large losses, and phase shift control is only applicable to full-bridge circuits, limiting its scope of use. Summary of the invention

[0009] In view of the deficiencies in the prior art, the present invention provides a composite modulation closed-loop control starting method consisting of pulse width modulation and pulse frequency modulation. This method can effectively suppress the surge current impact when the LLC resonant converter is started, and the output voltage rises smoothly and quickly, making the resonant converter startup process stable and reliable, and has high engineering use value.

[0010] The present invention provides a soft start control method for an LLC resonant converter. The resonant converter is an LLC full-bridge resonant converter or an LLC half-bridge resonant converter, comprising a full-bridge or half-bridge switching circuit, an LLC resonant circuit, a rectifier filter circuit, an output voltage and current detection circuit, a resonant circuit detection circuit, a control circuit (MCU) and a drive circuit. During the soft start period, the control circuit (the controller can be a DSP, a single-chip microcomputer, an ARM embedded processor or an FPGA programmable chip) detects the output voltage of the LLC resonant converter, and generates a signal of a certain duty cycle or switching frequency according to the rising speed of the output voltage to perform a switching operation on the switch tube of the full-bridge (or half-bridge) power conversion circuit, so that the resonant current of the LLC resonant converter when starting is kept within an acceptable range, the output voltage is established quickly and stably, and the soft start of the LLC resonant converter is realized.

[0011] The specific technical solution is: a composite modulation closed-loop control soft start method suitable for a resonant converter, comprising the following steps:

[0012] (1) According to the output voltage rating U of the resonant converter o and soft start time T soft Indicator, calculate the output voltage rise rate reference value S rise :S rise =U o / T soft .

[0013] (2) When the system starts, the controller first operates in the PWM (pulse width modulation) soft start mode. The control circuit outputs two complementary drive signals to drive the switch tubes of the full-bridge or half-bridge switch circuit to work. The drive signal starts with an initial frequency f startHigher than the resonant converter resonant frequency f r Generally, the initial starting frequency f start The value is 1.5f r ~3f r The initial duty cycle D start The value is between 0-0.2.

[0014] (3) In PWM soft start mode, the control circuit maintains the drive signal frequency at f start The initial value of the duty cycle is D start Output drive signal.

[0015] (4) Read the output voltage value Uout of the control circuit and calculate the current output voltage rise rate Uout rise , and according to U rise and the preset S rise The PID control algorithm is used to adjust the duty cycle D of the drive signal, output a complementary drive signal for closed-loop control, and maintain the drive signal frequency at f start unchanged, and read the resonant current value Ir at the same time;

[0016] (5) Determine whether the resonant current value Ir is greater than a preset resonant current maximum threshold value Ir_th. If so, reduce the duty cycle D of the drive signal and re-enter step (4). Otherwise, enter step (6);

[0017] (6) Determine whether the duty cycle D is equal to the preset maximum duty cycle Dmax. If so, proceed to step (7); otherwise, proceed to step (12).

[0018] (7) Entering PFM soft start mode, the control circuit maintains the duty cycle of the drive signal at D max The initial frequency is f. start Output drive signal.

[0019] (8) Read the output voltage value Uout of the control circuit and calculate the current output voltage rise rate Uout rise , and according to U rise and the preset S rise The PID control algorithm is used to adjust the drive signal frequency f, output a complementary drive signal for closed-loop control, and keep the duty cycle Dmax unchanged, while reading the resonant current value Ir;

[0020] (9) Determine whether the resonant current value Ir is greater than a preset resonant current maximum threshold value Ir_th. If so, increase the drive signal frequency f and re-enter step (8); otherwise, enter step (10);

[0021] (10) Determine whether the output voltage Vout reaches the rated output voltage value Vo? If yes, proceed to step (13) and the soft start ends; otherwise, proceed to step (11);

[0022] (11) Determine whether the drive signal frequency f satisfies f≤fr. If so, proceed to step (13) and the soft start ends; otherwise, re-enter step (8);

[0023] (12) Determine whether the output voltage Vout reaches the rated output voltage value Vo. If so, proceed to step (13); otherwise, re-enter step (8);

[0024] (13) Soft start ends.

[0025] The present invention adopts a PWM and PFM composite modulation method, takes the output voltage rise rate of the resonant converter as the control target, and performs closed-loop control on the soft start, so that the output voltage rises steadily during the start-up process, the output capacitor charging current impact is small, and the surge current impact of the resonant cavity is effectively suppressed, and the soft start process is more stable and reliable. The entire soft start process of the present invention is controlled by a digital controller, and the start-up requirements of different resonant converters can be met by flexibly setting the upper limit of the resonant current and the output voltage rise rate, thereby improving the applicability of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is the structure diagram of LLC resonant converter;

[0027] Figure 2 This is the schematic diagram of a half-bridge LLC resonant converter.

[0028] Figure 3 It is the LLC resonant converter soft start flow chart.

[0029] Figure 4 This is the soft start resonant current waveform with a start time of 30ms

[0030] Figure 5 This is the soft start output voltage waveform with a start time of 30ms

[0031] Figure 6 This is the soft start resonant current waveform with a start time of 15ms

[0032] Figure 7 This is the soft start output voltage waveform with a start time of 15ms

[0033] Implementation Examples

[0034] Figure 2 This is the LLC resonant half-bridge converter schematic diagram. Figure 2 The technical solution in the embodiment of the present invention is described.

[0035] The parameters of the resonant converter are: rated input voltage: 220VAC; output voltage 48V; rated output power 1000W. The resonant frequency is 65KHz. Set the initial starting frequency of the resonant converter to 2 times the resonant frequency, that is, f start =130KHz, set the initial soft start duty cycle to D start =0.05, set the maximum duty cycle D max =0.45, set the maximum allowable value of the resonant current I r_th = 15A. The following sets two different start-up times to implement soft start of the resonant converter for verification.

[0036] (1) Set the start time to 30ms, that is, the resonant converter output voltage takes 30ms to rise from zero to 48V, completing the output voltage establishment and ending the soft start. At this time, the rise rate of the converter output voltage is U rise =1.6V / ms.

[0037] according to Figure 3 The LLC resonant converter soft start flow chart shown implements soft start on the resonant LLC resonant half-bridge converter. Figure 4 This is the resonant current waveform during the soft start process with a start time of 30ms. It can be seen from the figure that the soft start time is 30ms. The resonant current rises steadily during the start process. The maximum resonant current is about 13.5A, which does not exceed the maximum allowable value of the resonant current of the resonant converter of 15A. Figure 5 This is the output voltage waveform during the soft start process with a start time of 30ms. It can be seen that the output voltage of the resonant converter rises smoothly during startup. At 30ms, the output voltage reaches 48V to complete the soft start and enter normal working state.

[0038] (2) Set the start time to 15ms, that is, the resonant converter output voltage takes 15ms to rise from zero to 48V, completing the output voltage establishment and ending the soft start. At this time, the rise rate of the converter output voltage is U rise =3.2V / ms.

[0039] Figure 6 This is the resonant current waveform during the soft start process with a start time of 15ms. It can be seen from the figure that the time required for soft start is 15ms. During the start process, the resonant current rises steadily, and the maximum resonant current is close to 15A, which is limited to the maximum allowable value of the resonant current of 15A. Figure 7 This is the output voltage waveform during the soft start process with a start time of 15ms. It can be seen that the output voltage of the resonant converter rises smoothly during startup. At 15ms, the output voltage reaches 48V to complete the soft start and enter normal working state.

[0040] It can be seen that soft start can be completed by taking different start times. The shorter the start time, the greater the peak value of the resonant current at start-up. Therefore, when implementing soft start, a suitable start time can be set according to the requirements of the resonant current at start-up of the resonant converter.

[0041] When the patented invention is used for soft starting, the surge current impact can be limited to an allowable range, the output voltage rises smoothly and quickly, the starting process is stable and reliable, and it has high engineering application value.

Claims

1. A composite modulation closed-loop control soft start method suitable for a resonant converter, characterized in that: The steps include: (1) According to the output voltage rating U of the resonant converter o and soft start time T soft Indicator, calculate the output voltage rise rate reference value S rise :S rise =U o / T soft ; (2) When the system starts, the controller first operates in the PWM (pulse width modulation) soft start mode. The control circuit outputs two complementary drive signals to drive the switch tubes of the full-bridge or half-bridge switch circuit to work; the drive signal starts with an initial frequency f start Higher than the resonant converter resonant frequency f r ; (3) In PWM soft start mode, the control circuit maintains the drive signal frequency at f start The initial value of the duty cycle is D start Output drive signal; (4) Read the output voltage value Uout of the control circuit and calculate the current output voltage rise rate Uout rise , and according to U rise and the preset S rise The PID control algorithm is used to adjust the duty cycle D of the drive signal, output a complementary drive signal for closed-loop control, and maintain the drive signal frequency at f start unchanged, and read the resonant current value Ir at the same time; (5) Determine whether the resonant current value Ir is greater than the preset resonant current maximum threshold value Ir_th. If so, reduce the duty cycle D of the drive signal and re-enter step (4); otherwise, enter step (6); (6) Determine whether the duty cycle D is equal to the preset maximum duty cycle Dmax. If so, proceed to step (7); otherwise, proceed to step (12); (7) Entering PFM soft start mode, the control circuit maintains the duty cycle of the drive signal at D max The initial frequency is f. start Output drive signal; (8) Read the output voltage value Uout of the control circuit and calculate the current output voltage rise rate Uout rise , and according to U rise and the preset S rise The PID control algorithm is used to adjust the drive signal frequency f, output a complementary drive signal for closed-loop control, and keep the duty cycle Dmax unchanged, while reading the resonant current value Ir; (9) Determine whether the resonant current value Ir is greater than a preset resonant current maximum threshold value Ir_th. If so, increase the drive signal frequency f and re-enter step (8); otherwise, enter step (10); (10) Determine whether the output voltage Vout reaches the rated output voltage value Vo. If so, proceed to step (13) and the soft start ends; otherwise, proceed to step (11); (11) Determine whether the driving signal frequency f satisfies f≤fr. If so, proceed to step (13) and the soft start ends. Otherwise, re-enter step (8); (12) Determine whether the output voltage Vout reaches the rated output voltage value Vo. If yes, proceed to step (13); Otherwise, re-enter step (8); (13) Soft start ends.

2. A composite modulation closed-loop control soft start method suitable for a resonant converter as claimed in claim 1, characterized in that: In step (2), the initial frequency f is started start The value is 1.5f t ~3f t The initial duty cycle D start The value is between 0-0.2.

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