LLC resonant power supply soft start method based on pre-charging mode

Abstract

An LLC resonant power supply soft start method based on a pre-charging mode comprises the steps that a filter capacitor Co of a filter part of an LLC resonant power supply is pre-charged, a pre-charging power supply uses a direct-current power supply which is the same as an input power supply on the direct-current side of the LLC resonant power supply, a switching tube PWM of a chopper circuit of the LLC resonant power supply does not work, a rectifying circuit of the LLC resonant power supply is in one-way conduction, and the direct-current side of the LLC resonant power supply is in one-way conduction. When the filter capacitor Co is charged to be close to the rated voltage Voref of the load, pre-charging is ended; a switching tube PWM of the chopper circuit starts to work, and an LLC resonant power supply is started; as the filter capacitor Co is pre-charged, the input impedance is not changed, the voltage acting on the resonant inductor is reduced to half of the original voltage, and the impact current is reduced compared with the direct starting of the LLC resonant power supply; according to the invention, through the LLC resonant power supply pre-charging mode, compared with a traditional series connection resistor starting mode, access of a resistor with a large resistance value is omitted, and loss is avoided; on the premise of not increasing the starting time, the impulse current of the resonant cavity and the impulse voltage on the capacitor Cr are more effectively reduced, and the method is simple and easy to implement.

Description

Technical Field

[0001] The invention belongs to the technical field of DC-DC conversion, and in particular relates to a soft starting method of an LLC resonant power supply based on a pre-charging mode. Background Art

[0002] LLC resonant power supply is widely used in distributed communication power supply. It has the advantages of small number of components, ZVS turn-on of primary side switch tube, ZCS turn-off of secondary side rectifier switch tube, ability to maintain output voltage stability when input voltage fluctuates, small operating frequency adjustment range, higher energy utilization rate and lower space occupancy ratio at the same output power. It is an extremely attractive DC-DC power supply topology.

[0003] However, in actual use, LLC resonant power supplies will also face the problem of starting current overshoot. Traditional converter soft start methods are generally divided into PWM start, APWM start, PFM start, series resistance start, etc., but the converter gain change during startup is inversely proportional to the startup time, that is, the startup current overshoot and startup time cannot be taken into account at the same time; in recent years, nonlinear control methods such as trajectory control and dead time control have been developed to suppress the startup current overshoot, and good results have been achieved in the startup current overshoot and startup time, but the soft start control algorithm is relatively complex, the amount of calculation is large, and the hardware computing power requirements are high.

[0004] Aiming at the current overshoot problem at the startup of LLC resonant converter, Li Guangyao, Shao Lihuan, Zheng Changfeng. Research on linear compensation soft start control of LLC resonant converter [J]. Measurement and Control Technology, 2023, 42(08): 89-93. DOI: 10.19708 / j.ckjs.2022.09.302., according to the principle of LLC resonant converter, the frequency regulation (PFM) gain and duty cycle regulation (PWM) gain are analyzed, and a linear compensation soft start control strategy for LLC resonant converter is proposed by using the relationship between frequency regulation gain, duty cycle regulation gain and hybrid regulation total gain. This strategy designs the duty cycle change function according to the gain characteristics during duty cycle regulation, designs the frequency change function according to the hybrid regulation total gain relationship obtained by the above analysis, and uses the frequency regulation gain to linearly compensate the duty cycle regulation gain, so that the output total gain can tend to change linearly and suppress the startup current overshoot. However, this method requires the use of simulation and mathematical fitting tools to design the duty cycle change function and frequency change function respectively, and it is necessary to ensure that the total gain change function of the mixed regulation changes linearly. The design process is relatively complicated, and the actual implementation of the soft start control strategy is relatively complicated, and frequency modulation and duty cycle adjustment need to be performed simultaneously. Summary of the invention

[0005] In order to overcome the defects of the above-mentioned prior art, the purpose of the present invention is to provide a soft-start method of LLC resonant power supply based on pre-charging mode. By pre-charging mode of LLC resonant power supply, the connection of a larger resistance is omitted compared with the traditional series resistance starting mode, thus avoiding loss; without increasing the starting time, the resonant cavity impact current and the capacitor C are more effectively reduced. r The surge voltage is increased to suppress the overshoot of the LLC resonant power supply startup current and voltage.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for soft starting an LLC resonant power supply based on a pre-charging method, the specific steps are as follows:

[0008] Step 1: The filter capacitor C of the LLC resonant power supply filter part o Pre-charging is performed, and the pre-charging power supply uses the same DC power supply as the DC side input power supply of the LLC resonant power supply. The switch tube PWM of the chopper circuit of the LLC resonant power supply does not work, and the rectifier circuit of the LLC resonant power supply is unidirectionally conducted;

[0009] Step 2: In step 1, the filter capacitor C of the LLC resonant power supply o After pre-charging, the switch tube PWM of the chopper circuit of the LLC resonant power supply starts to work, starting the LLC resonant power supply.

[0010] The filter capacitor C of the filter part of the LLC resonant power supply in step 1 is o The specific method for precharging is:

[0011] The switch tube PWM of the chopper circuit is turned off, the rectifier circuit of the LLC resonant power supply is unidirectionally conducted, and the pre-charge power supply pre-charges the load side of the LLC resonant power supply. o Charge to near the load rated voltage V oref When the pre-charge is completed, the filter capacitor C in the equivalent circuit of LLC resonant power supply o The upper voltage variation law is:

[0012]

[0013] Among them, U dc It is a pre-charged DC power supply, which is consistent with the DC side power supply of LLC resonant power supply. o is the internal resistance of LLC resonant power supply; C o is the filter capacitor of LLC resonant power supply, R L It is the load of LLC resonant power supply;

[0014] The filter capacitor C of LLC resonant power supply oCharged to a load voltage close to the LLC resonant power supply rated voltage V oref The time is:

[0015]

[0016] Pre-charged DC side power supply U dc The magnitude is the load rated voltage V oref 100 times, formula (13) is close to zero, and the pre-charge time t C Ignore.

[0017] The specific method of step 2 is:

[0018] The switch tube PWM of the chopper circuit of the LLC resonant power supply is turned on to start the LLC resonant power supply. When the LLC resonant power supply is started, the output capacitor voltage is not zero, and the initial voltage u of the filter capacitor Co is C (0) Load rated voltage V close to LLC resonant power supply oref :

[0019] u C (0)≈V oref ≠0 (14)

[0020] The PWM switching frequency of the switching tube in the chopper circuit of the LLC resonant power supply is close to the resonant frequency, so:

[0021]

[0022] LLC resonant power supply load voltage V o Close to the load rated voltage V oref ,have:

[0023]

[0024] When the LLC resonant power supply starts, the output capacitor voltage is not zero and is regarded as a voltage source UC. After conversion to the primary side, the DC side power supply Udc of the LLC resonant power supply is connected in series with the voltage source UC as the input power supply, and the polarity is opposite; at this time:

[0025]

[0026] The current meets:

[0027]

[0028] The input impedance remains unchanged, and the voltage applied to the resonant inductor is reduced to half of its original value.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] The present invention designs a soft-start method for LLC resonant power supply based on pre-charging mode. The specific technical scheme includes two links: first, in the pre-charging stage, the switch tube PWM of the chopper circuit does not work. Since the rectifier circuit is unidirectional, only the load side pre-charging circuit works. When the filter capacitor C o Charge to near the load rated voltage V oref When the switching tube PWM of the chopper circuit starts to work, the LLC resonant power supply is started. o Pre-charging is performed, the input impedance remains unchanged, and the voltage applied to the resonant inductor is reduced to half of the original voltage. Compared with directly starting the LLC resonant power supply, the inrush current is reduced.

[0031] The present invention uses the LLC resonant power supply pre-charging method, which saves the connection of a larger resistance than the traditional series resistance starting method, thus avoiding loss; without increasing the starting time, it more effectively reduces the resonant cavity impact current and the capacitance C r The method of applying impulse voltage is simple and easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is the overall structure diagram of the technical solution adopted by the present invention;

[0033] Figure 2 This is the LLC power supply topology diagram;

[0034] Figure 3 It is the relationship diagram between gain K and normalized frequency Fx;

[0035] Figure 4 It is the equivalent circuit after the LLC power supply is started up, from the secondary side to the primary side;

[0036] Figure 5 This is the LLC power supply equivalent circuit at time 0;

[0037] Figure 6 Without taking other measures, the LLC power supply directly starts the resonant cavity impulse current waveform;

[0038] Figure 7 Without taking other measures, LLC power supply directly starts the resonant capacitor Cr to impact voltage waveform;

[0039] Figure 8 This is the LLC power supply series resistance starting resonant cavity impulse current waveform;

[0040] Fig. 9 It is the impulse voltage waveform of LLC power supply series resistance starting resonant capacitor Cr;

[0041] Fig.10 It is the pre-charging stage equivalent circuit of the present invention;

[0042] Fig.11 It is the equivalent circuit of the LLC power supply startup stage of the present invention;

[0043] Fig.12 It is the waveform of the resonant cavity impulse current after adopting the present invention;

[0044] Fig.13 This is the waveform of the impulse voltage on the capacitor Cr after the present invention is adopted. DETAILED DESCRIPTION

[0045] The present invention is described in detail below with reference to the accompanying drawings.

[0046] Classic LLC resonant power supply such as Figure 2 As shown, it includes a DC power supply, a chopper circuit, a resonant cavity, a transformer, a rectifier circuit, a filter circuit (filter capacitor C o ) and load (Z L ); wherein, the DC power supply is used as the power input part of the LLC resonant circuit; the chopper circuit input is connected to the DC power supply, and the output alternating voltage is controlled by the switch tube PWM; the resonant cavity receives the alternating voltage output by the chopper circuit and transmits power in the form of alternating current; the transformer part receives the alternating current output by the resonant cavity and can convert the alternating voltage to a level suitable for the load demand; the rectifier circuit is responsible for converting the transformed alternating current into direct current; the filter circuit filters out the alternating components in the direct current to obtain a stable direct current to supply the load.

[0047] The control method of the classic LLC resonant power supply is PFM (pulse frequency) control, and the output voltage V o The relationship with the control pulse signal frequency of the IGBT is:

[0048]

[0049] in:

[0050]

[0051]

[0052] From formula (1), we can see that the gain K is related to the quality factor Q, the inductance ratio m and the normalized frequency F. x After the circuit design is completed, Q, m, etc. remain unchanged, so the normalized frequency F can be controlled x (i.e. IGBT switching frequency) controls the gain K, thereby adjusting the output voltage V o Specifically, from Figure 3It can be seen that when the inductance ratio m is fixed, the quality factor Q increases and the peak value of the gain K curve decreases; when the normalized frequency Fx=1, the gain K=1; near the normalized frequency Fx=1, the gain K curves corresponding to different quality factors Q are close to each other, and the rate of change of gain K with Fx is negative.

[0053] When the LLC resonant power supply is started directly, its equivalent circuit is as follows: Figure 4 Shown: DC power supply U dc Connect the switch K1, the resonant cavity resistor R in series, and then connect the resonant capacitor C in series r , resonant inductor L r , and then connect the excitation inductor L in series m With C * o , Z * L The parallel branch of C * o , Z * L The filter capacitor C o 、Load Z L The equivalent capacitance and impedance after the secondary side is converted to the primary side are as follows according to the circuit principle:

[0054] u C (0)=0,i C (0)≠0 (8)

[0055] Among them, u C (0) is the filter capacitor C at time 0 o Or resonant capacitor C r Voltage across the terminals, i C (0) is the filter capacitor C at time 0 o Or resonant capacitor C r Based on this, the circuit at time 0 is equivalent to Figure 5 In the form of: DC power supply U dc Connect the switch K1, the resonant cavity resistor R in series, and then connect the resonant inductor L in series r . Generally f r The design is relatively large (kHz~MHz). According to formula (6), Lr is generally very small, and the resonant inductance L r The input impedance satisfies:

[0056] Z in =|R+jωL r |<<1 (9)

[0057] The equivalent circuit input voltage at time 0 is U dc , the current satisfies:

[0058]

[0059] like Figure 1 As shown, a soft start method of LLC resonant power supply based on pre-charging mode is composed of two parts: LLC resonant power supply and pre-charging circuit. The specific steps are as follows:

[0060] Step 1: Use the same DC power supply as the LLC resonant power supply DC side input power supply for the pre-charge power supply, and adjust the filter capacitor C o In the pre-charging stage, switch K1 is opened and K2 is closed. The switch tube PWM of the chopper circuit does not work. Since the rectifier circuit is unidirectional, only the load side performs the pre-charging circuit U dc -R o -K2-C o / / Z L Working, when the filter capacitor C o Charge to near the load rated voltage V oref When switch K2 is turned off, the equivalent circuit is as follows Fig.10 Shown: DC power supply U dc Series power supply internal resistance R o , then connect the filter capacitor C in series o With load Z L When the filter capacitor C o Charge to near the load rated voltage V oref When the pre-charge is completed, the filter capacitor C o The upper voltage variation law is:

[0061]

[0062] Among them, U dc It is a pre-charged DC power supply, which is consistent with the DC side power supply of LLC resonant power supply. o is the internal resistance of the power supply; C o is the filter capacitor, R L is the load resistance; the filter capacitor C o Charge to near the load rated voltage V oref The time is:

[0063]

[0064] The DC charging power supply uses the same power supply as the LLC input, and the pre-charging DC power supply U dc The magnitude is the load rated voltage V oref 100 times, (13) is close to zero, and the pre-charge time t C Ignore;

[0065] Step 2, open switch K2, close switch K1, the switch tube PWM of the chopper circuit starts to work, and the LLC resonant power supply is started;

[0066] Based on the filter capacitor C of LLC resonant power supply in step 1, o After pre-charging, when the LLC resonant power supply starts, the output capacitor voltage is not zero, so the initial voltage u of the filter capacitor Co is C (0) Load rated voltage V close to LLC resonant power supply oref :

[0067] u C (0)≈V oref ≠0 (14)

[0068] The PWM switching frequency of the chopper circuit switch tube of the LLC resonant power supply is close to the resonant frequency, so:

[0069]

[0070] LLC resonant power supply load voltage V o Close to the load rated voltage V oref ,have:

[0071]

[0072] When the LLC resonant power supply starts, the output capacitor voltage is not zero and is regarded as a voltage source U C , after conversion to the original side, the equivalent circuit is as follows Fig.11 As shown: switch K1, resonant cavity resistance R, resonant inductor L r Series, LLC resonant power supply DC side power supply U dc With voltage source U C The series connection is used as the input power supply, and the polarity is opposite. At this time:

[0073]

[0074] The current meets:

[0075]

[0076] The input impedance remains unchanged, and the voltage applied to the resonant inductor is reduced to half of the original value. Compared with directly starting the LLC resonant power supply, the inrush current is reduced.

[0077] Directly start the LLC resonant power supply without taking other measures, the resonant cavity current will quickly reach a very large value, generally reaching dozens or even hundreds of times the rated current amplitude. Such a large impact current will break down the switch device, burn capacitors and other components, damage the circuit system or trigger circuit protection. Take 380V input, 60W, 12V output LLC (switching frequency 100kHz) as an example, the typical impact current waveform is as follows Figure 6 As shown, the normal current amplitude is about 1.5A, and the inrush current at startup reaches about 26A, and the ratio of the two reaches 17.3.

[0078] The rapidly increasing starting current will r A large amount of charge accumulates rapidly on the electrode, causing the voltage at both ends to increase rapidly, which can reach several to dozens of times the rated voltage. The voltage at both ends satisfies the following relationship:

[0079]

[0080] Typical resonant capacitor C r The impulse voltage waveform is as follows Figure 7 As shown, the normal voltage amplitude is about 260V, and the impulse voltage at startup reaches about 1500V, and the ratio of the two reaches 5.7.

[0081] In order to solve the above inrush current problem, a relatively simple method is to directly connect a certain resistance (50Ω) in series during startup, and cut off the resistance after a period of time, thereby increasing Z in formula (9) in , under the condition that the power supply voltage remains unchanged, the current, the impact current of the resonant cavity, and the impact voltage on the capacitor Cr are reduced. The effect is as follows Figure 8 , Fig. 9 As shown, the impact current and voltage dropped to 8A and 440V, respectively, which are 5.3 times and 1.7 times of the rated values, respectively. The impact current was reduced, but still exceeded the rated value by several times. At the same time, the startup time increased from 0.4ms to 1.2ms.

[0082] In order to more effectively reduce the starting impact current and voltage and shorten the starting time, the present invention adopts an LLC resonant power supply soft starting method based on a pre-charging mode.

[0083] First, the pre-charging stage is carried out. At this time, switch K1 is disconnected and switch K2 is closed. The switch tube PWM of the chopper circuit does not work, and the rectifier part is unidirectionally conducted. Only the load side pre-charging circuit U dc -R o -K2-C o / / Z L The circuit works, when the filter capacitor C o Charge to near V oref =12V, disconnect K2. The filter capacitor C used o The DC charging power supply uses the same power supply as the LLC input, namely U dc =380V, the theoretical charging time is 4×10 -6 s, which is much smaller than 0.4ms, so the time consumed for charging can be ignored.

[0084] Next, switch K2 is opened and switch K1 is closed, and the switch tube PWM of the chopper circuit directly starts LLC. oPre-charging is performed, the input impedance remains unchanged, the applied voltage is reduced to half of the original, and the impact current is reduced. The impact current of the resonant cavity and the impact voltage waveform on the capacitor Cr are as follows: Fig.12 , 13 As shown, the impulse current is 3.6A and the impulse voltage is 420V, which are 2.4 times and 1.6 times the rated values ​​respectively. The starting time is about 0.3ms, which is 1 / 4 of the starting time when a resistor is directly connected in series.

Claims

1. A soft-start method for LLC resonant power supply based on pre-charging mode, the specific steps are as follows: Step 1: The filter capacitor C of the LLC resonant power supply filter part o Pre-charging is performed, and the pre-charging power supply uses the same DC power supply as the DC side input power supply of the LLC resonant power supply. The switch tube PWM of the chopper circuit of the LLC resonant power supply does not work, and the rectifier circuit of the LLC resonant power supply is unidirectionally conducted; Step 2: In step 1, the filter capacitor C of the LLC resonant power supply o After pre-charging, the switch tube PWM of the chopper circuit of the LLC resonant power supply starts to work, starting the LLC resonant power supply.

2. According to claim 1, a method for soft starting of LLC resonant power supply based on pre-charging mode, Features: The filter capacitor C of the filter part of the LLC resonant power supply in step 1 is o Pre-charge, the specific method is: The switch tube PWM of the chopper circuit is turned off, the rectifier circuit of the LLC resonant power supply is unidirectionally conducted, and the pre-charge power supply pre-charges the load side of the LLC resonant power supply. o Charge to near the load rated voltage V oref When the pre-charge is completed, the filter capacitor C in the equivalent circuit of LLC resonant power supply o The upper voltage variation law is: Among them, U dc It is a pre-charged DC power supply, which is consistent with the DC side power supply of LLC resonant power supply. o is the internal resistance of LLC resonant power supply; C o is the filter capacitor of LLC resonant power supply, R L It is the load of LLC resonant power supply; The filter capacitor C of LLC resonant power supply o Charged to a load voltage close to the LLC resonant power supply rated voltage V oref The time is: Pre-charged DC side power supply U dc The magnitude is the load rated voltage V oref 100 times, formula (13) is close to zero, and the pre-charge time t C Ignore.

3. According to claim 1, a method for soft starting of LLC resonant power supply based on pre-charging mode, Features: The specific method of step 2 is: The switch tube PWM of the chopper circuit of the LLC resonant power supply is turned on to start the LLC resonant power supply. When the LLC resonant power supply is started, the output capacitor voltage is not zero, and the initial voltage u of the filter capacitor Co is C (0) Load rated voltage V close to LLC resonant power supply oref : in C (0)≈V oref ≠0 (14) The PWM switching frequency of the switching tube in the chopper circuit of the LLC resonant power supply is close to the resonant frequency, so: LLC resonant power supply load voltage V o Close to the load rated voltage V oref ,have: When the LLC resonant power supply starts, the output capacitor voltage is not zero and is regarded as a voltage source UC. After conversion to the primary side, the DC side power supply Udc of the LLC resonant power supply is connected in series with the voltage source UC as the input power supply, and the polarity is opposite; at this time: The current meets: The input impedance remains unchanged, and the voltage applied to the resonant inductor is reduced to half of its original value.

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