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

Abstract

The application discloses a pre-charging mode-based LLC resonant power supply soft-start method o The pre-charging mode-based LLC resonant power supply soft-start method is characterized in that: a filter capacitor C of a filter part of an LLC resonant power supply is pre-charged o The pre-charging mode-based LLC resonant power supply soft-start method is characterized in that: a filter capacitor C of a filter part of an LLC resonant power supply is pre-charged oref The pre-charging mode-based LLC resonant power supply soft-start method is characterized in that: a filter capacitor C of a filter part of an LLC resonant power supply is pre-charged o The pre-charging mode-based LLC resonant power supply soft-start method is characterized in that: a filter capacitor C of a filter part of an LLC resonant power supply is pre-charged r The pre-charging mode-based LLC resonant power supply soft-start method is characterized in that: a filter capacitor C of a filter part of an LLC resonant power supply is pre-charged

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of DC-DC current conversion, and particularly relates to an LLC resonant power supply soft start method based on a pre-charging mode. BACKGROUND

[0002] LLC resonant power supplies are widely used in distributed communication power supplies, have the advantages of fewer components, ZVS turn-on of the primary side switch tube, ZCS turn-off of the secondary side rectifier switch tube, the ability to maintain the stability of the output voltage when the input voltage fluctuates, a small working frequency adjustment range, higher power utilization, and lower space occupation ratio under the same output power, and are an extremely attractive DC-DC power supply topology.

[0003] However, in actual use, LLC resonant power supplies also face the problem of starting current overshoot. Traditional converter soft start methods are generally divided into PWM start, APWM start, PFM start, and series resistance start, but the converter gain changes inversely with the start time, i.e., the starting current overshoot and the start time cannot be considered at the same time. In recent years, trajectory control and dead time control have been developed to suppress the starting current overshoot, and good results have been achieved in starting current overshoot and starting time, but the soft start control algorithm is relatively complex, the calculation is large, and the hardware computing power requirement is high.

[0004] In view of the current overshoot problem of the LLC resonant converter during startup, Li Guangyao, Shao Li-huan, and Zheng Changfeng. LLC resonant converter linear compensation soft start control research [J]. Measurement and control technology, 2023, 42(08): 89-93. DOI: 10.19708 / j.ckjs.2022.09.302. According to the analysis of the frequency regulation (PFM) gain and the duty cycle regulation (PWM) gain based on the principle of LLC resonant converter, a linear compensation soft start control strategy for LLC resonant converter is proposed by using the relationship between the frequency regulation gain, the duty cycle regulation gain and the total gain of hybrid regulation. The strategy designs a duty cycle variation function according to the gain characteristics of the duty cycle regulation, and designs a frequency variation function according to the total gain relationship of the hybrid regulation obtained by the above analysis, and linearly compensates the duty cycle regulation gain with the frequency regulation gain, so that the total output gain can tend to be linearly changed, and the starting current overshoot is suppressed. However, this method needs to use simulation and mathematical fitting tools to design the duty cycle variation function and the frequency variation function, and needs to ensure that the total gain variation function of the hybrid regulation is linearly changed, the design process is relatively complex, and the soft start control strategy is actually implemented. It is relatively complex and needs to adjust the frequency and the duty cycle at the same time. SUMMARY

[0005] In order to overcome the defects of the prior art, the present application aims to provide a LLC resonant power soft start method based on pre-charging mode, which avoids the loss by eliminating the access of large resistance compared with the traditional series resistance start mode; without increasing the start time, the LLC resonant power soft start method based on pre-charging mode can effectively reduce the resonant cavity impact current, the capacitor C r overshoot voltage, and achieve the suppression of LLC resonant power start current and voltage overshoot.

[0006] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0007] A LLC resonant power soft start method based on pre-charging mode, the specific steps are as follows:

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

[0009] Step 2, on the basis of pre-charging the filter capacitor C o of the LLC resonant power in step 1, the switch tube PWM of the chopper circuit of the LLC resonant power starts to work, and the LLC resonant power is started.

[0010] The specific method for pre-charging the filter capacitor C o of the LLC resonant power in step 1 is as follows:

[0011] Turn off the switch tube PWM of the chopper circuit, the rectifier circuit of the LLC resonant power is unidirectionally conducted, the pre-charging power pre-charges the load side of the LLC resonant power, when the filter capacitor C o is charged to close to the rated voltage V oref of the load, the pre-charging is completed; the voltage variation law of the filter capacitor C o in the equivalent circuit of the LLC resonant power is as follows:

[0012]

[0013] Wherein, U dc is the pre-charging DC power, which is consistent with the DC side power of the LLC resonant power, R o is the internal resistance of the LLC resonant power; C o is the filter capacitor of the LLC resonant power, and R L is the load of the LLC resonant power.

[0014] Pre-charge the filter capacitor C oCharging to the load rated voltage V of LLC resonant power supply oref The time is:

[0015]

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

[0017] The specific method of the step 2 is:

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

[0019] u C (0)≈V oref ≠0 (14)

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

[0021]

[0022] The load voltage V o of the LLC resonant power supply is close to the load rated voltage V oref , and has:

[0023]

[0024] When the LLC resonant power supply is started, the output capacitor voltage is not zero, which is regarded as a voltage source UC, and the LLC resonant power supply DC side power supply Udc and the voltage source UC are connected in series as an input power supply after being converted to the original side, and the polarity is opposite; at this time:

[0025]

[0026] The current satisfies:

[0027]

[0028] The input impedance is unchanged, and the voltage acting on the resonant inductor is reduced to half of the original.

[0029] Compared with the prior art, the beneficial effects of the present application are:

[0030] The application designs an LLC resonant power soft-start method based on pre-charging mode, and the specific technical scheme includes two links: first, pre-charging stage, the switch tube PWM of the chopper circuit part does not work, because the rectifier circuit is unidirectional conduction, only the load side pre-charging loop works, when the filter capacitor C o is charged to close to the rated voltage V oref of the load, the switch tube PWM of the chopper circuit starts to work, and the LLC resonant power is started, because the filter capacitor C o is pre-charged, the input impedance is unchanged, the voltage acting on the resonant inductor is reduced to half of the original, compared with directly starting the LLC resonant power, the impact current is reduced.

[0031] The application avoids the connection of a large resistance by the pre-charging mode of the LLC resonant power, compared with the traditional series resistance starting mode, and the loss is avoided; without increasing the starting time, the resonant cavity impact current and the capacitor C r impact voltage are more effectively reduced, and the method is simple and easy to realize. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 It is the overall structure diagram of the technical scheme adopted by the application;

[0033] Figure 2 It is an LLC power topology structure diagram;

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

[0035] Figure 4 It is an LLC power starting equivalent circuit after being calculated from the secondary side to the primary side;

[0036] Figure 5 It is an LLC power equivalent circuit at 0 time;

[0037] Figure 6 It is the LLC power resonant cavity impact current waveform without taking other measures and directly starting the resonant cavity;

[0038] Figure 7 It is the LLC power resonant capacitor Cr impact voltage waveform without taking other measures and directly starting the resonant capacitor;

[0039] Figure 8 It is the LLC power series resistance starting resonant cavity impact current waveform;

[0040] Figure 9 It is the LLC power series resistance starting resonant capacitor Cr impact voltage waveform;

[0041] Figure 10 It is the pre-charging stage equivalent circuit of the application;

[0042] Figure 11 This is the equivalent circuit for the LLC power-on stage of the present invention;

[0043] Figure 12 This is the waveform of the resonant cavity impulse current after adopting the present invention;

[0044] Figure 13 This is the waveform of the impulse voltage on capacitor Cr after adopting the present invention. Detailed Implementation

[0045] The present invention will now be described in detail 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, and a filter circuit (filter capacitor C). o ) and load (Z) L The circuit consists of a DC power supply, which serves as the power input for the LLC resonant circuit. A chopper circuit is connected to the DC power supply and outputs an alternating voltage via PWM control of a switching transistor. The resonant cavity receives the alternating voltage from the chopper circuit and transmits power in AC form. The transformer receives the AC output from the resonant cavity and converts the AC voltage to a level suitable for the load requirements. The rectifier circuit converts the transformed AC voltage to DC voltage. The filter circuit removes the alternating component from the DC voltage, providing a stable DC power supply to 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 between the frequency of the control pulse signal of the IGBT and the frequency of the control pulse signal is as follows:

[0048]

[0049] in:

[0050]

[0051]

[0052] From formula (1), it can be seen that: the gain K is related to the quality factor Q, the inductance ratio m, and the normalized frequency F. x Since Q and m remain constant after the circuit design is completed, the normalization frequency F can be controlled. x (That is, the 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, as the quality factor Q increases, 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 directly started, its equivalent circuit is as follows: Figure 4 As shown: DC power supply U dc Connect switch K1 and resonant cavity resistor R in series, and then connect resonant capacitor C in series. r Resonant inductor L r Then connect the magnetizing inductor L in series m With C * o Z * L Parallel branches, where C * o Z * L It is the filter capacitor C o Load Z L The equivalent capacitance and impedance after being transferred from the secondary side to the primary side, according to circuit principles, are:

[0054] u C (0)=0,i C (0)≠0 (8)

[0055] Among them, u C (0) represents the filter capacitor C at time 0. o Or resonant capacitor C r Voltage across terminals, i C (0) represents the filter capacitor C at time 0. o Or resonant capacitor C r The current. Based on this, the circuit at time 0 is equivalent to... Figure 5 Form: DC power supply U dc Connect switch K1 and resonant cavity resistor R in series, and then connect 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 for an LLC resonant power supply based on pre-charging is presented, consisting of an LLC resonant power supply and a pre-charging circuit. The specific steps are as follows:

[0060] Step 1: The pre-charge power supply uses the same DC power supply as the DC input power supply of the LLC resonant power supply, and the filter capacitor C in the filter section... o During the pre-charging phase, switch K1 is opened and K2 is closed. The PWM switching transistor of the chopper circuit is not working. Since the rectifier circuit is unidirectional, only the load side is in the pre-charging circuit U. dc -R o -K2-C o / / Z L When the filter capacitor C is working... o Charge to near the load's rated voltage V oref When switch K2 is disconnected, the equivalent circuit is as follows: Figure 10 As shown: DC power supply U dc Series power supply internal resistance R o Then connect a filter capacitor C in series. o With load Z L In the parallel branch, when the filter capacitor C o Charge to near the load's rated voltage V oref At that time, pre-charging ends; filter capacitor C o The voltage variation pattern is as follows:

[0061]

[0062] Among them, U dc This is a pre-charged DC power supply, consistent with the DC side power supply of the LLC resonant power supply, R o C is the internal resistance of the power supply; o For the filter capacitor, R L The load resistor is used; the filter capacitor C is used. o Charge to near the load's 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-charge DC power supply U dc The magnitude is the rated load voltage V. oref 100 times, (13) equation is close to zero, pre-charge time t C Negligible;

[0065] Step 2: Open switch K2 and close switch K1. The PWM of the switching transistor in the chopper circuit starts working, starting the LLC resonant power supply.

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

[0067] u C (0)≈V oref ≠0 (14)

[0068] The switching frequency of the PWM transistor in the chopper circuit of the LLC resonant power supply is close to the resonant frequency, therefore:

[0069]

[0070] The load voltage V of the LLC resonant power supply o Approximately the rated voltage of the load V oref ,have:

[0071]

[0072] When the LLC resonant power supply starts up, the output capacitor voltage is not zero and is considered as a voltage source U. C After being referred back to the primary side, the equivalent circuit is as follows: Figure 11 As shown: Switch K1, resonant cavity resistor R, resonant inductor L r The DC-side power supply U of the series-connected LLC resonant power supply dc With voltage source U C When the power supplies are connected in series and have opposite polarities, then:

[0073]

[0074] The current satisfies:

[0075]

[0076] With the input impedance unchanged, the voltage applied to the resonant inductor is reduced to half of its original value, which reduces the inrush current compared to directly starting the LLC resonant power supply.

[0077] Directly starting an LLC resonant power supply without taking other measures will cause the resonant cavity current to rapidly reach a very large value, typically tens or even hundreds of times the rated current amplitude. Such a large inrush current will break down switching devices, burn out capacitors and other components, damage the circuit system, or trigger circuit protection. Taking a 380V input, 60W, 12V output LLC (switching frequency 100kHz) as an example, a typical inrush current waveform is as follows: Figure 6 As shown, the normal current amplitude is about 1.5A, while the inrush current during startup reaches about 26A, with the ratio of the two reaching 17.3.

[0078] The rapidly increasing starting current will rapidly accumulate a large amount of charges on the resonant cavity capacitor C r The voltage across the capacitor rapidly increases, and can reach several times to several tens of times of the rated voltage, and the voltage across the capacitor satisfies the following relationship:

[0079]

[0080] The typical resonant capacitor C r The impact voltage waveform is shown in Figure 7 The normal voltage amplitude is about 260V, and the starting impact voltage reaches about 1500V, and the ratio is 5.7.

[0081] In order to solve the above impact current problem, a relatively simple method is to directly insert a certain resistance (50Ω) during starting, and cut off the resistance after a period of time, thereby increasing Z in in formula (9), under the condition that the power supply voltage is constant, the current is reduced, the resonant cavity impact current and the impact voltage on the capacitor Cr, and the effect is shown in Figure 8 , Figure 9 The impact current and voltage are reduced to 8A and 440V, respectively, and are reduced to 5.3 times and 1.7 times of the rated value, respectively. The impact current is reduced, and still exceeds the rated value several times, and the starting time is increased from 0.4ms to 1.2ms.

[0082] In order to achieve the purpose of more effectively reducing the starting impact current and voltage and shortening the starting time, the LLC resonant power soft starting method based on the pre-charging mode is adopted.

[0083] First, the pre-charging phase is carried out, at this time the switch K1 is disconnected, the switch K2 is closed, the switch tube PWM of the chopper circuit does not work, the rectification part is unidirectional conduction, only the load side pre-charging circuit U dc -R o -K2-C o / / Z L circuit works, when the filter capacitor C o is charged to close to V oref =12V, K2 is disconnected. The filter capacitor C o used is 1.3mF, and the DC charging power supply uses the same power supply as the LLC input, that is, U dc =380V, and the theoretical calculation charging time is 4×10 -6 s, which is much smaller than 0.4ms, so the charging time can be ignored.

[0084] Next, the switch K2 is disconnected, and the switch K1 is closed, the switch tube PWM of the chopper circuit works to directly start the LLC. Since C oThe pre-charge is carried out, the input impedance is unchanged, the acting voltage is reduced to half of the original, and the impact current is reduced. The impact current and the impact voltage waveforms on the capacitor Cr are shown in Figure 12 , 13 , the impact current is 3.6 A, the impact voltage is 420 V, which are 2.4 times and 1.6 times of the rated values respectively, and the starting time is about 0.3 ms, which is 1 / 4 of the direct series resistance starting.

Claims

1. A LLC resonant power supply soft start method based on pre-charge mode, the specific steps are as follows: Step 1, filter capacitor C of filter part of LLC resonant power supply o Pre-charge, pre-charge power supply uses the same DC power supply as the 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, filter capacitor C of LLC resonant power supply in step 1 o On the basis of pre-charging, the switch tube PWM of the chopper circuit of the LLC resonant power supply starts to work, and the LLC resonant power supply is started. The filter capacitor C of the filter part of the LLC resonant power supply in step 1 o Pre-charging is performed in the following way: The switch tube PWM of the turn-off chopper circuit is turned off, the rectifier circuit of the LLC resonant power supply is unidirectionally conducted, the pre-charging power supply pre-charges the load side of the LLC resonant power supply, and when the filter capacitor C o is charged to be close to the rated voltage V oref of the load, the pre-charging ends; the voltage variation law of the filter capacitor C o in the equivalent circuit of the LLC resonant power supply is: wherein U dc For pre-charging DC power supply, consistent with the DC side power supply of LLC resonant power supply, R o For the internal resistance of LLC resonant power supply; C o For the filter capacitor of LLC resonant power supply, R L For the load of LLC resonant power supply; The filter capacitor C of the LLC resonant power supply o Charging to a load rated voltage V close to the LLC resonant power supply oref is: Pre-charge DC side power supply U dc The order of magnitude of the load rated voltage V oref is 100 times, formula (13) is close to zero, the pre-charge time t C is negligible; The specific method of step 2 is as follows: 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 C (0) The load rated voltage V of the LLC resonant power supply is approached oref : u C (0)≈V oref ≠0 (14) The PWM switching frequency of the switching tube of the chopper circuit part of the LLC resonant power supply is close to the resonant frequency, so: Load voltage V of the LLC resonant power supply o Approaching the load rated voltage V oref There are: When the LLC resonant power supply starts, the output capacitor voltage is not zero, which is regarded as a voltage source UC, and the LLC resonant power supply DC side power supply Udc and the voltage source UC are connected in series as input power supply with opposite polarity; At this time, there is: The current satisfies: The input impedance is unchanged, and the voltage acting on the resonant inductor is reduced to half of the original.

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