A phase-shift control method under high gain of a full-bridge LLC

Abstract

The application discloses a phase-shift control method under full-bridge LLC high gain, selects two phase-shift frequency points in a high frequency band, which are a phase-shift control starting point frequency and a phase-shift control terminal point frequency respectively, simultaneously determines a maximum phase-shift period, obtains a linear phase-shift angle in the frequency range through linear function calculation, and thus can better reduce the high gain of the LLC circuit and does not affect the working state under heavy load. Meanwhile, the phase-shift angle of the starting point frequency is set to 0, seamless switching between a normal mode and a phase-shift mode can be realized, a wide input and wide output range of electric vehicle charging can be adapted, voltage ripple under light load and no load can be improved, and stability is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power supply equipment, and particularly relates to a phase-shift control method under high gain of a full-bridge LLC. BACKGROUND

[0002] With the rapid development of society, electronic technology has been widely applied in various fields. The full-bridge LLC resonant converter is a high-efficiency, high-power-density DC-DC conversion topology, which is widely used in new energy power generation, electric vehicle charging, data center power supply and other fields.

[0003] The traditional full-bridge LLC control method mainly adopts the PFM frequency modulation method. The four switching tubes are all driven with a 50% duty cycle. The driving voltages of the same bridge arm are complementary, and the phases are 180 degrees apart. The driving of the other bridge arm is consistent with that of the previous bridge arm. When the output gain is adjusted by adjusting the frequency, due to the limited switching frequency and the conflict between efficiency, the driving frequency is limited to around 200KHz, which limits the adjustment range of the output voltage and cannot meet the requirements of wide input and wide output range of electric vehicle charging. SUMMARY

[0004] Therefore, it is necessary to provide a phase-shift control method under high gain of a full-bridge LLC, which can quickly and effectively adjust the phase-shift angle.

[0005] A phase-shift control method under high gain of a full-bridge LLC, the specific steps comprising:

[0006] Step one, sampling the output voltage of the full-bridge LLC resonant converter, taking the difference between the sampling voltage and the reference voltage as the error value, and calculating the required loop frequency through the 2P2Z compensator;

[0007] Step two, determining the starting frequency, the terminal frequency, the maximum phase-shift angle and the minimum phase-shift angle of the phase-shift control, so as to determine the linear function between the frequency value and the phase-shift angle;

[0008] Step three, taking the starting frequency of the phase-shift control as the limiting point to realize seamless switching between the phase-shift control and the 2P2Z loop control. When the loop frequency is less than the starting frequency, the 2P2Z loop control is adopted. When the loop frequency is greater than the starting frequency, the phase-shift control is adopted;

[0009] Step four, limiting the highest switching frequency of the MOS tube at the starting frequency of the phase-shift control, calculating the corresponding phase-shift angle, and respectively phase-shifting the first switching tube Q1 and the fourth switching tube Q4, the second switching tube Q2 and the third switching tube Q3 to reduce the conduction time and reduce the corresponding gain.

[0010] Preferably, the step one, the output voltage of the full-bridge LLC resonant converter is sampled, and the difference between the sampling voltage and the reference voltage is the error value, and the specific steps of calculating the required loop frequency by the 2P2Z compensator include:

[0011] Step 1.1, the output voltage of the full-bridge LLC resonant converter is sampled to obtain the output sampling voltage value;

[0012] Step 1.2, calculate the difference between the sampling voltage and the reference voltage;

[0013] Step 1.3, input the difference between the sampling voltage and the reference voltage to the 2P2Z compensator to calculate the loop frequency of the 2P2Z compensator.

[0014] Preferably, the step two, the starting frequency, the ending frequency, the maximum phase shift angle and the minimum phase shift angle of the phase shift control are determined to determine the linear function between the frequency value and the phase shift angle, and the specific steps include:

[0015] Step 2.1, determine the starting frequency, the ending frequency, the maximum phase shift angle and the minimum phase shift angle;

[0016] Step 2.2, determine the slope k and the offset b of the linear function according to the starting frequency, the ending frequency, the maximum phase shift angle and the minimum phase shift angle, and the linear function is shown in formula (1):

[0017] f (x)= k x + b (1) formula;

[0018] Wherein, x is the input frequency of the phase shift control, f (x) is the phase shift angle.

[0019] Preferably, the starting frequency is 200KHz, the ending frequency is 250KHz, the maximum phase shift angle is 1 / 2 cycle, and the minimum phase shift angle is 0.

[0020] Preferably, the full-bridge LLC resonant converter includes a control circuit MCU, and the control circuit MCU includes an analog-to-digital conversion module ADC, which is used to convert the sampling voltage of the output circuit into a digital signal and input to the control circuit MCU for processing.

[0021] Preferably, the step three, the starting frequency of the phase shift control is the limiting point, and the specific steps of realizing seamless switching between the phase shift control and the 2P2Z loop control include:

[0022] Step 3.1, in the full-bridge LLC resonant converter, the 2P2Z compensator dynamically adjusts the loop characteristics, and the frequency control mechanism of the pulse frequency modulation (PFM) stabilizes the output voltage;

[0023] Step 3.2, judging whether the loop frequency of the 2P2Z compensator is greater than the starting frequency of the phase-shift control;

[0024] Step 3.3, when the loop frequency is less than the starting frequency, adjusting and compensating the system through the frequency performance of the 2P2Z compensator;

[0025] Step 3.4, when the loop frequency is greater than the starting frequency, linearly controlling the phase-shift angle of the system through the phase-shift control.

[0026] Preferably, the step four limits the highest switching frequency of the MOS tube to the starting frequency of the phase-shift control, calculates the corresponding phase-shift angle, and respectively shifts the phase of the first switch tube Q1 and the fourth switch tube Q4, the second switch tube Q2 and the third switch tube Q3. The specific steps include:

[0027] Step 4.1, when the loop frequency is greater than the starting frequency, limiting the loop frequency to the starting frequency;

[0028] Step 4.2, calculating the phase-shift angle corresponding to the loop frequency by using the linear function of the phase-shift control;

[0029] Step 4.3, outputting the driving pulse with the corresponding phase-shift angle to the first switch tube Q1 and the fourth switch tube Q4, the second switch tube Q2 and the third switch tube Q3, so as to adjust the on-charging time length of the LLC circuit.

[0030] In the above phase-shift control method of the full-bridge LLC high gain, two phase-shift frequency points are selected in the high frequency band, which are the starting point frequency and the terminal point frequency of the phase-shift control, and the maximum phase-shift period is determined. A linear phase-shift angle in the above frequency range is obtained through linear function calculation, so as to better reduce the high gain of the LLC circuit without affecting the working state under heavy load. At the same time, the phase-shift angle of the starting point frequency is set to 0, so that seamless switching between the normal mode and the phase-shift mode can be realized to adapt to the wide input and wide output range of the electric vehicle charging, and the voltage ripple under light load and no load can be improved, and the stability is improved. The method is simple, easy to implement, low in cost, and convenient to popularize. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 is a flow chart of the phase-shift control method of the full-bridge LLC high gain of the embodiment of the application.

[0032] Figure 2 is a circuit structure schematic diagram of the full-bridge LLC resonant converter of the embodiment of the application.

[0033] Figure 3This is a schematic diagram of the phase-shift control of the switching transistors in the full-bridge LLC resonant converter according to an embodiment of the present invention. Detailed Implementation

[0034] This embodiment takes the phase-shift control method under high gain of full-bridge LLC as an example. The invention will be described in detail below with reference to specific embodiments and accompanying drawings.

[0035] Please see Figure 1 This invention illustrates a phase-shift control method for a full-bridge LLC under high gain, with the specific steps as follows:

[0036] Step S10: Sample the output voltage of the full-bridge LLC resonant converter, and use the difference between the sampled voltage and the reference voltage as the error value. Calculate the required loop frequency using the 2P2Z compensator.

[0037] Specifically, the following steps are included:

[0038] Step S11: Sample the output voltage of the full-bridge LLC resonant converter to obtain the output sample voltage value.

[0039] Step S12: Calculate the difference between the sampled voltage and the reference voltage.

[0040] Specifically, the reference voltage is the target voltage output by the power supply, which is the ideal output voltage of the power supply. In this embodiment, the range of the reference voltage is 30V~60V, and the rated voltage is 53V.

[0041] Step S13: Input the difference between the sampled voltage and the reference voltage into the 2P2Z compensator and calculate the loop frequency of the 2P2Z compensator.

[0042] Specifically, the loop frequency function of the 2P2Z compensator includes: adjusting the gain and phase characteristics through pole and zero configuration, controlling the bandwidth, suppressing high-frequency noise, compensating for phase delay, and ensuring sufficient phase margin, thereby maintaining system stability and optimizing dynamic response.

[0043] Calculating the loop frequency of a 2P2Z compensator involves the following steps:

[0044] 1. Determine the transfer function of the controlled object.

[0045] First, obtain the pass function of the controlled object. Gp ( s ), and calculate its frequency of target crossing. fc Gain at | Gp ( j 2 πfc | and phase ϕp .

[0046] 2. Set design goals: Crossing frequencyfc and phase margin PM .

[0047] 3. Calculate the phase boost needed for the compensator

[0048] The total open loop phase at crossover frequency fc must satisfy:

[0049]

[0050] The phase that the compensator must provide is: ϕc

[0051]

[0052] 4. Design the compensator zero and pole locations

[0053] The compensator transfer function Gc(s) is:

[0054]

[0055] where,

[0056] Gc Gc(s) is the transfer function of the compensator; s s is the complex frequency variable (Laplace operator), s Gm is the mid-band gain of the compensator; s Gm = 1 jω Gm = 1 j 2 πf ; K Gm is the mid-band gain of the compensator;

[0057] ωz 1, ωz 2 are the angular frequencies (rad / s) of the two zeros, corresponding to the frequencies: fz 1=2 π / ωz 1, fz 2=2 π / ω z 2;

[0058] ωp 1, ωp 2 are the angular frequencies (rad / s) of the two poles, corresponding to the frequencies: fp 1=2 π / ωp 1, fp 2=2 π / ω p 2.

[0059] At the zeros, a phase boost is provided. At the poles, a phase lag is reduced.

[0060] 5. Calculate the compensator gain K ​

[0061] at the crossover frequency fc , the total open loop gain needs to be 0dB:

[0062]

[0063] Solving:

[0064]

[0065] wherein, fz 1 is the first zero frequency, fz 2 is the second zero frequency, fp 1 is the first pole frequency, fp 2 is the second pole frequency.

[0066] 6. Calculate the phase of the compensator at the crossover frequency fc : ϕc

[0067]

[0068] Ensure ϕc that the above-mentioned compensator needs to provide the phase requirements.

[0069] Step S20, determine the starting frequency, the ending frequency, the maximum phase shift angle and the minimum phase shift angle of the phase shift control, to determine the linear function between the frequency value and the phase shift angle.

[0070] Specifically, the following steps are included:

[0071] Step S21, determine the starting frequency, the ending frequency, the maximum phase shift angle and the minimum phase shift angle.

[0072] Preferably, the starting frequency is preferably 200KHz, the ending frequency is preferably 250KHz, the maximum phase shift angle is preferably 1 / 2 cycle, and the minimum phase shift angle is 0.

[0073] Specifically, the starting frequency and the ending frequency of the phase shift control are two frequency points selected according to actual commissioning, and are usually selected in the high frequency band. Since the actual ideal maximum frequency of the MOS tube is 200KHz, if the frequency is too high, it will cause the temperature to be too high or the loss to be too large, therefore, the starting frequency is preferably 200KHz, and the ending frequency is preferably 250KHz.

[0074] Step S22, determine the slope k and the offset b of the linear function according to the starting frequency, the ending frequency, the maximum phase shift angle and the minimum phase shift angle, and the linear function is shown in formula (1):

[0075] f (x)= k x + b (1)​

[0076] wherein x is the input frequency of the phase-shift control, f (x) is the phase-shift angle.

[0077] Specifically, with the two groups of data of the starting frequency and the minimum phase-shift angle, the terminal frequency and the maximum phase-shift angle corresponding to each other, substituted into formula (1), the slope k and the offset b of the linear function can be calculated.

[0078] Specifically, the highest switching frequency of the MOS tube is related to the resonant frequency of the LLC resonant cavity, and the switching frequency is generally selected to be about 3 times the resonant frequency. In the embodiment, the resonant frequency is 70 KHz, and therefore the starting frequency is determined to be 200 KHz. The starting frequency is the highest frequency point 200 KHz, and the terminal frequency is adjusted according to the required phase-shift slope, which is determined to be 250 KHz in the embodiment, or adjusted according to the linear function f ( x )=0.05 x - 10000.

[0079] Step S30, the starting frequency of the phase-shift control is used as a limit point to realize seamless switching between the phase-shift control and the 2P2Z loop control; when the loop frequency is less than the starting frequency, the 2P2Z loop control is used; and when the loop frequency is greater than the starting frequency, the phase-shift control is used.

[0080] Specifically, the following steps are included:

[0081] Step S31, in the full-bridge LLC resonant converter, the 2P2Z compensator dynamically adjusts the loop characteristics, and the frequency regulation mechanism of the pulse frequency modulation (PFM) is used to stabilize the output voltage.

[0082] Step S32, it is judged whether the loop frequency of the 2P2Z compensator is greater than the starting frequency of the phase-shift control.

[0083] Step S33, when the loop frequency is less than the starting frequency, the system is adjusted and compensated by the frequency performance of the 2P2Z compensator.

[0084] Step S34, when the loop frequency is greater than the starting frequency, the system is linearly controlled by the phase-shift angle through the phase-shift control.

[0085] Specifically, in the entire working process of the full-bridge LLC resonant converter circuit, the 2P2Z compensator is always in a working state, and the PFM frequency control is performed by the loop frequency output by the 2P2Z compensator. Only when the loop frequency output by the 2P2Z compensator is greater than the starting frequency of the phase-shift control, the phase-shift control is used.

[0086] Specifically, since the phase shift angle corresponding to the starting frequency in the linear function of the phase shift control is 0, seamless switching of the 2P2Z loop control mode and the phase shift control mode at the limit point can be achieved.

[0087] Step S40, limiting the highest switching frequency of the MOS tube at the starting frequency of the phase shift control, calculating the corresponding phase shift angle, and respectively phase shifting the first switch Q1 and the fourth switch Q4, the second switch Q2 and the third switch Q3 to reduce the conduction time and reduce the corresponding gain.

[0088] Specifically, the following steps are included:

[0089] Step S41, when the loop frequency is greater than the starting frequency, limiting the loop frequency at the starting frequency.

[0090] Step S42, calculating the phase shift angle corresponding to the loop frequency by using the linear function of the phase shift control.

[0091] Step S43, outputting the drive pulse with the corresponding phase shift angle to the first switch Q1 and the fourth switch Q4, the second switch Q2 and the third switch Q3 to adjust the conduction charging time of the LLC circuit.

[0092] Specifically, by controlling the conduction phase difference of the diagonal bridge arm switch (such as the first switch Q1 and the fourth switch Q4, the second switch Q2 and the third switch Q3), the effective conduction time of the resonant cavity LLC is adjusted to control the energy transmission amount. For example, increasing the phase shift angle will shorten the time window of the energy transmission from the primary side to the secondary side, reducing the output voltage gain.

[0093] The switches of the same bridge arm adopt complementary conduction mode (such as the first switch Q1 is on while the second switch Q2 is off), and the dead time is inserted to avoid the risk of direct through. The implementation of the phase shift control is that the switch drive signal of the lagging bridge arm is delayed by a certain phase relative to the leading bridge arm to form a phase shift angle f (x).

[0094] For example, the drive signal of the leading bridge arm (such as the first switch Q1 and the second switch Q2) remains a fixed frequency signal with a duty cycle close to 50%; the drive signal of the lagging bridge arm (such as the third switch Q3 and the fourth switch Q4) is delayed by a phase f (x) compared with the leading bridge arm, and by adjusting f (x) can dynamically adjust the output power.

[0095] Under the phase shift control, the primary side voltage waveform presents an asymmetric square wave, and the effective pulse width is determined by the phase shift angle. The resonant network LLC adjusts the resonant current phase according to the phase-shifted voltage waveform.

[0096] The full-bridge LLC resonant converter circuit in the embodiment, as shown in Figure 2 The full-bridge LLC resonant converter includes a control circuit MCU, which includes an analog-to-digital conversion module ADC for converting the sampling voltage of the output circuit into a digital signal and inputting it to the control circuit MCU for processing.

[0097] In the full-bridge LLC resonant converter circuit, the output voltage is sampled in real time by the output voltage sampling circuit and transmitted to the ADC module for signal conversion.

[0098] In the embodiment, the starting frequency is 200KHz, the terminal frequency is 250KHz, the minimum phase shift angle is 0, and the maximum phase shift angle is 2.5us. The linear function is calculated by substituting the slope k and the offset b, and the final linear function is: f ( x )=0.05 x - 10000.

[0099] Through the above formula, the corresponding phase shift angle f ( x ) can be calculated by substituting the loop frequency x.

[0100] As shown in Figure 3 , when the loop frequency works at 190KHz to 200KHz, it is a normal 2P2Z loop control, and the phase shift angle is 0. When the loop frequency operates to 210KHz, the phase shift angle is calculated by the formula f ( x )=0.05x - 10000, and the calculation result is 500ns, so the drive output frequency is limited to 200KHz, and 500ns phase shift is performed. Similarly, when the loop frequency is 230KHz, the phase shift angle is calculated by the formula f ( x )=0.05x – 10000, and the result is 1500ns, so the drive output frequency is limited to 200KHz, and 1500ns phase shift is performed.

[0101] Generally, the gain value is calculated according to full load when designing the hardware, and when the power supply works at light load or no load, the output voltage exists voltage drift or jitter because of the overstrong gain. For the full-bridge LLC, the control gain is to raise the frequency, but the frequency cannot be raised unlimitedly, and the control gain is also limited, so when the loop frequency is raised to 200K or more, the phase shift is performed on the driving of the first switch tube Q1 and the fourth switch tube Q4, the second switch tube Q2 and the third switch tube Q3, so as to reduce the conduction charging time and reduce the gain. When the phase shift is to T / 2, the conduction time will be 0 in theory, and the gain tends to 0, so the driving frequency output result can be limited to 200K to reduce the loss, and the phase shift control does not need to add other loops for control, and can share the voltage loop, and only needs to linearly control the phase shift angle.

[0102] Because the highest 60V output voltage needs to be considered when selecting the hardware, the circuit gain is designed on the basis of meeting the 60V output voltage, and when the output voltage works at 30V, the gain is overstrong, and if the phase shift is not performed, the output voltage is difficult to stabilize at 30V. In order to reduce the gain when the output voltage is small (such as 30V), the highest frequency works at 200KHz to enter the phase shift mode, and the angle of the phase shift is adjusted through the 2P2Z loop, so as to stabilize the output voltage at 30V.

[0103] The working frequency is near the resonant frequency at heavy load, and is not related to the phase shift. Only when the output voltage is low, the load is light or no load, the working frequency is raised to 200KHz, so as to enter the phase shift control. Therefore, the scheme is used to realize wide input and wide output of the output voltage, and the effect is very obvious especially in the low output voltage section.

[0104] In the above phase shift control method of the full-bridge LLC high gain, two phase shift frequency points are selected in the high frequency section, which are the phase shift control starting point frequency and the phase shift control terminal point frequency, and the maximum phase shift period is determined, a linear phase shift angle in the above frequency range is obtained through linear function calculation, so as to better reduce the high gain of the LLC circuit, and the working state under heavy load is not affected. At the same time, the phase shift angle of the starting point frequency is set to 0, so as to realize seamless switching between the normal mode and the phase shift mode, so as to adapt to the wide input and wide output range of the electric vehicle charging, and improve the voltage ripple of the light load and the no load, and improve the stability. The method is simple, easy to realize, low in cost, and convenient to popularize.

[0105] It should be noted that the above only represents the preferred embodiments of the present application, and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A phase-shifted control method for full-bridge LLC high gain, characterized in that, The specific steps include: Step one, the output voltage of the full-bridge LLC resonant converter is sampled, and the difference between the sampling voltage and the reference voltage is taken as the error value, and the required loop frequency is calculated through the 2P2Z compensator; Step two, the starting frequency, the terminal frequency, the maximum phase shift angle and the minimum phase shift angle of the phase shift control are determined to determine the linear function between the frequency value and the phase shift angle; Step three, taking the starting frequency of the phase shift control as the limiting point, realizing the seamless switching of the phase shift control and the 2P2Z loop control; when the loop frequency is less than the starting frequency, the 2P2Z loop control is adopted; when the loop frequency is greater than the starting frequency, the phase shift control is adopted; Step four, limit the highest switching frequency of the MOS tube at the starting frequency of the phase shift control, calculate the corresponding phase shift angle, and respectively shift the first switch tube Q1 and the fourth switch tube Q4, the second switch tube Q2 and the third switch tube Q3 to reduce the conduction time and reduce the corresponding gain; Wherein, the first switch tube Q1 and the fourth switch tube Q4 are arranged in different bridge arms, and the second switch tube Q2 and the third switch tube Q3 are arranged in different bridge arms.

2. The phase-shifted control method for full-bridge LLC high gain as claimed in claim 1, wherein, The specific steps of the step one, sampling the output voltage of the full-bridge LLC resonant converter, taking the difference between the sampling voltage and the reference voltage as the error value, and calculating the required loop frequency through the 2P2Z compensator include: Step 1.1, sampling the output voltage of the full-bridge LLC resonant converter to obtain the output sampling voltage value; Step 1.2, calculate the difference between the sampling voltage and the reference voltage; Step 1.3, input the difference between the sampling voltage and the reference voltage to the 2P2Z compensator to calculate the loop frequency of the 2P2Z compensator.

3. The method of phase-shifted control of full-bridge LLC with high gain according to claim 1, characterized in that, The specific steps of the step two, determining the starting frequency, the terminal frequency, the maximum phase shift angle and the minimum phase shift angle of the phase shift control to determine the linear function between the frequency value and the phase shift angle include: Step 2.1, determine the starting frequency, the terminal frequency, the maximum phase shift angle and the minimum phase shift angle; Step 2.2, determine the slope k and the offset b of the linear function according to the starting frequency, the terminal frequency, the maximum phase shift angle and the minimum phase shift angle, and the linear function is as shown in formula (1): f (x)= k x + b (1); wherein x is the input frequency of the phase-shift control, f (x) is the phase-shift angle.

4. The phase-shifted control method for full-bridge LLC high gain as claimed in claim 3, wherein, The starting frequency is 200KHz, the terminal frequency is 250KHz, the maximum phase shift angle is 1 / 2 cycle, and the minimum phase shift angle is 0.

5. The method of phase-shifted control of full-bridge LLC with high gain according to claim 1, characterized by, The full-bridge LLC resonant converter includes a control circuit MCU, and the control circuit MCU includes an analog-to-digital conversion module ADC, which is used to convert the sampling voltage of the output circuit into a digital signal and input it to the control circuit MCU for processing.

6. The method of phase-shifted control of full-bridge LLC with high gain according to claim 1, characterized by, The specific steps of the step three, taking the starting frequency of the phase shift control as the limiting point, realizing the seamless switching of the phase shift control and the 2P2Z loop control include: Step 3.1, in the full-bridge LLC resonant converter, the 2P2Z compensator dynamically adjusts the loop characteristics, and through the frequency regulation mechanism of pulse frequency modulation PFM, the output voltage is stabilized; Step 3.2, judge whether the loop frequency of the 2P2Z compensator is greater than the starting frequency of the phase shift control; Step 3.3, when the loop frequency is less than the starting frequency, the system is adjusted and compensated by the frequency performance of the 2P2Z compensator; Step 3.4, when the loop frequency is greater than the starting frequency, the system is linearly controlled by the phase shift angle of the phase shift control.

7. The method of phase-shifted control of full-bridge LLC with high gain according to claim 1, characterized by, The step four limits the highest switching frequency of the MOS tube at the starting frequency of the phase shift control, calculates the corresponding phase shift angle, and specifically adjusts the phase shift of the first switch tube Q1 and the fourth switch tube Q4, the second switch tube Q2 and the third switch tube Q3. Step 4.1, when the loop frequency is greater than the starting frequency, the loop frequency is limited to the starting frequency; Step 4.2, the linear function of the phase shift control is used to calculate the phase shift angle corresponding to the loop frequency; Step 4.3, the drive pulse with the corresponding phase shift angle is output to the first switch tube Q1 and the fourth switch tube Q4, the second switch tube Q2 and the third switch tube Q3, so as to adjust the conduction charging time length of the LLC circuit.

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