WO2020186978A1 - Method for realizing interphase current sharing of illc resonant converter and prolonging power-down hold-up time - Google Patents

Abstract

Provided is an iLLC resonant converter, comprising two LLC resonant converters, a digital controller and two switch control inductors. Power transformers of the two LLC resonant converters are respectively provided with an auxiliary winding, the two switch control inductors are respectively connected in parallel to the auxiliary windings, the parameters and structures of the two LLC resonant converters are the same, input ends of the two LLC resonant converters are connected in parallel, and output ends thereof are also connected in parallel, and the parameters and structures of the two switch control inductors are the same. According to the resonant converter provided in the present invention, the respective output currents and common input voltage of two LLC resonant converters are sampled by means of a digital controller, the phase angle of switch control inductors is adjusted through operation to realize current sharing of a resonant cavity, and the direct-current voltage gain of the resonant cavity can be adjusted according to the voltage of a direct-current bus during input power-down, thereby prolonging the power-down hold-up time of the converter.

Description

Method for sharing current between phases of iLLC resonant converter and prolonging the maintenance time of power failure Technical field

The invention relates to power electronics technology and its application field, in particular to an iLLC resonant converter and a method for sharing current between phases and extending the power-off maintenance time.

Background technique

Switching power supplies such as server group power supplies and communication power supplies have low output voltages and large output currents, and the power supplies are required to maintain a normal constant voltage output for more than ten milliseconds after the power grid is powered off, so that the uninterruptible power supply (UPS) can be used during this period of time. Provide backup energy for these power sources. This period of ten milliseconds is called hold-up time. LLC resonant converter is an inductor-inductor-capacitor series resonant DC-DC converter with a narrow frequency range and wide voltage adjustment range. It can achieve zero-voltage soft switching in the full load range, and the secondary rectifier diode can achieve zero-current shutdown. , The output does not need filter inductance, so the whole machine has high conversion efficiency, small size and high power density, so it has been widely used in the above-mentioned power supply.

However, there are still two design difficulties in practical applications: (1) LLC resonant converter has no output filter inductor, so the current ripple is large. In order to meet the current ripple requirements, a larger capacity capacitor is required, resulting in volume and Cost increase; (2) When the input AC grid is powered off, in order to ensure that the output voltage can still be maintained within ten milliseconds, the LLC resonant converter is required to have a larger voltage gain adjustment range, but the voltage gain adjustment range of the LLC resonant converter It has a greater relationship with the excitation inductance of the resonant cavity. A smaller excitation inductance can make the resonant cavity obtain a larger voltage gain, but during normal operation (when the grid voltage is normal), the resonant cavity has a large circulating current and large conduction loss, which leads to conversion. The efficiency of the device drops. Larger magnetizing inductance can ensure the high efficiency of the LLC resonant converter during normal operation, but when the power grid fails, the voltage gain adjustment range is very small. To ensure sufficient power failure maintenance time, it is necessary to increase the DC bus capacitor The capacity of the DC bus capacitor increases and the cost increases. This design contradiction leads to a dilemma in the choice of resonant cavity magnetizing inductance.

In order to reduce the output current ripple, LLC resonant converters often adopt the staggered phase parallel connection method, so that the output current ripples can cancel each other, effectively reduce the output current ripples, reduce the volume and cost of the output filter capacitor, and extend the life of the capacitor. At the same time, it will bring a new problem: the parameters of the resonant cavity components in each phase are inevitably toleranced, and the parameters are difficult to ensure absolute consistency. Even if the deviation of the resonant cavity parameters is small, the cavity current will have obvious differences. As a result, the power load of each phase is not balanced, which reduces the reliability of the power supply system, and at the same time, the advantage of reducing current ripple will also be weakened.

In order to solve the problem of current equalization between phases, the document "A Control Strategy and Design Method for Interleaved LLC Converters Operating at Variable Switching Frequency" (IEEE Transactions on Power Electronics, 2014, 29(8), pp: 4426-4437) adopts switch control The capacitance (SCC) method adjusts the natural resonant frequency of the interleaved parallel resonant cavities to achieve current sharing of each phase, but this method has a small adjustment range for the resonant cavity gain. The document "Combined Multilevel and Two-Phase Interleaved LLC Converter With Enhanced Power Processing Characteristics and Natural Current Sharing" (IEEE Transactions on Power Electronics 2018, 33(7), pp: 5613-5620) gives a natural equalization of resonant cavity The design scheme is only suitable for two-phase parallel connection. The invention patent "A current sharing adjustment method of interleaved parallel LLC resonant circuit" (201810195443.9) adopts detecting the current or voltage error of each LLC branch, and adjusting the duty ratio of each LLC branch according to the error to adjust the gain of each LLC branch to achieve Current sharing, this is a control method to adjust the pulse width, which can solve the current sharing problem, but it does not help to extend the power-off maintenance time.

In order to solve the problem of the short duration of the LLC resonant conversion power supply output power failure, the document "Analysis on Half-Bridge LLC Resonant Converter by Using Variable Inductance for High Efficiency and Power Density Server Power Supply" (2017 IEEE Applied Exposition Power Supply APEC), 2017, pp: 170-177) provides a variable magnetizing inductance design. When the power is off, a smaller magnetizing inductance is used to make the LLC resonant converter obtain a larger gain. However, this method has only two states, and the magnetizing inductance cannot be adjusted continuously, and it is easy to cause overshoot when the mode is switched. The document "An LLC Converter Family With Auxiliary Switch for Hold-Up Mode Operation" (IEEE Transactions on Power Electronics, 2017, 32(6), pp: 4291-4306) uses LLC resonant inductor and a switch to form a boost circuit to achieve Extension of power-down sustaining time. However, this method can no longer integrate the resonant inductor in the transformer, which increases the volume of the converter.

The methods given in the above-mentioned documents and patents cannot simultaneously meet the needs of the current balance adjustment of the staggered resonant cavity and the extension of the power-off maintenance time. The present invention provides a method for realizing current sharing between phases of iLLC resonant converters and prolonging the power-off maintenance time, and at the same time solves the two design difficulties of the aforementioned staggered parallel LLC resonant converter power supply.

Summary of the invention

In order to overcome the defects in the prior art, the present invention provides a phase-staggered parallel LLC resonant converter, which can balance the current between the phases and maintain high efficiency, and can make the phase-staggered parallel LLC resonant when the power is off. The converter has sufficient output sustaining time. The following technical solutions are specifically adopted:

An iLLC resonant converter includes two LLC resonant converters, a digital controller and two switch control inductors. An auxiliary winding is provided on the power transformer of the LLC resonant converter. The switch control inductor and the auxiliary The windings are connected in parallel, the parameters and structure of the LLC resonant converter are the same, the input ends of the LLC resonant converter are connected in parallel and the output ends are also connected in parallel, and the parameters and structure of the switch control inductor are the same.

Specifically, the LLC resonant converter has a half-bridge structure or a full-bridge structure, the operating frequency of the LLC resonant converter is the same, and the phase difference of the driving signal of the LLC resonant converter is π/2.

Specifically, the switch control inductor includes a linear inductor and a bidirectional controllable switch in series, and the bidirectional controllable switch includes two low on-resistance power switch devices.

Specifically, the number of turns of the auxiliary winding is the same, and the number of turns of the auxiliary winding is not greater than the number of turns of the secondary winding.

Specifically, the method for realizing current sharing between phases of the staggered parallel LLC resonant converter includes the following steps:

1) The digital controller sets the initial values α _ini of the phase difference angles α ₁ and α ₂ between the voltage waveform across the auxiliary winding and the drive signals of the two switch control inductors;

2) Sampling the output currents i _o1 and i _{o2 of the} two LLC resonant converters respectively;

3) The digital controller compares the output current i _o1 with the half value of the total output current, and adjusts the phase difference angles α ₁ and α ₂ according to the comparison result.

Specifically, the initial value α _ini in the step 1) is determined by the following formula through a table look-up method:

Among them, L _SCI-max is determined by the following formula:

In the formula, a is the turns ratio of the primary winding and the auxiliary winding of the power transformer, L _m-eq.max is the maximum value of the equivalent magnetizing inductance required to realize the zero voltage conduction of the LLC resonant converter, L _{m -ini} is the initial magnetizing inductance value of the power transformer.

Specifically, the adjustment range of the phase difference angles α ₁ and α ₂ is π/2-α _ini .

Specifically, the adjustment rule in step 3) includes:

First, the output current i _{o1 is} less than half of the total output current, the digital controller adjusts the phase difference angle α _{1 to} increase by one unit, and the phase difference angle α _{2 to} decrease by one unit;

Then the output current i _{o1 is} greater than the half value of the total output current, the digital controller adjusts the phase difference angle α _{1 to} decrease by one unit, and the phase difference angle α _{2 to} increase by one unit;

Finally, the output current i _o1 is equal to the half value of the total output current, and the phase difference angles α ₁ and α ₂ remain unchanged.

Specifically, the method for extending the power-down sustaining time of the staggered-phase parallel LLC resonant converter includes the following steps: first, the closed-loop control system of the LLC resonant converter maintains the stability of the output voltage by adjusting the frequency, and then the digital control The regulator adjusts the phase difference angles α ₁ and α _{2 to} decrease to π/2, so that the DC voltage gain of the LLC resonant converter reaches the maximum value.

Specifically, the calculation formula of the maximum DC voltage gain of the LLC resonant converter is:

Where k _min is the ratio of the equivalent excitation inductance of the resonant cavity to the inherent resonant inductance when the phase difference angles α ₁ and α _{2 are} equal to π/2, and f _n-min is the minimum operating frequency of the LLC resonant converter and The ratio of the natural frequency of the resonant cavity, Q _max is the quality factor of the resonant cavity under full load.

The beneficial effects of the present invention are:

1) During the normal operation of the power system, the phase-staggered parallel resonant cavities maintain a large excitation inductance, which can effectively reduce the conduction loss of the resonant cavity and the turn-off loss of the primary switching device, so that the power system maintains high efficiency;

2) When the current of the out-of-phase resonant cavity is unbalanced, the excitation inductance of each resonant cavity is fine-tuned by the control system to make the resonant cavity current tend to be consistent and still maintain a large excitation inductance, which does not affect the efficiency during normal operation;

3) The entire adjustment process is programmable and continuously adjusted, which is excessively stable and will not over-adjust;

4) The switch control inductor replaces the air gap in the traditional LLC resonant transformer, reduces the edge effect of the transformer, improves the efficiency of the transformer, and reduces the volume of the transformer, which improves the power density of the power supply system;

5) When the AC power fails, the input DC bus voltage drops. The control system changes the DC voltage gain of the resonant cavity by adjusting the equivalent inductance of the switch to control the inductor, so that the power-off maintenance time is greatly extended, and the input DC filtering is effectively reduced The requirement of capacitance reduces the cost and improves the power density.

Description of the drawings

Fig. 1 is a structural block diagram of a method for implementing phase-to-phase current sharing and extending the power-down sustaining time of a staggered parallel LLC resonant converter according to an embodiment of the present invention;

2 is a schematic circuit diagram of an implementation method for phase-to-phase current sharing and extending power-down sustaining time of a staggered parallel LLC resonant converter according to an embodiment of the present invention;

FIG. 3 is a schematic diagram of comparison of output current ripples in different situations of the embodiment of the present invention;

4 is a timing diagram of working waveforms of a two-phase switch controlled inductor according to an embodiment of the present invention;

5 is a simulation diagram of the DC voltage gain variation of the resonant converter according to the embodiment of the present invention;

FIG. 6 is a simulation diagram of realizing resonant cavity current balance by changing the resonant cavity excitation inductance according to an embodiment of the present invention;

FIG. 7 is a flowchart of a procedure for adjusting the resonant cavity current by controlling the phase angle of the inductor through the switch according to an embodiment of the present invention;

8 is a simulation diagram of the relationship between the phase angle of the switch control inductor and the equivalent excitation inductance of the resonant cavity according to the embodiment of the present invention;

FIG. 9 is a schematic diagram of the control principle of a method for extending the power-off maintenance time according to an embodiment of the present invention;

Reference signs: 1-first LLC resonant converter, 2-second LLC resonant converter, 3-digital controller, 4-first switch control inductor, 5-second switch control inductor.

detailed description

The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work shall fall within the protection scope of the present invention.

As shown in Figure 1, an iLLC resonant converter includes a first LLC resonant converter 1 and a second LLC resonant converter 2, which have the same parameters and structure, and are used to realize the digital control of phase-to-phase current sharing and power-down sustaining time extension control. The first switch control inductor 4 and the second switch control inductor 5 have the same parameters and structure. The power transformers of the first LLC resonant converter 1 and the second LLC resonant converter 2 are respectively provided with an auxiliary winding The first switch control inductor 4 and the second switch control inductor 5 are respectively connected in parallel with the auxiliary winding, the number of turns of the auxiliary winding is the same, and the number of turns of the auxiliary winding is not greater than that of the secondary winding. The number of turns, the input ends of the first LLC resonant converter 1 and the second LLC resonant converter 2 are connected in parallel and the output ends are also connected in parallel. The first switch control inductor 4 and the second switch control inductor 5 consist of a linear The inductor is connected in series with a bidirectional controllable switch, and the bidirectional controllable switch is composed of two power switch devices with low on-resistance.

As shown in FIG. 2, the first LLC resonant converter 1 and the second LLC resonant converter 2 have a half-bridge structure, and the operating frequencies of the first LLC resonant converter 1 and the second LLC resonant converter 2 are the same, The phase difference of the driving signal is π/2 to realize the mutual cancellation of the output current ripple and reduce the output current ripple. However, due to the inevitable tolerance of the parameter values of the components, the input impedance of the resonant cavity cannot be completely consistent, so that the amount of energy transferred by the staggered parallel LLC resonant converter is inconsistent, the current of the resonant cavity is not balanced between the phases, and the output current ripple cancellation effect Weakened, resulting in increased output current ripple and reduced power system reliability. Define the output capacitor current ripple as:

ΔI=I _max -I _min

Current ripple rate:

Figure 3 shows the output capacitor current ripple of the single-phase LLC resonant converter, the current ripples cancel each other when the staggered parallel LLC resonant converter has no parameter tolerance, and the current ripple when the staggered parallel LLC resonant converter has parameter tolerance. Comparing the mutual cancellation situation, it can be seen from Figure 3 that the output current ripple of the staggered-phase parallel LLC resonant converter is greatly reduced (about 1/5) compared with the single-phase situation, but the existence of parameter tolerances leads to the output current ripple cancellation effect Poor, the output ripple is still large, and the two-phase power distribution is uneven, which reduces the reliability of the system.

The flow chart of the control program for the LLC resonant converter in this embodiment to realize current sharing between phases is shown in Figure 7:

1) The digital controller 3 sets the phase difference angles α ₁ and α between the voltage waveform at both ends of the auxiliary winding and the driving signals of the first switch control inductor 4 and the second switch control inductor 5 through a look-up table method The initial value α _{ini of 2} ; the initial value α _ini is determined by the following formula:

Among them, L _SCI-max is determined by the following formula:

In the formula, a is the turns ratio of the primary winding and the auxiliary winding of the power transformer, L _m-eq.max is the maximum value of the equivalent magnetizing inductance required to realize the zero voltage conduction of the LLC resonant converter, L _{m -ini} is the initial magnetizing inductance value of the power transformer;

2) Sampling the output currents i _o1 and i _{o2 of the} two LLC resonant converters respectively;

3) The digital controller compares the output current i _o1 with the half value of the total output current, and adjusts the phase difference angles α ₁ and α ₂ according to the comparison result to adjust the three elements of the LLC resonant converter resonant cavity in phase-staggered parallel connection (Resonant capacitor, resonant inductance, magnetizing inductance), so as to realize the dynamic adjustment of the LLC resonant cavity magnetizing inductance separately, the adjustment range of the phase difference angle α ₁ and α ₂ is π/2-α _ini .

The specific adjustment rule in step 3) is: when i _{o1 is} less than half of the total output current, the digital controller 3 adjusts the phase difference angle α _{1 to} increase by one unit, and the phase difference angle α _{2 to} decrease by one unit; When i _{o1 is} greater than the half value of the total output current, the digital controller 3 adjusts the phase difference angle α _{1 to} decrease by one unit, and the phase difference angle α _{2 to} increase by one unit; when i _o1 is equal to the half value of the total output current, the phase The difference angles α ₁ and α ₂ remain unchanged.

FIG. 4 shows a control timing diagram of the auxiliary winding voltage waveforms of the LLC resonant converter with two phases staggered in parallel and the driving waveforms of the first switch control inductor 4 and the first switch control inductor 5.

The DC voltage gain of LLC resonant converter can be effectively adjusted by adjusting the size of the excitation inductance of the resonant cavity. The ratio of the magnetizing inductance to the resonant inductance is defined as the value of k. Since the resonant inductance is not adjustable, this embodiment adjusts the value of k by changing the magnetizing inductance. Figure 5 shows the voltage gain simulation comparison of different k resonators under the same Q value. It can be seen that a smaller k value can obtain a larger DC voltage gain after the operating frequency is lower than the resonance frequency. Therefore, changing the magnetizing inductance can realize the adjustment of the DC voltage gain of the LLC resonant cavity, so that it is possible to realize the adjustment of the current balance of the LLC resonant cavity in staggered phase and parallel connection. Figure 6a shows the input current simulation diagrams of three sets of resonant cavity parameters with parameter tolerances. It can be seen that the currents of the three sets of parameters are inconsistent at the same frequency and there are large deviations. Figure 6b shows the three sets of parameters. The result of adjusting the lower excitation inductance shows that the current deviation of the three sets of parameters at the same frequency is very small, and the effect of adjusting the resonant cavity current is achieved.

Taking into account that the positive and negative half-cycle volt-second products of the power transformer are equal, the first switch control inductor 4 and the second switch control inductor 5 adopt the full-wave control method, and it is also considered that the switch control inductor can achieve zero current conduction and Turning off, but non-zero voltage when turning on, so the load winding voltage level of the power transformer in the two-phase converter is lower to reduce the conduction loss.

The relationship curve between the phase difference angle and the equivalent excitation inductance of the resonant cavity is shown in Figure 8.

In the case of input AC power failure, the DC bus voltage begins to drop. Fig. 9 shows a schematic diagram of the DC bus drop and the direction of k value adjustment and the change direction of the phase angle α of the switch control inductor. When the DC bus voltage drops, the method for extending the power-down sustaining time of the staggered parallel LLC resonant converter includes the following steps: first, the closed-loop control system of the LLC resonant converter maintains the stability of the output voltage by adjusting the frequency, and then The digital controller adjusts the phase difference angles α ₁ and α _{2 to} decrease to π/2, so that the magnetizing inductance of the two-way LLC resonant conversion in phase-staggered parallel is reduced, thereby obtaining a larger DC voltage gain, which can be reduced The frequency change range extends the power-off output maintenance time, reduces the required DC bus capacitance value, and achieves power density improvement and cost reduction. The required DC bus capacitance value calculation formula is:

Where n is the turns ratio of the primary winding and the secondary winding of the transformer, Po is the output power, t _hold-up is the required power-down maintenance time, and is the calculation of the maximum DC voltage gain designed by the staggered parallel LLC resonant converter The formula is:

Where k _min is the ratio of the equivalent excitation inductance of the resonant cavity to the inherent resonant inductance when the phase difference angles α ₁ and α _{2 are} equal to π/2, and f _n-min is the minimum operating frequency of the LLC resonant converter and The ratio of the natural frequency of the resonant cavity, Q _max is the quality factor of the resonant cavity under full load.

The foregoing descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modification, equivalent replacement, improvement, etc., made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (9)

An iLLC resonant converter is characterized in that it comprises two LLC resonant converters, a digital controller and two switch control inductors. An auxiliary winding is provided on the power transformer of the LLC resonant converter, and the switch control inductor The parameter and structure of the LLC resonant converter are the same, the input end of the LLC resonant converter is parallel and the output end is also parallel, and the parameters and structure of the switch control inductor are the same; The method for iLLC resonant converter to realize current sharing between phases includes the following steps: 1) The digital controller sets the initial values α _ini of the phase difference angles α ₁ and α ₂ between the voltage waveform across the auxiliary winding and the drive signals of the two switch control inductors; 2) Sampling the output currents i _o1 and i _{o2 of the} two LLC resonant converters respectively; 3) The digital controller compares the output current i _o1 with the half value of the total output current, and adjusts the phase difference angles α ₁ and α ₂ according to the comparison result. The iLLC resonant converter according to claim 1, wherein the LLC resonant converter has a half-bridge structure or a full-bridge structure, the operating frequency of the LLC resonant converter is the same, and the LLC resonant converter The phase difference of the driving signal is π/2. The iLLC resonant converter according to claim 1, wherein the switch control inductor comprises a linear inductor and a bidirectional controllable switch in series, and the bidirectional controllable switch comprises two low on-resistance Power switching device. The iLLC resonant converter according to claim 1, wherein the number of turns of the auxiliary winding is the same, and the number of turns of the auxiliary winding is not greater than the number of turns of the secondary winding. The iLLC resonant converter according to claim 1, wherein the initial value α _ini in the step 1) is determined by the following formula through a look-up table method:

Among them, L _SCI-max is determined by the following formula:

In the formula, a is the turns ratio of the primary winding and the auxiliary winding of the power transformer, L _m-eq.max is the maximum value of the equivalent magnetizing inductance required to realize the zero voltage conduction of the LLC resonant converter, L _{m -ini} is the initial magnetizing inductance value of the power transformer. The iLLC resonant converter according to claim 1, wherein the adjustment range of the phase difference angles α ₁ and α ₂ is π/2-α _ini . The iLLC resonant converter according to claim 1, wherein the adjustment rule of step 3) comprises: First, the output current i _{o1 is} less than half of the total output current, the digital controller adjusts the phase difference angle α _{1 to} increase by one unit, and the phase difference angle α _{2 to} decrease by one unit; Then the output current i _{o1 is} greater than the half value of the total output current, the digital controller adjusts the phase difference angle α _{1 to} decrease by one unit, and the phase difference angle α _{2 to} increase by one unit; Finally, the output current i _o1 is equal to the half value of the total output current, and the phase difference angles α ₁ and α ₂ remain unchanged. A method for using the iLLC resonant converter of any one of claims 1 to 4 to extend the power-down sustaining time, wherein the method comprises the following steps: first, the closed-loop control system of the LLC resonant converter The output voltage is maintained stable by adjusting the frequency, and then the digital controller adjusts the phase difference angles α ₁ and α _{2 to} decrease to π/2, so that the DC voltage gain of the LLC resonant converter reaches the maximum value. The method for extending the power-down sustaining time of an iLLC resonant converter according to claim 8, wherein the calculation formula of the maximum DC voltage gain of the LLC resonant converter is:

Where k _min is the ratio of the equivalent excitation inductance of the resonant cavity to the inherent resonant inductance when the phase difference angles α ₁ and α _{2 are} equal to π/2, and f _n-min is the minimum operating frequency of the LLC resonant converter and The ratio of the natural frequency of the resonant cavity, Q _max is the quality factor of the resonant cavity under full load.

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