An adaptive optimization charging method for high-voltage capacitor charging power supply

Abstract

The application provides an adaptive optimization charging method for a LCC resonant converter with a capacitive load based on an improved fundamental equivalent model, to solve the long charging time, large frequency variation range, narrow ZVS range and other defects of a traditional CC charging method. The charging method comprises the following steps: establishing an enhanced fundamental approximation (E-FHA) model; obtaining an analytical expression of a voltage gain M, a resonant current effective value I rms , an average output current I o and a switching frequency f s ; determining the relationship between the maximum average output current and the switching frequency during charging directly through the analytical expression and an AOC method, and realizing a pulse frequency modulation (PFM) frequency control curve. The method provided by the application fully utilizes the working current limit of the switch and the switching frequency limit to obtain a higher average output current, thereby improving the charging speed and reducing the switching frequency range and temperature rise.

Description

TECHNICAL FIELD

[0001] The present application relates to an adaptive optimal charging method for a high-voltage capacitor charging power supply, in particular to an adaptive optimal charging (AOC) method for a high-voltage capacitor charging power supply (CCPS) based on an improved fundamental harmonic equivalent model of an LCC resonant converter. BACKGROUND

[0002] Due to the higher efficiency and power density compared with the traditional low-frequency high-voltage pulse generator, the high-voltage capacitor charging power supply (CCPS) can achieve better steady-state and dynamic performance, and has been widely used. In particular, with the rapid development of power electronics technology, high-power high-voltage capacitor charging power supply (CCPS) has become a key device in the field of X-ray machines, electrostatic precipitators, etc.

[0003] As an important device of high-voltage capacitor charging power supply (CCPS), the LCC resonant converter has a wide load range and can achieve zero-voltage switching in a full power range under certain conditions. In recent years, it has attracted the attention of many researchers and has become a research hotspot in high-voltage CCPS.

[0004] Generally speaking, the main modeling methods of the LCC resonant converter of the high-voltage CCPS include time-domain modeling, state trajectory modeling, and fundamental harmonic approximation (FHA) modeling. Among them, the time-domain analysis method can accurately describe each state of the converter, but due to the large number of resonant elements and complex calculations, it is difficult to obtain the specific expressions of the output voltage and output current; the state trajectory analysis can reflect the detailed working state of the converter, but for the LCC resonant converter applied to the high-voltage CCPS, the range of the state trajectory is different at the initial stage and the end of the charging. Therefore, the control method based on state trajectory analysis also requires a large amount of calculation. In summary, the fundamental harmonic approximation (FHA) modeling is selected.

[0005] Some studies have shown that the prediction error of the FHA model mainly comes from the neglect of high-order harmonics and the influence of rectifiers and filter circuits, etc. In order to deal with the above problems, some researchers have proposed some improved fundamental harmonic approximation equivalent models. For example, the high-order harmonics of the primary input voltage are considered to make the model more accurate; the effects of filter capacitors and rectifier diodes are considered in modeling, etc. However, these models only consider resistive loads with small filter capacitors and do not consider actual pure capacitive charging loads.

[0006] Patent document 1 discloses a high-voltage capacitor charging power supply, which comprises a low-voltage rectifier circuit, a bridge inverter circuit, a resonant circuit, a step-up transformer and a high-voltage rectifier circuit. The low-voltage rectifier circuit is used for AC / DC conversion of alternating current, outputting direct current voltage, the bridge inverter circuit and the resonant circuit are used for resonant conversion of the direct current voltage, outputting bipolar voltage pulses, the step-up transformer and the high-voltage rectifier circuit are used for converting the bipolar voltage pulses into unipolar voltage pulses to charge the high-voltage capacitor; the bridge inverter circuit is composed of an RSD switching unit, the RSD switching unit comprises an RSD stack composed of one or more RSD devices in series and / or parallel, a reverse blocking diode, a reverse protection diode, a first saturable magnetic switch and an RSD pre-charge circuit. But patent document 1 adopts a conventional charging method, the charging speed is slow, the switching frequency range is large, and the utilization rate of the magnetic element is low.

[0007] Therefore, how to design a charging method of the LCC resonant converter with the fastest charging speed for the load of the capacitor becomes a problem to be solved.

[0008] Prior art documents

[0009] Patent documents

[0010] Patent document 1: CN201410348131.9 SUMMARY

[0011] The present application aims to solve the problems of long charging time, large frequency range and narrow ZVS range of the conventional CC charging method. A fundamental wave equivalent model based on an improved LCC resonant converter is proposed, and a fastest charging method based on the model is proposed, which is limited by the frequency and maximum working current of the switch.

[0012] The purposes of the present application include:

[0013] (1) An improved LCC resonant converter fundamental wave equivalent model is proposed;

[0014] (2) An adaptive charging method for charging a pure capacitor load is proposed to improve the charging speed;

[0015] (3) The advantages of the charging method in charging speed, switching frequency range and temperature rise are analyzed.

[0016] To achieve the above purposes, the present application adopts the following technical solutions:

[0017] (1) The relationship expression of the output current I o and the voltage gain M and the normalized switching frequency F n is established;

[0018] (2) Analysis of three stages of charging, upper and lower limits of switching frequency, and maximum switching current are solved;

[0019] (3) The charging time, switching frequency range and temperature rise of the charging method are obtained through experiments. BRIEF DESCRIPTION OF DRAWINGS

[0020] The following will be further described in detail in combination with the drawings and the embodiments implemented by the inventor.

[0021] Figure 1 Three charging stages of adaptive optimization charging proposed by the present application.

[0022] Figure 2 Improved fundamental equivalent model of LCC resonant converter applied in the present application.

[0023] Figure 3 The relationship between output current and voltage gain under different normalized switching frequencies under the present application.

[0024] Figure 4 CCPS experimental platform based on LCC resonant converter of the present application.

[0025] Figure 5 Resonant current and output voltage waveform diagram under the constant current charging method of the present application.

[0026] Figure 6 Resonant current and output voltage waveform diagram under the adaptive optimization charging method of the present application.

[0027] Figure 7 Temperature distribution of power semiconductor devices at the end of charging under the constant current charging method of the present application.

[0028] Figure 8 Temperature distribution of power semiconductor devices at the end of charging under the adaptive optimization charging method of the present application.

[0029] In the drawings, the corresponding Chinese meanings of each English word are as follows:

[0030] Drvier: driver

[0031] Carrier generator: carrier generator

[0032] Start-up mode: start-up mode

[0033] Stage: stage

[0034] Modulator: modem

[0035] Charging mode: charging mode

[0036] Adaptive optimal charging mode

[0037] Optimal calculation

[0038] 500W LCC resonant CCPS prototype

[0039] capacitive load

[0040] resonant capitor

[0041] Oscilloscope

[0042] half bridge modules and drivers

[0043] Auxiliary powersupply

[0044] DC power supply

[0045] Control board

[0046] hall and current sampling circuit

[0047] Voltage sampling circuit

[0048] step-up transformer

[0049] Output voltage

[0050] Resonant current

[0051] Temperture DETAILED DESCRIPTION

[0052] The adaptive charging method mainly has three charging stages based on the LCC resonant converter circuit topology, as shown in the following figure. Figure 1

[0053] The first charging stage, in the initial charging stage, in order to have a faster charging speed and a smaller resonant current effective value, a fixed switching frequency f​s1 Charging until the output voltage reaches U ref1 .

[0054] The second charging stage, the second charging stage is targeted at the maximum charging speed, the power conservation law is established, the maximum switching current of the switch, the maximum switching frequency of the switch, the minimum frequency of realizing soft switching is limited, the objective function and the constraint condition are established, the maximum output current is solved under different output voltage values, so as to realize the fastest charging speed.

[0055] The power conservation law needs to establish the switching loss model of the primary and secondary sides, as shown in equations (1)-(4).

[0056]

[0057]

[0058]

[0059] P Loss' =P S_Cond +P S_off +P D_Cond (4)

[0060] Based on the loss model, the power conservation is expressed as equation (5).

[0061]

[0062] The range of switching frequency is as equation (6).

[0063] f smin <f s <f smax (6)

[0064] The maximum switching current limit can be expressed as equation (7).

[0065]

[0066] Through the above equations, the maximum output current value under different voltage gain and the corresponding switching frequency at this time can be solved, and the maximum charging speed is realized through frequency control.

[0067] The third charging stage, the constant voltage charging is realized by intermittent current charging, the judgment condition for entering this stage is that the output voltage reaches U ref When the output voltage is higher than U ref2 , the converter works, reaches the voltage U ref , the converter stops working, and through such intermittent working, the working efficiency of the converter at light load is improved.

[0068] The application is based on the circuit diagram of LCC resonant converter CCPS, based on the circuit diagram, when charging the high-voltage CCPS capacitive load, according to the changing characteristics of the load, an improved fundamental equivalent model is established as shown in Figure 2 Due to the non-resistance characteristics of the load, the load resistance R e and the capacitance C e change with the change of output voltage and current, the expressions are formula (8) and formula (9). Wherein the input impedance can be expressed as formula (10). The relationship between output current and voltage gain and switching frequency of the converter is shown in Figure 3 .

[0069]

[0070]

[0071]

[0072] As shown in Figure 4 , the adaptive optimization charging method for high-voltage capacitor charging power supply proposed in the application is verified on a 500W LCC resonant converter CCPS experimental platform, and the experimental results are shown in Figure 6 and 8 . Specifically, Figure 6 the resonant current and output voltage waveform of the resonant converter using the adaptive optimization charging method of the application, Figure 8 the temperature distribution of the switching device using the adaptive charging method of the application is shown.

[0073] At the same time, in order to compare, the technical effect of using the prior art is shown. Specifically, Figure 5 the output voltage and resonant current waveform of the constant current charging are shown when the maximum current stress of the switching device is set to be the same as the prior art; Figure 7 the temperature distribution of the switching device using the constant current charging strategy at the end of charging is shown.

[0074] Comparing Figure 5 with Figure 6 , Figure 7 with Figure 8 , it can be seen that the adaptive charging method proposed in the application can realize faster charging speed without increasing the switching current stress, and compared with the traditional constant current charging strategy, it can save 40.8% of the charging time. The adaptive charging strategy proposed in the application can realize faster charging speed without increasing the switching current stress, in addition, compared with the constant current charging strategy, the switching frequency variation range is smaller, and the temperature rise of the switching device at the end of charging is lower.

Claims

1. An adaptive optimal charging method for a LCC resonant converter with load as capacitive, characterized by with the limitation conditions of the switching device including maximum operating current I max and maximum switching frequency f smax and minimum frequency satisfying soft switching f smin and the power conservation law, the adaptive charging method seeking the maximum output current to achieve the maximum charging speed, the objective function and the constraint condition are expressed as equation (1): （1） The charging method mainly includes three charging stages: Charging first phase: fixed switching frequency is used f s1 Charging until the output voltage reaches a first reference voltage U ref1 ; a second charging phase: when the output voltage reaches the first reference voltage U ref1 entering the second charging phase, in which charging is performed at a maximum charging speed. The third charging stage: intermittent current charging under light load, entering and maintaining constant voltage charging mode.

2. The adaptive optimal charging method for a load capacitive LCC resonant converter according to claim 1, characterized in that , in the charging second phase, the loss in the constraint function P Loss are obtained from equations (2) - (5); wherein P Loss conduction loss of the MOSFET P S_Cond , see equation (3) and turn-off loss P S_off , see equation (4) and conduction loss of the secondary diode P D_Cond , see equation (5) (2) （3） （4） （5） 。 3. The adaptive optimal charging method for a load capacitive LCC resonant converter according to claim 1, wherein, The above charging method is based on an improved fundamental equivalent model. Due to the non-resistive characteristics of the load, the load resistance equivalent to the primary side is R e and a capacitor C e Changes occur with changes in output voltage and current, and the expressions are formula (6) and formula (7); （6） （7）。 4. The adaptive optimal charging method for a load capacitive LCC resonant converter according to claim 1 or 2, characterized in that, The above charging method is based on the improved fundamental equivalent model, and in the third charging stage, the input impedance is as shown in formula (8): （8）。

Images