CN111371144B

CN111371144B - Control method for reducing capacitance value of charging equipment of new energy electric automobile

Info

Publication number: CN111371144B
Application number: CN202010220487.XA
Authority: CN
Inventors: 刘钧; 冯颖盈; 姚顺; 徐金柱; 胡飞
Original assignee: Shenzhen Vmax Power Co Ltd
Current assignee: Shenzhen Vmax Power Co Ltd
Priority date: 2020-03-25
Filing date: 2020-03-25
Publication date: 2023-05-02
Anticipated expiration: 2040-03-25
Also published as: CN111371144A; WO2021189711A1

Abstract

The invention provides a control strategy for reducing capacitance of a new energy electric vehicle charging device, which comprises the following steps of firstly, calculating a relational expression of capacitance and charging current according to the relation between input and output power and pulsating power and voltage on a capacitor; step two, constraint conditions of charging current and capacitance pulsation peak values are given; and thirdly, calculating the minimum value of the capacitance value under the constraint condition of the charging current and the capacitor pulsation peak value according to the relation between the capacitance value and the charging current. The invention transfers the pulsating power on the capacitor by changing the charging mode of the battery, can obviously reduce the capacitance and the volume of the capacitor, and simultaneously improves the power density of the charging equipment of the new energy electric automobile.

Description

Control method for reducing capacitance value of charging equipment of new energy electric automobile

Technical Field

The invention relates to the technical field of charging of new energy electric vehicles, in particular to a control method for reducing capacitance of a charging device of a new energy electric vehicle.

Background

Along with the promotion of national advocating energy saving and emission reduction and green travel policies, new energy automobiles gradually occupy more and more share in the automobile market, and electric automobiles are the main force armies of the new energy automobiles. Charging equipment for charging an electric automobile, such as a quick charging pile, a vehicle-mounted charger, a wall-mounted charger and the like, is an important component of an electric automobile system, and a high-power-density charging equipment is very important, so that the overall performance of the electric automobile is improved.

In order to ensure that the input voltage and the input current are in phase, the prior charging device internally comprises a PFC converter, which leads to the input power containing a ripple component with frequency multiplication of 2. Typically this portion of the pulsating power is stored in the capacitor on the dc side of the PFC, while a large capacitance and large volume of the capacitor is required because of the dc charging mode. For example, a typical 6.6kW vehicle-mounted charger requires a capacitance of about 2000uF, the capacitance volume being about 10% of the entire charger board volume. Meanwhile, with the rapid development of semiconductor technology, high-frequency and high-density charging equipment is an important point for pursuing various host factories and parts.

With the development of lithium battery technology, more and more researches show that, for example, IEEE Transactions on Vehicular Technology < < The influence of current ripples on the life-time of lithium-ion batteries > >, pulse charging current has little influence on the service life of a battery, and charging mileage, voltage grade, battery temperature and charging current are main factors influencing the service life of the battery.

Therefore, the capacity value of the capacitor of the charging equipment is reduced, the pulsed charging technology is feasible, and on the basis of original control, the capacity value of the charging equipment is reduced, the power density of the charging equipment is improved on the premise of reducing the cost, and the service life of the battery is not reduced.

Disclosure of Invention

The present invention is directed to a control method for reducing capacitance of a capacitor of a charging device, so as to solve the problems set forth in the background art.

The invention provides a control method for reducing capacitance of a capacitor of charging equipment, which comprises the following steps:

step one, calculating a relation expression of capacitance value and charging current according to the relation between input and output power and pulsating power and voltage on a capacitor;

step two, constraint conditions of charging current and capacitance pulsation peak values are given;

and thirdly, calculating the minimum value of the capacitance value under the constraint condition of the charging current and the capacitor pulsation peak value according to the relation between the capacitance value and the charging current.

Further, the relation expression of the capacitance value and the charging current in the first step is as follows:

the input and output real-time power are respectively

Wherein u is _ac And i _ac Input voltage and current respectively, their effective values are V _ac And I _ac Omega is fundamental frequency, i _b The charging current of the battery is Ib, ibrip is the pulsating component of the charging current, vb is the voltage direct current of the battery, and P2 is the power consumed by the second load; neglecting losses of the line and the switching tube, the following expression can be deduced from the power balance

Wherein, pcrip (t) is the pulsating power on the capacitor (average value is zero), uc (t 0) is the initial value of the capacitor voltage, vdc is the average value of the voltage on the capacitor (uc (t)), ucrip (t) is the pulsating voltage, icrip (t) is the current on the capacitor, and C is the capacitance value;

the relation between the capacitance value and the charging current can be calculated by the formula (2)

Further, the specific process of the second step comprises:

the charging modes of the battery are mainly divided into two types, namely direct current charging and sinusoidal charging; as can be seen from the formula (2), when the DC charging mode is adopted, i.e. _brip =0; at this time, the input pulsating power is absorbed by the capacitor, and u _crip (t) in the case of limited peak value, the capacitance of the capacitor will be relatively large; therefore, the pulsating power on the capacitor is transferred by changing the charging mode of the battery, and the charging current is ensured to be constantly greater than zero;

based on the above division plates, the following constraint conditions are obtained

From equations (2) and (6)

The minimum capacitance of the capacitor is only p _crip (t) waveform correlation less than zero; however, when p _crip (t) above zero, which must satisfy the condition of equation (7), and which averages zero and the waveform is symmetrical for half the fundamental period; that is to say p _crip (t) is greater than zero, and the waveform thereof is not limited at all as long as the above-mentioned requirement is satisfied.

Further, in the third step, the first step,

in the working process of the charger, the waveform of the charging current is controlled to track the reference signal, thereby realizing the purpose of adjusting the power absorbed by the battery and indirectly adjusting the pulse power p on the capacitor _crip (t)；p _crip The waveform of (t) directly determines the capacitance of the capacitor; the reference signal of the charging current indirectly determines the capacitance and the volume of the capacitor;

p _crip the waveform of (t) is achieved by adjusting the charging current; when p is _crip (t)>When 0, the waveform can be freely set under the conditions that the constraint condition satisfies the formula (7) and the average value of the waveform is zero and the waveform is symmetrical in half fundamental wave period; therefore, the charging strategy is also under the corresponding constraint condition that p _crip (t) can be freely set when the constraint condition is satisfied.

The invention has the following beneficial effects:

according to the control method for reducing the capacitance value of the capacitor of the charging equipment of the new energy electric automobile, provided by the invention, the pulsating power on the capacitor is transferred by changing the charging mode of the battery, so that the capacitance value and the volume of the capacitor can be obviously reduced, and meanwhile, the power density of the charging equipment is improved.

Drawings

FIGS. 1 (a) and 1 (b) are flowcharts of a control method for reducing capacitance of a charging device according to the present invention;

fig. 2 (a), 2 (b) and 2 (c) are block diagrams of the vehicle-mounted charger according to the present invention;

fig. 3 is a waveform diagram of input power and output power of the charger;

FIG. 4 is a waveform of the voltage on the PFC DC side capacitor;

FIG. 5 is a block diagram of the structure and control strategy of the vehicle-mounted charger;

FIG. 6 p _lim A waveform diagram of (t);

FIG. 7 is p _crip An example waveform diagram of (t);

FIG. 8 is i _b An example waveform diagram of (t);

FIG. 9 is a reference signal block diagram of a charging current;

FIG. 10 is a waveform diagram of a reference signal;

fig. 11 is a waveform diagram of a charging current;

FIG. 12 is a graph of current waveforms through a capacitor;

FIG. 13 is a graph of p derived according to equation (14) _crip An example waveform diagram of (t);

FIG. 14 shows the charging current i in one fundamental period _b A waveform diagram of (t);

FIG. 15 is a control strategy block diagram for reducing the capacitance of a vehicle-mounted charger;

FIG. 16 is a reference signal i of the charging current at the time of abrupt change of the second output load _bref A waveform diagram of (t);

fig. 17 shows the charging current i at the time of abrupt change of the second output load _b A waveform diagram of (t);

FIG. 18 is a graph of p derived according to equation (16) _crip An example waveform diagram of (t);

FIG. 19 is a graph of p derived according to equation (17) _crip An example waveform diagram of (t);

FIG. 20 is a graph of p derived according to equation (25) _crip An example waveform diagram of (t);

FIG. 21 is a diagram of i corresponding to equation (23) _brip A waveform diagram of (t);

FIG. 22 is a diagram of i corresponding to equation (24) _brip A waveform diagram of (t);

FIG. 23 is a diagram of i corresponding to equation (26) _brip A waveform diagram of (t);

FIG. 24 is a diagram of i corresponding to equation (20) _b A waveform diagram of (t);

FIG. 25 is a diagram of i corresponding to equation (21) _b A waveform diagram of (t);

FIG. 26 is a diagram of i corresponding to equation (27) _b Waveform diagram of (t).

Detailed Description

The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

Fig. 1 (a) and 1 (b) present a control strategy for reducing capacitance and volume, comprising the steps of: describing the relationship between the pulsating power and the voltage on the capacitor with a set of equations; obtaining an expression of capacitance and charging current according to the equation set; giving constraint conditions of charging current and capacitance pulsation peak value; obtaining the minimum value of the capacitance value based on the proposed charging strategy; the power density of the vehicle-mounted charger can be improved by using the minimum capacitance.

Fig. 2 (a), fig. 2 (b) and fig. 2 (c) are conventional circuit structures of a charging device, the circuit structures are composed of two conversion modules, namely a PFC (AC/DC) and a DC/DC, an input end of the PFC is connected with a power grid, an output end of the DC/DC conversion circuit is connected with a battery, and a capacitor is connected in parallel with an output end of the PFC and an input end of the DC/DC. The DC/DC conversion circuit comprises two paths of outputs, wherein the first path of output is connected with the high-voltage battery, and the second path of output is connected with the low-voltage battery. In the networking charging process, one path charges a high-voltage battery, and the other path can supply power for a low-voltage battery (connected with a second path of output load (less than 2 kW)). The capacitor can store energy and can absorb input pulsating power, and the capacitance and the volume of the capacitor are closely related to the absorbed pulsating power.

The relation between the power absorbed by the capacitor and the power absorbed by the battery can be deduced from the power balance, and the capacitance value of the PFC DC side capacitor can be determined. Assuming that the real-time power of the input and output are respectively

Wherein u is _ac And i _ac Input voltage and current respectively, their effective values are V _ac And I _ac Omega is fundamental frequency, i _b Charging current for battery, I _b For its average current, i _brip V is the pulsating component of the charging current _b For the voltage (DC) of the battery, P ₂ Power consumed by the second load. Neglecting losses of the line and the switching tube, the following expression can be deduced from the power balance

Wherein p is _crip (t) is the pulsating power on the capacitor (average value is zero), u _c (t ₀ ) Is the initial value of the capacitance voltage, V _dc For the upper voltage u of the capacitor _c Average value of (t), u _crip (t) is its pulsating voltage, i _crip (t) is the current on the capacitor, and C is its capacitance. Generally, the charging modes of a battery are mainly divided into two types, direct current charging and sinusoidal charging (given modes of reference currents). As can be seen from the formula (2), when the DC charging mode is adopted, i.e. _brip =0. At this time, the input pulsating power is absorbed by the capacitor, and u _crip (t) in the case of limited peak value, the capacitance of the capacitor will be relatively large.

Taking a vehicle-mounted charger with a power class of 6.6kW as an example, the voltage on a capacitor at a direct current side is 400V in normal operation, and the maximum fluctuation amount of the voltage on the capacitor is assumed to be 40V (ucrip (t) peak-to-peak value). When a direct current charging mode is adopted, the power absorbed by the battery is active power, and the input pulsating power is stored on a capacitor at the direct current side of the PFC, so that a pulsating component exists in the voltage on the capacitor. The value range of the capacitor can be calculated according to the pulse power absorbed by the capacitor, and the minimum capacitance value is 1313uF. The waveforms of the input and output power of the charger are shown in fig. 3, and the waveform of the voltage on the capacitor is shown in fig. 4.

The capacitance value-Vpp-Ipp-of the capacitor is limited by the ripple current of the capacitor, so that the capacitance value is obtained; pulsating power is also reflected.

As can be seen from fig. 2 (a), 3 and 4, the input pulsating power is absorbed by the capacitor, resulting in a pulsating component in the capacitor voltage. From the above analysis, it is clear that the capacitor needs to absorb all the pulsating power when using the direct current charging method, which results in a large capacitance and volume.

When a sinusoidal charging mode is adopted, as can be seen from the formula (2), the sinusoidal charging mode can transfer the pulsating power on the capacitor, so that the capacitance and the volume of the capacitor are reduced. It is theoretically possible to make the pulsating power on the capacitor zero (p _crip (t) =0), the average value and the ripple component of the charging current are respectively

In practical application, when the input power is constant, the load on the output side of the second path of the DC/DC module absorbs active power, i.e. P ₂ The value of (2) will be greater than zero. When P ₂ >At 0, as can be seen from the formula (3), the pulsating current i _brip (t) the amplitude is greater than the average current value I _b Then the charging current i _b (t) is less than zero, that is, a charge-discharge phenomenon occurs, which, however, reduces the cycle life of the battery. According to the requirements of national standard GB/T31484-2015, the judging standard of the cycle life of the power battery is as follows: complete cycle test when capacity decays to 80% of initial value>Complete cycle test 1000 times, or when the capacity decays to 90% of the initial value>500 times. False, falseLet the charging current charge the battery by using the formula (3), and P ₂ =2000W. The specific gravity a of the discharge amount and the charge amount in a half fundamental period or the specific gravity a of the discharge amount and the charge amount in one complete cycle period can be calculated by the formula (3).

Wherein t is _cro Representing i _b (t) the moment of the first zero crossing. As can be seen from equation (4), about 6 full discharges out of 100 full discharges also consumed about 6 cycle life. Thus, such a charging strategy employing equation (3) reduces the service life of the battery. To avoid this, it is necessary to change the charging strategy of the battery.

Fig. 5 shows a control block diagram of the entire vehicle-mounted charger and proposes a charging strategy that minimizes the capacitance value. The basic idea of the control strategy presented in fig. 5 is to transfer the pulsating power on the capacitor by changing the charging mode of the battery and to ensure that the charging current is constantly greater than zero. The relation between the capacitance value and the charging current can be calculated by the formula (2)

From the above analysis, the following constraints exist:

from equations (2) and (6)

To facilitate analysis of p _crip The maximum value range of (t) defines the equation to the right of the inequality as p _lim (t)。

FIG. 6 shows p in equation (7) _lim (t) waveform in one fundamental period (0.02 s). From the figure, p is found to be within the period of 0 to t1 _lim (t) is less than zero, and p is known by combining the formulas (7) and (2) _crip (t) and the ripple voltage u across the capacitor _crip (t) is less than zero, that is to say the capacitance is in a discharged state. If p is _crip (t) continuously less than zero, then the capacitor voltage continues to decrease. According to the constraint condition (formula (6)), and assuming that the maximum value of the fluctuation amount of the capacitance voltage is 40V (Δv=40v), the capacitance voltage reaches the minimum value. When the capacitance voltage is reduced to the minimum value, if p _crip (t) still less than zero, the capacitance of the capacitor must be increased, otherwise the constraint is not satisfied. The capacitance and p can be deduced from formulas (5) and (6) _crip Relation between (t)

As can be seen from formulas (7) and (8), the condition that the capacitance value C is minimum is:

where T is the period of the fundamental wave signal (t=2pi/ω), and k is an integer.

From the above analysis, the minimum capacitance of the capacitor is only p _crip (t) is related to waveforms less than zero. However, when p _crip (t) is greater than zero, it must satisfy the condition of equation (7), and it averages zero and the waveform is symmetrical for half the fundamental period. That is to say p _crip (t) is greater than zero, and the waveform thereof is not limited at all as long as the above-mentioned requirement is satisfied. FIG. 7 shows p in one fundamental period _crip A waveform (solid line) of (t) and p _lim The waveform (dotted line) of (t) is compared, but is not limited to such waveform, and its expression is

Wherein t is ₁ 、t ₂ 、t ₃ And t ₄ Can be all made of V _ac 、I _ac And P ₂ And (5) calculating.

FIG. 7 only shows p _crip An example waveform of (t), but is not limited to this waveform. When p is _crip (t)<At 0, the waveform thereof is as shown in fig. 7; when p is _crip (t)>When=0, p _crip The waveform of (t) only needs to satisfy the condition that it is zero on average and the waveform is symmetrical in half the fundamental period, while also satisfying the condition of formula (7).

In general, the second output load consumes power P in FIG. 2 (a) ₂ 2kW or less, it is necessary to satisfy the constraint condition (formula (6)) according to P ₂ The minimum capacitance value of the capacitor is designed by combining the maximum value of the capacitor with the formulas (8) and (9) to calculate the minimum capacitance value

The minimum capacity in equation (11) is calculated for a particular application, such as the input power and the power consumed by the second load. The above analysis and derivation is not limited to one particular application, but is still applicable to calculating the minimum capacitance of the capacitor for different levels of input power and output power.

From equation (2) and FIG. 5, p _crip The waveform of (t) is achieved by adjusting the charging current. When p is _crip (t)>At 0, the waveform can be freely set under certain constraint conditions (satisfying the formula (7), and the average value of the waveform is zero and the waveform is symmetrical in half fundamental wave period). Thus, the charging strategy is also under corresponding constraints (such that p _crip (t) meeting its constraint conditions) can be freely set.

During the working process of the charger, the waveform of the charging current is controlled to track the reference signal, thereby realizing the purpose of adjusting the power absorbed by the batteryRegulating pulsating power p on capacitor to ground _crip (t). From equation (11), p is _crip The waveform of (t) directly determines the capacitance of the capacitor. Thus, the reference signal of the charging current indirectly determines the capacitance and the volume of the capacitor. For p in FIG. 7 _crip An example waveform of (t) may give a corresponding one of the charging strategies (as such, not limited to this one in practical applications).

The pulsation component i of the charging current can be reversely deduced by combining the formula (2) and the formula (10) _brip Expression of (t):

from equations (2) and (12), the charging current i can be deduced _b Expression of (t):

i in formula (13) _b The waveform of (t) is shown in FIG. 8, which is a signal with a period of 0.01s, and only the waveform in one fundamental period is shown. The waveforms shown in the figure correspond to p in FIG. 7 _crip The waveform of (t), but is not limited to such a waveform. From the above analysis, the control algorithm in fig. 5 may employ equation (13) as shown in fig. 9, wherein the reference signal of the charging current is obtained from equation (13), but is not limited to this reference signal. The reference signal is differenced from the fed back charging current, an error signal is obtained, and the error signal is adjusted by Repeated Control (RC) to enable the charging current to track the reference signal. A charging strategy is provided to pulse the power p across the capacitor _crip The expression of (t) is shown as formula (10).

To verify the effectiveness of a charging strategy provided in FIG. 9, again taking a vehicle-mounted charger with a power level of 6.6kW as an example, the average voltage across the DC side capacitor is 400V, the maximum pulse is 40V (u _crip (t) peak-to-peak value), the capacitance of the capacitor is 220uF (taking into account a certain margin)Slightly greater than calculated by equation (11), the power P consumed by the second output load ₂ Is 2kW. The reference signal of the charging current is shown in fig. 10, and the waveform of the obtained charging current is shown in fig. 11 by repeatedly controlling such that the charging current follows the reference signal.

FIG. 12 shows the current i flowing through the capacitor _crip (t) whose value is proportional to the pulsating power absorbed in the capacitor (see equation (2)), the waveform in FIG. 12 is also proportional to p in FIG. 7 _crip The waveforms of (t) are similar. The waveform of the voltage on the capacitor is shown in fig. 13, which shows that the peak value of the fluctuation amount is about 20V, satisfying the constraint condition (formula (6)). Therefore, the minimum capacitance calculated by the theory described above is reasonable, and a charge control strategy provided in fig. 9 is also effective.

From theoretical analysis and verification, the minimum capacitance calculated in equation (11) is reasonable. During the actual charging process, the power consumed by the second output load (P ₂ ) When the change occurs, as can be seen from equation (13), the reference signal of the charging current also changes. When P ₂ When the charging current is suddenly added, the charging current does not necessarily track the reference signal immediately because the control needs to pass a certain adjustment time, so that the charging current is pulled down or even smaller than zero. To avoid this, the reference signal for the lower charging current needs to be adjusted. From the above analysis, the minimum capacitance of the capacitor is determined by P ₂ Is determined by the maximum value of (2), the pulsating power on the capacitor can be taken as

Wherein t is ₁₁ The value of (2) can be determined by p _crip (t ₁₁ ) Calculated by =0, t ₁₄ The values of (2) may be calculated in a similar manner and the waveforms in these two intervals are shown in equation (14). But t is ₁₂ And t ₁₃ The value of (2) and p during this time _crip The waveform of (t) is not limited to that given by equation (14), and as such, the waveform is under certain constraints (satisfying equation (7), but also halfWith zero mean and symmetrical waveform) during the fundamental period, fig. 13 gives p according to equation (14) _crip An example waveform of (t).

The pulsating component i of the charging current can be back-deduced by combining equation (2) and equation (14) _brip Expression of (t):

the charging current i can be deduced from the formulas (2) and (16) _b Expression of (t)

Wherein t is ₁₁ 、t ₁₂ 、t ₁₃ And t ₁₄ The value of (2) is the same as that in equation (14), and i is known from equation (2) _b The waveform of (t) is composed of p _crip (t) nor is it limited to the expression given in equation (16).

Equation (16) shows the charging current i _b The minimum value of (t) is the power (P) consumed following the second load ₂ ) Varying, even P ₂ The charging current is not pulled down to a range less than zero by the sudden addition. FIG. 14 shows the charging current i during one fundamental period _b Waveform of (t). In order to ensure that the charging current is not less than zero in the event of a sudden increase in the power consumed by the second load, the reference signal for the charging current in fig. 5 can be obtained from equation (16), and fig. 15 shows a corresponding charging strategy.

To verify the analysis, it is assumed that the power P consumed by the second load is initially ₂ 500W, second stage P ₂ Is projected to 1000W, third stage P ₂ Protruding to 2000W. With the charging strategy provided in fig. 15, reference signal i of charging current _bref (t) As shown in FIG. 16, the charging current i of the battery _b The waveform of (t) is shown in fig. 17. As can be seen from fig. 16 and 17, when the power consumed by the second output load is suddenly increased, the average value of the charging current is summedThe minimum value becomes smaller. When the power consumed by the load is suddenly increased to 2000W (maximum load condition), the minimum value of the charging current is zero. Thus, for the case of a sudden load on the second path, the charging strategy provided in fig. 15 may ensure that the charging current of the battery is greater than or equal to zero.

As a result of the above analysis, t in equation (14) ₁₂ And t ₁₃ The value of (2) and p during this time _crip The waveform of (t) is settable under certain constraints (satisfying equation (7), and its average is zero and its waveform is symmetrical in half the fundamental period). Equations (16) and (17) give p, respectively _crip The waveforms of the other two expressions of (t) are shown in fig. 18 and 19.

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The corresponding charging current i can be deduced from formulas (2), (16) and (17), respectively _b Expression of (t)

Corresponding charging current i _b Expression of (t): formulas (20) and (21) give i, respectively _b The waveforms of the other two expressions of (t) are shown in fig. 24 and 25.

The corresponding expression of the pulsating current ibrip (t), formulas (23) and (24) give two other expressions of ibrip (t), respectively, the waveforms of which are shown in fig. 21 and 22.

Wherein p is _crip16 (t) and p _crip17 (t) is p in formulas (16) and (17), respectively _crip An expression of (t). Similarly, the control algorithm in the charging strategy of fig. 5 may also employ equations (18) or (19) to obtain the reference signal for the charging current. Charging current i _b (t) and p _crip The relation of (t) is shown in formula (2), and p _crip The expression of (t) is freely settable under certain constraints, such as equations (10), (14), (16) and (17). Thus, the charging current i _b (t) is also not limited to that shown in equations (13), (16), (18) and (19), and the charging strategy in fig. 5 is also not limited to that provided in fig. 9 and 15.

According to the theoretical analysis and verification, the control strategy provided by the invention can obviously reduce the capacitance and the volume of the capacitor, and the charging strategy provided by the invention can ensure that the charging current is greater than or equal to zero in the case of abrupt change of the second path of load.

In the case where the input power and the output load are the same as those of the dc charging method:

the capacitance value of the charging strategy capacitor provided by the invention is as follows:

the minimum capacity of the direct current charging mode is 1313uF

Fig. 2 (b) is another common structural form of the vehicle-mounted charger, and when p2=0, the vehicle-mounted charger is a special form of fig. 2 (a), and the capacitor can be omitted under the structure.

Pulsating power p _crip Expression of (t): equation (25) gives p _crip An expression of (t) whose waveform is shown in FIG. 20.

Corresponding pulsating current i _brip Expression of (t): equation (26) gives i _brip An expression of (t) whose waveform is shown in FIG. 23.

Corresponding charging current i _b Expression of (t): equation (27) gives i _b An expression of (t) whose waveform is shown in FIG. 26.

The vehicle-mounted charger with the structure shown in fig. 2 (c) can be changed into the structure shown in fig. 2 (a) by converting two low-voltage loads into one low-voltage load, and the control method is the same as that shown in fig. 2 (a), and the description is omitted here.

The capacitance capacity of the invention is reduced by 83.8%. Therefore, on one hand, the charging strategy provided by the invention can obviously reduce the capacity and the volume of the capacitor, so that the power density of the vehicle-mounted charger is improved; on the other hand, the charging current is ensured to be greater than or equal to zero, the phenomenon of charging and discharging of the battery in each half fundamental wave period is avoided, and the service life of the battery is prolonged.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims.

Claims

1. A control method for reducing capacitance of a new energy electric vehicle charging device is characterized by comprising the following steps:

step one, calculating a relation expression of capacitance value and charging current according to the relation between input and output power of a capacitor and pulsating power on the capacitor and capacitor voltage;

2. The control method according to claim 1, wherein the relation expression of the capacitance value and the charging current in the first step is as follows:

the input and output real-time power are respectively

Wherein u is _ac And i _ac Input voltage and current respectively, their effective values are V _ac And I _ac Omega is fundamental frequency, i _b Charging current for battery, I _b For its average current, i _brip V is the pulsating component of the charging current _b For the direct voltage flow rate of the battery, P ₂ Power consumed by the second load; neglecting losses of the line and the switching tube, the following can be deduced from the power balanceExpression type

Wherein p is _crip (t) is the pulsating power on the capacitor, u _c (t ₀ ) Is the initial value of the capacitance voltage, V _dc For the upper voltage u of the capacitor _c Average value of (t), u _crip (t) is its pulsating voltage, i _crip (t) is the current on the capacitor, C is its capacitance;

calculating the relation between the capacitance value and the charging current

3. The control method according to claim 2, characterized in that step two includes:

based on the above analysis, the following conclusions were drawn:

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wherein DeltaV is the fluctuation of the capacitor voltage; p (P) _lim (t) represents V _ac I _ac (1-cos(2ωt))-P ₂ ；

The minimum capacitance of the capacitor is only p _crip (t) waveform correlation less than zero; however, when p _crip (t) above zero, which must satisfy the above formula conditions, and which averages zero and the waveform is symmetrical for half the fundamental period; that is to say p _crip (t) is greater than zero, and the waveform thereof is not limited at all as long as the above-mentioned requirement is satisfied.

4. A control method according to claim 3, wherein in step three:

in the working process of the charger, the reference signal is tracked by controlling the waveform of the charging current, thereby realizing the purpose of adjusting the power absorbed by the battery and indirectly adjusting the pulsating power p on the capacitor _crip (t)；p _crip The waveform of (t) directly determines the capacitance of the capacitor; the reference signal of the charging current indirectly determines the capacitance and the volume of the capacitor;

p _crip the waveform of (t) is achieved by adjusting the charging current; when p is _crip (t)>When 0, the waveform of the wave satisfies the formula condition under the constraint condition, and can be freely set under the conditions that the average value of the wave is zero and the waveform is symmetrical in half fundamental wave period; therefore, the charging strategy is also under the corresponding constraint condition that p _crip (t) can be freely set when the constraint condition is satisfied.