WO2021189711A1

WO2021189711A1 - Control method for reducing capacitance value of capacitor of charging device for new energy electric vehicle

Info

Publication number: WO2021189711A1
Application number: PCT/CN2020/101128
Authority: WO
Inventors: 刘钧; 冯颖盈; 姚顺; 徐金柱; 胡飞
Original assignee: 深圳威迈斯新能源股份有限公司
Priority date: 2020-03-25
Filing date: 2020-07-09
Publication date: 2021-09-30
Also published as: CN111371144A; CN111371144B

Abstract

A control policy for reducing a capacitance value of a capacitor of a charging device for a new energy electric vehicle. The policy comprises: step I, according to a relationship between an input power and an output power and a relationship between a pulsating power and voltage on a capacitor, performing calculation to obtain a relational expression between a capacitance value of the capacitor and a charging current; step II, giving constraint conditions for the charging current and the pulsation peak value of the capacitor; and step III, according to the relationship between the capacitance value of the capacitor and the charging current, calculating the minimum value of the capacitance value of the capacitor under the constraint conditions for the charging current and the pulsation peak value of the capacitor. In the control policy, the pulsating power on the capacitor is transferred by means of changing a charging mode of a battery, thereby significantly reducing the capacitance value and volume of the capacitor, and also increasing the power density of a charging device for a new energy electric vehicle. Moreover, the policy ensures that the charging current is constantly greater than zero when a low-voltage load at an output end is dynamic, thereby prolonging the service life of the battery.

Description

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

Technical field

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

Background technique

As the country advocates energy conservation, emission reduction, and the implementation of green travel policies, new energy vehicles are gradually occupying more and more shares in the automobile market, and electric vehicles are the main force of new energy vehicles. Charging equipment used to charge electric vehicles, such as fast charging piles, on-board chargers, wall-mounted chargers, etc., are an important part of the electric vehicle system. A high-power-density charging device is very important, and it helps to improve The overall performance of electric vehicles.

In order to ensure that the input voltage and the input current are in the same phase, the charging equipment of the prior art includes a PFC converter inside, which causes the input power to contain a pulsating component of 2 times the frequency. Usually this part of the pulsating power is stored in the capacitor on the DC side of the PFC. At the same time, because of the DC charging mode, a large capacitance and large volume capacitor is required. For example, a typical 6.6kW vehicle-mounted charger requires a capacitor of about 2000uF, and the volume of the capacitor accounts for about 10% of the volume of the entire charger board. At the same time, with the rapid development of semiconductor technology, high-frequency, high-density charging equipment has become the focus of various OEMs and parts.

With the development of lithium battery technology, more and more studies have shown that, for example, in IEEE Transactions on Vehicular Technology <<The influence of current ripples on the life-time of lithium-ion batteries>>, the impact of pulsed charging current on the battery Life expectancy is very small, charging mileage, voltage level, battery temperature, and charging current are the main factors affecting battery life.

Therefore, it is feasible to reduce the capacitance value of charging equipment and the use of pulse charging technology, and on the basis of the original control, the advantage is to reduce its own capacitance value and reduce the cost, and increase the power density of the charging equipment without reducing it. The service life of the battery.

Summary of the invention

The object of the present invention is to provide a control method for reducing the capacitance value of the charging device, so as to solve the problems raised in the background art.

The present invention provides a control method for reducing the capacitance of a charging device, which includes the following steps:

Step 1: Calculate the relationship expression between the capacitance value of the capacitor and the charging current according to the relationship between the input and output power and the pulsating power on the capacitor and the voltage;

Step 2: Give the constraint conditions of the charging current and the peak value of the capacitor ripple;

Step 3: According to the relationship between the capacitance value of the capacitor and the charging current, the minimum value of the capacitance value is calculated under the constraints of the charging current and the peak value of the capacitor pulsation.

Further, the relationship between the capacitance value of the capacitor and the charging current in step one is expressed as follows:

The real-time power input and output are

Among them, u _ac and i _ac are the input voltage and current respectively, their effective values are V _ac and I _{ac respectively} , ω is the fundamental frequency, i _b is the battery charging current, Ib is the average current, and ibrip is the pulsation of the charging current Component, Vb is the DC voltage of the battery, and P2 is the power consumed by the second load; ignoring the loss of the line and switch tube, the following expression can be derived based on the power balance

Where pcrip(t) is the pulsating power on the capacitor (the average value is zero), uc(t0) is the initial value of the capacitor voltage, Vdc is the average value of the voltage on the capacitor (uc(t)), and ucrip(t) is The pulsating voltage, icrip(t) is the current on the capacitor, and C is the capacitance value;

The relationship between the capacitance value of the capacitor and the charging current can be calculated by formula (2)

Further, the specific process of step two includes:

The battery charging method is mainly divided into two types, DC charging and sinusoidal charging; from the formula (2), when the DC charging method is adopted, i _brip = 0; at this time, the input pulsating power is all absorbed by the capacitor. When the _{peak value of u crip} (t) is limited, the capacitance of the capacitor will be relatively large; therefore, it is necessary to change the charging method of the battery to transfer the pulsating power on the capacitor, and ensure that the charging current is always greater than zero;

Based on the above sub-boards, the following constraints are drawn

It can be seen from formulas (2) and (6)

The minimum value of the capacitor only with p _crip (t) of less than about zero waveform; inner However, when p _crip (t) is greater than zero, it must satisfy the formula (7) are satisfied, there is a half cycle of the fundamental wave which The average is zero and the waveform is symmetrical; that is, when p _crip (t) is greater than zero, as long as the above requirements are met, there are no other restrictions on the waveform.

Further, in step three,

In the working process of the charger, the waveform of the charging current is controlled to track the reference signal, so as to adjust the power absorbed by the battery, and indirectly adjust the pulsating power on the capacitor p _crip (t); p _crip (t) The waveform directly determines the capacitance of the capacitor; the reference signal of the charging current indirectly determines the capacitance and volume of the capacitor;

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

The present invention has the following beneficial effects:

The control method for reducing the capacitance value of the new energy electric vehicle charging equipment provided by the present invention transfers the pulsating power on the capacitor by changing the charging method of the battery, which can significantly reduce the capacitance value and volume of the capacitor, and at the same time increase the power density of the charging equipment .

Description of the drawings

Figure 1 (a), (b) is a flow chart of a control method for reducing the capacitance of a charging device according to the present invention;

Figure 2 (a), (b) and (c) are structural block diagrams of the on-board charger of the present invention;

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

Figure 4 shows the waveform of the voltage on the DC side capacitor of the PFC;

Figure 5 is a structural block diagram and control strategy of the on-board charger;

Figure 6 is _{a waveform diagram of p lim} (t);

Figure 7 is an example waveform diagram _{of p crip (t);}

Fig. 8 is an example waveform diagram _{of i b (t);}

Figure 9 is a block diagram of the reference signal of the charging current;

Figure 10 is a waveform diagram of the reference signal;

Figure 11 is a waveform diagram of the charging current;

Figure 12 is a waveform diagram of the current flowing through the capacitor;

Fig. 13 is an example waveform diagram of _{p crip} (t) derived according to formula (14);

Figure 14 is a waveform diagram of the charging current i _b (t) in a fundamental period;

Figure 15 is a block diagram of a control strategy for reducing the capacitance of an on-board charger;

_{FIG. 16 is a waveform diagram of the reference signal i bref} (t) of the charging current when the output load of the second path changes suddenly;

Figure 17 is a waveform diagram of the _{charging current i b} (t) when the second output load changes suddenly;

Fig. 18 is an example waveform diagram of _{p crip} (t) derived according to formula (16);

Fig. 19 is an example waveform diagram of _{p crip} (t) derived according to formula (17);

Fig. 20 is an example waveform diagram of _{p crip} (t) derived according to formula (25);

Figure 21 is a waveform diagram _{of i brip} (t) corresponding to formula (23);

Fig. 22 is a waveform diagram _{of i brip} (t) corresponding to formula (24);

Figure 23 is a waveform diagram _{of i brip} (t) corresponding to formula (26);

Fig. 24 is a waveform diagram _{of i b} (t) corresponding to formula (20);

Fig. 25 is a waveform diagram _{of i b} (t) corresponding to formula (21);

Fig. 26 is a waveform diagram _{of i b} (t) corresponding to formula (27).

Detailed ways

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.

Figure 1 shows the control strategy to reduce the capacitance value and volume of the capacitor, including the following steps: use equations to describe the relationship between input and output power and the pulsating power and voltage on the capacitor; according to the equations, the capacitance value and charging of the capacitor are obtained. The expression of the current; the constraint conditions of the charging current and the peak value of the capacitor ripple are given; the minimum value of the capacitor value is obtained based on the proposed charging strategy; the use of the smallest capacitor can improve the power density of the on-board charger.

Figure 2 shows the circuit structure of a conventional charging device. The circuit structure consists of two conversion modules, PFC (AC/DC) and DC/DC. The input of the PFC is connected to the grid, and the output of the DC/DC conversion circuit is connected to the battery. It is connected in parallel with the output terminal of PFC and the input terminal of DC/DC. The DC/DC conversion circuit includes two outputs, the first output is connected to a high-voltage battery, and the second output is connected to a low-voltage battery. During the networked charging process, one way is to charge the high-voltage battery, and the other way can supply power to the low-voltage battery (connected to the second output load (less than 2kW)). Capacitors can store energy and can absorb the input pulsating power, and its capacitance and volume are closely related to the absorbed pulsating power.

According to the power balance, the relationship between the power absorbed by the capacitor and the power absorbed by the battery can be derived, and the capacitance value of the PFC DC side capacitor can also be determined. Suppose the real-time power of input and output are respectively

Where u _ac and i _ac are the input voltage and current respectively, their effective values are V _ac and I _{ac respectively} , ω is the fundamental frequency, i _b is the battery charging current, Ib is the average current, and ibrip is the pulsation of the charging current Component, Vb is the voltage (DC) of the battery, and P2 is the power consumed by the second load. Ignoring the loss of the line and the switch tube, the following expression can be derived according to the power balance

Where pcrip(t) is the pulsating power on the capacitor (the average value is zero), uc(t0) is the initial value of the capacitor voltage, Vdc is the average value of the voltage on the capacitor (uc(t)), and ucrip(t) is The pulsating voltage, icrip(t) is the current on the capacitor, and C is the capacitance value. Generally, the charging method of the battery is mainly divided into two types, DC charging and sinusoidal charging (reference current given method). It can be seen from formula (2) that when the direct current charging method is adopted, that is, ibrip=0. At this time, the input pulsating power is all absorbed by the capacitor. When the ucrip(t) peak value is limited, the capacitance of the capacitor will be relatively large.

Take an on-board charger with a power level of 6.6kW as an example. During normal operation, the voltage on the DC side capacitor is 400V, assuming that the maximum voltage ripple on the capacitor is 40V (ucrip(t) peak-to-peak value). When the DC charging method is used, the power absorbed by the battery is active power, and the input pulsating power will be stored on the capacitor on the DC side of the PFC, resulting in a pulsating component in the voltage on the capacitor. According to the pulsating power absorbed by the capacitor, the value range of the capacitor can be calculated, and the smallest capacitance value is 1313uF. The waveform of the input and output power of the charger is shown in Figure 3, and the voltage waveform on the capacitor is shown in Figure 4.

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

It can be seen from Figure 2(a), Figure 3 and Figure 4 that the input pulsating power is absorbed by the capacitor, resulting in a pulsating component in the capacitor voltage. From the above analysis, it can be seen that when the DC charging method is used, the capacitor needs to absorb all the pulsating power, resulting in a large capacitance and volume.

When the sinusoidal charging method is used, it can be seen from formula (2) that the sinusoidal charging method can transfer the pulsating power on the capacitor, thereby reducing the capacitance and volume of the capacitor. Theoretically, the pulsating power on the capacitor can be zero (p _crip (t) = 0), then the average value and the pulsating component of the charging current are respectively

In practical applications, when the input power is constant, the load on the second output side of the DC/DC module will absorb active power, that is _{, the value of P 2} will be greater than zero. When P ₂ >0, it can be seen from formula (3) that _{the amplitude of the pulsating current i brip} (t) is greater than the average current value I _b , then the charging current i _b (t) will be less than zero, that is, charging and discharging will occur Phenomenon, but this phenomenon will reduce the cycle life of the battery. According to the requirements of the national standard GB/T31484-2015, the judgment standard of power battery cycle life is: when the capacity decays to 80% of the initial value, complete cycle test>1000 times, or when the capacity decays to 90% of the initial value, complete cycle test >500 times. Assume that the charging current uses formula (3) to charge the battery, and P ₂ =2000W. Formula (3) can be used to calculate the specific gravity A of the discharge and the charge in a half fundamental cycle or the specific gravity A of the discharge and the charge in a complete cycle.

Where t _cro represents _{the moment when i b} (t) crosses zero for the first time. From the formula (4), it can be seen that there are about 6 full discharges in the process of 100 full charging, which also consumes about 6 cycles of cycle life. Therefore, this charging strategy using formula (3) reduces the service life of the battery. In order to avoid this situation, the battery charging strategy needs to be changed.

Figure 5 shows the control block diagram of the entire on-board charger and proposes a charging strategy that minimizes the capacitance of the capacitor. The basic idea of the control strategy proposed in Figure 5 is to transfer the pulsating power on the capacitor by changing the charging method of the battery, and to ensure that the charging current is always greater than zero. The relationship between the capacitance value of the capacitor and the charging current can be calculated by formula (2)

From the above analysis, we can see that there are the following constraints:

It can be seen from formulas (2) and (6)

In order to facilitate the analysis _{of the maximum value range of p crip} (t), the formula on the right side of the inequality is defined as p _lim (t).

_{Figure 6 shows the waveform of p lim} (t) in formula (7) within a fundamental period (0.02s). It can be seen from the figure that in the time period from 0 to t1, p _lim (t) is less than zero. Combining formulas (7) and (2), it can be seen that both p _crip (t) and the pulsating voltage u _crip (t) on the capacitor are less than zero. , Which means that the capacitor is in a discharged state. If p _crip (t) continues to be 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 capacitor voltage ripple is 40V (ΔV=40V), the capacitor voltage reaches the minimum value at this time. When the capacitor voltage drops to the minimum value, if p _crip (t) is still less than zero, then the capacitance value of the capacitor must be increased, otherwise the constraint conditions are not met. The relationship between the _{capacitance value and p crip} (t) can be derived by formulas (5) and (6)

According to formulas (7) and (8), the conditions for the minimum capacitance value C are:

Where T is the period of the fundamental signal (T=2π/ω), and k is an integer.

According to the above analysis, the minimum capacitance value of the capacitor is only related to _{the waveform when p crip} (t) is less than zero. However, when p _crip (t) is greater than zero, it must meet the condition of formula (7), and the average is zero in half of the fundamental wave period and the waveform is symmetrical. That is to say, when p _crip (t) is greater than zero, as long as the above requirements are met, there are no other restrictions on the waveform. Figure 7 shows a waveform (solid line) _{of p crip} (t) in a fundamental wave period and _{compares it with the waveform of p lim} (t) (dashed line), but it is not limited to this waveform, and its expression is

Among them, the values of _{t 1} , t ₂ , t ₃ and t ₄ can all be calculated _{by V ac} , I _ac and P _2.

Fig. 7 only _{shows an example waveform of p crip} (t), but it is not limited to this kind of waveform. When p _crip (t)<0, the waveform is shown in Figure 7; when p _crip (t)>=0, _{the waveform of p crip} (t) only needs to satisfy the average value of zero in half of the fundamental period. And the waveform is symmetrical, and it also satisfies the condition of formula (7).

_{Generally, the power P 2} consumed by the second output load in Figure 2(a) is less than or equal to 2kW. In order to meet the constraint condition (Equation (6)), the minimum capacitance value of the capacitor _{must be designed according to the maximum value of P 2, and then combined} Formula (8) and (9) can calculate the smallest capacitance value

The calculation of the minimum capacitance value in formula (11) is for a specific application, for example, the input power and the power consumed by the second load are set. However, the above analysis and derivation are not limited to a specific application. In the case of different levels of input power and output power, they are still suitable for calculating the minimum capacitance of the capacitor.

From formula (2) and Figure 5, we can see that _{the waveform of p crip} (t) is achieved by adjusting the charging current. When p _crip (t)>0, its waveform can be set freely under certain constraints (satisfying formula (7), and its average is zero in half of the fundamental wave period and the waveform is symmetrical). Therefore, the charging strategy can also be set freely under the corresponding constraint conditions (such that p _crip (t) satisfies its constraint conditions).

In the working process of the charger, the waveform of the charging current is controlled to track the reference signal, thereby achieving the purpose of adjusting the power absorbed by the battery, and indirectly adjusting the pulsating power p _crip (t) on the capacitor. From formula (11), we can see that _{the waveform of p crip} (t) directly determines the capacitance of the capacitor. Therefore, the reference signal of the charging current indirectly determines the capacitance and volume of the capacitor. _{For an example waveform of p crip} (t) in FIG. 7, a corresponding charging strategy can be given (similarly, it is not limited to this charging strategy in practical applications).

Combining formula (2) and formula (10) can reverse _{the expression of the pulsating component i brip} (t) of the charging current:

From formulas (2) and (12), the expression _{of charging current i b (t) can be derived:}

_{The waveform of i b} (t) in formula (13) is shown in Figure 8. It is a signal with a period of 0.01s. The figure only shows the waveform within a fundamental period. The waveform shown in the figure corresponds to the waveform of p _crip (t) in Figure 7, but is not limited to this kind of waveform. According to the above analysis, the control algorithm in Fig. 5 can adopt formula (13) as shown in Fig. 9, wherein the reference signal of the charging current is obtained by formula (13), but it is not limited to this kind of reference signal. Make the difference between the reference signal and the feedback charging current, and then obtain the error signal, and then use the repetitive control (RC) to adjust the error signal to make the charging current track the reference signal. A charging strategy provided in the figure can make _{the expression of the pulsating power p crip} (t) on the capacitor as in formula (10).

In order to verify the effectiveness of a charging strategy provided in Figure 9, also take an on-board charger with a power level of 6.6kW as an example. The average voltage on the DC side capacitor is 400V, and the maximum ripple is 40V (u _crip ( t) Peak-to-peak value), the capacitance value of the capacitor is 220uF (considering a certain margin, slightly larger than that calculated by formula (11)), the power P ₂ consumed by the second output load is 2kW. The reference signal of the charging current is shown in FIG. 10, the charging current is made to track the reference signal through repeated control, and the waveform of the obtained charging current is shown in FIG. 11.

Figure 12 shows the current i _crip (t) flowing through the capacitor. Its value is proportional to the pulsating power absorbed by the capacitor (see formula (2)). The waveform in Figure 12 is also similar to p _crip (t) in Figure 7. The waveform is similar. The waveform of the voltage on the capacitor is shown in Figure 13, which shows that the peak value of its pulsation is about 20V, which satisfies the constraint condition (Equation (6)). Therefore, the minimum capacitance value calculated by the above theory is reasonable, and the charging control strategy provided in Figure 9 is also effective.

Through theoretical analysis and verification, it can be known that the minimum capacitance value calculated by formula (11) is reasonable. In the actual charging process, when the power (P ₂ ) consumed by the second output load changes, it can be seen from formula (13) that the reference signal of the charging current will also change. When P _{2 is} suddenly increased, because the control needs to pass a certain adjustment time, the charging current may not immediately track the reference signal, resulting in the charging current being pulled down or even less than zero. In order to avoid this situation, the reference signal of the charging current needs to be adjusted. It can be seen from the above analysis that the minimum capacitance value of the capacitor is determined according to _{the maximum value of P 2} , and the pulsating power on the capacitor can be taken as

The value of t ₁₁ can be determined, and it can be calculated by p _crip (t ₁₁ ) = 0. _{The value of t 14} can also be calculated according to a similar method, and the waveforms in these two intervals are as shown in formula (14) Shown. However, the values of _{t 12} and t ₁₃ _{and the waveform of p crip} (t) during this period are not limited to those given by formula (14). Similarly, the waveform is subject to certain constraints (satisfying formula (7), and there is half The average of the fundamental wave period is zero and the waveform is symmetrical) can be set freely. Figure 13 shows an example waveform _{of p crip (t) according to formula (14).}

Combining formula (2) and formula (14) _{, the expression of the pulsating component i brip} (t) of the charging current can be deduced:

According to formulas (2) and (16), the expression _{of charging current i b (t) can be derived}

Among them _{, the values of t 11} , t ₁₂ , t ₁₃ and t ₁₄ are the same as those in formula (14), and from formula (2), it can be seen that _{the waveform of i b} (t) is determined by p _crip (t), and also It is not limited to the expression given by formula (16).

Equation (16) shows that _{the minimum value of the charging current i b} (t) changes with the power consumed by the second load (P ₂ ). Even if P _{2 is} suddenly added, the charging current will not be pulled down to less than zero. Scope. Figure 14 shows the waveform of the charging current i _b (t) in a fundamental period. In order to ensure that the charging current is not less than zero when the power consumed by the second load is suddenly increased, the reference signal of the charging current in Fig. 5 can be obtained by formula (16), and Fig. 15 shows the corresponding charging strategy.

In order to verify the above analysis, suppose that the power P ₂ consumed by the second load is 500 W at the _{beginning, P 2 is} suddenly increased to 1000 W in the second phase, and P _{2 is} suddenly increased to 2000 W in the third phase. Using the charging strategy provided in FIG. 15, the reference signal i _bref (t) of the charging current is shown in FIG. 16, and _{the waveform of the charging current i b} (t) of the battery is shown in FIG. 17. It can be seen from Figures 16 and 17, that when the power consumed by the second output load is suddenly increased, the average and minimum value of the charging current will both become 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. Therefore, for the sudden increase of the second load, the charging strategy provided in Figure 15 can ensure that the charging current of the battery is greater than or equal to zero.

As the above analysis shows, _{the values of t 12} and t ₁₃ _{in formula (14) and the waveform of p crip} (t) during this period are under certain constraints (satisfying formula (7), and within half of the fundamental period The average is zero and its waveform is symmetric) can be set. Equations (16) and (17) respectively give the _{other two expressions of p crip} (t), and the waveforms are shown in Figure 18 and Figure 19.

According to formulas (2), (16) and (17), the corresponding charging current i _b (t) expressions can be derived respectively

Corresponding expressions of charging current i _b (t): Formulas (20) and (21) respectively give the _{other two expressions of i b} (t), and the waveforms are shown in Figure 24 and Figure 25.

Corresponding to the expression of the pulsating current ibrip(t), formulas (23) and (24) respectively give the other two expressions of ibrip(t), and the waveforms are shown in Figure 21 and Figure 22.

Where p _crip16 (t) and p _crip17 _{(t) are the expressions of p crip} (t) in formulas (16) and (17), respectively. Similarly, the control algorithm in the charging strategy in FIG. 5 can also adopt formula (18) or (19) to obtain the reference signal of the charging current. The relationship between charging current i _b (t) and p _crip (t) is shown in formula (2), and _{the expression of p crip} (t) can be set freely under certain constraints, as shown in formula (10) , (14), (16) and (17). Therefore, the charging current i _b (t) is not limited to those shown in formulas (13), (16), (18) and (19), and the charging strategy in Figure 5 is also not limited to those provided in Figures 9 and 15 .

According to the above theoretical analysis and verification, the control strategy proposed by the present invention can significantly reduce the capacitance and volume of the capacitor, and in view of the sudden change of the second load, the proposed charging strategy can also ensure that the charging current is greater than Or equal to zero.

Compared with the DC charging method, when the input power and output load are the same:

The capacitance of the charging strategy capacitor proposed by the present invention:

The minimum capacitance of DC charging method is 1313uF

Figure 2(b) is another common structural form of a vehicle-mounted charger. When P2=0, the vehicle-mounted charger of this structure is a special form of Figure 2(a), and the capacitor can be eliminated under this structure.

The expression of pulsating power p _crip (t): Formula (25) gives _{an expression of p crip} (t), and its waveform is shown in Figure 20.

Corresponding pulsating current i _brip (t) expression: formula (26) gives _{an expression of i brip} (t), and its waveform is shown in Figure 23.

The expression of the corresponding charging current i _b (t): Equation (27) gives _{an expression of i b} (t), and its waveform is shown in Figure 26.

Figure 2(c) The vehicle-mounted charger of this structure can be transformed into Figure 2(a) by converting two low-voltage loads to one.

The capacitance of the capacitor of the invention is reduced by 83.8%. Therefore, on the one hand, the charging strategy provided by the present invention can significantly reduce the capacity and volume of the capacitor, thereby increasing the power density of the on-board charger; on the other hand, ensure that the charging current is greater than or equal to zero, and avoid the battery in every half of the fundamental cycle. The phenomenon of charging and discharging can help improve the service life of the battery.

For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-limiting, and the protection scope of the present invention is determined by the appended claims.

Claims

A control method for reducing the capacitance value of a new energy electric vehicle charging device is characterized in that it comprises the following steps:

Step 1: Calculate the relationship expression between the capacitance value of the capacitor and the charging current according to the relationship between the input and output power and the pulsating power on the capacitor and the voltage;

Step 2: Give the constraint conditions of the charging current and the peak value of the capacitor ripple;

Step 3: According to the relationship between the capacitance value of the capacitor and the charging current, the minimum value of the capacitance value is calculated under the constraints of the charging current and the peak value of the capacitor pulsation.
The control method according to claim 1, wherein the relationship between the capacitance value of the capacitor and the charging current in step one is expressed as follows:

The real-time power input and output are

Among them, u ac and i ac are the input voltage and current respectively, their effective values are V ac and I ac respectively , ω is the fundamental frequency, i b is the battery charging current, Ib is the average current, and ibrip is the pulsation of the charging current Component, Vb is the DC voltage of the battery, and P2 is the power consumed by the second load; ignoring the loss of the line and switch tube, the following expression can be derived based on the power balance

Where p crip (t) is the pulsating power on the capacitor (the average value is zero), u c (t 0 ) is the initial value of the capacitor voltage, V dc is the average value of the voltage on the capacitor (u c (t)), u crip (t) is the pulsating voltage, i crip (t) is the current on the capacitor, and C is the capacitance value;

Calculate the relationship between the capacitance value of the capacitor and the charging current
The control method according to claim 2, wherein step two comprises:

The battery charging method is mainly divided into two types, DC charging and sinusoidal charging; from the formula (2), when the DC charging method is adopted, i brip = 0; at this time, the input pulsating power is all absorbed by the capacitor. When the peak value of u crip (t) is limited, the capacitance of the capacitor will be relatively large; therefore, it is necessary to change the charging method of the battery to transfer the pulsating power on the capacitor, and ensure that the charging current is always greater than zero;

Based on the above sub-boards, the following conclusions are drawn:

The minimum value of the capacitor is smaller than with only p crip (t) related to the waveform zero; however, when p crip (t) is greater than zero, it must satisfy the condition of the above formula, as well as the half cycle of the fundamental wave of zero mean And the waveform is symmetrical; that is, when p crip (t) is greater than zero, as long as the above requirements are met, there are no other restrictions on the waveform.
The 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 achieving the purpose of adjusting the power absorbed by the battery, and indirectly adjusting the pulsating power on the capacitor p crip (t); p crip (t) The waveform directly determines the capacitance of the capacitor; the reference signal of the charging current indirectly determines the capacitance and volume of the capacitor;

The waveform of p crip (t) is realized by adjusting the charging current; when p crip (t)>0, its waveform satisfies the formula conditions under the constraint conditions, and its average is zero during half of the fundamental wave period and the waveform It can be set freely under the condition of symmetry; therefore, the charging strategy can be set freely under the corresponding constraint conditions such that p crip (t) satisfies its constraint conditions.