1. Inductive coupling charging standard
In order to realize the marketization of electric vehicles, the American Society of Automotive Engineers has formulated corresponding standards according to the system requirements. Among them, for the chargers of electric vehicles, two charging standards, SAEJ-1772 and SAEJ-1773, have been formulated, which correspond to the contact charging method and the inductive coupling charging method respectively. Electric vehicle charger manufacturers must comply with these standards when designing, developing and producing electric vehicle chargers.
The SAEJ-1773 standard gives the minimum physical size and electrical performance requirements for electric vehicle inductive charging couplers in the United States. The charging coupler consists of two parts; the coupler and the car socket. Its combination is equivalent to a transformer with separation of primary and secondary sides working between frequencies of 80~300kHz.
SAEJ-1773 recommends three charging methods as shown in Table 1. For different charging methods, the design of the charger will be different accordingly. Among them, the most commonly used method is the home charging method, the charger power is 6.6kW, and the chargers with higher power levels are generally used in charging stations and other occasions. According to the SAEJ-1773 standard, the inductive coupler can be represented by the equivalent circuit model shown in Figure 1, and the corresponding component values are shown in Table 2.
|charging mode||charging method||power level||grid input|
|Mode 1||emergency charging||1.5kW||AC120V, 15A single phase|
|Mode 2||home charging||6.6kW||AC230V, 40A single phase|
|Mode 3||charging station||25~160kW||AC208 ~600V three-phase|
|Lp±10%(μH)||0.8||0.5||Voltage per turn (V)||100||100|
|Rsmax(kΩ)||1.6||1.3||Coupling efficiency (%)||＜/99.5||＜/99.5|
|Ls±10%(μH)||45||55||Insulation resistance (MΩ)||100||100|
|Rmmin(mΩ)||20||40||Maximum charging current (A)||400||400|
|Lm±10%(μH)||0.8||0.5||Maximum charging voltage (V)||474||474|
The primary and secondary sides of the transformer are separated, with a large air gap, which is loosely coupled, and the magnetizing inductance is relatively small. When designing a power converter, the influence of this small magnetizing inductance on the circuit design must be fully considered.
The power transmission cable must be considered in the design. Although this item is not included in the SAEJ-1773 standard, the volume, weight and equivalent circuit of the power transmission cable must be considered in the actual design. Since the size of the transmission cable is mainly related to the level of the transmission current, reducing the charging current can correspondingly reduce the cable size. In order to minimize cable power loss, coaxial cables can be used and optimized in the operating frequency band. In addition, the cable introduces additional impedance, increasing the equivalent leakage inductance of the transformer, which must be considered in the design of the power stage. For a 5m long coaxial cable, the typical resistance and inductance values are Rcable=30mΩ, Lcable=0.5~1μH.
2. Requirements for inductively coupled charging power converters
According to the equivalent circuit of the inductive coupler given by the SAEJ-1773 standard, the characteristics of the connecting cable and the battery load, the inductively coupled charging power converter should meet the following design criteria.
(1) High frequency chain of current source. The secondary filter circuit of the inductively coupled charging power converter is installed on the electric vehicle, so the use of a capacitive filter circuit in the filter link will simplify the on-board circuit, thereby reducing the weight of the entire electric vehicle. For capacitive filtering, the power converter should be a high-frequency current source. In addition, this current source circuit is relatively less sensitive to changes in the operating frequency and power level of the power converter, so it is easier to consider three charging modes at the same time for circuit design. No overvoltage clamping is required.
(2) Soft switching of the main switching device. The high frequency of the inductively coupled charging power converter can reduce the volume and weight of the inductive coupler and the on-board filter element, and realize the miniaturization of the power supply system. However, with the continuous increase of the frequency, the switching loss of the power converter using the hard switching mode will be greatly increased, and the efficiency of the power converter will be reduced. In order to achieve higher frequency, higher power level charging, soft switching of the main switching device must be guaranteed to reduce switching losses.
(3) The working range of constant frequency or narrow frequency change. The inductively coupled charging power converter works in a constant frequency or narrow frequency range, which is conducive to the optimal design of magnetic components and filter capacitors. At the same time, it must avoid working in the radio bandwidth and strictly control the electromagnetic interference in this area. For frequency conversion work, light load corresponds to high frequency work, and heavy load corresponds to low frequency work, which is beneficial to basically the same efficiency under different load conditions.
(4) Wide load working range. Inductively coupled charging power converters should be able to operate safely over a wide load range, including the extremes of open and short circuits. In addition, the power converter should also be able to work in trickle charging or equalizing charging modes. In these modes, the power converter should be able to guarantee high efficiency.
(5) The turns ratio of the inductive coupler. The larger turns ratio of the primary and secondary sides can make the primary side current smaller, so that the power transmission cable with thinner wire diameter and the power device with lower current rating can be used, and the efficiency is improved.
(6) Enter the unity power factor. The inductively coupled charging power converter works at high frequency, which will cause harmonic pollution to the power grid. Effective measures must be taken, such as power factor correction or reactive power compensation technology, to limit the total harmonics of the inductive coupling charging power converter of electric vehicles entering the power grid. quantity. At present, the charging power converter must meet the IEEE519-1992 standard or similar standards. To meet these standards, it is necessary to increase the complexity of the input part of the inductively coupled charging power converter and the whole machine, increase the cost, and according to different According to the charging level requirements, the inductively coupled charging power converter can choose a two-stage structure (the front stage is PFC + the rear stage is a charger circuit) or a single-stage circuit integrating the PFC function and the charging function.