SLLA497 September   2020 TMS320F28P550SJ , TMS320F28P559SJ-Q1

 

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AC/DC and DC/DC Power Stages

DC charging stations require high-power converters which are capable of charging to 80% SOC in under 30 minutes. These fast charging applications require modular power converters which can be paralleled to cater to different power levels, thereby enabling fast charging. The most important parameters are the energy density and system efficiency. Energy density is the amount of energy that can be transfered for a given volume of converter. If we can double the power output for the same size, it results in significant cost savings and also helps in fast charging. This is accomplished by operating the converter at high switching frequencies which reduces the size of magnetics and thereby helps achieve high power density. A higher system efficiency means lower losses and a smaller heat sink solution for a given application. It also reduces the thermal stress on devices and contributes to longer life expectancy of parts. The latest trend in automotive technology is the concept of Vehicle to Grid (V2G) which allows for the flow of energy from the battery to the grid for stability of grid when the vehicle is parked or not in use. This requires both the power stages to be bidirectional for supporting such applications. The converter must also be capable of providing galvanic isolation between the input and output stage through a high-frequency transformer with the required voltage conversion ratio as per the application. The converter must operate at high efficiency through inherent soft switching (like ZVS/ZCS) over a wide input and output voltage range.

The AC/DC stage (also know as the PFC stage) is the first level of power conversion in an EV charging station. It converts the incoming AC power from the grid (380–415 VAC) into a stable DC link voltage of around 800 V. As shown before, the PFC stage is very important to maintain sinusoidal input currents, with typically a THD < 5%, to provide controlled DC output voltage higher than the amplitude of the line-to-line input voltage, single-stage power conversion, no galvanic isolation, unidirectional and bidirectional power flow possibly with (limited) capability of reactive power compensation, simple circuit topology, simple modulation and control scheme, and the possibility to achieve high efficiency and high power density.

The DC/DC stage is the second level of power conversion in an EV charging station. It converts the incoming DC link voltage of 800 V (in case of three-phase systems) to a lower DC voltage to charge the battery of an electric vehicle. The electric vehicle charging standards are governed by standards such as Combined Charging System (CCS) and CHAdeMO. The DC/DC converter must be capable of delivering rated power to the battery over a wide range, for example 50V-500V to accommodate batteries from 48V (e-bikes) all the way up to 400V (PHEV) with the capability of charging the battery at constant current and at constant voltage mode, depending on the State Of Charge (SOC) of the battery.