We can get an answer through the data of fast charging pile and slow charging pile on the market: slow charging is AC charging with charging power less than 10KW;  Fast charging refers to DC charging with charging power above 30KW.  

"Why is DC charging faster than AC?"  "Why are all fast charges DC charges?"  

The car's power battery, the essence of the battery, it is the output of direct current.  Therefore, when the power battery needs to be charged, the input must also be direct current.  The national grid is in the consideration of power generation and transmission, the use of alternating current, so when charging the car, must use the "AC/DC DC converter" to convert the alternating current of the grid into direct current.  At this time, there are two charging modes according to the position of "AC/DC", as shown below.

First, alternating current is directly input to the charging port of the car, while AC/DC is integrated in the car, and then the AC/DC conversion is completed in the car. Second, alternating current is converted to direct current at the charging station, and the converted direct current is input to the charging port of the car to charge the battery directly.  

In order to improve the charging speed, it is necessary to improve the charging power P (P=U*I), high power means high voltage, high current, which has a high requirement on the performance of AC/DC module.  Because the space on board is very limited, only low-power AC/DC can be integrated;  However, the AC/DC module external to the charging station has no space limitation and can achieve high power.  So the speed of DC charging is much higher than that of AC charging.