High frequency conversion uses significantly smaller transformers -
LF vs HF transformers shown here
High Frequency Conversion
Traditional industrial battery chargers use large transformers to electrically isolate and step-down a 50 or 60 Hz AC line voltage to the appropriate DC voltage required by the battery pack. A basic rule of physics says that the higher the frequency of power put through a transformer, the smaller the transformer can be. In high frequency power conversion, the effective line frequency passed through the transformer is typically increased by several orders of magnitude, using silicon transistors to chop up the AC waveform into small manageable bits. The high frequency transformer can therefore be a small fraction of the size of a low frequency transformer. With the overall charger weight and size dramatically reduced, overall cost is significantly reduced. This is due to silicon transistors falling in price at the same time as the prices of commodities, such as copper and steel used in transformers increase.
The fast response available from high frequency power conversion can quickly adapt to a slow changing AC waveform. This provides other important benefits such as world-wide operation with a wide 100-264VAC 50/60 Hz input voltage range, precise regulation of the battery charging voltage and near-unity Power Factor.