Transformer vs Switching

In the previous sections we provided an overview of transformer and switching system-based AC-DC conversion in terms of operation and circuits. In this section we attempt to summarize their pros and cons.

Figure 8. Transformer system-based AC-DC conversion

Figure 9. Switching system-based AC-DC conversion

Figure 10. Transformer and switching system compared

In terms of circuit configuration, due to differences in the conversion methods employed, the switching system is more complicated. In addition, the switching system requires a control circuit (basically an IC).

Although the two systems are similar in the components used, the switching system requires some higher voltage tolerant parts, with some cost implications, as would be expected.

Nevertheless, the most significant difference lies in efficiency, and in terms of volume/weight the switching system is considered more advantageous.

As an example, have you noticed that charging wall adapters for portable devices have recently become smaller and lighter? Figure 11 illustrates commonly available wall adapters. The left unit is a transformer system while the right unit is a switching system. In terms of specifications, the right unit, smaller in size, features an output value that is greater by as much as 1W.

Figure 11

In the switching system, the AC input (50/60Hz) which has been converted to DC is converted back to high-frequency AC, permitting the use of smaller transformers and output capacitors, thus substantially reducing the form factor. In the section on basic operations we stated that “the switching system, after rectifying and smoothing of the input AC, operates in the same manner as a DC-DC converter,” and this is also precisely true for the switching wall adapter. In terms of efficiency also, the switching permits the extraction exclusively of the required power, for improved efficiency, and obviously, reduced heat dissipation.

In any design process, a tradeoff between efficiency, size, and cost must be taken into consideration. With a solid understanding of the differences and pros and cons of these systems, we believe that optimal choices can be made. Recently, the issue of power consumed by AC adapters during their standby status has been noted. A solution may be at hand in the adoption of the switching system.