JAJU732C June 2019 – July 2022
In a power supply design, transformers and inductors are major contributors to size. Increasing the operating frequency reduces their size, but increasing the switching frequency beyond a particular value affects the efficiency of the power module. This is because the skin effect becomes very high at that frequency where the current flows through the surface of the conductor. Similar to the skin effect, there is a proximity effect, which causes current to only flow on surfaces closest to each other. Furthermore, from a proximity standpoint in high-frequency designs, conductor size and the number of layers must be optimized. With a planar transformer, more interleaving to reduce the proximity effect can be achieved. This interleaving can be tailored to produce a specific amount of leakage so as to aid in power transfer and to contribute to ZVS.
Planar transformers offer the following advantages over conventional transformers and hence were used in this reference design:
focuses on the actual planar transformer chosen for this application with details on the loss numbers.
The leakage inductor alone cannot ensure soft switching up to light loads. As seen previously, increasing the soft switching range by increasing inductor value increases the RMS currents. In practice, leakage inductor is chosen to provide soft switching only up to ½ or 1/3 of rated load. Beyond this point, the transformer magnetizing inductance is used for ensuring soft switching near light loads. The magnetizing inductance is chosen generally ten times the value of leakage inductance as a starting point for this optimization.