The transformers designed especially for this type of use, i.e. the so-called “integrated resonant transformers”, make use of leakage inductance, which normally represents an undesirable parasitic effect, instead of a discrete inductor, integrating two of the three resonant tank elements in just one inductive component.
As well as convenience in terms of costs and dimensions, it must also be highlighted that the magnetic flux of the leakage inductance goes substantially through free air, thus eliminating every problem linked to saturation of the core, which must be kept in mind using a discrete inductor.
In order to achieve good results, the design structure and details must be skilfully managed so as to obtain the required leakage inductance, in relation to all the other design parameters, under conditions of minimal loss.
While in other situations the use of empirical experience and simple generic calculating methods bring about approximations that can be more
or less accepted in many specifications, these are not acceptable in high-efficiency applications. In fact, in this case, a few more lost Watts - sometimes a fraction of a Watt - can have a significant effect on the power supplier’s overall efficiency; it can easily compromise the careful choices made during the design of the converter.
Optimal efficiency for inductive components can only be achieved by surpassing a number of simplified design methodologies, such as the equal division of the loss target between the core and the copper.
Literature and experience teach us that the best efficiency point can be identified through the ad hoc definition of the losses depending on the induction value
Fig. 3 – Leakage/ inductance relationship
In the specific case of integrated transformers, there are a number of restrictions which require close co-operation with the inductive components manufacturer during the electronic design.
The definition of the best parameters of a resonant tank cannot be made without considering the restrictions linked to the structural elements of each transformer, firstly the curve showing the relationship between inductance and leakage inductance
Fig. 4 – Losses in an inductive component depending on induction value
Without this dialogue, at best, you will be forced to work with an inappropriate inductance value, which can result in very bad energy and cost efficiency.
The most critical issue in the design of these transformers is the realistic calculation of winding losses, without which any design optimisation becomes unfeasible.
During this calculation, the eddy current loss resulting from the “proximity effect”, should be considered as well as the “skin effect”, a recognised phenomenon that is easy to manage. These calculations become even more complex in the presence of litz wire windings (multistrand), whose use is inevitable given the typical working frequencies in the order of 100 KHz and over.
These critical aspects along with other minor issues often lead to transformers with poor efficiency from an economical, energy and dimensional point of view.
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