The presence of the active PFC stage at the input of the high performance SMPS is almost mandatory. There are also some critical design considerations relating to this component, especially for some of the most used types.
The most popular type of PFC adopted for power levels up to 200-300 W is the "Transition Mode" (sometimes also named “Critical Mode” or “Boundary mode”), where the usual core loss calculation methods cannot be used due to the complexity of the current waveform.
In fact, even with constant load the current has a particular wave shape, substantially triangular but with continuously variable frequency and amplitude, depending on the instantaneous input voltage value [|sen (VinRMS)|]
Fig. 5 – Inductance current wave form of a Transition Mode PFC (simulation from ITACOIL software)
This increases possible errors in the loss calculations, making the use of advanced calculation methods necessary.
It must be taken into consideration that the loss curves published by the manufacturers of magnetic cores refer to sinusoidal waveforms as well as specific frequencies and temperatures and therefore are not directly applicable.
The calculation problems for losses in the windings mentioned above in relation to the integrated transformers also exist for this component. Therefore, optimal design requires the access to specific resources and tools, normally not available to electronic design teams.
From a general supply chain point of view, the manufacturer of inductive components should take charge of the design, in order to generate evident scale benefits, with the purpose of a fruitful collaboration.
The investments required in order to generate the necessary specific skills, to implement sophisticated calculation tools and to build moulds for bobbins and specific accessories, adequate both in terms of performance and safety (isolation, creepage, dimensions, etc. that allow to exclude many problems during type-approval tests, also in demanding sectors such as that regarding electromedical devices) can be more easily sustained by the supplier of inductive components.
ITACOIL has acted in this respect, investing in recent years in order to give customers the opportunity to realise their projects with really small design efforts.
With the resonant tank already correctly designed, according to all the requirements and structural restrictions of the integrated transformer, the design of this type of converter becomes objectively easy also for designers who approach this topology for the first time.
The use of standard integrated transformers found in the catalogue allows to easily solve most of the needs in the LED lighting, home appliances, home automation, information technology sectors and many others, but the highly qualified technical support and rapid sampling enable the same benefits even where a custom product is required.
(see integral version on http://www.itacoilweb.com/files/resonant_power_supply_eng.pdf)
Valentino Radaelli
technical manager
ITACOIL srl
www.itacoilweb.com
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