The resonant switching topology is, to date, one of the
most efficient solutions for switch mode power supply design.
"Although in existence for many years, only recently has the LLC resonant converter, in particular in its half-bridge implementation, gained in the popularity it certainly deserves. In many applications, such as flat panel TVs, 85+ ATX PCs or small form factor PCs, where the requirements on efficiency and power density of their SMPS are getting tougher and tougher, the LLC resonant half-bridge with its many benefits and very few drawbacks is an excellent solution."
In fact the introduction of new regulations, both voluntary and mandatory, has brought about a revaluation of the LLC series resonant topology.
In LED lighting, to name just one of the infinite applications, the need for more and more efficient power supply systems is pushing power designers in this direction.
All the main manufacturers of active components currently available on the SMPS market have included efficient chips in their product catalogues.
With an effectively contained degree of circuit complexity, they allow the realisation of power supplies with 90-96% efficiency, (which can be improved further using synchronous rectifiers instead of output diodes) and reduced EMI/EMC problems in comparison to other topologies thanks to the "Zero Voltage Switching" and to the substantially sinusoidal high frequency currents.
"Generally speaking, resonant converters are switching converters that include a tank circuit actively participating in determining input-to-output power flow."
The operating principle is based on the characteristic gain curve of the resonant tank", which allows to change the gain by a moderate variation of the switching frequency, thus resulting in an effective regulation of output voltage or current in relation to load and input voltage changes.
The resonant "tank" is a set of two inductive elements and one capacitor (LLC). Even if the use of three different components, i.e. a discrete inductor, a conventional transformer and a capacitor is technically possible, poor results would be obtained on all fronts: cost, size and energy efficiency.
The use of an integrated transformer is much more convenient, which has specific features and integrates resonant inductance as described below.
To give an idea of the advantages, if a 150 W integrated transformer is well-dimensioned it can have dimensions of less than 28x29x23 mm, with costs that are obviously more competitive with respect to solutions with discrete inductance.
It is therefore evident that the only reason that can lead to this solution is the design difficulty of a coherent tank.
While the designs of the largest manufacturers of electronic equipment generally show a correct structuring of the integrated transformer and other inductive components, mid-sized or small manufacturers - even if all SMPS manufacturers enjoy the same technology advantages in relation to active and passive components - very often do not have the same benefits in regards to fundamental components, such as the integrated transformer and PFC stage inductor, given the specificity of the inductive components and the limitation of resources destined to the project.
- typical range of efficiency for the simplest circuitry 94-96%, with possible improvements through synchronous rectification and other small adjustments;
- utilising correctly sized magnetic components, the design is rapid and notably simplified;
- the current waveform at high frequencies is basically sinusoidal with significant reductions in harmonics with respect to other topologies;
- MOSFET commutation ON "ZVS" (Zero Voltage Switching) with associated elimination of commutation loss, reduction/elimination of dissipators and reduction of stress and EMI, which are often the causes of the most hostile design problems;
- the possibility to reduce consumption with low/zero load by utilizing the functions "burst mode" and "PFC stop" implemented on many controllers;
- the possibility of optimal sizing for continuous and temporary power including some significant improvements on conventional solutions (eg. an optimised transformer with volume similar to a classic EF25 can supply up to 200-250W);
- with respect to other topologies, with the attributes described above, LLC supplies have reduced dimensions with a notable reduction in EMI/EMC issues.
- typical cost savings on dissipators, EMC filters, smaller transformers etc.
- one necessary factor to be considered for the optimised design of power supplies is the magnetics, the ommission of which means to renounce a significant potential in improved efficiency;
- the design of the optimised integrated transformer requires specific competence;
- two MOSFETs are required (half bridge) rather than the single MOSFET for the Flyback;
- the controller is slightly less economic than the flyback (this cost increase tends to cancel itself, when it is not transformed into a saving, thanks to the minimal spendings on other components).
Lots of documents are available to extend the topic, let's take a look on some examples.
For a short reading it could be suggested the design considerations from Fairchild Semiconductor ® or from Texas Instruments ®.
For a reading more interesting and complete it could be useful to see these application notes from ST Microelectronics ®, Fairchild Semiconductor ® and Infineon ®.
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.
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.
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.
A lack of consideration of these issues or a lack of competence in magnetics design often lead to transformers, and so to power supplies, with poor efficiency from an economical, energy and dimensional point of view with an undesirable waste of resources.
These comparative tests are interesting examples of how it's possible to increase the efficiency thanks to an optimised integrated transformer.
Fairchild FEB212-003 24V-8A resonant converter demo-board based on chip FSFR2100 - comparative test
NXP® UM10450 - 19,5V 90W resonant converter demo-board with PFC based on TEA1713 - comparative test
STMicroelectronics EVL130W-SL-EU 130 W SMPS for LED street lighting applications based on L6599AT-6562AT - comparative test
For more info about the integrated resonant transformers, please, click on the flag of your own language: llc resonant transformers-en llc resonant transformers-de llc resonant transformers-it llc resonant transformers-es llc resonant transformers-fr
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)|].
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.
For more info about the PFC inductors, please, click on the flag of your own language: llc resonant transformers-en llc resonant transformers-de llc resonant transformers-it llc resonant transformers-es llc resonant transformers-fr
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