LLC and LCC Resonant Topologies

Which are the pros and cons? When to use one or the other? Let's find out.

Find out now the COST and PERFORMANCE benefits of the LCC topology

Read the comparative test report

A highly regarded topology…

The LLC topology is widely recognized as the most beneficial for switching converters in the 80…1000W power range and it’s increasingly used in applications up to a few dozen kilowatts, such as electric vehicle charging equipment, for the benefits it offers when it comes to EMC, efficiency, dimensions of the magnetic components of the resonant tank and the filters.

Since the cost of the magnetic components is in broad terms proportional to their dimensions, a cost advantage follows.

Click here for more details on the resonant topologies’ benefits.

…and a less popular one

Even if often recommended in the literature for some applications, the LCC topology is less known and applied.
Compared to LLC, the LCC topology requires an additional capacitor. The commonly used “LCC” definition is in fact reductive: it would be more appropriate to call it “LLCC”, technically speaking.

This resonant topology presents the same benefits listed above for LLC, besides being optimal for certain types of application described later.

However, there are some challenges related to the design.

Typical schematics of LLC and LCC tanks – Compared to LLC, LCC requires an additional capacitor.

Even more efficiency: the Integrated Resonant Transformer

By applying integrated resonant transformers, which use the leakage inductance for eliminating the need for a descrete resonant inductor, even greater efficiency, dimensional and economic advantages are possible.

Added to this is the bonus of a robust and high insulation between input and output, side effect of the primary and secondary windings being placed into separate sections of the bobbin for generating a higher leakage inductance.

It should be noted that the design of LLC or LCC tanks with an itegrated transformer presents additional complications compared to a tank with a non-integrated transformer, because the degree of freedom for the definition of the resonant inductance value is lost.
It is in fact bound to some construction details of the transformer.

Furthermore, the layout of the windings in an integrated transformer causes by its nature a distribution of the magnetic field that implies more losses compared to a traditional transformer.

However, both these aspects can be resolved by adequate optimization procedures, making the use of the integrated transformer better in every respect.

Comparison of the winding layouts in the traditional transformer and in the integrated transformer.

In the integrated transformer, the primary and secondary windings are placed into separate sections of the bobbin.

A weak point: wide voltage ranges

The resonant topologies show their main weakness in applications where there are large variations in the input or output operating voltage.

If the tank is properly designed, the “Zero Voltage Switching” can be sustained also in presence of rather large voltage variations, but this involves a reduction of the benefits compared to other topologies, due to the negative impact on cost and performance that this entails.

The extent of this impact is often acceptable, but it should be considered that it increases roughly proportionally with the enlargement of the voltage range.

As an example, we’re often asked if it’s possible to effectively support 1:3 ratios on the INPUT voltage range (classic 90-265Vac) with a single-stage resonant converter and without jumpers.
The answer is NO: for supporting a similar input voltage range, a second stage is needed (boost or PFC, indeed often required by standards) or a capacitive voltage doubler have to be implemented on the input, which also requires a removable jumper to switch from the 90-130V range to 190-265V.

Applications with a wide OUTPUT voltage range are a little less critical. 1:3 ratios are achievable with good results, as shown in this example: LLC tank with 15…48Vdc_out that we designed for NXP back in 2013.

Though, in the next paragraphs we show you a better solution.

Example circuit scheme showing how input extended range can be supported in a resonant converter.

A capacitive voltage doubler can be implemented on the input in order to support the extended range.

The LLC topology’s trade-off

For applications with a wide output voltage range, such as power supplies for LED strings of variable lengths (also with deep dimming) or high-performance battery chargers, the LCC topology is undoubtedly the most suitable.

It’s no secret that, if developing a well optimized LLC tank with its wound components is less trivial than people generally think, the LCC topology is significantly more expensive and time-consuming.
Because of the additional capacitor, the tank switches from 3rd to 4th order requiring, among other things, a much higher computing power for doing simulations and optimization calculations.

No worries!

Now we’re ready to support you also if you go for the LCC topology.

Our all new design skills and increasingly powerful proprietary CAE software will make your design activities incredibly easier, while providing the highest levels of optimization (also economic) and a uniquely quick development time.

Seeing is believing!
Check the comparative report about our latest LCC Demo Board optimization.

Other examples here:
Original Demo Boards
Improved Demo Boards

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