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LLC vs LCC Converters for High Output Voltage

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state_of_flux:
Hello all,

I am currently designing a power supply. The aim is to build a solution that minimises the volume, weight, and size of the solution. I am currently investigating the use of resonant converters, such as series, parallel, hybrid resonant (LLC and LCC) converters, and full bridge phase shifted converters.

I understand that the LLC converter leads to the best efficiencies and thus have improved power densities, when compared to LCC. And incorporation of the magnetic components into the transformer also help to reduce size and weight of the solution.

My question relates to deciding which could be utilized to its best potential for my application, which is a high output voltage (1-10kV range, 500W+) with dual outputs. I am leaning towards the use of the LLC due to the aforementioned issues, however I often see LCC implemented for high voltage, low current outputs whereas LLC for low voltage, higher current outputs (with synchronous rectification, for example). There are a select few papers in the literature that utilize LLC converters successfully in high output voltage applications - but the scarcity of these kind of makes me err on the side of caution. I have also investigated phase shift converters however their soft switching capabilities seem to be less impressive than LLC and again seem to be used for high current output applications.

I know that the winding capacitance of the transformer in a high output voltage design tends to be large - therefore suggesting the possible use of the LCC converter to include this in resonance. However, I am trying to decide whether the issues this capacitance would cause to circuit performance would outweigh that of the benefits of increased power density and efficiency overall.

Hope someone can point me in the right direction.

Thanks in advance  |O  |O

MagicSmoker:
I wouldn't attempt an LLC (or LCC) converter as a total SMPS novice, but I REALLY wouldn't attempt a phase-shifted full bridge, as it deceptively difficult to get working properly, and tends to blow up lots of expensive switches along the way. LLC is probably the most forgiving of the 3 you listed, and is a decent choice for HV outputs on account of it incorporating the leakage inductance into normal operation, but it might not be the best depending on the type of load. Are you looking for more of a constant current type output - which is very common for HV supplies - or do you need precise voltage regulation?

state_of_flux:
Hello MagicSmoker,

Thanks for your input! I did see a lot of documentation mentioning how phase shifted design can be troublesome. In this application the regulation of the output voltage is more important. Furthermore it is desirable to operate the converter at a fixed frequency, most likely above resonant frequency to obtain soft commutation of the switches. Hope this information can help in giving more assistance.

MagicSmoker:
You can achieve soft switching above or below resonance, so don't let that concern dictate your design. Whether the LLC or LCC topology is the better choice depends more on if stray inductance or capacitance is more of a problem in your transformer design. Using a split core bobbin to separate the primary and secondary is great for isolation, but exacts a terrible price in leakage inductance; conversely, interleaving the primary and secondary drastically cuts leakage but (generally, not always) results in higher stray capacitance - especially between primary and secondary.

However, if you need the converter to operate at a fixed frequency then neither LCC or LLC is appropriate, and you might want to look into using auxiliary switch and LC networks to effect lossless transitions. For example, the active-clamp forward is a good choice for this power level and while it mainly benefits switch turn-off (so best for IGBTs) it can be coaxed into achieving lossless (or reduced loss) turn-on, which is best for MOSFETs.

state_of_flux:
That makes sense, the transformer parasitics are what were concerning me most with the resonant designs. Since the output voltage in my case is large, the stray capacitance tends to be more of an issue than the leakage inductance (I believe). I find it difficult to decide whether the leakage inductance or the stray capacitance would be more of an issue without first designing the transformer - but then don't you need to know which topology to use, before you can design the transformer? This is something that does confuse me.

I do seem to see some applications in my research area use different control methods of the resonant converters, however they are more sparse than the frequency modulated ones - another reason I was unsure whether these converters would be applicable.

With the forward converter there exists an output filter inductor - which is troublesome in my case, considering the high output voltage, the inductor would be too large, and impractical. I have looked into forward converters, alongside push-pull converters, however literature states that these are used in applications with power levels <500W, rather than >500W where the transformer size becomes a limiting factor. I have looked into interleaving methods for boosting the power capability, but this is typically to share higher levels of input current, which I don't really have - furthermore an additional transformer may be unnecessary.

I really value your help with this, if you have any more input it would be greatly appreciated.

Thanks! :-+

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