#41🚀 600W LLC ZVS range
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The resonant converters with isolation are very common in high step-down applications, especially server applications, where 12 V is required from a 400V bus.
I’m very interested in the LLC topology because due to the high frequencies with GaN devices, it achieves the highest power density of the state of the art. I found this article [1] about ZVS for LLC which talks about the ZVS range and I would like to discuss it with you. Especially interesting, the impact of the magnetics in their performance.
Achieving ZVS at high frequencies for LLCs is key to keeping their advantage. The problem found with these converters is the loss of ZVS operating at the resonant frequency when in theory, there are no restrictions for ZVS at the resonant frequency.
However, when the secondary parasitic capacitors are considered, during the dead time (Where ZVS occurs), the magnetizing inductance is clamped by the output before the ZVS is realized [1]. In this situation, ZVS is deteriorated, especially at high frequency, when series resonant inductance (Lr) is small.
Basically, at high frequency with small Lr, the ZVS occurs with the magnetizing inductances and there are possibilities of losing ZVS easily.
In most of these designs, where there is only one magnetic component, the magnetizing inductance (Lm) will be used for the ZVS instead of the Llk.
In the following picture can be observed the magnetizing current ILM is clamped because of the secondary and the voltage VLM become negative before ZVS occurs.
The traditional LLC, working at resonant frequency has ZVS by default. Nevertheless, if we consider the parasitic capacitance of the secondary diodes, ZVS is limited, therefore it’s important to calculate the range of Lr and Cr where you can get ZVS.
In the article, the authors fix the Lr and explore the relation between Cr and ZVS. There is a minimal value of Cr for achieving ZVS.
For verifying their theory, they designed a prototype with the following specifications:
They play with two different Cr values to demonstrate their theory. The first value is 22 nF, which is above the limit for achieving ZVS, and 15 nF, which is below the limits.
In this first one, it’s appreciated the ZVS condition.
While in the second one, there is no ZVS and therefore, lower efficiency.
I would like to know the Llk of their design to try to replicate the simulation and verify what they say. In any case, it’s clear that magnetics are key for obtaining a good result.
The analysis of the article is interesting for understanding the ZVS limits, however, the main lesson for designers is the importance of the resonant values, which are mainly determined by the transformer.
Should the designer start the design from the Transformer? What do you think?
Conclusion
There are limits in the ZVS for LLC HB topologies and the selection of the resonant tank is key.
My recommendation: check your possibilities regarding the transformer: Leakage inductance, magnetizing and resonant frequency, and simulate your possibilities before moving to the detailed design.
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References
[1] W. Qin, L. Zhang and X. Wu, "Re-examination of ZVS Condition for MHz LLC Converter Operating at Resonant Frequency," 2018 IEEE International Power Electronics and Application Conference and Exposition (PEAC), 2018, pp. 1-4, doi: 10.1109/PEAC.2018.8590254.