Aerospace, Military & Defence
The gain of the tank varies with frequency, so the amplitude of the output voltage from the transformer can be modified by varying excitation frequency, achieving regulation through feedback. The ‘Q’ of the combined resonances critically affects the curve shape and the transfer function. ‘Shaping’ the curve correctly with a designed Q value is an important factor to achieve the +/-15 per cent maximum frequency variation specified; a steeper slope to the gain curve implies a smaller frequency variation for a given gain change. Q is low with no load and it reaches the highest value at full load, so the curves effectively represent the gain as load changes.
Above resonance, a tuned circuit looks inductive, and below, capacitive. This is shown in Figure 2 in the orange and white areas respectively and affects the drive voltage/current relationship. In the inductive region, but above fres1, low-loss zero voltage switching (ZVS) is achieved. Below fres1 but above fres2 ZVS is also achieved along with zero current switching (ZCS) of the output rectifiers, as long as the capacitive region is avoided, identified by a negative incremental slope to the gain curve. The optimal point is to operate at fres1 under nominal conditions for best efficiency, achieving primary ZVS, secondary ZCS, and an output current with no ‘dead time’. Consequently, the RMS value is minimal and conduction losses low. Care must be taken under all conditions, including transients and faults so that the circuit does not enter the capacitive region, as losses can immediately exceed damaging levels. The complete analysis of an LLC converter is highly complex, so approximations and assumptions are usually made. The main one is that the current in the tank is sinusoidal, with little harmonic content: the so-called ‘First
gate drive power requirements. Resonant capacitor Cr was chosen as a parallel combination of C0G dielectric ceramic types for accuracy and stability with voltage and temperature.
Matrix transformer
The transformer design in a high-efficiency LLC converter is critical, so a matrix type was selected for best performance. These are effectively distributed transformers with the primary winding passing through each in series but with individual secondaries that are externally paralleled. Matrix transformers can have very low leakage for high efficiency, and low profile, which suits the application and can be fabricated in a ‘planar’ arrangement for a good packing factor and efficient heat sinking. The transformer was further optimized by an interleaving of the two ‘push-pull’ single turn secondary windings with the primary, reducing proximity losses by lowering the ‘H’ field in the winding window and by customizing the magnetic core geometry.
Experimental results Figure 3: Algorithm flowchart developed for optimum LLC converter design
Harmonic Approximation’ (FHA). Other design choices are to set the parameter ‘m’, the ratio of magnetizing to resonant inductance, affecting the slope of the gain curve, and the circuit Q value, affected by the load resistance reflecting through the transformer turns ratio n, back to the tank. GAIA Converter and Universidad Politécnica de Madrid generated a complex, interactive design algorithm to reach optimal results, as summarized in Figure 3.
The final design was set with fres1 at 830kHz, n = 4, Lr = 2.4µH, Lm = 3µH, and Cr = 15.6nF, and a paper was presented at APEC 2022 with the full
calculation details2 . To fulfil the converter
technical specifications, Lr and Lm must have similar values. This leads to a conclusion that Lr could not be formed from deliberately introduced transformer leakage inductance, as it would increase losses and reduce coupling unacceptably, and a separate discrete component was designed. This took the form of four custom stacked E-Cores, with four turns of Litz wire to reduce skin effect losses. The core material was TP5E, gapped to achieve 2.4µH. On the primary side, GaN HEMT transistors were chosen for the switches for their low on-resistance, high speed, low device capacitances, and low
Along with the developed design algorithm, circuit, thermal, and EMI simulation were employed yielding promising results. A prototype was built demonstrating that the converter would fit in the specified volume and electrical and thermal tests performed. Transformer temperature rise was shown not to exceed 30°C and target efficiency figures were met. The critical specification for variation of frequency of +/-15 per cent maximum was met and a final power density achieved of 32 kW/dm3. Results are summarized in Table 1.
Conclusion
The work described has shown that the rigorous requirements of DC-DC power conversion in MEA applications can be met with a SWaP-optimized LLC design, helping to reduce emissions today and to bring forward the time when all- electric propulsion in aviation becomes a commercial reality.
www.gaia-converter.com 1
References
Aircraft batteries: current trend towards more electric aircraft
https://ietresearch.onlinelibrary.
wiley.com/doi/pdf/10.1049/iet-est.2016.0019 2
Table 1: Overview of the experimental results 22 July/August 2022 Components in Electronics
A. de Juan, D. Serrano, P. Alou, J.N. Mamousse, R. Deniéport and M. Vasic, “High-Frequency LLC Converter with Narrow Frequency Variations for Aircraft Applications”, 2022 IEEE Applied Power Electronics Conference and Exposition (APEC), Houston, Tx, March 2022
www.cieonline.co.uk
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