The essence of the vector method is to follow the angle between voltage and cuurent on the primary side of the LLC converter. The major benefit of this method is simplicity and clarity.
The vecor method is based on the basic FHA and AC circuit analysis. The critical transfer point between inductive and capacitive mode is identified as a simple vector criteria where the inverse transfer function vector and the vecrot of serial combinations of load components are orthogoal.
Then the calculation procedure becomes straightforward. Key formlas are derived by using simple trigonometry, and the minimum operating frequency (omega_0) is such that it gives a minimum size for the resonant tank. Simple calculations further enable the design procedure:
1. Select the topology (half-bridge or full-bridge).
2. Provide I/O voldatge ranges.
3. Use nominal input voltage to set operating modes (boost + buck, or boost only)
4. Provide the output current requirement.
5. Select nominal operating frequency.
6. The optimum minimum operating frequnecy will be selected by the calculator.
7. The LLC calculator will give LLC converter components.
8. In order to have Cr as a standard value, the calculator slider enables fine-tuning.
The transfer function variation as a function load is also provided. It enables investigation of the min./max. load conditions.
AN_1802_PL52_1803_235257, Part II: Using the LLC calculator with Rules of Thumb(ROT) and fast verification with LTspice, Aguide for adapting the LLC calculator to design rules based on exact mode analysis of LLC converter
A number of key points were driving the development of this technique, in consideration of the improvements in both the desig process and the results:
* FHA misses important time-domain behavior at the heart of LLC functioning – the resonant mode charge pump formed with Cr.
* FHA under-predicts the output capability, and predicts a lower Q required, resulting in higher Cr, lower Lr and lower Lm, increasing primary-side losses.
* FHA models assume variable-frequency sine wave and miss the RMS factor increase in current on the secodary when using m-ratios with large values in order to “optimize” the primary-side current with FHA.
* Reverse FHA analysis does not accurately predict the gain capability and power margin of the LLC tank, and flags designs with more efficient tank designs in the real world – the bumble bee can fly well, and with lower losses overall with the correct tank configuration.
* With the use of some design ROT, the LLC calculator based on vector analysis calculations can be used to visualize and propose more optimimum LLC tank designs than a conventional FHA approach, and runs well in an Excel environment.
* LTspice and other non-linear capable tools will tell the truth in verification and, using the right model format, do so quite quickly to confirms the main LLC converter parameters.
* Two high-performance alternative tank alignments have been presented for the Infineon/Finepower 12V 600W LLC converter using the existing transformer turns ratio n=16:
– one focused on improving light-load efficiency
– one focused on minimizing secondary-side RMS current in the converter at low-line operation as well as any input voltage
– both had lower RMS operating currents than the original design at any operating point, but with different end-point optimizations.
* A wide input range 12V 600W example has been proposed for WBG semiconducters using a transformer turns ratio of n=17, illustrating how with wide input range capability, components might be reduced in size for the complete SMPS, with improved efficiency and power density, while demonstrating how ROT works with the LLC calculator gain results, and coparing these results to detailed exact mode calculations.
* The use of LTspice with a specifically designed simulation methodology is shown, which can verify tank behavior in under 30 seconds in typical cases with a high-performance notebook computer.