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7kW 45kHz LLC Transformer Overheating in 750V to 48V 140A DC-DC Converter Design

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  • #1 21682353
    Slavko Skala
    Anonymous  
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  • #2 21682354
    Rick Curl
    Anonymous  
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  • #3 21682355
    Harry Dellamano
    Anonymous  
  • #4 21682356
    Slavko Skala
    Anonymous  
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  • #5 21682357
    Harry Dellamano
    Anonymous  
  • #6 21682358
    Slavko Skala
    Anonymous  
  • #7 21682359
    Harry Dellamano
    Anonymous  
  • #8 21682360
    Slavko Skala
    Anonymous  
  • #9 21682361
    Harry Dellamano
    Anonymous  
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  • #10 21682362
    Michael52 Gerald
    Anonymous  
  • #11 21682363
    michael Jones
    Anonymous  

Topic summary

A 7 kW DC-DC converter design using LLC topology converts 750 V DC to 48 V DC at 140 A, operating at 45 kHz full load and up to 200 kHz at idle. The main issue is transformer overheating, with surface winding temperatures reaching 70-80°C within 10 minutes under full load, while other components remain around 50°C. Initial integrated transformer designs incorporating the resonant inductor via leakage inductance showed rapid heating due to high leakage inductance and proximity effect losses. The transformer core is a generic Chinese P4 ferrite (EE85), with flux density around 150 mT at 45 kHz. Increasing frequency to reduce turns worsens heating. Current density is approximately 7.5 A/mm² for secondary and 5 A/mm² for primary windings. Suggestions include simplifying to a single primary and single secondary winding with a full-wave voltage doubler to reduce secondary turns and leakage inductance, and employing zero-voltage switching (ZVS) synchronous rectification to improve efficiency. Weight constraints limit the use of dual high-current inductors. Alternative ideas involve bifilar winding to minimize magnetomotive force and leakage inductance, and increasing frequency to reduce turns while managing thermal issues. The project is for a tethered drone power supply, with recommendations to use 1200 V SiC MOSFETs instead of IGBTs for better performance and ZVS operation. Reference to Cree Charger Application Note CPWR-AN25 REV-B highlights a design with two-winding transformers and SiC MOSFETs for similar applications. Finite element method tools like MagNet and ThermNet are mentioned for 3D magnetic and thermal modeling.
Summary generated by the language model.
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