Hello dear engineering friends !
Iam designing 7kwt dc-dc converter that converts 750v DC into a 48V DC (140A) using LLC topology for a hobby project.working frequency is 45khz when at full load and up to 200khz when idle.and the problem is with transformer design.i have tried 6 different transformer constructions and all of them heat up very fast and severe.
All other components of the converter are at 50C,which is very fine as for me. transformer gets hot up to 70-80C on the surface of the winding just in 10 minutes under full load.
My first transformer designs where so called integrated designs,where resonant inductor was incorporated in a transformer using leakage inductance between primary and secondary.that design showed rapid heat build up on the surface of the primary which was facing secondary (primary and secondary where separated in space,thus big leakage inductance present). then i have read about proximity effect and made a conclusion that i have to make separate external resonant inductor and transformer should have lowest value leakage inductance as possible,because if i got it right,leakage inductance makes eddy currents and thus heating nearest layers of the winding leaving all the rest layers cold.
All the transformer designs had air gap at the ferrite core (1mm). Main core is EE85 big peace of ferrite (860mm2 Ae).
So i have proceeded to a foil secondary winding and finer litz wire (600x0.1 instead of 0.46x32). secondary winding is center tapped (half bridge synchronous rectifier) 2+2 turns 0.2mm foil 53mm wide.10mm2 for a 140 and 50% duty cycle and primary winding is 16 turns (8+8 in series) 0.1x600 litz wire,half of primary is outside of secondary and half of it is inside in it.
That kind of design i have looked up in the infineon evaluation board of a 3 kwt llc converter appnote (https://www.infineon.com/dgdl/Infineon-ApplicationNote_Evaluationboard_3kW_dual_phase_LLC-AN-v01_00-EN.pdf?fileId=5546d462580663ef01582eb629b70118) you can check it on the 25 page. infineon design has even more secondary turns with a foil (5+5), it also has air gaps on all three limbs of ferrite,it also has external resonant inductor. but somehow it work on a fantastic 100khz+ frequency,where all the skin effect and proximity effect are very limiting.
i have used spacer to pull inner layer of a primary on a bobbin away from a fringing air gap field. but that doesnt helped alot.transformer runs very hot,isolation start to boil after some working time.
iam so exhausted from all those failed experiements,that i gave up and cant go on without a help. i have been struggling with this project for 13 months by now,half of it with a transformer.
If there was some power electronics engineer among you that is familiar with a magnetics and transformer design i would really appreciate to get some help.
Iam designing 7kwt dc-dc converter that converts 750v DC into a 48V DC (140A) using LLC topology for a hobby project.working frequency is 45khz when at full load and up to 200khz when idle.and the problem is with transformer design.i have tried 6 different transformer constructions and all of them heat up very fast and severe.
All other components of the converter are at 50C,which is very fine as for me. transformer gets hot up to 70-80C on the surface of the winding just in 10 minutes under full load.
My first transformer designs where so called integrated designs,where resonant inductor was incorporated in a transformer using leakage inductance between primary and secondary.that design showed rapid heat build up on the surface of the primary which was facing secondary (primary and secondary where separated in space,thus big leakage inductance present). then i have read about proximity effect and made a conclusion that i have to make separate external resonant inductor and transformer should have lowest value leakage inductance as possible,because if i got it right,leakage inductance makes eddy currents and thus heating nearest layers of the winding leaving all the rest layers cold.
All the transformer designs had air gap at the ferrite core (1mm). Main core is EE85 big peace of ferrite (860mm2 Ae).
So i have proceeded to a foil secondary winding and finer litz wire (600x0.1 instead of 0.46x32). secondary winding is center tapped (half bridge synchronous rectifier) 2+2 turns 0.2mm foil 53mm wide.10mm2 for a 140 and 50% duty cycle and primary winding is 16 turns (8+8 in series) 0.1x600 litz wire,half of primary is outside of secondary and half of it is inside in it.
That kind of design i have looked up in the infineon evaluation board of a 3 kwt llc converter appnote (https://www.infineon.com/dgdl/Infineon-ApplicationNote_Evaluationboard_3kW_dual_phase_LLC-AN-v01_00-EN.pdf?fileId=5546d462580663ef01582eb629b70118) you can check it on the 25 page. infineon design has even more secondary turns with a foil (5+5), it also has air gaps on all three limbs of ferrite,it also has external resonant inductor. but somehow it work on a fantastic 100khz+ frequency,where all the skin effect and proximity effect are very limiting.
i have used spacer to pull inner layer of a primary on a bobbin away from a fringing air gap field. but that doesnt helped alot.transformer runs very hot,isolation start to boil after some working time.
iam so exhausted from all those failed experiements,that i gave up and cant go on without a help. i have been struggling with this project for 13 months by now,half of it with a transformer.
If there was some power electronics engineer among you that is familiar with a magnetics and transformer design i would really appreciate to get some help.
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