So i a m using this transformer : https://www.we-online.com/components/products/datasheet/760895641.pdfand for the IC i am using the: https://www.ti.com/product/UCC25660?login-check=true#design-tools-simulationThere is a tool that is a xcel that helps calculate component values.Here is the problem i was planning to use the built in leakage inductance BUT then xcel gave me this: || || |Maximum Switching Frequency|fSW(max)|82.9|kHz| |Minimum Switching Frequency|fSW(min)|60.3|kHz |As for the IC:• 50kHz to 750kHz full-load switching frequencyAll fine and reasonable values but in the transformer datasheet at typical application i have this: Switching Frequency f switch 70 - 120 kHzShould i be concerned? i am not sure if those 10Khz are a problem , or should i play with a external inductor and see if i can get some numbers in the transformer range?This is my first LLC design and i am not so sure what i can round off / proximate and what needs to be exact.Also since i am already posting this here is another thing i am not sure off:Cr is often split in 2 since it has benefits at start up and you simply spread power dissipation in 2 components but is there any risk in this technique? ( technically i can use caps with 1/2 the voltage rating and add some protection diodes but i would rather use something safe/ stupid proof)Also i am having a hard time finding something that can handle 1.9A RMS and is not apple sized or very expensive.
Sniper2 , 10-27-2024, 10:19 AM
And regarding the mosfets that i should use i have many options but the problem is that i dont have a estimate of loses , i would like to use SMD but i am not sure i can keep them cool , or some THT without heatsink but again in TI s reference designs they did it both ways are 2 different power LVLs ( then again one heatsink was simple flat piece of metal shared with all other stuff from APFC AND LLC should be low loses so i am a bit split here since as of now i can only get some conduction loses estimate RMS and my MX loses would be under 2W per FET.Any advice?
QDrives , 10-27-2024, 02:57 PM
Not an LLC expert...So your numbers show that 70k...82.9kHz switching is ok."*Cr is often split in 2...*" -- Split in series or parallel? What does Cr do?As for the MOSFET losses, what is the supply voltage? Quantity production?Do note that TH MOSFETs often have inherent higher inductance which is not beneficial with higher switching frequencies.
Sniper2 , 10-27-2024, 03:47 PM
Cr - the resonant capacitor
Sniper2 , 10-27-2024, 03:48 PM
and lets say it is in 2 parallel ones ( not classical parallel since it is a coupling cap but close enough)
Sniper2 , 10-27-2024, 03:49 PM
Vin is 350-410Vdc from my APFC 390V nominal
Sniper2 , 10-27-2024, 03:49 PM
my MOSFETs should end up in ZVS anyway so i am not even sure how to consider loses here
Sniper2 , 10-27-2024, 03:51 PM
TI uses both SMD and THT FETs so i guess it is fine 100kHz is sort of slow , then again this is my first time doing high voltage too
Sniper2 , 10-27-2024, 03:51 PM
@QDrives
Sniper2 , 10-27-2024, 03:52 PM
NOTE: math said my switching will be in 60-83kHz range since it is a frequency variable topology with 50% duty
Sniper2 , 10-27-2024, 03:52 PM
BUT the transformer datasheet at typical application i have this: Switching Frequency f switch 70 - 120 kHz
Sniper2 , 10-27-2024, 03:53 PM
there is the TYPICAL that maybe means there is some leaways but that interval is the most efficient of all?
QDrives , 10-27-2024, 07:21 PM
I see MPS also has some information on LLC's https://www.monolithicpower.com/learning/resources/understanding-llc-operation-part-i-power-switches-and-resonant-tankAt higher voltages the switching losses can become huge.
Sniper2 , 10-27-2024, 09:12 PM
will read that
Sniper2 , 10-27-2024, 09:12 PM
then again i am supposed to get soft switching with this topology....
Sniper2 , 10-27-2024, 09:13 PM
BTW how practical do you think is the idea of useing large SMD PADS to cool SMD FETs ?
QDrives , 10-28-2024, 08:19 PM
I have done that many times. But... I must admit, never tested/compared, calculated or simulated anything to know exactly if, and how much, it helps.
Sniper2 , 10-28-2024, 08:22 PM
@QDrives so you questimate it and say more or less this should be good enough ?
QDrives , 10-28-2024, 08:24 PM
Nope, even worse, I see how much space I have for it.
Sniper2 , 10-28-2024, 08:30 PM
LOL
Sniper2 , 10-28-2024, 08:31 PM
now i feel over professional
QDrives , 10-28-2024, 08:34 PM
Then again, I cannot say I ever had to increase the size or add a heatsink (unless this was already also in the design).In one of my later designs, I did 10A continues without heatsink. Just SMD MOSFETS (8m Ohm) and copper.H bridge and something like 25x60mm???
Sniper2 , 10-28-2024, 09:38 PM
well that is easy to estimate the loses
Sniper2 , 10-28-2024, 09:38 PM
maybe i am also paranoid about heat since i did math for APFC
Sniper2 , 10-28-2024, 09:39 PM
and man that things really needs a heatsink
Sniper2 , 10-28-2024, 09:39 PM
since i am not brave anough yet to interleave multiple phases