Two transistor forward chipset (LTC3765 LTC3766) cant do mains isolated SMPS - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: June 24, 2023, 12:14:31 am
content: Hi,As can be seen , the LTC3765 and LTC3766 combo can do 2 Transistor forward (active clamp) with synchronous rectifiers.......but due to the isolation transformer used to pass the drive signals between pri and sec, this couldnt be used in offline applications, because the higher leakage inductance in the signal transformer would mean the signal getting obliterated.Do you not think its slightly ridiculous that there does not exist a true offline 2 transistor forward chipset that does it for offline applications?LTC3765https://www.analog.com/media/en/technical-documentation/data-sheets/3765fb.pdfLTC3766https://www.analog.com/media/en/technical-documentation/data-sheets/3766fc.pdf...The above chipset will take you for £12+.....and you cant even use it for offlien apps...isnt this a bit ...well....off?

author: tggzzz
author_rank: Super Contributor
timestamp: June 24, 2023, 08:13:34 am
content: Quote from: Faringdon on June 24, 2023, 12:14:31 amWarning! Warning! Treez alert.

author: jc101
author_rank: Frequent Contributor
timestamp: June 24, 2023, 08:52:47 am
content: If Analog say the parts can be used, but your simulation says it can't, then speak to Analog. They are very helpful people and will point you in the right direction.

author: Faringdon
author_rank: Super Contributor
timestamp: June 24, 2023, 09:30:51 am
content: As can be seen the signal transformer used (PA2907) has Lp=1200 and Max Leakage L of 600nH......this isnt likely to be possible with the full 3500VAC of mains isolation.https://eu.mouser.com/datasheet/2/447/SPM2007_61-2904141.pdf

author: Faringdon
author_rank: Super Contributor
timestamp: June 25, 2023, 12:43:49 pm
content: Hi, ThanksOur volumes arent enough to garner replies from semico's.The attached shows a 400w 360vin , 100khz, two transistor fwd with active clamp.The LTSPICE Sim of this chipset runs so slowly it is of very limited practical use.The dissipation in the clamp cap snubber is obviously too much, and was calculated with the damping factor of the datasheet of ltc3765.The LTspice sim cct doesnt work at all unless you short out the ISMAG resistor.The circuit works very badly.There are enormous current spikes in the synch fets.The Secondary power synch_fet turns on before the primary power fet....this is never wanted........you want the primary one to turn on first, then the sec diode to conduct, then the sec synch fet should short out its diode.

author: jc101
author_rank: Frequent Contributor
timestamp: June 25, 2023, 01:20:26 pm
content: Just post on their forum, they don’t ask about volumes etc.They have replied with a day or two to anything I’ve posted there in the past.

author: Faringdon
author_rank: Super Contributor
timestamp: June 26, 2023, 08:55:28 am
content: thanks, but i never get replies from them.

author: tggzzz
author_rank: Super Contributor
timestamp: June 26, 2023, 10:28:22 am
content: Quote from: jc101 on June 25, 2023, 01:20:26 pmQuote from: Faringdon on June 26, 2023, 08:55:28 amIt would appear you aren't asking questions that cannotusefullybe answered.Did they, by any chance, respond to your earlier questions and not respond to later questions?