LLC design not working please help - Page 1
Forum section: electronics / projects

author: Sniper1
author_rank: Regular Contributor
timestamp: May 12, 2025, 06:36:10 pm
content: So I designed a AC DC SMPS with APFC and a LLC Isolated stage and it should output 24V ( or +-12V if i invert some rectifying diodes, D7 D8, and disable the synchronous rectifier) .The problem is that neither the APFC and LLC dont seem to work. I asked on the TI forum and i was told the schematic is correct but it still does not work for some reason.For LLC tank i used TI provided calculator to pick all values and there was supposed to be a lot of design margin. I never put a proper load on this design. I even disconnected the inrush relays since i was thinking it drew to much current from the transformer but still i can measure any voltage on the isolated outputs.Currently i have the synchronous rectifiers mounted and no normal diodes mounted on the outputHere are a few measurement and observations that i made:- APFC FB voltage is 4.1V so it is clearly not working since on the bulk capacitors i read about 320-330V (from 230V mains EU)- APFC voltage on the Vcc pin is 11V ( not the best since it is not higher then the max value but should still work ) this happened while R70 is mounted- in a interesting note the mod point of the C21 C24 capacitors is at 0V to 4V- on D6 Katode i can measure about 4V for some reason but not all the time- no component seems to get hot ( measured with a laser thermometer)-BLK on the LLC chip is 1.3V si the LLC chip should be active- the 5V LLC power supply seems not to work , measure on C14Does anyone have any idea what i am doing wrong?Or any idea what i can try and make this work? Or what else to try and measure.I dont own an oscilloscope yet (Looking at a DHO800/900 atm ) + i would also need an isolation transformer to properly measure things safelySo i must note that that the LLC chip requires min 33uF on the 15V rail and i have only 10uF on the schematic but IRL i have 68uF additional electrolytic capacitor also soldered on the PCB.No changes 15V rail is still stuck at 4VAlso i cut the trace that powers the relay so there is like no load on the 15V rail.I also tried to power the LLC+APFC ICs with an external battery , died across C50, did not work and it blew the LLC chip , apparently there was a LDO mosfet between mains and 15V , for start upAnd the diode of that mosfet got a huge current surge and the IC blew.I didnt try again after replacing the chip ( this time with mains plunged since i am very concerned about my safety + IC should start from mains anyway.I did not test yet with the battery across C5 ( where D17 would keep the LLC chip safe).I am without ideas and i am stuck ....I am sort of focusing on the LLC since yeaa it can work at lower power lvls with just the rectifierAny ideas what is not ok in my design?

author: homebrew
author_rank: Frequent Contributor
timestamp: May 15, 2025, 10:35:34 am
content: Where is the 15V rail sourced from?The only point I could spot was via the depletion mosfet linear regulator. But if so, is D17 *really* in correct orientation?Apart from that, the layout looks quite dangerous to me in some areas. Especially on the primary side, where the distances between L and N are sometimes VERY small. Why bother routing isolations slots when the adjacent components are placed with even less separation? (e.g. fuses and mains input terminal)Also I personally don't like the depletion mode 15V "regulator". I'm not into PSU design at all ... but I would be worried that if that thing fails shut -> big time explosion of components.General question: Why are you designing something safety relevant from scratch that is absolutely bulk standard and readily available for cheap?It's neither worth the risk, nor the price ... AND you apparently lack the appropriate equipment to do the required measurements.

author: Sniper1
author_rank: Regular Contributor
timestamp: May 15, 2025, 09:02:57 pm
content: The 15V should come from the transformer auxiliary after it is rectified by diode D6. So yea D17 has the correct orientation.Regarding that *regulator* yea it is not something that i was reling that much it was more like in case it does not start normally i can try with this.Regarding layout: yea it is my first time doing mains stuff without AC-DC modules no with those slots i tried to fix my best layout attempt.I do mostly digital at work and only deal with low* voltages 95% of the time.Regarding safety i am doing my best with fuses and breaker externally and extreme caution .```General question: Why are you designing something safety relevant from scratch that is absolutely bulk standard and readily available for cheap?```It is mostly for the learning process and since i am making it why not to make it right ( with safety , and isolation ) .I like do do SMPS and at work i deal only with low-mid power low voltage stuff and i sort of wanted to make some big mostly personal ambition.IF all went well the plann was to take the 24V output and make another board with PSB PD/PPS this time a digital SMPS.yea if i wanted only 24V i could use a module but there is not that much to learn from that.Regarding equipment untill not what i did with personal stuf was mostly digital so porgramind and i sort of got it to work only with LEDs and my DMM or simple stuff.

author: Sniper1
author_rank: Regular Contributor
timestamp: May 17, 2025, 10:38:15 pm
content: UPDATE:If i use external 15V AFPC works well , not exact but within expected tolerancesLLC still not working at all.I also added a diode+ capacitor to see if my synchronous rectifiers are at fault but still nothing.Also i noticed that the LLC chip is relatively hot 35-40C. while it is doing nothingBias voltage is still; about 4-5V when it was supposed to be 12-15V ( from the transformer)I also tried to change the split capacitor into a single one but no changes