Sanity check on boost converter - Page 1
Forum section: electronics / beginners

author: JJalling
author_rank: Unknown
timestamp: February 20, 2022, 02:50:24 pm
content: Hello, I'm trying to make a boost converter (5V -> ~180V).I have made a small board with the attached circuit, but I cannot get it to work. I'm only getting ~3V out, and the transformer gets hot.Am I doing something obviously wrong?BR Jonas

author: JJalling
author_rank: Unknown
timestamp: February 20, 2022, 03:05:22 pm
content: Checking the datasheet again, I have now added C1 (22pF) to the circuit, but without any change.

author: Ian.M
author_rank: Super Contributor
timestamp: February 20, 2022, 04:40:02 pm
content: Its telling that theLMR62014datasheet doesn't show a single flyback application. SW (pin 1) is rated for abs. max. 22V and you didn't even give it a snubber! You should probably also have connected the secondary in series with the primary to make a tapped inductor boost converter.AlsoATB3325-nnnnnCTis a range of RF baluns with a minimum operating frequency of 5MHzm not intended for power applications. If that's what you are using as it doesn't even specify a DC resistance and the LMR62014 max. frequency is a fraction of its min. frequency, its saturating and you are S.O.L.I *HOPE* you've got the so-called 'ATB3225' step-up transformer:ATB3225nn-110!

author: Siwastaja
author_rank: Super Contributor
timestamp: February 20, 2022, 05:10:24 pm
content: Assuming part with 1:10 turns ratio, 180V on the output would theoretically reflect 5V+18V on the input with perfect snubbing or no leakage inductance; this already exceeds the absolute maximum Vsw rating significantly. There is no way this could ever work, not even close.Pick a part with higher Vsw rating and provide an RCD snubber typical to flybacks (google for typical flyback schematics to get the idea).Feedback path / loop compensation with boost and flyback is also a bit finicky because of the Right Half Plane Zero.

author: JJalling
author_rank: Unknown
timestamp: February 20, 2022, 07:51:59 pm
content: Hi Ian,Thanks for your reply, but I'm a bit puzzled why you are bringing ATB3325 into the thread. Can you elaborate on that. The schematics I showed, states ATB3225.BR Jonas

author: JJalling
author_rank: Unknown
timestamp: February 20, 2022, 07:53:04 pm
content: Quote from: Siwastaja on February 20, 2022, 05:10:24 pmThanks, that's good info. I'll dig further in my part drawers and see what I can find!BR Jonas

author: Ian.M
author_rank: Super Contributor
timestamp: February 20, 2022, 08:27:22 pm
content: AAgh!! I made a typo searching google and didn't notice then copy/pasted the bad part number and result.Please *IGNORE* my stupidity.Siwastaja has nailed it. I'm surprised its even outputting 3V as I would expect overvoltage to blow the LMR62014's output transistor, dead short to ground. Ground /SHDN and see if the SW pin is lower than Vin or if the transformer is getting hot.The LMR62014 may still be viable if you can find a higher ratio pulse transformer. Maybe one with a center tap on the low turns count side?Another option, keeping the ATB3225 would be to use a voltage doubling rectifier so the LMR62014 only 'sees' 5V+90V/10 (=14V) reflected peak at SW, but you still get 180V out.

author: JJalling
author_rank: Unknown
timestamp: February 21, 2022, 07:58:42 am
content: Thanks Ian for your feedback!BR Jonas