My boost converter is SMOKING :D  - Page 1
Forum section: electronics / beginners

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 08, 2022, 07:51:20 pm
content: I made it, finally got the voltage I wanted, no noise at all, everything was amazing, and then I was like, what is that smoke...----Hello!So it's my first time building a switching converter.My specs are:* Input voltage USB Vbus (so 4.5V min)* Output voltage about 5.8V* Output current:** peak: 90mA** peak average over 25ms: 30mA** average (over 1 second f.x.): 10mAThe parts I am using are:* IC:https://datasheet.lcsc.com/lcsc/1806070746_XI-AN-Aerosemi-Tech-MT3540_C181744.pdf* Diode:https://datasheet.lcsc.com/lcsc/1807271843_Tak-Cheong-SD103AW_C248792.pdf* Inductor: 100uH +/-20%, package 1201, Q=35, RDC=1.5 ohm, rated current=150mA* Resistor divider: 183kOhm and 47kOhm - should set the voltage at 5.87V* Input cap: 4.7uF MLCC* Output cap: 4.7uF MLCCI am not sure if the thing that's smoking is dead now, but I removed the whole load now and it is still smoking when I power it up.Not sure exactly which component is smoking either.Making the math followinghttps://www.ti.com/lit/an/slva372c/slva372c.pdf, I get:* D=34%* inductor riple current: 12.8mA* maximum switching current: 52mAThat is based not on the peak output current of 90mA but the peak average of 30mA.I guess I missed something somewhere in the calculations?Thank you!Simon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2022, 04:44:29 am
content: L is way too big for something of that rating. Design based on total rating, not what [you think] you're using it at. So, the 4.7-22uH they say in the datasheet.The error amp is compensated for L and C values within the expected range. Using values very different from that is likely to result in oscillation. Though not abject failure, usually.Sadly they don't give complete values for example designs, nor an application section, so you're on your own there. There's only so many values to guess at, so it's not that terrible. Set up a test jig and poke around. (Say, 5V input from a bench supply, fixed load resistor plus step load to test dynamics. Adjusting values by a factor of 2 or so is plenty of resolution to see how things behave, say starting from the headline circuit values.)What's your layout look like?Tim

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 09, 2022, 07:15:20 pm
content: Layout is messy, but the inductor loop is very short.It's done on a test board with SMD inductor, SMD diode and SMD capacitors, but using TH resistors for the sensing.(the npn on the picture is the filter "capacitor multiplier" on the output of the boost converter)

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 09, 2022, 07:44:14 pm
content: And thanks for the info TimWhat do you mean by "Design based on total rating"?I'll make a new circuit with 4.7uH. I made the math, still following that same documenthttps://www.ti.com/lit/an/slva372c/slva372c.pdf,I get:* inductor riple current: 280mA* maximum switching current:** Using average output current: 190mA** Using peak output current: 280mAThe 4.7uH inductor I'm gonna be testing with is a big one so it can easily withstand the current. The diode is more on the edge? The datsheet says 200mA average forward current and 400mW power dissipation. Not sure how to calculate the power dissipated by the diode though?Also what are acceptable values for the DCR of the inductor?

author: Benta
author_rank: Super Contributor
timestamp: March 09, 2022, 09:51:30 pm
content: EU solution:slap a sticker on it reading "SMOKING KILLS".

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2022, 11:14:55 pm
content: What's going on with the feedback resistors, and the PN2222?Tim

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 10, 2022, 07:49:42 am
content: @Benta:@Tim: As I wrote the resistor on the left and the NPN is for the filter after the boost converter.The feedback resistors are TH, 150k+33k from OUT to FB, and 47k from FB to GND. That should make 1.2V * (1+183/47) = 5.9VRegarding my calculations above - isn't the inductor ripple current way too high? TI says it should be between 20% to 50% of the output current (again I'm wondering if they mean average or peak here), which is in my case:* 20%-50% of average output current: 6-15mA* 20%-50% of peak output current: 18-45mAWhich is where I start to understand why it's a bad idea to use an IC designed for 1.5A and use it for 100mA... Right?EDIT: apologies for creating another topic (https://www.eevblog.com/forum/beginners/boost-converter-understanding-tis-calculations/) - but in a way it's not related to this project, more just me trying to understand TI's equations. I'll still use this topic for this particular design/problem.EDIT 2: for those passing by this post - based onthe long answer from Tim on the other post, it seems the problem was because of the too large inductor and the fact this IC is peak current mode control, so it became unstable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 10, 2022, 03:03:18 pm
content: Thing is it's only compensated for full load. You can't force it to use bigger inductors, it just oscillates. It will run in burst mode at such light loads. (Which is oscillation anyway, but intended, for whatever that's worth?)Tim