Simplest way to get -28V 0.1A from +5V, without using transformer? - Page 1
Forum section: electronics / beginners

author: LinuxHata
author_rank: Frequent Contributor
timestamp: March 02, 2025, 08:30:47 am
content: Hello.I need a negative voltage source for my VFD display project. I only need fairly well (within +-5%) regulated -28V at 0.1A max. Input voltage is 5 volts. I'd like to skip usage of a transformer as much as possible and focus on integrated switch solutions.I've already tried to use MC34063A, MC34167 as inverting boost converters, using circuits from the datasheets, but none worked properly - there was problems with output regulation, super low efficiency, like 50%, IC overheating and burning.Maybe there are some other boost converter chips, which also can be used as negative boost converters?

author: John B
author_rank: Frequent Contributor
timestamp: March 02, 2025, 09:30:32 am
content: The MC34063 is garbage, your inductor was probably saturating and the output transistors were passing excessive current.Try purchasing a newer design IC. Something with FET pass transistors, a more modern control loop and ideally current monitoring through the inductor.

author: Chalcogenide
author_rank: Regular Contributor
timestamp: March 02, 2025, 09:34:52 am
content: The LT8365 is quite expensive, but can directly regulate negative voltage in an inverting configuration. There is an LTspice model available and you can simulate it quite easily.

author: voltsandjolts
author_rank: Unknown
timestamp: March 02, 2025, 09:37:59 am
content: Maybe this simple switcher can give some ideas:https://www.eevblog.com/forum/testgear/juntek-dpa-2698-10mhz-signal-amplifier-digilent-analog-discovery/msg3649978/#msg3649978

author: LinuxHata
author_rank: Frequent Contributor
timestamp: March 02, 2025, 09:42:19 am
content: Yes, that Linear chip is too expensive.For the symmetrical circuits, like that with XL6012, I already tried, and they have no regulation on negative output, since they regulate only positive output, so they're meant to be used when load on both polarities is symmetrical.

author: voltsandjolts
author_rank: Unknown
timestamp: March 02, 2025, 09:55:02 am
content: You're not drawing current much from the -28V, and you don't need precision regulation, so

author: squadchannel
author_rank: Frequent Contributor
timestamp: March 02, 2025, 10:06:17 am
content: asked webench.LM43601 seems to easy. $3.129 (1pcs, ti.com)

author: LinuxHata
author_rank: Frequent Contributor
timestamp: March 02, 2025, 11:41:53 am
content: Thanks, but it seems to be step down converter, how can it work in boost mode?

author: tooki
author_rank: Super Contributor
timestamp: March 02, 2025, 11:55:04 am
content: Quote from: LinuxHata on March 02, 2025, 11:41:53 amBy changing where ground is. TI knows what it’s doing, and it looks like they used that chip as an example for showing how to reconfigure a buck converter to an inverting converter. If you look at the product page, the documents section is full of application notes about doing just this, like “Inverting Applications Made SIMPLE with LM4600x and LM4360x”.Note also that the WEBENCH tool squadchannel screenshotted does actual simulations, so you can not only see that they suggest it, but try out the effects of changing component values, for example.Note also the TI forum discussions about the inverting configuration driven from 5V; there are some good tips in those.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: March 02, 2025, 01:27:38 pm
content: Well, I know how to use buck chip as inverting chip, and have used that feature many times, but I still can't get idea how buck chip can work as boost chip.

author: langwadt
author_rank: Super Contributor
timestamp: March 02, 2025, 02:59:48 pm
content: Quote from: LinuxHata on March 02, 2025, 01:27:38 pmhttps://www.ti.com/lit/an/slyt286/slyt286.pdf