Generating 3 voltage rails, will this work ? - Page 1
Forum section: electronics / beginners

author: BravoV
author_rank: Super Contributor
timestamp: June 29, 2018, 09:19:16 am
content: The +15V and -15V rails are classic topology, linearly regulated to power the analog section, while the +5V rail uses an isolated DC to DC step-down switching converter to power logic parts.Will this work ? Other considerations if any ?Thanks in advance.

author: Ian.M
author_rank: Super Contributor
timestamp: June 29, 2018, 09:33:29 am
content: It will work, but a fully regulated isolated DC-DC converter is more expensive than a simple switching (non-isolated buck) regulator run off the unregulated +V rail.If you need a *LOT* of current from the +5V rail, you may have to go with the isolated DC-DC converter to utilise the whole secondary, but as high current isolated DC-DC converters are expensive, its probably more cost-effective to simply up the mains transformer VA rating and use a simple switching regulator.

author: CJay
author_rank: Super Contributor
timestamp: June 29, 2018, 09:47:32 am
content: I've just done pretty much exactly that to remove the need for a custom mains transformer.Using a centre tapped mains transformer to feed 7915 and 7815 regulators and 'stealing' some power from the positive rectifier to feed a buck converter (no need for isolation in my application) for a 5V rail.

author: Benta
author_rank: Super Contributor
timestamp: June 29, 2018, 10:56:16 am
content: I've done it the "other way round". A 5 V wall-wart to power the logic, and an isolated 5 V in, +/-15 V out DC-DC module.

author: BravoV
author_rank: Super Contributor
timestamp: June 30, 2018, 07:26:41 am
content: Quote from: Ian.M on June 29, 2018, 09:33:29 amThanks for the confirmation.Yeah, the +5V rail needs about 4 to 5 Amps while the pos & neg 15V rails only merely at sub 100mA.My situation is I already have a suitable transformer with the right size, its just its only has 3 taps includes the CT.Also coincidentally at local here there is a really cheap offering for a NOS Vicor high density DC to DC module that I think suitable for +5Volt rail.Yeah, buying a new one of this isolated DC to DC is very expensive.

author: BravoV
author_rank: Super Contributor
timestamp: June 30, 2018, 07:28:53 am
content: Quote from: CJay on June 29, 2018, 09:47:32 amSo the negative side of the 5V rail is also isolated from the ground reference for +&- 15V rails ?

author: Ian.M
author_rank: Super Contributor
timestamp: June 30, 2018, 07:58:52 am
content: Unless you've got a 15-0-15V >60VA transformer + the 30 to 48V in, 5A@5V out DC-DC converter hanging about essentially free, it probably doesn't make much sense to do it that way. If the transformer is run too close to its rating, the 7815 and 7915 regulators will be prone to dropout at the ripple troughs if the 5V rail is heavily loaded, so I suspect a 50VA transformer would be marginal.The easy low cost option would be a 19V small laptop PSU, 7815 for a clean +15V rail, buck converter for the +5V rail and inverting switching regulator to get a -18V feed to a 7915 for a clean -15V rail.

author: CJay
author_rank: Super Contributor
timestamp: June 30, 2018, 08:33:04 am
content: Quote from: BravoV on June 30, 2018, 07:28:53 amNot in my most recent incarnation as there was no advantage but I have used isolated DC-DC converter that way a few times when I've had expensive digital hooked up to analogue that might be subjected to lots of volts by accident,

author: David Hess
author_rank: Super Contributor
timestamp: June 30, 2018, 09:08:45 am
content: That is a fine way to do it if you can find an economical isolated converter. But beware that the line regulation of linear regulators decreases at higher frequencies so do not count on them being able to remove switching noise from their inputs; additional LC filtering may be necessary.

author: BravoV
author_rank: Super Contributor
timestamp: June 30, 2018, 10:25:41 am
content: Quote from: Ian.M on June 30, 2018, 07:58:52 amYeah, I agree on your suggestion above, actually had thought about that arrangement too.The major factors I've opted this arrangement at the 1st post, as I already have the suitable toroid transformer which the size + the circuit fit in the space allowed, and also the Vicor module is on it's way, really, I'm lucky to score it as its dirt cheap and NOS, about 10 bucks compared to new one say like from Digikey.

author: BravoV
author_rank: Super Contributor
timestamp: June 30, 2018, 10:27:59 am
content: Quote from: David Hess on June 30, 2018, 09:08:45 amDavid, yeah, I've been thinking about the quite line at analog power rails too, as above circuit, where that LC filtering parts should be placed ideally ?Around C1 at input and C6 at output ?

author: BravoV
author_rank: Super Contributor
timestamp: July 01, 2018, 03:38:44 pm
content: Just arrived, the DC converter.

author: David Hess
author_rank: Super Contributor
timestamp: July 01, 2018, 04:38:02 pm
content: Quote from: BravoV on June 30, 2018, 10:27:59 amI would start with an LCR decoupling network at the input to the switching voltage regulator to isolated it from the inputs to the linear regulators.

author: BravoV
author_rank: Super Contributor
timestamp: July 02, 2018, 08:04:48 am
content: Quote from: David Hess on July 01, 2018, 04:38:02 pmSomething like this ? Added L4 ?You mentioned LCR.. where the R part should be placed ?Also L6 to reduce the output switching noise.

author: bd139
author_rank: Super Contributor
timestamp: July 02, 2018, 08:15:58 am
content: Ideal world: Meanwell RT-65C + pi network LC filters on the +15V, -15V outputs.

author: BravoV
author_rank: Super Contributor
timestamp: July 02, 2018, 08:19:40 am
content: Quote from: bd139 on July 02, 2018, 08:15:58 amAgree, its just I already have the suitable (ideal) transformer collecting dust, and a really-really damn cheap NOS Vicor isolated DC to DC converter.

author: bd139
author_rank: Super Contributor
timestamp: July 02, 2018, 08:22:19 am
content: Yeah that was a bargain

author: David Hess
author_rank: Super Contributor
timestamp: July 04, 2018, 01:14:51 am
content: Quote from: BravoV on July 02, 2018, 08:04:48 amExactly.QuoteThe resistor goes across the inductor to limit Q and prevent noise peaking. The value can be selected based on the self resonate frequency since above this point, the inductor looks capacitive anyway. Or just assume that you will never get more than 20 or 30 dB of attenuation and treat it as an RC filter with the inductor shorting out the resistance at low frequencies to prevent excessive voltage drop.QuoteThere is nothing wrong with that.

author: OuchItReallyHz
author_rank: Newbie
timestamp: July 07, 2018, 06:49:17 pm
content: Quote from: Ian.M on June 30, 2018, 07:58:52 amWhat inverting switching regulator would be a good option? I'm looking for something like this as well, +/- 15V, but haven't come up with anything simple save for a AC transformer, but as a relative noob, I'd rather not deal with the weight and issues of a transformer.

author: bd139
author_rank: Super Contributor
timestamp: July 07, 2018, 07:09:40 pm
content: TI do a range of SMPS parts that just workhttp://www.ti.com/ww/en/simple_switcher/index.html?HQS=sva-bmp-sspl-simpleswitcher-vanity-lp-null-wwe#

author: Ian.M
author_rank: Super Contributor
timestamp: July 07, 2018, 07:40:11 pm
content: Also for one-offs, many EBAY LM2596 modules can be rewired in inverting mode. Seehttps://iw0ffk.wordpress.com/2014/09/19/negative-voltage-from-lm2596-dc-dc-ebay-module/However your supply to them must be beefy as they wont start up successfully in inverting mode from low current supplies.