Most efficient and simple and cheap way for making -5 and -27V from +5V supply? - Page 1
Forum section: electronics / beginners

author: LinuxHata
author_rank: Frequent Contributor
timestamp: April 30, 2022, 12:46:03 pm
content: Hello.I need to drive some VFD display controllers, which require -5V and -27V. Approximately 1A at -5V and 100mA at -27V.The simplest solution seems to use B0505S and B0524S type isolated dc-dc converters, but they have very low efficiency and unregulated output.Of course, there are converters with higher efficiency and regulator output are available, say, made by TRACO, but they do have very high prices, definitely not beginner-friendly.I tried to use MC34063 as inverting boost converter. It do works, but efficiency is very low, and it heats a lot.I tried to source some negative output converters or controllers, but they seem to be also quite rare and expensive, like MAX776, LT3580, LT1370 and similar.As I've researched, there's a cuk topology converters, which output negative voltage, but all circuit samples I've found online, are only positive to positive converters.Since power is not that high, I'd like to have something with integrated switch, to reduce number of components.So I'm looking for some simple and common component based CUK converter, which can deliver either of the mentioned voltages.Any ideas?

author: madires
author_rank: Super Contributor
timestamp: April 30, 2022, 01:40:05 pm
content: Flyback?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 30, 2022, 01:55:25 pm
content: You can turn it around.You can start with a 27Vdc power supply, and then just dedicate the highest voltage as "Ground".You can add an lm7905, LM337 or whatever to power your microcontroller.The microcontroller really does not care whether you designate the power supply as a "positive" or "negative" voltage. All it knows are the voltage differences between it's pins.When you do this, you do have to take care that level conversion between differen parts of your circuit is done properly.If you only want an extra -5V at a small current, then the ICL7660 is the defacto standard for making it, There are lots of variants of this IC in these days though.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: April 30, 2022, 04:23:02 pm
content: It is not microcontroller.It is specialized VFD display driver, which requires -27V.You can check PT6312 datasheet, as an example.Also there's need for -5V for filament supply.For flyback, I don't see any examples of circuits which would be able to deliver negative voltage at output (should use optocoupler as sensing circuit, but all circuits I've see with optocoupler, say for UC3843, are dedicated for high side switching).

author: Bicurico
author_rank: Super Contributor
timestamp: April 30, 2022, 04:39:11 pm
content: Is it acceptable that I stop by to ask: what is the reason and meaning of having a negative voltage?What is the difference of driving +5V or -5V in regard to GND? Isn't it the same, with pseudo electrons just flowing in the opposite direction?Why does the VFD expect -27V? Is it so, because it expect like +12 on a different pin and the overall difference has to be 39V?I really don't understand and I am not afraid to ask...Sorry if this is off-topic.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: April 30, 2022, 06:02:22 pm
content: You can download datasheet for PT6312 and find answers to all your questions there

author: Bicurico
author_rank: Super Contributor
timestamp: April 30, 2022, 06:55:32 pm
content: Not sure if you meant me. I did search for the datasheet, but the first one only contained 3 pages and the last one I found had indeed several pages, but those were dedicated to the serial protocol, mostly.No info about the questions I asked.Regards,Vitor

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 30, 2022, 07:27:07 pm
content: https://www.digikey.es/es/articles/generating-multiple-outputs-from-a-single-synchronous-buck-converter-is-simplehttps://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/456329/lm2595-adj-inverting-buck-boost-feedback-questionReverse the secondaries in the example to get dual negative outputs.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 30, 2022, 07:42:08 pm
content: Quote from: LinuxHata on April 30, 2022, 04:23:02 pmYes, I understand that, but I also assumed that you needed a micrcontroller or some other 3V3 or 5V logic to send data to that display.And instead of first building a 5V power supply and then making -27V out of it, it may be simpler to build a 27V power supply, and then add something to make whatever other voltages you need to it.

author: mariush
author_rank: Super Contributor
timestamp: April 30, 2022, 07:55:48 pm
content: Well... I just checked the PT6312 and it doesn't say anywhere that you need -27vSee page 16 -https://serkeros.files.wordpress.com/2015/01/pt6312-datasheet.pdfAbsolute maximum ratingsLogic supply voltage VDD -0.5 to +7 VDriver supply voltage VEE VDD +0.5 to VDD -40 VLogic input voltage V1 -0.5 to VDD +0.5 VRecommended :Code:Parameter Symbol Min. Typ. Max. UnitLogic supply voltage VDD 3.0 5 5.5 VHigh-level input voltage VIH 0.7VDD - VDD VLow-level input voltage VIL 0 - 0.3VDD VDriver supply voltage VEE VDD -35 - 0 VSo you could use Vdd = 3.3v or 3.6v and you could use less than -27v ... in fact on page 18 it shows in the schematic that they use -24v , and 3.3v/5v for the rest ... don't see any -5v.24v or -24v power supply would be easier to get.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: April 30, 2022, 08:09:06 pm
content: -27V is common voltage for VFD displays. You can't use lower voltage - it won't lit up. Some use 24V, some - 40 or even 60.For the PT6312 datasheet, it indeed contains the answers. Here's the screenshot with the circuitry.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: April 30, 2022, 08:31:22 pm
content: Quote from: DavidAlfa on April 30, 2022, 07:27:07 pmThanks, this looks really interesting!But I need far more current on -5V rail, about 1 amps - it has to drive filaments of the VFD tubes. (I'm using separate CV/CC step-down converter to deliver required voltage and current to VFD filaments- 1.5v 100mA each)I've read the original AN1118 and it says, that "upping" IC to LM2596 will boost output currents to 275mA.So I assume, if I use MT3410L as downconverter, which will make required filament voltage from -12V, it all should be fine, since 12*0.275=3.3w, and still, after losses, power should be enough.

author: Benta
author_rank: Super Contributor
timestamp: April 30, 2022, 08:45:27 pm
content: Needing -27...30 V is obvious.But why the -5 V? Do the filaments double as cathodes? In that case, why -5 V when the filaments are 1.5 V?I'm confused.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: April 30, 2022, 09:12:40 pm
content: Yes, filaments are also used as cathodes, and they need to be powered with AC voltage, because powering them with DC will cause lower brightness at one edge.So I need some "headroom" in terms of voltage to generate AC for them from DC (by using simple h bridge controlled with MCU).

author: Benta
author_rank: Super Contributor
timestamp: April 30, 2022, 09:43:24 pm
content: So to summarize:You need +5 V for the logic supply (or you already have it).You need -27 V for the VFD.You need 1.5 VAC for the filaments (frequency a mystery).Why not say so from tha start? It would probably have resulted in much better answers.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: May 01, 2022, 05:51:16 am
content: +5V will be sourced from an USB phone charger.-1.5V AC can be any frequency above 2khz or below 400hz, to avoid mechanical resonances of filaments (as per VFD tube datasheet). Square wave or sine wave - all are ok. Currently I'm using L9110 as a H bridge driver for filaments, which is being controlled by PIC12F683, which generates appropriate duration pulses with dead time included.So I still need -27V and -5VAs initially stated.The circuit with LM2595 seems to be fine, but as I've checked, there are almost no coupled inductors with different turns ratios. So if I generate -27V with that circuit, I'll have 27V on the other output too. I can wind it by myself, but I'd like to avoid "handwork", for more "factory-look device" aesthetics

author: inse
author_rank: Super Contributor
timestamp: May 01, 2022, 08:57:32 am
content: @BicuricoSee it like this: debts and assets are the same thing, pseudo money flowing in opposite directions ;-)

author: Bicurico
author_rank: Super Contributor
timestamp: May 01, 2022, 09:44:04 am
content: But why does they VFD care for the flow direction of the electrons? Just connect +/- the other way around, would be my instinct, which I know is wrong.Still I don't understand it at all.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: May 01, 2022, 10:00:39 am
content: It is not VFD, it is VFD driver chip was made that way - for ease of making.