Idea for converting a single 24V supply into a dual +/-24 V supply - Page 1
Forum section: electronics / projects

author: SharpEars
author_rank: Unknown
timestamp: October 25, 2016, 03:13:52 pm
content: I have a single 24 V switch-mode power supply rated at just over 4A of output that I would like to convert to a dual output (i.e., +/0/- 24 V supply with at least 2A of output capability). My idea is to do the following:Starting with the V+ and V- at a 24 V (4+ A) potential, we connect a boost converter to go from 24V to 48V. The boost converter is powered by V+/V- and let's call its 48V outputs VB+ and VB-.Then:VB- and V- are tied together (which they most likely are already) and form our -24 V output. The original V+ is our new ground (0V). VB+ is the new +24 V output. So, the combination of VB+, V+, and VB- forms the +24V/0V/-24V dual output supply. The result, given a capable boost converter should be able to put out at least 2 A on each of the two rails (+24/0 and 0/-24).Does this sound reasonable or is there a better way to do this (relatively efficiently and not overtly expensively)?

author: StillTrying
author_rank: Super Contributor
timestamp: October 25, 2016, 04:19:54 pm
content: "we connect a boost converter to go from 24V to 48V"If your 24V ? boost converter has an isolated/floating output anyway, then you might as well connect it's +ve output to 0V and use it's -ve output as the -24V, ....I think...

author: SharpEars
author_rank: Unknown
timestamp: October 25, 2016, 04:56:12 pm
content: Quote from: StillTrying on October 25, 2016, 04:19:54 pmSadly, an isolated boost converter would be far more expensive at these current levels.

author: helius
author_rank: Super Contributor
timestamp: October 25, 2016, 05:03:45 pm
content: How do you ensure that the source impedance of the -24V to 0 leg is the same as the 0 to +24V leg? I think that common-mode noise problems will be worse using your idea as opposed to a voltage inverter chip like LTC3260*. But there's no reason it can't work.* Of course, they have much lower current limits

author: SharpEars
author_rank: Unknown
timestamp: October 25, 2016, 05:17:19 pm
content: Quote from: helius on October 25, 2016, 05:03:45 pmThe source impedance would most likely differ, but it is not clear to me why that matters as long as both the boost converter and original power supply regulate their output based on the load(s). Perhaps you can elaborate on the issue here.The LTC3260 is a very low current device (50 mA per rail), otherwise it would be perfect.

author: C
author_rank: Super Contributor
timestamp: October 25, 2016, 05:37:20 pm
content: In place of boost positive, just use a boost negative to get -24vRemember that there is no 100% here.Also need to remember that most power supplies can increase output, but load is used to decrease output.A floating 48v regulated supply would need a power op amp to maintain center output. Look at what happens when you have 1 amp on one side and 2 amps on other side, then swap the currents and look again.

author: StillTrying
author_rank: Super Contributor
timestamp: October 25, 2016, 05:55:29 pm
content: If this is what you mean I think it should work, just don't use it on anything expensive.

author: SharpEars
author_rank: Unknown
timestamp: October 25, 2016, 06:13:21 pm
content: Quote from: C on October 25, 2016, 05:37:20 pmI am sorry, but I don't get it. The original PS will regulate the original 0 (V-) and +24V (V+) outputs to be 24 V apart. The boost converter will regulate its 0 and +48V outputs (VB-, VB+) to be 48 V apart. It seems like the center output will always be V- + 24 V = 0 V in the final output topology (see StillTrying's diagram, above), independently of loads on each half of the dual supply.What am I missing and/or why would the center float at all?

author: rob77
author_rank: Super Contributor
timestamp: October 25, 2016, 06:34:30 pm
content: don't forget your -24V will be groundreferencedconnected and not the 0V !!

author: Cliff Matthews
author_rank: Unknown
timestamp: October 25, 2016, 06:43:33 pm
content: Take 2 old laptop 19vdc @3.5amp adapters (most are isolated) and feed 2 ebay 10amp boost converters (the red PCB's with heatsinks running down both sides) and for under $20 you're good to go for 3amps without a lot of worry on the puny heatsinks.

author: nali
author_rank: Frequent Contributor
timestamp: October 25, 2016, 06:55:18 pm
content: What is the current path when you put a 12 ohm load between +24v & 0v ?

author: C
author_rank: Super Contributor
timestamp: October 25, 2016, 07:08:20 pm
content: Most power supplies can only increase the voltage.Think of simple positive power supply. You have a transistor that can increase the positive output(V+) an only load decreases output. Your positive regulator can do increase positive.Think of simple negative power supply. You have a transistor that can increase the negative output(V-) an only load decreases output. Your negative regulator can do increase negative.Now your design has0 to 24 positive regulator with outputs called -24 & 0.a 0 to 48 positive regulator with outputs called -24 & +24.What happens when you connect a 1 ohm resistor between 0 & 24. The regulators can not stop the increase of 0, only a decrease.It takes an OP amp output that do both increase & decrease to do a center where you have loads on each side of center.just pick a positive regulator that has a negative regulator and look at inside parts.LM317 & LM337lm7805 & lm7905or easy to seeLT3081 & LT3091http://www.linear.com/product/LT3081http://www.linear.com/product/LT3091

author: SharpEars
author_rank: Unknown
timestamp: October 25, 2016, 07:10:45 pm
content: Quote from: nali on October 25, 2016, 06:55:18 pmIt is from the output of the boost converter (VB+ which is at a fixed +48V absolute potential) to the output of the original power supply, which is at a fixed +24V absolute potential, for a total potential difference of 24 volts. The two are related (i.e., part of the same circuit), because there is no isolation here. Or, am I misunderstanding how this model works.

author: C
author_rank: Super Contributor
timestamp: October 25, 2016, 07:12:31 pm
content: Read my post above

author: suicidaleggroll
author_rank: Super Contributor
timestamp: October 25, 2016, 07:36:06 pm
content: I believe what C is trying to say in his post is that most power supplies can only source current, they can't sink current. So when a load tries to push current into your "0V" rail, unless you have a load at least that large between 0 and -24 to sink it away, it will start increasing the voltage of your "0V" rail without control. That is unless your 24V supply is capable of both sourcing and sinking current, you should check.

author: SharpEars
author_rank: Unknown
timestamp: October 25, 2016, 07:38:06 pm
content: Quote from: C on October 25, 2016, 07:08:20 pmI always thought that proper regulators can sourceandsink current. Otherwise, how can you possibly connect two supplies in series and use the center as ground for a dual supply circuit? Inevitably, some of the + voltage of the dual supply may return to the series connection point (i.e., 0V), pulling it up with respect to its negative connector.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: October 25, 2016, 07:41:38 pm
content: Quote from: SharpEars on October 25, 2016, 07:38:06 pmNope. Some can, many can't. The datasheet should say if it can, but most likely it can't.Quote from: SharpEars on October 25, 2016, 07:38:06 pmThat's completely different. When you put two isolated supplies in series, a load on the top supply does not push current into the bottom supply. Every amp that comes out of the top supply's + goes back into its -, not into the bottom supply's +, which is what you're trying to do here.Remember, current flows in a loop. Every amp that comes out of a given supply's + needs to get back to its - some way or another. With your proposed setup, that current would have to flow out of the 48V supply's +, into the 24V supply's +, then travelbackwardsthrough the 24V supply's circuitry before finally making its way back to the 48V supply's -.

author: SharpEars
author_rank: Unknown
timestamp: October 25, 2016, 07:42:22 pm
content: Quote from: suicidaleggroll on October 25, 2016, 07:36:06 pmOK, that makes sense. Here is my supply, the output of a VHB100W-Q48-S24 isolated DC-DC converter:http://www.cui.com/product/resource/vhb100w.pdfNowhere, do they state whether the converter can sink current, so it's hard to say.

author: nctnico
author_rank: Super Contributor
timestamp: October 25, 2016, 07:42:28 pm
content: A power supply can only source current. Current always flows in a loop so if you draw that in a block diagram it is easy to see if something will work or not.

author: C
author_rank: Super Contributor
timestamp: October 25, 2016, 07:46:17 pm
content: Thanks suicidaleggroll, like your wording.A positive regulator can source current.A negative regulator can sink current.An op amp can source & sink current.QuoteYou are talking about two power supplies that are only connected at output terminals. Not what you have.

author: SharpEars
author_rank: Unknown
timestamp: October 25, 2016, 07:49:02 pm
content: Quote from: C on October 25, 2016, 07:46:17 pmOK, so what I am trying to do is not possible without at least one more isolated converter? I was hoping not to have to use up two of them.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: October 25, 2016, 07:50:57 pm
content: Quote from: SharpEars on October 25, 2016, 07:49:02 pmUnless you canguaranteethat you will always have more current flowing from 0 to -24V than from 24V to 0, then no it won't work. If you can guarantee that, then you should be fine.

author: C
author_rank: Super Contributor
timestamp: October 25, 2016, 07:54:03 pm
content: You can do what you want.You can use a negative boost converter, some call this and inverting regulator.OrYou use a DC to DC with isolation.

author: SharpEars
author_rank: Unknown
timestamp: October 25, 2016, 07:55:41 pm
content: Quote from: Cliff Matthews on October 25, 2016, 06:43:33 pmYou can get these on AliExpress for asong(i.e., $3.98 US with free shipping [to the US]):https://www.aliexpress.com/item/10A-Buck-Converter-Adjustable-200W-For-Battery-LED-Car-Power-Module-DC-DC-7-32V-Y102/32704854046.htmlBut, how do I solve my problem with them and without the laptop adapters (i.e., with just the one 24V supply)?

author: C
author_rank: Super Contributor
timestamp: October 25, 2016, 08:00:34 pm
content: Ti's Products for Buck/Boost, Inverting or Split-Railhttp://www.ti.com/lsds/ti/power-management/buck-boost-inverting-or-split-rail-products.page#p1129=Invertingmany in that list will work, you want input to include +24 and output to include -24Edit sorry typed 12 in stead of 24

author: SharpEars
author_rank: Unknown
timestamp: October 25, 2016, 08:02:47 pm
content: Quote from: C on October 25, 2016, 08:00:34 pmAh yes, good old charge pumps. Perfect, if you need less than 80 milliamps (250 mA at ridiculously low voltages - i.e., 5V).

author: C
author_rank: Super Contributor
timestamp: October 25, 2016, 08:11:51 pm
content: LM5175 20AYou will find some standard switching controllers can be configured to go negative and do power.All the circuit is doing is using inductive kick to kick down instead of up.Designing a negative boost converter from a standard positive buck converterhttp://www.ti.com/lit/an/slyt516/slyt516.pdf

author: StillTrying
author_rank: Super Contributor
timestamp: October 25, 2016, 08:18:54 pm
content: I think the input 0-24V supply only has to always supply current (mostly to the booster!) and never sink any - in theory.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: October 25, 2016, 08:20:17 pm
content: Quote from: SharpEars on October 25, 2016, 08:02:47 pmGo to TI's Webench and enter your parameters. 22-26 Vin, -24Vout, 2Aout. It'll build you a circuit with the LM5088 in an inverting configuration.

author: C
author_rank: Super Contributor
timestamp: October 25, 2016, 08:32:15 pm
content: Quote from: StillTrying on October 25, 2016, 08:18:54 pmWith a load between +24 and 0You might have +20 , 0 , -28 Note total is still 48 vThe +24 volt is now 28 and if it has over voltage protect you just tripped out on over voltage

author: suicidaleggroll
author_rank: Super Contributor
timestamp: October 25, 2016, 08:51:51 pm
content: Quote from: C on October 25, 2016, 08:32:15 pmI think he's saying that since any current out of the "+24V" rail comes from the 48V supply, which is pulling from the 24V supply, it will be impossible for there to be a net current into the 24V supply (since the 48V supply has <100% efficiency). In other words, a 1A draw on the "+24V" rail will go back into the "0V" rail, but not into the 24V supply, since >1A will have to feed into the input of the 48V supply in order to generate that 1A in the first place.I think steady-state it looks fine, but I'd worry about switched loads and timing issues related to WHEN the 48V supply kicks up its current draw to feed the +24V rail, versus WHEN the load on that +24V rail turns around and dumps that current back into the 0V rail. Unless it's just a pure 100% duty resistive load you could still run into problems.

author: StillTrying
author_rank: Super Contributor
timestamp: October 25, 2016, 08:52:34 pm
content: The way I'm thinking of it is that if you draw1Abetween the +24V and 0V outputs, the OV won't rise up towards the +24 output because the 48V booster will then be drawing2Aoff the 0-24V input supply.Edit: I agree with suicidaleggroll other than >1A = >2A drawn off the 0-24 input supply by the booster.

author: C
author_rank: Super Contributor
timestamp: October 25, 2016, 09:25:35 pm
content: If there is less load or no load on 0 to -24, nothing prevents +24 increase towards +48Draw in the transistors and look.When the +24 voltage increases the supply decreases source current by using pass transistor adjusting towards off.When the pass transistor is off nothing prevents more voltage increase.just to make it more fun, +48 is from +24 so at some point, the boost to 48 is not getting the input current it needs and starts dropping.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: October 25, 2016, 09:39:30 pm
content: Quote from: C on October 25, 2016, 09:25:35 pmThe 48V supply would be feeding itself in addition to what's coming from the 24V supply.Think of the system in steady-state with a 24R load between the +24V and 0V rails. You have 1A flowing out of the +24V rail and back into the 0V rail. That 1A would then go straight into the input circuitry for the 48V supply, where it combines with an additional 1A from the 24V supply, to provide the 2A the 48V supply requires to generate the 1A feeding the load. In reality the 24V supply would be feeding a bit more than 1A to account for the 48V supply's inefficiency.This only works with resistive steady-state loads, as soon as you get into switching loads everything gets tossed out the window.

author: StillTrying
author_rank: Super Contributor
timestamp: October 25, 2016, 09:40:37 pm
content: "When the pass transistor is off nothing prevents more voltage increase."Other than the booster no longer being powered!

author: Zero999
author_rank: Super Contributor
timestamp: October 25, 2016, 11:06:40 pm
content: Yes, it should work. As stated above, the input power supply never has to sink any current, as it's always sourcing more current to the boost converter.As long as the boost converter and primary supply are properly decoupled, I don't see how dynamic loads could be a problem. Obviously the voltage regulation will be no better than the primary supply.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: October 25, 2016, 11:35:26 pm
content: Quote from: Hero999 on October 25, 2016, 11:06:40 pmYes I think you're right, my head was in a different place.

author: Cliff Matthews
author_rank: Unknown
timestamp: October 26, 2016, 01:55:58 am
content: Quote from: SharpEars on October 25, 2016, 07:55:41 pmUniversal 65-90w Laptop adapters go for a song too. Boosting a few volts for either rail is light work for the other boards too.Even with one rail using an isolated converter, there seems no recipe will convert 24v@4a to (+/-)24v@2a without loss or concession.Unless I've not read everything, we still don't know the end application, work duty and specs required for the end result.

author: Someone
author_rank: Super Contributor
timestamp: October 26, 2016, 02:45:12 am
content: Quote from: ebclr on October 26, 2016, 12:16:52 amYep, and some references for how its done:www.vishay.com/docs/76946/creatingnegativeoutputvoltage.pdfwww.ti.com/lit/slyt516

author: Kalvin
author_rank: Super Contributor
timestamp: October 26, 2016, 03:38:34 pm
content: One idea is to use a simple dual output dc-dc converter 24V input, +/- 24V output. The converter can be either isolated or non-isolated as the following idea shows:http://www.eleccircuit.com/wp-content/uploads/2007/10/lm2587.gifPlease note that the link above shows only an idea and needs to be redesigned to suit the OP's purposes.

author: SharpEars
author_rank: Unknown
timestamp: October 26, 2016, 06:27:28 pm
content: Quote from: Hero999 on October 25, 2016, 11:06:40 pmI am not sure what you mean by properly decoupled. The circuit schematic has been drawn and is in the thread. Are they "properly" decoupled?

author: Kalvin
author_rank: Super Contributor
timestamp: October 27, 2016, 02:14:43 pm
content: Inverting Buck-boost converter:Principle: "Using the TPS5430 as an Inverting Buck-Boost Converter"http://www.ti.com/lit/an/slva257a/slva257a.pdfHowever, the maximum voltage rating of the TPS5430 is only 36V which not high enough for this particular application as the voltage across the switcher device will be at 48V. Let's select a device with a maximum voltage rating of 50V or more.LMR16030 SIMPLE SWITCHER 60V, 3A Step-Down Converter with 40uA Iq from TI, the price less than $2 in 1000's:http://www.ti.com/product/lmr16030Disclaimer: I have not verified by any means whether the LMR16030 is suitable for the application. The device looks similar to the TPS5430 so it might be usable for this application as well.Edit: The LM5576 SIMPLE SWITCHER® 6V to 75V, 3A Wide Vin Step-Down Regulator may also be usable:http://www.ti.com/product/lm5576See thehttp://www.ti.com/lit/ds/symlink/lm5576-q1.pdfpage 20 for the inverting configuration.

author: StillTrying
author_rank: Super Contributor
timestamp: October 27, 2016, 03:30:12 pm
content: Its steady state and slow changing conditions below 2Ashouldbe fine.The problem is that if there's a sudden surge of current or short on one side, then for a small time the other side sees up to 48V, as the 0V line is pulled. Hefty 27V zenners and decouplingshouldmake it safer.