2.5kW DC to DC Boost Converter Project - Page 1
Forum section: electronics / beginners

author: Transmitter Man
author_rank: Contributor
timestamp: October 01, 2017, 05:31:19 am
content: My first post so please be gentle.I'd like a little input from the brain trust on a project I'm working on.12V to 48V at 50A (yes, it's a big one) DC to DC converter.This supply will be power a high frequency power amplifier used for single sideband communications.If any of you are amateur (ham) operators then you'll know the deal. Current draw will be constantly swinging between virtually zero load to max.I am having colleague of mine supply this custom isolated converter and due to the nature of the load, I require a very clean and stable output.Ripple AFAIK will not help obtain decent IMD and any noise will probably to a degree get amplified. Of course I will have switched LPF on the amplifiers output but I don't wish to create other problems by having a 'dirty' power supply.My question is, what filtering and regulator topology do you suggest as a minimum to give a clean & stable output on the above.I will have a very stiff DC input supply so there are no worries at all in that department.Provisional board design enclosed for your viewing pleasure.Appreciate your thoughts and input.David

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 01, 2017, 08:10:49 am
content: Why do you want to start from 12V?

author: Transmitter Man
author_rank: Contributor
timestamp: October 01, 2017, 08:24:40 am
content: Hi Mike,Well in answer to your question I just want to see if a one box solution can be accomplished to power a pair of high power LDMOS RF devices without involving any other external equipment in a motor vehicle such as a 28V alternator (easier to boost to 48V) or a 48V alternator. The fact is I drive a diesel so no ignition to interfere with and the fact that with SSB operation while the peak current draw is high the average current requirement is I believe going to be considerably less and the 'shock' can easily be handled by a higher than stock quality automotive charging and battery storage system.Yes, I know it's a crazy concept.I happen to like crazyDavid

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 01, 2017, 08:35:50 am
content: What run-time do you need ?Four lead-acid batteries, charged in parallel and discharged in series may be a viable solution - very simple, very electrically quiet. If it's a push-to-talk type of thing you could have them switch between charge and discharge on the mic key - you may want to use MOSFETS rather than relays to avoid a loud clunk.Or RC Lipos - more complicated charge scheme but potentially faster charge

author: Transmitter Man
author_rank: Contributor
timestamp: October 01, 2017, 08:47:37 am
content: It would be a press-to-talk operation with less talk to and more listenExcuse my ignorance but what is RC Lipos. Google doesn't know nor do I?I'm afraid I'm not really interested in carrying four batteries and as I stated above I really want a one box solution, with built-in DC converter with the end result power amplifier having around 15db gain.100W RF in and 1.5kW RF out, simples.David

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 01, 2017, 08:55:31 am
content: Quote from: Transmitter Man on October 01, 2017, 08:47:37 amlithium-polymer batteries for radio-control models etc.QuoteYour >200A input cables are going to be pretty bulky though, and it will be hard to get good efficiency at that current.What is your incoming supply going to drop to when you are pulling 200A from it?I would guess you could probably build something with lipos in a similar volume to a 2.5kw DC-DC converter

author: daqq
author_rank: Super Contributor
timestamp: October 01, 2017, 09:24:57 am
content: This might be interesting:http://cds.linear.com/docs/en/design-note/dn1043.pdf

author: Transmitter Man
author_rank: Contributor
timestamp: October 01, 2017, 09:26:35 am
content: Hi Mike,DC power cable run will be circa 10ft long from battery to input of converter.I will be using 4/0 power cable rated at 400+A.DC voltage drop using this calculator:http://www.marinewireandcable.com/p/voltage-drop-calculator.htmlis .05V !The above is based on 12V at the battery end however as you probably know batteries with charge going in typically measure between 13.8-14.2.Your 200A draw calc is of course correct but this is instantaneous peak current draw and way higher than average draw.If you have ever used a HF, VHF or UHF transceiver in the mode of single sideband (SSB) then you will know that when you're not speaking their is little if any current draw on the power supply, the peak current draw in this particular case may be 200A for speach peaks (I've yet to crunch the numbers) but the average current is far less. It really is, honest.My stock vehicle and it's not an executive saloon comes standard with a 140A alternator however the only mod I introduce, and I have used several in the past is to connect a superior external voltage regulator system:http://www.adverc.co.uk/which offers greatly improved charging of one or more battery banks. Your headlights run far brighter than normal!Please in this particular case forget peak current draw and concentrate on average current draw.A good quality commercial battery can handle instantaneous peaks pretty well even without introducing a snake oil stiffening capacitor as sold to many high end automobile stereo op'sBTW, I love that baggage handling X-ray machine of yoursDavid

author: Transmitter Man
author_rank: Contributor
timestamp: October 01, 2017, 09:32:36 am
content: Hi daqq,That paper talks above a voltage divider. I'm not sure it pertains to boosting voltage.Maybe I read it wrong.Thanks for your input.David

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 01, 2017, 09:34:16 am
content: Quote from: Transmitter Man on October 01, 2017, 09:26:35 amOne isssue is that a DC-DC converter must be sized for the peak draw, but capacity of batteries(used within their peak current spec) may not have to be all that large.

author: Transmitter Man
author_rank: Contributor
timestamp: October 01, 2017, 09:45:53 am
content: Yes, agreed Mike.I will admit I had not factored that very important point in, so I'll see if the first converter falls down when used in anger.I have a feeling it may not like the load but we shall see.David

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 01, 2017, 10:14:39 am
content: Quote from: Transmitter Man on October 01, 2017, 09:45:53 amOf course you'll also need to take precautions to stop any significant amount of RF getting back to the converter, as that could cause Very Bad Things to happen. That, along with keeping it stable with the rapidly chaging load is going to make the converter design rather challenging.

author: ocset
author_rank: Super Contributor
timestamp: October 01, 2017, 10:43:47 am
content: Parallel the boosts as in the attached

author: Transmitter Man
author_rank: Contributor
timestamp: October 01, 2017, 11:16:54 am
content: A filter on the output of the converter or and input of the amplifier should stop any RF getting back into the power supply.I have requested as good regulation as he can design in so we shall see how that holds up.I can feed a low power load in and monitor on both scope & spectrum analyzers both of which I have available.Treez,There's malware in that file!David

author: Transmitter Man
author_rank: Contributor
timestamp: October 01, 2017, 11:56:59 am
content: To go back to my original question.Can anyone suggest a better than average output regulator and hash filter circuitry or topology as I'd like to have this first build as good as I can get it minimising changes next time round.David

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 01, 2017, 12:52:36 pm
content: Did you provide a suitable noise spec and measurement condition for your colleague to test the power supply design at?If the PSU doesn't meet its specs, it's a bad PSU. End of story.Tim

author: dmills
author_rank: Super Contributor
timestamp: October 01, 2017, 01:57:38 pm
content: A couple of pointers:While SSB has high peak to average ratio, most class AB power amps have an unfortunate behaviour in that the drain current is proportional to the square root of the power backoff ratio, so at 10% power output you are still drawing 33% of whatever PEP current is, which is pretty horrible.Given the silly large 12V currents in play I would think long and hard bout a polyphase design it will raise the ripple frequency making it far easier to filter, as well as reduce the strain on the caps, Linear Tech have the sand you need for this..For the same reason, something transformer based rather then a classical buck might be a better choice, maybe something in the ZVS PSFB style.You really want the DC/DC close to the batteries, nothing good comes from long leads on the input side where filtering is even more of a pain.As far as filtering goes, measure first then design based on what you find, higher switching frequencies make filtering easier (again arguing for multiphase converters), but you are probably looking at a few stages of LC filters (You do not want too much Q here or the amp board will probably act out).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 01, 2017, 02:23:56 pm
content: What is the current envelope like ? if the peak currents are the peaks of the AM input, then a big cap on the output may significantly reduce the peak current requirement for the DC/DC converter

author: ocset
author_rank: Super Contributor
timestamp: October 01, 2017, 02:45:13 pm
content: PSU...do it with parallel smps's...I have sent you 5 posts which got kicked out due to being >1MB..let me try with this gem of knowledge attached.

author: Transmitter Man
author_rank: Contributor
timestamp: October 01, 2017, 05:15:24 pm
content: Treez,Thanks for posting again but I opened up both files and there was nothing there.Is anyone else seeing this information?I do not normally have issues reading pdf's.Hi Tim,Thanks for your input.In answer to your question I did not give him spec for the output other than from an isolated design I required as clean and regulated output as he manage.So, not very scientific I'm afraid.I need an attainable noise figure from you guys?I am also aware that a supply rail with poor regulation will have a negative affect on intermod distortion from my amplifier.I intend using a pair of underun 1500W LDMOS to obtain better than average IMD at 1500W peak output so do not wish to defeat this objective with a poorly regulated supply.I have asked what load/s and other test equipment he has available to test and monitor the output.Hi Dan,I was hoping that you might just pop by :-) as I know you know your RF.I will run your comments past my converter man although I may just have the existing board made & populated and see where is falls down myself.I hear you regarding the power cables. My last mobile amplifier was strictly low voltage bipolars but at around 800W my DC supply rail dropped only 0.2V down to 14V on key down CW however I have never experimented with switchers at this power so it will be interesting to see any results.David

author: fcb
author_rank: Super Contributor
timestamp: October 01, 2017, 05:33:30 pm
content: I would start by making the design modular - perhaps divide it down into 4 or 8 identical modules. They can share an input and output buss.Use a boost controller IC (something like the LM5122) that supports synchronous rectification, and lock all the units to the same clock (and if necessary phase shift each of the clocks (master oscillator and some div-by-8 logic).The current sharing might be interesting to get right, but in practice I've done something a bit like this and the resistance of the output wiring (or your first stage of output filtering) along with carefully trimming of each module worked in my application (I ended up with four regulators, all set with slightly different output voltages).

author: dmills
author_rank: Super Contributor
timestamp: October 01, 2017, 05:44:02 pm
content: Current mode controllers are the way to go if you are doing boost converters for this, usually very easy to parallel, and do run them phase shifted, much easier to filter.The nasty trap with LDMOS is AM to PM conversion due to Cds and the miller capacitance Cdg varying with applied voltage, this usually shows up not as classical intermod but as cross modulation causing low level copies of the transmitted spectrum spaced at multiples of the switcher frequency to appear in the output, do check for this.I take it you are going for the MRF1K50 in some form (I like the plastic package ones, 30% lower thermal resistance is not to be sneezed at).If you have an RC network from drain to gate to provide feedback you will want to watch this as it can be a route for supply noise to get coupled to the gates, which never ends well, make the bias supply very stiff at AC to minimise this (Big cap to ground from the centre tap of the gate transformer), then make sure you can remove the bias quickly.Regards, Dan.

author: ocset
author_rank: Super Contributor
timestamp: October 01, 2017, 06:03:07 pm
content: QuoteI dont understand, i just opened them from the post....its there.i agree about interleaving the smps's...as the attached ltspice shows...much less ripple

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 02, 2017, 03:54:12 am
content: Hi,Before you go to far down the road with this project, I would check if you can even get 2.5kW out of a 12V battery. You have to consider the ESR of the battery. The ESR will vary with health of the battery, state of charge and the temperature.LTspice modelHere is a model of the battery with an estimated 50mof series resistance. The model also includes a voltage controlled resistor as a load.LTspice ResultsThe upper graph shows efficiency. It is the power in the load divided by the total power dissipated converted to a percentage.The lower graph shows the input current, red trace and the power dissipated in the load, blue trace.The maximum power occurs when the load is equal to the ESR. At this point the same power is dissipated in the ESR of the source, 50% efficiency.Regards,Jay_Diddy_B

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2017, 04:01:26 am
content: Quote from: Transmitter Man on October 01, 2017, 05:15:24 pmBluh...That's a dangerous spec! You might just end up getting little more than an automotive DC-DC converter. By that, I mean the kind of thing commonly used in car amps: 12V is chopped up, transformed and rectified. Nothing else: no regulation, no filtering (just a wad of capacitors), no limiting, no protection. It's a wonder they manage to even start up, but that's somewhat helped by their poor design (slow switching speed), as is EMI, I suppose.The regulation of such a design is poor. The MOSFET Rds(on), DCR and leakage inductance of the transformer, and Vf of the rectifiers, all add up in series, determining the output regulation (i.e., change in output voltage over change in load current). The line regulation (change in output voltage over change in input voltage) is always 1. In other words: it's nothing more than a DC transformer.That's perfectly possible as all he could "manage"!It takes effort to do better: you must add a filter choke, current sense and feedback loop to provide regulation and limiting or protection.Best case: the PSU is designed as modules (as suggested above), where each module has a control voltage input, power input (nominal 12V), and a power current output (Io = V(control) * gain). Note that gain has units of conductance, so this is a transconductance stage. Any number of transconductance amplifiers can be connected in parallel. Currents in parallel add, no sharing to worry about. Finally, an error amplifier sets V(control) based on output voltage feedback, thus regulating output voltage.Because V(control) is always bounded (the error amplifier can't set any voltage outside its supply voltage range), Io(total) is always bounded. You getimplicit current limitingthis way -- it's really quite fantastic, and it doesn't take much more effort than the alternative, which is significantly worse.The main alternative is a voltage mode control. This also has to be wired in parallel, naively, because there's no way to sense current sharing. It's a very poor design, as the current sharing between sections is at the mercy of propagation delays and resistances in each section. If one delay is a little out from the rest, that transformer hogs current and melts transistors; cascade failure ensues. No matter the number of stages, voltage mode control is itself at the mercy of transients, as all it takes is a sudden change in source or load condition to cause an excessive current draw, and psst, bang, there goes all the transistors again.So your options are: bargain basement, unregulated and unprotected; voltage mode, regulated and unprotected; and current mode, regulated and protected.QuoteWell... you tell us, it's your amplifier!How much supply voltage variation is tolerable?How much PSRR does it have? At what frequencies?How sensitive is your receiver? Do you expect to be able to listen to conversations while this supply is running? In which bands?If you don't even have any basic guesses, then test and measure! Vary the supply voltage and see where IMD degrades; couple RFI into the supply and see where it goes; test your receiver's dynamic range and discrimination; etc.We are privileged to work in a field where every possible statement can be measured, tested and calculated. Don't let that opportunity pass you by!Cheers,Tim

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 02, 2017, 04:24:02 am
content: Hi,You can estimate the bandwidth of a boost power supply by calculating the frequency of the RHP zero:Freq RHP Zero = Vin x Vin--------------------------2 x Pi x L x Vout x IoutAssume that1) A four phase approach is used2) The switching frequency is 100kHz3) Ripple Current = 40%L= 2uHVout = 48VIout = 12.5A (1/4 the total)Vin = 9V (12V source with ESR loaded down 33% of the power in the ESR)Freq RHP zero = 10KHzIt is traditional to have a loop bandwidth = Freq RHP zero / 10 = 1kHzThe control loop will have a bandwidth of about 1 kHz. This will limit the transient response of the power supply.Regards,Jay_Diddy_B

author: Transmitter Man
author_rank: Contributor
timestamp: October 02, 2017, 09:10:16 am
content: Tim,The output of this smps should answer some or all of your technical questions:http://power.murata.com/data/acdcsupplies/cps_d1u-2000-48-hx.pdfThe FCC Certified & CE marked broadcast transmitters that are made by my last company used one or more of these until they recently changed to Delta's and they powered similar RF PA's albeit for the FM broadcast band.I am of course looking to produce higher output from a very stiff automotive battery source.David

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2017, 09:21:07 am
content: Oh good, FCC Part 15, that's an excellent starting point. And not hard to verify!Tim

author: Transmitter Man
author_rank: Contributor
timestamp: October 02, 2017, 10:02:24 am
content: Hi Tim,Not Part 15 but Part 73 verifiedPart 15 are pretty much toys Part 73 is for 'professional' broadcast transmitters however as my amplifier will only operate in the licensed amateur radio bands I think the technical requirements regarding emissions are less stringent. I am no expert in this area. I know spurious & harmonic measurements to meet FCC standards however at this time I have no intention of going into production nor am I based in the US I am just content on proving the concept of this mobile HF amplifier project can work.David

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2017, 11:41:22 am
content: Quote from: Transmitter Man on October 02, 2017, 10:02:24 amWell, that's the thing, isn't it?Can? Absolutely. It's been done before, and will be done again.Will? Who knows. Depends on how good the power supply is. Hopefully you get a satisfactory design, or if it's noisy, you find it's easily filtered/shielded, and all is well.That's the only way anyone can possibly answer your question: with numbers, ratings and measurements. That's why I ask! Until you get the physical article, there is no possible way anyone can address your original question with just the information presented here.Tim

author: Transmitter Man
author_rank: Contributor
timestamp: October 02, 2017, 02:22:06 pm
content: Tim,I will have the first converter built and arrange some testing and will report back, possibly with a video link.Thanks for your thoughts and comments.David

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2017, 02:23:31 pm
content: Cheers and good luck.Tim