Low power isolated DC/DC converter - Page 1
Forum section: electronics / projects

author: jaromir
author_rank: Unknown
timestamp: September 16, 2019, 10:39:34 am
content: For those not interested in reading wall of broken english text, TLDR version:Please suggest isolated low power +-15V DC/DC converter from 6-10V battery supply. LT8300 or LT3001 may work, while it's being delivered I made experiments with DIY DC/DC converter with mixed results.-----I'm thinking of a project where I'd need isolated analog portions, each one with isolated, floating +-15V supply. Power consumption is say 10mA per portion, that is 2x10mAx15V=300mW, let's say under 0,5W. To make things complicated, it needs to be powered of batteries, probably 6xAAA or 6xAA cell, that implies working range 6-10V.At first I thought like this can be done using off-the-shelf DC/DC converters, but somehow I can't find good candidate. First problem is input range. Majority of the "black bricks" is designed for stable input voltage, say 5V+-10%. I can add switching preregulator to bring 6-10V input down to 5V for DC/DC converters, but having two DC/DC converters in series looks like unwieldy solution. What is worse, most of the "black bricks" do have quite bit of current consumption. No load consumption is usually 100-200mW (often not specified) or thereabouts and full load (1W) efficiency 70% (getting worse at my 300mW conditions), together with switching 5V preregulator that would give horrible total efficiency.There are types with wider input range, but with somehow worse efficiency, not much of improvement over solution with preregulator.Fortunately, there are ICs designed for low power DC/DC converters, like LT8300 or LT3001. Though not exactly cheap, those seem to be designed for low power supplies, so I ordered a few pieces. While mail delivery does its job, I thought like "let's see if I can do it better", at least for education purposes. It turns out that... not yet and probably I need some consultation.I opted for symmetrical push-pull converter like thishttps://imgur.com/a/lLrmKPxand thought of having LC low pass filter and perhaps voltage regulator after the rectifier.First things first, I took two coreshttps://www.tme.eu/sk/en/details/efd15_8_5/ferrite-cores/feryster/and bobbinhttps://www.tme.eu/sk/en/details/efd15-k-8p-smd/coil-formers-and-accessories/feryster/and made two bifilar windings with 0,18mm diameter enameled wire. First (primary) is 2x40 turns, seconday is 2x80 turns. I know the transformation ratio is not enough for 6V->15V conversion, but the bobbin was already full and it's definitely good enough for tests. For primary, I connected end of one winding to start of another, creating longer winding with tap in center; the same for secondary. To check whether the polarity is OK, I grounded both primary and secondary taps and applied 80kHz signal to one end of primary, observing voltages on other winding ends. See attachment 1_check.png Yellow trace is excitation signal, blue is another end of primary - having opposite polarity, I guess that is correct. Purple and dark blue are ends of secondary, both being in opposite phase to each other and having correct transformation ratio.I breadboarded the push-pull converter with two BC337-40 transistors, fed with 8V from lab power supply. I inserted 4R7 resistors into transistors emitters to observe current waveforms and experimented with diodes from transistor collectors to ground. Bases of transistors were fed from dual channel arbitrary waveform generator, set to 80kHz square wave, 5V, adjusting duty cycle.The good news is that the converter basically works. When fed with 20% duty cycle, for 8V input I have roughly 15,4V after rectifier with 10uF smoothing cap and 4k7 load resistor. With 10V input, output is above 19V; with 6V, something around 11V. So far so good. That being said, there are aspect of this simple circuit that escape my understanding.Let's look at waveforms, see attachment 2_transistor1.png Yellow trace is excitation of one transistor (blue is another transistor), purple is voltage on emitter resistor and dark blue is its collector voltage. The messy waveforms indicate suboptimal probing, but perhaps it's obvious that voltage on emitter resistor continued flowing after base voltage fell down. Is that expected? I though emitter current would rise only until excitation impulse goes off.I varied the duty cycle, hoping to vary the output voltage, but the voltage remained more-less constant in range 5-30% of duty cycle, falling down quickly under 5%. I expected somehow stronger output voltage dependency on duty cycle. Above 30% of duty cycle I got increased power consumption, even with slight or no secondary load. I'm aware this kind of converter does need some dead-time, but this seems too high.Last point is efficiency. I measured 65% at most; that is not exactly convincing.How can I achieve better regulation range? Is transformer such as this suitable for this job? Can I achieve better efficiency?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2019, 01:54:30 pm
content: How isolated does it really need to be, if it's battery powered to begin with? (Are you using the battery power, directly or with a common-ground converter, for something else? Is it because the battery can be substituted with a possibly-common-ground supply?)First off I'd suggest an ordinary flyback supply, and this can be made with off-the-shelf coupled inductors (1:1 or 1:2 ratio). Use two inductors, primaries wired in parallel, secondaries independent (a diode and cap, each), then the DC outputs wired in series to get the +/-15V.Any flyback controller or regulator will do, just make sure it's current mode control, of some sort or another. Usually peak current with slope compensation. If they don't say this in the description, the block diagram will show a current sense signal and a comparator, and a ramp output from the oscillator section being added into the current sense signal.The tricky part is feedback. You could do primary side feedback, but regulation stinks (limited by leakage inductance, which is not usually terrific; a lot of coupled inductors are kind of intentionally bad, it helps for some applications but hurts this one). Secondary side feedback (usually with a TL431 + optocoupler) is very accurate, but kind of wasteful (TL431 needs >= 1mA for accurate regulation, and optos are usually driven at 5-20mA for better... I don't really know, speed I guess?).What's the lowest current consumption you will have? Should the supply have an enable signal, so it can be turned off when not needed? If it's always on, and the load can be light (like <1mA), the bias current drawn by the feedback circuit will be relatively substantial! So it would be nice to have a way around that.Maybe just using a better error amplifier (AP, ZR, TLV and other -431 prefixes?), and if possible, a more compact opto that still performs well at low currents.If you don't need low output ripple, you may also consider a hysteretic control; this could use very little idle consumption (a suitable comparator might draw 10s uA), and could also be wired so that the opto is only turned on when load current is demanded. (This makes a bit of a chicken-egg problem at startup (how can the secondary side command more current, if it doesn't yet have any supply voltage to work with?), but that can be worked around.)Regarding push-pull: a self-oscillating circuit incurs full idle current due to the controller, drivers, and core loss. It could of course be triggered with hysteresis to give a mostly-stable output while consuming less power under light load. The circuit tends to be more complicated, I think, for the same feature and power level, so I would suggest a more conventional flyback.BTW, what's wrong with off-the-shelf modules?BTW, one thing thatiswrong with them, or a lot of them in this range (<= 1W converters): they often do use a push-pull circuit, self oscillating with two transistors and some bias and signal coupling parts, and diodes for the output. Theseare not regulated, and typically have a tighter supply voltage range (say 8-16V, versus say 5-30V for a flyback). So as your batteries droop towards EOL, so will the +/-15, which I'm guessing is undesirable.Note that such a circuit (like the one linked) is acharge pump, transformer coupled -- not a switched inductive converter. This is why the output voltage follows the input, and this is what you observed. What's missing? An output filter inductor, between the rectifier and capacitor, to make a forward converter.Forward converters are perfectly fine as well, and controllers like UCC3808 come to mind, or some of LT's fanciest offerings (the slew-rate-controlled regulators come to mind), or integrated with some of AD's signal isolators (ADuMxxxx family). And probably a heck of a lot of other things, since it's been closer to a decade since I last looked over these product lines, hah.You can also make a one- or two-transistor forward converter, but you still have the primary downside of the topology which is needing a transformeranda filter choke. Flyback combines them in one, basically, at the expense of higher input and output ripple; but that's easily tolerable at this power level, so it's a common choice.Tim

author: jaromir
author_rank: Unknown
timestamp: September 16, 2019, 02:34:47 pm
content: Thank you. This is really detailed and valuable post, as is vast majority of your posts.Because of length and number of inputs in your post I snipped a few bits of it and tried to response to main points below:Quote from: T3sl4co1l on September 16, 2019, 01:54:30 pmI need to have two separate voltages isolated from each other. The battery referenced part will have exposed USB port and external power jack, so I concluded it may be good idea to have battery "earthed" and the two circuit portions "floating". Your deduction is correct, I should be more explicit in my post.Quote from: T3sl4co1l on September 16, 2019, 01:54:30 pmThanks for suggestion, I'll try flyback.Quote from: T3sl4co1l on September 16, 2019, 01:54:30 pmI found relatively low power optoisolators, like SFH618 being specified at 1mA, so it may be good candidate for secondary side regulation.Also, TLVH431 are 431-like devices with much lower specs.Quote from: T3sl4co1l on September 16, 2019, 01:54:30 pmEnable signal is not needed. Device will have clicky-clacky mechanical ON-OFF switch.Current consumption will change depending on what the analog floating portion will do, but will not get under something like 5mA and not above 10mA.Quote from: T3sl4co1l on September 16, 2019, 01:54:30 pmThanks. It will power analog circuitry, but LC filtering after DC/DC will be needed anyway, perhaps even linear regulator, so hysteretic control may do the job.Quote from: T3sl4co1l on September 16, 2019, 01:54:30 pmYes, this is what's wrong with them. In my first post I thought of workarounding this with switched buck pre-regulator to step down voltage to 5V, for example, so that off-the-shelf DC/DC modules do have stable input voltage. This is somehow unwieldy, though. What more, efficiency will suffer - quiescent power consumption of typical DC/DC 1W bricks is in the same order of magnitude as my circuit will consume, preregulator will knock it down a bit more.Quote from: T3sl4co1l on September 16, 2019, 01:54:30 pmAha, now I see it!When searching for forward converter I usually found flyback with reset winding+diode, like thishttps://www.maximintegrated.com/en/images/appnotes/3983/3983Fig01a.gifand I disregarded the secondary partWhat I'm really missed is the choke, this is very good point.Quote from: T3sl4co1l on September 16, 2019, 01:54:30 pmOK, so my main takeaway from this is:1, employ choke, as per picturehttps://www.maximintegrated.com/en/images/appnotes/3983/3983Fig01a.gif2, try flybackI'll try both and will let you know.

author: Zero999
author_rank: Super Contributor
timestamp: September 16, 2019, 02:36:40 pm
content: Those ready made isolated DC:DC converters are fine, as long as you get ones with a wide supply voltage range and don't cheap out. A filter might be needed to git rid of any high frequency ripple, but that about it. Here's a link to the datasheet for the range I've used before.https://docs-emea.rs-online.com/webdocs/16ed/0900766b816ed87a.pdf

author: jaromir
author_rank: Unknown
timestamp: September 16, 2019, 02:49:27 pm
content: Thanks for the tip, Traco is my long time favorite when it comes to DC/DC bricks.The TRN 1-0523 looks fine, but its no-load current consumption of 35mA means power consumption roughly 200mW, while I expect it to deliver something about 150-300mW of output power. Not great, not terrible. I have a few of TMA0515D in my junkbox, with no-load consumption of 30mA. This, with switched pre-regulator could do the job, too. TRN has better load variation regulation.I consider both TRN and TMA+regulator as acceptable, but first I'd like to explore other ways.

author: Zero999
author_rank: Super Contributor
timestamp: September 16, 2019, 03:22:47 pm
content: I consider 200mW of idle power dissipation to be quite bad. I suppose in the applications where I've used these DC:DC converters, quiescent power wasn't a consideration, so I didn't even bother checking the specification on the data sheet.Out of the data sheets you've linked to in your first post, the LT3001 seems like the it's the most suitable for your application, because it's optimised to lower supply voltages. The LT8300 will work, but the higher voltage rating of the internal MOSFET, means it also has a much higher on resistance, so is less suited for low voltage operation. I wonder how well it would work with a centre tapped transformer?There are also lots of unregulated transformer driver ICs available, but then you'll want something to regulate the secondary voltage. Linear regulators are probably out of the question, as you want low power dissipation, so a transformer with two secondary windings, each powering a buck regulator, could be used.https://www.analog.com/media/en/technical-documentation/data-sheets/3439fs.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: September 16, 2019, 03:51:19 pm
content: One point to watch with ready made DC/DC bricks is capacitive coupling and common mode signal injection. Some of the cheaper ones are really bad in this respect (probably due to using a really tiny transformer). The injected common mode signal is very hard to filter in a really isolated application.One can combine the push pull configuration with an additional series inductor (like in the maxim picture) on the secondary side. This way the PWM ratio can also be use for voltage regulation (a little like a buck converter, using the same switches). The old style TL0494 can work this way - chances are there are more modern lower power versions available.For low power loss chances are it gets better with a relatively low frequency and a slightly larger core, as this allows for more spacing / isolation.If the primary side uses USB anyway, it could be possible to power the converter from the USB side instead of batteries. As USB tends to be noisy anyway, another switched mode converter from an extern 6-12 V to 5 V if need may not make things much worse.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 16, 2019, 08:18:43 pm
content: You can also take a look at XP Power products. Not bad stuff.Just a note. The 6V min input voltage will be a bit on the low side to find a suitable converter with a +/-15V output.https://www.xppower.com/Products/DC-DC-Converters

author: jaromir
author_rank: Unknown
timestamp: September 16, 2019, 10:52:49 pm
content: I spent another evening with this.At first I made another transformer for flyback. The same wire, core and bobbin as for previous one, primary is 30 turns, secondary 60 turns. I built flyback with BC337-40 as before and results were not exactly convincing. The circuit worked, but efficiency was around 60%. I noticed the same thing as mentioned in my first post about push-pull converter - the emitter current continued to flow after base voltage decreased to zero - seehttps://snipboard.io/dZzK8Y.jpgAfter replacing BC337 with MOSFET IRLD120 this delay disappeared. I knew BC337 isn't stellar transistor, with not exactly great switching times, but I really underestimated it.With new transistor in place I was able to achieve higher efficiency. With 23,5V on output, feeding two parallel 4k7 resistors - that is 235mW of output power - I had consumption of 30,5mA from 10V input (305mW), meaning efficiency 77%, being a bit lower at 6V. I call it acceptable result, though I'm sure there is a room for improvement. Efficiency doesn't include feedback and control, though. The transformer has a few pF of capacity between primary and secondary.With better choice of transistor I returned back to push-pull circuit. I was able to get better efficiency than before (around 75%). After employing rectifier with accumulation choke I was able to achieve quite wide and more linear output control range, but since flyback has simpler transformer and overall circuit, I'm going the simpler way.Now I'll wait for LT8300 and LT3001 to arrive, but this simple flyback sounds like good backup plan. Thanks all for comments.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2019, 12:25:24 am
content: What was your circuit?Remember that base charge needs to be removed, just as gate charge needs to be removed from the MOSFET. Usually a voltage divider is used, to get a logic-level input while providing a discharge path for the base. A speed-up cap can be placed in parallel with the top divider resistor to yank some charge out immediately. A delay of 100ns should be easily achieved this way.Tim