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.
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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 this
https://imgur.com/a/lLrmKPx and thought of having LC low pass filter and perhaps voltage regulator after the rectifier.
First things first, I took two cores
https://www.tme.eu/sk/en/details/efd15_8_5/ferrite-cores/feryster/ and bobbin
https://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?