So in the continuing saga of cotton spark detection, I need to make an isolated 2-output DC-DC converter to power an RS-485 link and a small SBC--about 2W altogether. This isn't a terribly low-noise application, so I was thinking about using the Bourns SRF0703-471M "coupled inductor" as a flyback. The question is, what's the coefficient of couping? There's no way to find out from the datasheet. I'll get a few to try out, but in the mean time, does anybody know the approximate value of k for these beasts?
Thanks. The Bourns ones I was looking at are the cheesy "shielded" kind with the big gap at the top of the core + cup, so I was hoping it would be around 0.95-0.98.
It's sort of an interesting case--very cost sensitive, but has to be tough enough to withstand a monsoon thunderstorm in a cotton mill with very iffy grounding and long runs of thin wire. We're going to bus around +24V, panel ground, and a twisted pair for 200 kb/s RS485 data.
So I'm looking at some combination of MOVs, TVS zeners, and depletion MOSFETs on all four lines, plus fully isolated power and comms.
The first thing I tried was a half-bridge driving two 150 uH double-wound inductors (L1A + L2A in series, L1B + L1B in parallel for the output), followed by bridges, filters, and LDOs. That worked well with no ringing or other nonsense, and managed to get to 5V okay, but it was a bit marginal at the low voltage limit. Using MOSFETs for the bridge on the output helped some, but it was just getting too complicated.
I'll probably just put in one of those 27-cent A&O buck chips and then drive a couple of 1:1 transformers. I can use the unregulated input to power the high side gate, which helps some.
The bad news about regulated switchers is that they have negative input resistance, which may become a problem when the wiring runs get too long. (I can see somebody running the power on a spare CAT6 pair, 20 ohms per 1000 feet.)
Yup. It isn't, though--the inductance in series is 2 L1 + 2 M, and in parallel it's (L1+M)/2. They list the series inductance as 4x the parallel, so k is around unity. The specs are +-20%, so k could be as low as 0.6 and it would still pass.
We like to start with a polyfuse (radial lead, not surface mount) followed by a unipolar transzorb. Seems to protect against user blunders, short of 120 VAC on a 24 volt bus.
Lightning can be a nightmare in tropical climes and old buildings. Horror stories about gear in Florida. Protect those RS485 chips, too.
That recently confused our test folks. We have some bench supplies that come up slow, and some products with purchased switcher bricks, and the combo wouldn't start with the supplies set for apparently reasonable current limits. We try to include UVLO and/or soft-start when we design switchers.
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John Larkin Highland Technology, Inc
picosecond timing precision measurement
but that drops rapidly as the input voltage declines--it's 66 ohms at
12V input.
Thus when the wiring resistance gets into the tens of ohms, life starts to get interesting, and of course when it gets above 72 ohms, it can't supply 2W into any load whatsoever.
Thanks. Thought of that--I've done it before, but it really only works when the main supply is more heavily loaded than the isolated ones. I need two isolated outputs and zero non-isolated ones.
Also I can't be sure that one supply will always be drawing more current than the other--a double terminated RS485 link can draw over 80 mA, whereas it's much less with AC termination. The SBC can draw 160 mA max, but there's no minimum specified. So it looks like a buck followed by two 1:1 transformers, bridges, and LDOs. _Not_ the elegant solution I was hoping for, but there you go.
I've also put unipolar TVSes in series with the ground pin of a PolyZen. You keep the super-fast switching action, which is great. You do have to be careful not to exceed the standoff voltage rating of the polyfuse part, of course.
Not PolyZens--they're magic. The zener heats up the polyfuse super fast on any sort of large overload, so you don't have the problem of the diode unsoldering itself before the polyfuse switches. They aren't as fast for reverse voltage, but then you don't care as much since the diode isn't dissipating much.
I've measured quite a few types, not your p/n but similar ones. Like John says, they tend to be bifilar wound, and therefore with quite low leakage inductance. I've measured 0.3 to 0.5% of the magnetizing inductance.** So coupling = 0.96 to 0.98. One caution: They have more winding-to-winding capacitance than you'd normally expect. Easy to measure.
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