Non-isolated AC/DC 3.3v 30mA with all standards in mind and limited space - HOW? - Page 1
Forum section: electronics / general

author: OleMi
author_rank: Contributor
timestamp: January 19, 2021, 01:50:27 pm
content: Subj is old as the world, however still many concerns. Browsed half of Internet and found no answer, neither off-the-shelf solution. Seems no manufacturers take such demands seriously.1) Only 3.3v/30mA to power a logic IC on the PRIMARY side is needed.2) Space is limited to ~ 9x17x17mm/ 0.35"x0.67".0.67" (idea to put it to a SIPx enclosure together with the logic IC, space for which is not considered in these dimensions - so the real space will be a bit more).3) Should satisfy IEC 62368-1 (i.e. varistor, fuse and bleeder resistors are all should be integrated). Probably external fuse can be considered only.4) EMC situation is bad, need EMC protection built-in, not external. Class B.5) Wide input range, say 90-250VAC.6) No ground exists from mains. Class II.All off-the-shelf solutions are not finished and moreover are tailored for high current and therefore bulky, or they require ground for EMC.Is that possible at all to fit to the given space?Does it really change dimensions of the major Safety/EMC components when required current is that small or there is always some minimum space that the power unit will take exists?Am I understand correctly that capacitors-based PSU will be better in terms of EMC in compare with a switching one, but space is questioned?Anyone ever met that challenge and have any good SCH to share? Obviously the main challenge is space here.

author: OleMi
author_rank: Contributor
timestamp: January 19, 2021, 02:39:17 pm
content: Efficiency and standby is secondary in this question.Full CE/FCC compliance required (the RED, FCC Part B), compliance for AU/CA as well.

author: fcb
author_rank: Super Contributor
timestamp: January 19, 2021, 04:00:51 pm
content: Power Intergrations, LinkZero rangehttps://ac-dc.power.com/products/linkzero-family/

author: OleMi
author_rank: Contributor
timestamp: January 19, 2021, 04:27:59 pm
content: Quote from: fcb on January 19, 2021, 04:00:51 pmHow does it solve the above task? It's just another power management IC to build a switching power supply around it. There are tons of them out there.

author: nctnico
author_rank: Super Contributor
timestamp: January 19, 2021, 05:01:50 pm
content: What kind of logic chip do you need to power? What needs 30mA?

author: OleMi
author_rank: Contributor
timestamp: January 19, 2021, 05:46:40 pm
content: Quote from: nctnico on January 19, 2021, 05:01:50 pmEnergy metering IC

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 19, 2021, 06:03:30 pm
content: Quote from: OleMi on January 19, 2021, 04:27:59 pmPower integrations do a range of chips aimed at non-isolated mains applications ( e.g. Linkswitch TN series) - without the need for an isolating transformer it's much less of a headache to build these from off-the-shelf components than using a module, hence there aren't many (any?) modules out there.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 19, 2021, 06:05:28 pm
content: Quote30mA is going to be too much for a capacitive dropper, but your size envelope should be doable with one of the Power integrations chips or similar.

author: ocset
author_rank: Super Contributor
timestamp: January 19, 2021, 06:36:36 pm
content: attached is a cap dropper for vac input = 162-265vac input.Its in the free download LTspice sim. (please ignore the solenoid)I didnt make it for down to 90vac as the caps would have got even bigger than they already are so it shows you that its not viablefor your size profile....but as you say its better for emc so maybe you get some more room allowance?But yes, power integrations i believe offer the smallest solution (that doesnt involve inventing something from scratch at a cost of a small fortune)

author: f4eru
author_rank: Super Contributor
timestamp: January 20, 2021, 11:36:52 am
content: Treez, your circuit doesn't work.Common ground between both rectifiers will just short half of the dropper caps through the common ground.

author: ocset
author_rank: Super Contributor
timestamp: January 20, 2021, 07:02:27 pm
content: QuoteI just ran it in LTspice...it works fine. What do you mean?Here is attached with param statements, so you can chop and change cap and res to your liking...and get down to your 90vac input, etc.QuoteThe inrush at "on-at-mains-peak" is large if those resistors are much lower in valueDissipation in the 22R's is only some 34mW at 162vac

author: Gyro
author_rank: Super Contributor
timestamp: January 20, 2021, 07:32:36 pm
content: Quote from: treez on January 20, 2021, 07:02:27 pmTo simplify it for you.Look at the ground symbol below the LT1121, look at the ground symbol by D6. Note how one bridge rectifier (D3-6) is fed directly, then note how the other bridge rectifier (D7-10) is fed via series RCs on both in both inputs.... Then go and work out what you are actually simulating and what it is telling you!P.S. I've attached the pdf that you posted in #9 for reference...

author: ocset
author_rank: Super Contributor
timestamp: January 21, 2021, 09:16:09 pm
content: I see what you mean, why should the cap dropper current return back through the other cap and resistor, its effectively shorted out by the D6.Thing is, the circuit is shown working fine, just as it should. The solenoid is being driven satisfactorily, and the 3v3, 30mA is being produced satisfactorily. There are no bad overcurrents or overvoltages. There wouldnt be any other way of doing this without adding more components.I did this years ago and seem to remember the situation you speak of, but it seemed to work anyhow, and they picked a cheaper resistive linear regulator in the end anyway, as they only wanted 5V, 5mA, so the capp dropper was binned.In fact the attached was the story of the options at the time (sorry to OP for digressing)......i'm going to have to come back to this because, yes, i can't yet see why the sim is working, when by rights, it appears that it shouldnt. And if its a bogus method, then what's the fix, that involves least extra added components (but no more as i apologise to OP for digress...but then, in truth, there is no product available to the OP other than the LNKswitchs already depicted above)

author: fcb
author_rank: Super Contributor
timestamp: January 22, 2021, 01:21:13 pm
content: There was an IC made by Harris many moons ago called the HV-2405E (https://www.rocelec.com/part/HARHV3-2405E-5).No transformer required, 5-24V / 50mA (non-isolated).Always wondered why this technique wasn't used more. Could this design be replicated with a handful of discretes?

author: f4eru
author_rank: Super Contributor
timestamp: January 22, 2021, 06:42:59 pm
content: Quote from: blueskull on January 22, 2021, 03:20:34 pmCoolI need this for a special application...Question : do you use zero detect and hysteresis to switch the transistor ?how do you power this circuitry itself ?

author: fcb
author_rank: Super Contributor
timestamp: January 22, 2021, 08:44:03 pm
content: Just playing around with simple non-isolated design, slightly inspired by the Harris part I mentioned earlier.R13 represents the load. The zener + Vbe drop set the output voltage. I can get variations of this design to work from 90-260Vac. Standby power could be reduced if the cascode transistors gain was higher (or darlingtons), but the current limiting provided by the worst transistor hFE is useful to keep down start-up surges.The output is a bit rough, but no overshoot and it would require a small linear regulator (MIC5504?) to provide a nice regulated supply.My guess is that this generate very low EMI as nothing switches hard. Transistors all kept to same type and probably need to be Vceo of at least 120V and a few hundred mA. Something like a DZT5551. Q5 could be a cheaper one if it's Vceo is respected.Voltages developed across Q1-Q4 aren't equal, so Q1 would get the hottest, probably use a couple more stages rather than go up a transistor. Probably make it work with SOT23's if there's a bit of copper about.

author: fcb
author_rank: Super Contributor
timestamp: January 23, 2021, 10:59:01 am
content: Yeah, lots of other ways to do it. Wanted to spread the heat - essentially was trying to be more efficient than a zener shunt.Got some other ideas using a 500V mosfet and s-r flip-flop.

author: fcb
author_rank: Super Contributor
timestamp: January 23, 2021, 05:43:36 pm
content: This design uses a single MOSFET of at least 450V. There are a ton of $0.25 TO220 and SMT MOSFET's that would do.There is a fairly sharp current spike once per cycle on the input, possibly enough to require a bit of supression. Might build this and see how it fairs. Efficiency isn't bad at low current draws, it's easy to modify to get 12V at 50mA.I had a zener on the gate originally, but no need as it's well behaved. Also R11 is set to work from 90VAC to 230VAC, if it's just for 230VAC then R11 can be 2M2. C1 slows the switching and reduces peak current, not overly sensistive, but if it gets too big it doesn't work. R1 helps slow down the turn-on spike.I haven't played with full-wave rectifier, but it would reduce ripple on the Vout. You can tune the efficiency at very low current draws with C1/R12.

author: nctnico
author_rank: Super Contributor
timestamp: January 23, 2021, 07:40:59 pm
content: Just a remark: some MOSFETs have integrated zeners on the gate to help protect it. Could save some parts.

author: fcb
author_rank: Super Contributor
timestamp: January 23, 2021, 07:47:20 pm
content: Quote from: nctnico on January 23, 2021, 07:40:59 pmThey do indeed. I had an 11v zener on the gate-source on this design, but it never went above 6v.