Pot core design for simple old style SMPS - a whole bunch of questions? - Page 1
Forum section: electronics / projects

author: SingedFingers
author_rank: Frequent Contributor
timestamp: December 23, 2016, 07:23:16 pm
content: I'm trying to resurrect a piece of homebrew kit from the dark ages of the 1970s that my father built. It's a primitive but rather cool looking DVM. This used a Mullard LA4 pot core as the primary transformer. This provided a +/-12v supply at 100mA a rail (4000 CMOS + opamps) and a ~180v supply at 50mA for nixies looking at the design. It also provided galvanic isolation for the instrument.Unfortunately to use the technical term, the transformer is bollocks'ed. The tape that was used appears to have eaten all the insulation off the wire used and shorted some of the windings. It also blew up the oscillator transistors. Fortunately I have a schematic with winding info on it.Now I'd like to replace this with a suitable modern equivalent to see if I can get it back up again.Where do I start? I have zero experience with magnetic design other than hacking some transformers in the dark ages to reduce secondary voltage and incinerating some naively built E core transformers recently.The original appears to oscillate at about 50KHz from some simulations but I have a million questions with no answers:1. What core type do I choose? (I'm looking at RM8 cores in particular as they appear to be roughly the same size as the original)2. How do I calculate volts/turn?3. How do I calculate total power and flux etc?4. What bits do I need as most RM8 cores appear to be fragmented into bobbins, clips, cores, insulators, all sorts.5. What does a gapped core do versus a non-gapped one and do I need one for this job?6. What wire size do I need for the primary and secondary taps?If there is a book or reference online for the whole lot that's good. No problem with mathematics as long as it's calculus or simpler

author: KhronX
author_rank: Frequent Contributor
timestamp: December 23, 2016, 07:30:22 pm
content: I don't suppose going linear is much of an option, is it? Linear, as in, good ol' 50Hz transformers."Even" two separate transformers should be quite small, at those minute power levels.

author: tautech
author_rank: Super Contributor
timestamp: December 23, 2016, 07:35:28 pm
content: Quote from: SingedFingers on December 23, 2016, 07:23:16 pmDoes the info have the wire sizes and turns # ?Found this chart for Mullard X reference to modern cores:http://electrojumble.org.uk/Mullard_PotCore_Equivs.pdf

author: SingedFingers
author_rank: Frequent Contributor
timestamp: December 23, 2016, 07:53:04 pm
content: It has number of turns but no Al value or pot info. The wire size I can measure but it looks like scraps which may have contributed to the inferno that finished it off. The oscillator looks like a Royer Converter so may rely on saturation to regulate frequency. Thanks for that link didn't see that! Will see where I get with that.I'm considering buying some RM8 core parts from Farnell and winging it at the moment. Possibly even ripping the front end off and putting a multivibrator in it so that I don't need to rely on saturation (as I don't totally understand it!)Can't go 50Hz. There's not much room in the case to squeeze anything bigger than a small pot core in it. Plus the supply voltage is 12v DC (everything he built was 12v DC supply standard) which makes finding suitable ratios off the shelf difficult.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: December 23, 2016, 08:13:40 pm
content: Ok so thanks to that I now know it's the same core as an LA1412. Latest data book I found on archive.org quotes a 35mm pot core with mu-zero (effective permeability) of 63. It's a vinkor core.Now to work out if a suitable modern replacement exists!

author: tautech
author_rank: Super Contributor
timestamp: December 23, 2016, 08:30:13 pm
content: Quote from: SingedFingers on December 23, 2016, 08:13:40 pmTry Ferroxcubehttp://www.ferroxcube.com/FerroxcubeCorporateReception/index.jsp

author: SingedFingers
author_rank: Frequent Contributor
timestamp: December 23, 2016, 08:38:02 pm
content: Yeah looking there. EPCOS are much cheaper thoughDoes this make any sense or am I talking bollocks here? Like I said before I know nothing of magnetic design.I've calculated the power transfer to be around 8W with 20% headroom. That means 700mA flowing in primary at 12v assuming 100% efficiency. Reactance therefore should be about 70 ohms at 50KHz. Working back from an N87 RM12 EPCOS core with an Al of 5300 I'll need 4 turns on the primary. Two secondaries of 4 turns and one of much more for the HT supply (haven't worked this out yet). That's 0.5mm wire.Sounds completely wrong. Old transformer has 100 turns on primary.Hmm. Idiot hat on.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 23, 2016, 08:49:31 pm
content: If only the windings are gone - you could likely reuse the old magnetic core. So it is more like an exercise in winding and to get the insulation right.The requirements for the "transformer" depend on the type of converter. Some (e.g. flyback) need specific AL values to get a good balance of inductance and saturation. Usually this means having an air gap. Others, like forward converters only need a minimum AL and have no problem with moderately higher values.It would make quite some sense to use a royer converter, as this is low noise. This is more like a resonant forward converter with the inductance setting the frequency.Reactance and input power consumption don't work together this way for a forward type converter - it can in a very limited way work for a flyback. Reactance would be more like setting the magnetizing current in a transformer - this is supposed to be much smaller than the normal current under load.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: December 23, 2016, 09:40:10 pm
content: Gotcha that makes sense now. Slowly learning hereThanks for your time.

author: chris_leyson
author_rank: Super Contributor
timestamp: December 24, 2016, 12:20:59 am
content: Hi SingedFingers, if it's a Royer converter it will rely on transformer saturation for the oscillator timing and because the Royer converter is a square wave oscillator then you can use the volt time product to calculate the number of primary turns for a given primary voltage, effective core area, saturation flux density and maximum ON time. There is a well known formula based on Faradays law for caluculating the minimum number of primary turns, N = Vt/BAe whereV = primary volts (V)t = ON time (us)B = Peak working flux density (T)N = number of turnsAe = effective core area (mm2)So from the information provided, the core is a 35mm diameter Ferroxcube LA1412 so best guess for effective core area, Ae, is somewhere around 190mm2but you can measure the center leg dimensions to get a better estimate. Working frequency is 50kHz so for a half cycle the maximum on time is 10us. Working flux density would be BSATfor a Royer converter. Couldn't find any useful data on Mullard Vinkor ferrite but a Google search brought up MMG-Neosid as alternatives, available in P11 or F58 material, P11 has a nice square loop B/H characteristic best for power whereas F58 is more linear and best suited to filter applications. Looks like Bsat is around 300mT. Rearrange N = Vt/BAe to get V/N = BAe/t or volts/turn in terms of working flux density, core area and maximum ON time. Putting in the known or estimated quantities, B = 300mT, Ae = 190mm2and t = 10 us gives 5.7 V/t. That doesn't add up I would expect the 100 turn winding would be for the secondary 180V nixie supply. 2V/turn would make more sense. If you are going to rebuild it count the transformer windings and work out the topology.Edit: The above calculation is not correct. The peak flux swing should be from -Bsat to +Bsat so working flux density is 600mT and the volt/turn is 11.4 V/t.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: December 24, 2016, 10:15:08 am
content: Thanks for the calculations. I'll run that through on paper today when I have a few minutes. Starting to understand this all now slowlyI'm going with another approach for this by the looks which is more empirical. The oscillator is not a Royer looking at it now; it's a multivibrator driving it. The schematic was drawn by an alcoholicSo the primary inductance needs to be the same at the oscillating frequency from what I can see so the power transistors see the same load. I'll pick a gapped RM12 core by sticking my finger in the air and landing it on the screen on RS which is approximately the same size, wind the primaries on it and see if it oscillates properly and doesn't go up in smoke. If it does work, I'll wind the secondaries using the appropriate turns ratio, then check the unloaded voltage. If that's ok I will attach suitable resistive loads to the secondaries to simulate connection to the device and check the thing doesn't melt over the space of an hour or so.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 24, 2016, 11:58:48 am
content: The required core is quite different for converters using push pull drive (usually operating in forward mode) and those using only one field polarity (e.g. flyback converters and similar). The second group usually needs a gaped core to get magnetic energy storage or at least have low remanence.The first group is less critical, as this is more like a high frequency transformer. In this case inductance is less critical except for resonant designs.So it would help to show the circuit.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: December 24, 2016, 06:43:07 pm
content: Stuck on my phone at the moment so lacking in drafting ability but I will describe it. It's a standard transistor multivibrator using two 2n3055's. The load resistors are replaced with two halves of the centre tapped transformer primary. Vcc is applied directly to the centre tap.It is just a high frequency push-pull transformer. Dumb as chips.As the 2n3055's are smoked (C-E short) I will be replacing with BD437's as I've got some few floating around.

author: chris_leyson
author_rank: Super Contributor
timestamp: December 28, 2016, 06:04:33 pm
content: Forgot to mention staircase saturation in balanced transformers where balanced infers working in two quadrants and this applies to either push-pull or half bridge topologies. If the transformer drive is not balanced then there will be a small DC bias added to the flux swing on each cycle and if the imbalance is not corrected then the transformer will eventually saturate. One way to avoid staircase saturation is to sense the transistor currents and employ cycle by cycle current limiting.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: December 28, 2016, 08:14:45 pm
content: Thanks for that. Was completely unaware of that behaviour.Expecting delivery of RM12 cores and some spare power transistors (I'm going to toast at least two doing this) tomorrow so will see how far I get with this one. Fingers crossed!

author: SingedFingers
author_rank: Frequent Contributor
timestamp: February 04, 2017, 11:11:17 am
content: Digging this thread up from the grave again, I just spent two hours on this again and managed to get it oscillating and the 180v tap wound (kill me - took forever). Loaded voltage is about right (15v low but thats based on scribblings. Transistors are running cool. Can also light neons off the 180v tap with a series resistor. Things are looking up!Used an Epcos RM12 core in the end. No science. Over-wound the primaries and took turns off until the measured inductance on my LCR meter was good.Bad news though, I just noticed the pip is missing off one of the nixies so got to source a replacement on ebay for one of them. And damn me they're not socketed so this is going to be annoying.