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| Design of SMPS - Not sure what these spikes are being caused by. |
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| T3sl4co1l:
--- Quote from: Zog on May 15, 2018, 02:07:50 am ---I would imagine that it also separates the HV and LV side ground planes which is good right ? --- End quote --- If you're going to use slots, better to position the LV section opposite the HV area, not hugging around it. That is, put the +12V in the middle. --- Quote ---Now.. off to search for a torroid with a tap, If such an animal exists in the wild. Somehow I doubt it. EDIT: ... 2 toroids in series... DOH :palm: Hi Ho, Hi Ho it's off to Digikey I go :) --- End quote --- Nooo, not in series, you need coupling! Winding coils isn't hard, otherwise look for tapped inductors. DK has 1:1 to 1:10 ratios stocked, IIRC. Look for the same "primary" (smaller winding) inductance as the original design. You probably won't find toroids, but there are lots of ferrite and molded SMT inductors, and inverter transformers (use the "secondary" as primary, e.g. a 120 (or 85-265) to 12V transformer would do very nicely here). Incidentally, "torroid", is that phonetic, or is there a regional or language influence? It's a very common typo but I'm unsure and curious why... --- Quote ---EDIT: not quite sure what you mean about how to arrange the circuit. --- Quote ---A doubler is incompatible with a peak current mode supply, at least for any reasonable output current. Instead, use a tapped inductor (transistor switches the tap, supply to one end, diode to the other, "long" end), so that the flyback voltage is higher than what the transistor sees. --- End quote --- --- End quote --- Consider what happens in the instant the transistor turns on: it draws a huge gulp of current through both doubler capacitors. The controller detects this and shuts off the pulse short. Result: huge EMI (hard switching, high peak currents), very low output power (low duty cycle). Once it's up to voltage, it can be okay, because the current will be small when the capacitor voltage is very close to the peak-to-peak voltage already. But it won't take much load to pull it down into the range where it falls over. Whereas a tapped inductor, just works the way the circuit normally does, and you can get nearly any voltage you want. ;) Tim |
| Zog:
Torroid is the shape .. so I just call them that. Most people I know do too. Toroidal is of course the correct term though, I guess . Back to the chase. I am replacing that LV design and putting in a better one. That one has no input protection UV or OV and I am going to need a VDD of 12VDC for the Tube HV drivers. I have already moved it out of the way so I can do that. No board made yet until I get it this part right. --- Code: ---Nooo, not in series, you need coupling! --- End code --- Still don't get it. Coupling ? You mean I have to use a transformer ... right ? The thing is, the way it is at the moment drawing that amount of current it gets as hot as hell. I mean really hot. the mosfet gets to about 85c and the current sense resistors get to 80c ! The "Torroidal" inductor gets to about 50c ... but that is not on the board so that's fine. Obviously efficiency is not ... err great ;D Man this thing can pump out some current though even if it's not efficient ! The 5w load resistor gets to 200c This was all tested at 25c after only 30 minutes. Output dropped from 140mA to 135mA after that time ... so I guess the inductor was having a hard time and was derating. Will start looking for transformers. Though I am not really sure how I am going to wire it up. Thanks again for your interest Tim, Phill |
| T3sl4co1l:
Yes, a transformer. Well, sort of. A transformer that stores energy (in other words, a tapped, coupled inductor). Not an ideal transformer (which does not store energy and has infinite inductance)! :) Part of the problem is huge input currents and awkward duty cycles, because the circuit attempts to make a large voltage ratio between input and output. The tapped inductor fixes this, making it a very normal (ideally 1:1.5 to 1:3 ratio) converter, as far as everything else is concerned. Tim |
| Zog:
Got it ! :-+ Looking on DK now |
| Zog:
Still looking. The 1:1 ratio inductors will cause the same problems so they are out. --- Code: ---Once it's up to voltage, it can be okay, because the current will be small when the capacitor voltage is very close to the peak-to-peak voltage already. But it won't take much load to pull it down into the range where it falls over. Whereas a tapped inductor, just works the way the circuit normally does, and you can get nearly any voltage you want. ;) --- End code --- For the same reason above I assume ? Just checking EDIT: ... stupid question of course it will. Just can't find one that is suitable. Would someone please be good enough to rub my nose in one that would do the job that is reasonably small ? PCB mount SMT or through hole ? EDIT AGAIN:... I think this is an impossible task. For something 1. smallish 2. the right current rating 3. the right voltage rating 4. the right saturation rating 5. the right inductance. This came close but no cigar. https://www.digikey.com.au/products/en/transformers/switching-converter-smps-transformers/168?k=&pkeyword=&pv1393=21&FV=1f140000%2Cffe000a8%2C15c0001b%2C15c0002d%2C15c0004b%2C15d00006&quantity=0&ColumnSort=0&page=1&stock=1&nstock=1&pageSize=500 Perhaps a snubber on the switch node would be a possibility ? At least that would knock of the EMI right ? Even More EDITING: "For those playing along at home" ;D Direct link to the article. https://www.fairchildsemi.com/application-notes/AN/AN-4162.pdf |
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