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| Problems with DIY SEPIC Converter |
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| Eldi4:
Well, first of all, probably i can't fully understand what you said, with my non-engineer brain and english is not my native langauge, so im sorry if my answer does'nt really relate to what you say You do the calculation just in a minute while im doin it in hours, and not even fully understand what do i calculate :-DD --- Quote from: T3sl4co1l on December 30, 2018, 03:28:20 pm ---Ok, so for a boost/flyback/SEPIC configuration, 1:1 winding ratio and ~1:1 voltage ratio, that'll be a peak switch current of about quadruple the output current, or 32A (assuming BCM)? '3843 current sense voltage is 1V max, shouldn't that use (1V) / (32A) = 0.031 ohms, not 0.05? Probably even smaller than that, considering minimum input voltage? It can be a little more in CCM though. Details, in any case; you're still at a fraction of the full output. DCM, BCM and CCM (discontinuous, boundary or continuous conduction modes): are determined by the frequency relative to the inductance, supply voltage and load. If Ipk > V*dt/L (where dt = 2/Fsw), it's in CCM, else DCM. (BCM is the threshold between the two, where switch turn-on current is just barely ~zero.) So, it looks like... --- End quote --- Well, probably this explain why the voltage drop to 22V when i accidentally draw 7A at the output, So..., mine is CCM right? --- Quote from: T3sl4co1l on December 30, 2018, 03:28:20 pm ---Ahhh yeah, that'll happen... ;D - Hmm, 20AWG wouldn't really be enough for 8A, especially including ripple; doubled up (since this is SEPIC), it should be fine though. - How was it wound? A ball of wire on either side of the core? Twisted pair? - The T90 core sounds way too small to begin with, and mix #26 makes almost a better resistor than inductor at this frequency. Try mix #8, or any of the fancy-named low-loss materials (Kool-Mu, Sendust, MPP, etc.; choose mu_r ~ 20 to 60 for best results), or gapped ferrite. - What kind of 4700uF capacitor? Just one? What about supply (input and output) filters? What kind of ceramic? - Just one of those transistors is WAY MORE than enough to do the job. Two in parallel is hurting you more than it is helping! You may be better off with several IRFZ46N's (or STP50N06 or whatever cheap, old fashioned types are similar) in parallel, since the paralleling reduces stray inductance. Stray inductance is critical at this peak switching current. In applications like this, I like to use a current transformer to save on power lost in the current sense resistor. That looks like this: Note the CT primary winding in series with the MOSFET drain. The secondary feeds a diode and burden resistor, which the 3842 senses. The 1:150 turns ratio means the burden resistor sees 1/150th of the switch current, so effectively the 2.2 ohms acts as 2.2/150 = 0.015 ohms, for a peak switch current of 68A*; meanwhile, it dissipates 150 times less power than a single, direct resistor would! *Hmm, sounds pretty high for a mere IRFZ46N. What was the designer thinking? ;D Note the inductor is spec'd as mix #26; I tried that initially, and it got way too hot. I replaced it with severe overkill (a T156-2 with 10AWG equivalent Litz cable), which runs stone cold. :P See side view here: https://www.seventransistorlabs.com/Images/Magamp_PSU2.jpg The snubber (SB540 and 1uF cap) probably aren't necessary, but this type of clamping -- placed as close as possible to the transistor(s) -- can extend the voltage rating a bit, allowing you to accommodate more stray inductance between the transistor, coupling capacitor, output diode and output filter capacitor. Keeping peak voltages in check is priority with a mere 60V transistor that you're running at 48V peak nominal -- that certainly leaves no room for automotive voltage variations, if this is an automotive application. Anyway, ~30uH nominal, at 100kHz and 24V (nominal) input, suggests dI = 4A, much less than the 30A peak allowable here, so it's going to be CCM at more than light loads. Which means... --- End quote --- I choose 20AWG because the Inductor RMS current is 11A, and winding two 20AWG wires simultaneously (1:1 turn) is already a pain for me, as i don't have many experience in winding inductors hmm.... I wound it just like normal winding on toroid Gapped ferrite?, like switching transformer core?, what about EI33? Yep, the coupling capacitor is just one 4700uF NELC brand, i know it must be low esr and high capacitance as possible, but this is only what i had on the components drawer. the input filter is just 1000uF, it's just salvaged smd ceramic capacitor from V_Core CPU Laptop filter. Hmm, but when i use single mosfet instead of two, the efficiency decreased by 6% (but same position, just unwired it), so what do you mean is single mosfet can handle the power dissipation alone? For now i'd like to stick with shunt resistor, but it is really a good suggestion tho, as the resistor participate on 5% deficiency. That's a lot of winding too tho.. Probably i'll try EI33 core instead, Previously my FET is 200V IRF640, but after the leg was broken i changed it to IRF1010E as it is the only one available in my components drawer --- Quote from: T3sl4co1l on December 30, 2018, 03:28:20 pm ---...Chaos! The peak current mode converter is a real, practical* implementation of the Logistic map, a beautiful bit of mathematics, which you don't really have to understand, just know that, when a control parameter (namely, inductor current) goes into a certain range... nutty things happen, and the result is a complex (not quite random; chaotic!) pattern that doesn't help us much, indeed it increases current ripple and makes the control loop somewhat unstable so we'd prefer to avoid it. (It also makes a screeching or hissing noise, which is annoying!) *Not that it helps us, just that, it's a good circuit... except for this behavior. ::) So, what to do? 1. Choose a higher ripple current fraction. 4A ripple out of 8A load implies 50% ripple. The instability goes away when inductor current is able to reach zero, i.e., in DCM, i.e., at >100% ripple. Downside: more inductor losses, higher peak switch current. 2. Add slope compensation. This is in the UC3842 datasheet or appnote -- check it out. :) Downside: this worsens the current limiting behavior, which may cause the peak switch current to be much higher under some conditions (low input voltage and high load current), which you need to consider, to keep safe operation. 3. Don't use peak current mode at these power levels. It's a good control but it's just not well suited to higher power levels -- there is an economy of scale working against you here. More common are push-pull forward converters with average-current-mode control. This can be implemented with the old standby TL494 (or '598 with integrated gate drivers, or because they're pretty weak, just add an external driver chip to either one). You'll need an extra error amp to regulate output voltage -- the TL494 can't be wired that way by itself, unfortunately. Depending on what capacitors you're using, I would guess the inductor, and maybe the switch and diode, in that order, are the major power losses in your circuit, and yes, efficiency that low is not unexpected given the above analysis. (Well, I didn't really analyze much here, just talking about things I've analyzed or demonstrated in the past. But yeah, I've been here, almost exactly.) --- End quote --- Oh no... 1. you mean by lowering the inductance? 2. gonna check that out 3. i'm just using my noobies instict when choosing control tho :D, so UC3843 is just really not suited for my application?, it seems more complexity = more efficiency? How did capacitor hugely impact to efficiency? --- Quote from: T3sl4co1l on December 30, 2018, 03:28:20 pm --- '3843 isn't very powerful, about 1A peak gate drive current. That's equivalent to about 10 ohms resistance. 5cm of wiring will have a stray inductance of around maybe 40nH, and the gate equivalent capacitance is around 10nF (this is Cg(eq) = Qg(tot) / Vgs(on) ), so the gate circuit impedance is around Zc ~ sqrt(L/C) = 2 ohms. To push this circuit into ringing, you need a driver below 2 ohms, and '3843 just isn't fast enough or strong enough to do it. More importantly, with ~10nF hanging off it, that's a 10 ohm * 10nF = 100ns time constant, so we expect a gate rise/fall time on the order of 200ns, and a drain switching speed on the order of 50-100ns (the drain switching is faster, because most of the drain current change happens over a narrow span of gate voltages -- which is to say, the transistor has gain, and acts to sharpen the gate waveform). At 100kHz, this isn't really terrible. It's a maximum switching loss of around 100ns * 32A * 48V * 100kHz = 15W. Still, it dominates over conduction loss, which is why I said the transistor is too big. Out of a total ~200W capacity, 60% efficiency is 80W lost, about 60W of which is now unaccounted for. Again, the inductor, and probably the diode, capacitors, I don't know what else -- will probably be the dominant losses here. --- End quote --- This is what i mean "You do the calculation just in a minute while im doin it in hours" :-DD 5% out of resistor 10% out of mosfet 25% out of nowhere hmm... --- Quote ---Unfortunately, designing and building SMPS is nearly futile without a scope. |O It can be done by an expert... but even then, there are so many small variables left uncontrolled that a 'scope is really necessary. Good luck, hopefully you can find something useful! Tim --- End quote --- That's really what is my true problem, im not an expert and no scope.... Thank you! |
| T3sl4co1l:
EI33 is probably very big for the purpose, but that does mean it will run cool. :) It will need to be gapped with about 0.5mm between the core faces, or 1mm in the center. Check if your cores sit flat together, or have a gap -- depends what the core was originally used/sold for. You may not need to add anything! Tim |
| Eldi4:
Changed it to EI33, it's not the core that's heating, but the wire on inductor, any idea?, only 1.5A output. |
| T3sl4co1l:
Got a picture of the winding? Tim |
| Eldi4:
Sorry, been sleeping after the new years Here you go, It's winded just like normal switching transformer winding, first layer Inductor 1, second layer Inductor 2. This is weird, How did winding causes wire to get hot?? |
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