2kw 60v to 380v DC-DC full bridge converter design - Page 1
Forum section: electronics / projects

author: ambrozy
author_rank: Contributor
timestamp: October 27, 2022, 11:46:07 pm
content: I have four EE42x20 F806 cores, F806 is some obscure, old (80s) polish material, all I know is that it suppose to be similar to 3C90 except much higher core losses so it may be not suitable for higher than 30khz, those cores have Ae=234mm2 and measured Al=4500nHI want to make from those cores DC-DC full bridge converter 60v to 380v capable of ~2kw (definitely 1kw continuous), I think of making four transformers of ratio 1:1.6 and connecting primaries in parallel and secondaries in series, I calculated 24 turns of primary per transformer for 107mT (keeping it low because of material) and 25khz, I want to use IPP086N10N3GXKSA1 mosfets and FES16JT-E3/45 diodes for rectification.Does all this make sense, or am I missing something? magnetics is not my thing so I would appreciate any help.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2022, 01:50:04 am
content: That is correct.The switching loop (both sides) is of paramount importance. Understand that the inductance between supply bypass capacitor, and high/low side transistors, is excited by peak switching current, at the instant of turn on/off. Keep this loop short, and keep its impedance* (with transistor Coss) low, and maybe add an RC or RCD (rate or peak clamp) snubber very close to each transistor (with, for an RC, R = Zo and C ~ 3 Coss, or other for RCD types), and you'll have it reliable.*Take an inductance and a capacitance, rub them together, you get a characteristic impedance Zo = sqrt(L/C), and frequency Fo = 1 / (2 pi sqrt(LC)). In the bridge, these give the ratio of peak voltage to peak current, and ringing frequency, of the switching transient. If Fo >> t_r and t_f, there will be little harmonic energy to excite this resonance and overshoot will be negligible (which is what the "minimize inductance" mantra really means, but it is not the most general solution).Use current mode control. Here, that means the output inductor, the current of which can be sensed by a Hall effect sensor for example (since control is likely primary side). Control that first, and then control the current setpoint to regulate output voltage (or whatever else you're doing, or a joint combination of several, like if MPPT, or CV/CC, etc.). This separates the output LC filter into the individual components, which are compensated separately, with simple RC circuits -- no stupid ESR phase margin antics to do and performance is faster/tighter than voltage mode. Plus you get maximum current limit 100% for free, no hackery required. It's great.At these power levels, I would also start to suggest desat protection. Wire a latch to the control enable, so it can be shut down in an instant, and stays off until reset manually (or by a long one-shot timer if you want self resetting). "Desat" means, transistor voltage drop being much higher than nominal, heading into the constant-current range (thus, leaving  saturation, and dissipating a ton of instantaneous power). This is usually done with an integrated gate driver IC/module, thus monitoring Vds locally. A less strict version might sense it "remotely" (i.e. a comparator watching the phase voltages from the inverter), or just monitor the current sense (e.g. a current shunt resistor at the bottom (source pin(s)) of the inverter). (Inverter current sense can also be used for peak current mode control, but that's probably undesirable here -- I would suggest average current mode, so you can use a relatively cheap powdered iron inductor for the output.)I've seen some engineers brag of their experience in terms of "buckets" of dead transistors. While you can gain experience burning transistors, it's a very low-information method. Better to understand why and how the transistor failed, right? I can probably fit all the transistors I've blown, in two hands -- and that mostly because two were large industrial modules. I owe that success in part to my integration of desat protection since early projects. Usually, the protection loop fires, stopping gate drive in mere microseconds, within enough time that -- even if one transistor has cooked off and failed fully shorted, its companion might switch into that short circuit, be immediately turned off, and survive -- only one transistor need be replaced here. Instead of exploded semiconductors, you have the nuisance of a circuit quietly going "tick" as it draws fault current for some microseconds and stops in its tracks. A nuisance which is repeatable so you can attach scope probes and see what's going on, where it's misfiring, how much current is flowing and where, etc. Much more informative.You may also consider wiring the secondaries to individual rectifiers; this probably doesn't matter here (the individual ratios are pretty mild, and even the full length of the combined secondary won't be much compared to Fsw -- that is, comparing electrical length of the secondary wire as a transmission line, to the switching frequency), but becomes significantly more important at high voltages (as the ratio goes up, the impedance goes way up!).Schottky rectifiers are also a consideration, but to be fair, PN junction types are still quite effective at this sort of thing -- modest voltages (100s) and lowish frequencies. If you wanted to do this at say 200kHz or more, SiC schottky would be pretty attractive. (Or a couple 200-300V Si schottky FWB, one per winding or two, stacked. There are even a few 600V sort-of-schottky in Si out there, impressively enough!)You probably want to use litz wire, like 7 strands or more, of whatever gauge gives the equivalent total required. Try to interleave primary and secondary: this reduces leakage inductance and proximity effect (eddy current losses in the windings). Use a primary gauge (and stranding) adequate to wind one full-width layer (less whatever width (margin) you need to leave for isolation purposes), and alternate P and S sections until you have the total parallel ampacity required for the transformer. Probably all P sections in parallel, and all S sections in parallel, would be fine here. So the S's will use finer wire and fit 1.6 times more turns per layer I guess. Or use the same wire strands but 1.6 times more of them for the primary -- same idea.Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 28, 2022, 02:59:06 pm
content: Hi ambrozy,I am building some 300 Watt DC Converters each with (1) transformer, somewhat similar to your (4).The one just tested is running at 32 kHz, the others might run at 70 kHz.The breadboard Heat run test is completed and I have made a handwritten page of the transformer details with some other info.That may be useful to compare with your design.https://app.box.com/s/w8ibpf3nwqgbvh6q8y7bnuvs7xp9axlkThe peak flux density is about 0.22 T at the highest duty, full load.

author: ambrozy
author_rank: Contributor
timestamp: October 28, 2022, 05:27:54 pm
content: Thank you T3sl4co1l very much!I'll start with the last thing:Quote from: T3sl4co1l on October 28, 2022, 01:50:04 amOnly type of litz wire I am able to get in small quantities is something like this:(it's in polish, but decimal numeral system and metric system are international as far as I know)As per seller of this wire it's wrapped in cotton, I don't know if this type is suitable for power transformers and at what temperatures it can work.Other possibility is just to use solid wire, I have 29mm width so I should be able to wind 24 turns (it will be more like 23 3/4 to be exact) of 1.2mm diameter wire on one layer, and use 3 layers of that in parallel for primary, how bad would that be? I understand that smaller wires will reduce skin effect but this particular litz wire has just about half cross sectional area of solid wire of the same diameter. I suspect that only finite element analysis would give me definitive answer, but maybe there is some simple way to estimate that?

author: ambrozy
author_rank: Contributor
timestamp: October 28, 2022, 05:57:42 pm
content: Quote from: mag_therm on October 28, 2022, 02:59:06 pmI don't think core material is similar, only piece of information about my F806 material is core losses at 15khz 200mT T=100C - ≤140µW/mm3 I'm not an expert by any means, but comparing that to any "modernish" material I can find it looks very bad.

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 28, 2022, 06:26:47 pm
content: Hi ambrozy,The grade prefix "F" was used by Neosid UK and Neosid Germany.I could not find your F806, only Neosid F8 which was for inductors, similiar to 3B1 and 3H1Your core loss at 140e-6W/mm^3 is similar to my core at 128e-6 W/mm^3 @ 100 kHz @ 0.1T . It is only about 3 Watt for the whole core set.Power transformer Mn Zn core loss has not changed much for 50 years. The reason we couldn't usually use > 50kHz frequencies back than was transistor, not core loss.For litz wire, it might be useful (and has been used) when in close proximity to current sheet secondaries eg in lateral layered windings (stacked pancake)Fow a concentric layer construction with round conductors, I doubt that it has useful increase in efficiency or reduction in leakage inductance.(But I have not modelled that in FEM.)For organic insulation cotton, paper, wood,, it is usually limited to temp class A, even when varnished, as the organic materials decay with time-at-temperature.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2022, 07:58:22 pm
content: Quote from: ambrozy on October 28, 2022, 05:27:54 pmWell those would be fine, if a bit expensive (or maybe not?, and, the downside to cotton as mentioned above). But why are those the only two possibilities, can you not think of another? Say, gathering up a few strands of finer wire, and twisting that? Like 7 x 0.4mm would be about right, I think? Or 12 x 0.35mm and use 7 x 0.35 for the secondary, that's handy. The only annoyance is having to lay out and twist enough for a winding at a time.BTW, count on losing maybe 10% of the width to uneven winding, so, 22 turns of 1.2mm would be comfortable, 23 would be a squeeze. Or 18 if leaving margin. Less still with twisted strands, but they're more effective, a worthwhile trade. (Skin depth at 30kHz is 0.37mm so I'd say 0.6mm max, and preferably half that because of proximity effect.) Maybe 12 x 0.35 wouldn't lay 24 turns per single layer and you'd use something finer still, and make up the difference with more layers?Oh; do you have bobbins? Occurs to me if these are pretty old parts, maybe not. Epoxy and FR4/G10 are fine materials to build one with, but mind the wall thickness. Or white glue and cardboard, hehe, again with the same class A limit as above.Tim

author: ambrozy
author_rank: Contributor
timestamp: October 28, 2022, 08:31:50 pm
content: Quote from: T3sl4co1l on October 28, 2022, 07:58:22 pm~8usd per 100g of this litz wire, there are available more suitable wires like 15x0.2mm without cotton wrap for example that cost half the price but minimal order is 1000g, I'm not so sure I will need that amount in the future.Maybe I'll try to make my own litz wire, but I'm a bit skeptical about it, I need ~2.5m for one primary layer and ~4m for the secondary, I don't really see what technique I should use to make it happenI don't have bobbins, I will make them from pressboard that supposedly can whistand 190C continuous heat