USB Hub Design - Part 4/x - Buck Switching Supply Design
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- Опубликовано: 28 июн 2024
- USB Hub Design - Part 4/x - Buck Switching Supply Design
This is the fourth part of an extended series going over the design of a complex USB hub. Your feedback decides the route that this project goes! Please share thoughts and comments in the comment section or the discord channel!
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IMHO the inductor should not be before mosfets but after them. In 5V buck.
Woah, don't know how I missed that! I told you this has kicked my butt! Good catch, will update it.
This design has aged you. You can see it in the thumbnail. Haha. Anyhow, thanks for sharing this complex design, your viewers appreciate it.
Good to see a challenging project instead of the usual fare we get from RUclips
13:50 The 5 uF "total derated cap" is already the sum of the 3 10 uF caps derated to 1.7 uF each.
Hm. Glad you pointed that out, that does seem what they're doing. But, and I'll have to check the math, 5uF total on the input of the buck is going to be way too low I think
FWIW, modern tantalum capacitors from decent brands (Kemet, Murata, etc.) use an alternative solid electrolyte chemistry to the older type that had a tendency to turn into fireworks. This has been the case for the past ten years or so now.
All tantalum caps have very small fissure defects inside them, and on the older materials these fissures would tend to form oxygen at the surface discontinuities under DC bias. Furthermore, these oxidised surfaces have higher resistance/impedance, so the ripple current can start to cause hotspots. You can see where this is going: oxygen and heat = fire. The oxygen production is typically only stimulated by higher DC bias. It's an electrochemical process, and to push it further you need more voltage, so once the cap has seen 5V it won't degrade any further if it never sees a voltage higher than that. As such, if your board didn't catch fire in testing then it shouldn't catch fire in operation... in theory. Typically the "firework" scenario only occurs when something else fails and a voltage rail goes out of spec, causing the cap to see a higher voltage, a bunch of oxygen to form and the hotspots to worsen, and eventually you get thermal runaway and fire. However, in a small number of cases where the tantalum cap's internal fissures were unusually bad and you had a bunch of ripple current, you could set them off just with the increased voltage rail ripple you'd tend to see in a system after the electrolytic capacitors aged and their ESR worsened. Motherboard and graphics card VRMs were notorious for that type of failure.
The capacitor industry kinda got sick of this, partly because people started avoiding tantalum caps, and figured out a better solution. The newer solid electrolyte chemistry does the reverse under bias: it absorbs oxygen on the fissure surfaces. This is often referred to as a self-healing effect. During production the caps go through a DC bias ramp to "cure" the internal imperfections, which makes them *far* safer than they used to be for handling high ripple currents close to their rated voltage. You can still make them catch fire if you really hammer them with ripple current, but that's no different than an aluminium electrolytic or solidpoly.
I might be wrong but the 10uF input cap says Quantity 3, so maybe the derrated capacity might be 3 times the one you found on the Murata table (3x1.7) ? Great video!
Yeah, see my comment on another mention of that!
You can put an inductor in series with the 5V buck after the first output cap. This will increase the ripple seen by the controller and help filter the output.
A 1.5uH as your switching inductor should be fine.
Great video! Can't wait to see the PCB layout.
Totally, that's the way you do it. I don't see a need to get the ripple down that far on this board, so I won't be adding it.
Learning a lot about power supplies. Thx
If I am not wrong, you could use a different type of Buck controller that uses a current sensing resistor and does not have issues with output capacitors being "too good"
Hi, this project amazing and i learn to much things, espacialy dc bias. But i have a question that may you mantioned this before why there is no typeC out with PD.