And yes, we're going to need different coff resistance for different voltage outputs probably anyway. I think rsense and the Roff are going to have to be user configured. Also the Mtn-Max has rsense as user configured. I give values in the tables for Roff for some frequencies assuming 470pf Coff (this is how Ti's example did it, start with 470pf and derive Roff as needed). You can interpolate pretty much linearly. I see absolutely no need for 1% resistors here as Ti uses.
Rsens=0.248/Imax For this you want 1% definitely.
I'm kind of liking 10k and 3.3k for Rf2 and Rf3 and a 10uf Coff cap Cf2 cap.
This gives you less than 1% control ripple and about 1% minimum output if that 5uA iadj current is accurate.
I'll revisit my notes on this and might re-optimize a little, but this should work.
All that mcu hardware is up to you but I guess is the same as your other drivers.
Come to think of it, maybe a through hole for Roff would be nice, to allow easy changes? Or just a big footprint for easy soldering? You could always modify max current then just in software. But as before, I guess through holes are difficult. I haven't done much of this smt soldering yet. My hot air station is in the mail. I have modified about 3 tiny smt components before with an iron and just scraped through by holding my breath.
I find SMD components to be FAR easier to solder then through hole but I also have a reflow station. It is WELL worth the investment if you plan to do SMD work.
I like to release things in a complete all-inclusive fashion so that new or experienced user alike know what they need in order to do and complete the project.
Naturally the sense resistor will need to be configured per driver, an easy to follow calculator for that is ok. or just a list of resistors and the corresponding current they will give.
Roff is something that should be able to have a simple table that says if you want X setup you need Y Roff.
Basically I am trying to put together a nice list of components that is easy to follow with a pre-made shipping cart with them included for the most common setups.
I am acting like anyone building these is a complete noob with no prior parts selection skills at all. Just a soldering station and the will to give building a driver a try.
Even I am having a hard time figuring out exactly what the final parts list is going to look like when it comes down to actually placing the order. I could figure it out naturally but I don’t want people to have to do that, I want them to have a list and simply click a link to have 90%+ of the components pop up for them.
This is nothing against you, you have done amazing work. I am just trying to find a final list that I can use to create a shopping cart out of and a clear explanation in the OP of exactly what the builder needs and needs to do to get this working.
I understand your point entirely. It's just not there right now. Only just figured out what mosfet to use yesterday and only worked out the leakage current issue on iadj a couple of days ago (before that I wanted 0.5s RC for soft transition as the premium touch, but without a huge cap it probably isn't happening. I may still look into the huge cap options). Mostly though, just have to sit down now and write up a short list mostly done above, but will make it more organized shortly.
My hot air reflow station bit the dust Sunday. Something in the iron handle shorted, the handle got hot, and turning it off didn’t solve the smoking issue at that point, had to yank the plug. The main unit smells bad so I know it’s fried. Just when I was really getting dependent on it, too!
Yeah, I totally understand and no pressure. I was simply saying to let me know when you get it all figured out so I could get it all put together into a nice easy-as-possible to understand post for people to try out.
Well one like it, the first one that came up in ebay anyways.
It has served me very well. Not an issue with it to date. It works good, is stable and temperature control is good.
Only real complaints are it is not very high wattage so it takes a minute for it to heat up the iron and the iron is a normal china knock off without the best tips to heater fit (so it temp sags more on big things).
For the price though I am fully satisfied. A hakko is great but not 4x+ greater.
I find that the soulder you use makes a far larger differnce then the station. I recently got some new solder, after a buttload of research I settled on kester 63/37 #66/245 Flux in .6mm. It was hard to find but it was worth it. Flows super good, the iron stays clean and makes a great joint.
When I first learned that I could actually solder it was because of the right solder and the Hakko 888 station. After that, I bought the big roll of Kester and now I stick with it, wont’ buy anything else. Kester solder paste too, have tried others and the dynamics of the Kester just hold it head and shoulders above everything else I’ve used.
Kester is good indeed, just got to make sure you get the right stuff. Kester sells many many (way more then I first thought) types of solder and they all work differently.
The stuff I listed above is a no-clean solder with high flux content to allow easy hand soldering and good wetting.
The classic 44 kester works fine but you have to clean up the residue or it can screw up the parts over time.
I will tell of my own stupidity in the hopes that someone may learn something I obviously did not.
My cheap Yihua reflow/solder station has an on/off switch to power up the transformer in the rear of the unit. Individual on/off switches and temperature settings with digital readouts for each station on the front. I found that when I went to use the soldering iron Sunday the unit was in standby mode with - - showing on the screen. I fired it up anyway, and it overheated and smoked and died. Opening it up, the transformer overheated and melted down. This was obviously a result of my leaving it powered up overnight and then using it without a cool down. Now, I will say that there are 2 wires within the soldering handle that have all the insulation burned away, so something shorted in the handle and this may be the real reason it died. The handle of the iron was very cheaply made, with a plastic male/female thread connection that wouldn’t stay together.
I’m leary of purchasing another el-cheapo unit, but how do I know that a more expensive unit won’t still have these issues?
For the record, I didn’t buy this reflow station. It was in the modding kit I bought from Old-Lumens when he sold out.
Interesting, I am always careful to turn mine off after I am done with it, I will continue to do that.
I don’t think I would get a more expensive knock off unit, from my research none of them seemed to really stand out as being better then the others. Go for a brand name or cheap IMHO.
I think I found my hot air replacement, a XYtronic from the States (LF852D). Dedicated hot air, I already have the Hakko 888 so between the two I’ll be in good shape. Once you use the hot air station, there’s no going back! lol
I just got a clone of a clone off aliexpress. I'm going to go check my fire extinguisher now, lol. I'll check for shorts from case to hot at least. I've been zapped by cloned chinese stuff before.
A little progress on caps, or at least on what doesn't exist.
Until that's more ready. I did have a thought. It looks like there are still quite a few spare pins on that MCU.
It is actually possible to program frequency in the same way we program iadj. It would require a second buffer cap that Cf2 charges from. The buffer cap gets charged through a resistor (or possibly divider) from the mcu. I say possibly divider because Roff is always taking current from the buffer cap anyway serving as the second half of the divider, pulled to ground during on time (by the IC, to reset Coff) , and never above 1.24 V during off time. The fet duty cycle is very fast and coff discharge cycles won't impact the buffer cap. Coff just sees a constant voltage source with an Roff impedance (exactly as it should), and the buffer cap just sees an average resistive connection to ground as if the second half of its voltage divider, as it needs to fix its voltage. That average does depend on fet duty cycle, and so working out exactly what voltage and off time a given pwm makes has to be calculated still, but this doesn't need high precision either.
This probably isn't necessary for normal uses, but it could
a) offer some frequency vs power correction if it ends up useful (not clear)
b) simply allow frequency to be optimized without resoldering which if nothing else is useful while first figuring things out.
c) allow software reconfiguration if re-purposing the driver for a different voltage output.
Anyway, I'm still working on a shopping list for how things are now. Programmable frequency could be a nice touch though.