I don't know what you expect me to say about the above statement; it doesn't make a lot of sense.

The 7.5V onboard supply - a buck regulator - provides up to 100mA of current. The driver uses about 35mA, leaving about 65mA for comparators - 7.5V should be fine, right?

If you want to make this thing somewhat flexible, you might plan on using discrete MOSFETs instead of arrays, as you'll have a much wider selection.

The design would be ideally flexible, but the PCB would almost certainly change for higher powered models.

Did you look at the specs? If you want good performance and long life, then de-rate the components by 50% or more. Irms(max) for that inductor is 3.2A, and it will saturate @4.1A. You do not want to get close to saturating it, or you will see smoke.

Ah, good idea. Yes, I did read the datasheet. However, I am still a noob - a student learning electronics, so some things don't make sense to me. The datasheet says: "Operating Temperature -40 °C to +125 °C (Temperature rise included)" which would seem to indicate that running at Irms(max) would allow it to operate up to 125°C, unless I'm missing something. Although I wouldn't run it at maximum load, inductors are basically finely wound coils of wire, so the only thing that could go wrong is the wire could melt or break, right?

See Ronald Dekkers' "Flyback Converters for Dummies" page for a 'scope image of inductor saturation. Power dissipation in the upper MOSFET will skyrocket.

I'll be way clear of Isat.

You'll have to see how it does on a test bench. Start out using resistors, and then decrease their values to see what ringing you get.

When I get the time to make some PCB's I'll do this.

Why don't you test the model using LTSpice, and see if it agrees with the datasheet?

I'm not entirely sure how to do this. I only started using LTspice about 3 days ago.

 

It is possible. Some of the parts are almost as hard to get as Buck Pucks. I have the current drawing posted, but I'll be updating them with the current parts list tonight or tomorrow and reposting them.

I finished the PCB tonight. I'll give it the smoke test tomorrow. For the last week I've bee reacquainting myself with home brew PCBs. The results have been very impressive so far. Once you have the procedure down it is much faster than wiring a circuit from scratch.

I'm in the market for some Buck-Pucks, when do you think you would have some kits for sale...? Do you need Beta-Testers!!!

I don't find the Buck-Pucks hard to come by, I think they make them in my State. The real issue is finding a competitor, perhaps you are the guy!

iONic

 

I'm not entirely sure how to do this. I only started using LTspice about 3 days ago.

I expect you to figure out how to do it.

It's not so hard. You have all of the tools you need.

You just need to figure out how to use them.

You want me to hold you by the hand, but I have other things to do.

 

I expect you to figure out how to do it.

It's not so hard. You have all of the tools you need.

You just need to figure out how to use them.

You want me to hold you by the hand, but I have other things to do.

You don't need to hold me by the hand...

But it would be helpful to know if you've done this before. For all of what I know, you could have made those models yourself.

For now I will assume they are non-ideal. The oscillation frequency is mainly a product of the inductor and capacitor; therefore, as long as the comparator is reasonably fast, it will be fine. Worst case, I can design it for the standard pin-out for a single or dual comparator and then if necessary replace the comparator with a high speed one.

I suppose a pulse source might work, but this is why I prefer using breadboards and not simulators. Unfortunately you can't easily breadboard tiny SMT components, especially not high speed supplies.

For another inductor, SDR1006-6R8ML would be an option, or a similar 6.8µH inductor. It has an Isat of 5.9A, but is much bigger than the previous inductor, so obviously less preferable.

Good night all. I shall see in the morning...

 

Thanks for simulating my circuit SgtWookie and eblc1388.

I built one of these at some point to run a 3W white LED, so I knew about the current rising for Vin (poor Vin regulation) when I drew that schematic from memory. Also from my memory, the resistor "R comp" that you have called "R2" adds a negative bias of Vin:current, so if the right value is chosen it biases the current regulator (BC337) off more as Vin rises and compensates that effect.

I know the one I built worked pretty well for 12v battery use, and current regulation was within 10% or so from 11v to 14v and much better than 10% for the expected 12v-13v battery range in actual use.

I also stuck with the larger inductor, 330uH if I remember to keep switching freq low and efficiency was pretty high. But like I said this is all from memory and was years ago...

 

It would be helpful to know if you've done this before. For all of what I know, you could have made those models yourself.

I didn't make them myself. Have a look in the .sub files; you'll usually see credits as to where they came from.

For now I will assume they are non-ideal.

All SPICE models are a trade-off; they don't model every possible parameter or else it would take virtually forever for a simulation to run. For example, practically no models will account for the inductance of the interconnecting wiring from the semiconductor die to the pins. At low frequencies, it doesn't make much difference - however, if you have fast rise/fall times, you will very quickly start to see differences.

The oscillation frequency is mainly a product of the inductor and capacitor; therefore, as long as the comparator is reasonably fast, it will be fine. Worst case, I can design it for the standard pin-out for a single or dual comparator and then if necessary replace the comparator with a high speed one.

Yep.

I suppose a pulse source might work, but this is why I prefer using breadboards and not simulators. Unfortunately you can't easily breadboard tiny SMT components, especially not high speed supplies.

When you're dealing with fast rise/fall times, it's tough to get decent results on a breadboard anyway, as they have lots of parasitic inductance and capacitance. You really need to build these types of circuits on a custom PCB, or the parasitics will wreak havoc.

For another inductor, SDR1006-6R8ML would be an option, or a similar 6.8µH inductor. It has an Isat of 5.9A, but is much bigger than the previous inductor, so obviously less preferable.

Well, you need to decide what range of output current and voltage that you want to support, and I suggest choosing an inductor rated for at least twice your RMS or average current rating.

Enter the DC resistance, max current and RMS current limits in the inductor in your SPICE model, and see how well (or poorly) it works.

Increasing the inductance means you'll operate at a lower frequency, but you'll have to increase the size of the capacitor to filter the greater amount of ripple. It's kind of a vicious spiral, gobbling up real estate on the board.

 

I've decided to go for a 1500mA maximum output. This gives me 1.8Arms through the inductors, allowing me to use 4A inductors, and a smaller dual-MOSFET.

1500mA works from as low as 8V. I was also thinking of making a portable version running off a 6V to 9V battery, with reduced maximum output of 600mA (which would be pushing it for a 9V battery, anyway.)

 

Here's a schematic for a battery version, with the ability to output 0-600mA. It has lots of "options", if you want those options, you can add the required components.

 

Ugh, I can hardly read that schematic with the background being black.

 

Ugh, I can hardly read that schematic with the background being black.

Okay, here's one black on white.

I guess I like the high contrast scheme, but maybe you don't. gschem also has a light mode which looks quite similar to LTspice. I also attach that version.

 

Here is a PCB design for the 600mA version.

The 1500mA version will probably be identical apart from two discrete fets, a larger overall footprint, a dedicated 7.5V buck reg and a bigger inductor. Also it might have optical isolation for the inputs.

It is 44mm long and 36mm tall. It has a DC jack input - for plugging in a 7.2V - 9.5V power supply (it must be regulated - unregulated supplies will damage the controller IC), and a battery connection point for connecting a 9V battery. The inputs are diode-OR'd and also reverse polarity protected.

It should be made on a 2-layer FR4 process. Component side is for most tracks; solder side is used for the ground plane and occasional traces. It should meet 10/10 rules.

Image and gerbers attached. Released as open hardware, licensed under Creative Commons V3 BY-SA. The initials on the board "TO", "BM" and "SW" are attribution keys.

 

Some updates to the 600mA version. I changed the colors to make it more contrasting. Also, it should be easier to get manufactured as I've removed some traces that failed DRC. And I added vias under the gate driver and MOSFET, to help with cooling.

 

Anyone want to help me build/debug these prototypes?

 

Anyone want to help me build/debug these prototypes?

Exactly what would this consist of. Would you supply the parts and boards?


iONic

 

Exactly what would this consist of. Would you supply the parts and boards?


iONic

I'm working on another project at the moment, so I probably can't afford to get PCBs made. But if someone wants to take the design and make it, I'd be happy to debug it or help with problems that may arise.

 

I have a few projects on my plate right now, but if things clear up in the next few weeks, I would be glad to give 'her a build.

 

I'm working on another project at the moment, so I probably can't afford to get PCBs made. But if someone wants to take the design and make it, I'd be happy to debug it or help with problems that may arise.

I was going to say, order the parts and ship to me, send me the schematic, and I work with it, but I'm beginning to wonder how much time I will have for it.
My dog is scheduled for a lump removal(Sarcoma/cancer) and don't know what sort of support he will need for a while. Perhaps I'll have more time as I will be stuck with him in the house, don't know.


i

 

Tom, I've been occupied with family over the Thanksgiving holiday last week, and then I've been sick as a dog since then. Sorry, but I'm out of the picture for a bit.

 

I'd like to bump this thread to see how everyone is doing. I'd like to build this, but I don't have the time or money. It would be nice to see both the original version and the new design working.

 

My circuit is just sitting there, waiting. No Excuse, but it will be tested (and we'll see if it works).