uSupply USB Rev B Update

[EEVblog]

For those following along, here is the Rev B USB µSupply.
Still preliminary, and not 100% checked & tidied up, but close.

Key points:
- Back to my original concept of dual 3 digit LED display (nice big easy to read 0.56") and dual (right angle) knobs to make it look'n'feel like a real power supply. No squinting at a small LCD, see the values across the room.
- One function push button for basic modes or set V/I display etc, same as original designs.
- 120mm x 50mm
- Battery powered from a standard 18650 cell
- Selectable battery or mains input with jumper. Allows 1A charging + full output operation.
- Max output 15.25V @ up to 500mA. Capable of higher current at lower voltages if really needed.
- Right angle riser board for the banana plugs, soldered onto the main board (no wiring).
- TO-220 LT3080 with bottom plate PCB used as heatsink.
- Construction is kinda like the attached, open style sandwich but with a PCB heatsink bottom plate. The top is clear acrylic.
- Micro USB charging (400mA) and isolated serial comms for, eh, whaever.
- Faster 1A charging using a 5V DC plugpack, which switches automagically
- Self test capability
- Small expansion board for, whatever. Maybe a fixed 5V USB output connector or something.

Schematic:
http://eevblog.com/files/uSupplyUSBrevB.pdf

Anything I've goofed please let me know.
I don't want to do any more videos on it until I actually have a the new board built and working, then I'll likely do a flood. That prevents any more videos that lead in the wrong direction.
No correspondence will be entered into on the design or feature set 

It's almost 1:30am, today's achievement ticked off, need sleep. 

Dave.

[Tooms]

So the adapted the Sick of Beige for the case ?

if yes then it will fit into the stanards sizes ?

http://dangerousprototypes.com/docs/Sick_of_Beige

http://dangerousprototypes.com/docs/Sick_of_Beige_standard_PCB_sizes_v1.0

 


Tooms

[fenclu]

Wow, top job. That's the best USB PSU of all your designs. As long as you keep it thin it'll be very portable because footprint ain't really that important in this case, I think.

[mikeselectricstuff]

I'd be inclined to put a high resistance (e.g. 5x100k 1206 in series) across the isolation barrier to drain any charge accumulated on the cap.
I don't see any reverse polarity or overvoltage protection on the ext DC input. Polyfuse+zener perhaps?
Selfttest probably not very useful.
R11 2K5 is an oddball non E24 value - I'd use 2K4, 2K7, or a combination if you really care. 
TXD/RXD polarity across the isolators appears to be wrong unless there's some non-obvious inversion going on. To minimise power draw you want the optos off when the bus is idle. So just have the LEDs pulled down instead of up, which should fix both issues.
I've not looked at the FT230X data but on all the other FTDI chips, status LEDs are pulled down, not up.
Is the DC-DC quiet enough without a cap across the output?
Is C1 big enough as an input decoupler to the boost reg?

 


 

[baoshi]

I'm looking at the hostile/dumb user case. If BATT/EXT power is set at EXT side but no external 5v is applied, the output will be powered by USB with 2W isolator. Shall some protection be included to limit the output?

[Mr Smiley]

Hi,

Sorry to nit-pick Dave, but middle right of your schematic, you have constant current load  I = 0.6/R14.

shouldn't it be R25.

Maybe some spare pads for possible hardware de-bouncing (R/C ) for you encoders, just in case you have problems, not populated as such, but there in case. They could become problematic with use.

 

[mikeselectricstuff]

Maybe some spare pads for possible hardware de-bouncing (R/C ) for you encoders, just in case you have problems, not populated as such, but there in case. They could become problematic with use.

 

I'd agree - the edges from some contacting encoders can be too fast to reliably keep track of and can lead to erratic behaviour -  some simple RC filtering can save a lot of grief. 

[Jad.z]

Dave is probably using some software debounce routine.

[fenclu]

Yep, looks like it's designed to tweak it in the software.

[EEVblog]

Dave is probably using some software debounce routine.

Same encoders as previous bench design. You can see it working in previous videos. No problems.

Dave.

[EEVblog]

So the adapted the Sick of Beige for the case ?
if yes then it will fit into the stanards sizes ?
http://dangerousprototypes.com/docs/Sick_of_Beige

I looked at those.
Would probably fit in the 100x80 version, but I didn't like that square form factor.

Dave.

[EEVblog]

Wow, top job. That's the best USB PSU of all your designs. As long as you keep it thin it'll be very portable because footprint ain't really that important in this case, I think.

I wanted it thin, but there is only so much you can do using an 18650 battery.
I contemplated cutting the cell into the board (as an idea I had for the bench version), but it's just not workable.
So I decided to just let it be what it turns out to be, rather than get all fancy.

Dave.

[EEVblog]

Selfttest probably not very useful.

It's for essential for production.

Dave.

[mikeselectricstuff]

Selfttest probably not very useful.

It's for essential for production.

Dave.

Surely part of the test/calibration process needs a calibrated  external load or current reference?

[EEVblog]

Surely part of the test/calibration process needs a calibrated  external load or current reference?

Haven't decided if I'll simply leave it up to the user to tweak the cal values if they want the best accuracy.
Either way, having the self test stuff in there allows an almost complete power on self test of the major features (reference, output voltage, output current, current limiting) without the need for a bed of nails or other dicky test jig connection. Combined with more precise batch testing that pretty much ensures units are quick and easy to test with high confidence.
All for the cost a transistor and a few resistors I already had in the BOM

Dave.

[JoannaK]

Nice schematic, with the helpful pictures.


AFAIK Those serial data isolation leds are indeed put non optimal (as Mike posted) since TTL-level (or cmos level) RS-232 idles at high voltage.

I noticed that PA7 is used as a high current pulsed led driver .. Perhaps not a problem as is, but I do hope it won't cause any loss of A/D accuracy since it's on the same 8-bit port as the Analog inputs? (wild speculation, yes, I'm good at it)

I'm not sure how Dave has decided to do the 7-segment muxing, so my assumptions may be wrong.  But anyhow, I'd add cheap fet (or bc846) buffers at the common (kathode?) side for safety. At the moment those Digit selector pins (Dig1-Dig6) need each to be able to sink all the current that high side delivers. Most likely the real current will be a lot less than calculated (5.8mA * 8 ), but >40mA is quite a lot for a digit selecting pin.

May well be a stupid question, but what are those CN3 and CN4 used for? And having  2*3 pin headers with 5 pins each?

[hans]

I really think the boost converter could use some extra capacitance. Although.. ceramic caps are good , I don't know if 1 can handle switching currents of 2-3App at 1.25MHz.
The value itself doesn't sound too odd, because the frequency is quite high.
Mike mentioned the input cap, but in a boost converter the input cap has to smooth out the ripple current through the inductor. The output capacitor is much more 'stressed'.
For a buck this is vice versa if I recall correctly.

What is the exact reason for 2.5V Logic by the way? Lower power consumption? Because it seems harder for people to start hacking the firmware because I am pretty certain a lot of 5$ AVR programmers work on fixed voltages. Maybe some series resistors in the input lines of the program header will protect/fix the problem.

Wouldn't it also be cheaper to replace the LM324 with for example a LM358, because you only use 2 opamps?

Other than that, it looks good.

[mikeselectricstuff]

I'm not sure how Dave has decided to do the 7-segment muxing, so my assumptions may be wrong.  But anyhow, I'd add cheap fet (or bc846) buffers at the common (kathode?) side for safety. At the moment those Digit selector pins (Dig1-Dig6) need each to be able to sink all the current that high side delivers. Most likely the real current will be a lot less than calculated (5.8mA * , but >40mA is quite a lot for a digit selecting pin.

At the very least it would probably be wise to keep the display mux synchrounous with  the PWM to avoid ripple artifacts - if the MCU is sourcing 40mA, I'd bet the voltage on the PWMs will shift noticeably. You might even see some variation in PWM voltage depending on the number of segments lit.

One possible issue running the MCU at 2.5V vs. 3.3v is the output FETs on the ports may not source/sink as hard as they do at higher voltages, sourcing is probably worse than sinking due to the P-channel output FET.
 

[Bored@Work]

since TTL-level (or cmos level) RS-232 idles at high voltage.

It idles at low voltage. A line driver like a MAX232 then inverts and the real RS232 signal then idles at space, i.e. the high voltage of RS232. But for conversion to USB one would not add a line driver, but stick with the logic levels going to the USB interface chip.

[EEVblog]

I really think the boost converter could use some extra capacitance. Although.. ceramic caps are good , I don't know if 1 can handle switching currents of 2-3App at 1.25MHz.
The value itself doesn't sound too odd, because the frequency is quite high.

The values are as recommended in the datasheet app for a max power output.
I have not gone through additional calculations.

What is the exact reason for 2.5V Logic by the way?

Direct power from the battery.

Wouldn't it also be cheaper to replace the LM324 with for example a LM358, because you only use 2 opamps?

Not really. Same price. The LM324 is more common. Arbitrary choice.

Dave.

[HLA-27b]

Dave, I hereby solemnly take the credit for badgering you to continue with this. 

Now then, can we have the mounting hole positions / diameters and top and bottom clearances please. I'd like to see how it fits into that other project being designed right now.

[hlavac]

I like the 2.5V rail idea, with regard to losses on the LED current limiting resistors. Cuts it to somewhat acceptable levels.

Too bad you use a linear regulator to get the 2.5V, negating all the savings

I was experimenting with minimizing the LED display current limiting losses on a 5V rail, and had to resort to kind of high efficiency DCDC converter driving the segments. Works, but is complex to drive and adds components to the BOM.
I basically connected the LED segments in place of inductor free-wheeling diode to maximize efficiency

[robint91]

Why have you chosen for two optocouplers and not of those digital isolators? Which I find better for isolating datacommunications. Optocouplers are sometimes too slow if the pull-up circuitry isn't properly chosen.

https://www.silabs.com/Support%20Documents/TechnicalDocs/Si8410.pdf
http://www.digikey.com/product-detail/en/SI8421AB-D-IS/336-1755-5-ND/2170662

[EEVblog]

Why have you chosen for two optocouplers and not of those digital isolators? Which I find better for isolating datacommunications. Optocouplers are sometimes too slow if the pull-up circuitry isn't properly chosen.

Cheaper, and more jelly bean.

Dave.

[mikeselectricstuff]

since TTL-level (or cmos level) RS-232 idles at high voltage.

It idles at low voltage. A line driver like a MAX232 then inverts and the real RS232 signal then idles at space, i.e. the high voltage of RS232. But for conversion to USB one would not add a line driver, but stick with the logic levels going to the USB interface chip.

Wrong. TTL UART signals idle high. RS232 idles low.