Still trying (and NO LONGER failing) with the LM2588... 5V -> +/- 12V

[jerry507]

Look, it's not that you lack the tools. You said you've got copper clad board laying around and you've got a soldering iron. Maybe a little wire and that's all you need. When we say build it Jim Williams style, or dead bug style or whatever phrase, that means:

http://cds.linear.com/docs/Application%20Note/an47fa.pdf

Look at page 18 (of the PDF).

Keep the leads as short as you can, everything as tight as you can. Your circuit will work. That's all you're missing. You're thinking to yourself "Man, it can't be the circuit board I'm using, right?" Yes, it is. Switching supplies are easy to build assuming you have an acceptable construction technique and you choose good components. You've got good components, just a poor layout/construction technique.

The flyback is a great way of doing what you're doing, and dropping it for a different switcher won't solve your problem and you certainly won't learn anything more. Stick with it, and listen to what we're saying.

[Skimask]

Ok, I will disassemble the board and give it a try, ugly as it may end up, I'll try it.  Not really a lot of connections to be made, and a lot of them are directly to the "ground plane" at that.
But, I think I'm going to 'mount' (where's that hot glue gun?) the components on the one side, drill holes in the board and solder the ground connections to the copper side.
Lessen the chances of shorting anything out.

[Skimask]

Well...here it is...
Haven't tried it out yet.  Just had to get some pictures up here.  Gonna spend awhile with the circuit under the microscope looking for anything miswired.
Obviously not the tightest layout.  Now that I look at it, I likely could've squeezed it together a bit better, but I think the wiring is about as short as I can get it.

Changes from the example schematic in the LM2588 datasheet:

Ran out of anything near 330uF cap's for the output.  Used 4,700uf in parallel w/ 4.7uf.
In the other version, the negative rail wouldn't regulate since it didn't have any load, so added a 10K on the negative rail to ground.
Added the extra Cin and 3.9ohm across the battery Vin and LM2588 Vin (as suggested by the datasheet).
82 ohm sand resistors are there for load testing only.
Feedback, R1=1K, R2=a pair of 4.7K.  Should give me about 12.792v at the output, assuming all of the values are dead on.
The twisted wires are a pair of 22ga jumper wires feeding the coil primary side.

Now for the good stuff...
EDIT:  Now that I see the pictures in full screen, I'm going back to clean off that flux.  Jeeze that makes it look ugly---er.
EDIT#2:  And now there is a ground on pin#4 of the LM2588.  How could I possibly forget that?
EDIT#3:  Also reconnected the switch to Vin vs. ground...

[Skimask]

Ok...works!!!
Still got a bit of high frequency noise on the outputs, not a lot, surely not NEARLY as much as before.
The trace coming off the switch @ pin 5 is sort of recognizable, but doesn't resemble the trace in the datasheet.  I can see the switch points, but I can also see a sold 3 waves of rings after the switch turns on.

Nonetheless, it works.  I get 13.02 volts at both outputs, well within the 5% resistor tolerance, easily trimmed out, but no need since I'll be post-regulating this down to +12/-12 after I raise the output voltage a bit more to cover the headroom for the regulator.

Output load on both rails is 82ohms, ~160mA per rail.
13.02 / 82 * 2 = 317mA load + 2.5mA resistive load for feedback + about 100mA for the LM2588 itself = ~419 mA
13.02 * .419 = 4.45 watts

Input voltage is a steady 4.63v @ 1.81amps = 8.38 watts input

8.38 / 4.45 = 53%

Not great, but not terrible and doesn't bug me since all I want is the voltage and I'm driving this with a 4 cell pk of D's rated at 10000mAh (whether I can actually get the rated capacity, I'd doubt it, but I'll take what I can get).

Top of the head (e.g. bottom of the barrel) math says, assuming full load, max out the output rails at 300mA each (double what it is now), 50% efficient, double the 1.81 amp input, call it 3.7amps, add in another 500mA from the PICs, LCD, other circuitry, 4.2 amps, drawing off a 10AH battery pack, gives me about 2.4 hours with the numbers, maybe 1/2 that, a little over an hour semi-realistically (.4 C draw from the batt pack, inefficiencies, other loads, etc.), on a charge.
I'm good with that...especially since I don't anticipate the end unit to run at full load maybe 10% of the time at the very most...who knows...
Full load would entail the least amount of series resistance on the part I'm testing, all of the opamps putting out maximum current, LCD at full brightness with all pixels lit up, the PICs cranking away.

Now then...
MAN WAS I WRONG!  Or rather massively ignorant!  I now see, after reading thru the aforementioned documents, what a decent ground plane can do for you.
Even though my circuit is far from optimal (need compensation values tweaked, etc), it works, and I think I can assume it will work well once placed on a proper PCB with a generous amount of copper dedicated for a ground plane only.

Next up...wait for the batteries to fully charge up, add in more resistors to get it to a full 300mA per rail for a load, and see what it does.
(And small booster, low current, <100mA, for a USB charger to kinda trickle the battery pack up a bit while I'm connected to the PC.
Ya, 100mA max going into a dead 10,000 mAh batt pack.  That'll take awhile. Like ~7 days if the batt pack was completely dead, and the circuit was off, and I left it on the USB connector continuously.  A lot of IF's there.)

On another side note...on that last try with the circuit, the regulator heated up faster than the resistors and I generally shut everything off once it hit about 160F (100F above ambient).  Took about 3 minutes under light load to get to that point.
This time around, with the 82 ohm loads, it barely got warm.  Last reading I took after about 15 minutes of running was 80F...20F above ambient, and both resistors were up to about 150F.
It also draws 250uA (assuming that's anywhere near correct with my crappy meter) with the battery connected and the LM2588 "switched" off.

[Colfaxmingo]

Looks good!  Glad you could get it going.

[mariush]

Why would you need 4700uF/35v on the outputs? I don't think you would need that much.

Did you test how the esr of the capacitors affects the circuit? Have you tried using one or a couple of 820-1000uF 6.3v polymers for the 5v with this setup? And maybe a 1500-2200uF 16v low esr cap for the 12v?

[Skimask]

Just using stuff I had in the spare parts bins.
After I play with it a bit, I'll put parts in it closer to what they actually should be (e.g. 330uf on the outputs vs. 4700uf).

[Bored@Work]

You still haven't understood the prototyping technique.

You take the copper clad board, copper side up! Up as in that you look at it. And thats where it stays.

You solder some terminals or sockets to ground to have some standoffs for a start. For the supply rails you of course need to have them isolated from ground, but fixed to have some standoffs.

You directly solder parts to these connectors. When you have part that needs to have a pin connected to ground you  rotate it, bend the pin etc until you can solder that pin directly to the copper. No wire, no hole. Put the damned pin on the copper, solder it.

You take a part that needs to be connected to a non ground pin of that part. You directly solder the pins together, avoiding wires. If you need additional stand ups to hold non grounded parts in the air you solder a resistor in the gigaohm range to ground.

[Skimask]

Yes, I get the prototype technique which has been eluded to, both by the comments and the linked text......something solderable but insulated for standoffs as needed, direct connections between parts, yes, I get it.

But, with that being said, doing what I did worked, therefore, how much change do I actually need to make?
I've only got 6 extra interconnect wires as it is.  2 of them going directly to the ground, 2 of them running from the output caps to my load resistors (which if this were to be used in an actual circuit would like go to multiple other parts anyway), and 2 are actually interconnect wires that were needed because the pins on the coil itself are too short to do anything with.  4 of those wires are really needed because the coil's pins are only .1" long in the first place.

Yes, I could've done it with the copper side up.  I didn't want to do it that way.  I didn't feel comfortable with the copper side up as such.

Where did I go "wrong" again?

[Shuggsy]

Very good that you got it going! Good job. Layouts and schematics go hand-in-hand to make circuits work, and the more high frequency components (fast switching, even at "slow" rates for example) you have going around the more attention needs to be paid to good layout practice.

As for why you should do builds like this with the copper side up, well:

Ok...works!!!
Still got a bit of high frequency noise on the outputs, not a lot, surely not NEARLY as much as before.
The trace coming off the switch @ pin 5 is sort of recognizable, but doesn't resemble the trace in the datasheet.  I can see the switch points, but I can also see a sold 3 waves of rings after the switch turns on.

That speaks for itself. YES, your circuit works! And really, for this application that may be all you're after. If, however, find yourself working on another sensitive circuit with high frequency components later on and build it the same way then you may not be so lucky. In this case, it may not be "wrong" so much as just bad practice. Maybe best to do it the right way when it doesn't matter so much so you have experience when a circuit really needs to be built carefully.

One of Linear Technology's latest app notes deals specifically with layout considerations for power supply designs: Application Note 139 - Power Supply Layout and EMI. In their case, it's written for PCB manufacture instead of the dead-bug/copper-clad/Jim Williams style which is more for prototyping and one-offs. Rather than thin traces like a wire would essentially be, nearly all the layout is done using big, low-impedance copper pours. Attention is paid to where the major current paths are and how to best separate things.

Anyway, good job again on getting the switcher going! Very good troubleshooting experience.

[Skimask]

Can't disagree with any of that.
Changed the feedback resistors to get ~18-19v at the outputs, added a 7815 and a 7915 to each rail and loaded it to about ~225mA on each rail.
The post-regulated outputs look clean on the 'scope (Tek 2246A, A/C coupled, 20mv/div).  Shut the circuit down and keep the probes connected, and I can see the ambient noise here in the shop (concentrated around 58Khz and 60Hz) on the 'scope is pretty close to the noise that I'm seeing with the circuit powered up.  Hooked up an identical set of probes to Channel 2, grounded it to the PCB, left the tip 'hanging' a couple feet away from the PCB, selected add and invert, and the noise all but disappears, even down to 2mv/div.  Same thing with the circuit powered off.  So, I think I can conclude that the bulk of the post-regulated noise on the rails is due to the shop's ambient electrical noise.

For grins, bumped up the output to +/- 24.4v, still worked, but killed that battery pack fast!
Playing around...I'm having fun...

EDIT: Slightly confused on the efficiency calculations...
Input = 4.66v @ 1.81amps = 8.435 watts
Output#1 = 13v @ 82 ohms = 159mA = 2.067 watts
Output#2 = -13v @ 82 ohms = 159mA = 2.067 watts
Total output = 4.134 watts
Output / Input = 49%, which seems awful low...considering the noise, probably dead on.
Been running for an hour, heatsink on the LM2588 is steady at 62C

[Skimask]

Another note...
I had trouble believing myself, that I did something wrong in the first place (other than the obvious ground plane issues that I got schooled on!)...
So, I built up another one of these circuits last night, on the same type of protoboard I used 'back in the day' when I built up that 5v->12v boost converter roughly 10+ years ago.

It worked.

So now I've got three identical versions of the exact same circuit here...

1 - built up on a "protostack" pcb (http://www.protostack.com/boards/prototyping/prototyping-board-full-size-style-1).  Doesn't work, lots of noise, etc.

2 - built up on a piece of single sided copper clad PCB, dead bug style.  Works well, a little bit of noise, taken care of easily, and will be the type of construction I'll use in the "final" version.

3 - built up on a piece of protoboard similar to the protostack pcb, same layout as far as pins and traces go, but much older, maybe 10+ years old.  Put it together in the same manner as version #1 as far as parts placement and wires go.
This version works as well.  Just a tad bit more noise than version #2, but the input vs output numbers (volts, current draw, etc) match up between #2 and #3, at least as far as the resolution of my BK2709 can get.  And they both run about 3 1/2 hours loaded down on a charge from the battery pack before the LM2588 won't regulate any more (down to ~3.7v input voltage).

Therefore, all other things considered, I can only conclude that the 'protostack' boards are the issue.  Whether it's thinner copper, different substrate, I don't know since I don't have the spec's on the older protoboard.
I do know that the 'protostack' protoboards work well for 'purely' digital circuits...e.g. PICs running at 64Mhz (internal of course), with SD cards driving them at max SPI rates, LCDs, GPS, WIFI, Camera modules, fram chips, etc.

If it was just as easy to dead bug 40 pin PICs, I'd do the whole thing on the single sided board.  It's not, and I've got a stack of 7 protostack boards to use up.

[jerry507]

Eh, if you're having trouble believing all this layout stuff means anything then all you need to do is look around the datasheet more. They're getting inefficiencies of 75-90%. Look at their diagrams. If you're not getting waveforms that look like that, you're doing it wrong.

Yes, some of it will be ESR in your output caps (read the linked Linear app note and look at the current loops to see why these matter). The xformer you're using is from the datasheet, and will be what they used. All that ringing and stuff is layout related. Extra inductance in the leads, mostly.

You don't need a PCB to get optimal performance, and the dead bug style can do just as good if not better. But if you don't "believe" it, not much we can do.

[Skimask]

Yes, I know I'm doing it "wrong"...in a way.  That's what I'm saying.  But, what I'm also saying is that I know I had a similar project working using my "old methods" on those prototyping PCBs.  I just had to prove it to myself, mainly because back then I thought I knew a lot, and even if it worked, maybe I still had a LOAD of noise that I wasn't aware of.
I get the "goodness" of a good ground plane, component placement, cap ESRs, flux leakage, internal resistance, skin effect, current loops....All (ok, most) of that good stuff.
I'm eventually wanting this thing on a PCB so it can be fairly durable, more repeatable, something I can get more than PCB made up to play with in the future.
Like the Monkees said...I'm a believer.

[jerry507]

Well it "worked" on your old PCB too. It regulated and did stuff, it just died under load and was very inefficient. That's exactly what you'd expect to see from all the parasitic effects. You see the same stuff happen with poor quality transformers. All that ringing really messes with your circuit.

[Skimask]

One thing I can't find in the LM2588 datasheets is how to calculate the correct values for R(comp) and C(comp) coming off pin 2.
well, that and the need for a proper PCB design

[jerry507]

Assuming you're not changing the frequency (100khz default) by putting a resistor from pin 1 to ground, you don't need to change the compensation network.

[Skimask]

Nope, just the isolation circuit as described in the datasheet for the power on/off function, a diode and a switched resistor to Vin.

Did a few more runs of the circuit with the battery packs.  Output voltage = 13v, load = 82 ohms per rail = ~317mA total output load, a bit over 4.1 watts.
Using 4 cell D NiMH batteries, got a tad over 3 1/2 hours before the output dropped off @ 3.7v battery pack voltage.
Tried a 4 cell AA NiMH pack for the heck of it and it ran about 1/2 hour, close enough to what I had calculated.

I'm satisfied enough, except for the efficiency.  ~8.3 watts in , 4.1 watts out, a bit less than 50% efficient, and I can only account for about 2 watts of that using the formulas in the datasheet.  The rest is ringing, noise, etc.  Build it, rebuild it...  Eventually I'll get a configuration that takes care of business.

[ptricks]

One thing often overlooked on switching supplies is the input side, just using a battery on the input doesn't solve the problem.
A good pdf on switching designs and layout :
https://roboturk.googlecode.com/files/Boost%20Converter%20Design%20Tips.pdf

[Skimask]

More good reading material...

One note that hit me in the face in that document was the one about "always" putting a 1uf in parallel with any aluminum electrolytic to bypass the cap's inductance.
Google'd it, read up on it a bit, and of course the intarwebs agrees.
So, I added 1uf across the input and output caps.
Sure enough...  I've only got one battery run on it, so it's not by any means a statistical analysis, but after adding the caps, I got 4 hours on the charge before it went out of regulation.  Input current dropped to ~1.6 amps (from 1.81amps).  Calculated efficiency is up to about 55%.

Hey...every little bit counts.

[Psi]

75% efficiency should be possible without much trouble, so there's something causing your low 55% figure

I've not read this entire thread, so ignore me if this has been said before.

I would put a pot on the frequency input and graph the efficiency across the frequency range.
You'll get better efficiency if you tune the frequency to match your transformer.

[Skimask]

Possibly, but if you look at the circuit board I whipped up a couple of pages ago, I think you'll see why the efficiency is so low.  Ugly construction to say the least.
I've got really good inputs from here, and a bunch of tech doc's that I've read thru with good design tips and such.
For now, this circuit will work just fine for my application.  When I get around to making it properly, I'm sure the efficiency numbers will go up markedly.

[Shuggsy]

Before this topic gets buried with time, I'd like to add one more bit of reading material. I skimmed through the other pages and didn't see this one linked. Again, this is an app note from Linear Technology, but delves into the big world of switching regulators in a very nice, explanatory, and fun to read way. It was written by the late, great Jim Williams who has been mentioned previously in this thread. I highly recommend this app note as well as the entirety of app note 47, High Speed Amplifier Techniques, which was previously linked (much more than copper-clad build construction info inside it at almost 100 pages!). This app note is app note 27, Switching Regulators for Poets -- A Gentile Guide for the Trepidatious.

It doesn't go much into layout, but does make some mention of it (quote: "Layout is vital.") and some measures to ease the layout constraints. If you want good layout practice guides, app note 47 provides many examples as it consistently deals with high frequency techniques. App note 27 also goes over frequency compensation, an overall checklist for switching regulator designs, and common design issues. Being a Linear Technology app note, it does exclusively use Linear Tech parts, but the lessons can be applied to general switching regulator design.

Skimask, any chance we'll see some of the results from the circuit using this regulator's outputs? Also, perhaps you should edit the topic line to read something like"Still trying (...finally SUCCEEDING!)  with the LM2588... 5V -> +/- 12V" 

[Skimask]

Resulting circuit/project - that's gonna even more fun...or not...
This whole battery powered supply plus a couple of small linear regulators, charging circuit, etc., 2 DIP28 PICs, 1 DIP14 PIC, 3 DIP8 PICs, 3 TL084s, 2 6 channel digital pots, LCD w/ touch panel, 4x4 matrix keypad, SD card, USB, and 2 banks of 8 reed relays (unless somebody has a good part number for a digital pot that can handle + and - across the pot terminals, as well as up to 1 watt per digital resistor).

[Skimask]

Next issue...finding an opamp that'll do close to +/- 12v and 300mA at the outputs in a DIP8 package...
Literally hundreds to choose from.  Initial look says a TCA0372 looks like it'll fit the bill...