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Still trying (and NO LONGER failing) with the LM2588... 5V -> +/- 12V
Posted by Skimask on 10 Jan, 2013 07:13 -
Still trying to make a flyback boost / inverting converter using the LM2588. Been at it for the past couple of days now.
The circuit I'm using...almost exactly...is described here...
http://www.ti.com/lit/ds/symlink/lm2588.pdf
Figure 13, Page 17.
The only difference is that I'm using the LM2588-ADJ with the appropriate feedback resistors, R1=8.756K, R2=1K
12v = 1.23 * ( 1 + ( 8756 / 1000 ) (ok actually 11.99988v calculated, but my parts don't exactly have .000001% tolerance).
It's a dual output flyback converter, with 4-6v DC input, +12/-12v output @ 300mA.
In my proto circuit, I have 2 82ohm power resistors on the +12v and -12v rail, giving me about 146mA of load on each rail.
At most, I can only get 6.2v on each rail. 6.2v @ 82ohm = ~76mA.
When I open circuit both rails, I can get the full +12v and -12v. I've tried a handful of other resistors on the outputs, and of course, the voltage drops with decreasing resistance.
But, I'm not getting anywhere near the expected results.
The circuit is soldered down. I checked for excessive resistance on the traces.
I get the same voltage at the battery and the "switch" pin of the LM2588, so no drops there.
The battery pack hasn't dropped below 4.4v while I've been testing the circuit. When it does, I switch out battery packs for a freshly charged one...with the same results at the outputs.
Nothing on the board gets hot. The LM2588 warms up to about 10-15 above ambient (80F) then steadies up. No change in output voltage while it warms up.
I can see the ~100Khz waveform on the 'scope at the switch pin of the LM2588, and other relevant waveforms are similar to the waveforms noted on the datasheets.
So...what's my problem?
(besides the obvious)
If I didn't know any better (and I don't), I'd almost say that the coil itself was wired backwards.
4.4v input gives me +/- 6.2v output. 12.4v total differential / 4.4v input = 2.8, and the coil input/output ratio is 1:2.5
I'm still wokring on trying to build this curve tracer, but make it battery powered rather than using a PC/AT PSU (with +12v and -12v rails), and need/want at least +12v and -12v rails for the op-amps. I'm using a 4 cell battery pack for now, but if the circuit can handle it without letting all the smoke out, I'm not opposed to switching over to an 8 cell NiMH battery pack. Might end up doing that anyway...4 cell pack might not last all that long. -
Did you use one of the recommended transformers? (page 19, transformer type T2)
I can't see anything else wrong. Maybe a picture of your setup would help. -
Yep, got a couple of samples of the Q4337 directly from Coilcraft...part is the recommended T2 on pg. 19.
And other than using the LM2588-ADJ and appropriate feedback resistors vs using the LM2588-12 in the datasheet, the circuit I built is identical to the one in the datasheet as noted above.
I'm fairly confident in the wiring as I've disassembled and reassembled the circuit three times now. It's working as described in the datasheet, except that it won't support any sort of load above around 12ma (1K load) on each rail before the voltage starts to drop off from the desired set-point of the feedback resistors. I'm wanting the 300mA described by the example circuit, but I'll settle for 200mA, possibly as low as 100mA.
All of the grounds are tied back to a single point. I've paralleled an extra 120uF with the existing 100uF for Cin, also tried paralleling the 330 uF Cout caps with a couple of big ol' 4700uF caps.
Also tried a 2nd LM2588 as well as the 2nd coil.....just in case I got a bad part from the distributor.
No dice... -
If you load the output with the 82R resistors, do you see the feedback (pin 3) drop to about 0.6 volt?
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+ and - rails loaded with 82R, output voltage on each rail is about +6.4v and -6.4v, fdbk @ pin 3 = 396mV
With both rails open-circuited, I get + and - 12.1v on the outputs, and 731mV at the feedback pin. -
With both rails open-circuited, I get + and - 12.1v on the outputs, and 731mV at the feedback pin.
Feedback should be 1.2V I think, and I wonder how you get 731mV with the rails open, does the feedback pin draw current?
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Nothing measurable other than floor noise on my meter, and the datasheet confirms nothing more than nA at the FB pin.
Double-double checking the grounds now. Even though I've checked them once, I'm wondering if I've got a high resistance wire somewhere..... -
So how do you explain the 700mV? 8K7 and 1K, 12V, should be something like 1.2V isn't it?
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I can't explain squat!
On a whim, I just hooked up my 'scope to the switch pin on the '2588. LOTS of noise. I don't have a fairly nice square wave like shown on the datasheet diagrams, and certainly nothing resembling a 100khz switching signal.
Gonna do some more reading of the 'sheet. Might have to add the 'snubber' RC filter across the postive output diode...probably a dozen other things as well. Might be that all that noise is causing the '2588 to do some stupid stuff. The output voltage on both rails is relatively fairly clean, albeit low...but, start putting all that noise into the feedback and it ends up getting amplified and causing all sorts of crap elsewhere... -
Tell us some more about the parts you're using. Specifically what diode, output bulk capacitor, compensation capacitor. Pictures of your circuit would also help. Also which exact version of this chip are you using? I think you said the Adjustable version, just want to be sure.
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You don't happen to build that thing on a solderless breadboard?
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If you post a picture with the switch node and the trafo-anode node for the +12- it might give a clue to your problem. Of course you have short ground connection of the probe
bored@work asked a very good question. If the answer is yes, that's most certainly the problem. If you've built on a solid copper board, then I would say that you might have the answer in your first post: not that the windings are reversed, but rather that you put the diodes the wrong way, so that you get exactly what you measured: a simple 1:2.5 trafo (instead of a coupled inductor).
Another problem might be that you get switch noise into the feedback input, even with such low impedance FB-network. Minimize the FB-node; connect the output to the FB top R away from any high di/dt and du/dt.
//C
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@jerry - Using exactly the parts noted on the datasheet's schematic, LM2588-ADJ, Coilcraft Q4337, 1N5819's.
The big caps are genuine Nichicon's and fairly new, about a year old, 100uf on the input (tried 330uf as well), 330uf on the output (tried 4700uf in parallel too).
I don't have an ESR tester, but I've tried more than one in each position and they all act the same way.
The .1uf and .33uf are ceremic disc types, and I tried a couple of big ol' square poly (
) types as well.
@Bored - uh...ashamed to say, yes I did at first
and an old one at that, then got smart and moved it over to a regular proto-board. Good solder, tinned connections, the whole bit. Same thing.
@CarlG - trafo-anode node? Wha? Probe-yep, the ground connection that feeds right off the tip of the probe itself. I double-checked the polarity markings on the transformer and diodes to make sure everything matched up.
@AcHmed99 - Thought about the battery pack too. AA's might have too much internal resistance, crapping out when the switch is on, the V drop too fast for the 'scope to see, so switched over to a 4 pack of D cell NiMH (and tried a 4 cell NiCad pack as well).
Gonna tear it down again tonight and build it up one more time.
I gotta be doing something fundamentally wrong here. -
Ya know, I just looked at LM2588 and Coilcraft Q4337 datasheets at the same time.
The polarity marks at on pins 1,6,7 on the LM2588 datasheet.
Polarity marks at on pins 2,6,7 on the Coilcraft Q4337 datasheet.
Hmmm.... -
Why is this circuit inappropriate to breadboard? is it really that sensitive?
are you sure this is not a bit of paranoia? -
I'm talking about solderless breadboard vs soldered protoboard.
The solderless breadboard version's waveforms were practically unrecognizable, and like I said, the board was an old one. Contacts probably ugly, dirty, worn out.
Someday I'll throw it out...( NOT! Who throws anything like that away? ) -
I'm talking about solderless breadboard vs soldered protoboard.
The solderless breadboard version's waveforms were practically unrecognizable, and like I said, the board was an old one. Contacts probably ugly, dirty, worn out.
Someday I'll throw it out...( NOT! Who throws anything like that away? )
I would have thought solderless board be OK for this application because its only 200khz. I also noticed that all the capacitors used in the circuit are way bigger then the parasitics present on a breadboard.. I thought they were OK up to +10 megahertz.
I guess age has something to do with it. -
Normally, I would agree, if the circuit was all digital.
But, there's an analog factor going on here, and I think the extra noise on the solderless breadboard gets amplified a bunch internally in the LM2588 in the feedback circuit and causes all sorts of stupidness to go on. -
The switching frequency has nothing to do with the breadboard type required (or fundamentally, the ground plane). It's the rise- and falltimes that matters (which naturally must be low if fs is high). Even if you're switching something with 1kHz, there's nothing saying that you don't have tr and tf of 1-10ns, e.g. an ordinary logic gate. In this design you have a voltage swing of from -1.25*Vin to nominally 12V in say 10 ns or so, and of course also high di/dt that induces high frequency EM fields.
Here's also the requirement of keeping the ground connections of the regulator, the FB net, and the in- and output capacitors together in an optimal way, which IMO is not possible with a "ordinary" breadboard (i.e. those white/blue/whatever plastic ones). I'm not saying they're useless, but avoid them when you're dealling with high di/dt or du/dt. -
Right. I couldn't agree more...except that you put it in a much more technically correct format that I by far.
Now the only question I've got is how you got the subscripts to work! (e.g. the small letters behind the big letters, Vf, Tf, Tf, and so on )
Working on the 3rd iteration now...a few changes in order. Heavier gauge jumper wires, adding the snubbers and other items noted in the datasheet, whether they'll be needed or not. If it's noted, I'm going to add it this time around.
EDIT: Looking thru the datasheet... Thinking I might add another cap across the output right at the feedback resistor network as close to the LM2588 as I can get it.
Low cap value, maybe 10pf or so, high enough VDC value to keep it from frying itself...
Or would that be a really bad idea since it'll be messing with the feedback voltage...keeping it from changing as fast as it can?
If the switching freq is 100khz, the cap value plus the feedback resistor network, run it thru the math, keep the RC constant above 5x the switching speed, or 500khz.
Should help smooth things out? Yes? Or ruin it completely? -
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Now the only question I've got is how you got the subscripts to work!
When you edit the reply the buttons for making something sub or superscript are on the 2nd row just below right justify text/font.
Highlight the text you want, then click the button. -
Aha!
I see it now.
Learn something new every day -
Pictures of construction will help a lot. Switching regulators are heavily dependent on layout for success.
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EDIT: Looking thru the datasheet... Thinking I might add another cap across the output right at the feedback resistor network as close to the LM2588 as I can get it.
No, a feed-froward cap is quite common to reduce overshoot and ringing. The RC is not related to the switching frequency. Sometimes Cff is mandatory for stability, but if the datasheet doesn't say so, you may check the ringing frequency and try out the cap value. If you still got that low impedance feedback voltage divider as before I don't think 10 pF will do any difference. For me, last time I think I tried with an RC =1/(2*pi*f), where f is the ringing frequency, and it worked out quite well. If Cff gets too big, the step-response will be degraded.
Low cap value, maybe 10pf or so, high enough VDC value to keep it from frying itself...
Or would that be a really bad idea since it'll be messing with the feedback voltage...keeping it from changing as fast as it can?
If the switching freq is 100khz, the cap value plus the feedback resistor network, run it thru the math, keep the RC constant above 5x the switching speed, or 500khz.
Should help smooth things out? Yes? Or ruin it completely? -
Ya know, I just looked at LM2588 and Coilcraft Q4337 datasheets at the same time.
Trust the datasheet on this one. It's seems like it's the classic transformer-instead-of-coupled-inductor case...
The polarity marks at on pins 1,6,7 on the LM2588 datasheet.
Polarity marks at on pins 2,6,7 on the Coilcraft Q4337 datasheet.
Hmmm....