LM 2587/2588 Multiple output flyback regulator regulation. - Page 1
Forum section: electronics / projects

author: peter.mitchell
author_rank: Super Contributor
timestamp: January 25, 2014, 04:09:42 pm
content: So for a while I have been wanting to build a replacement power supply for circuits that use 9v block batteries for obvious reasons (more expensive, lower energy density, harder to find than other batteries, can't be charged as fast as other batteries, chargers aren't as common ect ect ect). Often they are used where either a high voltage is needed, or a reasonably large voltage swing pos and neg.So I went looking at boost and flyback regulators. For me, the first place I looked was nationals line of simple switchers, they're pricey but easy, I settled on the LM2588, because i had a few in my parts bin and it seemed to fit the bill. I know the switcher has horrible standby performance but thats fine, a real power switch and a small RC "delayed start" to the enable pin would take care of that.I started building one of the example configurations shown in thedatasheet, Figure 26, of course using the adjust version i needed to put in the voltage set resistors, however, this is where I got a bit lost/confused.It appears only one of the two outputs on this regulator is actually regulated, unless there is something I'm not getting... This isn't very acceptable to me, especially if say... the regulated rail is heavily loaded and the unregulated one isn't, because then it could easily go overvoltage. The opposite situation whilst isn't as dangerous is just as bad, because one rail would drop miserably...There are a few ways to fix this and there are a lot of alternative products, but I thought I'd ask if anyone had anything to share on it here; I would usually also post a bit more info, but it is late ect.Thanks in advance.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 25, 2014, 04:27:17 pm
content: Quote from: peter.mitchell on January 25, 2014, 04:09:42 pmWhat else do you expect? You have a single feedback pin, i.e. a single input into a single error amplifier. And the IC has a single output driver. How should it regulate two outputs independently? It regulates the energy going into the single flyback transformer.Some options you have:Build two independent regulators with two ICs.Get an IC with two independently regulated outputs.Bodge it by weighted-summing up the error terms of the two outputs on your own and feed the result into the error amplifier/feedback pin. Probably requires opto-isolation and separate voltage references. The summing weights dictate how both voltages are shifted if the load on one changes.Design the unregulated (or both output) voltage(s) with some voltage headroom, so you can add one (or two) linear LDO regulator(s) on the secondary. And ensure some minimum load on the regulated output.

author: peter.mitchell
author_rank: Super Contributor
timestamp: January 26, 2014, 12:39:56 am
content: I had realized that already B@W, no shit.What did I expect? Maybe them to have a dido clamp or something on the output on the unregulated output in the typical apication, not having one is pretty irresponsible.What I wanted to hear from the forum was more along the lines of;In a real world application, how much does it really matter? How much will the unregulated rail swing? The more load you put on it the more the other rails will suffer (because flyback transformer) too so it should stay pretty close provided you have a minimum load on the regulated rail...That and how bad are the other aspects, I mean, if it has neigh undampenable ripple or is a noise machine then the regulation point is moot.If you follow best practice when laying out the PCB for it, would its noise be any worse than a comparable power boost converter because of the transformer(bigger antenna)? If you use good caps (a nice low esr electro, a big x7r and a small np0) on the output would that be sufficient to kill the noise (10-30mv p-p)? Or would you have to throw in an inductor or two too?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: January 26, 2014, 12:59:22 am
content: Peter,You would get some regulation from the datasheet schematic. You could consider the negative output to be semi-regulated. How well it is regulated depends on how tightly coupled the two secondary windings are.Without any load on the negative output, the circuit will tend to peak detect any noisy spikes, with a few milliamps of load it will probably be o.k.What are your requirements?If the load current is small I would consider making an 18V supply followed by an active (op-amp) splitter to make the two rails.With a little more information I might be able to offer more advice.Jay_Diddy_B

author: Harvs
author_rank: Super Contributor
timestamp: January 26, 2014, 01:53:32 am
content: Quote from: peter.mitchell on January 26, 2014, 12:39:56 amComes down to your min load requirements. As Jay_Diddy_B suggested, if you have a well made transformer without significant leakage inductance, and you have a reasonable min load on all secondary windings (a typical requirement might be 10% of full load), then you can usually get +/- 5% regulation on the unregulated secondaries. An example of this is almost any ATX psu.It only becomes a problem when you have no load on any one of the windings. You can get over-voltage on unregulated windings due to relatively low energy spikes charging up the output caps. Or if you have no load on the regulated secondary, but a load on one of the unregulated windings, then the feedback circuit will only see the correct voltage on the unloaded regulated line and thus keep the duty cycle too low.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 26, 2014, 08:18:38 am
content: Quote from: peter.mitchell on January 26, 2014, 12:39:56 amSo why do you write shit like "This isn't very acceptable to me"? Picture little old lady holding a china tea cup with pinky finger raised up at a church social function. "Ethel, what do you thing about the flyback regulation?", "Well Agathe, it isn't very acceptable to me!" "Yes dear, does no one think of the children?".

author: Skimask
author_rank: Super Contributor
timestamp: January 26, 2014, 08:43:28 am
content: I'm no genius (and likely zero help) when it comes to the LM2588 (re:https://www.eevblog.com/forum/beginners/still-trying-(and-failing)-with-the-lm2588-5v-gt-12v/75/), in my circuit, as long as the + and - loads were equal, the + and - voltages stayed where they should. Start goofing with the loads on the opposing rails, the positive side stayed good, the negative side went wonky, specifically it got a lot noisier.Like I said, likely no help at all...I ended up post-regulating both rails down to +/- 8v. Using the reference circuit, that worked up to about 1/2 amp on each side. Anything more and the noise starts up and the average voltage falls.

author: peter.mitchell
author_rank: Super Contributor
timestamp: January 28, 2014, 07:25:21 am
content: Ok, so a little more detail here;The specific product I wanted to use it in is a process thickness meter, it has two sets of 4 laser diodes on either side, very very well aligned with each other, it does a sweep of pulses going from very low power (1ma) up to a fairly high power level (600ma) on one side and uses the diodes on the other side as a sensor, then swaps. It does some math and some logging and gives you a thickness of the material. Problem is, it runs off 2x 9v batteries, and has an RMS current draw of around 400ma when sensing, so you need very good batteries, and even then they go flat very quickly if used for any reasonable length of time.The current requirements probably put it outside the range of an Op-amp buffered virtual ground, especially since the load is pulsed.The min load on both rails is around 20ma, however, when that rail is at min load the other is at max load, this is why i was concerned about regulation on different rails and was suspicious of the minimum load/load balancing.On the weekend I didn't have much time so I wound some transformers and that was it (i already a PE-68412 which was specifically mentioned in the datasheet, but i thought while i was going i might check out a few others). This morning I put together some prototypes(on a copper clad board, with some punched lands) to see how it looked;Input power was a linear bench PSU set at 5v with remote sense.The first one I put together I used the transformer from Pulse and some 330uf panasonics on the outputs, a 470uf electro and a 22uf x7r soldered between the ground tab and the vin pin. With a small load on both rails (10ma), they both sit nicely on the set voltage (12.47v according to the resistor values) as the load climbs, the "semi-regulated" rail tracks reasonably closely (0.1-0.2v), but as it passes 200ma it gets a bit wobbly, it has a low frequency ripple of around 0.5vp-p@400hz~ by 300ma it is now at about 1vp-p. Both rails also have a lot of HF crap on top.Things change fairly dramatically when the load is changed from an "ideal" balanced, resistive load. As expected, the "semi-regulated" rail climbs when it has light load compared to the regulated rail, and falls when it is heavily loaded compared to the regulated rail.With a minimum load of 20ma on the "unloaded" rail and the loaded rail at "full load"(300ma), the difference between rails is as high a 5v! Clamping or secondary regulation is most certainly needed if the load will be this imbalanced with the datasheet build. Altering output capacitance had little effect, though, adjusting the switching frequency did help.I believe the problem to be the transformer, it gets quite warm under full load, also, as you wind up the frequency a bit it does alleviate the rail voltage difference, the ripple and the temp slowly drop as well, however, the regulator itself did warm a little.The second prototype was identical to the first, but I used my own transformer, keeping to the same ratios and inductance, but using thicker wire, a slightly larger core and skimping on barrier tape for tighter windings.The two outputs tracked much more nicely at low load, being almost identical (0.05v or so), as load is increased, there isn't any low frequency ripple, however the noise seems a fair amount worse - either that or it isn't being masked as much by the low freq ripple. The transformer didn't get warm and the switch no more so than in the first setup. When changing to an imbalanced load, if the heavier load was on the "semi-regulated" rail, the rail wouldn't sag anywhere near as much (only about 0.5-0.75v). However, having the "semi-regulated" rail with light load and the regulated rail with the heavier load, it increased far more than before, instead of rising to just under 17v, it hit almost 19v!The third prototype was the same as the above two but with my own transformer again, this one had a lower turn ratio (1:2.2 vs 1:2.5) and a slightly higher inductance - the average dutycycle would be higher, so more switch and winding losses, but tighter regulation. This worked a charm; the switch was hotter over all, but the regulation at split loads was much tighter (only 0.5-1v) and the noise was lower as well, but still pretty high at high loads.As the dutycycle was already up, it didn't respond as well to transients, to "work around" this, i just bumped up the caps on the output, putting a 1000uf electro and a 22uf ceramic on each rail, with the 1000uf electro only, the transient response was much better, but noise was worse, the ceramic fixed that though.Overall it's quite workable, efficiency when using an electronic load is highest at 220ma from both rails, measuring around 80-83% (hard to accurately measure), and when powering the tool it is around 75% due to spikes and idle time, SW freq seems best for this transformer at around 190k. It does have a bit of a practical setback though; whilst I can build the circuit very small for what it is, it still takes up around the same area as a single 9v battery and weighing a little bit more. So now im looking for ways to bring the size down.Thanks for the help Jay (transformer windings makes a big difference), harvs (keeping the avg duty cycle up helped a lot) and ski (lots of great info in that thread, i also had the same observation as you though, at default switching freq, the switch waveform is a dogs breakfast, but it "snaps" clean at a certain point and stays that way to max freq, didn't bother to investigate.)Quote from: Bored@Work on January 26, 2014, 08:18:38 amBecause there maybe workarounds/changes/compromises that may make it more acceptable than the specs suggest?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: January 28, 2014, 11:51:39 am
content: Peter,I suggest winding the transformer like this:1) Wind the primary. use multiple strands of wire so that the winding fills the full width of the bobbin.2) One layer of thin tape3) Wind both of the secondary windings together, by taking two pieces of wire side-by-side bifilar winding. Select a wire size that will fill the full width of the bobbin. Each wire is one of the secondary windings.4) One layer of thin tape5) Wind an identical primary to step 1 over the secondary windings.6) Connect the two primaries in parallel7) Final wrap of tape.This arrangement will get you the best possible coupling.Regards,Jay_Diddy_B