YAPS Part Eleven - Further Testing

RE: YAPS Part Eleven - Further Testing

Andrew J — Wed, 31 Jul 2019 20:18:53 GMT

So, keeping things up to date...I hope nobody thought I'd given up. I've made a few changes following input from you guys, particularly Jon. I now have a supply which will go to the full 15V and 3A!

[View:/resized-image/__size/620x513/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/7652.contentimage_5F00_204192.jpg:620:513]

Note the iMon current and U3/U4 temperatures: I did indeed blow those analog pins when I was probing but as far as I can tell the Linear Regulators are fine. Fortunately, the zener diodes (I think) protection on the 4Duino saved the chip.

The issue that I was having was caused by an LT and data sheet application note error in the schematic. They have tied the iLim pins of the linear regulators together with a 3k92 Ohm in parallel with a 5K pot, both then in series with a 100Ohm. The calculation they conveniently provide buried in the data sheet is 360mA / 1kOhm + 450Ohms = output current and even that isn't clear! That gives current = (2197 + 100 + 450) / 1000 * 0.36 = 0.989A per linear regulator. I'd used a 3K3 (1%) and a 5K pot (actually 4973Ohms) so I could achieve: (1984 + 100 + 450) / 1000 *.36 = 0.912A per Linear Regulator = 1.82A which, not coincidentally, was what I was achieving with the 5Ohm load. Clearly that's enough for 10Ohms which only needs 1.5A.

The annoying thing is that using LT Spice with LT components and an LT schematic (and my updated schematic) shows it working perfectly fine with the parallelised resistors! The lesson here is that you should still do the calculations if they are given, you can find them and understand them!

Doing the calculation with just the 5K pot would give a total of 3.996A. I desoldered the 3K3 and bingo. Sort of....rise time to nearly 3A (actually 2.9 eventually) was around a minute. In fact it never got there and was stuck at <15V and <3A (actual values following ohms law based on 5Ohm load) Not good - I have access to a very simple dummy load that was able to draw 3A out of it fairly easily - it wasn't stuck with 5Ohm resistance - so I knew it could do it.

I'd also looked at the feedback circuit with some more simulation - I mention this in a comment above in response to Jon. I can't yet change the Inductor or the Output cap but I have bodged in a 22nF cap across one of the 100K feedback resistors and re-routed the 1nF capacitor with a bit of wire (and cut the trace to its original destination). Firing it up and it shoots immediately to 15V, 3A! Yay. It still drops to 0V and 0A as well through the voltage and current controls.

Ok, so I think I've resolved the output functionality, leaving the noise. Here's some piccies. Firstly, ripple at the output terminal with the 5Ohm load. Pretty poor BUT I don't see the switching spikes.

[View:/resized-image/__size/620x372/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/0871.contentimage_5F00_204193.png:620:372]

Next, ripple at the output terminal with the 10Ohm load:

[View:/resized-image/__size/620x372/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/4113.contentimage_5F00_204194.png:620:372]

If you look carefully it is possible to see a bit of the switching noise just before the rise to a peak - I'd assume it is there on the 5Ohm waveform as well but the timebase is too large to notice it. Still poor ripple but the switching noise looks a lot more under control in my opinion (I'd be interested in other's opinion though.)

I also checked the voltage on the feedback pin of the Switching Regulator, Vfb. The whole point is to try and bring that under a bit more control. Prior to the bodges:

[View:/resized-image/__size/620x372/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/0804.contentimage_5F00_204195.png:620:372]

This is ranging from 460mV to 2.16V (10Ohm load). After the bodges with the 10Ohm load:

[View:/resized-image/__size/620x372/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/3036.contentimage_5F00_204196.png:620:372]

It's now ranging between 0.97V and 1.37V - firmly in the range for a 200kHz operation - I should say, I think it is still switching at 100kHz, I need to do more measurement. LTSpice (which I'm not sure whether I trust or not anymore) has this at 1.15 to 1.22V but I haven't modelled the output cap ESR or parasitics as per the actual build. Waveform is similar though. It gives me a bit more confidence that changing the Inductor and output cap for low ESR along with, say, a 20nF on the output, the Vfb would improve.

There's still the output ripple though. I need to find a way to squash that so this is what I intend to do:

Redesign the PCB layout, particularly parts position and the grounding.
Add in low ESR Input and Output caps for the Switching Regulator - I've found some that are 28mOhm.
Add in a 20nF cap to sit alongside the Output Cap
Provision optional pads for 2 more Output caps of 100uF (but could be other values)
Change the inductor to 50uH to help reduce output ripple
Test points for various pins so I don't blow it up anymore.
Use small pots for the feedback circuit so I can make adjustments. I think I'd ultimately want to swap these for resistors at some point so I shall design around that.

I'm hoping to make use of LT's samples provision to get replacement parts to save cost! I want to keep the existing board as-is for comparison purposes and I can't be sure the parts I have are not damaged in some way. Remember, the first thing I did was solder the Schottky in the wrong way and blew up the Mosfet and sense resistor - it could just as easily have damaged the switching regulator.

I have been reading about proper grounding, including a paper from Analog Devices (LT.) It's difficult: there are definitive recommendations for the use of a ground plane but just as definitive recommendations for star grounding (Gene, for example, is a definite fan of star grounding as he's commented above.) Analog Devices are very much in the ground plane camp. One thing I'm fairly sure of, and open to correction, is that copper fills on the top layer that are via'd to the ground plane and used as a grounding plane by the top layer components is not a good thing - I think that's a capacitor. So I intend to tie ground pins with a short track and a via. Is it worth, then, 'filling the gaps' with copper on the top layer?

Finally, I will re-layout the PCB but I have previously gone with 2-layer to keep the cost down. That inevitably means I have to place tracks on the bottom layer which, if I go with a ground plane, will cause breaks - you can see in the images earlier in the comments. Should I bite the bullet and go for a 4-layer board - I could keep the current carrying tracks on one layer and signal tracks on a different layer?

RE: YAPS Part Eleven - Further Testing

Andrew J — Thu, 25 Jul 2019 17:26:28 GMT

I've made the change to the ground connections as suggested - in two places actually:

[View:/resized-image/__size/503x900/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/6761.contentimage_5F00_204184.jpg:503:900]

This has improved things somewhat. First image, before change; second image after change:

[View:/resized-image/__size/620x372/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/2068.contentimage_5F00_204185.png:620:372][View:/resized-image/__size/620x372/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/1563.contentimage_5F00_204186.png:620:372]

Pk-pk has reduced a lot, although still not acceptable. The actual switching spike (at the point of the max rise) is measurable at 4.3mV and the waveform is more sinusoidal. I took the 4Duino and thermistors off the board and re-measured:

[View:/resized-image/__size/620x372/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/5706.contentimage_5F00_204187.png:620:372]

A little more difference in the pk-pk and a cleaner waveform.

I also measured the feedback voltage with 10Ohm load:

[View:/resized-image/__size/620x372/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/7824.contentimage_5F00_204188.png:620:372]

According to the data sheet, the LTC1624 should be able reach 200kHz with a feedback voltage of 0.75V; with a 0.696V minimum it should be getting close to 200kHz.

I also measured the voltage reaching each of the LT3081s and the output voltage from each:

[View:/resized-image/__size/196x113/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/7652.contentimage_5F00_204189.png:196:113]

Pretty even-stevens so I'm ok with that. What I'm not ok with though is the probe slipping and hooking 18V onto the TEMP pin which has either:

promptly blew the LT3081 and iMon analog pins on the 4Duino; or
damaged the LT3081s.

Temps are now reading over 145C and iMon is reading 2.9A (max possible through the calculation on the 4Duino.) Bum.

I did some measurements with the 5Ohm load as well:

Feedback voltage:

[View:/resized-image/__size/620x372/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/6266.contentimage_5F00_204190.png:620:372]

Here we have a minimum value of 0.460V - enough for 170kHz.

And voltages at the LT3081s - this time I didn't measure at the pin, just the ingoing voltage at the capacitor just before the track splits to feed each. I also measured the outputs again:

[View:/resized-image/__size/228x113/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/1300.contentimage_5F00_204191.png:228:113]

Again, even-stevens.

So where does that leave me. I can't be sure the LT3081s aren't damaged or if it's just the 4Duino analog pins, I'll have to check the latter but measurements of temp and iMon voltages on the board is very unstable and the temp readings for the Mosfet, Rectifier and Case are still working on the 4Duino. But I come back to the Switching Regulator circuit, specifically, feedback and layout.

With the 5Ohm load attached, the Vfb pin is seeing a peak of 2.16V and at 10Ohm it is 1.46V: it is rated for an absolute max of 2.7V to -0.3V so it's within range. The range given in electrical characteristics is quite tight actually, 1.1781V - 1.2019V, and is tested "...in a feedback loop which servos Vfb to the midpoint of the error amplifier (Vith = 1.8V) Measuring Vith with the 5Ohm load, it is giving a voltage of 1.18V to 2.14V (nominal range is 1.19V to 2.4V); with the 10Ohm load it is 1.6V to 2.34V.

The datasheet actually says: "...The main control loop is shutdown by pulling the Ith/Run pin below its 1.19V clamp voltage..." so it could be unstable for the 5 Ohm load, hence the low voltage.

I need to have a think.

RE: YAPS Part Eleven - Further Testing

jc2048 — Tue, 23 Jul 2019 12:17:25 GMT

Hello Andrew,

Having now looked at your schematic and their design article [I'm doing this all backwards!], here are a few quick observations.

In the context of their circuit (I found your schematic confusing), the feedback circuit looks reasonable. Do you understand, from your simulating, how that tracks the converter output so that it's just above the linear regulator outputs or would you like me to talk you through it? (Understanding it is useful because you could then [temporarily] modify it to give more headroom for experimenting with the circuit - always keeping in mind the dissipation of the linear regulators, of course.)

The converter you've substituted operates at a much lower switching frequency - theirs is 700kHz and yours is 200kHz. That will have an effect from the point of view of the ripple (higher) and the control side (the voltage loop control bandwidth is consequently much lower - so it will respond to changes at the Vf input much more slowly).

A second obvious difference is the reference voltage (yours is higher at 1.2V - theirs 0.97V). Curiously, because of the way this all operates, that isn't too much of a problem - it just gives a small amount more of headroom which, if anything, is probably helping you a little.

There's also an odd quirk (look at the graph called 'Frequency vs Feedback Voltage' in the datasheet) where it shows the frequency dropping when Vf is low. (May not matter, but it's the kind of thing to keep in the back of your mind, just in case.) It could be part of the explanation for why you're seeing the 100kHz switching frequency.

Loop stability, once you move away from recommended circuits and simple, resistive, potential-divider feedback, is difficult with a switcher because of a) the mix of analogue and PWM making analysis difficult anyway and b) they probably don't tell you all you need to know about what goes on inside the chip. It was easy for the LT engineers because they have to understand it intimately to design the chips in the first place. You may well be lucky and it may all be unconditionally stable. If not, you'll need to experiment with it. Keep it in the back of your mind as a possible cause of problems.

The capacitors, the one on the transistor emitter and the one you've got at the Vf input, look to me to have been intended as noise filters to reduce high frequency noise rather than modify the loop response.

The trickiest situation would seem, to me at least, to be when the output load changes. The linear regulators will counter that change fairly quickly but the poor old converter will just trudge along slowly behind and there's then a real risk the regulators run out of headroom as the capacitor on the output of the dc-dc converter discharges before it can be replenished. Perhaps try switching your load resistors with a logic-level MOSFET with the gate driven from an Arduino, or something like that, and see how the linear regulators and the dc-dc converter respond to the load going on and off. You should be able to work out what's going on from the scope traces if you probe around. [Perhaps, for experiemnting, you could make the headroom variable (with a pot), rather than the fixed 1.7V or whatever it currently is.]

As far as noise goes, you need to think differently about the switching ripple noise and the switching spike noise. The ripple comes from two things: indirectly, the coil current ramping up and down (a natural part of the converter operation) converted to a voltage by the ESR of the output capacitor(s) and, directly, the output capacitor(s) voltage as it charges and discharges. The linear regulators will remove much of that.

The spikes come from the very rapid changes at the switching node when the switching takes place and are difficult to deal with once you've generated them. The very fast edges will have frequency components that are radio frequency signals and they'll readily skip around through quite small parasitic or intrinsic capacitances. If you've got some clip-on ferrites, you might experiment with passing the output wires from your board (ground as well as the Vout) through one and see if that helps reduce it a bit. Keep the wires coming out away from those going in or you'll get some capacitive coupling across, reducing the effectiveness.

Quick observations he says, and then writes an essay! Hopefully there's something in that lot that helps.

RE: YAPS Part Eleven - Further Testing

shabaz — Sun, 21 Jul 2019 15:17:13 GMT

Hi Andrew,

One thing (which I can't see on the schematics in Part 2) is this diode:

[View:/resized-image/__size/620x373/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/1172.contentimage_5F00_204171.png:620:373]

It relates to some weird property where the set pin voltage must not exceed 10V difference to the output voltage (but can be lower).

[View:/resized-image/__size/620x164/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/2642.contentimage_5F00_204172.png:620:164]

It could be worth having a pair of diodes (back to back) to ensure that the voltage difference cannot exceed a low value.

I've not used this voltage regulator before, but I do recall a few years ago looking through the specs, and eventually deciding that any adjustable bench supply would be possibly better designed with the LT3089 (with more of them in parallel since it has less current capability) than any of the other LT parts in this LT30xx series. There were some reasons I thought that, but I can't recall them : (

I'll carry on looking through the design, in case anything else can be spotted.

RE: YAPS Part Eleven - Further Testing

Andrew J — Sun, 21 Jul 2019 12:53:23 GMT

I've been having a bit of a discussion with Jon Clift, and lately Shabaz, on this thread as a continuation of a comment made after I blew up my Mosfet - I want to bring it over to this post to keep things together.

In summary: I changed the Mosfet for one which is more in line with requirements of the LTC1624 Switching Regulator. I'm still seeing lots of switching noise on the output. The TG pin voltage and the SW pin voltage are in step with each other with the gate 5.6V higher than SW which is what it should be according to the datasheet. There was some querying on the feedback mechanism and some misunderstanding on my part of the Mosfet Gate-Source voltage (Vgs.)

Just to confirm my understanding of Mosfet specs, following clarification from Jon (bit of a 'of course' moment for me.) Vgs is the voltage between the Gate pin and the Source pin of the Mosfet. The absolute max is +-20V so I need to keep it in that range. I was (originally) linking it solely to the voltage at the gate from the LTC1625 which was swinging up to 30V-ish. As I was pulling this project together, I had asked a question about this and the answer was, to paraphrase a fair bit, that swings up to 30V didn't matter, look at the simulation reported average. I did grasp the following bit but hadn't related it so concretely: as the voltage on the Mosfet Gate rises it eventually reaches a threshold with the Source voltage - Vgs(th) - which turns the Mosfet on and it start to conduct. As the voltage on the Gate continues to rise, the Mosfet turns 'on more' and the voltage at Source rises - as long as the difference in voltage between that on the Gate and that on the Source is greater than the (minimum) threshold value, the Mosfet is on. The greater that voltage difference, the greater the voltage and current that passes through the Mosfet is. As the voltage difference hits +-20V, the Mosfet starts being damaged, ultimately failing.

Clearly, the important spec is Vgs, and everything else seems to flow from that. Anyway, the upshot of this is, I'm not supplying more than 20V Gate-Source difference and what I am supplying is enough for the voltage/current I require from the supply (15V 3A). I've measured it and compared it to graphs in the datasheet.

The Feedback Circuit

[View:/resized-image/__size/620x572/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/3666.contentimage_5F00_204169.png:620:572]

This is designed to keep the output voltage (at the inductor output) 1.7V above the load voltage (so, 16.7V with a 15V load.) This provides a minimum dropout for the Linear Regulators (2xLT3081) and helps prevent them overheating: so the Switching Regulator output follows the load requirement. I have taken measurements: with a 15.6V load, the voltage at the Inductor output is 17.3V. As a design, I'm picking up from comments that it looks a bit strange, and certainly isn't a feedback mechanism documented in the LTC1628 datasheet.

However, it is a feedback circuit created by LT themselves. In fact, a large chunk of the design of this supply has come from LT paper here. This is something LT have created and sell as a demo board, presumably for marketing purposes, so I have assumed it is a working design that does no show their components in a bad light.

If you compare their overall design with the design I have (for the whole power supply), you will see lots of similarities. Not by chance: I think I mentioned initially I was building on others' shoulders to gain experience and I didn't want to just copy it exactly (no fun or learning.) In particular, I have the same building blocks but I have used different LT parts for the Step Down Switching Regulator and Switched Inverter:

Switched Inverter (not in the above image): this takes a +5V input and turns it into a -5V output which then draws -8mA from the LT3081s (2 x -4mA) which allows them to regulate voltage downs to 0V. It's not enough to short output to ground, a small current must be pulled from them. I've used a different part, as the LT part is too hard to hand-solder. Functionally, this is working as I can get output down to 0V and 0A with the SET potentiometers and the measurements I've taken match those in the LTSpice simulation.
Step Down Switching Regulator: LT have used a part that is too hard to hand-solder hence the choice of part in the design I have. If you look at the LT schematic in that link (or PDF accessed at that link) you can see they have the same differential feedback circuit - that is not in the datasheet for their part, the LT8612 - which shows a simpler voltage divider mechanism. I have assumed that it is a workable approach: the big difference of course is in the Step Down part used and that isn't something that can be allowed for until it is tried!

In measuring the Step Down Regulator output (at the inductor) against the output at the terminals (load) I see:

[View:/resized-image/__size/199x97/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/5775.contentimage_5F00_204170.png:199:97]

As you can see, it drifts a bit at lower voltages but not in the wrong direction! As long as it stays above 1.5V difference, the LT3081s can do their job. An alternative would be to set the feedback up as a simple voltage divider but if there were no way to follow the actual output voltage, then the drop across the LT3081s would become pretty significant. At least, that is what I assume would happen based on reading LT's paper. It's worth noting that during the soak test I wasn't seeing a significant heating of either the LT3081s or the LTC1628.

What I suspect is that the LT8612 has much lower ripple on its output than the LTC1624, by which I mean switching noise, and I haven't accounted for that properly. It's not something that was apparent in the LTSpice model I created but I almost certainly haven't set that up to show it. The models for the LT parts used have come from LT themselves (naturally, as LT Spice is their product) so I assume they are accurate.

None of the parts I use are from anywhere except reputable suppliers: mostly Farnell and CPC, with Mouser, Digikey and RS for those that Farnell or CPC don't stock or do so in quantities I don't need. I never buy any parts from Amazon, eBay, Aliexpress, Banggood etc as I have zero trust in them.

Hopefully, that has provided a bit more useful information for anyone following along. Putting aside the issue with a 5V static load for the moment (a colleague at the museum is going to bring a DC load to try with to confirm), the thing killing me at the moment is that switching noise. I'm busy researching that to see what I can do about it but I think my lack of experience is telling here. At least I know what to be considering in future projects [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/0027.contentimage_5F00_1.png:16:16].

One thought I did have looking again at the PCB ground plane in the comment above, is I'm wondering if the switch noise is being coupled onto the ground plane and causing issues across the whole circuit. So a thought to bodge low uF/nF capacitors in parallel at the inductor would be pointless. Perhaps I should have kept the ground plane separate for the Switching Regulator. Any thoughts on that or ideas for testing that theory?

RE: YAPS Part Eleven - Further Testing

shabaz — Sun, 21 Jul 2019 12:32:22 GMT

Hi Andrew,

That noise at 183kHz is not as bad as it appears, the persistence view of the scope shows it shifted, so really it may be about 60mVp-p instead of 110mVp-p. The 100Hz output is unusual, what is the Vin voltage on the DC-DC converter chip, is it higher than the output, by more than the dropout voltage? Is there such 100Hz ripple on the Vin?

RE: YAPS Part Eleven - Further Testing

Andrew J — Sat, 20 Jul 2019 17:03:32 GMT

Can someone confirm that I've made a rookie mistake putting my PCB together:

[View:/resized-image/__size/341x470/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/contentimage_5F00_204166.png:341:470][View:/resized-image/__size/432x488/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-935fd969-01c7-4e01-b71c-b2e6608dead6/contentimage_5F00_204167.png:432:488]

Note the path from the inductor, through the electrolytic capacitor, off to the linear regulators (REGout2) in the schematic. Note the paths in the PCB - REGin disappearing off the right side is off to the linear regulators.

RE: YAPS Part Eleven - Further Testing

genebren — Thu, 18 Jul 2019 21:41:45 GMT

I have enjoyed following along on the development of your project. While things look a little bleak right now, keep in mind how much you have achieved and don't lose confidence yet. Think on the problems, go back to books and keep trying. Any good project has a lot of potential issues and challenges so consider this as a great opportunity to learn and grow.

I will revisit your schematics and see if I have any suggestions.

Good luck!

Gene

RE: YAPS Part Eleven - Further Testing

three-phase — Thu, 18 Jul 2019 21:14:40 GMT

Some interesting test results you have shared with us. Some of my thoughts.....

Did you measure what the voltage was at the rectifier with the 5Ohm resistor attached, are you dropping volts there that are limiting the output? Unless you measured the 5Ohms with a 4 wire test setup, I wouldn't worry too much about the value you got.

When measuring over / undershoot, I usually set the scope trigger to single shot with a fast timebase, so it only captures the initial event.

When measuring the noise, was the bandwidth scope limited? Usual practice is to limit it to 20MHz. Your setup is also very open, with fairly long leads that will pickup all kinds of noise from all directions.

I am sure others will come along with some much better observations than I have made. Good luck with your fault finding and thanks for sharing.

Kind regards.