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.

Thanks for the feedback Jon, that's really useful and appreciated - and essays are good, sorry for responding with one of my own.

I've been mulling over a few things since Sunday:

• The feedback circuit: I’m no stick in the mud - if a common refrain is “something up with the feedback?” from you and Shabaz (and a couple of other people who haven’t even seen the circuit) I think that perhaps there is indeed “something up with the feedback”!
• The ground plane: I think I’ve got this hideously wrong and I’m coupling noise from the switching circuit into the rest of the control circuit.  This is just something I was thinking about and have been researching.

I thought I understood what the feedback circuit is doing but I've been mulling over the data sheets again and in trying to articulate it here, it doesn't add up in my head - I can't see how it operates to reach the 1.7v difference, sorry.  Essentially, it is 'programming' a different output voltage for the LTC1624 as the load voltage changes.  I had thought the two 100K resistors were providing a voltage to the PNP transistor approximately half way between the output voltage from the inductor (switching circuit) and the load voltage.  The 1K resistor is reducing the voltage from the inductor to just enough to turn the PNP on and allow enough of a current to flow over the 5K resistor to tune the voltage to the feedback pin.  That is being internally referenced to balance the output of the LTC1624 such that it adjusts the current to the Inductor so it remains in balance with the output current.  That naturally adjusts the voltage to allow for that balance to occur.  I guess I took it for granted because looking at the simulation in more detail, I can't work the maths out,

If I look at the ground plane I have - first image is bottom layer with silk screen; second is the top layer - I kept the power ground separate (as the datasheet said) but I'm thinking that the way I've laid this out is contributing to the problem.  You can see where the net tie is (circled), so path to ground for the switching circuitry is underneath the rest of the control circuitry but is also mixed on the top layer as well.

I'm now thinking I should have done the following (replicated on the top layer), and tying in multiple places:

Next Monday, I'm meeting up with a colleague who is bringing along a DC load so we can see more of what is going on and what happens as the load varies.  It always struck me that to build a power supply, one could do with a DC load, but to build a DC load, one needs a power supply - I didn't want to buy either!

A few other thoughts I've been mulling over the last couple of days:

• is the voltage to the Linear Regulators imbalanced such that one isn’t able to provide enough current - i.e. both equally up to 1.5A each?  Clearly getting the trace from the Inductor output to reach the Vin pin of each LT3081 over a precise, equal distance is difficult (I’m thinking of this in the same was as provisioning power in a star layout.)  If that was the case, it could be acting as a current limiter such that there is enough headroom for 15V/10Ohm but not enough for 15V/5Ohm so the voltage is drooping. 
• In soldering the Schottky incorrectly, blowing up the Mosfet and sense resistor was a primary result but perhaps it’s damaged the LTC1624 as well.  I’m not really convincing myself on that though.  From what you have written here, the likely thing is that the feedback circuit isn’t operating fast enough - it’s as subject to the ripple out of the inductor as the rest of the circuit, although simulation does show its minimum level is > 1V.  As a rhetorical question, is there a simple resistor divider feedback that can track the output?  The key thing is to either (a) set it up to provide the 15V full time and let it drop over the Linear Regulators and deal with the power dissipation; or (b) find a mechanism to have the LTC1624 output  follow the load output.
• Along with the ground plane, have I laid out the switching circuit properly?  The datasheet use nice relative terms such as ‘close’ but what does that mean in practice?  It’s not helpful that it relates this to both pins on diagonally opposite points of the chip - if you look at the sizes of D1, L1 Q1, C10 and R2 all which need to be 'close' to pins 4 and 8 of U1!
• Given what you say, perhaps there is a faster part available.
• 50uH Inductor and a 0.033Ohm sense resistor: from the LTC1624 datasheet, Rsense value is chosen based on Imax - it’s not clear from the wording what they actually mean by Imax (the grammar is confusing to me), but I’ve taken it to mean the maximum output current I want, i.e. 3A.  That would give a sense resistor of 100mV/3 = 0.033Ohms; the choice of inductor is based on ‘ripple current’: lower inductor value, higher ripple.  So in my calculation a 0.033 Ohm sense resistor with 50uH seemed like the way to go but resulted in a LT Spice simulation output that was unstable compared to using a 0.010Ohm and 10uH.   I assumed there was something going on with the inductor that I didn’t understand - and given they were values used in the LT application notes that’s what I went with. 

What I will do is take a few more measurements around the feedback circuit and the linear regulators.  In particular see if the feedback is dropping too low and adjusting the frequency - although if the feedback voltage is too low, wouldn't that affect the output voltage - actually, potentially what is happening with the 5 Ohm resistive load?  One of the issues I'm finding is that probing the circuit can actually have quite an impact on the what is measured.  And trying to probe that LTC1624 chip is proving difficult with a spring mounted ground pin on the probe that has a tendency to 'ping' around!  I've already blown the sense resistor.

I'd appreciate if you could clarify the feedback circuit in some more detail.  I'll post up more results as I get them and decide what to do.

It would be a shame if this just can't work, especially as it works great in LT Spice (real live vs simulation!)  If I could get a bit more confidence it could work, it might be worth taking another look at the PCB, adding some test points, and rebuilding it up.

I am sure you will get it working soon. There is some phenomenal help available to you within this forum. Everything that is designed from scratch will have a few mishaps as they are developed, and a lot on here will have gone through that.

I would prioritise the problems you have in to ones that need to be done to get it functioning, and the nice to do to improve its performance. Concentrate on the ones to get the power supply working and then move on to the improvements.

Kind regards.

Hi Andrew,

Regarding the comment

"output voltage on the current control SET pin but it immediately drops output voltage by 3v so it’s not possible?"

is this the ILIM pin you're probing?  It should have no impact on the output voltage so this is concerning : ( To me it sounds like a faulty IC if it is doing that : (

The circuit under test is isolated from ground, right (i.e. you're using a transformer supply with no connection of the output 0V to ground), so the 'scope ground connection should not have any impact.

According to the datasheet chart (although the wording in the paragraphs was vague.. they ought to get the right people to assist datasheet creation) and the PDF here: https://www.analog.com/media/en/technical-documentation/tech-articles/lt-journal-article/LTJournal-V24N2-02-df-BenchSupp…

the chart on one of the pages shows that there is foldback behaviour, so that the output should go to 0V if overloaded, so a lesser drop like 3V is not part of the behaviour : (

Once you have the load (or if you've got some resistors to test this with) it could be worth checking what resistance the potentiometer is set to, and then seeing if the current limit feature does drop the voltage to 0V or not, once that load is exceeded.

Sorry, I was just giving an example, I'll be more specific.  On the 3092 Current Source there is a Set pin.  Trying to measure the voltage on that pin with the DMM drops the load output by 3V - which I can see on the DMM and on the 4Duino.  The circuit is isolated from ground with no connection of the 0V to ground.  I can't say I've noticed it when scoping the SW or TG pins on the LTC1624.  The question was more general really, but in retrospect I wish I hadn't asked it as I'll just use the scope in future which I assume won't cause that type of issue.

Oh, I see. You're right, that may happen with the DMM. The output current there is very low, so maybe if the internal resistance of the DMM is low, some of that current is diverted, so it could affect the output voltage.

The help on here is awesome and I can't thank everyone enough.

The help on here is awesome and I can't thank everyone enough.