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YAPS Part Eleven - Further Testing

Andrew J

EDITS: 21/07/19 - added a PDF of the schematic

            11/08/19 - updated navigation links

 

Introduction

My intention was to document, in some detail, testing of the power supply with both a 10 Ohm and 5 Ohm load (to drive the current from 0A to 3A.)  To summarise, I’m seeing a number of problems - see later - and I thought I’d document where I’ve got to as I need to do some thinking about this.  See my conclusion at the end.  TL;DR; It isn't working as well as I hoped!

 

I’ve broken the testing down as measured testing and Noise and Ripple testing.  If you can see problems with how I’ve done this testing, or ways of improving it, or indeed other tests you’d like to see, please let me know so I can improve or report additional results.

 

For the measured testing, I’ve captured data for the following tests:

  • Voltage and current comparison test
  • Steady state operation/voltage stability
  • Rise Time
  • Overshoot/Undershoot when turning on the mains switch; turning off/on load switch; lowering voltage level down to 0V; raising voltage level up to 15V
  • Soak test over 1 hour to measure temperatures and voltage stability

 

My scope was warmed up for 30 minutes before taking any measurements.

 

Environment and Setup

By necessity, the following equipment is turned on and plugged in to the same power strip as the scope and power supply: FTTP Modem, Router.  Also operating, but plugged in to a different power strip: iMac, AOC monitor, USB hard drive.

 

The probe was used with a short ground connector

image

 

Images on other blog posts show the setup and layout of the prototype board.

 

10 Ohm Load

The load is a 10.08 Ohm, 100W resistor, connected through its solder lugs (although not soldered.)  For all these tests, using the scope, the probe was set at 10x, full bandwidth, and a timebase that allowed for 1GSa/s where possible, measuring DC, and connected across the control stage output terminals.

 

Voltage and Current measurement

Essentially, this is a comparison test of DMM measurement vs INA260 vs 4Duino.  I took readings at various voltage settings, reported in the table below.

  • DMM voltage was measured at the Control Stage output terminal
  • DMM current was measured in series between Control Stage output and load
  • INA260 voltage was measured at Vin to the chip
  • INA260 current was calculated using ohms law from the measured voltage at INA260 Vin.
  • 4Duino voltage and current as reported on the screen
  • 10.08 Ohm load

 

image

The Extech, measuring current, has a resolution to 2DP (i.e. 10mV).  These results seem pretty good to me, particularly current.

 

The table following shows measurements with NO load attached (voltage only of course:

image

Again, very good.

 

Steady State Measurement

These images were captured off the scope

15V - 3.02V / division

image

10V - 2V / division

image

5V - 1V / division

image

0.5V - 100mV / division

image

As you can see voltage is steady but the noise is more apparent with the scope set in the 100mV division range.  More on noise in later tests.

 

Rise Time

3.02V/div.  Normal trigger set at 181mV.  Timebase at 5mS.

image

LTSpice simulation rise time:

image

The actual rise time to the set voltage is 12.7ms actual vs 2.1ms simulated.  There are similarities in the waveform but it's not clear why the rise time is so slow - my suspicion falls on the MOSFET, see Conclusions.

 

Overshoot

The scope timebase was set at 100uS and sample rate at 1GSa/s.

Voltage control on the supply was set so 4duino was reporting 15v;  Trigger set to 15.46V, rising edge, normal mode.

 

Mains Switch test: mains turned off then back on.  No triggering occurred.  Trigger set to 15.4V and test repeated.  Waveform displayed - noise is triggering the waveform.

Load Switch test: mains is on, load switch turned off then back on.  No triggering occurred.  Trigger set to 15.4V and test repeated.  Waveform displayed - noise is triggering the waveform.

Rising Voltage test: voltage control turned to 0V and waveform confirmed on the scope.  Trigger set back to 15.46V, normal.  Voltage control rapidly turned to increase volts.  No waveform triggered before measured volts reached 15V.  No overshoot observed.  Reset voltage control back to 0V and trigger to 15.4V and repeated.  Waveform was displayed at a reported 15V (4Duino).  Again, noise is triggering the waveform.

 

There doesn't appear to be any actual overshoot occurring - no high spiking - but noise is making it difficult to accurately assess if there’s a low-mV overshoot.

 

Undershoot

The scope timebase was set at 100uS and sample rate at 1GSa/s.

Voltage control set so scope was seeing 15V.  Trigger set to -0.6V, falling edge, normal mode.

Mains Switch test: with voltage control set at 15V the mains switch was turned off.  Waveform displayed.

Load Switch test: with voltage control set at 15V and mains switch turned on, the load switch was turned off.  Waveform displayed

Lowering Voltage test: Voltage control rapidly turned from 15v to decrease volts.  No waveform triggered until around 0.24V - again, this seems to be the result of noise (and is in line with the triggering on the overshoot.)

 

I’m not convinced I’m measuring these undershoot tests correctly, when operating the switches, as whatever position I set the trigger to (e.g. -60mV, -340mV) a waveform is displayed at that value when the switch is activated, but there is no spiking, just a steady voltage line.  Lack of experience with scopes I think, but thoughts would be appreciated?

 

Soak Test

This didn’t involve the scope, just readings from the 4Duino - I'm particularly interested in reported temperatures.  Power supply was left off to cool to ambient and then turned on for 1 hour at 15.5V and measurements recorded.  At the end of the hour, I used a thermocouple on a multimeter to cross-check recorded temperatures and sample other components on the Control Stage PCB.  Other components’ are the Schottky Rectifiers, 5V regulator and LT3092 current control.

 

image

Other components measuring between 40c and 45c.

These results seem pretty reasonable to me - no components seemed to overheat anywhere near their limits.  Voltage had drifted down by 50mV from its initial peak setting - I’m wondering if that is due to the heating of the load resistor which is quite warm given it’s dissipating over 23W of power.

 

5 Ohm Load

The load is actually 5.08 Ohm, 100W resistor connected through solder lugs (although not soldered).   I had intended to undertake the same testing as for the 10 Ohm load but the supply will only provide for 8.5V / 1.6A (approx) rising slowly over 6 minutes to 8.87V / 1.7A, and still rising.  Something clearly isn’t right so there doesn't seem much point until I investigate further.  I tested with a 1000 Ohm resistor (I only have 0.25W unfortunately) and that was ok.  I don’t have another power resistor to try with.  Not only that, I have noticed that touching the -ve line from the current control potentiometer drops about 2V on the output - see the video

 

No other leads do this and I have tested continuity (power off) and it’s solid.  I’ve even replaced the connector.   If I connect the 10 Ohm load and try this experiment, it has no affect; ditto no affect if no load is attached.

As a thought, I’m wondering if the bad output is the result of current limiting from the circuit? I can drop it down from whatever the current reading is to 0A with the potentiometer.  Could the 5 Ohm resistor be faulty?  It’s a 1% resistor measuring 5.08 Ohms so is slightly out of spec but I put this down to the DMM?

 

Noise and Ripple Testing

Noise and ripple are poor which I need to sort out.  For the sake of documenting the testing, I’ve included images below to show what I mean.

Baseline

I took some measurements with the power supply off to get a baseline level.  Same timebase and voltage division; bandwidth limited; 1x probe, AC coupled.

 

Background noise, no probe connected to the scope, power supply off:

image

Background noise, probe held, power supply off:

image

Background noise, probe on ESD mat (connected to ground), power supply off:

image

Background noise, probe held away from power supply, power supply on:

image

Background noise, probe held by power supply, power supply on (good heavens!):

image

Supply Measurements

Supply on at 15v.  Timebase and/or voltage division selected to show noise; bandwidth limited; 1x probe, AC coupled.

 

10 Ohm Load attached, powered through RCD (1st image), no RCD (2nd Image):

imageimage

No load attached, powered through RCD (1st image), no RCD (2nd Image):

imageimage

And here's a couple with the timebase changed, both without RCD.  With load (1st image) and no load (2nd image):

imageimage

 

I think it's fair to say these are NOT good results and I need to do something about it.  Again, I suspect the MOSFET.

 

Current Position

As I noted at the start, I decided to document the testing progress so far as there are clearly issues and I guess part of this whole project was to document how it came together, good and bad!  The 5 Ohm load results I’m not sure what to make of or how to proceed - it clearly doesn’t like such a low resistive load but I don’t have any other way I can think of of testing it.  I can’t understand what is up with the current control connection either - why would touching it with such a low load affect the voltage?

 

I’m wondering if the Mosfet is causing some of the issues.  In this thread, Jon Clift points out it may well cause problems and it is definitely out of conformance to the requirement of the LTC1624 which requires a maximum 2000pF of input capacitance on the connected Mosfet (the one I have is 3247pF so way over.)  I’ve hunted down another one, IRL8721PBF, but it’s not easy!  There are 10s of thousands of the things and the parameteric search on Farnell/CPC is hopeless for purpose.  I’m also not sure what the important specs to look for are but I have read around and here’s a comparison of what I think the important ones are:

image

The rise/fall/delay times seem to be heavily dependent upon the specific circuit they were tested with but I quote them for comparison purposes.  The Total Gate Charge seems to be a better indicator of switching time and the IRL is significantly better.  The input capacitance is well within requirements as well.

 

My approach now is to:

  • replace the MOSFET with the IRLB8721.
  • replace the power switch with an filtered module as per Michael Kellett’s suggestion in this thread.
  • bodge in a 0.4mH Common Mode Choke between the Power stage output and Control Stage input.
  • test voltages at various points of the circuit to compare with what the LTSpice simulation is reporting.  It may give me a clue as to where issues lie - perhaps I’ve mis-soldered a resistor or capacitor (shame these can’t be tested in-circuit.)

 

Why a filtered power module and Choke?  The frequency I was seeing the noise at seems to range from 60kHz to 130kHz and the datasheet indicates this would attenuate; I’m wondering if some noise is coming in through the power lead; I’m also wondering if this thing switching is potentially introducing noise on other equipment which I need to deal with.  I may have grasped the wrong end of the stick here completely though

 

If anyone has any other suggestions then I’m really open to hearing about them.  I’ve come a long way and I feel so close - It’s a bit disheartening if I’m honest: it all seemed to be going so well!

 

Follow on results I’ll post as a comment.

 

Next: Part Twelve - Design revisited: reworking the layout and PCB

Back: Part Ten - The Control Stage and Initial Functional Testing

Attachments:
imageSchematic.pdf

  • Hi Andrew,

     

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

    image

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

    image

     

    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.

  • in reply to Andrew J

    Hi Andrew,

     

    Please ignore my comment, I wasn't thinking straight : ( I meant the linear regulator, I was just wondering if there was any dropout issue, but since it is at 1.7V higher than the desired voltage, that's good.

    Sorry to hear about the shorting issue : (

    I'll re-read through your blog posts and comments, to hopefully understand the design a bit more.

  • in reply to shabaz

    Not sure what you mean by the DC-DC converter chip?  Do you mean the Step Down Switching Regulator?  If yes, then it is at Vin to the supply, 24.5V and doesn't change so yes, much higher than the output voltage.  If you mean the Linear Regulators, then their Vin is 1.7V above the output (so 16.7V for a 15V output) - that's set by the feedback circuit (see my lengthy comment below) - and remains 1.7V above the output and ok for those chips.

     

    When you ask about the 100Hz ripple on Vin, do you mean Vin to the supply (i.e. at J2 in the pcb image above) or at one of the chips?

     

    I may struggle to answer this question except from memory: I've just managed to blow something - the MOSFET, sense resistor or something - trying to scope on the Mosfet's Gate pin.  Probe slipped: I should have put some test points in to the circuit.  image

  • 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

    image

    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:

    image

    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 image.

     

    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?

  • 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?