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First post on here and not very experienced so let me know if there's anything wrong with my post as well as my design.

I am trying to layout a boost converter using the PAM2421AECADJR IC I bought as a prototype, but I'm having issues with the output dropping under any significant load.

Vin = 3.3 V, expected Vout = 12 V. I based the design on the recommended one in the datasheet

Layout Schematic full size

It seems to work fine with a 100 kΩ load, but drops to 6 V with a 10 kΩ load and quickly drops to ~3 V with anything below 10 kΩ.

Parts List:

This is the prototype layout:

Prototype

And this is the switching node voltage with a 100 kΩ load:

100K load SW Node

And with a 100 Ω load:

100 Ohm load SW Node

The inductor is one of the B78108E1103K000s which on paper seems like it should be good enough for this circuit (saturation current 2 A, 136 mΩ DCR).

I've tried to follow as much advice as I could find in terms of layout.

  • switch loop as small as possible
  • separate the PGND and AGND as much as possible (but connect at a single point)

Not sure what to do from here to get it to work, so hopefully someone can give me some suggestions.

Just for clarification, in the reference schematic, the inductor is 6.8uH (not 68), it's a little hard to see. And the actual value used in the breadboard is 10uH (as that's the closest I had)

UPDATE

I managed to get the circuit working, no change to the schematic or components used, just by swapping to a protoboard and putting a massive blob of solder and bare wire together to make a large ground plane. After doing that, the output voltage was stable with both the 100 kΩ and 100 Ω loads as well as a 12V audio amplifier with good results

protoboard frontprotoboard back

switch node voltage

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    \$\begingroup\$ Don't try to be cute with the GND, it should be a bus-bar that will crowbar anything with sparks if there's a need to do so. In some cases you want to consider your return currents e.g. sensitive analogue electronics but that's not the case here. From your results there's something wrong with the circuit, your 10uH "inductor" does not look like an inductor at all, it looks like a fat resistor. I could be wrong but that's what caught my eye right away. Where's your 68uH inductor? Is 10uH supposed to be 10uF cap and you used 68uF elcap as your 68uH inductor? \$\endgroup\$
    – Barleyman
    Dec 31, 2021 at 4:42
  • \$\begingroup\$ @Barleyman The 10uH is the substitute for the 6.8uH (sorry, the . might not be that readable I guess) inductor suggested by the datasheet (and it is an inductor, part number B78108E1103K000) \$\endgroup\$
    – failsafe
    Dec 31, 2021 at 4:54
  • \$\begingroup\$ Ok, but as @MarkU says in his answer, this is not a power inductor, it's for filtering noise. Your inductor IS ferrite-core but please do look for something that says SMPS or DC-DC. \$\endgroup\$
    – Barleyman
    Dec 31, 2021 at 5:01

5 Answers 5

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On top of what's been discussed in comments, here's my take on this.

First, based on the datasheet your 10uH inductor is a noise filter part, not a SMPS "power" inductor. Please find something that says "power" "DC-DC" or something to that effect. These are often SMPS and/or relative chunky through-hole parts Your SMPS is something like 500kHz so the inductor SFR must be much higher than that, usually it's like 10MHz and above.

Second, what are your electrolytics like? 470uF tells us nothing basically. For SMPS duty you need LowZ, Low ESR, Low Impedance type electrolytic, which all mean basically the same thing. Some kind of generic mystery Elcap is not going to get the job done. Usually for SMPS duty you choose elcap based on ripple current rating, not so much for ESR or capacitance per se, but this depends on power levels, for low voltages, high power density, high frequency you might want ceramics or polymerics. Don't confuse your design with this for now, file it for future reference. Elcap is fine for generic fiddling.

Third, what's your 3.3V supply like? Don't forget the input current is ideally the output current multiplied by the ratio of input and output and in practise there's a harsh current spike on boost circuits. Your 470uF input cap should take care of it if (a) you are not trying to be clever with grounds (b) no long wires (c) you have low impedance elcap.

Fourth, consider using a proper solder breadboard. I know it's more work than this kind of plug-in-wire board but you can actually create perfectly decent connections on a breadboard. And as a bonus, you learn some useful soldering skillz. Something along these lines: Vero Board

The Vero board is one style with the entire row (or column) is shorted so you will need a scalpel (small box cutter works) to cut tracks manually but as an upside you get PCB-like copper connections. The other type is with every pin being isolated which requires you to create solder bridges everywhere.. Just look for protoboard on google.

Hint, get proper soldering iron that comes with a power supply unit that has a temperature control, even if it's a cheap one. If you're trying to make solder bridges, set the temperature too low, inadequate temperature tends to make solder follow the iron. Also, get a solder wick and a flux pen. Solder wick has bit of a gotcha for uninitiated, it works best where wetted with solder so don't try to get it to suck solder as-is, solder some on it as a starter and go from there. Ideally it'll look like a fat wire of solder afterwards.

Well as far practical soldering goes the kit still needs tweezers, wire cutters and one of those crappy PCB holder/cum rubbish magnifier glass things. Presumably you already have access to many of those items. Ideally a decent magnifying lamp thingy, in a pinch that fold-up magnifying glass thing (you want 5x or more, avoid plastic lenses and small lenses!) you'll use that fold up magnifying glass your entire career either way. Ebay is perfectly valid source for these and they cost little but remember caveats.Electronics pocket magnifying glass

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  • \$\begingroup\$ Thanks a lot for all that info. I definitely going to try on protoboard next and see what happens. I'll try and answer all you questions, not sure the best place, so I'll leave a comment for each \$\endgroup\$
    – failsafe
    Jan 1, 2022 at 3:10
  • \$\begingroup\$ 1. The inductor datasheet seems to hit all the relevant specs as far as I can tell. It mentions DC-DC converter as an application, has a Isat of 2A and a minimum resonance frequency of 20MHz. Am I missing something? Or is this a suitable inductor for my application (despite it's small appearance)? \$\endgroup\$
    – failsafe
    Jan 1, 2022 at 3:12
  • \$\begingroup\$ 2. As for the caps. The 470uF is (KSY477M025S1A5H12K). The impedance is ~60-70mOhm. I think the one thing that seems off about it is the ripple current rating, which is only stated for frequencies up to 100KHz, and that seems to be listed as ~1A at 100KHz if I'm reading it correctly, but I don't know what any of that means yet. \$\endgroup\$
    – failsafe
    Jan 1, 2022 at 3:21
  • \$\begingroup\$ 3. 3.3 V supply is currently a bench supply that can handle up to 4A@12V so it shouldn't be my problem I wouldn't think \$\endgroup\$
    – failsafe
    Jan 1, 2022 at 3:23
  • \$\begingroup\$ 4. Going to give the solder breadboard a try and will report back how I go \$\endgroup\$
    – failsafe
    Jan 1, 2022 at 3:24
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  1. Solderless breadboard just won't work for switch-mode power supplies, there is too much parasitic inductance and capacitance. This absolutely must be laid out on a PCB. Check if the manufacturer has a reference design / evaluation kit / application notes. As an applications engineer (20+years at Maxim), this is the most common problem customers have trying to build boost or buck regulators.

  2. That 10uH wire-wound air-core choke is not the right type of inductor. Its LI^2 energy storage is not enough, and its internal resistance is too high. You need a ferrite or iron core inductor made for switch-mode power supplies, such as TDK CD75 series. Usually these are surface-mount because of the high currents.

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  • \$\begingroup\$ The end goal is to get it on a pcb, but I wanted to make sure I had the design right before I put a layout together and ordered it. So would you say that if I laid it out on a pcb and used a ferrite/iron core inductor it would have a decent chance of working, given it currently works for very light loads? Also, how would tell the inductors energy storage is not enough, if all the numbers on the datasheet seem to indicate it is? \$\endgroup\$
    – failsafe
    Dec 31, 2021 at 4:48
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    \$\begingroup\$ You cannot build a modern (fast switching) DCDC on a breadboard like that. Specifically you need the input decoupling caps (the 1 uF) to be within mm of the IC. \$\endgroup\$
    – jp314
    Dec 31, 2021 at 4:52
  • \$\begingroup\$ That energy storage thing is something I ran into years ago but it was by far more aggressive conversion ratio, I designed 12V to 450VDC boost and found many ferrite/iron core inductors insufficient, you have to dig into it once you go out of your comfort zone. Even if the said boost had very little load. You can do SMPS on proto-board but he's got this idea of "AGND" and "PGND", which is leading him wrong right away. \$\endgroup\$
    – Barleyman
    Dec 31, 2021 at 4:52
  • \$\begingroup\$ @jp314 Nah. But you do want to have 4-layer PCB with dedicated power and ground floods. Replace "fast" 1MHz SMPS with 1GHz RF stuff and we'll start talking about relative merits of a millimetre. \$\endgroup\$
    – Barleyman
    Dec 31, 2021 at 4:56
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    \$\begingroup\$ @Barleyman - An SMPS that switches with ns switching times is effectively a GHz RF circuit -- and is high power for RF. \$\endgroup\$
    – jp314
    Dec 31, 2021 at 4:59
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Designing this circuit on a breadboard takes way more engineering effort than laying it out on a PCB. This is a 500kHz switcher with switching currents on the order of amperes.

The primary problem I see is that “10uH” is a sufficient description for an inductor in this application. It’s not. 10uH by itself is maybe fine for high impedance signal chains like audio filters. Once you’re beyond that, you need a few more specs, and the most important one is the saturation current, or the maximum rated DC current, whichever is the lower of the two.

At the minimum, we’d need to see your actual parts list, and then we could comment on what’s wrong and why.

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  • \$\begingroup\$ Thanks. I'm getting the feeling that even if there is something inherently wrong with what I've done, it'll be easier to tell if I don't do it on a breadboard. Going to try with a protoboard instead to see if it clears things up a bit. I'll also update the question with a parts list (need some time to get it together) \$\endgroup\$
    – failsafe
    Jan 1, 2022 at 2:59
  • \$\begingroup\$ I've updated the question with a full parts list \$\endgroup\$
    – failsafe
    Jan 1, 2022 at 4:01
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That's a tiny output current causing it to droop. I've built very similar circuits (using a different switcher chip) on a solderless breadboard without problems. You have 2 wires connecting the grounds, so that isn't the problem.

You aren't asking for an amp out at 12V, you are asking for 1.2mA and it's failing. Recheck your wiring.

Also, have you checked your input voltage? Is it stable? You don't have current limiting set on your 3.3V supply, right? You need more input current than the output current you are asking for. For a 10mA output at 12V, for example, you'll need something like 60mA input at 3.3V. Check the input with your oscilloscope to make sure it isn't drooping.

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  • \$\begingroup\$ Thanks @bob_monsen. The input seemed stable when I checked it and I don't have the current limit turned on (but regardless I had it set at 1.5A) so that shouldn't be an issue either. Working on trying a protoboard instead of the solderless breadboard to see if it helps as I was getting a bunch of significant noise on the FB pin and the voltage at that pin was not working as expected \$\endgroup\$
    – failsafe
    Jan 1, 2022 at 10:06
  • \$\begingroup\$ By the way, the schematic at the top of the datasheet you've provided indicates it requires 3.6V minimum input. I'm not really sure if this could be the problem. The chip itself specifies it'll go down to 2.7V. Try upping the input voltage just to see. \$\endgroup\$
    – bob_monsen
    Jan 3, 2022 at 21:51
  • \$\begingroup\$ That's true. That could have something to do with the schematic (which comes from the datasheet, and references Vin as 3.6-5.5) being used to provide 800mA@12V and the max current rating for the IC not being able to provide that below 3.6? Just a guess, but given I'm not trying to pull 800mA@12V (looking for ~200mA@12V) I'd think I'd be able to use Vin at 3.3. I've also tried using a Vin at 3.7 and 5.3 to confirm that wasn't the problem and it has the same problems. \$\endgroup\$
    – failsafe
    Jan 4, 2022 at 4:39
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The #1 problem I see is the ratio of the Self Resonant Frequency, SRF, to switching frequency fs, is to low. Then when the FET output capacitance Coss is added, it reduces the ratio with a new SRF = 2MHz and fs appears to be 1/(4.6x400ns)=543 kHz or approx 4.3:1 instead of >20:1. You can see 4.3 cycles between pulses with No load (100k). This means the inductor cannot store the energy between pulses and just oscillates between pulses.

You have 2 considerations:

1: pick a better inductor with a 30 MHz SRF , instead of 6.5 but that would need to be a smaller L value than 68 uH as I only see 8MHz in 2A chokes in D-K's lineup, so it would have to be much larger than necessary (bigger winding gaps) and costly.

2: choose a much smaller L by a factor of 5 like 10 to 12 uH

So you have only the second choice as I see it.

This creates a new problem with faster risetimes and more crosstalk making parasitic jumper ESL and raises the Q if the DCR is also 5x lower. That makes the PCB layout more critical.

You know what happens when you measure a square wave on a 10:1 probe with a long ground lead. it rings at 20MHz due to the <10 nH ESL of a ground lead and the probe coax capacitance. So traces must be fat and short as this w/l ratio controls the inductance (logarithmically) from 1.5 to about 0.3 nH/mm (off the top of my head) for most PSU traces. Then you can compute the SRF from this and the Q = Z(f)/(DCR+RdsOn) for the reactance/resistance ratio.

This is why there is a tradeoff between ripple, resonance and efficiency.

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  • \$\begingroup\$ Interesting. Thanks for the extra ideas. To clarify, the inductance used in the design is 10uH and in the reference schematic it's 6.8uH (not 68uH, I know it's hard to see). So option 1 should be a possibility as well. I've been looking into my design a bit more, and noticed a significant level of noise on the FB reference pin when higher loads are present, and it's actually looking like that is causing the control loop to misbehave as well. So it seems there are a bunch of things I need to address. \$\endgroup\$
    – failsafe
    Dec 31, 2021 at 11:11
  • \$\begingroup\$ Regardless of getting my assumption wrong the value, resonance is still the problem to we Know who is afraid to comment but likes to downvote \$\endgroup\$ Dec 31, 2021 at 13:53
  • \$\begingroup\$ Now I see your 1103 p/n suffix is 10 uH with a SRf of 20 MHz, so the Coss must be quire high to bring SRF down to 2 MHz . This is your problem ! What do the design notes say for SRF/Fs ratio or the p/n of recommended inductor? Be aware if crosstalk on wires that causes positive feedback to Vfb. this also has a similar effect. \$\endgroup\$ Dec 31, 2021 at 16:04
  • \$\begingroup\$ @TonyStewartEE75 I told him not to try to be too clever with GND routing. Someone said the AGND and PGND should be separate and connected only on one point. Cool, but normally you'd replace "PGND" with "Earth" i.e. incoming unfiltered ground and AGND would be rest of the board. There are exceptions but low-power medium-frequency SMPS is not maybe it and controlling return currents and establishing ground islands as reference planes is not relevant here. \$\endgroup\$
    – Barleyman
    Jan 1, 2022 at 2:20
  • \$\begingroup\$ @TonyStewartEE75 unfortunately the datasheet for the boost ic is super limited and doesn't say much. There's definitely some cross talk on the FB line (as it lines up with some of the transitions of the switching) so that's definitely an issue. Hoping having a more decent ground plane will clear some things up. Going to try a protoboard and try to make a decent ground with a large solder blob to see if that will help. But I need to read up on SRF, cause I don't know anything about that currently. \$\endgroup\$
    – failsafe
    Jan 1, 2022 at 3:05

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