Diode choice for buck regulator? - Page 1
Forum section: electronics / beginners

author: ssashton
author_rank: Frequent Contributor
timestamp: January 26, 2021, 01:25:25 pm
content: Hi All!The quick question: What aspects does one consider to choose a flyback diode on the output of a buck regulator?Longer explanation:I designed a circuit with a DSP audio chip and powered it with LM317 regulators. However they got a little hot, so I swapped out the reg for the digital section with anMPS2359and in my prototype it worked just as well as the LM317.I then got 100 PCBs made, but the noise level from the audio outputs was about 7dB worse than before. After much fussing about I found that using a different diode in the buck converter caused most of the difference. In my prototype I used a1N4148WSbut in the production batch I usedB5819W. In my trying to find this issue, I also tried aDB2X41500Land that was even worse giving about 10dB increased noise level!What aspects should I look for in the datasheets to choose the best diode for this application? I'm only drawing about 50mA, not sure if that matters?

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 26, 2021, 02:00:33 pm
content: That B5819 has a very high capacitance and that wont help, the 4148 is working due to its low capacitance and speed even though its silicon, the 2359 is just chunky silicon and to slow! You need a small low capacitance schottky, bat85 would be ideal but wrong case! Also your output cap could be influencing noise if its just some garbage grade, try a low esr type.

author: ssashton
author_rank: Frequent Contributor
timestamp: January 26, 2021, 10:47:35 pm
content: Thanks for your reply.I've ordered some bat85 and I'll see how they perform.Going back to understanding the datasheets to choose the right part; when you talk about 'speed' are you talking about the reverse recovery time or something else?Other than capacitance and speed is there anything else such an on resistance that effects performance here?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2021, 11:18:53 pm
content: What's wrong with the capacitance?It's a 1.2A rated regulator, using a diode rated less than that is inviting disaster. Maybe not under nominal load, but who knows what might happen.They show B230A in the application diagram. Something around there is a fine choice. What exactly, doesn't matter all that much, actually.There's not much difference between schottkys -- even the archaic '5819 (presumably that one is in reference to the 1N5819, a 1A 40V schottky which was introduced---anyone know?) is still relevant. Capacitance and Vf have been optimized only very little in the last decades. (The main improvements have been towards higher voltages: they used to top out at 80 or 100V, but are now available up to 300V in Si, and >=1700V in SiC. Formerly, ranges only served by PN junction types.)The elephant in the room here is the layout, can you show it?Tim

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2021, 12:20:16 am
content: Quote from: ssashton on January 26, 2021, 10:47:35 pmThe recovery type can matter. Some of the faster rectifiers are "soft recovery" which means they have a more gradual turn-off instead of a snappy turn off which results in lower high frequency components. Slower "fast" recovery rectifier like the 1N4933 series may be more suitable.Sometimes a ferrite EMI suppression bead is placed in series with the rectifier to limit di/dt and lower EMI.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 27, 2021, 04:54:55 am
content: Beware -- the above is generally true for discrete circuits, but regulators have tight limits on the switch node voltage. PN junction diodes, and any added series impedance (like a ferrite bead), cause the switch node to swing further below GND.Conveniently... the MP2359doesn'tspecify this voltage. So, who knows.Most are limited to something like -2V continuous, peaking to say -5V for brief transients (<100ns?).Switching regulators are generally designed for tight (minimum area) layout, and there's not much you can do about it. Like FB on the diode, or some kind of snubbing network on VIN (which might turn out to be okay, but who knows -- internal logic voltage is drawn from here as well as switch current, and it's probably not going to appreciate the noise a snubber would introduce here). These are among the options you can consider in a discrete circuit. (Discrete circuits still usually use an IC for controller, or at least gate drive; these may have more relaxed requirements, for example some bootstrap drivers that are rated to -30V "Vsw", under certain restrictions.)Tim

author: ssashton
author_rank: Frequent Contributor
timestamp: January 28, 2021, 03:34:00 pm
content: I tried a few diodes in D1 with the following noise floor results at the end of the analog audio outputs (level referenced to nothing at all, lol) :1N4148 -83dBBAT85 -82dBB230A -76dBDB2X41500L -73dBB5819W -75dBThe B230A didn't do well at all! I wonder if the BAT85 suffered a little becasue it was a leaded component so sitting a few extra millimetres above the ground plane, although performance also seemed a little inconsistent with a few dB variation each measurement.This is the layout. The main digital DSP power is actually drawn from T1 whose base is driven by the DSP chip (R23) to get 1.8V. So how the regulator drives T1 is probably more important than how it drives the D3.3V input on the DSP, which is mostly just for I/O logic voltage. :R16, R24 are feedback.R22 is enable (whcih I stupidly grounded rather than connected to V-in!)C22 is bootstrapC17 is 10uF Input smoothing.C16 is 10uF output smoothing.D1 flyback diodeL1 4u7 inductor

author: ssashton
author_rank: Frequent Contributor
timestamp: January 28, 2021, 04:00:28 pm
content: Quote from: David Hess on January 27, 2021, 12:20:16 amSo a soft turn off diode produces less spiky ground currents, but a diode that is too slow will stop the boost circuit working properly. So there must be a 'sweet spot'? Perhaps this point depends on things like current draw and switching speed of the regulator?

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 28, 2021, 04:51:01 pm
content: Glad the BAT85 helped, high capacitance on switch nodes doesn't help noise, sometimes an ultra-fast can be better for a less snappy recovery.Your layout is pretty cool but I would like to see a topside track between D1-C16 to improve grounding and maybe an extra via there (you can trial with a thick short wire link).C16 is a single high value ceramic with one specific resonance, I would be tempted to add some other lower values in paralleled (just solder on top) for example 100 & 10nF.

author: ssashton
author_rank: Frequent Contributor
timestamp: January 28, 2021, 06:00:47 pm
content: Oh momentous day! I think this is the first time anybody EVER complimented me on a PCB layout.So far the 1N4148 is still the best. It would be nice to really understand why, but perhaps without scoping across the diode it's difficult to say for sure.1N4148 in place, the performance is only 1-2dB worse than a LM317. So there i not much room for improvement anyway.I did try putting a 10uF Panasonic FC electrolytic in parallel with the 10uF ceramic output cap but it made no difference whatsoever. You are right I should try adding 100nF.I also tried replacing the input cap with the electrolytic and performance got significantly worse. I guess it didn't work well for the switching freq. of this regulator.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 28, 2021, 08:47:43 pm
content: I like the ground plane under the regulator itself, but why not extend over the whole board? In particular, the input (C17) will have some noise, maybe a volt peak, maybe less, so there's a noise loop between VIN (whatever that goes to, bottom left) and thelong loop ground returnup over from C16 to "U1" (the label).Oh, what the hell is going on with C20, C21? Ground is completely off to the side and not tied with the pour at all?Try scraping off the soldermask in the indicated regions, and shorting across with copper foil:Tim

author: ssashton
author_rank: Frequent Contributor
timestamp: January 28, 2021, 10:25:58 pm
content: Hi Tim,This board includes analog audio sections, so over-all it implements multiple star ground points. The buck reg and digital power decoupling C29, C30 returns near the high current dirty star point, while the linear reg for analog sections and decoupling C20, C21 returns nearer to the low current clean star point.Note that the 'ground plane' under T1 is actually not ground it is the T1 output... for some reason labelled 3.3D2 when in fact it is 1.8V I should fix that! That 3.3D2 line inconveniently interrupts the ground plane under the DSP chip and the buck regulator. However I don't really mind this because it allows me to ensure the return to the buck reg is nearest to C16 the output cap.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 28, 2021, 11:51:04 pm
content: Well I don't have a clue what this DSP is or is doing, at best I have to assume it's swarming with digital I/Os which will easily throw around more noise than the regulator, exacerbated substantially by the lack of ground plane.Without any additional information, that is a satisfactory explanation for a noise floor. I suppose I'm surprised it's not even worse. But again, I have no idea what the signals even are. I guess there's analog I/O, since it's a DSP. It looks like it probably doesn't expose enough pins to be accessing, like, external RAM, ADC and DAC, so there wouldn't be that level of noise at least, and that might explain an analog grounding scheme (depending on how the chip is designed). But that doesn't explain why analog and digital grounds aren't joined underneath it, for example?There is a proper way to do star grounding, that respects both DC and RF behavior; alas, few "audiophile" designs actually implement it this way. For the most part they are EMI disasters, which only escape notice because they are audio frequency amplifiers with passive power supplies. At worst they oscillate, then some damping component is found to suppress it, and that's that. I have no idea where you're coming from, like if this is relevant, or if you're just following appnotes, or whatever. Anyway, when I see "star grounding", it raises my hackles because it's almost never implemented in the best way.Tim

author: ssashton
author_rank: Frequent Contributor
timestamp: January 29, 2021, 12:52:55 pm
content: Hi Tim,I didn't mean to sound like I don't appreciate your advice on the ground plane, it's just that joining with that of the buck regulator is not possible on this 2 layer board.I did try joining the analog decoupling ground under the chip, but it didn't work as well as keeping the digital and analog returns seperate back to the more global star points.Actually they are not really a star, it's more like a line coming off the main smoothing cap with each return joining at a distance along that line depending on how clean or noisy the current are.Perhaps you misunderstand my terminology talking about a noise floor. Even when working to perfect spec there will be a noise floor, it's just a matter of getting it as low as possible by optimising grounds and in this case seemingly the buck converter diode choice.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 29, 2021, 05:24:52 pm
content: Note that since schottky diodes do not have reverse recovery, as a result they don't have the "softness" factor either. They recover "immediately", and in this respect, are "perfect".They do have junction capacitance though, which for practical purposes, behave the same, causing current spikes when you short it.But a classical PN diode which has "worse" reverse recovery may in some cases still be better if it has considerably less capacitance. This would be the case in very small converters, mostly.Maybe you are on a wrong track trying to find the "best" diode; maybe you should be concentrating to the other classical ways to reduce EMI, e.g., increase gate resistance to slow down switching, add RC snubbers (over the diode, and over the FET)...I'm assuming efficiency is not critical? I don't mean going into linear, but maybe losing a few percentage points in added switching losses or snubber losses, and added losses of extra filters.1.4MHz is fine and you can't go much higher, but have you confirmed you are actually running at 1.4MHz, i.e., not in a low-load pulse skipping mode?

author: ssashton
author_rank: Frequent Contributor
timestamp: January 30, 2021, 01:33:55 am
content: Good point about a pulse skipping mode, but I dont think this reg has that feature. It says fixed 1.4mhz.Interesting also you say shottkey essentially have instant reverse recovery. Many datasheets for shottkey diodes simply don't list that spec. However the Bat85 does.I think you guys have helped me to understand the datasheets better and looking at the specs for each diode and the noise I got from them, it seems very much like a combination of reverse recovery time and capacitance. In order of performance best to worse:1N4148 (4pF, 4ns PN) -83dBBAT85 (10pF, 5ns shottkey) -82dBB230A (200pF, recovery not listed schottkey) -76dBB5819W (120pF, recovery not listed schottkey) -75dBDB2X41500L (70pF, 25ns PN) -73dBIt seems like the lower the capacitance and the faster the recovery time the better.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 30, 2021, 04:25:24 am
content: Hmm... Check the "start-up through enable / no load" plot in the datasheet. It may not be a pulse-skipping mode (implying the actual switching rate goes strictly subharmonic to the oscillator; which, when this is done explicitly, the ratio is often powers of 2, like I think TI's eco-something controllers do?), but it certainly looks to stop switching when it doesn't need to. I mean, it's not synchronous, so it can't go full-wave for literally any load including zero, it has to have some way to cut off completely.The question remains, how does frequency change with load? It may turn out it reaches every-cycle operation even for fairly light loads, like a few percent of nominal or something; I don't know. (For example, this is my experience with UC3842 family controllers.) They don't seem to plot that, unfortunately. Also, if it's cutting off due to the error amp saturating, it basically becomes a hysteretic controller (i.e., Vfb just barely drops into the operating range, emit a pulse, wait; etc.), and the switching frequency (given that it's still triggered coherent to the oscillator) is very sensitive to ambient noise, and can be pretty chaotic. (A common experience, hearing a whine from idling equipment -- including a crunchy whine, or even full-out hissing (signs of either noise, or true dynamic chaos). The sound is emitted by the switching inductor (via magnetostriction) or filter caps (via electrostriction or piezoelectricity).)But anyway, at least it's easy to test, if you've got a scope handy. Just see what happens at light load conditions.(They also show a frequency foldback, but that's a startup condition when Vfb is low, so not applicable to normal operation.)Regarding recovery -- some advanced type schottky (interesting new structures developed for high voltages) do actually exhibit recovery, but it's greatly reduced compared to PN types. So sometimes you see the rating, despite the "schottky" title.Signal diodes are different: note the type of test, it's a small(ish)-signal pulsed waveform in a controlled impedance environment. Different from rectifier recovery tests, which use higher voltages and a current ramp waveform. The measurement will simply register any capacitance as recovery, which, is fair enough -- recovery sort of acts like a dependent capacitor, though it has a lossy component that schottky diodes lack. This is hard to measure just from the transient waveforms, so just noting the apparent recovery time (which is really just charging CJ from zero) is what they do.Also also, (just to muddy things further?) -- (most? all?) schottky still exhibit conventional recovery, when driven to high enough currents. This occurs because they're actually constructed with a PN diode in parallel, the so called guard ring. This structure serves a physical purpose (allowing higher breakdown voltage), but also a protective purpose (it usually has some avalanche capacity, protecting against ESD or switching transients), and can be forward-biased when the diode is simply driven hard enough that Vf goes into its forward-bias range. After which, on sudden reversal, it will recover as usual for a PN diode, much slower than expected given the diode's rated capacitance. In short, recovery depends on I_f, and gets worse above a certain point. (Which in part, is a reason why schottky are less preferred for pulsed applications, like snubbers.)Also about recovery, it gets worse with rising temperature, so PN diodes dissipating a lot of heat due to it can thermally run away.This probably doesn't matter here, but it might be a good reason to avoid using a 200mA signal diode for rectification with a1.2A regulatorthat'll beat the piss out of it if simply loaded enough. (Likewise, the 1N4148, though it will handle the peak current just fine, the transient (forward recovery) and steady-state Vf aren't great as mentioned, and it's simply not big enough, it'll still melt at 1.2A. It is not a safe combination.)Hm, regarding consequences, actually -- diodes typically fail shorted, at least at first. If the initial failure causes adequate fault current to flow, they can be blown open like a fuse; but that takes some milliseconds to happen, and a lot of capacitance, or a battery or some source of fault current. In this case, that shorts the output, which collapses rapidly, and the regulator goes into its startup condition -- frequency foldback, and it's peak current limited so it'll just emit runt pulses forever. Which, if it doesn't overheat under that condition, it'll survive, replace the diode and it works again. If it overheats and fails, it probably fails shorted as well, and now you've got +V dumped into the shorted diode and output. Which, maybe that'll cause more damage, maybe not. (Runt pulses draw a lot of switching loss -- it's not obvious at a glance if the regulator would survive. It does have a 150°C temp limit, above which it'll stop, cycling on and off; but that's not a guarantee of survival, thermal cycling can still kill it over time.)So that's a deep dive on lesser documented diode characteristics, and some failure effects analysis.Tim

author: ssashton
author_rank: Frequent Contributor
timestamp: January 30, 2021, 11:44:07 am
content: Wow thanks Tim I'll have to read that one a few times to digest it all!I do have a scope but it's a pretty pathetic Picoscope 2203 5mhz 8bit (48db dynamic range) so I'm not sure if I'll see the details of the regulator noise without some sort of analog amplification first?I may well have seen the 1N4148 thermal run away yesterday. It was all working fine then I heard hissing noise from the speaker outputs and a moment later the buck regulator became a tiny firework.I had been doing a lot of hot air rework in that area to try all the diodes and caps etc.. so thought it might have just been a weakened component. I rebuilt the board and it was working for many hours yesterday. I know that the DSP draws a pretty invariable 50mA so unless somebody mods the board there will be nothing else connected to that buck converter. Although it could be a rather transient 50mA I only measured with a multimeter.But I think given your point on failure I may need to check this carefully. I have a tiny temp probe that came with my BM235, but could thermals be more of a highly transient issue that doesn't have time to dissipate from the junction??I'll see if I can grab some measurements from the scope and post here.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 30, 2021, 12:45:18 pm
content: There really is no otherproperway of choosing the parts (inductor, diode) for the converter IC than to look at the maximum worst case current limitation value and design for that.It sucks because there are no (small, cheap, well integrated, easily available) parts on the market with smaller ratings than some 1.5A, which is ridiculous! The need for small (say a few hundred mA) converters is obvious and evident, yet the manufacturers just force us to oversize our supplies by 10x.Usually this means a some 1.5 - 2A rated schottky and inductor saturation current, increasing the total footprint and cost.If you choose the part ratings according to your expected load, you have a serious risk of killing the parts in edge cases such as start-up into capacitive load or output short. To avoid that, the parts need to be rated for the maximum current the ICis able toprovide, this is called "switch current limit" or similar in the datasheet.You really can't avoid having an oscillosscope when designing and debugging switch mode converters, even if you use off-the-shelf ICs as much as you can. You need some 100MHz of bandwidth, really. Parasitic oscillations tend to be around 100MHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 30, 2021, 01:24:13 pm
content: I wouldn't say that; there's LMR14203XMK for instance, a nice cool 300mA. Same package and pinout, too! (The SOT-23-6 pinout is surprisingly standard actually, IIRC.)Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: January 30, 2021, 01:57:19 pm
content: Oh, that's quite cool, switch current limit specified at 525mA typical, maximum not specified but maybe you could work with 0.5A rated parts with that one.Yes, I have learned the hard way to always choose integrated regulators in the SOT-23-6 package; functionally equivalent, pin-compatible replacements are easily available in this era of sourcing uncertainties.

author: ssashton
author_rank: Frequent Contributor
timestamp: January 30, 2021, 07:23:13 pm
content: Okay I got out the scope today, as well as the audio interface and bothered to actually save the data so I can share it with you guys!I made a change from the last data I posted by adding a 1K buffer resistor between the DSPs analog audio outputs and the DC blocking caps. This seems to have helped, but there are still some big differences here!First I measured the buck converter output simply driving 3.3V into 50ohms. I put the resistive load across the output cap and also measured there. The bigger clumps of transients, visible with a 50R load for BAT85 and B5819 but particularly visible with the actual DSP load, appear to be at roughly 28KHz intervals.Next I swapped the resistive load for the actual DSP load.I also checked the frequency content. All seemed to have a strong 1.4MHz spike, so presume continuous mode?I followed up by measuring the effect on the audio output, which besides stability is really what matters. It's all very well probing the regulator, but in reality it needs to result in good performance at the end of the chain. Note the 'Average RMS Amplitude' in the window on the left. The FFT averages random noise so is not suitable for judging smaller noise floor differences.It's interesting also to note an effect not only on noise floor but also harmonic distortions.Looks like the B5918 is the winner! Yes it has 2dB higher noise floor than the 1N4148 but the harmonic distortions added by the latter are not desirable. The scope waveforms clearly show the 1N4148 is not actually helping the buck reg work that well. I wonder why it results in the lowest noise... any why the B230A (recommended in the reg Datasheet!) is so bad?!EDIT: Whoops that last image was wrong, updated now.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 31, 2021, 11:14:26 am
content: Did you try the multiple output capacitors I suggested ? I mean mlcc's of course not something with legs.Another tack is to add a post filter, either RC or LCYet another is to use a 5V switcher with an LDO linear 3V3

author: Siwastaja
author_rank: Super Contributor
timestamp: January 31, 2021, 01:36:24 pm
content: Quote from: fourtytwo42 on January 31, 2021, 11:14:26 am... carefully selected with good PSRR rating available at the relevant high frequencies!They exist but most LDOs on market filter > 1MHz only by the parasitic RLC filter they form, in such case you could replace the LDO with a straight wire or low-value resistor.

author: iMo
author_rank: Super Contributor
timestamp: January 31, 2021, 02:06:43 pm
content: Quote from: ssashton on January 30, 2021, 01:33:55 amAs has been mentioned here the reverse current is pretty different with 1n4148 and the others too - some 5nA with 4148 vs. XXXuA with others..

author: ssashton
author_rank: Frequent Contributor
timestamp: January 31, 2021, 03:31:22 pm
content: Yeah I did try bypassing the input and output 10uF MLCC with 100nF. Also tried 10nF on the input side. The difference was minimal but in all cases the single 10uF actually worked betterI did find something interesting though, the previous plots were all taken with the bare PCB on my wooden desk.I have since mounted the PCB on the grounded steel equipment panel with 5mm stand-offs. The those transient clumps have reduce significantly. Obviously it acts as a ground plane, even though it is 5mm away.Thinking about it, I should have run the V-IN and Ground lines back to the main smoothing cap in parallel or mirrored top and bottom. Even though the buck has its own input bypass cap there must be an RF loop area of some level created by the traces here.Makes me realise that despite having a ground plane under the buck converter itself, really the entire PCB must have one for best performance. In the future maybe I will do a multi-layer board wit ha ground plane for HF stuff and star grounds on a separate layer for audio freq.No metal panel:On Metal Panel:Ignore my typos on the diode number!

author: Siwastaja
author_rank: Super Contributor
timestamp: January 31, 2021, 05:49:46 pm
content: Quote from: ssashton on January 31, 2021, 03:31:22 pmThis isn't surprising, the wrong advice of combining different capacitance values of parallel MLCCs still lives strong. Ignore it. Mimimum impedance SRF is the wrong parameter to look at, everything that matters isoveralllow impedance and you get that with maximized capacitance and minimized inductance.10u || 100n very low ESR MLCC can realistically oscillate, so the downside is real.Is there any upside? Adding 100n in parallel with 10uF does not increase capacitance, and only decreases inductanceif it is in significantly smaller package, placed/laid out in a significantly better spot.The advice was good when 10uF was basically just a through hole electrolytic with long leads, then the added 100n or 1u ceramic had some benefit, you could place it physically closer, and the ESR of the elcap damped the circuit.Now, MLCCs come in such large capacitances and such small packages (i.e., small inductance) that there is no such benefit anymore.For example, if you currently have 10uF in 1206, you canactuallyreduce the impedance by changing to 2 x 4.7uF 0805 in parallel. Via placement is also important. Even 0603 is available in quite high capacitances, but do check the DC bias effect.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 31, 2021, 08:33:33 pm
content: Quote from: Siwastaja on January 31, 2021, 05:49:46 pm