TVS protecting MOSFET gate - effect of diode's leakage current on switching? - Page 1
Forum section: electronics / beginners

author: HwAoRrDk
author_rank: Super Contributor
timestamp: August 05, 2023, 07:01:53 am
content: I'm using some N-channel MOSFETs to switch a load. In order to add some protection to the MOSFET gate, I'm thinking of adding a bidirectional TVS diode in parallel with the gate-source. The MOSFET gates will be driven with a maximum of a 5V signal, so I'm thinking of using an SMF6.0CA diode.Looking at the datasheets for various brands of SMF6.0CA, they all specify a reverse leakage current of around 100-400 uA. What effect will this have (if any) on the gate charging behaviour of the MOSFET? I know that there will be some effect due to the junction capacitance of the diode (which, from the datasheet, can be up to approx. 1000 pF), but I don't know if the leakage current will make any difference to the switching behaviour.I don't want there to be too much of a deleterious effect on switching, because the MOSFETs I am using are already marginal for very low duty cycles (e.g. 1%) at PWM frequencies of over 1 kHz with the gate drive current I'm able to provide.Also, does anyone know what the curve looks like for leakage current on a TVS diode? The datasheet figures are specified at the reverse working voltage (i.e. 6V for SMF6.0CA), but does the leakage current tail off at lower working voltages? That is, if it only ever sees 5V maximum, will the leakage be lower? This information is not in any datasheet I have seen.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2023, 07:13:13 am
content: The leakage current of the TVS will go down with lower voltage. The capacitance may be more of an issue if used with a relatively small MOSFET.Anyway the leakage current should not have an effect, as the driver side should be well strong enough to overcome it. In most cases there is no need for the extra TVS diode: modern power MOSFETs essentially all include a protection diode as part of the chip. In addition to day the driver is usually a CMOS chip and these usually have diodes towards there supply. So the driver can also clamp spikes.Instead of an extra TVS it may be more like checking the choice of the gate drive and MOSFET.A TVS on the gate side would be more protecting the driver in case the MOSFET failed.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: August 05, 2023, 11:45:08 am
content: Quote from: Kleinstein on August 05, 2023, 07:13:13 amOkay, that's good to know.By the way, how come it's only the lower-voltage TVS diodes that have significant leakage current? Once you get to 8V and above (e.g. SMF8.0CA) they're all only specified as having 1 uA leakage current. That kind of seems contrary to a lower working voltage getting less leakage.Quote from: Kleinstein on August 05, 2023, 07:13:13 amNow I think about it, yes, it just means that the drive current to the gate will simply be reduced by a tiny bit, but a fairly insignificant amount. Still, I guess I should try and minimise the leakage current by picking a TVS diode with leakage as low as possible (i.e. 100 uA rather than 400).Quote from: Kleinstein on August 05, 2023, 07:13:13 amThe ones I am using do not have protection diodes on the gate. At least, the datasheet does not mention having any, and I can't imagine why the MOSFET would have protection diodes on the gate and not mention it in the datasheet.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2023, 12:20:15 pm
content: Zener diodes are not all the same. Below some 4-5 V they use the zener effect (tunneling through a very thin isolating layer). Above some 7 V they are based on avelance breake through. in the transition region there is a combination of both. The lower voltage zener diodes also have a not so ideal behavior, with relatively high resistance and leakage. The same would likely also applied to the TVS diodes as a type of zerner made for high peak current.Even if one only used 5 V gate drive one could use a TVS for a high voltage (e.g. 9 V), just to give protection for the gate.The protection Zener in MOSFETs is so common, I would not be surprized if it is still in there. Unprotected types are rare and usually extra mention that and need special ESD handling on assembly. They are mainly for uses where super low gate current is needed - not for simple load switching. The zener saves on the extra protection against ESD.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2023, 09:22:37 am
content: What are you really doing?There's no point in protecting a gate arbitrarily. So I wonder what it is you're doing, and, it's probably something "wrong". But that's a uselessly subjective word, so let me illustrate some possibilities that come to mind:Is gate wired to an external connection, so that ESD or surge is a problem?--> Underlying problem: Why is it wired that way? Use a receiver and gate driver instead. Protect the receiver as any logic-level input would be: maybe it's just some resistance and ESD diode, maybe a whole surge rated diode (TVS), maybe a current limiter and MOV (ESD+surge plus cross-wiring protection), etc.Is it wired to a load that can blow up the transistor, so that backflow from drain to gate (during device breakdown) occurs?--> Underlying problem: protect the transistor. Add a control circuit to limit current and temperature; add TVS or series limiter or crowbar to limit voltage or energy; add clamp diode to circulate or recycle flyback energy instead of dumping it as avalanche energy; etc.Bonus points: add bandwidth limiting (filtering and Schmitt trigger, or pulse width timers, or etc.) to limit switching loss from pathological sources; add current limiting, temperature limiting, etc. (Burdensome when enumerated out like this? Perhaps; but indeed there are common enough applications that many such features are integrated into a variety of chips; shop around, you may find something that solves your problem even better than you were expecting!)Just slapping on a TVS, isn't likely to be all that wise, for two reasons:1a. The peak clamping voltage is much higher than the nominal rating. This, not to mention the effect of stray inductance, may not protect the MOSFET against input ESD/surge.1b. Conversely, in the reverse direction (transistor breakdown), the drain circuit can have quite low impedance (fractional ohms), from quite high voltages (potentially the full supply, whatever that is). A TVS will do a poor job here, and significant current will flow on into the driver.2. The TVS's capacitance forms a resonant circuit with the stray inductance between gate and source. This will oscillate at some 100s MHz, not much amplitude (power MOSFETs don't have much gain left up there -- but still more than enough to oscillate!), but more than enough amplitude to blow out emissions regs.The simple solution to both, actually (well, the 1b part), is to put the TVS in the middle of the gate resistor split down the middle. The resistance between TVS and MOSFET dampens oscillation, while the resistance between driver and TVS limits fault current into the driver (from whatever peak clamping voltage the TVS has). The driver might still end up damaged (depends on ratings, bypass cap and resistor values, etc.), but it at least has a better chance of success this way.I use this approach occasionally -- not in production, mind, but in those few cases where a control/protection circuit isn't feasible and all that, and where device destruction is likely: SMPS prototyping. To avoid burning through too many drivers/controllers, I may add a TVS in middle of the gate resistor, which does slow switching a bit (capacitance), but absorbs the brunt of the voltage/current during breakdown. The increased switching loss is tolerable as a matter of prototyping, and further tweaking is expected in going from proto to final version. Point being, once the ballpark values are figured out (transistor ratings, PCB layout, etc.), device destruction is far less likely, and so the additional protection feature can be removed, while further optimizing component selection.Tim

author: HwAoRrDk
author_rank: Super Contributor
timestamp: August 06, 2023, 12:43:11 pm
content: Quote from: T3sl4co1l on August 06, 2023, 09:22:37 amA question I ask myself almost every day, about life in general...Quote from: T3sl4co1l on August 06, 2023, 09:22:37 amThe gate is wired to an off-board connection. So, I'm primarily interested in over-voltage clamping (in case someone connects something inappropriate by mistake - e.g. >12V) and also ESD protection. I don't have any driver/receiver - one is not necessary for the usage scenario (DC load switching or low-freq PWM). The gate will be driven with a nominally 3.3V or 5V logic level directly from a microcontroller (yes, the MOSFET is suitably rated for 'logic level input', e.g. Vth is 1V typ). There is a 470R series gate resistor which I've determined is an acceptable trade-off between gate current and switching speed.Quote from: T3sl4co1l on August 06, 2023, 09:22:37 amMaybe? But probably not really. The supply current each MOSFET will be switching is individually fused, at an amperage well below the rating of the transistor. In terms of voltage, the nominal operating voltage is also below the rating (12V vs 20V), and the power supply involved has a 'power good' signal that is used to cut off drive to the transistors (or, specifically, power to the MCU) when out of range. A flyback diode will definitely be used for inductive loads.Quote from: T3sl4co1l on August 06, 2023, 09:22:37 amDon't worry, I am aware the peak clamping voltage of a TVS is higher than the working voltage. Hence why I tentatively chose an SMF6.0CA, which has a max clamping voltage of 10.3V, well below the 12V Vgs rating of the MOSFET. And why I can't really go for a 7V or 8V TVS to reduce leakage current, as they have a clamping voltage >12.Quote from: T3sl4co1l on August 06, 2023, 09:22:37 amI had already been considering this (e.g. 220R->TVS->220R->gate), but I don't have room for two resistors on the PCB.I mean, sure, I could use smaller components, but I like to have (for prototype at least) things hand-assemble-able.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 06, 2023, 01:01:36 pm
content: I would not worry about it, there is no point adding a TVS. Optimally the microcontroller would be on the same board, as close as possible, but nah, just make sure the connector you use is of decent quality.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 06, 2023, 01:22:52 pm
content: Some schools solder TVS diodes directly on TO-200) fets before they hand them out to students. Protecting the gate is quite useful in such surroundings (Handeling loose FET's, no ESD controlled environment).However, once the FET is soldered on the PCB , then whatever driver is used for the FET takes over the gate protection task.But I see current leakage as a valid problem. An uC pin can easily handle 20mA DC.I am concerned about using an uC as a gate drive. I know it's tempting, but be sure to use one of the "Logic Level" fets. Quite a lot of fets have a threshold voltage of 4 to 4.5V and that is marginal for driving with an uC. In general R(dson) also halves if you drive it with about 12V to 15V.If you want to drive the FET fast, then a proper gate driver is needed, that can deliver high peak currents. Fet Gate drivers can deliver peak currents of 1A or more to charge the gate capacitance (including Miller effect) quickly.An NE555 is a quite decent gate driver. It can boost the voltage to over 12V, and it's output can also deliver 500mA (DC, I've never seen specs for peak current, but it's probably higher). A dedicated gate driver is "better", but NE555 is cheap and widely available (you probably already have them).If you have an oscilloscope, you can measure the gate drive. If there is a horizontal "plateau" on the gate drive when the FET is switched on, then gate charge current is insufficient. For occasional switching it may be adequate, but when you want to PWM a higher current motor, the FET will heat up because it has both a voltage drop and is conducting current during that "charge time".

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2023, 05:07:05 pm
content: Quote from: HwAoRrDk on August 06, 2023, 12:43:11 pmHuh... which one is it? A microcontroller doesn't sound like something that should get miswired. Or, wouldn't the MCU pop first?If this is like a one-time thing (before/during assembly?), put in ESD procedures. Yield will increase significantly!QuoteI don't like the sound of that, either -- fuses pass hundreds of times their rating, peak. Fuses aren't current limiting. Transistors rarely pass ten times their rating. The transistor loses. Unless you've guaranteed adequate minimum resistance in the path so the transistor stays saturated*, which... seems unlikely maybe if you're that pressed for space?*Voltage saturation, i.e. Rds(on) ohmic region.20V isn't much overhead either, and, [inductive clamping] only counts if the diode is right beside the transistor, and a supply bypass cap beside that. Permissible loop length (stray inductance) increases as risetime increases, though it sounds like getting it more than small enough won't be a problem...QuoteIf it's only ESD, you might also consider an even smaller part, like a DFN-2 TVS (assuming such fine SMT is also acceptable). Peak voltage won't be as good but perhaps Rg * Ciss can be tweaked for effective filtering.QuoteFair enoughTim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2023, 05:11:15 pm
content: Quote from: Doctorandus_P on August 06, 2023, 01:22:52 pmAnd draws about four orders of magnitude more leakage than the diode!But a CMOS one might be fine. Much less drive strength though. Or since a function isn't really needed, just a 74HC1G something would do, I suppose...Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 06, 2023, 05:25:33 pm
content: The main concern was not "total" leakage current, but the what the effect of the extra leakage is on the gate drive, and that answer is simple. It's negligible non-existant.Also:Quote from: HwAoRrDk on August 05, 2023, 07:01:53 amSo you do indeed want to PWM.The trick to decent PWM'ming a FET is to have a high current gate drive. a microcontroller is just not going to deliver that. Just have a look at any dedicated gate driver. You will find almost none that are below 500mA, and some go over 2A peak current. And with MOSfet's it's all about the peak current, as a static drive of the gate is near zero current, and that is just not interesting.74HC1G is not very useful here. It still has a low output voltage. 8V is a nice minimum to strive for, but 10V to 12V is a more common voltage to drive a MOSFET gate. (That still leaves the area between 12V and 20V as "headroom" and margin to add exra protection if you wish so.@ HwAoRrDkWhich MOSFET are you using? And what sort of voltages and currents are on "the other side"? With that info we can give better answers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2023, 03:54:25 am
content: You're making a lot of assumptions there -- and to be fair, so am I, but mind I'm being flexible with them. This is all just very generic and wishy-washy until an application and circuit has been defined (we're waiting). In particular:Quote from: Doctorandus_P on August 06, 2023, 05:25:33 pmYou seem to be operating from an SMPS sort of frame here -- where typical transistors might be, say, 50nC Qg, and Vgs(on) >= 8V is required, and where very fast switching is dictated by losses (even 100ns might be "a long time").This seems at odds with several things OP has mentioned so far: low voltage, potentially low current (but this is an interesting point, because the mention of fusing implies MUCH greater Id(pk) -- if it has been chosen properly), and slow speed (at most a few kHz?).Low voltage devices are quite promising with logic-level drive, so I have no qualms with that. 5V drive -- 3.3V or potentially even less, actually -- is well within the realm of possibility. There are some non-logic-level parts in that range too, I think, so it depends; I'm not going to make a hard assumption on that, just, well like I said -- general thoughts and wishy-washies.Low current devices are also quite promising with logic level drive, as in, direct from the logic pin. A couple 74LVC17s in parallel will drive say 10 or 20nC at up to fractional MHz just fine. 74HC, just a bit less. An MCU or other single gate will do less Qg at the same speed, or more Qg at proportionally slower speed. Given OP's mention of 470R gate resistor, and apparently that being satisfactory, drive strength doesn't seem to be the issue here!(I'm also fond of using CD4069s for this purpose, usually with a BJT follower for final boost. One gate to invert, then the remaining five in parallel to boost, gives an output impedance around 40 ohms at 12V supply -- stronger than most discrete/BJT pre-drivers would do! Which after the follower, if using high current low-Vce(sat) types, is several amperes peak into the gate. Probably limited more by CD4000 output risetime than anything else, hah.)I know from experience, how little gate current is really needed, at times -- from linear amplifiers where surprisingly weak drivers are feasible (the input impedance is low, Miller effect is relevant, and it's more charge-controlled than voltage; and, because modern devices have so little Crss, the charge gain is quite high), to ersatz switchers where only meager performance was possible (but, still more than adequate for the purpose).Two examples that come to mind: one, a linear current limiter, with fractional-mA gate drive and a settling rate of 5µs or so (into a single STP50N06); another, a switching current limiter, with a proprietary pulsed driver circuit delivering ~3mA from the pre-drivers, boosted up to ~200mA by an emitter follower (see attached) (giving ~80ns drain voltage commutation on the simulated PSMN011-80YS, and under 200ns measured in the real circuit). See also LT4363 and similar parts (and load switch, hot-swap, wired-OR, etc. controllers), which typically use quite small gate currents for 10µs-range switching performance.For Fsw of several kHz, I have no problem with ~mA gate drive, even up to 50nC+, which would likely be quite a large part (lots of amps / low Rds(on)) at 20V.So, just to give some familiarity with the design space, other than bleeding-balls-edge SMPS stuff. MOSFETs have high gain these days, you don't have to beat them up every time!Tim

author: HwAoRrDk
author_rank: Super Contributor
timestamp: August 07, 2023, 05:52:07 am
content: I should probably clear up a few things about the usage scenario:The voltages being switched are anywhere between 5V and 12V - perhaps a little more allowing for tolerances, but definitely below 20V. The design is aimed at switching maximum loads of around 5-6A. The intended usage is for DC power switching, but also catering for low-frequency PWM too (for e.g. driving LEDs).I already have a prototype board assembled and working. With a 470R gate resistor and 5V gate drive I am getting rise times of approx. 2 us on the load (although, curiously, the gate rise time is ~4 us, which doesn't make sense to me - how can the load's be less than the gate's?). I'm successfully PWM-ing a load of about 5A (several metres of LED strip) at 500 Hz and the MOSFETs are barely warm, even after a couple of hours.Adding TVS diodes to the MOSFET gates was just a "while I'm making some revisions, it would probably be a good idea if..." kind of thing.Quote from: T3sl4co1l on August 06, 2023, 05:07:05 pmThe off-board microcontroller would be something like a Teensy, Arduino, Blue Pill, etc. It's up to the end-user to connect it up, so plenty of scope for mis-wiring.Quote from: T3sl4co1l on August 06, 2023, 05:07:05 pmI know fuses aren't current limiting. But we're talking a 5A fuse (although I'm aiming for maybe 7.5A, but couldn't get hold of any) versus a MOSFET rated for 20A continuous and 140A pulsed. So I'm confident that the fuse will blow before the transistor does.Quote from: T3sl4co1l on August 06, 2023, 05:07:05 pmWhat do you mean "clamping"? As I understand it, a flyback diode (as in a regular rectifier diode in anti-parallel to the load being switched) re-circulates the flyback current through the load, so there's no clamping going on, and I don't see how a bypass comes into it.Sure, I've done a clamping arrangement before with a zener diode in anti-parallel across the transistor (to get faster pull-in of a relay at switch-off), but that's not what I'm talking about.Quote from: Doctorandus_P on August 06, 2023, 05:25:33 pmI'm using anAO4402(although the specific one I have on the board right now is a clone byHXY). It doesn't need gate drive voltages of 10V; it's specified down to Vgs of 2.5V. I specifically chose it for this reason so as to be compatible with 3.3V logic level drive.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2023, 08:50:38 am
content: Am I missing something? Like half the labels on the plots are missing. Or maybe they weren't there to begin with. WTF?The "clone" looks to have copied their homework in the most literal sense, which doesn't give me any encouragement that the part they sell is actually what the datasheet thinks it is. Or they've managed to obtain identical dies, through various means (including test rejects?).Anyway, it's impossible to evaluate the fusing rating of that part with the missing labels. The SOA plot gives some hints: 50A/10ms seems to be on the Rds(on) curve. I would then need to see the fuse datasheet, and how much total loop resistance there is, to say more. Small chip fuses (say 1A 28V) can clear this quickly, but I'm not sure about a 5A fuse, not from a mere 10x overload.Resistance is important, as for example a polyfuse might have enough resistance to limit short-circuit current to this level, and might itself dissipate enough power to open in that time. (Actually probably not, polyfuses act more in the 100ms range.) 240mΩ would handily limit 12V short circuit to 50A. Though a 5A polyfuse probably has less than that, and a fuse-fuse certainly less still.Quote from: HwAoRrDk on August 07, 2023, 05:52:07 amOk, so think "PLC switch output", but it's transistor not relay, and it's not industrial, it's weenio-adjacent. Simple enough.Howwouldn'tdrain rise/fall be faster?Just because you think you're using it as a switch, doesn't mean it's not an amplifier. It's a large-signal / nonlinear case, but it can still be treated as a linear amplifier in a crude sense.Another rude awakening to this truth is parasitic oscillation, when applicable.Indeed, I've seen risetime ratios closer to 8:1, accompanied by real delay between rising drain and falling gate -- the trick here is, it's using SJ transistors at 400V, and the super-high-Coss region (under 20-30V) hardly registers on the waveform. When the transistor turns off, its voltage has barely risen (and Miller effect is kind of N/A for the same reason; though keep in mind Crss has the same dramatic change with Vds), and Coss is holding it low almost like waiting out a diode reverse recovery phase. Eventually Vds starts rising again, Coss plummets, and Vds explodes away (finishing the rest of the edge in say 10ns).This sounds like fantastic ZVS switching operation (the transistor channel, as such, is turned off long before Vds starts to rise substantially), but it's not actually quite perfect. The SJ structure itself has effectively dielectric loss, so there is some minimum / residual switching energy despite having done everything perfectly as far as gate drive and drain snubbing / rate-of-rise / time-until-rise.(So, interestingly, older generation (planar) transistors have lower ultimate losses -- catch is, they're such pigs (many times larger die area for same Vds and Rds(on)), even just sheer gate drive power, let alone Coss energy on the occasion you can't ensure ZVS switching (light load condition, startup?), and needless to say, cost (again, die area), means they're still worse overall (for most / almost all applications), and hence are largely obsolete types, today. MOSFETs under 200V, or maybe including 200V, depends -- finer pitch is coming -- are still planar, so you are free from this loss mechanism at lower voltages.)...Anyway, I digress; the actual rise depends on load current/phase, Coss vs. Crss vs. Ciss (and their curves), and y_fs, but it's typically 2-3 times faster than the gate waveform.More intuitively: most of the drain swing occurs during the Miller plateau, and that's only about a third of the total gate swing in a typical switching application. The rest of the swing is just to make sure "on" is on and "off" is off.QuoteAlso a good way, though with the downside that you can't do bucking with it (say for a motor -- well, you can add an external recirculation diode, then the TVS is just dead weight -- a little extra capacitance increasing switching losses, who cares), and it can be a concern with leakage current, or supply voltage range, if relevant. (Example, I did a battery powered gadget last year and a 5.1V zener at 4.2V had more leakage than the MCU while operating... Changed that to a regular diode, no problem.)Anyway, clamping, yes -- flyback comes from the source, the event that caused it, the transistor. The bulk of the inductance might be in a load, but that doesn't excuse the connecting leads up to it, nor the strays on the board itself. The diode must therefore be beside the transistor, with a local bypass cap. Now, if you don't have supply handy (the other terminal of the load), this could still just be an RCD clamp snubber sort of thing -- which behaves much as the TVS, but maybe the diode's capacitance is lower? -- but for the regular old case of antiparallel diode, yeah, that's where it should be.Again, critical loop length (how adjacent is "adjacent") depends on switching rate, which, if you're commutating fault current (50A+) in say a microsecond, that can still go pretty high without much inductance -- 8V * 1us / 50A = 160nH, or a couple inches of hookup wire. Enough that you could have an outboard diode, or, say it's on a terminal block, just stick a leaded diode into there, but not like feet away.And maybe utilizing the avalanche rating is acceptable, too -- it's probably not going to be terrifically repetitive for just on/off stuff, and even if it is, well, the end user can just blow it up and go get another one I suppose. Depends I suppose, how much blame you want to receive for them blowing up your device (because clearly it's not "misuse", it's the manufacturer's fault for not designing it for idiots!...), versus how cheap it is, or something? *Shrug*, I suppose that could happen. (Double shrug, probably bound to happen regardless.)Anyway. Is this overdesigned? Perhaps. I know I've seen just, plain ass MOSFET on a board with terminal blocks on Ali/etc., and, I guess people use 'em alright? Alternately: what are you doing that they aren't, and what value is it to what users? Well, that's also getting more into questions that are more important to yourself. But I guess as long as you're not sure what you're doing, maybe that's some useful inspiration.Tim

author: HwAoRrDk
author_rank: Super Contributor
timestamp: August 07, 2023, 10:02:37 am
content: Quote from: T3sl4co1l on August 07, 2023, 08:50:38 amYeah, it's not just you, some labels are indeed missing.Have a look at the datasheet forAO4402G. It's a very, very similar variant (seems like a slightly less 'beefy' version - lower pulse rating, lower avalanche rating, slightly higher Rds(on), etc.), but they actually have the charts labelled properly there.Quote from: T3sl4co1l on August 07, 2023, 08:50:38 amI guess that's good then, because the 5A fuses I'm using are specified to blow in 10 ms at a fault current of just over 50A (bit hard to tell exact figure due to fuzziness of graph - I hate it when manufacturers put low-res bitmap charts in their datasheets).Quote from: T3sl4co1l on August 07, 2023, 08:50:38 amYes, I understand that it's better to have the flyback diode near the transistor rather than the load, because there will be additional inductance from wiring, etc, and the job of the diode is to protect the transistor, not provide a tight re-circulation loop to the load. But I've never heard a recommendation for any kind of capacitor in tandem with a flyback diode. Where exactly is this bypass capacitor supposed to be? You're not talking about in general on the power supply to the load, but a separate one paired with the diode, right?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2023, 12:37:29 pm
content: Ah, that looks better...Yeah, that has a chance of survival then. At room temperature anyway. It's close, hard to say if it's marginal, or good enough. But it doesn't look like a reliable magic smoke generator at least.Bypass, like this,+12V and GND should be bypassed nearby. Doesn't have anything to do with the remote supply, it just needs the voltage reference, and a little current to recirculate back (at a rate now determined by the local cap, so, it can be ~us, ~ms time constant even; tau is basically C * ESR). Of course, you might as well route the wires/cables point to point -- that is, run a pair from the power supply, to the switch, then a pair from the switch to the load. +V is commoned at the switch (to pick up the bypass cap), but currents are balanced along both cables so you don't get weird stray fields or anything like that.Tim