Circuit for MosFets in parallel for extra current capacity.(Solved) - Page 1
Forum section: electronics / projects

author: Jim-0000
author_rank: Regular Contributor
timestamp: December 30, 2018, 01:23:25 am
content: I wish to find a design for a MosFet circuit to have around 100 amp capacity. I will be drawing 60 amps, so some headroom will be good.I know there are big MosFets that will do this job and more, but they are expensive.I understand that it is possible to design a circuit for two or more smaller, much cheaper one.Just found this one for example:https://www.sparkfun.com/products/10213FQP30N06L for 95 cents.Three of these (if they prove to be suitable, if not, something similar) will be ok for my pensioners budget.I understand that provisions must be made to ensure all gates are turned on at the same time, or very nearly so, to avoid one semi-conductor coping with the turn on spike.Also, heat dissipation factors have to evaluated and most likely, some sort of heat sink provided.I already have some of these in stock:FQP30N06L Data sheet:http://www.mouser.com/ds/2/149/FQP30N06L-112819.pdfThis is my first post here.I hope some reader can assist or direct me to another source etc.Jim.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 30, 2018, 01:51:49 am
content: Okay but for what?Tim

author: MarkF
author_rank: Super Contributor
timestamp: December 30, 2018, 01:54:31 am
content: WelcomeConsider something in a TO-247 package for better heat dissipation. Like an IRFP064N @ $2.59UShttps://www.mouser.com/ProductDetail/Infineon-IR/IRFP064NPBF?qs=sGAEpiMZZMshyDBzk1%2fWi5%252bqVgN3%252bWS8G8awMUgotiM%3dThe TO-247 package is much bigger.Need more details.

author: Jim-0000
author_rank: Regular Contributor
timestamp: December 30, 2018, 02:28:43 am
content: Ok Tim and Mark. Thanks for the replies.Tim:"For what?"It is for a 4 cell LiPo powered ESC (Electronic Speed Controller) in a model aircraft - glider.Mark,I will look at the suggested alternative MosFet now, thanks.Is it possible to make multiple quotes on these forums?Jim.

author: Jim-0000
author_rank: Regular Contributor
timestamp: December 30, 2018, 02:54:15 am
content: Mark,I found the Mouser link formidable to navigate. However, I did find this data sheet:https://www.st.com/resource/en/datasheet/stp160n75fHowever, there seems to be some confusion about the max current capabilities.Also, I am confused about the nomenclature (identification of these devices). Even on the above data sheet, it is not clear which ones are being rated. TO-247 or STB160N75N3?Screenshot attached.Jim.Edit: I did find my way into the Mouser site. My search lead me to the data sheet on SUG90090E. For all I know, it is an equivalent, or similar to the device number you suggested above Mark.I surely does seem to be a candidate for my application. Price is ok too.

author: David Hess
author_rank: Super Contributor
timestamp: December 30, 2018, 02:56:50 am
content: If it is a switching application which seems certain for an ESC, then nothing special needs to be done as long as the combined gate charge requirements are consistent with the desired switching time and driver. The problem then becomes a layout challenge.Besides the obvious recommendation of keeping the source inductance low and minimizing the loop area of the gate driver connection, use individual low value gate resistors close to the power transistors. If you want to waste some time, you can buy more power transistors than needed and match them for Vgs but it is not really necessary.I have seen early ESCs designed with a row of TO-220 parts practically touching with no heat sink on the exposed tabs because the parts were selected for a low enough on resistance that power dissipation was not a problem. They did not use gate resistors but I am not sure if that was a good idea; they got away with it anyway.

author: Jim-0000
author_rank: Regular Contributor
timestamp: December 30, 2018, 03:33:57 am
content: Quote from: David Hess on December 30, 2018, 02:56:50 amAffirmative; it is for switching on an ESC.QuoteI do have lots of these: FQP50N06Out of curiosity, how does one match them for Vgs?QuoteIt would be no problem to include one.I will read further before deciding on wether to use 2 or 3 of the FQP50N06 or order one of the SUG90090E from the US supplier. Shipping costs from the US to here are a bit of a slug though, I just might be able to find them in China with a bit of luck.But as I say, I am far from decided. The multiple FQP50N06 sounds like a bit of fun, and I have them already.Many thanks David.

author: David Hess
author_rank: Super Contributor
timestamp: December 30, 2018, 04:07:04 am
content: Quote from: Jim-0000 on December 30, 2018, 03:33:57 amHmm, I am used to doing this with JFETs and bipolars. Connect the gate to the source and drive a fixed current through it from your power supply which can be done easily enough with just a resistor. Then measure the Vgs.https://www.diyaudio.com/forums/solid-state/299546-nelson-pass-easy-peasy-mosfet-vgs-measurement.htmlUpdate: T3sl4co1l has it right; short the gate to the drain and not the source for this test.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 30, 2018, 04:31:32 am
content: Oops, gate to drain, otherwise correct.Datasheet says Vgs(th) measured at some Vds and Id. You get the Id from the supply (Id = (Vsupply - Vds) / resistance), and adjust Vgs until Vds matches spec. Measure Vgs and there you have it.If they don't spec what Vds it was measured at, it's probably safe to assume Vds = Vgs. In other words, "diode strapped" mode.A DMM's diode test function will do as well (it supplies the test current), as long as it will read high enough voltage drops.Tim

author: David Hess
author_rank: Super Contributor
timestamp: December 30, 2018, 04:36:10 am
content: Quote from: T3sl4co1l on December 30, 2018, 04:31:32 amYea, gate to drain. I did that over in my mind several times and then found the example that I linked and still got it backwards.QuoteFor this the test current does not matter as long as it is consistent. What is important if it is important is to throw out any outliers. For this I would probably test at 10 to 100 milliamps.Datasheets specify the gate threshold voltage at a specific low test current which is not the real gate threshold voltage but it is close enough for practical purposes.QuoteNone of my handheld meters have enough voltage compliance but my old Tektronix bench meters could do it in ohms mode because they will happily destroy a transistor base-emitter junction with 10 volts. Well, two of them could do it with one generating the current and the other measuring the voltage because in ohms mode the maximum reading is 200mV or 2V.

author: spec
author_rank: Frequent Contributor
timestamp: December 30, 2018, 05:27:48 am
content: Hi Jim-0000Just read through your thread above- sounds like your project will be great fun.You say that you have a load of FQP50N06 NMOSFETs and implied that one option would be to use those, so I did some quick calculations to see what would be needed:Four FQP50N06 NMOSFETs in parallel would be needed so the nominal maximum current through each NMOSFET would be 25A.The maximum RDSS of FQP50N06 is 22mR, so at 25A this gives a static dissipation of 13.7W which will be OK.But you only get that RDSS with a gate drive of 10V or more (maximum limit = 25V), so I would advise a 12V gate drive.That takes care of the static conditions in theory, but there are the physical conditions to attend to as well. Briefly, large heatsinks would be required and the wiring of the parallel MNMOSFETs is critical, especially the drain wires, which must have the same resistance to aid current sharing between the four NMOSFETs.Then there are the dynamic conditions to consider.If you know all this, just skip it, but here goes.MOSFETs do not switch on and off immediately, mainly because of the massive parasitic capacitors that they have, and this is especially the case with high-power MOSFETs, like the FQP50N06. As a consequence, while switching they dissipate a lot of power because they have a voltage across them and a current flowing from their drain to their source (power= I * V). For this reason, it is important to turn the NMOSFEts on and off as fast as possible, especially in pulse width modulation (PWM) applications where the MOSFETs would be continuously turning on and off.The bottom line is that you need to hit the MOSFET gates with a relatively high voltage (12V) and have a large current available to charge and discharge the effective gate capacitances. In short, you would need a gate driver chip for each NMOSFET (in practice driving a MOSFET gate is quite complicated and a bit of wave shaping is required, but forget that for the present).Just a word about power transistor characteristics on the data sheet. The first thing is that the power rating is a sales figure and has no use in the majority of designs and, to a lesser degree, the same goes for the maximum current figure. The reason for this is that other factors constrain the performance way before these two limits.The characteristics that are vitally important though are [figures for FQP50N06]:VDSmax  (good)VGSmax [+-25V] (good)Vthmax (Voltage where the MOSFET just start s conducting current from its drain to source) RDssmax(resistance between drain and source when the MOSFET is fully turned on by a high specified GS voltage) TJmax (maximum junction temperature) [175 deg C] (good)ThRJCmax (thermal resistance from junction to case) [1.2degCW] (not bad)Safe operating area (SOA) [see data sheet]Gate Charge (capacitance)  (a lot!)https://www.onsemi.com/pub/Collateral/FQP50N06-D.pdf

author: MarkF
author_rank: Super Contributor
timestamp: December 30, 2018, 05:32:11 am
content: Quote from: Jim-0000 on December 30, 2018, 02:54:15 amHere is the datasheet for the IRFP064N for reference.The SUG90090E you mentioned looks like a good candidate too. Although twice the cost.Better yet, the IRFP044N @ $1.63US

author: spec
author_rank: Frequent Contributor
timestamp: December 30, 2018, 06:37:17 am
content: Hi again Jim-0000,It will be important for us to know what type of drive you are planning on using. The options are low side NMOSFET, high side NMOSFET, high side PMOSFET. Technically, low side NMOSFET is far superior, and high side NMOSFET is the most complicated, but is still reasonably practical with the appropriate gate driver chip and power supply lines.

author: Jim-0000
author_rank: Regular Contributor
timestamp: December 30, 2018, 09:19:12 am
content: Quote from: spec on December 30, 2018, 05:27:48 amWhat is RDSS? Pardon my ignorance.QuoteThe onboard main power in the model aircraft this is planned for is 14.4 volts. So, that is no problem. I could just wire a supply off the main LiPo battery supply.QuoteThanks for the figures, and all the other details. The size and configuration of the heatsink might be a limiting factor. I will have to think further.

author: Jim-0000
author_rank: Regular Contributor
timestamp: December 30, 2018, 09:21:23 am
content: Quote from: spec on December 30, 2018, 06:37:17 amSpec,Thanks for your question. However, I have no idea about this..........sorry!

author: nick_d
author_rank: Regular Contributor
timestamp: December 30, 2018, 10:27:08 am
content: I agree with all that has been said here. One thing I am slightly confused by is the need for a gate resistor, it has been said that a design seen in the wild did not use gate resistors and got away with it, but why would this be bad? If you just connect all gates in parallel and drive them with one hefty gate driver chip, I would have thought that all the gate capacitances would appear in parallel and be charged in parallel, leading to all MOSFETs switching on at the same time (subject to slight differences in their Vgs as discussed).Anyhow. On the subject of gate drive generally. Because of the huge gate capacitance as mentioned, what you are doing (directly or indirectly) is using a smaller MOSFET, which has a smaller gate capacitance and hence is easy to turn on and off (perhaps by a digital output from your ESC), and then using that smaller MOSFET to drive the gate of the power MOSFET. And as already mentioned, the reason this is important is because the MOSFET becomes a resistor when partially turned on, so it can heat up during the switching. Hence the switching must occur as quickly as possible.The problem then is that to drive the gate, you need a separate smaller P-type MOSFET for charging and N-type MOSFET for discharging the gate. Not only that, but since both will be connected to the gate simultaneously and hence also to each other, dead time is desirable between turning one of the gate driver MOSFETs off and the other on. While you can build all this from discrete parts, the dedicated gate driver chip is highly recommended for simplicity. (And it has also been mentioned above that it has improved waveform, I didn't know this but maybe it compensates gate inductance).Note the ESC might have the gate driver built in. Check it.Now for my suggestion, the "poor man's gate driver" and that is the 74AC logic family. The thing is though, you would need to have logic-level power MOSFETs, which require a gate voltage of only 5V to turn on, not the 12V as calculated above. I know you already have the traditional (not logic-level) MOSFETs in stock, but if the choice is to order a gate driver or a new MOSFET then the logic-level approach may be attractive. You could get a 74AC541 octal buffer and put 2 of those buffers in parallel to drive each of your 4 gates. No gate resistors required, and each gate driven separately.cheers, Nick

author: spec
author_rank: Frequent Contributor
timestamp: December 30, 2018, 10:31:26 am
content: + Jim-0000Thanks for your replies- very helpful. There is no need for any apologies- just ask any questions that you like. By the way it is me that is uninformed about modern model aircraft, although I have built and flown quite a few as a nipper- no electric motors in those days, just thermals, elastic bands, and diesel. And radio control was an electronic marvel well beyond my means or understanding.In general, I do not see any problems in any area that cannot be resolved, and will cover your points in replies #13 and #14 in further posts.But can I ask a few more questions about the application:Am I right is saying that the MOSFETs will be used to control the speed of an electric motor which spins the propeller in a model aircraft which flies.How big is the model aircraft?Will pulse width modulation be used to control the speed of the electric motor? Or will the motor just be turned on and off relative infrequently, say no more than twice a second?What is the construction of the model aircraft.If the model aircraft is not skinned in aluminum, would it be possible to skin a part of the aircraft in aluminum or are there any thermally conductive areas on the aircraft. The thinking here is to use the aircraft as the heatsink for the MOSFETs. A large area would not be required.What signal would be used to control the MOSFETs- do you have details of voltage etc?A picture of the model aircraft would be great.Once I get sufficient information about your application, I will do an outline circuit which we can discuss if you like.

author: spec
author_rank: Frequent Contributor
timestamp: December 30, 2018, 11:08:08 am
content: Quote from: Jim-0000 on December 30, 2018, 09:19:12 amWhen an NMOSFET gate and source are at the same voltage the resistance between the NMOSFET drain and source is, essentially, an open circuit so that any voltage between the drain and source will not cause any current to flow from the drain to the source.But if you make the gate more positive than the drain (4V for the FQP50N06) the NMOSFET will conduct a small current. This voltage is the threshold voltage (Vth). But if you increase the G/S voltage the resistance drops to a very low value. In the case of the FQP50N06 the resistance with 10V G/S drops to 0.022 Ohms (22 mili Ohms), which is essentially a short circuit. This low resistance is RDSS (resistance drain/source): can't remember what the second S represents.And that is all there is to RDSS (it is a bit more complicated far a critical design, but that need not concern us at this initial stage).

author: spec
author_rank: Frequent Contributor
timestamp: December 30, 2018, 11:20:31 am
content: Quote from: Jim-0000 on December 30, 2018, 09:19:12 amExcellent

author: spec
author_rank: Frequent Contributor
timestamp: December 30, 2018, 11:30:17 am
content: Quote from: Jim-0000 on December 30, 2018, 09:19:12 amFrom what you have explained about the application I don't see any problems, so far, with the heat sinking that cannot be solved.When you mention heat sinks people think of big, expensive aluminum structures with fins all over the place, and in many high-power applications this is the case. But in other applications, you can use the unit structure as the heat sink. For example, I have built many 100W audio amplifiers, which had no separate heat sinks. Instead, the whole aluminum top cover was one big and efficient heatsink.

author: Jim-0000
author_rank: Regular Contributor
timestamp: December 30, 2018, 11:49:37 am
content: Quote from: spec on December 30, 2018, 10:31:26 amSpec,No, that is the function of the ESC (Electronic Speed Controller).This proposed MosFet switch is to turn the power from the 14.4 volt LiPo battery to the ESC. It is basically a safety measure. Most electric powered models of this category are simply plugged in to the battery (to the ESC as described above) to "switch them on". The danger is if the throttle stick is moved accidentally while the propeller is in near someone, it could cause and injury. The carbon fiber folding propeller on have on my almost built big glider would be like a pair of rotating scimitars.QuoteThis one is 3.8 mtr wing span. Not completed yet, but it has been given a CG test launch with a temporary fuselage.Full disclosure build thread here:https://www.rcgroups.com/forums/showthread.php?2787059-4-metre-span-built-up-thermal-plank-build-threadAnd the thread that inspired me to investigate a MosFet based switch for my application (60 amps max) is here:https://www.rcgroups.com/forums/showthread.php?3016539-High-Power-MosFetQuoteThe ESC generates the PWM to control the brushless motor speed.QuotePlease see all construction details and pictures on the build thread......linked above.......................QuoteLots of pictures in the thread linked to above.QuoteI much appreciate your assistance. I have a growing feeling that I am out of my depth here. If I miss answering anything you or someone else asked, please be assured it was only due to some degree of overwhelm and not intentional rudeness.

author: Jim-0000
author_rank: Regular Contributor
timestamp: December 30, 2018, 11:55:52 am
content: Quote from: nick_d on December 30, 2018, 10:27:08 amNick,Not applicable. This planned MosFet device will be to act as the switch that supplies power to the ESC. Not the other way around.QuoteThanks again Nick. I will do some reading up on the "poor mans gate driver" that you nominate above.

author: spec
author_rank: Frequent Contributor
timestamp: December 30, 2018, 12:08:34 pm
content: Quote from: Jim-0000 on December 30, 2018, 11:49:37 amNo worries.Thanks again for the information. That has cleared up and simplified the situation a lot.There is only one remaining area to be resolved now and that is the high side low side issue, but I am preparing a post for discussion which should enable us to resolve that issue.

author: Jim-0000
author_rank: Regular Contributor
timestamp: December 30, 2018, 01:36:51 pm
content: I will attach a picture of a smaller rating Mosfet switch I have made to fit to another type of aircraft. This plane will be internal combustion engine powered, but will of course have radio control. The radio receivers and servos require 6 volts in this application and I have chosen to use an electronic switch instead of a common mechanical one for reliability reasons.I will also fit this type of switch to my unpowered slope soaring gliders. Some of the higher performance slopers use high power demand servos, so a good reliable supply is essential.This design is by Vollradth and was posted in the link above.

author: spec
author_rank: Frequent Contributor
timestamp: December 30, 2018, 02:22:02 pm
content: + Jim-0000Attached is a schematic showing the three switch architectures, mentioned previously. As you can see High Side Switch PMOSFET (HSSP) is the simplest and the best from a functional point of view, but obviously it does not use your FQP50N06 NMOSFETs. Also PMOSFETs do not quite have the same performance as NMOSFETs, but that is unlikely to be a problem (I haven't checked though).Low Side Switch NMOSFET (LSSN) is also simple and would be the one to go for, in you want to use your FQP50N06 NMOSFETs, provided the ESC was happy feeding an open circuit (a resistor from the ESC output to 0V may be required).High Side Switch NMOSFET (HSSN) is as functionally good as HSSP, and could use your FQP50N06 NMOSFETs, but with the complication of requiring a high side NMOSFET driver chip.If you would like to consider these switch architectures and let us know which one you favor, I will knock out an appropriate schematic. For HSSP and LSSN though, the circuits shown are pretty much what the final circuit would be, except that the paralleled MOSFETs (if required) are not shown.

author: spec
author_rank: Frequent Contributor
timestamp: December 30, 2018, 04:27:42 pm
content: Quote from: Jim-0000 on December 30, 2018, 01:36:51 pmInteresting. That circuit agrees with my calculations which show that 25A per MOSFET is a practical limit. This means that your 100A version would need four parallel MOSFETs to get a good solid reliable design. But if you used some really beefy MOSFETs, the heatsinking requirements would be small. Time allowing, I hope to post a High Side Switch PMOSFET design using four £3UK PMOSFETs, which will hardly need any heatsinking.

author: Jim-0000
author_rank: Regular Contributor
timestamp: December 30, 2018, 08:21:59 pm
content: Hi Spec,Thanks for providing the three draft circuits.However, judging by the way the circuitry is arranged, I fear that I have still no described my application and where about in the model power system wiring, this planned switch will go.The motor is 3 phase. The ESC is an electronic commutator. There is no direct connection from battery + or - to the motor.The 3 connections from the ESC go to the motor.The two wires from the battery go to the ESC.The attached diagram might clarify this. The switch will go between the battery and the ESC. It will not go between the ESC and the motor.Edit: The current from the battery to the ESC is DC. While the current from the ESC to the brushless motor is 3 phase AC! .

author: Jim-0000
author_rank: Regular Contributor
timestamp: December 30, 2018, 08:52:09 pm
content: This is an oscilloscope display of the waveform to a typical 3 phase brushless motor.The first is at partial throttle.The second at full throttle.I hope this clears up some confusion.Jim.

author: spec
author_rank: Frequent Contributor
timestamp: December 31, 2018, 06:52:08 am
content: Thanks Jim-0000 for the information in replies #26 and #27.Yes, that does explain the situation well and means that the only option is high side switching, either with NMOSFETs or PMOSFETs. I would recommend a high side PMOSFET design with beefy PMOSFETs which would need minimum heat sinking: a number of suitable PMOSFETs are available and at a reasonable price too. What do you think?The circuit that I have in mind would be configurable, where every PMOSFET handles 25A, so you just add PMOSFETs to suit the application: 1 x PMOSFET = 25A switching capability, 2x=50A, 3x=75A, 4x=100A, and so on.If you are interested in the theory, it is the RDSS of a MOSFET, when it is turned on hard, that ultimately dictates the current handling capability of a MOSFET. The formula for the power generated in a MOSFET is ID2* RDSS. Where ID is the drain/source current and RDSS is the resistance between the drain and source (RDSS is often just labeled RDS on data sheets).A beefy PMOSFET can have an RDSS as low as 0.01 Ohms (10 mili Ohms), so at 25A ID the power dissipation would be 252* 0.01 = 6.25 Watts per PMOSFET. Multiply by four PMOSFETs gives a total power dissipation of 25W, which I suggest would be manageable in your model aircraft.If you are wondering why four beefy PMOSFETs are required to provide a 100A switching capability, rather than just one beefy PMOSFET, here is the answer:1002* 0.01 = 100 Watts.

author: Jim-0000
author_rank: Regular Contributor
timestamp: December 31, 2018, 09:53:19 am
content: Quote from: spec on December 31, 2018, 06:52:08 amSpec,I will have to do some reading up on the definitions of the terms your use above.QuoteYes, I understand that. Thanks. Leave it with me for a few days to do some catch up reading.

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2018, 01:35:35 pm
content: Quote from: nick_d on December 30, 2018, 10:27:08 amThe gate resistor suppresses parasitic oscillation during switching and prevents gate drive overshoot. With a low capacitive impedance at the source and drain, the input impedance is negative at high frequencies like a Colpitts or Clapp oscillator. Slightly larger values of gate resistance may be used to match the impedance of the line between the gate and driver and suppress any series inductance lowering overshoot.Gate stopper resistors for tubes and especially for tetrode tubes served the same purpose. The same problem comes up with bipolar transistors driving a capacitive load when the base impedance is too low.The whole subject was discussedherewhere I posted an old article with details and math.This is one of those problems which might only be described as weird until you put a fast enough oscilloscope on it.

author: spec
author_rank: Frequent Contributor
timestamp: December 31, 2018, 09:47:35 pm
content: Hi nick_dI missed your question of post #15, which DH fielded so informatively.Gate stoppers are often a source for discussion because, at first inspection, they appear to have no function, so below are a few points.As well as helping to prevent parasitic oscillations, gate stoppers can have other functions:Along with the MOSFET's parasitic capacitances, suppress unwanted induced signals from transmitters: radio, TV, RADAR, WiFi, switches, etcAlong with other components, shape the gate waveform to optimize high speed switching (not applicable in the OP's application)Protect the gate from ElectroMagnetic Pulse (EMP) (caused by lightning mainly)Where you have MOSFETs in parallel, individual gate stoppers can help prevent odd interactions between the MOSFETsIn push pull applications, gate stoppers can be used, with other components, to delay turn on and thus provide some dead time so that both MOSFETs are not on at the same time (a fatal condition for the MOSFETs and possibly any transformer involved)One thing to bear in mind is that MOSFETs are phenomenally fast but have huge parasitic capacitances in the nF region, especially power MOSFETs. This is a formula for parasitic oscillations.The other thing to bear in mind is that the nice schematic that you see on paper is nothing like the real life physical circuit: even a piece of wire has resistance, capacitance, and inductance, and acts as an antenna, that can both transmit and receive electromagnetic signals. And when you consider, transistors, transformers, inductors, capacitors, and resistors the situation is even more complex.Finally, the physical layout of this high current switch is critical and, hopefully, will be described in later posts, but just to say that gate stopper resistors will only be effective if they are connected directly to the MOSFET gate terminals using leads as short as possible.You mentioned that there are MOSFET circuits without gate stoppers that seem to work OK. This can be the case, but will the circuits work in all situations- probably not. The other point is that a circuit may be oscillating but the user may be quite unaware of it.

author: nick_d
author_rank: Regular Contributor
timestamp: January 01, 2019, 12:13:48 am
content: Thanks guys, I did not know that. I will use gate stopper resistors in future.The theory warrants a closer look than I can give right now. I understand negative resistance more or less, as I was at one stage planning to build an audio power amp that used op-amp circuits to generate say -3 ohms to drive a 4 ohm speaker. Apparently this gives a form of motion feedback to partially compensate box and driver response.I don't understand Colpitts oscillators well or why they have negative resistance. I will revisit it some time.cheers, Nick

author: spec
author_rank: Frequent Contributor
timestamp: January 01, 2019, 07:29:14 am
content: Quote from: nick_d on January 01, 2019, 12:13:48 amNo probs from meYou need to adjust the gate stopper resistor value according to the MOSFET and the circuit function. With switch-mode power supplies, typically switching at a frequency of 50kHz to 4MHz, the gate stopper is used mainly for shaping the gate waveform, rather than preventing parasitic oscillations.For low frequency switching, as in this application, switching times are relatively unimportant but, all the same, you don't want the MOSFETs to turn on/off too slowly or they may exceed their safe operating area (SOA). Besides which, it is just a waste of power. So a good value of gate stopper in these cases is 50R. 50R crops up a lot in electronics and 50R to 100R, with most resistors, is the magic value range where the reactances of the resistor's inductive and capacitive components tend to offset one another, so you get good frequency characteristics. When you do high speed designs with ECL or PECL you use 50R PCB lines with a ground plane and, of course, you get 50R coaxial cable for video and RF.All oscillators rely on positive feedback to sustain oscillation. The Colpitts oscillator is just another method of providing positive feedback, as are the Phase Shift, Wayne Bridge, Hartley, Clapp, and so on. The Colpitts oscillator just happens to have good high frequency characteristics.Negative resistance is no big deal. With a normal resistance as you reduce the voltage the current goes down according to Ohm's law. With a negative resistance when the voltage is decreased the current goes up. And that is all there is to it.And one final bit of cracker-barrel advice is to pay particular attention to decoupling. This vitally important area is quite often missed on the vast majority of circuits that you see on the net and in books, probably because decoupling components tend to clutter the schematic and make it harder to follow.

author: spec
author_rank: Frequent Contributor
timestamp: January 01, 2019, 08:29:40 am
content: If all this stuff about gate stoppers, decoupling, routing etc sounds a bit fussy and over the top perhaps the following two stories will illustrate the consequences of not doing it right.The first story concerns a friend's high-end MOSFET audio power amplifier. He said that it sounded very good but there was something about the sound that was not right somehow, and he couldn't quite put his finger on it. I had a listen and on his outfit it sounded superb... at first. But, after a while, I too detected an odd characteristic to the sound, so I took his amp home to investigate.To short a long story, I found that the output power MOSFETs were bursting into oscillation at random points of the output voltage waveform. The amplitude of the oscillations varied and, for a while, they would be absent. There were no gate stoppers and relatively long traces between the MOSFETs and the driver circuit. So, obviously, I fitted gate stoppers. This greatly reduced the tendency for the MOSFETs to burst into oscillation and lowered the frequency too. But it was not a complete cure.The MOSFETs were decoupled, but only with high-value aluminum electrolytic capacitors, so I added polypropylene decoupling capacitors. The result was that there was no sign of the parasitic oscillations and the owner was over the moon that his amp had regained it's original pure sound.The other story concerns a maritime system built into a tall 19 inch cabinet. Right from the start this system was troublesome and gave spurious results. And after many fancy investigations with no conclusion, I was asked to arrange an investigation. As it happened, we had just taken on a new graduate, and he got the job. Once again, to short a long story, he found that far from being some esoteric technical issue that was causing the problem, it was the basics that were wrong.The first thing he found was that the 74 series TTL chips on the cards that made up the equipment only had around 4V between their actual 0V pin and 5V supply pins (min allowable is 4V75), although the central 5V high-current power supply was pushing out 5V2. He also found that the analog and digital circuits had inadequate decoupling. Armed with these findings, he deigned a modification scheme to correct the issues, and when that was implemented- guess what? The system was transformed and never caused a problem again. And even better, we never heard another squeak from the 'experts' who had been deriding our elementary approach.

author: Niklas
author_rank: Frequent Contributor
timestamp: January 01, 2019, 01:42:26 pm
content: I used to have the same hobby, flying 10 cell (NiMH) F5F electric gliders, but have not been active for at least 10 years. Brushless motors were new back then and the LiPos were just introduced.There might be an easier way to implement the safety feature, but it depends on the speed controller's safety. Usually there is a microcontroller that does the control signal processing and commutation calculations. Upon power on, the software waits for the control signal to be "motor off" before enabling the speed controller function. It is also common that it will use the motor windings to beep when this is done.There is also another software function that performs a watchdog timeout function. The control signals from the receiver is sent out at approx 20-50 times per second, depending on the manufacturer. The signal is a 5V pulse, positive logic, with pulse width varying between 1.2 and 1.8 milliseconds, depending on the demanded speed or servo angle. If that signal is not received, or the pulsewidth is out of range, then the software should disable the speed controller.A small switch, inserted on the control signal between the receiver and the speed controller, and a pulldown resistor (10k, signal-GND on the ESC side of the switch) could be an alternative. The control signal wire is either yellow, orange or white and the GND wire is usually black or brown. If you use an extension cable, then you don't need to modify the speed controller itself.

author: spec
author_rank: Frequent Contributor
timestamp: January 01, 2019, 02:13:21 pm
content: Quote from: Niklas on January 01, 2019, 01:42:26 pmBrilliant

author: David Hess
author_rank: Super Contributor
timestamp: January 01, 2019, 02:22:50 pm
content: Quote from: spec on January 01, 2019, 08:29:40 amMy stories involving MOSFET linear output stages without gate resistors usually involve the MOSFETs breaking into a roughly 10 to 100MHz howl and self destructing and the better the low loss high frequency decoupling is, the more quickly they self destruct. Testing shows no problems until the output cable is attached.For switching stages time spent in the linear region is short so oscillation is very rarely a problem but larger values may be used to control ringing in the gate drive signal. But an ESC is usually driving a relatively long lead (transmission line) to a motor which can lead to problems.The safe rule of thumb is to include a lossy element on at least one lead of any transistor. Strong decoupling and a capacitive load are especially problematical.

author: spec
author_rank: Frequent Contributor
timestamp: January 01, 2019, 04:00:25 pm
content: Ha Ha I love these stories. It is a consolation to know that others have problems too.Before, I got the hang of decoupling, stopping, etc, I went though a phase where everything I touched oscillated, unless I tried to make an oscillator that is.We had a name for intermittent circuits- 'Thursday Circuits/Equipments'. Thursdays were often the day for customer demonstrations/handover and, guess what, that would be the day that the gremlins would show themselves.By the way, I started a new thread on this general topic:https://www.eevblog.com/forum/chat/thursday-circuits-gremlins-repairs/

author: spec
author_rank: Frequent Contributor
timestamp: January 01, 2019, 04:14:54 pm
content: Quote from: David Hess on January 01, 2019, 02:22:50 pmGood idea, presumably a lossy feritte bead would be one way.Quite right about strong decoupling being critical, but you often need strong decoupling to get a good frequency response, supply line rejection, etc. Also poor decoupling can cause parasitic oscillations/ringing too.

author: David Hess
author_rank: Super Contributor
timestamp: January 01, 2019, 08:06:06 pm
content: Quote from: spec on January 01, 2019, 04:00:25 pmThe first time I ran into this problem was when I was like 13 and had hooked up a 7805 regulator to my 723/2N3055 based bench supply. How does a 5 volt regulator running on 8 volts put out 12 volts? One 0.1 microfarad ceramic disc capacitor at the regulator's input solved it.Quote from: spec on January 01, 2019, 04:14:54 pmThere are a bunch of different ways to handle it as shown in the article attached below.A ferrite bead is usually more expensive than a resistor and a resistor also serves as a fuse but I have used either way depending on the application.

author: Jim-0000
author_rank: Regular Contributor
timestamp: January 01, 2019, 08:35:48 pm
content: Quote from: Niklas on January 01, 2019, 01:42:26 pmExcellent suggestion Niklas! I know that if the good quality ESC's that I use do not receive a control signal, the motor will not operate.I think this might be the solution...........and a very elegant one at that!QuoteI will probably cut the servo lead signal wire and hard wire the switch in series with it including one end of the resistor and the other end to earth. I try to avoid servo lead plugs and sockets wherever possible. We know there are too many poorly crimped servo extension leads around on the market.I have drawn a little sketch to confirm if I have interpreted your suggestion correctly or not. Please check it for me.I will be trying this out on the bench before the day is finished. I will cut up an extension lead for test purposes first, before hard wiring the switch and resistor in the ESC receiver lead as described above.Many thanks,Jim.

author: bitbanger
author_rank: Regular Contributor
timestamp: January 02, 2019, 04:17:18 am
content: Three phase BLDC ESCs can be quite complex (much more-so beyond just this parallel issue) even for the hobby realm. I'm never one to knock anyone's ambition but have you considered that you can buy these readily? However perhaps this is being integrated into an existing PCB? Either way, this doesn't address the parallel question directly but you may find the below reference designs helpful:https://www.st.com/content/ccc/resource/technical/document/user_manual/group0/06/a3/c1/ae/7d/27/4c/e0/DM00384353/files/DM00384353.pdf/jcr:content/translations/en.DM00384353.pdfhttps://www.infineon.com/dgdl/Infineon-Application-Motor_Control-Drone_Electronic_Speed_Controller_ESC-TR-v01_00-EN.pdf?fileId=5546d462580663ef015843a229fe54ea

author: Jim-0000
author_rank: Regular Contributor
timestamp: January 02, 2019, 05:21:16 am
content: Quote from: bitbanger on January 02, 2019, 04:17:18 amThanks for the post Bitbanger.However, we appear to be at crossed purposes. Either that, or you have replied to the wrong thread. Your post has me mystified; I apologise if I may have communicated the wrong message along the line.Please explain, what is this "parallel issue"?I already have ESC's (Electronic Speed Controllers plural!). I have no intention of building one any day soon, and most likely, never will. They are readily available, cheap and of good quality: as you suggest.The aim of this project is to design and build a device that will safely switch the power supply (in this case, a Lithium Polymer 14.4 volt battery), to an already existing ESC.Not to make one.Jim.

author: Jim-0000
author_rank: Regular Contributor
timestamp: January 02, 2019, 09:10:49 am
content: Quote from: Niklas on January 01, 2019, 01:42:26 pmNiklas,Success! I just did a test on the bench with a cut and modified servo extension lead as per my little circuit diagram above. It works perfectly..........the motor will run with the switch on and not run with it off regardless of the throttle stick positon.Very good, thank you for the creative suggestion.Thanks to all contributors.Jim.

author: bitbanger
author_rank: Regular Contributor
timestamp: January 02, 2019, 12:01:37 pm
content: Jim - appears my post was the result of insufficient reading/skimming, and the late hour! My apologies for the confusion, please disregard.Cheers,James

author: Jim-0000
author_rank: Regular Contributor
timestamp: January 02, 2019, 06:18:06 pm
content: Quote from: bitbanger on January 02, 2019, 12:01:37 pmJames,Apology accepted, of course. All good in the end.Thanks for explaining.Jim.

author: Jim-0000
author_rank: Regular Contributor
timestamp: January 03, 2019, 03:53:04 am
content: This is a short video of my vastly simplified safety switch in operation on the test bench. Circuit thanks to Niklas.