Inductive MOSFET snubber design - Page 1
Forum section: electronics / beginners

author: Samuel43
author_rank: Newbie
timestamp: June 21, 2025, 03:55:22 am
content: Hi,My brother has tasked me with designing him a soft starter for a fan in the engine bay of his car. It will be programmed to ramp up from 0 to 100% power over a 10 second period when the input labeled 'SENDER' is pulled low by a temperature sensor switch in the engine.I have not designed high speed PWM inductive load switching circuits before, I have only designed simple on-off ones that can get away with using a 1N4007 diode snubber. I understand that for circuits like this it is recommended that you use RC snubbers.Of note, the fan is 12V 80W.Questions:1) For this application, can I get away with a diode snubber or do I need to seriously look at using an RC snubber.2) Are there some simpler rules of thumb that I can follow to figure out RC values, as various documents online have some very nasty looking equations and seem to imply that I would need to build the PCB and measure/quantify certain PCB trace inductance values, which I have no easy way to do at home.3) Is there a recommended PWM frequency range that would make designing a snubber circuit easier?4) Currently for my high-side switching setup I have a the large P-Channel MOSFET that switches the load and that is then switched by a smaller n-channel FET from the microcontroller. Is this an appropriate setup or should I be using a MOSFET driver IC?Please see attached initial schematics and pngs of the board.Thank you

author: David Hess
author_rank: Super Contributor
timestamp: June 21, 2025, 12:13:16 pm
content: Quote from: Samuel43 on June 21, 2025, 03:55:22 amI have used diode snubbers in similar circuits without problems, but I was operating at 20+ kHz.Here I think I would do the same thing with the power diode placed *across* the motor so the flyback current is contained and not directed back around the MOSFET into the battery. Then I would operate the PWM at a relatively low frequency.Note that this must be a high current fast diode.QuoteI think there is an old Motorola application note which covers it well, but I don't know how far you can get without knowing the motor inductance so how much energy it is storing per switching cycle.QuoteThe frequency does not matter, however in your case you can use a pretty low frequency, like 100s of Hz. The motor will hum, but only during startup.QuoteWhatever you want to do will work. I would use a high side p-channel MOSFET large enough to blow the fuse if there is a short, rather than some type of current limiting.

author: PGPG
author_rank: Frequent Contributor
timestamp: June 21, 2025, 12:49:11 pm
content: Quote from: Samuel43 on June 21, 2025, 03:55:22 amHigh speed is for me several hundred kHz while here you can run at hundred Hz, I think.So may be from electronic point of view it is like DC switching on/offIn 70s/80s I have been playing with car electronic in my father car (thyristor ignition, wiper intermittent, LED battery voltage indicator, ...). I don't know car electronic standards but I think that the greatest danger will come from the power supply side, where you should expect occasional surges of high voltage and current (e.g. when switching off the starter).I have no experience with protecting against it, but you should not ignore it if you want your circuit to work for long time.And one more thing. I don't know if the insurance company can't use every non-approved device installed in the car to avoid paying compensation in some cases.

author: johansen
author_rank: Super Contributor
timestamp: June 21, 2025, 03:21:41 pm
content: You can use a N channel fet and switch the - side of the fan.As for the snubber: the loop area between the mosfet, back emf diode and the battery is an inductance that must be absorbed by a capacitor located close to the mosfet and the diode.The switching frequency doesn't matter except with regard to turn on and off losses.If there is enough parasitic inductance to cause the fet to go into avalanche breakdown, lowering the switching frequency doesnt make it go away.(Except that since you have to set your oscope for 10 or 1ms per division to see your LF pwm, does makes it go away...because now you cant see the half microsecond long voltage spike on your scope lol!)....

author: David Hess
author_rank: Super Contributor
timestamp: June 21, 2025, 04:30:04 pm
content: Quote from: johansen on June 21, 2025, 03:21:41 pmThat is discouraged in automotive applications because they use a negative ground and nobody will be expecting it.QuoteThe motor is not designed for high frequency switching, so hysteresis losses can be high, although this did not stop me from using 15+ kHz to drive an alternator field winding.

author: johansen
author_rank: Super Contributor
timestamp: June 21, 2025, 05:44:27 pm
content: As the frequency goes up the ripple current goes down, so it's a wash.can you give some real numbers for how much extra losses at say a 50% duty cycle into a 12v 10amp fan motor at 1 vs 10khz?Due to the switching turn off ans turn on times you will need an rc or lc integrator to precisely measure the rms volts at the fan motor to ensure the same duty cycle at 50% at both frequencies. A 1% difference would exceed the extra hysteresis losses.Car fan motors may have .040" thick laminations, and low inductance due to ceramic magnets.

author: David Hess
author_rank: Super Contributor
timestamp: June 21, 2025, 07:09:34 pm
content: It is actually both hysteresis losses and eddy current losses. It will not matter for this application because they will only exist during the soft startup period. I did not worry about it with the alternator field winding because an alternator already runs hot and has forced air cooling.I suspect it would be just as good or better to use a power resistor startup with two power MOSFETs. This way there would be no switching nonsense to deal with.

author: floobydust
author_rank: Super Contributor
timestamp: June 21, 2025, 08:08:29 pm
content: Samuel43 my thoughts on the circuit:The back-emf diode D1 would see peak current equal to the motor's current, so it should be an 8-10A diode, and also fast recovery, Schottky is good too. 1N4007 is not suitable here.I don't see any protection for transients, the circuit is pretty fragile. You'd never run a raw MCU input out to a sensor on a car engine. Vreg input must have a series diode to prevent -ve spikes from killing it. The mosfets I would have gate zeners, and a low value R series for the SQD40081 it has big 10nF gate capacitance.R3 1k can dissipate 200mW so it needs to be a bigger part.The fan can windmill and act as a generator so DRIVE_IND might unexpectedly glow when the fan is off.The PCB traces will not take the high current unless you add solder on top of them or a copper wire busbar in that.

author: langwadt
author_rank: Super Contributor
timestamp: June 21, 2025, 08:34:06 pm
content: often easier to just use something like;https://jlcpcb.com/partdetail/InfineonTechnologies-BTS6143D/C95232then it's protected against most abuse

author: Samuel43
author_rank: Newbie
timestamp: June 23, 2025, 02:03:41 am
content: Okay,Thanks everyone for all the replies. I updated the design to address the feedback. I changed out the P-Channel FET to something with lower input capacitance and with built-in zener protection. I also changed R3 to 100R and selected a larger package. I also changed D1 to a much larger Schottky diode and added a diode to the input of the regulator. Finally, I added an SSR to couple the engine sensor to the MCU.I did already plan on tinning the high current traces with extra solder. I realize that that did not show up in my previous renders from my starting post. I used a different method so that the tinned traces show up correctly.

author: floobydust
author_rank: Super Contributor
timestamp: June 23, 2025, 08:42:37 pm
content: I think R3 is wrong and more like this. I'm not sure how much current an internal zener can take on a mosfet and always use an extra one. BSS138K also has one. Point is expect at least 40V transients on a modern car, much greater on a vintage car.The skinny PCB trace could be done different. Use the room and copper is free on a PCB.The wire to the temperature switch is a good antenna and will pick up ignition noise etc. Also there can be -ve spikes because the engine block ground and your board's ground, the fan motor's ground- are likely all different places. I don't know the car or layout.The R7 1k needs to be higher power or higher value and I would have a resistor across the opto LED so it is not so sensitive. An ordinary opto-coupler could be used like PC817, 4N25 etc.I would consider using just resistors on the MCU input, 12V at the temp switch but divide it down for the MCU input.With the Vreg input I add a 10R resistor to limit the current for the 1N4007 section in case that cap is taking all the +ve voltage spikes. You don't want all the car's +ve spikes being fully absorbed by your input circuit.

author: MrAl
author_rank: Super Contributor
timestamp: June 25, 2025, 05:07:00 pm
content: Quote from: Samuel43 on June 21, 2025, 03:55:22 amHi,A few things to think about...First, the need for a Mosfet driver comes in more with high frequencies which would be 20kHz or something like that, especially with 100kHz. You can get away with 200Hz here for example, which means the turn on and turn off times are not as important. It's still an issue though, and you need to check that because it could easily overheat the Mosfet. You may find you need a heat sink anyway. Driving with a resistor like 1k would be too high in most cases because that would cause the turn on and turn off times to be too long. You can easily make a custom driver with two transistors: one to turn off, and one to turn on.Second, motors have a lot of inductance which means once you get current flowing you can't stop it too easily, and that affects the ability to get a proper PWM power drive to the motor. The circuit has to be able to supply current and also be able to decrease the current rapidly. Since the energy in an inductor has to be forced to decrease as well as increase we can look at the energy in the inductor to get a good idea what has to happen. The energy can be viewed as the volt seconds, and that is simply the volts times the time in seconds. So during regular PWM, if you apply 12 volts for 1 millisecond the volt seconds so far comes out to 12*0.001=0.012 volt seconds. In order to sweep the energy out of the inductor, we need to remove 0.012 volt seconds. That means again 12 volts for 0.001 seconds but this time the inductor has to supply the energy. Since volt seconds is simply the volts times the seconds though we allow other voltages like 1v typical for an approximate single diode voltage drop. However, since we applied 12v for 1ms at 1v we would have to allow a longer time period, which would be 0.012/1=0.012 seconds. So we get away with 1ms turn on, but to turn off completely we need to wait for 12ms which is 12 times longer.If we could up the voltage to 2 volts, that would decrease to just 6ms, and at 4 volts it would decrease to 3ms, 8v 1.5ms, and finally at 12v again it would only require 1ms.The question is, how do we up the voltage during turn off.It turns out it could be easy. Since the Mosfet is used as a high side driver, when the Mosfet goes to turn off the voltage across the inductance (the motor) will try to shoot up extremely high, even 1000 volts if it was allowed to. We usually refer to this as a high voltage spike. It's the inductor trying to dump it's energy as fast as possible. We can't allow that, but we could clamp it to a higher voltage like with a zener. Interestingly, we already have a 12v source with a very low impedance which is the car battery. This means we can run a diode from the motor to +12v and we will then see the inductance discharge in about 1ms with that 0.012 volt second energy we were assuming previously.The question now is, is that going to be fast enough. This now becomes a question of how good of a control over the motor do we need. If the accuracy of the PWM does not have to be really perfect, this is probably good enough at say 200Hz.The other thing now is that since this diode will connect from the motor positive terminal to the battery positive supply node, it will be a bit longer and so it's a good idea to use a snubber as well. The snubber connects very close to the transistor, across the drain and source.The other question is can you get away with just a single diode across the motor. Again that depends on how fast you need it to lose energy. At a low frequency like 200Hz it may work ok anyway. You can also use several diodes in series which will work even better.The whole reason for thinking about all this is that when we supply a PWM of 50 percent, we hope to get an average of 50 percent of the voltage across the motor. If we want 75 percent, then we hope to get 75 percent of the voltage across the motor. 50 percent would be 6v with the engine off, 75 percent would be 9v with the engine off. If we do not need that correlation, then we can get away with a lot more which could mean less circuitry.

author: Samuel43
author_rank: Newbie
timestamp: August 11, 2025, 12:54:22 am
content: Okay,Sorry for the hiatus. My brother put the project on hold after the head gasket blew while he was tuning a new ECU he installed, which he was able to fix. I looked into langwadt's suggestion of using the BTS6143D and decided to proceed down that route since it gives a few more protections than my original designs, thank you for letting me know this exact type of component exists. I ended up using a VN7007AHTR since it is advertised as automotive grade and natively can handle the negative spikes from an inductive load without other external components. I did also set it up to do current sensing so I can report a short or open load condition.Another question I have is should I add a TVS diode or can I just rely on the ESD protections from the linear regulator and VN7007?Thanks

author: Psi
author_rank: Super Contributor
timestamp: August 11, 2025, 01:09:34 am
content: I use VN7003 in some of my automotive products. Similar to VN7007 except VN7003 is a bit higher current and lower RDSon. They are great chips.The VN7003 drives a 5-10A 6mH solenoid in my case.I have a 24V 5000W TVS that clamps at 36V across the 12V input supply on the PCB. (ATV50C240JB-HF)I also have a 20A 100V diode across the load as extra protection from the negative spikes. The VN7003 can handle some negative spikes but 10A into 6mH was just too much for the chip so I needed extra protection. (I used a VB20100S-E3/8W)From purely a Rds standpoint you don't need any extra heatsinking for 80WVN7007AH is 7mR and your 80W is 6.6A so 6.6^2 * 0.007 = 0.3WBut if you are switching at 200+Hz (something i have done on the VN7003) it does get quite a bit hotter from switching losses.So you might want to expand the tab area to give the thing more copper area to dissipate heat, since it's free to do so.Below is how I did it, with via's to bottom layer and more copper there as well.

author: Samuel43
author_rank: Newbie
timestamp: August 12, 2025, 03:48:48 am
content: Thank you Psi for your input. Please see updated schematic and boardviews. A few final questions:1. For connecting battery voltage to the soft starter I have selected a B02P-VL connector (https://www.digikey.ca/en/products/detail/jst-sales-america-inc/B02P-VL/1883075). Is this appropriate or does someone else have any other suggestions? I find searching for connectors on Digikey difficult for some reason and datasheets aren't always helpful on telling you the matting connector part number.2. I have attached a simplified schematic of how the soft starter will connect to the rest of the system. I'm planning on having it connect through a DPDT (ON-OFF-ON) switch, so that if the soft starter fails, the fan can still be controlled. Currently, I only have the switch only control the high-side of the motor and have the low-side go through to the M- connection and to ground regardless of switch position. Can this potentially fry the soft starter when the fan gets manually switched off but the M+ terminal is not connected to the high-side of the fan? Don't worry, there will be a fuse as well.Thanks

author: Psi
author_rank: Super Contributor
timestamp: August 12, 2025, 06:02:21 am
content: Connector looks fine to me.Quote from: Samuel43 on August 12, 2025, 03:48:48 amNot entirely sure what you mean