Inductive Ringing

cmartinez

Joined Jan 17, 2007
6,789
For this application, 1Khz 0.5A 12V
If that's the case, I wouldn't worry too much about it. It's a rather low load at a low frequency. Just make sure you place a TVS diode across the load to protect the mosfet

Is it possible to create a MOSFET LTspice model, or modify an exisiting model for this device?
Yes it is, and there are several people in this forum that could help you accomplish that. But it's not a minor thing to do. It requires patience and experience. If you really want to dive into this subject, I suggest you open a different thread dealing exclusively with it. I'd love to help you on this one. I use LTspice for all my sims, but my knowledge is quite limited regarding the creation of new components.
 

SteveSh

Joined Nov 5, 2019
105
Position the diode more correctly next to the transistor. This diode is designed to protect the transistor, not the choker.
I don't agree. The diode belongs as close to the coil of the relay as possible. It protects the transistor no matter where it's placed. But the further away from the coil it is, the bigger the loop area for the circulating current and so the bigger the concern for EMC.
 

MisterBill2

Joined Jan 23, 2018
5,229
I don't agree. The diode belongs as close to the coil of the relay as possible. It protects the transistor no matter where it's placed. But the further away from the coil it is, the bigger the loop area for the circulating current and so the bigger the concern for EMC.
Going back to post #1, it is a solenoid of some sort, not a relay. And if the goal is to protect the transistor then put the diode close to it. If the goal were to keep the solenoid quiet then put it at that end.
 

TeeKay6

Joined Apr 20, 2019
572
I don't agree. The diode belongs as close to the coil of the relay as possible. It protects the transistor no matter where it's placed. But the further away from the coil it is, the bigger the loop area for the circulating current and so the bigger the concern for EMC.
@SteveSh
The diode protection is not the same for both diode locations, especially when the solenoid is 10M from the transistor. When placed at the transistor, the diode protects against flyback both due to the solenoid and to the long connecting wires. When placed at the solenoid, the diode protects only the solenoid...and the solenoid almost certainly needs no protection. With regard to EMI/RFI, I agree with you, but the TS has not said that there is any significant problem with emissions. Also, the TS has not indicated whether the 10m connecting wires are parallel wires or a twisted pair; that could be relevant for the issue of EMC.
 

TeeKay6

Joined Apr 20, 2019
572
Thank you all,

I think I see now that the ringing is not an issue for the solenoid itself, I was concerned about the amount noise in the cable driving the solenoid. However, I think I see this is not so bad.

I did add a MOSFET driver, although this cleaned the gate square wave slightly, no real change in the MOSFET output slopes. But this led me to more questions - I searched "MOSFET Gate Resistor" and found hundreds of results that just made me to be more confused.

If I understand everything correctly: The gate resistor for the VNN1N04 is specified as 330r minimum, if I was choosing this I would have worked on 20mA (assuming the input would be initially shorted to ground - mcu output pin 25mA) so 250r.

So if 330r is the minimum, why use anything else? The VNN1N04 is a logic level device, if I use anything bigger, like the 820r I am using, in theory this would slow the switching slopes.

I think I see that in a different application with a different MOSFET, higher Id, higher frequency my driving device may not be able to meet the drive requirements, and I would have to use a MOSFET gate driver.

Am I understanding this MOSFET correctly, why would you increase the gate resistor?

I can't find an LTSpice model for this device, so I have not been able to test this.

Thanks
"So if 330r is the minimum, why use anything else? " I agree; the data sheet says use 330Ω minimum, so use 330Ω. The drain current is limited internally by this "smart" MOSFET and the switching speed may not be much affected by the gate drive resistor.
 

Thread Starter

Gorden

Joined Oct 30, 2019
19
@SteveSh
The diode protection is not the same for both diode locations, especially when the solenoid is 10M from the transistor. When placed at the transistor, the diode protects against flyback both due to the solenoid and to the long connecting wires. When placed at the solenoid, the diode protects only the solenoid...and the solenoid almost certainly needs no protection. With regard to EMI/RFI, I agree with you, but the TS has not said that there is any significant problem with emissions. Also, the TS has not indicated whether the 10m connecting wires are parallel wires or a twisted pair; that could be relevant for the issue of EMC.

Yes, i did say it is a simple control flex - not twisted pair. I am not sure if i will have an EMC problem, this was the basis of the original post, did the noise look like a problem. I am thinking that it's not - it is 1Khz or less, 500mA at 12V.

The diode will be at the MOSFET end - becuase that's the most practical place to fit, however is there another job the diode does other than provide a path for the back emf.

Does it keep the solenoid engerised longer during the PWM off time?
 

Thread Starter

Gorden

Joined Oct 30, 2019
19
Sorry - just to be clearer, the diode is across the solenoid, but at the MOSFET end, not accross the MOSFET. This device does have internal clamps.
 

Bordodynov

Joined May 20, 2015
2,502
We can talk about where to put the diode for a long time. The best way to do that is to just check in reality when there is more interference. When I counted (I took a twisted pair), it was better to find the diode next to the transistor. But there are options for just two wires, depending on their location.
The best way is to practice!
 

cmartinez

Joined Jan 17, 2007
6,789
And on the other hand (other than a few cents spent, and perhaps a lack of elegance), I see no reason not to put a diode on both ends, that is next to the transistor and also at the inductor.
 

Thread Starter

Gorden

Joined Oct 30, 2019
19
Position the diode more correctly next to the transistor. This diode is designed to protect the transistor, not the choker.View attachment 193063View attachment 193063View attachment 193063View attachment 193065
This helps a lot. Measuring the ringing at the coil end, with the 100r 0.01uf fitted across the coil end (0.1uf looks even better). Only problem here is in an ideal world - nothing fitted to the coil end.

Fitting the snubber at the MOSFET end obviously has no real effect, fitting a 0.1uf on it's own across the diode at the MOSFET end improves the noise!, again measured at the coil end.
 

BobaMosfet

Joined Jul 1, 2009
1,012
If someone who can explain the purpose and function of the gate resistor will do that it will help indeed. Guesses are not welcome at this point, it needs to be an answer from real knowledge. I can provide a number of guesses with no promise that any of them are right, but they would all sound good. And it would not help at all, so I will not offer them.
R2 limits current to the gate; R1 is a pull-down to keep the gate in a specific state when not powered by the MCU. Notice the difference in resistor values- R2 allows more current to flow, than R1. So R2 overwhelms R1 if 5V is applied by the MCU. Otherwise, R1 ensures a small current flows to keep the gate in a grounded state.

1574873305329.png
This is not rocket science. I have to assume the MCU is digital (0-5VDC), since it's an MCU pin being referenced.
 
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BobaMosfet

Joined Jul 1, 2009
1,012
The series resistor, R2 is the one that needs explanation. R1, the resistor from the gate to the source, is a pull-down resistor to remove the gate charge, thus removing the gate to source bias, switching the FET off, if the driving source does not include a means to remove the gate voltage. That part is not a guess. The value of that resistor, 10 K ohms, seems a bit high to me, and it may be related to the slightly slower turn off time shown. But that is an educated guess.
Look at my explanation (entry #34) for R1 & R2 in this thread, all the rest of you are wrong.
 
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