Snubber circuit for MOSFET with inductive load - Page 1
Forum section: electronics / beginners

author: Darlington
author_rank: Contributor
timestamp: September 24, 2024, 02:29:41 pm
content: Hi Everyone,I'm making a snubber circuit for an inductive load (I need to open and close a magnetic lock).Various RCD snubber configurations can be found online, but I don't understand what difference there is between the two attached circuits.Also, I would like to make this design on a PCB, but since I do not have a fixed load, how could I leave open the possibility of adapting the snubber to the load?The MOSFET that I have chosen is the IRLML0100TRPbF.

author: PGPG
author_rank: Frequent Contributor
timestamp: September 24, 2024, 03:43:28 pm
content: Quote from: KKK on September 24, 2024, 02:29:41 pmI have no experience with snubber circuits, but if you assume R in snubber is 0 you will see the difference.Quote from: KKK on September 24, 2024, 02:29:41 pmIt is specified for Vgs=4.5V and you said nothing about your GPIO voltage.

author: Darlington
author_rank: Contributor
timestamp: September 24, 2024, 04:55:03 pm
content: VGS comes from an opto-isolator with a collector referred to a 5V power supply

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 24, 2024, 05:07:29 pm
content: Both circuits will do their job of reducing EMI and protecting the FET if everything is correctly dimensioned.The lower one will have a longer release time but I think this doesn't matter for an electric lock. The lower one will have the advantage that you can feed the +12V for the lock completely from an external source, the upper circuit requires you to route the +12V through your control circuit.

author: Darlington
author_rank: Contributor
timestamp: September 24, 2024, 05:45:11 pm
content: From some simulations done on the first circuit (snubber1) I notice that if you have a low switching frequency, less than 1Khz, if I increase R the peak increases.The fast recycling diode alone performs much better than the RCD circuit....

author: PGPG
author_rank: Frequent Contributor
timestamp: September 24, 2024, 06:34:08 pm
content: Quote from: KKK on September 24, 2024, 04:55:03 pmIf something driving LED in opto-isolator has the same GND as this circuit than opto-isolation is not needed.You can consider using Inteli-FETs like ZXMS6006D.

author: Darlington
author_rank: Contributor
timestamp: September 25, 2024, 06:41:00 am
content: Quote from: PGPG on September 24, 2024, 06:34:08 pmThe ZXMS6006D is perfect for my needs. I assume it has to be polarised like a normal MOSFET, right?As far as the opto-isolator is concerned, the complete scheme actually allows the two ground references to be coupled or decoupled

author: PGPG
author_rank: Frequent Contributor
timestamp: September 25, 2024, 09:19:28 am
content: Quote from: KKK on September 25, 2024, 06:41:00 amI was using ST Omni-FETs (they are stronger - 7A limit - not a problem) when working with 5V, but according their datasheet you can't be 100% sure of operation with 3V3. Searching for something to replace them I found these ZXM that can be driven from 3V3. As you have 5V you can check also Omni-FETs.Everything you should have in datasheet. Normal MOSFET needs 0 DC gate current but fully protected FETs need some small current to supply their protection circuitry.I'm sure that Omni-FETs reported back (by increasing current driven from gate pin) that some protection is triggered, but I'm not sure how with ZXM serie (I didn't used that feature).Because of this you can't drive them directly from digital IC output but you have to have there resistor.

author: Salitronic
author_rank: Contributor
timestamp: September 26, 2024, 07:10:45 am
content: The proper turn-off snubber is the second one (snubber across FET), the RCD circuit across the load as in your first image has a different purpose.The diode across the inductive load is strictly not part of the snubber circuit (although in some aspects it works towards the same goal), that diode is a freewheeling diode and is an essential part of the circuit, irrespective of any snubber you might add. It is this diode that takes on the biggest share of clamping the voltage across the inductor.The problem with using just a freewheeling diode across the inductor is that this clamps the inductor voltage to a very small value and correspondingly a low di/dt which results in a long time for the energy in the inductor to dissipate fully. Adding a series R/C circuit in series with the freewheeling diode as in your first image allows further control of this clamping, so that a higher clamping voltage can be used to discharge the inductor more quickly. That is the primary purpose of those R/C parts and although this is similar to a snubber circuit it is not a turn-off snubber.When the FET turns off, the freewheeling diode does not react instantly, there is a brief period in which the FET has turned off and the freewheeling diode is building charge to start conducting. During this period the voltage across the inductor will peak to some high value. The purpose of the turn-off snubber as depicted in your second image is mainly to handle this brief change-over period so that the voltage across the FET is limited to a safe value until the freewheeling diode effectively clamps the inductor voltage for the rest of the discharge.