MOSFET and flyback diodes - Page 1
Forum section: electronics / beginners

author: JohnSL
author_rank: Regular Contributor
timestamp: August 24, 2016, 07:55:37 am
content: I'm going to be using a MOSFET to drive solenoids and DC motors (just on or off, no PWM). My question is about the need for a flyback diode. Here is the MOSFET I'm thinking about using:https://www.digikey.com/product-detail/en/toshiba-semiconductor-and-storage/SSM3K361R,LF/SSM3K361RLFCT-ND/6109128It looks like it will conduct reverse current. So I'm wondering if I need an external flyback diode, or if I can just use the reverse diode built into the MOSFET for that purpose.The solenoids I'm planning on using are not very large, and run at 24V:http://www.robotdigg.com/product/566/High-frequency+Solenoid+Valve+24VDCAnd the motor I'm planning on using is a 2A motor:http://www.robotdigg.com/product/599/Diaphragm+pump+4+desktop+PNP+MachineThanks.

author: rs20
author_rank: Super Contributor
timestamp: August 24, 2016, 08:01:31 am
content: The diode in the MOSFET is across the MOSFET, not across the inductive load. So it's in the wrong place, and will do absolutely nothing to protect the MOSFET. You still need a separate flyback diode, placed across the inductive load as shown in diagrams online.

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 24, 2016, 08:09:34 am
content: Also, 3.5A absolute max mosfet dc current doesn't seem to be enough for a 2A motor, at start you will have peaks higher than that, unless you plan to cap the current with a resistor or something

author: JohnSL
author_rank: Regular Contributor
timestamp: August 24, 2016, 08:27:43 am
content: Quote from: rs20 on August 24, 2016, 08:01:31 amThanks, I should have seen that. So my next question is this. What should I spec for the diode in terms of max voltage and current?

author: JohnSL
author_rank: Regular Contributor
timestamp: August 24, 2016, 08:35:39 am
content: Quote from: MasterTech on August 24, 2016, 08:09:34 amI wasn't planning on using anything other than the MOSFET. What's a good rule of thumb in terms of the max current for the MOSFET to handle a 2A DC motor? These MOSFETs are cheap, so I have no problem going to a higher current.Also, I'm wondering what the best maximum voltage rating would be if I'm using 24V for the motor. Is 30V high enough, or should I go higher?Thanks again!

author: rs20
author_rank: Super Contributor
timestamp: August 24, 2016, 08:46:33 am
content: Quote from: JohnSL on August 24, 2016, 08:35:39 amThe max current is basically marketing nonsense. To calculate if a MOSFET is suitable, you need to be focussing more on working out if you meet the max power and max junction temperature specifications.The idealistic approach to selecting a MOSFET is to take all available MOSFET, filter out the ones that will blow up (i.e., exceed max current, power OR temperature), calculate how hot the remaining ones will get (using Rds(on), not max current ratings), and choose a compromise between that factor and cost. Obviously I'm not suggesting you do this exhaustively for all MOSFETs; I only point this out to make the point that asking for a max current rule of thumb here is really missing the point.In particular, since you're not doing PWM, you can tolerate a fairly high gate capacitance, so if I went through my selection process, I'd probably end up using any cheap old MOSFET "rated" for 50-100A. IRFZ44N is something I have lying in my MOSFET drawer here. Not because I targeted 50-100A, but because I was looking for low Rds(on) (which translates to lower wasted heat via the formula P=I^2 R).Edit: Amongst SOT-23's at Digikey, Si2342DS is the best available option. 17 milliohm Rds(on) at 4.5V, and a nominal rating of 6A. Not sure if that's sufficient or not, depends on the current profile of the motor at startup.

author: krolu
author_rank: Contributor
timestamp: August 24, 2016, 09:07:31 am
content: Quote from: rs20 on August 24, 2016, 08:46:33 amI was always wondering why MOSFET transistors blow so often. It seams that BPJ transistors will handle a lot more than those. This is a real good advice, just do not take current rating as primary spec and instead rely on something that not so many take into consideration.

author: JohnSL
author_rank: Regular Contributor
timestamp: August 24, 2016, 09:16:06 am
content: Thanks rs20. I don't think I need to go with super low Rds(on) since the motor gets up to full speed very quickly, and I won't be using PWM. At 2A the Rds(on) would have to be pretty high in order to exceed the thermal limits of most of these MOSFETs. I was looking at the MOSFETs used on my 3D printer board (a Rambo), and they use these: PSMN7R0-60YS. I certainly don't need something with those specs.Here's one that looks like it's way more than I need, but still not that expensive:https://www.digikey.com/product-detail/en/nxp-semiconductors/BUK9M85-60EX/568-13030-1-ND/6130529

author: langwadt
author_rank: Super Contributor
timestamp: August 24, 2016, 10:32:51 am
content: Quote from: JohnSL on August 24, 2016, 09:16:06 amuse something like a VND7NV04 VNB14NV04 instead and it'll be protected against almost anything

author: JohnSL
author_rank: Regular Contributor
timestamp: August 24, 2016, 04:38:45 pm
content: Quote from: langwadt on August 24, 2016, 10:32:51 amOr this part, which seems to be in the same family:http://www.digikey.com/product-detail/en/stmicroelectronics/VNL5090S5TR-E/497-14321-1-ND/4552566It's a little less expensive, but a whole lot of power. For my circuit board, I'm going to need a total of 5 or 6 MOSFETs. Most of the time these will be used for solenoids, so I can probably go with a cheaper MOSFET for most of them. I may just use two of the current limiting chips to drive the vacuum pump, as there will only be one or two.

author: rstofer
author_rank: Super Contributor
timestamp: August 24, 2016, 05:50:14 pm
content: To get an idea of the inrush current of a DC motor, simply measure the DC resistance. Then divide the applied voltage by the resistance and, voila', inrush current. As the motor comes up to speed, it generates a back EMF that reduces the running current.

author: rs20
author_rank: Super Contributor
timestamp: August 24, 2016, 08:51:49 pm
content: Quote from: JohnSL on August 24, 2016, 09:16:06 amThe MOSFETs you have chosen look fine to me; but I have to point out that the reasoning above is flawed. PWM does not particularly imply a need for low Rds(on). If anything PWM implies a need for good switching performance (low gate charge/capacitance, fast turn-on/turn-off time). If you're not doing PWM/rapid switching, Rds(on) is just about theonlything you can care about.

author: JohnSL
author_rank: Regular Contributor
timestamp: August 25, 2016, 06:45:51 am
content: Quote from: rs20 on August 24, 2016, 08:51:49 pmGood point. The correct statement is that 2A isn't enough to get anywhere close to the power limits of the chip. Thanks for all your help!