What is necessary for driving a mosfets directly at logic level? - Page 1
Forum section: electronics / projects

author: 4b686f61
author_rank: Newbie
timestamp: April 04, 2025, 03:34:03 pm
content: Here I have a PCA9685PW so multiple solenoids can be driven with only I2C pins from an Arduino. I chosen a mosfet that is rated for more than needed as from my testing, mosfets rated 10x the current get killed by the solenoid instantly while ones rated for say 50+ amps survive. The PXN012-60QLJ mosfet is rated for 60V@42A with a maximum gate threshold voltage of 2.5v leaving wiggle room for max saturation to prevent heat. In this circuit the mosfet is being driven by a 100 ohm resistor from the IC and a reverse diode to quickly turn off the gate. There is also a small RC snubber to work with the body diode.What is the least amount of parts I can get away with for this circuit while being redundant (diode failing short/open supply fused with a 1.5A PPTC, only 1 channel is activated at any moment for 250ms than 50% pwm), and can the mosfet be driven directly from the PCA9685 outputs?Edit: the chosen MOSFET has a VGS of 2.5v at mosthttps://assets.nexperia.com/documents/data-sheet/PXN012-60QL.pdf#page=4while the ones that didn't survive the test was the CSD19538Q3A, 14A at most.

author: Benta
author_rank: Super Contributor
timestamp: April 04, 2025, 06:23:05 pm
content: You need a logic level MOSFET. The gate threshold voltage is when it starts to conduct, not when it's saturated/fully on.

author: tooki
author_rank: Super Contributor
timestamp: April 04, 2025, 06:44:11 pm
content: Quote from: Benta on April 04, 2025, 06:23:05 pmThe specified partisa logic level MOSFET.

author: Benta
author_rank: Super Contributor
timestamp: April 04, 2025, 07:34:19 pm
content: Quote from: tooki on April 04, 2025, 06:44:11 pmA web search gave me no results, so it was just a comment.

author: tooki
author_rank: Super Contributor
timestamp: April 04, 2025, 07:42:51 pm
content: Quote from: Benta on April 04, 2025, 07:34:19 pmhttps://www.google.com/search?q=PXN012-60QLJbrings up theproduct pageas the first result, exactly the same with Bing and DuckDuckGo. So I don't know why you couldn't find it.

author: 4b686f61
author_rank: Newbie
timestamp: April 04, 2025, 10:54:25 pm
content: https://assets.nexperia.com/documents/data-sheet/PXN012-60QL.pdf#page=42.5v gate threshold at most

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: April 04, 2025, 11:03:58 pm
content: Driving solenoids with Mosfets is a very common practice in the automotive environment.Major semiconductor companies, like STMicro and Infineon and others, have developed fully autoprotected Mosfets.I would check some of their offerings.

author: Benta
author_rank: Super Contributor
timestamp: April 04, 2025, 11:33:15 pm
content: Yeah, I had a typo in my search. It happens.Back to the original question.You can safely remove R1, R3, D1, C2, C3.Replace D3 with a bidirectional TVS.

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 05, 2025, 12:30:45 am
content: Quote from: 4b686f61 on April 04, 2025, 03:34:03 pmKeep in mind that the 42A rating assumes the case temperature is held at 25C, which is not realistic. Usually device datasheets will all provide more conservative specifications about current/power handling which account for temperature rise of the case, but it seems that Nexperia omits that entirely from this datasheet, quite sneaky of them. I'd estimate this part could handle around 5A continuous in a standard JEDEC test.You didn't mention what supply voltage will be used with the PCA9685PW. You mentioned using PWM, what frequency?Hard to say whether R1/D1/C2/R3 are necessary or helpful. C3 definitely should be removed though. Replacing D2 with a bidirectional TVS/zener might would only make sense if not using PWM at all (with PWM a TVS/zener would burn a lot of power).

author: oPossum
author_rank: Super Contributor
timestamp: April 05, 2025, 06:56:46 am
content: Quote from: 4b686f61 on April 04, 2025, 10:54:25 pmGate threshold voltage is when the MOSFET is just starting to turn on. Only 1 mA drain current.See figure 6 on page 5. Needs at least 5 volts for minimal Rds. 6 or more would be better.What voltage is the PCA9685 running at?

author: tooki
author_rank: Super Contributor
timestamp: April 05, 2025, 08:33:24 am
content: Quote from: mtwieg on April 05, 2025, 12:30:45 amBut OP doesn’t need anywhere near 5A continuous. Their schematic clearly shows the solenoid as being 12V/500mA.

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 05, 2025, 12:32:55 pm
content: Quote from: tooki on April 05, 2025, 08:33:24 amActually I did not notice thatThen that begs the question of why OP is having issues with devices overheating and failing. It certainly doesn't have anything to do with Vth or Rdson. I'm guessing they're using a PWM frequency far too high for their gate driver circuitry.

author: tooki
author_rank: Super Contributor
timestamp: April 05, 2025, 02:09:58 pm
content: Quote from: mtwieg on April 05, 2025, 12:32:55 pmYep. I am puzzled by the failure, too. Note that OP did not state that the MOSFETs overheated, only that they "died instantly".Quote from: mtwieg on April 05, 2025, 12:32:55 pmAgreed.Quote from: mtwieg on April 05, 2025, 12:32:55 pmNot likely, since that chip only goes to 1.5kHz PWM, and driving a MOSFET is an explicitly supported configuration.

author: 4b686f61
author_rank: Newbie
timestamp: April 06, 2025, 12:43:14 am
content: D3? I assume it's the power supply TVS?

author: 4b686f61
author_rank: Newbie
timestamp: April 06, 2025, 12:48:26 am
content: In the case that the mosfet fails shorted at all pins, R1 would protect the ic

author: 4b686f61
author_rank: Newbie
timestamp: April 06, 2025, 01:21:00 am
content: Quote from: UnijunctionTransistor on April 04, 2025, 11:03:58 pmMost of the parts available I can't find an option to pwm individual outputs or they are awfully expensive and low current

author: benwillcocks
author_rank: Newbie
timestamp: April 08, 2025, 05:52:15 pm
content: It's surprising you are having problems with the MOSFETs "getting killed by the solenoid", since you are (correctly) using a freewheel diode D2. You could try replacing D2 just in case it had been damaged.The only other explanation I can think of is that your schematic shows no supply decoupling. If you have significant inductance (ie. long wires) between the power supply and your circuit, and no local decoupling, there will be a +ve-going voltage spike on VCC when the MOSFET turns off. This might damage the MOSFET if it exceeds the Vds spec.

author: mikerj
author_rank: Super Contributor
timestamp: April 09, 2025, 09:36:46 am
content: Quote from: 4b686f61 on April 04, 2025, 10:54:25 pmYou've been asked twice what Vdd you are supplying to the PCA9685, but still no answer.

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 09, 2025, 12:00:57 pm
content: Quote from: tooki on April 05, 2025, 02:09:58 pmRight again, I glanced at the datasheet and confused the 1MHz max I2C clock speed with PWM frequency.I'm certain there's some issue with the circuit which won't be fixed by using oversized MOSFETS. A couple scope captures of Vgs and Vds would probably explain a lot....

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: April 11, 2025, 03:53:54 pm
content: Quote from: 4b686f61 on April 06, 2025, 01:21:00 amWrong on both statements. These are individual MOSFETs that operate exactly like a plain MOSFET, but have several integrated protective features.I am out from home and googling from a phone is clumsy, otherwise I could give you several suggestions.But firstly….You also need to provide the supply voltage your circuit operates with and the load current.