Help w DRV8251A motor driver, please. - Page 1
Forum section: electronics / projects

author: HCB
author_rank: Contributor
timestamp: August 27, 2025, 10:35:35 pm
content: I'm not at the house so I can't post a schematic yet but I did want to get the ball rolling. I apologize if this isn't the right area to post this but it did seem to be the best spot.Here's the gist of the problem: if I run test code to extend or retract a linear actuator on 60 second or 120 second intervals, it will run for hours and hundreds of "cycles" (extend, pause, retract, pause). No problems. Each operation, extend or retract, takes about 16-17 seconds so I turn the output on for 20 seconds. Here's the problem: if I change the pause duration to 30 seconds, leaving about 13-14 seconds from the end of one motion to the beginning of the opposite motion, it smokes the DRV8251A. I have two separate boards doing the same thing. Also, in the actual "run" scenario, I can press a button to extend the LA and another button to retract it. Operating those buttons back to back with only a few seconds in between also the fries the DRV8251A (which is how I found the problem existed at all).Some more detail: I have DRV8251A feeding 12v dc to a linear actuator. The LA draws about 450mA when running, about 1/8th the max for this chip. I feed IN1 and IN2 straight 5V from pins on an ATMega328P-AU. I send one pin high to operate the LA in one direction and the other pin high to reverse it, with the opposite pin sent low. I never have both pins high, and I do not use PWM. It operates the LA for 20 seconds, regardless of direction, and then stops. By design, the LA should be operated twice per day. However, as stated above, there are manual override buttons. Those override buttons can be pressed at anytime as long as the LA is not currently under power.The whole circuit is powered by a lab power supply (on the bench) and a 12vdc lead-acid battery in the field. Ipropi is connected to ground via a 1k resistor and Vref is fed 3.3v via a voltage divider (5v -> 4k7 -> Vref -> 9k1 -> GND).I'm sure I've not included all necessary info but I can't think of anything else at this moment. What's causing me such trouble is that the design works great if I let the chip set for 40 or so seconds (or more) after each operation yet is destroyed if it sits only 13-14 seconds despite not feeling hot and being used well under its current rating. I smoked 4 of these chips today before I found that extending the test interval back to where it was set days ago (from 30 seconds to 60 and then 120 seconds) stopped it from happening. I first found the problem when installing it after bench-testing: extending then retracting the LA to be sure it was working properly: extend...retract...draggy-sound...stop...buzzing.Does anyone have ideas of what may be wrong? What further info would help?Thank you.--HC

author: Smokey
author_rank: Super Contributor
timestamp: August 27, 2025, 11:19:55 pm
content: Show us a picture of how the chip is mounted and connected.Without knowing anything else, I'm going to take a guess and say the thermal heatsinking is just poor.https://www.ti.com/lit/an/spra953d/spra953d.pdf12V * 0.45A = 5.4WThey don't do a good job in the data sheet explaining how much board area they used for their Junction-to-ambient thermal resistance, but lets just use that 40.4degC/W number.40.4degC/W*5.4W = 218CSo if you had poor heatsinking the junction would get up to 218C, which exceeds the 150C Tjmax. That coupled with brush type motors having high peak startup current might put a high thermal stress on the die and pop it, even through I would expect an integrated TI driver like this to protect itself better.If heatsinking is poor, redesign the board to have more copper under the part, and add a heatsink and some airflow if you can. Also instead of just jamming it full on at start up, pwm for a bit to limit current while the brush-type motor starts spinning for a lower peak power.

author: HCB
author_rank: Contributor
timestamp: August 28, 2025, 12:02:58 am
content: Some follow up info now that I'm home on the computer:The first pic is the schematic of the portion around/connected to the DRV8251A.The second is what I found when I got home (and the stench of burnt electronics). Thank God it didn't catch fire.The third pic is the top of the board where the chip mounts.The fourth is the bottom of the board beneath where the chip mounts. There are 4 vias kind of clustered: two wider spacing than the other. These are the thermal vias from the top side where the thermal pad solders to the board.--HC

author: Smokey
author_rank: Super Contributor
timestamp: August 28, 2025, 12:11:07 am
content: Check out table 9-6 from the data sheet for how much copper they want on the thermal pad:https://www.ti.com/lit/ds/symlink/drv8251a.pdf

author: HCB
author_rank: Contributor
timestamp: August 28, 2025, 12:23:38 am
content: Smokey, first: thank you for your reply and time.I've messed with electronics for decades but a) not much, b) not often, and c) I suck. I read that datasheet several times but that doesn't mean I did a good job of implementing the info into my circuit. To be honest, it never occurred to me to worry about the thermal heatsinking because I was so far beneath the amperage listed for this chip.I follow what you're saying about the wattage dissipated and it's clearly way over the Tjmax. I can add a heatsink but a redesign would be my last choice. The nature of the design is to be contained in a 3D printed enclosure in a chicken coop which bakes in the sun: thermal management is...sketchy. I will look into adding a heatsink.What causes me confusion is that I ran test code on two boards, including the crispy critter in the pic I've added as reply to my post, for hours on end with no problem. Granted it was indoors with air conditioning but it was in the same environment today when that one burned. On Sunday I left one board cycling extend, wait 60 seconds (about 43-44 seconds after the extend finished), retract, wait 60 seconds (same caveat), for over 2 hours. On Monday I set the other board to the same regimen except the pause was 120 seconds minus cycle time (103-104 seconds after the motion stopped), and it ran for over 6 hours. They both worked great, no failures. I've looked for solder balls, shorts, bad joints, etc with no luck. Touching the chip doesn't feel warm when it's running but then it could be a thermal cascade and I wouldn't feel much until it was smoking.I chose to use D12 and D13 to connect to the motor driver (PB4 and PB5) which I cannot use with the hardware PWM as I understand it. I may have painted myself into a corner.I'm going to go read the app note you linked. Thank you.--HC

author: HCB
author_rank: Contributor
timestamp: August 28, 2025, 12:33:29 am
content: Yeah, I think you nailed that. There's a lot of info there in and around 9-6 for me to go over. But that's where I missed the boat in my design; I didn't think I'd have thermal problems with such low current. Hmmm, I'm going to have to do some southern engineering.Thank you very much for your help.--HC

author: Smokey
author_rank: Super Contributor
timestamp: August 28, 2025, 12:40:56 am
content: Stick a thermocouple to the chip case top and monitor the case temp as you run it. That should give you a lot of info about thermal margins.If you don't have one, you can get a $20 multimeter from amazon that comes with a thermocouple that will do just fine for this. And you get another meter!

author: HCB
author_rank: Contributor
timestamp: August 29, 2025, 11:33:48 pm
content: First, the thermocouple is a great idea. My current meter doesn't have one but I do have several thermocouples with amplifier circuits I could use if attached to a mC. Unfortunately, they are in a SS well and I have to cut them...wait...I could rob the modified one from my reflow oven. I'll see about doing that. I should have thought of that.Still eating me: 2+ hours on 1 minute intervals one day, another unit 6+ hours on 2 minute interverals the next day, zero failures. Install and fail. Then test and fail on the bench. 12v battery in the field (13.8v), 12v bench power supply at the house. Doesn't matter, still weird failures. Wednesday I replaced a fried chip and did the mod shown in the pics: I scraped solder mask off where the thermal vias connect from the chip to the ground plane and soldered on some desoldering braid to add thermal mass. I modified my mC code to not allow cycles faster than every 2 minutes to allow for cooldown. I attempted to install it yesterday...fail. It operated once and never again. That it ran one full stroke but never ran again seems to indicate that it's failing at the end of the stroke, not from overheating. The problem seems inconsistent to me, especially with the hours and hundreds of bench runs which didn't burn anything up.My next attempt is going to be to cobble together a full wave bridge rectifier to try to eat the back EMF/field collapse instead of counting on the body diodes in the DRV8251A. I will also try the thermocouple on the chip...still not happy with myself for not thinking of that.Thank you for your help.--HC

author: HCB
author_rank: Contributor
timestamp: September 26, 2025, 06:03:00 pm
content: FWIW, for future readers who might have similar problems:It's been a month since I last posted what I was doing. Adding heatsink material and changing the code so that the unit could not operate within 2 minutes of a cycle (dragging it out to allow thermal dissipation) did not correct the problem. I did make a full-wave bridge rectifier using roughly the equivalent of 1N4007 (1,000V) diodes. This seemed to solve the problem until about 4 days ago when one of the units (complete device) failed after only 2 days in the field. I currently have 3 devices built but I only remembered modifying 2 of them so I thought the one that failed was the un-modified unit. Wrong. The unit which failed did, in fact, have the full-wave bridge rectifier cobbled onto it.Load on the linear actuator is a joke, it claims a capacity of 160 pounds yet it's lifting a piece of 9" x 12" x 5/8" plywood. It's free-moving with only gravity as a force acting on it. The linear actuator draws 0.4xx amps when running, well under the DRV8251A max of 4.1A. I do not believe it's overloading the chip, I believe I'm still dealing with flyback voltage problems.Adding to the confusion, partly mentioned before plus some more: I ran hundreds of tests on the bench with one of the linear actuators and these units with no failures. Installing them in the field often yielded failures after one cycle. The only real difference between the bench tests and the field installation is that the bench tests were done with a power supply of 12V while the field installation is running on a 12V lead acid battery which has a voltage of about 13.6V. New information is that two of the units are now installed and have been functioning for a bit; one for 4 days and one for about 2+ weeks. I feel I'm on the edge of some limit.FWIW, the linear actuator, while it draws very little current, has a fairly large motor on it, about 1.25" by maybe 2.5". I don't know enough about inductors or inductance to know how this affects flyback voltage but I would expect that a very small inductor would produce either lower flyback voltage or a shorter flyback voltage pulse than a similar voltage/amperage inductor which is physically larger. But, again, I know squat.My next attempt to solve this problem is to try to build a snubber but to do that I need to know the inductance of the motor. I have an oscilloscope which leaves me lacking a) a pulse generator and b) a way to be sure the motor shaft doesn't rotate. There are a number of other motor driver ICs I could try but solving this problem seems useful for making better designs in the future.--HC

author: thm_w
author_rank: Super Contributor
timestamp: September 26, 2025, 09:01:49 pm
content: The linear actuator can't be backdriven right?Most multimeters should have a thermocouple option, or a IR thermal gun can be had for <$20, I would set your PSU to 14 or 15V and then run it at full load just to get an idea of what things are running at.