• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Subscribers 29 subscribers
  • Views 1923 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DRV8825 problems

Gregor Kocjancic
Prodigy 60 points
Other Parts Discussed in Thread: DRV8825

Dear Sirs,

we designed a circuit with your DRV8825 chip to drive some stepper motors. At the beginning it looked that everything was fine, but after stress testing we "burnt" two DRV8825 chips. In both cases the DRV8825 controllers were working for more than 10 minutes and after powering down and enabling the power the controllers were in short-circuit. In both cases micro-stepping (1/16) was enabled, no fault condition was ever present, the controllers were also additionally cooled by a fan and until powering down everything was working flawlessly. The VMx voltage in our design is 24V, the digital side is on 5V and the Vref pin voltage is between 3.5V - 1.7V.

Now our question is what can be the cause of this problem? The DRV8825 controller has an internal shutdown protection for over-current, short circuit, under-voltage and over-temperature this is why we are very surprised that something like this happened twice.

Any suggestions and replies are very much appreciated.

Kind regards,

Gregor

  • 0
    TI__Expert 7455 points

    Hi Gregor,

    What you describe is rather odd, as this device is quite rugged. I must admit there is one set of conditions for which I can imagine this set of observations to be plausible, so please allow me to ask the following questions:

    1. How well is the chip soldered to the power pad on the board? How many layers is the board? Is there enough copper in the top and bottom layer to work as a heat sink?

    2. Can you share more information on what the stress test consists? For example, what was the environment temperature? What was the peak current set to?

    3. What is the SENSE resistor in this circuit?

    Thanks for this information! What I am trying to determine is whether some form of localized heating may have occurred. For this type of circuit to work reliably, we need a good thermal connection with the board. When that happens, the heating caused by power dissipation is slow enough that we can catch it with the internal protection circuitry.

    If on the other hand there is localized heating, then there is a probability for damage. Notice this is quite easy to do if the device's power pad is not properly soldered into the board. Under this condition, the thermal impedance is so large, the system is already above Abs Max, in which case performance cannot be guaranteed.

    Hope the info helps. Best regards,

    Jose Quinones

  • 0 in reply to Jose Quinones
    Prodigy 60 points

    Dear Mr. Quinones,

    thank you for your answer.

    1. This is a two layer board with a lot of copper around the chip and under the chip. Bellow is a picture of a part of the board. I think that the boards are soldered correctly, but because this are prototypes there is a possibility that maybe the power pad is not soldered perfectly. (Additionally we also had an idea to put a heat-sink on the top of the chip - is this a good idea?)



    2. The ambient temperature is bellow 35 degrees Celsius and the peak current is set to 2.1A. We are then driving the motor up and down continuously and with pauses.

    3. The sense resistors on pins ISENA and ISENB are 0.33 Ohms.

    Is there a way to determine if the heat is not speeding correctly? Can we measure the temperature of different parts of the chip and somehow observe if this is happening?

    Kind regards,

    Gregor

  • 0 in reply to Gregor Kocjancic
    TI__Expert 7455 points

    Hi Gregor,

    Thanks for this information. The layout seems like a good way to achieve low thermal impedance so all we need to ensure now is that the device's Power Pad is in fact soldered to the board. Here are some suggestions of what I would try:

    1. The optimal solution is to run the board through an X Ray and see what is the solder density. We recommend anything larger than 50%. I do realize taking an X Ray is easier said than done, so another option that I like to try is to use my heat gun and remove the chip. While I do so I look at the PCB and the device's Power Pad. Sometimes I see the device's Power Pad has no solder in which case the thermal impedance is too big for the system to work properly.

    A few observations which I have made throughout the years and which you can make sure are not haunting your application:

    1. When we design these layouts, it is crucial to tie both top and bottom layers with vias. I see you have such vias, so this is great. I would like for you to study the possibility of solder wicking through this vias during reflow. I have seen this quite a bit and is one of the main reasons for seeing the Power Pad not soldering properly into the board. What can you do? Ideally you would make the vias small enough to ensure the solder can not wick through. I believe the recommended size is 0.33 mm. Is this the size you are using?

    2. Another option is to tent the vias. There are different ways to tent them, but if done, make sure this happens on the top side and not the bottom side. Tenting on the bottom side alone can cause bubbling, which can cause undesirable effects while reflow.

    3. You mention these are prototypes. Were they hand assembled? Was a hot air gun used? I realize soldering these devices can be tricky sometimes. What I like to do is put a lot of solder with my soldering iron, heat the blob with the hot air gun and then sit the device of the molten ball. It may require a little bit of pressure and patience, but eventually the chip sits in and the Power Pad is soldered quite good the great majority of times.

    4. How much time passed between the moisture sensitive pack was open and the devices were reflowed into the board? These devices have a classification of MSL3 which means they must be soldered in place no longer than a week after the ESD bag is opened.

    To measure temperature we use a thermal camera. If you have access to this equipment, it may be able to tell you a little bit, although in my experience this kind of localized heating is so quick you will not be able to grab it with a camera.

    Also, using a heat sink from the top will not help too much as the plastic case has too high of a thermal impedance. We have heard of other users applying heat sinks from the bottom and getting rather good results. Another option is one of those surface mount heat sinks.

    Hope this info helps in honing the problem.

    Best regards,

    Jose Quinones

  • 0 in reply to Jose Quinones
    Prodigy 60 points

    Dear Mr. Quinones,

    the x-rays are a good idea, but beyond our reach.

    1. Our vias on the prototypes are bigger (0.6mm), but the vias are planed to be smaller.

    2. I am not sure if I understand correctly what you are meaning with tent the vias on the top side (tend the vias = cover them with solder-mask)?

    3. The boards were hand assembled with a hot air gun and solder paste.

    Probably the cause of the problem is poor heat dissipation, because we have a couple of boards that are working perfectly and we can not reproduce the problem.

    Kind regards,

    Gregor

  • 0 in reply to Gregor Kocjancic
    TI__Expert 7455 points

    Hi Gregor,

    Thanks for the answers! Yes, tenting means adding solder mask on top of the vias. There are other mechanisms such as plugging, but I am not aware of how this is done or what is the associated cost. I think solder mask is the most economically viable solution.

    If you cannot reproduce the problem with the other boards, then that is good news. But do keep us posted if you see this problem again.

    Best regards,

    Jose Quinones

  • 0 in reply to Jose Quinones
    Prodigy 60 points

    Dear Mr. Quinones,

    if we add solder mask to the vias then we are actually putting solder mask under the power pad. Is this a good idea?

    Kind regards,

    Gregor

  • 0 in reply to Gregor Kocjancic
    TI__Expert 7455 points

    Hi Gregor,

    As long as there is more than 50% solder coverage between the board's copper pad and the device's Power Pad, we are in good shape.

    That being said, if you make the vias small enough, you may not need to tent them at all. According to SLMA002, the application note regarding Power Pad layout and soldering techniques, vias in the range of 0.33 mm diameter should be small enough to not require tenting. If you want to maintain the larger via size, then tenting is a good option.

    Best regards,

    Jose Quinones