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DRV110: Rpeak resistance values and peak current levels not as expected

Adam Bierman
Intellectual 710 points
Part Number: DRV110
Other Parts Discussed in Thread: UCC27517

Hello,

I'm driving a solenoid on a 12-24V DC supply with the DRV110 as a PWM source driving into a UCC27517 Gate Driver.  During cycle testing the PWA has been failing repeatedly.  Each time it appears to be related to a drop in peak current.  On some boards, replacing the Rpeak resistor has resolved the issue.  On others it does not.  The waveforms look correct, with the duty cycle changing between peak and hold correctly, and the hold current is correct.  However the value read on a DMM for the Rpeak is showing ~40-42k Ohm depending on the unit when the value is in fact 100k.  The reading across Rhold is ~420k Ohm which is correct.  The resistor at Rpeak reads correctly at 100k when removed from the board and the resistance between the Rpeak pin and ground is ~50M Ohm when the resistor is removed.  The PWA sits on top of the solenoid piston and coil so is subject to some impact vibration, though it is isolated from most of that shock.  I am unable to figure out why my Rpeak value is shifting, and almost always down to the same ~40k level, after cycling for a while (8-12 hours).  Is it possible something in the IC could drive this change in resistance reading?

  • 0
    TI__Expert 3670 points
    Hi Adam,

    I am busy the rest of today but I will look into this more tomorrow. If possible can you send me a snapshot of your DRV110 schematic? If not what is your Rsense value and what peak current are you measuring when it fails?

    For clarification, is it failing because the peak current isn't getting high enough to drive the solenoid before getting to the hold period or is it hitting peak and then dropping? Can you send solenoid current waveforms from power on to hold (zoomed out enough to see rise to peak, keep at peak, and drop to hold) of a failure and one working?

    Regards,
    Kevin
  • 0 in reply to Kevin Schmidgall
    Intellectual 710 points

    Hi Kevin,

    Here is a snapshot of the DRV110 schematic: 

    The Rsense value is as indicated, the peak current I am measuring is 23.2A. The resistance I measure across R18 is ~42.3kOhm, when the resistor is removed the component reads ~99.7kOhm.  

    Here is a waveform of the problem solenoid, I'm working on getting one for a working solenoid. This is the solenoid current and DRV110 output signal.

  • 0 in reply to Adam Bierman
    TI__Expert 3670 points

    Adam,

    Based off that schematic I am calculating that your targets are Ipeak = 40A, Ihold = 1.65A. Is that correct?

    A question I have is, when are you measuring the resistance of Rpeak when it reads ~42k? Is the part turned on while you measure that? When the part is turned off and you measure Rpeak, with Rpeak on the board, is it still 42k or do you see 100k? 

    One other thing I want to point out to is I noticed your Rsense is rated for 3W. If your Peak current is 40A you have a power across that of P = (I^2)*R = (40A^2)*(.015) = 24W. Right now I dont think that is related to the issue but just wanted to point it out.

    Regards,

    Kevin

  • 0 in reply to Kevin Schmidgall
    Intellectual 710 points
    Hi Kevin,

    Those values are correct. That is measured when the part is turned off. When the part is turned on, which I just checked, I am reading 2MegOhm. We are aware of the issue with Rsense and I have changed that part for higher wattage which hasn't been implemented yet. I agree it's not likely related to the problem which is why before I do a spin I'm trying to figure this one out. One thing I should mention is that these parts are failing at ~10k cycles, that is when we see the fall from nominal peak current.

    Thanks,

    Adam
  • 0 in reply to Adam Bierman
    TI__Expert 3670 points

    Adam,

    For the screenshot you sent what did you measure Rhold as? Originally you said Rhold is measuring ~420kohm which is around its expected value so I'd expect Ihold to be around 1.65A but for that screenshot your Ihold is off from its expected value too. When the part is turned on what do you measure for the voltage on HOLD and PEAK?

    When you say it is failing after 10k cycles, do you mean after 10k power cycles it stops regulating to 40A/1.65A and goes to ~23.2A/4A?

    Regards,
    Kevin

  • 0 in reply to Adam Bierman
    TI__Expert 3670 points

    Adam,

    To add to my reply to your most recent response...what is interesting to me is that when the part is breaking the Ipeak and Ihold values are around what would be their default. Ipeak and Ihold = Vref/Rsense, so for a Vref(peak) of 300mV your Ipeak = .3V/15mohm = 20A and for Vref(hold) = 50mV your Ihold = 0.05V/15mohm = 3.33A. Where the red and black line of the graph, below, fall off is around 43kohm so what you are seeing (Rpeak ~ 42kohm, and ~20A) is in line with what the graph expects. This also would make sense to me because Ipeak was decreased and Ihold was increased which would be expected if the resistances changed from the values on the schematic to somewhere on the left side of that graph.

    Have you done an A-B-A swap yet? If not I would recommend doing that if possible. Take a board with a broken part and a board with a working part and swap the DRV110 to see if the problem follows the part. If the part is broken then the broken board should start working when you put the good part on, and the working board should not work with the bad part. 

    Regards,

    Kevin

  • 0 in reply to Kevin Schmidgall
    Intellectual 710 points
    Hi Kevin, RHold is consistently measuring approximately 420K for that screenshot. And that is what I mean, the regulation goes off from both, it's the pull-in we are most concerned about in context of functionality but I wouldn't be surprised if the issues are related. I will try an ABA swap if I can, I'm not sure I have a working board, but I do believe I have extra ICs that I can populate.
  • 0 in reply to Adam Bierman
    TI__Expert 3670 points
    Adam,

    Can you measure the voltage on the HOLD and PEAK pins of one of the broken parts? With the R values you are measuring I'd expect Vpeak ~ 0.4V and Vhold ~ 1V.

    Do you only start seeing this problem after you have cycled it many many times? And what percentage of ICs do you end up seeing this failure in?

    Regards,
    Kevin
  • 0 in reply to Kevin Schmidgall
    Intellectual 710 points
    Kevin,

    The voltage on Hold is: 1.1V on PEAK: 1.004V. RPeak is now reading at 39kOhm and RHold is 399kOhm. The values have shifted since the last time I measured them.

    So far we only see this problem after cycling many times. I don't believe we've seen a failure below ~9000 cycles. We've only had a small sample to work with, 6 prototypes, but we've seen it on 4 of them.
  • 0 in reply to Adam Bierman
    TI__Expert 3670 points
    Adam,

    Have you checked any thermals throughout the lifetime of cycling? Earlier I said I didn't think the 3W resistor was a problem but I'm starting to think it may be. Have you been using that 3W resistor for all testing so far? You said you were aware of the issue but hadn't implemented the higher wattage resistor so I'm guessing you're still using that 3W resistor which leads into my next paragraph.

    You said you were cycling for 8-12 hours which means that 3W resistor is probably getting really hot with 24W being dissipated across it. Even if not immediately, I imagine after 8-12 hours it's hot; the theta(j-a) or theta(j-b) is going to be too high for the amount of power dissipated across it. Through board heat dissipation or heat radiation through the air the resistor could be heating up the DRV110. After prolonged exposure this may be getting to the point where it is damaging the DRV110.

    So, if possible, I would recommend running the cycling tests and either look at some thermal imaging at the beginning, middle, and end or, at the least, come close with your hand to see if you can feel how hot it's getting at different points in the test.

    Can you give me the part number for the sense resistor you are using currently?

    Regards,
    Kevin
  • 0 in reply to Kevin Schmidgall
    Intellectual 710 points

    Hi Kevin,

    I have done thermal testing with thermocouples, however not during extended cycle testing. I have also switched to a 7W resistor (still not 24W I know)  I have swapped the DRV110 off a bad board and put a new one one.  Now the solenoid is pulling in but the values are still off from what I would expect. The screenshot of the current waveform is below.  The value read off the peak resistor is: 40.9kOhm (it read 96.6kOhm immediately prior to testing after replacing the DRV110, both values without power).  The hold resistor is reading: 400.6kOhm.  The current values shift from 34.6A to 26.8A on peak, and 4.2A on hold.  The sense resistor currently on the BOM is CRA2512-FZ-R015ELF

  • 0 in reply to Adam Bierman
    TI__Expert 3670 points

    Adam,

    Was the DRV110 used for that screenshot populated on a board that went through the cycling tests? Did you see this behavior, in the screenshot, right when you put the new DRV110 on the board? What's interesting to me now is, before, you said it would work properly for at least ~9000 cycles but it sounds like you are seeing this immediately.

    Separate question: when you did repeated cycling testing how long did the part stay on before you power cycle it (how often did you power cycle)?

    Regards,
    Kevin