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Original question:

PGA411-Q1: Issue on exciter power-supply overvoltage threshold

PGA411 Boost converter generates excessive VEXT

Steffen Frei
Prodigy 10 points
Other Parts Discussed in Thread: PGA411-Q1

Dear Ladies and Gentlemen,

I am currently searching for a problem I can't wrap my head around. I had the PGA411 working fine on a self-developed PCB for almost a year now. From one day to another, the device started entering FAULT state within a fraction of a second upon initialization. I read back the FAULT flags and was able to identify the FBSTOV flag as the fault origin. Therefore, I setup an oscilloscope measurement for the generated DC voltage. The voltage level is smooth and the switching pattern looks regular, however the output voltage level is not correct. There seems to be a voltage offset of about 3V between the voltage setpoint and the voltage generated:

Setpoint [V]         Actual Voltage [V]
10 N/A
11 N/A
12 N/A
13 N/A
14 17,4
15 18,3
16 19,1
17 20,0

The feed-forward was turned off for this measurement, turning it on resulted in a voltage reduction of approximately 0,3V, both with or without load. For voltage setpoints of 13V and below, the voltage pattern was not smooth anymore, as the integrated switch periodically paused switching for approximately 2,35us. This happened independent of the choice of 7Vrms or 4Vrms excitation. See the oscilloscope shot shown below for a setpoint of 12V:

I checked and resolvered all IC pins and passives for cracked solder joints, but to no avail. Therefore, I would like to ask whether this failure mode is common or at least explicable and whether you have any other suggestions of things I could try. As it worked fine for so long and no software modifications were performed, I doubt this is caused by a bad configuration. As a remark, the data sheet and the trouble shooting guide give inconsistent information about the voltage monitoring that sets FBSTOV: In the trouble shooting guide SLAA687 on page 6, the VEXT pin is stated as the pin in question, the datasheet states it to be VEXTS. A clarification on this would be appreciated. In my opinion, the degree of detail given concerning the boost circuit leaves room for improvement.

Lastly, I a m using this device in a government funded research project and was surprised to find the chip was changed to be a NDA product from one day to the other. As a personal note, I dislike the removal of data on obsolete or changed products, as there may be customers still relying on support for the products they purchased. I would be very pleased if you were to get in touch with me in order to get access to the full documentation as well as the opportunity to order or back-order, as we are already low on stock and would like to avaid having to change the resolver interface on short notice. Thank you in advance!

Yours sincerely,

Steffen F.

  • 0
    TI__Mastermind 40295 points

    Steffen,

    We have seen no failure modes like you have described. 

    Is the problems you described are for a different configuration than you have used in the past, would it be possible to revert your set-up back to what was done before to see if it will work as expected?
    For example, are you using exactly the same exciter that was used before, or is this physically a different one, even though it may have the same specs?

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 10 points

    Hi John,

    thank you for your quick reply. I did not make any changes to the configuration or the hardware. The internal amplifier was used and no modifications were done to the PCB. The connected resolver as well as the wiring were also not changed. I will, however, exchange the PGA411-Q1 today in an effort to determine whether or not this behaviour was caused by the IC itself. I suspect the internal reference voltage generator or voltage error calculation to cause the problem, as the PWM generation and switching works fine and the regulated voltage is smooth and noise-free (apart from settings with low voltage setpoints). The design is based on the PGA411-Q1 EVM but misses the 20V zener diode D8 which was marked DNP in an earlier version and therefore left out. I do understand, however, that the diode is also present in Figure 70 on page 85 of the datasheet. Is this diode required as a minimum load or some sort of transient overvoltage protection?

    Speaking about exchanging the IC, do you have any information on availability and purchase options for the part? Thanks in advance!

    Greetings,

    Steffen

  • 0 in reply to Steffen Frei
    TI__Mastermind 40295 points

    Steffen,

    First, you will need to sign an NDA with TI before you can order any PGA411-Q1 devices.
    I believe our Customer Support Center can help with this.
    Once the NDA is in place, you can also work with them to order devices.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 10 points

    Hi John,

    thank you for the clarification. I will contact the Customer Support Center and perform the necessary steps.

    Greetings,

    Steffen

  • 0 in reply to Steffen Frei
    Prodigy 10 points

    Swapping the IC did indeed fix the issue. I assume that the boost converter circuit must have been damaged by transient overvoltage, even though the 5V supply it was fed from is well filtered, decoupled and overvoltage protected. I added the zener diode in order to keep this from happening again.