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Original question:

UCC28631 Vsense or Rp resistor selection

UCC28631: UV detection threshold

Stefan Kutscher
Prodigy 10 points
Part Number: UCC28631
Other Parts Discussed in Thread: PMP10200, PMP22288

Dear TI Team,

I'm using the UCC28631 primary-side regulation flyback controller in an existing design and need to cope with some updated requirements, one of them being a lower input voltage.

As far as I understood the input voltage is being measured during the on-time portion of the switching cycle at the sense pin. See also table "PIN functions": "VSENSE - Sense pin for ... and input voltage sense/UV protection"

But I see considerably different minimum operation voltages depending on

  • values of R_a and R_b (same ratio to keep desired output voltage constant, also respecting R_th = R_a || R_b between 10kOhm and 20kOhm)
  • values of R_p

For minimum operating voltages I measured values between 40 to 180 V AC peak/50Hz (repeatedly and stable), which is equivalent to 6.4 to 9.6V at the SENSE pin. I was expecting a constant voltage threshold at the SENSE pin above which the IC detects an UV condition. But this is not the case. (However for output voltage measurement during the off-time portion of the switching cycle I find the precise value for V_OUT(ref)=7.5V in the datasheet, chapter 7.6 Electrical Characteristics, which I can confirm based on my measurements)

My questions:

A) How exaclty does the IC detect an undervoltage condition during operation at the SENSE pin (voltage measurement, current measurement, is the HV pin involved after all)?

B) How did the IC designers derive the value for AC_off in table 6 Key Device Parameters: AC_off = 65 V_AC/RMS, so 65*sqrt(2) = 92V AC peak?
Based on my measurements and the description of the input voltage measurement mechanism, I don't understand how this value can be fixed if the voltage at the SENSE pin depends heavliy on peripheral componenents.

C) In order to repect R_p=3.9kOhm, can I connect more than one diode between DRV and SENSE to lower the input voltage range (again, manipulating the voltage at the SENSE pin during the on-time phase)?
This would have the benefit that I don't have to adjust R_a and R_b in my current design.

D) What is the reason for R_p being fixed to 3.9kOhm?

Many thanks in advance
Stefan

  • 0
    TI__Mastermind 24245 points

    Hi Stefan,

    Thank you for the query on UCC28631. 

    You are definitely correct in your understanding regarding the measurement of input voltage during the ON time through the VSENSE pin.

    A ) First of all, the UCC2863x devices were not intended for low input voltage DC, they were targeted at universal AC input (90-264 Vrms). This is why the UV on /off thresholds were set for turn-on at 113 Vpk (80 Vrms) and turn-off at 92 Vpk (65 Vrms). The IC basically senses the input voltage by injecting some current and measuring the voltage level into the the effective thevenin equivalent of Ra||Rb.

    The Ra/Rb/Rp network has to be set to scale both the line-dependent on-time swing on the aux winding, plus the output-dependent flyback-interval swing, so the ability to tweak there is also limited. That is why for these applications it is advised to keep the thevenin resistance to between 10k to 20k.

    But you are correct in your observation that it changes with Ra/Rb and Rp combination.

    However adjusting Rp resistor can be used to adjust the turn on/turn off voltages. If Rp is made larger, this makes the input voltage appear higher than it really is, and allows the IC to start at lower voltage, and run down to a lower minimum voltage. 

    40Vpk (28Vrms) is going to be pretty low for this IC to handle as it it would require a huge offset in the Rp resistor which will have an impact on line reg and on the CC-mode limit point. I think you can try your point C definitely.

    I currently do not have design equations on how Rp was fixed at 3.9k, but will try to find it. Please let us know your input/output voltage specifications for this design with UCC28631.

    Let us know if you have any further questions.

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    Prodigy 10 points

    Hello Harish,

    Thank you for your reply.

    I now understood where the AC input range is coming from. As you said, it is the standard 100-240 V (RMS) +/- 10% tolerance.

    As mentioned, I will need to able to go below the value stated in the datasheet. Ideally, I

    (1) need to survive a drop to 15% of 230V RMS for 200ms (t_f not defined, as fast as possible), then rising linearly to 85% within 3 seconds, and
    (2) be able to operate at 50V RMS

    Regarding the output voltage, the secondary transformer side has several taps with positive and negative voltage of about +/-15V and maximum 20W total output power.

    The AC input range mentioned above seems to cover the most common AC input range across different countries. But can you tell me what technically limits the AC input range at the lower end? In other words, what in particular do I need to take care of when I push the UCC28631's limit towards lower input voltage?

    I'm going to cover load transients in high and low temperature while keeping an eye on the maximum rating of the IC and also the external MOSFET. You mentioned that line regulation could be impacted negatively if the R_p value is too high. I will definitely do a comparison with the performance of my current design and the modifications, but maybe you can recommend any specific AC input voltage profile that I can use to make sure line regulation is still OK? Any suggestions are welcome.

    Kind regards
    Stefan

  • 0 in reply to Stefan Kutscher
    TI__Mastermind 24245 points

    Hi Stefan,

    Thank you for the reply.

    Before trying to answer your questions, I would recommed a couple of designs which might be in the voltage range you are looking at since trying to get UCC28631 work might be difficult.

    PMP10200: https://www.ti.com/tool/PMP10200

    PMP22288: https://www.ti.com/tool/PMP22288

    Please give me a day/two, I will reach out to folks to understand internal limitations from preventing it to start at low voltage.

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    TI__Mastermind 24245 points

    Hi Stefan,

    To answer your first part regarding the limitation at ow voltage, modifying Rp/Ra/Rb affects the internal gains, specifically Kline and this in turn affects the compensation loop. Second as these are internal gains and not having access to the compensator makes it more difficult. I am not sure on the AC voltage profile as the requirement with 50V input as it may affect line regulation and we do not have any data to substantiate it for now. Please let us know if you have any questions.

    Thank you

    Regards,

    Harish