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UCC256404: Questions about Usage in LLC Application

Darren (FAE)
Expert 5483 points
Part Number: UCC256404
Other Parts Discussed in Thread: UCC256403

I have a customer working on an LLC (LCC-type for wide Vin) resonant design, and considering the UCC256404. But he has some questions, and your help would be hugely appreciated.

The UCC256404 will be used for a DC input voltage. (Range: 120Vdc ~ 420Vdc)
The UCC256404 was selected vs other variations because the "Bulk Voltage Sense" was lower.
Seems like it will work better with 120Vdc ~ 420Vdc input range....<-- comment from customer

[1]
The HV pins can handle an input voltage range of 120Vdc ~ 420Vdc no problem, yes?

[2]
Is there a way to disable the X-discharge function of the xx404 device?
The datasheet says it works up to 5uF capacitance, but they plan on a much higher capacitance. (The input is DC, not AC)
If there is a higher capacitance, does it just mean it will take longer than IEC60950, IEC60065, IEC62368 specified to discharge the cap?
(Again, they prefer the UCC256404 vs the 403 because of the Bulk Voltage Sense being lower...but if you have any comments on this too...?)

[3]
On the EVM, there are three resistors connected to the HV pin (R26 ~ R28) for about 5kΩ. How do you select what resistance to use here?
https://www.ti.com/tool/UCC25640EVM-020#technicaldocuments

[4]
Section 8.4.2 X-Capacitor Discharge in the DS says you need max 10MΩ parallel resistance for every 100nF. For 330nF, the DS says this is 3MΩ (basically three 10MΩ resistors in parallel), but in the EVM, for 470nF, instead of the 2.5MΩ you would expect, they use 4 series 10MΩ resistors = 40MΩ...is this right?

[5]
How do you disable the burst mode?
Hard to tell from the datasheet...do you just follow Option #7 and connect 2422 ~ 3038Ω between the BW pin and GND?

[6]
In the EVM, it looks like the resonant cap C17 is all that is needed to function, but there is also C1...is C1 needed? 
If so, is there any literature, or could you explain, what the function of this is?

I know this is a little long, but again, I really appreciate the help!

Regards,

Darren

  • 0
    TI__Mastermind 26770 points

    Hello Darren,

    Thank you for your interest in the UCC256404 LLC controller.

    The answers are:

    1) Yes, HV is rated to > 600V.

    2) Unfortunately, X-cap discharge cannot be disabled.  The UCC256404 must detect a zero-crossing periodically or it will actively attempt to discharge the DC input bus, resulting in overheating and damage to components.
    For AC inputs, if Cx > 5uF, it will take longer to discharge than the standards allow.

    3) Section 9.2.2.14 (page 55) of the datasheet indicates the total resistance should be about 5kR. The number of series resistors used depends upon their voltage rating.

    4) The 0.47uF cap C39 on the EVM is a dummy bulk cap and the 40MR bleeder resistors are a dummy load for this X-Cap discharge demonstration circuit. It is added to the LLC EVM because normally the LLC follows after a PFC stage which follows an EMI filter.  This is needless complication to show the X-cap discharge function.  Instead, this dummy input filter is added to showcase X=caps C40 and C41 and how the HV pin works for start-up and X-cap discharge.

    5) Yes, just follow Option #7 and the correct resistance range.

    6) Theoretically the LLC needs just one resonant cap C17, but there are some advantages to splitting that cap into two to complete the half-bridge topology.  So Cr = C1 + C17 in value.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    TI__Expert 5483 points

    Hi Ulrich,

    Thank you for the replies. I have a few follow ups...

    [1] The UCC256404 device HV pin is what is used to sense the AC waveform (Figure 38) - yes?

    [2] If there is no AC waveform for ~1s (txcap...), then the HV pin is used to draw current through and discharge the x-caps...yes?
    I assume the device is "turned off" during this time, and doesn't actively work to regulate the output voltage?

    [3] If the input is DC, the UCC256404 will never work, because it will always be trying to discharge the input ...yes?

    [4] In this case, you either need to wire a trace from HV to the AC input stage somehow, or use UCC256403, which has X-cap discharge disabled, and you just connect HV to GND...yes?

    Thanks,

    Darren

  • 0 in reply to Darren (FAE)
    TI__Mastermind 26770 points

    Hello Darren,

    For your follow-up questions:

    1) Yes, in UCC256404 HV is used to sense the AC waveform, after being used to charge up the VDD cap for start-up.

    2) Yes, HV draws current to discharge the X-caps when a DC state is detected.  But NO, the controller continues to actively work to regulate the output even during X-cap discharge.  It does not stop switching.

    3) No, if the input is DC, the UCC256404 will power up as usual and regulate the output. Meanwhile it begins to test for DC condition and then attempts to discharge the X-caps.  Yes, it will always be trying to discharge the input (after start-up), but no, it will not suspend regulation while doing this.

    4) To avoid damage in the X-cap discharge path, you need to connect HV to an AC input appropriately.  Otherwise, yes, consider using UCC256403 with HV grounded.

    Regards,
    Ulrich