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UCC256403: shortcut is not detected

Lukas Kreiner
Prodigy 30 points
Part Number: UCC256403
Other Parts Discussed in Thread: UCC256402, , UCC256303, UCC256404

Hi Team,

in case of shortcut at the output of my circuit this happens:

CH1: signal on PIN 6 /ISNS CH2: Gate lowside FET

during the time in which the higeside FET is on (between the ursors), the average of the ISENS signal is zero so the IC does not detect the shortcut.

can you help me with this?

  • 0
    TI__Guru 65955 points

    Hello,

    When you short the output of the LLC you are shorting the transformer and also taking power away from the opto feedback.  Because there is no opto current the controller will decrease the switching frequency to try to increase the gain.  The VCR pin will try to control the current even though the output is shorted.  So I am not surprised it is not shutting down.

    The ISNS pin is 3.2V so OCP1 one will not trip.  It looks like the VCR is controlling the current in the primary so OCP2 and OCP3 don't trip either.

    You could try to increase the RISNS resistor to see if you could get OCP1, COP2 or OCP3 to trip under output short circuit conditions.

    However you could ensured the UCC256403 shut down with an output short by power VDD of the UCC256402 off an aux winding from the LLC transformer.  This is shown in the simplified schematic on the front page of the data sheet.  In this way when the output is shorted VDD will become 0 V through VDD = Vout*Na/Ns and the controller will shut down because VDD is too low.  This is shown in the simplified schematic on the front page of the data sheet.

    Regards,

  • 0 in reply to Mike O'
    Prodigy 30 points
    Dear TI-Experts,
     
    although I understand your description of the problem, I can't believethat a modern IC is not able to reliably recognize a short circuit condition,
    but needs to be resetted externally.
    In our design we power the IC by a tiny dedicated DC/DC-converter. To power it by an additional aux winding would mean a new transformer
    a complete new layout and a time delay that our customer will not accept and might let us run in a contractual penalty.
    When working with the UCC256303 we managed to get the shortcircuit detection running in hick up mode after some amount of optimization.
     
    There must be a possibility to get into hick-up-mode without converting the whole design to the aux-supply.
    Any help will be greatly appeciated!!!
     
    Kind Regards,
    Mark Vorpeil (on behalf of Lukas Kreiner)
  • 0 in reply to Lukas Kreiner
    Prodigy 30 points

    Dear TI-Experts,

    some more information regarding our short circuit problem. See files of ISNS attached.

    As you can see the frequency drops from 0A to 8A, but rises extremely (up to 300kHz) in short circuit while the voltage of the signal drops.

    When we compared this with our unit using the previous version of the controller, we found that ISNS behaved quite different and allowed us to detect short circuit condition:

    0A    1,04V    @  130kHz

    8A    1,6V    @  111kHz

    SC   3,2V    @  158kHz

    Why does the new controller deliver such a low ISNS signal, while the old one does not?

    Kind Regards

    Mark Vorpeil + Lukas Kreiner

    ISNS-0A.TIF

    ISNS-8A.TIF

    ISNS-10A(sc).TIF

    ISNS-10A(sc)2.TIF

  • 0 in reply to Lukas Kreiner
    TI__Guru 65955 points

    Hello,

    I check the devices and the OCP trip points of the UCC256303 should be higher than the UCC25403.

    So if the design was the same the UCC256404 should enter OCP protection earlier.

                         UC256303               UCC256404

    OCP1           4.03V                        4V

    OCP2           0.84V                        0.6V

    OCP3           0.64V                        0.43V

    So the waveform you sent will not trip OCP1.  OCP2 and OCP3 are based on average current being measure only when HO is high.   This waveform is more triangular than sinusoidal.

    So the average would be roughly 2V*0.5*0.5 = 0.5V.  This should not have triggered OCP3 in the UCC256303 but should trip OCP3 of the UCC256404.

    You should be able to adjust the Risns resistor to fine tune this for a short circuit.  So you are correct.

    If OCP3 is exceeded for 50 ms, switching you stop for 1 s and a restart initiated.  I believe this is cycling of VCC between VCCReStartJFET(9.65V) and VCCStartSwitching (26V.)  I also believe the 1 s is based on VCC capacitor discharge and recharge time.  This will also set a soft start.

    Is there anything in your design that is preventing a VCC discharge if OCP3 is tripped.  There is something different between you two designs.  The UCC256403 should have initiated and OCP3 before the UCC256303.

    Based on OCP3 and adjusting Risns you should be able to cause the device to enter a hiccup mode during output short circuit.

    Regards,