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LM5026: Strange UVLO behavior

Nick Naylor
Prodigy 65 points
Part Number: LM5026

I have a forward converter with an input range of 120-190Vdc. I have the UVLO set at approximately 115V. If I turn on the supply slowly, it works fine, seen in the first picture PIC 1. However, if I do a rapid increase, it turns on around 90V, seen in the second picture PIC 2. There is no over-shoot on the input supply. CH1 BLUE is the input supply. CH2 RED is the UVLO pin.

Per Figure 26, R1 is 2M and R2 is 21.5K, which should give 40V of hysteresis and a turn-on of about 115V.

What is going on here?

PIC1

PIC 2

Circuit

  • 0
    TI__Mastermind 23875 points

    Nick

    Are we talking about the EVM or your own design? Did you add the capacitor (C10 in the schematic snip you posted) in parallel with the 21.5kΩ? Maybe add C10, adjust the value or add a HF bypass cap as close to the UVLO pin as possible?

    Steve

  • 0 in reply to Steven Mappus1
    Prodigy 65 points

    Hi Steve,

    This is my own design. I do have C10 populated with the 1000pF right now. I can play around with that, but I'm not currently seeing any noise on the UVLO pin.

    Thanks

  • 0 in reply to Nick Naylor
    TI__Mastermind 23875 points

    Nick,

    How are you applying VCC relative to VIN? With your 4-channel scope measure VIN, UVLO, VCC, REF and then VIN, UVLO, VCC, OUT. You need to make sure the UVLO comparator is biased before VIN reaches the UVLO_ON threshold. Another test could be to apply VCC bias separately (in place of the AUX winding) with a DC lab supply set to ~12V (diode in series with DC lab supply V+)...this will assure the PWM is actively biased before VIN is applies to UVLO. The information gained will help determine next steps.

    Steve

  • 0 in reply to Steven Mappus1
    Prodigy 65 points

    Here is schematic. VBLK is the main input, PVCC comes from a bias winding.

    First 2 are CH1 = VIN, CH2 = UVLO, CH3 = VCC, CH4 = REF

    Second 2 are CH1 = VIN, CH2 = UVLO, CH3 = VCC, CH4 = OUT

    Rapid increase

    Slow Increase

    Rapid Increase

    Slow Increase

  • 0 in reply to Nick Naylor
    Prodigy 65 points

    Forgot this one. CH1 = VIN, CH2 = UVLO, CH3 = VCC, CH4 = main input (VBLK). This is at 90V, so should not be turning on.

  • 0 in reply to Nick Naylor
    TI__Mastermind 23875 points

    Nick,

    For applications where an external regulator is not  applied (recommended when VIN<100V), UVLO is directly sensing VIN=VBULK so that whatever dV/dt is applied to VBULK, VIN is the same. Now you've added a 24V regulator between VBULK and VIN, so that VIN only sees 24V. If you look at the last scope pic you sent, you see that VBULK (CH4) is rapidly applied as 90V, UVLO>1.25V and this is happening while VCC<7.6V (VREF is not regulating yet, so the internal UVLO comparator is not biased and probably does not have it's 1.25V on V- yet). The UVLO comparator is about to become biased with V+>1.25V already present so that the comparator output will be HIGH when bias is applied.

    Two things to try:

    1. Since you are seeing inadvertent turn-on at VBULK 90V, try bypassing your linear regulator and connect BULK directly to VIN and you should see no turn-on under this condition. Do not try this test for VBULK>90V, just verify if you see a change in the turn-on and that you should see no OUT pulses when VBULK=VIN=90V
    2. Secondly, go back to your linear regulator and make sure it is configured as shown below. In your snippet from your schematic, I do not see that your regulated VIN is externally connected to VCC as shown below. Also, why did you opt to regulate VIN to 24V? You only need ~9V as shown below.

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    Prodigy 65 points

    Hi Steve,

    Regarding your ideas:

    1. I did this and the IC does not turn on. This tells me it is something to do with my input regulator.

    2. I tried this and it operates in the same manner. The 24V was simply because it's well over the 13V startup and so it doesn't have to drop as much voltage.

    I did get to thinking about slowing down the UVLO signal. I increased C10 considerably, 0.1uF instead of 1000pF. That seems to work. The 5V reference is up before the UVLO signal comes up and the problem is gone. I'll have to do some testing to make sure it didn't change anything else.

  • 0 in reply to Nick Naylor
    TI__Mastermind 23875 points

    Nick,

    Thanks for the waveforms and quick feedback. Clearly we see a sequencing condition in your converter startup that I believe you are now on the right path to resolving. I'm going to close this thread for now but if you need additional support, refence back to this E2E if needed or start a new topic. Hope this helped resolve your issue and if yes could you please click "resolved my issue?" Thanks for contributing tot he E2E forum.

    Regards,

    Steve