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Original question:

SN6505B: Does this pushpull driver need a snubber to be added?

SN6505B: Overvoltage peaks - Most prominent during startup

Cedric Gasser
Prodigy 110 points
Part Number: SN6505B

Hello,

We have created the following supply with the SN6505B:

Do reduce the ringing on D1 and D2, a RC snubber has been determined and added. R900 & R901 = 39e, C900 & C907 = 1nF

In regards to the maximum voltage on D1 and D2 of 16V (12.8V if you want to have a 20% reserve) we now still see Voltage spikes of 13.5V at 0.15A load and 19.25V at 0.27A load.
Even thoug these voltage spikes are too high, especially with increased loads, the mein Issue is, that we see these spikes also everytime at the startup of the circuit. Even with no load

Startup without load: CH1 = D1, CH2 = D2, CH3 = Vin, CH4 =Vout

Startup with 0.15A load: CH1 = D1, CH2 = D2, CH3 = Vin, CH4 =Vout

0.15A load: CH1 = D1, CH2 = D2, CH3 = Vin, CH4 =Vout

0.27A load: CH1 = D1, CH2 = D2, CH3 = Vin, CH4 =Vout

In further Tests we also added a TVS Diode on Q91 & Q905, or four 5.6V  Z-Diodes between Vin and D1/D2. Still we struggle to limit the peaks to <16V during startup and we cannot reach the 20% reserve of 12.8V at all.

According to the "SN6505B: Does this pushpull driver need a snubber to be added?" Thread, snubber and TVS Diode should not be neccesary at all.

Any advice on how to handle this overvoltage issue?

  • 0
    TI__Guru 53115 points

    Hi Cedric,

    Welcome to TI E2E forum!

    Thanks for sharing information related to the issue you are facing. To better understand the issue, could you please also share the below information?

    1. Transformer part number or datasheet which includes Vxt product information as well
    2. Desired output voltage (12V or 14V?) and current
    3. Share PCB layout to review

    The ringing on switching waveform typically happen due to high leakage inductance in transformer or PCB traces. RC snubber circuit can suppress these transient noise but a good design shouldn't require the snubber.

    Please share the requested information so that we can better comment, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 110 points

    Hi Koteshwar,

    1.) As we need a rather high isolation and creepage distance, our transformer choices are somewhat limited.
    Currently we have the HCTSM110103HAL-E from Bourns in the design. As the Vxt product is not part of the datasheet we measured 5 samples and the Vxt product is 10uVs. According to the datasheet the minimum Vxt product is 7.6uVs. Am I correct to assume that we're good here?

    HCTSM110103HAL(D6474)_DSS(20221017)X5.pdf

    2.) The desired Output Voltage is ~14V (Anything between 8V and 15V works)
    The needed Output power is 2W (~150mA output current). Althoug for an other similar applications we hope to use the same transformer for 3.3W (~270mA)

    3.) The layout is intended to be similat to the recommendation in the SN6505 datasheet.

    Best regards,

    Cedric Gasser

  • 0 in reply to Cedric Gasser
    TI__Guru 53115 points

    Hi Cedric,

    Thanks for providing the requested information. Please see my inputs below,

    1. Yes, your understanding is correct. The device SN6505B requires a minimum Vxt product of 7.6Vµs and a transformer with 10Vµs should work fine provided this is guaranteed across temperature and units.
      1. I see that the transformer has a leakage inductance of 6µH max for a primary inductance of 80µH. This seems to be on the higher side and it is quite possible that this is leading to the ringing that you are observing.
        1. Please do try to test a TI recommended transformer on your design to verify if the transformer is the cause for ringing.
      2. Could you also please confirm how did you measure Vxt product of the transformer?
      3. It is typically estimated and provided by the manufacturer.
    2. SN6505B should be able to support these requirements.
    3. The PCB layout mostly looks good and I do not see any major concerns.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Guru 53115 points

    Hi Cedric,

    I didn't hear back from you in a while, I am hoping that you were able to test a transformer with lower leakage inductance and avoid the ringing / transients. I will go ahead and mark this closed but if you still have questions, please feel free to respond back. If you have other new questions, please feel free create new E2E posts with your questions. Thanks.


    Regards,
    Koteshwar Rao