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Original question:

SN6505B: Can the SN6501 be readily replaced with the SN6505B?

SN6505B: How to solve this problem : D2 short to GND

Allen Chu
Intellectual 890 points
Part Number: SN6505B


Dear, when I used SN6505BDBVR Isolated drive transformer，the circuit is used in BMS, Supply voltage 5V。After mass production，SN6505BDBVR  had been damaged 3PCS On the client side. After analyzed, SN6505BDBVR  D2 short to GND. Can you help analyze this problem? Thanks.

SN6505B Circuit

D2 waveform

  • 0
    TI__Guru 55825 points

    Hi Allen,

    Thanks for reaching out and for sharing schematic and waveform.

    Please see below my comments and request for further information.

    1. I think you mentioned the input supply is 5V, could you please confirm what is the desired output voltage and what is the maximum load applied?
    2. Can you please confirm 3 pcs damaged out of total how many tested?
    3. Do you see all 3 showing up short from D2 to GND? Do you know if D1 or VCC are also shorted to GND?
    4. In the waveform, can you please indicate what are the 3 channels - CH1 (blue), CH3 (magenta), CH4 (blue)?
    5. I also noticed that the schematic has a resistor R5 valued 120Ω in series with 100nF cap C8. This is a decoupling cap for device, we do not recommend having any resistance in series with this cap.
    6. We also recommend having a bulk capacitance of 10µF or more on the transformer pin 2 for a stable input supply. Since the input supply isn't sufficiently filtered, it is possible for the transients at input to appear at D1/D2 pin which could exceed device voltage ratings.

    Please do share the above requested information to help debug the issue, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Intellectual 890 points

    Thanks for your repling.

    1.output voltage 12.5V.

    3.VCC shorted to GND,D1 or D2 shorted to GND.

    4.CH1 (Blue)  ,   D2 waveform.

    Others we found  from system clock mode changing to  External Clock Mode,D2 voltage had Voltage peaks, how  do we change the clock mode?

  • 0 in reply to Allen Chu
    TI__Guru 55825 points

    Hi Allen,

    Thanks for your inputs.

    Like I mentioned in my previous response, please request customer to remove R5 and add a bulk cap of 10µF. This most likely should resolve the issue.

    Regarding the external or internal clock operating mode, if the CLK is left unconnected then the device work with internal clock. If the CLK pin is connected to external clock source, then it will work at the frequency of external clock connected. There is no separate clock enable / disable, the CLK pin itself detects and works. I see that customer schematic has CLK connected some device, please ask customer to disconnect CLK from other devices and it should work fine with internal clock.

    Please request customer to follow the suggestions provide and let me know if they any further issues, thanks.


    Regards,
    Koteshwar Rao

  • +1 in reply to Koteshwar Rao
    Intellectual 890 points

    Hi Koteshwar,

    Thanks a lot.

    We have removed R5 and add a bulk cap of 10µF.

    When the chip starts, we found that D2 spikes occur during the internal clock switching to an external clock, and what is the reason for the spikes in D2?

    CH1 (Blue)  was D2 waveform.

    SN6505 EN, CLK operate timing sequence may be required？

  • +1 in reply to Allen Chu
    TI__Guru 55825 points

    Hi Allen,

    The spikes that you see during the rising edge of D2 waveform is the ringing caused by leakage inductance. The leakage inductance along with D2 parasitic capacitance forms resonance leading to oscillations at that frequency (usually much higher than switching frequency).

    The cause for leakage inductance could be many but the two I can think of are poor transformer design with high leakage inductance  and the other is poor PCB layout contributing leakage inductance due to long traces or loops. Please share customer PCB layout and I can review to see if there are any obvious issues in it. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Intellectual 890 points

    Hi Koteshwar,

    Can you tell me design requirements for start timing and frequency control relationship？

    Thanks & Best Regards

  • 0 in reply to Allen Chu
    TI__Guru 55825 points

    Hi Allen,

    The device has internal soft-start feature which sets the device start-up time to a fixed value as stated in the datasheet. This cannot be changed although it could vary slightly based on the output load conditions. This doesn't have any relationship to the internal or external switching frequency. The device will start switching at the frequency that is set by the design (internal or external). Thanks.


    Regards,
    Koteshwar Rao