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LP8860 / Braking EVM

Kuramochi Tadahiko
Guru 28715 points
Other Parts Discussed in Thread: LP8860-Q1

Hi,

My customer and I broke the several EVM...

The conditions are too large heating, error of output, etc...

I think that the EVM is broken when input voltage is low.

Have you ever heard similar case?

Best Regards,

Kuramochi

  • 0
    TI__Expert 4565 points

    Hi Kuramochi-san,

    Yes, low input voltage makes SW current higher and causes more power loss on SW FET. High power loss on SW FET heats up itself and can damage SW FET eventually. This usually happens at high SW freq condition such as 2.2MHz. At low SW freq, SW current or input current limit will hit first before this point.

    High SW loss on SW FET is caused by switching on SW FET itself and reverse recovery current on output diode. Default diode used on EVB is OK for low freq, but may cause high reverse recovery current at high freq. You may be able to try another diode to improve this. We've tested FSV10100V(Fairchild) and could lower SW FET temperature significantly.

    If you can't change diode, lowering SW freq or increasing input voltage will help.

    Thanks,

  • 0 in reply to Sung Ho Yoon
    Guru 28715 points

    Hi Sung-san,

    I received the additional questions from my customer.

    ・How much volts is the lower voltage breaking the EVM @2.2MHz?
    ・Is the EVM safe if the switching frequency is set lower than 2.2MHz?

    ・Is there any other important points in operating the EVM?

    Please answer on your experience for our reference.

    This EVM is braked frequently...


    Best Regards,
    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Expert 4565 points

    Hi Kuramochi-san,

    How much volts is the lower voltage breaking the EVM @2.2MHz?

    - If this is the question for low input voltage condition, it really depends on load size. At 2.2MHz SW freq, SW loss can be huge depending on boost components and SW current. Based on our test results, Vin can be as low as 5.1V for Vout = 37V, ILED per string = 120mA at 25C. If input voltage lower than this, boost OCP fault may trigger or SW FET may be damaged by thermal accumulation.

    - If this is the question for max high input voltage, any voltage higher than 52V can break LP8860-Q1 IC itself.


    Is the EVM safe if the switching frequency is set lower than 2.2MHz?

    - Actually, lower SW freq than 2.2MHz will be safer in terms of thermal characteristics. EVM can support wider input voltage range at lower SW freq.

    Thanks,

  • 0 in reply to Sung Ho Yoon
    Guru 28715 points

    Sung-san,

    Thank you for your answer.

    How about the following question?
    - Is there any other(fsw, Vin) important points not to break the EVM?

    Best Regards,

    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Expert 4565 points

    Other than mechanical short or high temperature exposure, EVM should be OK.

    Voltage/current ratings of all components should support worst condition.

    Thanks,

  • 0 in reply to Sung Ho Yoon
    Guru 28715 points

    Sung-san,

    Thank you for your support.

    I compared the FSV10100V with  the SS5P10(Vishay) which is installed the EVM as follows.

    I think that the SS5P10's SW loss is lower than the FSV10100V's one because the IR and Cj of the SS5P10 are smaller than the FSV10100V's one.

    Is my understanding wrong?

    Best Regards,

    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    Guru 28715 points

    Sung-san,

    I have additional questions.

    Is there the capacitor which you can recommend in the follows?
    - RB056L-40-TFTE25
    rohmfs.rohm.com/.../rb056l-40.pdf
    - RB050L-60DDTE25
    rohmfs.rohm.com/.../rb050l-60.pdf

    Please refer the following thread about condition.
    e2e.ti.com/.../573010

    You said that " Vin can be as low as 5.1V for Vout = 37V, ILED per string = 120mA at 25C.",

    In this case, which inductor was implemented the FSV10100V or the SS5P10 ?

    Please let me know if you have any question.

    Best Regards,
    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Expert 4565 points

    Kuramochi-san,

    Generally speaking, your understanding is correct, but in this case, Vf change over current seems affect AC characteristics more than parameters above.

    Vf of SS5P10 becomes much higher(~>0.6V) at peak diode current(not average rectified current), and lose characteristics of Schottky diodes which has fast transition time.

    As a result of long transition time, reverse current from boost is injected back to SW FET during its on time, and increase switching loss more than FSV10100V.

    Based on our test result, using FSV10100V lowered SW FET temperature tens of degree C at max load, max SW freq(2.2MHz) on other LED driver EVB.

    Hope this helps,

  • 0 in reply to Kuramochi Tadahiko
    TI__Expert 4565 points

    I guess you are asking about Schottky diode.

    Both seems too small for the apps condition.

    Voltage rating of 40V one is too low because max output voltage of LP8860 is 48V in case LED open fault happens.

    Current rating of 60V one is only 2A at worst case. Peak diode current can be higher than 2A though average rectified current is only 0.3A.

    I recommend using at least 5A or higher one.

    Thanks,