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Original question:

TPS25925: TVS and Schottky diode selection

LM5069: LM5069

Joseph Fearon
Prodigy 20 points
Part Number: LM5069
Other Parts Discussed in Thread: TPS25925, , TPS4811-Q1

Tool/software:

Hi Mark,

The path inductance at the input and at the output determine the need of TVS diode at the i/p and schottky diode at the output.

As you see in datasheet,  Equation 18 gives the transient voltage spike generated at the input when the device breaks the fault current. This demands TVS with appropriate clamping voltage to keep the input voltage with in the abs. max. of the TPS25925 device. Similarly, schottky diode is needed to freewheel the o/p current to clamp the negative voltages at the output.

TVS diode SMBJ16A-13-F and schottky diode B320A-13-F are recommended. The attachment provides information about the selection of TVS diode

TVS Clamping in Hot-Swap Circuits.pdf

Best Regards,

Rakesh

What are the criteria for selecting the Schottky diode that would be placed on the output in parallel with the TVS diode ?

The answer above addresses the TVS selection but not the schottky selection. Is there a similar article explaining the selection of Schottky for the output rail in Hot-swap applications ? 

Thanks 

  • 0
    TI__Guru** 104931 points

    Hi Joseph,

    Welcome to E2E!

    Here are the Schottky diode selection guidelines

    • A DC blocking voltage rating much higher than the Vin(max)
    • Non-Repetitive Peak Surge Current rating > Circuit breaker current
    • -VF should be close to min abs voltage rating of VOUT pin at the circuit breaker current
    • Use combination of capacitors and multiple Schottky diodes in parallel if needed

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 20 points

    Thanks for this explanation. What do you mean by -VF. Also how do you select the capacitor value and type. The circuit breaker current in my application is quite high, the max circuit breaker current would be ~250A at peak operating voltage of 60V. 

  • 0 in reply to Joseph Fearon
    TI__Guru** 104931 points

    VF means forward voltage of Schottky diode.

    Cout is driven by the load requirements. How much pulsed load you want to support and to attentuate noise if the load is DC-DC converter.

    you mean you are designing LM5069 for 250A ?

    Have you looked at TPS4811-Q1 ?

  • 0 in reply to Rakesh
    Prodigy 20 points

    Hi Rakesh

    The application is a Hot-Swap controller using LM5069MM-2 (see schematic below). The max circuit beaker current or fault current is 250A, permissible for 0.39ms before the controller turns off the pass FET. The load is a DC Brushed Motor. As it's a Hot-swap application to limit the inrush current I don't really want to add more capacitance to the rail as this will increase the inrush current.

    I've not looked at the TPS4811-Q1 as I've inherited this design from a previous engineer. The system is changing from a 24V battery to a 48V lifePo4 chemistry that has a full charge voltage of 60V. I need to be able to suppress the large transients caused by the back emf from the motor in order to protect the LM5069 as it's max voltage rating is 100V.

    So far I am planning to use a 30KPA58A. It has a min breakdown voltage of 64.8V and a Max clamping voltage of 92.4V. The existing design also uses a Schottky diode V60D60C in parallel with the TVS. See screenshot of schematic.

    So my question is how would I select a suitable Schottky diode to suppress transients due to switch off of the pass FET's. And how to calculate the power dissipation to ensure correct sizing of component.

    Is it possible to calculate or estimate the Transient voltage and Power without knowing all the details of the motor connected ?

    Thanks

  • 0 in reply to Joseph Fearon
    Prodigy 20 points

    Using your guidelines I'm thinking to use this diode V35DM120, The VRRM is 120V this is 2x VINMAX, the peak surge current IFSM is 320A this is greater than the Circuit breaker current 250A .The out pin abs min voltage is -1V. The datasheet for the diode doesn't give the VF for currents of 250A which is my circuit breaker current. So I'm unsure if this diode would be suitable.

    www.vishay.com/.../v35dm120.pdf

  • 0 in reply to Joseph Fearon
    TI__Guru** 104931 points

    Did you verified SOA margin using LM5069 design calculator ?