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UCC28180EVM-573: ucc28180 RC Snubber

Bright Robert
Intellectual 340 points
Part Number: UCC28180EVM-573
Other Parts Discussed in Thread: UCC28180, , UCC21550

Tool/software:

Hi,

I wanted to ask whether or not the ucc28180 does not require a snubber circuit as shown in the Evaluation for normal operation and without voltage spikes?

Kind regards,

Bright

  • 0
    TI__Mastermind 40070 points

    Hello Bright, 

    There is no RC snubber on the UCC28180EVM-573 board.  

    A snubber is not required in any topology, but is often a good idea to use to reduce or suppress voltage spikes that occur when current in an inductance is suddenly stopped or changed.  In the UCC28180 EVM, a snubber was found to not be necessary. 

    Regards,
    Ulrich   

  • 0 in reply to Ulrich Goerke
    Intellectual 340 points

    Hi Ulrich, 

    Thank you for the response. 

    Do you have any appropriate means for calculating/determining the RC snubbers values? especially for half bridge configuration such as ucc21550?

    Kind regards,

    Bright

  • 0 in reply to Bright Robert
    TI__Mastermind 40070 points

    HI Bright, 

    Here is one article on R-C snubber design.  https://www.ti.com/lit/pdf/ssztbc7?keyMatch=rc%20snubber 
    If you search TI.com or the internet, I'm sure you can find many more. 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 340 points

    Hi Ulrich, 

    Thank you for your response. 

    From the Step 2, What is a typical capacitance at full reverse voltage of a MOSFET, for example Part number sihp24n80ae, what capacitance is it referring to?

    • Input Capacitance (Ciss): 1836 pF
    • Output Capacitance (Coss): 65 pF
    • Reverse Transfer Capacitance (Crss): 5 pF

    or something else?

    Kind regards,

    Bright

  • 0 in reply to Bright Robert
    TI__Mastermind 40070 points

    Hi Bright, 

    Typically, a MOSFET has an R-C snubber applied from drain to source, so the MOSFET's drain-to-source capacitance is used for calculation.
    That capacitance is the Coss.   

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 340 points

    Hi Ulrich,

    Thank you for your response.

    1) What should be the voltage rating of the Caps and resistor?

    2) What is the best way to calculate the power dissipation of the resistor?

    E=1/2 x ​CV^2

    P = E x Fs, where Fs is the switching the frequency?

    Kind regards,

    Bright​

  • 0 in reply to Bright Robert
    TI__Mastermind 40070 points

    Hi Bright, 

    Your equations are correct.

    Please see this paper on snubber design for design details: https://www.ti.com/lit/slup100 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 340 points

    Hi Ulrich, 

    Thank you for your response. 

    While the specific arrangement of the resistor and capacitor relative to each other does not change the fundamental operation of an RC snubber, their placement relative to the switch and each other in the circuit can impact performance in terms of response time. 

    But does it matter which one that should come first across the drain of the MOSFET, either the capacitor or the resistor or it needs to be the capacitor? 

    I have seen designs that connects either way, so I am wondering which is the effective way. 

    Kind regards,

    Bright

  • 0 in reply to Bright Robert
    TI__Mastermind 40070 points

    Hello Bright, 

    The order of resistor and capacitor across the MOSFET drain to source is irrelevant. It does not matter at all to the MOSFET.
    Snubber performance will be exactly the same. 

    Some designers do not think about it at all.  Some may find that the order of placement is dictated by the way the package outlines fit better into their pcb layout.  Some may want the capacitor to be at the source if the source is GND, so that they can easily probe the ringing voltage on the cap.  Some may want the resistor to be at the source if the source is GND, so that they can easily probe the ringing voltage on the resistor and calculate the current.
    If the MOSFET (or diode) is not GND_referenced, then it will be equally onerous to probe either component differentially. 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 340 points

    Hi Ulrich,

    Thank you for the clarity. 

    I know that the voltage rating of a snubber resistor should be at least equal to the maximum voltage expected across it during operation, which is often less than the full DC link voltage unless the resistor directly bridges the high and low potential sides of the link. In this case if my DC link is 400V DC, I can use R_snubber rated at upto 300V - 400VDC?

    Kind regards,

    Bright 

  • 0 in reply to Bright Robert
    TI__Mastermind 40070 points

    Hi Bright, 

    After the MOSFET (or diode) has been ON for some time, the snubber capacitor may be fully discharged, so the maximum voltage across the resistor at the time of turn OFF can be the entire DC link voltage of 400V or even higher while the spike current flows during the snubbing action. 

    I recommend that you rate your resistor for 500 or 600V.  If a single resistor can't be found to sustain this, then string 2 or 3 equal-value resistors in series to split the peak voltage burden.  This is a widely-used practice. 

    The same with the capacitor voltage rating.  It needs to be rated higher (with sufficient margin) than the highest peak voltage that may appear across it (usually higher than the DC link).   

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 340 points

    Hi Ulrich,

    Thank you for your response. 

    Kind regards,

    Bright