Practical Power Dissipiation in SMD MOSFET

Discussion in 'General Electronics Chat' started by RamaD, Jan 8, 2015.

  1. RamaD

    Thread Starter Active Member

    Dec 4, 2009
    254
    33
    How much power dissipation is practical with SMD MOSFETs?

    The data sheet for this MOSFET CSD18531Q5A states :
    http://www.ti.com/lit/gpn/csd18531q5a

    Max. Pd (abs max ratings) 3.1W
    with 1sq in, 2 oz copper
    RθJC Thermal Resistance Junction to Case 1 °C/W
    RθJA Thermal Resistance Junction to Ambient 50 °C/W

    Even if the Pd is 1W, then the pcb track or case temperature would be 50 °C above ambient? On a max. ambient of say 45 °C, the case temperature would be 100 °C?
    In other words, what is the practical max. temperature allowable on the pcb track or the component?
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    no, the 50 degree c/w is a max rating, the actual is determined by the curent flow. you have to design the pc board with enough copper to use as a heatsink for the current it is designed for.
     
    RamaD likes this.
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    Remember, the power dissipation is not the load, but the chip.
     
    RamaD likes this.
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    Yes, it is as you describe. At 45 C ambient the case would be 95C, the chip inside would be 96C.
     
    RamaD likes this.
  5. RamaD

    Thread Starter Active Member

    Dec 4, 2009
    254
    33
    Thank you all.
    Yes, I understand that it is not the output power, but the power dissipated by the MOSFET.
    Since it is on the pcb, what would be the allowable temperature or practical maximum temperature?
    Is it possible to add a heatsink on these SMD devices apart from increasing the copper?
     
    Last edited: Jan 8, 2015
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,770
    970
    Typically standard FR4 circuit boards have UL RTI values around 105 deg C to 130 deg C.. You need to look at the approvals of the company producing the boards for you and the substrate/build up they are using.
    Of course any applicable standards your product is being built to may dictate other requirements for temps,etc...

    And yes you can add heatsinks too.. I used these in a past project for a SMT mosfet and they worked great. http://www.ohmite.com/cat/sink_d.pdf
    I could easily dissipate 9-10W..
     
    RamaD likes this.
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    Some tiny heat sinks are available to be glued on or clipped on.
    [​IMG]
     
    RamaD likes this.
  8. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    If you're that concerned about power then why are you using an SMD part which is difficult to heat sink?
     
    RamaD likes this.
  9. RamaD

    Thread Starter Active Member

    Dec 4, 2009
    254
    33
    I was intrigued by the small size of a 20A MPPT charger with SMD Components :)
     
Loading...