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Continuous Drain Current of CSD18533Q5A

Golem Chen
Prodigy 100 points
Other Parts Discussed in Thread: CSD18533Q5A, CSD18534Q5A, CSD18537NQ5A

Hi,
I'm looking for a power MOSFET used in brushless DC motor application.
And there is a module from TI used CSD18533Q5A for BLDC motor.
(www.ti.com/.../TIDM-LPBP-BLDCMOTORDRIVE;tisearch=Search-EN-Everything)

My question is what's the Continuous Drain Current of this MOSFET.
There are two information about Drain Current in the datasheet, one is in the table of "Absolute Maximum Ratings", it says 17A.
But in the other hand from "Figure 10. Maximum Safe Operating Area", if I use Vds=24V(DC), the max current is about 1A.
What's the different about these two information?
Thanks.

Have a nice day.
Golem

  • 0
    TI__Mastermind 18200 points
    Golem,
    The 17A is calculated from the RthJA and it is assumed the FET is fully on and being driven at 10Vgs or greater. In the SOA, when Vds = 24, you are operating in the linear region. The FET is not fully on (otherwise the current would be much higher and the FET would be saturated). Also, consider how much power that is - even 1A DC would be burning the FET with 24W.
  • 0 in reply to Brett Barr1
    Prodigy 100 points

    Hi Brett ,

    I think I make a mistake, since there is a loading(motor), the Vds will not equal to 24V.
    So I need to calculate the actual Vds, and decide the switch frequency, then see if it's in the SOA right?

    And I'm not familiar at "RthJA", is this means 17A is the saturation current at Vgs=10V or higher?

    Thanks.

    Have a nice day.
    Golem

  • 0 in reply to Golem Chen
    TI__Mastermind 18200 points
    Golem,
    No the saturation current would be much higher. The 17A is a thermally limited current assuming that the FET can dissipate 40 degrees C per Watt.

    Datasheet currents are all theoretical calculations. What current are you running the motor at? Basically you need to figure out how much power the FET will need to dissipate and if that is within reason for a QFN device.
  • 0 in reply to Brett Barr1
    Prodigy 100 points

    Hi Brett,

    We select a 70W BLDC motor operated at 24V. On the datasheet, the rated current of this motor is 3A, but the peak current is 20A.

    What makes me confused is that if I-peak=20A,  how do I choose a suitable MOSFET.

    Thanks.

    Have a nice day.
    Golem

  • 0 in reply to Golem Chen
    TI__Mastermind 18200 points

    Golem, 

    You should plan on worst case scenario, so if the peak current is 20A, you need to make sure the FET can handle that much. 

    Currents on datasheets are generally calculated from very specific board conditions, and thus they cannot be generalized to apply all the time. 

    20A isn't too much, especially if it will only be for small durations, and 3A isn't much at all. Th CSD18533Q5A shouldn't  have any problem with this. You can probably even go smaller with the CSD18534Q5A or the CSD18537NQ5A. Your best bet is probably to try more than one device and see which one gives you the thermal performance that is satisfactory for your board. 

  • 0 in reply to Brett Barr1
    Prodigy 100 points

    Hi Brett,

    Thanks for your help!

    I get more information about selecting a MOSFET. 
    I may start with CSD18533Q5A and see if there is room to change to other MOSFET you said.

    Thanks again.

    Have a nice day.
    Golem