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TIDA-00792: Power rating of Two parallel MOSFETs (CSD19536KTT) is not less?

imran riaz
Prodigy 100 points
Part Number: TIDA-00792
Other Parts Discussed in Thread: CSD19536KTT, BQ76940, BQ76200

Hello TI experts,

I am designing a battery storage system where my max voltage of system is 50.4 V. For my application the max current from battery storage to load is about 15A. I am using TIDA-00792 as a reference design. 

If we look at the "Figure 10. Maximum Safe Operating Area"  in datasheet of CSD19536KTT we can see that for DC current maximum current for Vds of 50V is less than 2.5A and for Vds>50 V is even less than 1 A. 

You have used two MOSFETs in parallel so current rating will be double. Please correct me if I am wrong. My question is "Is using MOSFET as a switch for DC a good solution?". What do you think about using IGBT instead of MOSFET because we do not need fast switching?.

If I can not use  CSD19536KTT for my application then kindly suggest any other component from TI. Thanks

Best regards

Imran Riaz 

  • 0
    TI__Mastermind 38300 points

    Hi Imran,

    Thanks for the inquiry. I was not involved in this reference design but I will provide an answer for the FET related questions. I am including some links below to blogs on how TI specifies our MOSFETs. I have found this to be very useful information. In this reference design, the charge and discharge FETs are fully ON (conducting current in the linear region with constant on resistance) or fully OFF (blocking voltage in the cutoff region) and spend a very short period of time switching between these two states. The switching times for the driver IC are on the order of 50us when driving a 10nF load. The CSD19536KTT SOA current for 50V/100us is > 100A. SOA is normally considered when the FET is used in the saturation region where there is significant drain-source voltage while current is flowing thru the device. Typical applications include linear regulator, hot swap and OR'ing. The FETs used in this reference design, CSD19536KTT, are ideally suited for this application because of their low on-resistance and excellent thermal capabilities. You should have no problem switching 15A at 50V with these devices.

  • 0 in reply to John Wallace1
    Prodigy 100 points

    Hello John Wallace,

    Good Morning. Sorry I did not understand fully. Just to be more clear. Are you saying that, in the charge and discharge application the FETs are used to conduct current in pulsed form and not in DC?. If this is so then in TIDA-00792 how bq76940 device can turn ON and OFF the CHG and DSG pins so fast? Because to turn ON and OFF these pins the host MCU will set and reset bits in bq76940 device registers through I2C interface which is not enough fast I guess. I am sorry this question will not be related to your field but you can forward it to other expert. Thanks

    Best regards

    Imran Riaz

  • 0 in reply to imran riaz
    TI__Mastermind 38300 points

    Hello Imran,

    The charge and discharge FETs are used to conduct DC current when turned on and block DC voltage when turned off. Please see TIDA-00792 schematic. The FETs are driven by the BQ76200 high side driver IC.Typical turn on and turn off times for the BQ76200 when driving a 10nF load is on the order of 50us. During these transition times, the FETs have their highest stresses: there is voltage across the device while current is flowing thru it. For your other questions, I will share this thread with the applications team responsible for the BQ products.

  • 0 in reply to John Wallace1
    TI__Guru 54715 points

    Hi Imran,

    The BQ76200 does not turn on the FETs fast. The BQ76200 has internal resistance and some recommended external resistance which will limit the switching speed of the FETs.  Slower switching avoids high inductive spikes from the system.   The BQ76200 should not be switched on and off quickly (PWM the battery) since the charge pump will deplete.  When the battery is on the FET VDS is low and current can be high with little power dissipation.  When the battery is off the VDS can be high but current will be 0, so there is little power dissipation.  As John indicated, switching is where the FETs experience significant power since VDS is changing while current is changing, be sure the switching time is in the safe operating area.