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TIDA-00281: Want to know detailed explanation how Power Stage is designed

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Part Number: TIDA-00281

I have two doubts about which I want very detailed explanations.

(1)According to TIDA-00281 its 1KW BLDC motor application with Vdc=48V. With this parameter, we can approximately find out current which will be 20Amp around.

In this case why MOSFETs are selected with Id rating of 197Amp?

(2)How RCD Clamp is designed for this application.tiduay9.pdf

  • Hello Abhi,

    Thank you for the interest in TI products. I am an AE supporting the UCC27201A and will address your questions.

    Mosfets in most cases are selected regarding power dissipation and thermal considerations. In almost all cases the device rated current is much higher than in the application. Just assuming a DC operation case of the 30A maximum specified in the TIDA-00281 document, and the maximum RDSON of the CSD19535 this will result in a power dissipation of 3.24W. It looks like the MOSFETs are surface mounted which limits the power dissipation ability.

    The RCD snubber functions to limit the voltage overshoot on the switch node caused from energy stored in the parasitic inductance, or leakage inductance of the switch node. This type of snubber is used extensively in flyback converters, and the principle is the same. The energy in the parasitic inductance from the current flowing is transferred to the capacitor in the RCD snubber to limit the voltage peak. There is an application note on Ridley Engineering at: http://www.ridleyengineering.com/images/phocadownload/12_%20flyback_snubber_design.pdf

    Confirm if this answers your questions, or you can post additional questions on this thread.

    Regards,

    Richard Herring

    Richard Herring

  • In reply to Richard Herring:

    Hi Richard,

    Thanks for your answer. That gave me a good knowledge of MOSFET selection.

    As per your answer, you told that Mosfet Id rating must be very high than the application current.

    But, your answer doesn't justify how you achieved MOSFET Id rating of 197Amps.

    It would be good if you show some calculations or flow by which you selected this Particular MOSFET with Id=197Amps for 1KW BLDC Motor application.

    Thanks,

    Abhi

  • In reply to Abhi Choksi:

    Hello Abhi,

    Thank you for the response. I don't think the 30A maximum operating current Vs the 133A rating (at 100 deg C) of the device was the parameter of consideration. Other than the application current being well below the device rating. The major design consideration will be the operating current, Rdson of the MOSFET and resulting power dissipation, as I mentioned in the previous post. In the end application, these factors as well as heatsinking and airflow will determine the thermal resistance and temperature rise of the device above the ambient temperature.

    Please confirm if this addresses your question, or you can post additional questions on this thread.

    Regards,

    Richard Herring

    Richard Herring

  • In reply to Richard Herring:

    Hi Richard,

    Thanks for your help.

    I understand that MOSFET Id=133A @Tc=100deg is not parameter for consideration.Rather than that, we must go for Rds(ON) and device power dissipation.

    Actually, I am curious to know how Particular MOSFETs are selected for applications.

    Let's say I am designing BLDC driver power stage for E-Bike. It contains 3-phase 250W BLDC Motor which I want to run on 60V DC Battery Supply. Also, I don't want to use any heat sinks and want to maintain an ambient temperature between 30deg - 40deg.

    For this above application, how would I choose MOSFETs?. Please share some detailed calculations and Flow for selection.

    Thanks, 

    Abhi

  • In reply to Abhi Choksi:

    Hello Abhi,

    The selection of the MOSFETs for the application will be based on the power dissipation in the MOSFETs, the maximum ambient air temperature, and the thermal resistance of the MOSFET package to ambient air.

    There are several variables in this case so to start the process you need to start with targets such as maximum junction temperature, 100 deg C is a common target. Also if you can estimate how much PCB trace area you can dedicate to the heatsink pad this will help set a thermal resistance of the trace heatsink. As a start maybe you can assume 1 in2 which for 2 oz copper has a thermal resistance of ~37 oC/W.

    With these assumptions now you can determine the power dissipation can be Pd=(Tj-Tamb)/Rja or ~2.2W

    The MOSFET Rdson has a positive thermal coefficient so you need to normalize the Rdson by the multiplier which for the CSD19535 is Figure 8 in the datasheet. Since this is motor drive the MOSFET losses should mostly be conduction losses which is Pd=IRMS2 x Rdson.

    For determining the peak and the RMS current, I would assume with 250W and 60V it will be ~4A, but you need to confirm your highest expected currents.

    This is an iterative process, and you need to be careful to assume an ambient temperature that is realistic. For an E-Bike, I would assume the outside air could be 30 to 40 deg C, and the air close to the PC board could be much higher. I would assume a higher ambient, but I am not an E-Bike product expert.

    There are some links I found on Electronic Design and EDN that discuss this exact process. Confirm the power dissipation or certainly the IRMS for your application since these articles discuss specific topologies.

    See the following links: https://www.edn.com/design/components-and-packaging/4341997/A-simple-guide-to-selecting-power-MOSFETs

    https://www.electronicdesign.com/boards/calculate-dissipation-mosfets-high-power-supplies

    Please confirm if this addresses your concerns or you can post additional questions on this thread.

    Regards,

    Richard Herring

    Richard Herring

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