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CSD19506KCS Thermal Considerations

Marius Jansen van Rensburg
Prodigy 210 points
Other Parts Discussed in Thread: CSD19506KCS, SM72295

Hi, I have a question relating to thermal considerations for the CSD19506KCS N-Channel NexFET Power Mosfet.

For the following values:

  • Drain-to-Source Voltage, V(DS) = 12V
  • Continues Drain Current, I(D) = 70A

What would the Drain-to-Source On Resistance, R(DS)on be to allow for the above values?

I am working on the assumption that the V(GS) voltage for the above would be: V(DS) + V(GS)TH = 12V + 2.5V = 14V.

Is this assumption correct?

Assuming an R(DS)on value of 2.0 mΩ (closest match on the DataSheet), can the Power Dissapation P(D) be calculated as follows:

P(D) = I² x R(DS) = (70)² x 0.002Ω = 9.8W ? (if not please provide clarity).

Working from a starting ambient temperature, T(A) of 25°C, how can I calculate the temperature increase for the calculated power consumption?

Do I work on 0.4°C/W temperature increase (as per DataSheet) or do I need to consider additional parameters?

Any insight into this would be greatly appreciated.

Also, can anyone recommend a good application note on balancing parallel power mosfets?

  • 0
    TI__Mastermind 18200 points
    Marius,
    Before I go into greater detail answering your question, please help me understand your application.

    Do you really plan on maintaining 12Vds over the FET, even when it is fully turned on / enhanced? If so, you will be operating in the "saturation region", not the ohmic or linear region of the MOSFET. Is this your intention? Only certain applications (hotswap, ORing, etc...) do this deliberately.
  • 0 in reply to Brett Barr1
    Prodigy 210 points
    Hi Brett,

    I am an Industrial Engineering student with limited Electrical Engineering knowledge.
    I am planning to use these FETs in an H-bridge configuration for an inverter application.
    I will utilize a mosfet driver to control the H-bridge. Currently looking into the SM72295 as a potential solution (based on TI's 800kVA inverter application note). So to answer your question, I believe operation in the Linear region will be fine (but you need to advise please).
    I am trying to establish a worst-case-scenario on the thermal requirements for the "on-time" of the FET. It will be driven by a PWM signal (via the dedicated driver) and it would be great to get the increase in temperature as a function of the "on-time" of the FET (based on the 12Vds and 70A requirement).
    I hope this makes sense. English is not my first language.

    Regards
  • 0 in reply to Marius Jansen van Rensburg
    TI__Mastermind 18200 points
    Marius,
    I understand. If you are using these FETs in a half bridge configuration, I don't think you will be operating in the linear region.

    For some information on what I mean by this, you can check out this video I did on SOA a little while ago.

    training.ti.com/understanding-mosfet-datasheets

    In your application, when you turn on the FET, it will no longer be blocking the 12VDS, and you will not be dissipating 12VDS * 70A = 1400W through the FET. That would be an insane amount of power for the MOSFET to dissipate for any more than a few us. Instead, the Vds will drop to a fraction of this value.

    Here's the question to ask: what VGS are you going to be driving the FET at? Then you can use Figure 7 to estimate the resistance of the MOSFET. Then you can use P = I^2 * R to estimate the power loss.

    You also should take into account switching losses, but usually inverter applications are switching at pretty low frequency. Do you know what frequency you will be switching at? I would say if it is under 10kHz, you can probably neglect switching losses.

    Estimating temperature increase is a little more difficult. That depends on the thermal environment of your board, which dictates the junction to ambient thermal impedance (RthJA).

    You can read more about thermal impedance as they pertain to MOSFETs here: e2e.ti.com/.../understanding-mosfet-data-sheets-part-6-thermal-impedance

    But before we get there, I'd say lets nail down how much Ploss you can expect to dissipate in your MOSFET.
  • 0 in reply to Brett Barr1
    Prodigy 210 points
    Hi Brett,

    Thank you. Let me digest this information and I will revert back to you if I have any further queries.

    Regards
    Marius