IRFZ44N Cooling Requirement

Discussion in 'General Electronics Chat' started by tracecom, May 9, 2010.

  1. tracecom

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    I am using an IRFZ44N to switch 12 to 14.5vdc at 5a maximum; the load is purely resistive. The duty cycle will be about 50% on/50% off and will be switched on/off about twice per minute. The gate voltage is 8v and the ambient temperature will be 40 degrees C max. How do I calculate cooling requirements? The datasheet and schematic are attached; if you would direct me to the data I need and the calculation method required, I would be grateful.
     
    Last edited: May 9, 2010
  2. jpanhalt

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    You may very well get by with no additional cooling. Your total Power Dissipation (PD) is PD(switching) + PD(resistive). Since you are switching so infrequently, your PD approximates PD(resistive) only, and it is only on 50% of the time. When on, the power is I^2 X RDS(on), which is approximately 0.5W.

    Here is a link to a more complete explanation the calculation:

    http://www.maxim-ic.com/app-notes/index.mvp/id/1832


    Go about 2/3 down to the section called Thermal Resistance.

    John
     
  3. mik3

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    Calculate the RMS current through the MOSFET and then calculate the power it dissipates by P=Irms^2*Rdson.

    The maximum junction temperature is 175 C.

    To find the required heat sink thermal resistance:

    Rth=[(175-40)/P]-1.5-1

    1.5 is the junction-case thermal resistance.
    1 is the assumed thermal resistance between the case and the heat sink

    To run the MOSFET cooler, you can choose a heat sink with less thermal resistance.
     
  4. SgtWookie

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    Why only 8v on the gate? And what are you using to drive the gate of the IRFZ44?
     
  5. tracecom

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    Thanks to everyone for their input. I will study what you have posted and try to understand it. I am encouraged by the opinion that I might get by with no additional cooling; that would be great.

    Presently, I have the circuit prototyped and running on an LM7812 outputting 12.08v to a total load of less than 100ma. The gate voltage is 10.78v from the output of an op-amp through the 10Ω resistor. However, I have now realized that I will need to change the LM7812 to a LM7809 because the input voltage may drop as low as 12v. So, I made what I thought was a worst case estimate of the gate voltage to be 8v, however on second thought, I guess it could fall as low as 7.5v. What is your opinion?

    ETA: Powering the circuit with a fresh 9v battery (9.31v) produces a gate voltage of 8.01v.
     
    Last edited: May 9, 2010
  6. jpanhalt

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    If you are limited in gate voltage, an IRLZ44 might be a better choice. It turns on completely at logic levels.

    John
     
  7. tracecom

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    Given the relatively low current through the MOSFET, would a IRLZ34N do just as well? And by logic levels, do you mean 12v or less? 5v or less? Or what?

    Thanks.
     
  8. retched

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    Logic levels are typically 5v(high) and 0v(low) for TTL and CMOS.
     
  9. SgtWookie

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    Logic level = 5v (TTL).

    You might consider using a more modern logic level MOSFET that has a somewhat lower Vdss, but much lower Rds(on) and a lower gate charge.

    You also might consider using a comparator such as an LM2903 rather than an opamp. The LM2903 has open-collector outputs, so the pull-up resistor can allow the output to reach the positive supply.

    If you are going to use the circuit in a vehicle, be certain to use components that are rated for use over the full automotive temp range (-40°C-125°C). The LM2903 dual comparator is such a part.
     
  10. tracecom

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    In both cases I have tried (12.08v supply and 10.78v output, and 9.31v supply and 8.01v output) the voltage drop through my circuit is 1.3v. Assuming that changing to an LM7809 produces similar results, the gate voltage on the MOSFET will be ≈7.7v. Should I change the MOSFET to an IRLZ34N?

    ETA: I see that the IRF has a maximum gate-source voltage of +/-20v and the IRL has a maximum gate-source voltage of +/-16v. I don't know exactly what that means. Does that mean the maximum difference between the gate and source voltage cannot be more than the number given?
     
    Last edited: May 9, 2010
  11. jpanhalt

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    Yes, the gate-source voltage limit is the maximum, but you are not going to exceed it with an 8V drive.

    I suspect the IRLZ34 will work too. Its RDSon is a bit higher, which will result in more resistive heat production (about 1.3W). But, that is still not a lot compared to what it can handle with a heat sink.

    John
     
  12. tracecom

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    Please tell me where on the IRLZ34N datasheet the minimum gate voltage is stated. And if I switch to the IRLZ34N, do I need to change the value of the gate resistor? Thanks.
     
    Last edited: May 9, 2010
  13. jpanhalt

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  14. tracecom

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  15. tracecom

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    From the IR datasheets, I find that the IRFZ44N has a Gate Threshold Voltage of 2.0 (Min.) and 4.0 (Max.) and the IRLZ44N has a Gate Threshold Voltage of 1.0 (Min.) and 2.0 (Max.) In my ignorance, I think that means that the IRFZ44N is guaranteed to turn on with a gate voltage of 4.0v or more, while the IRLZ44N is guaranteed to turn on with a gate voltage of 2.0v or more. Now, what I don't understand is why the IRLZ44N is any more likely to fully turn on in my circuit (which has a gate voltage of ≈7.7v) than the IRFZ44N that I am currently using. It seems to me that 7.7v of gate voltage will turn either MOSFET on completely. What am I missing? Thanks.
     
  16. SgtWookie

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    Take a look at what the threshold current value is.
     
  17. jpanhalt

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    From the IRFZ44 datasheet:
    [​IMG]

    Note that it starts to turn on at the threshold voltage, but is not fully turned on until Vgs is about 10 volts. The IRLZ44 starts conducting at a lower voltage and is fully turned on at about 5V. The meaning of threshold voltage for Vgs is one of the most common misconceptions encountered on this forum related to mosfets.

    John
     
  18. tracecom

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    Thanks for helping me to understand the graph; I had looked at it, but couldn't understand it. In fact, I still have a couple of questions. If I am reading it correctly, at a gate voltage of 7.7v, the IRFZ44N will have a drain-to-source current of ≈100 amps, but the maximum continuous drain current is only 49 amps (at 25 deg. C). So I assume this 100 amp current must be pulsed. And because my circuit only requires the MOSFET to conduct 5 amps (continuously), does it really matter that the IRFZ44N is not completely turned on by a gate voltage of 7.7v?

    Please don't think I am being argumentative. I really want to understand.
     
  19. jpanhalt

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    It is not required that you turn the gate on completely. However, refer to the graphs and estimate the effective resistance. Then calculate your power dissipation. NB: the scale is logarithmic. If you just double the resistance, then you will be generating a lot more heat. You also have to start considering the voltage drop across the mosfet too. That is why, when operating as a switch, it it usually considered best to turn the mosfet completely on.

    John
     
  20. tracecom

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    Okay, I understand. Please answer one more question. The 10Ω resistor in the gate voltage lead in my circuit came from a similar circuit that I saw somewhere; I don't remember where. Can you please tell me what the purpose of that resistor is, and how to calculate the correct value for it? Thanks.
     
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