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MOSFET safety margin, over design?

Catwell
What is an acceptable safety margin more a motor control MOSFET. For example; if the motor has a peak current draw of 7 amps, would a MOSFET rated for 8 amps be acceptable or would a MOSFET rated for 10 amps be more appropriate.
 
 
In the past, over-designing was the choice I went with, but is it really necessary?
 
Cabe
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  • 0

    Hi Cabe,

     

    Using the 8 Amp transistor gives you a safety margin of +10%, I personally would never feel comfortable  with such as a small margin for a motor. Even though the manufacturer has spec'd a peak of 7 amps remember that this value is generally based on a sample which is then mapped to a gaussian distribution(typical but each manufacturer does it differently). What this means is that approx. 70% of the motors fall within this range(Hope I remember correctly if not you look up the percentage of the population that falls within one standard deviation for a normal or gaussian distribution). Of the remaining 30% half will draw less than 7 amps while the other half will draw more than 7 amps. Last time I checked having 15% of your devices fail is not good business.

     

    Another thing to take into consideration are transient conditions which are generally unpredictable and can cause damage to transistors. Also remeber that the above discussion also applies to transistors, you want to make sure that even the low 15% of the transistor stock can also handle the motor current. I generally go for at least a 30% safety margin when I have worked with motors.

     

    If you're doing a one shot project you may be able to get away with an 8 amp transistor but for a commercial product definitely go for the 10 amp transistor.

     

    Regards,

     

    Jorge Garcia

  • 0 in reply to Jorge_Garcia
    For me....safety margin! Only very low gamma products have a pinpoint design values! I use ALWAYS 2x maximum voltage and 50% more of maximum peak current and 20% more of maximum current RMS value. So form me definitely safety margin!
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  • 0 in reply to Former Member

    Hi,

     

    Keeping a safe margin sound reasonable but if economics are important you should also have knowledge about the failure modes of a mosfet. Most mosfets are considered rugged and don't suffer from second breakdown, but be aware that the avalanche energy handling is specified at room temperatures and that the device can not handle any drain-source overvoltage spikes at maximum die temperature. Die temperature is a very important parameter for a device, i would say never go beyond 100C at maximum ambient temperature. The channel resistance has a positive temperature coefficient, you might also have thermal runaway at very high temperatures. Be very careful driving a mosfet as the gate-source voltage might have oscillations due to parasitic effects. This is very important when driving mosfets in parallel. If you don't know exactly how to do it forget it.

    Derating the drain-source voltage with 50% seems clever but a high voltage mosfet has a higher channel resistance and the on-losses will be higher. Keep in mind that the maximum dv/dt (during turning off of the device) is not only limited for EMI consideration but also for the device itself. I always place a capacitor across the drain-source to limit the dv/dt to 3kv/us.

     

    In applications which require a high lifetime (20 years) i derate the drain current to 80%, the drain-source voltage to 80%, the gate-source voltage to 80% and the die temperature 100C maximum.

     

    Best regards,

     

    Enrico Migchels

  • 0 in reply to enrico.migchels

    Enrico,

     

    Thank you for the suggestion. These mosfets will eventually be used in driving stepper motors, to let you know what they will be designed for.

     

    I suppose I will go with the old addage of overdesigning on the safe side.

     

    Cabe

  • 0 in reply to Catwell

    Cabe,

     

    Are all posts on element-14 conserning your projects or do you work in a team? You seem to be working on a lot!

     

    Best regards,

     

    Enrico Migchels