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About mosfet selection [message #168903] Tue, 24 January 2017 16:32 Go to next message
srinivasan m
Messages: 5
Registered: January 2017
Junior Member
Guys i'm currently designing charger circuit for li ion batteries. My charging current is 10 A. i have decided on the ratings of Mosfet 40v. Now i dont know to choose which type of Mosfet for my design.whether to go with either TO220AB type or LFPAK56. I'm too confused about selecting the right mosfet. I'm designing my first circuit for such high ratings. I'm having doubt about the wattage issue too. my board output ratings desinged is 25.2v, 10A which equals 250 watts. But in the data sheet of 40v rated mosfet , it is stated that the wattage is around 90watts-150 watts. But they have mentioned about maximum current ratings as 100A. I'm going to use my board as an thermal relief to dissipate heat. Which Mosfet is advisable for such kind of pcb.Suggestions are welcome.

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Re: About mosfet selection [message #168907 is a reply to message #168903] Tue, 24 January 2017 17:06 Go to previous messageGo to next message
Don Hersey
Messages: 31
Registered: March 2014
Member
The conduction channel of a mosfet has a positive tempco of resistance.  This means you can parallel them because no branch will 'current hog.'  Your main challenge is to keep your transistor cool, again due to this positive tempco.  You probably need to experiment.  I recommend a large heat-sink.  Mosfets like to be all-of-the-way on or off.  If you are using this device in a continuous mode, you may want to look into using a bipolar

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Re: About mosfet selection [message #168909 is a reply to message #168903] Tue, 24 January 2017 17:45 Go to previous messageGo to next message
rachaelp
Messages: 532
Registered: March 2015
Location: UK
Senior Member
If you are using the MOSFET such that it is either turned fully on or fully off then the power dissipation is due to the Rdson parameter when turned fully on. This specifies the "on" resistance of the MOSFET and the power dissipation due to this is then calculated the same as for any resistor. If you are switching the MOSFET on and off then the input capacitance and gate charge will affect the switch on/off time and larger values will cause more power to be dissipated during switching. Google for MOSFET switching losses and you'll find resources to help you work out the power dissipation in your MOSFET.

Here are a couple of items I found when I looked quickly just now:

http://www.ti.com/lit/an/slyt664/slyt664.pdf
http://application-notes.digchip.com/070/70-41484.pdf

Best Regards,

Rachael

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Re: About mosfet selection [message #168913 is a reply to message #168909] Tue, 24 January 2017 18:00 Go to previous message
Don Hersey
Messages: 31
Registered: March 2014
Member
Depending on circumstances, I derate this type of transistor.  If there is a possibility of high ambient temperatures, or sometimes the more capacious device isn't much more expensive.  They are much more rugged than the earliest ones on the market.  When looking at transistor specs pay close attention to the conditions of the test such as heatsink size

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