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Old 04-16-2016, 11:43 AM #1
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Default Never get the driver with external mosfet working. Possible Reason?

So I start building my own diver and play with different chip. Some of them are work but I never get the driver with external mosfet working. Most of them are using N-mos I have tried AO3400 and IRLML2502 (sot23) but both are not working. I thought nmos is the correct mosfet the schematic are calling for below mosfet:



Can someone please tell me whether I am using the correct mosfet? Thanks
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Old 04-16-2016, 03:49 PM #2
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Default Re: Never get the driver with external mosfet working. Possible Reason?

Are you sure you haven't mixed up the pins connection?
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Old 04-17-2016, 01:21 AM #3
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Default Re: Never get the driver with external mosfet working. Possible Reason?

Those are the pin numbers you could get when they put them in different style packages (i.e. a single fet in an 8 pin case etc).

But what seems to be the problem? Are you sure you are getting enough gate to source voltage? Mosfets can be wonderful switching components, but don't play that well at low voltages unless you use ones specifically designed for that.

General purpose mosfets often have their D-S resistance rated at a gate-source voltage of 10 or 15 volts. Logic level ones obviously have it at 5 volts, but in single battery operated devices you should look at low-voltage (3.3 or lower) logic level performance.
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Old 04-18-2016, 02:12 AM #4
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Default Re: Never get the driver with external mosfet working. Possible Reason?

Quote:
Originally Posted by Benm View Post
Those are the pin numbers you could get when they put them in different style packages (i.e. a single fet in an 8 pin case etc).

But what seems to be the problem? Are you sure you are getting enough gate to source voltage? Mosfets can be wonderful switching components, but don't play that well at low voltages unless you use ones specifically designed for that.

General purpose mosfets often have their D-S resistance rated at a gate-source voltage of 10 or 15 volts. Logic level ones obviously have it at 5 volts, but in single battery operated devices you should look at low-voltage (3.3 or lower) logic level performance.
How about in boost situation? Gate-source voltage should refer to the voltage before of after the boosting?
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Old 04-19-2016, 02:46 AM #5
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Default Re: Never get the driver with external mosfet working. Possible Reason?

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Originally Posted by Thunder22 View Post
So I start building my own diver...Can someone please tell me whether I am using the correct mosfet?
Not until you give some actual information about what driver setup you're working on.

"using the correct mosfet" FOR WHAT, exactly?
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Old 04-24-2016, 05:19 PM #6
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Default Re: Never get the driver with external mosfet working. Possible Reason?

In a boost regulation you should look at the input voltage, the FET is normally switched only by that, minus any voltage drop in the control IC (usually very low).

Your average FET will hardly conduct at, for example, 3.5 vGS. A 3.3 volt logic compatible type will. For example, a common BUZ11 fet will only drive a current of 100 mA for a GS voltage for 3 volts, even if the DS voltage is considerably higher.

There are some special designes that use a feedback winding to get a higher primary drive voltage, but those are uncommon, unless perhaps in stuff that needs to operate from a single alkaline cell.
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