How to read a datasheet for MOSFET's? - Page 1
Forum section: electronics / beginners

author: ryanmills
author_rank: Unknown
timestamp: March 02, 2017, 10:37:45 pm
content: Upfront disclaimer, I am very new EE.I'm playing around with a BJT NPN transistor and a N-Channel MOSFET. Tinkering with both to use them as a switch just to learn and understand them. I'm trying to use them in the same way you would use a relay. The BJT was easy enough to sort out with plenty of info on google. I'm not understanding MOSFET's fully so I'm not sure if i'm reading the datasheet to sort out the limits correctly. The project I am hoping to use them for is driving a higher voltage motor so they are bigger MOSFET's. The one in question is a Vishay Siliconix IRF510PBF (Datasheet:http://www.vishay.com/docs/91015/sihf510.pdf). The important thing is how much voltage can i drive to the gate. Would that be the "Gate-Source Voltage"? And if that is the case its listed as ± 20. If so,forgive what is likely a stupid question but what is the ± 20 based on? Or is it the "Gate-Source Threshold Voltage" that has a limit of 2 to 4 volts?A secondary question, that again may be stupid. Does the amount of voltage apply to the gate change how much current flows from the source to the drain? Or does is act more like a switch where say at X voltage you get full current flow thru to the drain? If so how do you determine what the min voltage to the gate is?

author: Wor3d
author_rank: Contributor
timestamp: March 02, 2017, 10:43:26 pm
content: Look up page 3, figure 3. I use it as a reference to designing "MOSFET as a switch", especially if I know the load, very easy to do and read from the graph how much gate voltage do you need for required current. For safety I usually go higher, or most of the time I just check if the MOSFET I want to use is open enough to pass current I want with voltage I am using (most often 5V or 3.3V from MCU).What you can read from the graph figure 3 - at 25degC you need 5V for 1A, 6V for 2.5A, 7V for 5A etc...That +-20V GS is absolute maximum which the gate can handle without damaging itself. The voltage is measured between Gate and Source, but usually you have Source grounded when switching some load (maybe some current measuring resistor), so it is +-20V from ground

author: ryanmills
author_rank: Unknown
timestamp: March 02, 2017, 10:46:51 pm
content: Forgive me, I'm not following when you say "Look up page 3, figure 3" do you mean in the datasheet or someplace else?

author: Wor3d
author_rank: Contributor
timestamp: March 02, 2017, 10:49:35 pm
content: Sorry, yes I mean the datasheet which you've linked above. Page 3, figure 3 - bottom leftGraphs are super useful, I usually check the brief description of the part which I want to use, maximum ratings if I am designing something unusual (high frequency, gate/load specs etc) and then check the graphs

author: Andy Watson
author_rank: Super Contributor
timestamp: March 02, 2017, 11:01:57 pm
content: Quote from: ryanmills on March 02, 2017, 10:37:45 pmThere is a very (very very) thin layer of insulation between the gate and the body/channel of the device. If you exceed 20V (either polarity) you risk puncturing this insulation. Once punctured, the mosfet is no more. Note - no significant current is required - static electricity will kill the device.

author: Benta
author_rank: Super Contributor
timestamp: March 02, 2017, 11:05:39 pm
content: Datasheet reading 101:All datasheets I've seen have the following elements:General description (easy)Absolute Maximum Ratings. When you exceed these, you'll destroy the device.Operating Parameters/Values/Curves/Whatever.These will tell you how to use the device.Easy, no?

author: ryanmills
author_rank: Unknown
timestamp: March 02, 2017, 11:20:47 pm
content: Quote from: Benta on March 02, 2017, 11:05:39 pmSure it's easy, when you know what those values apply too. I'm sure you do. But as someone new I don't know what Zero Gate Voltage Drain Current applys to anymore than I understand what I would use the Forward Transconductance for.

author: ryanmills
author_rank: Unknown
timestamp: March 02, 2017, 11:22:57 pm
content: Quote from: Andy Watson on March 02, 2017, 11:01:57 pmThanks, that answer the main question. I was confused why they would list it as +/- I was not thinking about a negative voltage.