Understanding MOSFET parameters - Dimming/Switching LED strip

Question

I'm trying to create a dimmable/ switchable 12v LED strip using an Arduino board and a MOSFET. Many tutorials can be found doing so such as this youtube link.

I've bought a LM317T before I had any substantial knowledge regarding MOSFETs. Trying to understand using some tutorials ( each one used different MOSFET ) made it partialy clear what parameters to look for in such project mainly using this link, but some left in the dark - and this is the reason for this post ( and to understand why my MOSFET is not suitable ).

I'll compare with one suggested, FDC855N

1) It has to be "Logic Level" MOSFET, meaning Vgs has to be 5v: from what I understand, 0v at the gate you get 0v at the drain and at 5v at the gate you get maximum Vsource. it is correct ?

1a) FDC855N Vgs's value is +/- 20v, how is it Logic Level ? will Vdrain @Vgs=5v will be different when @Vgs=17v ?

1b) Why in FDC855N Vgs have 2 values when Rds(on) is noted? one is 10V and other is 4.5v ?

2) Vgs(th) stands for minimal voltage needed for current flow at to drain. it is correct ?

2a) why is it noted as @Vgs=Vds ?

3) why Rds(on) is important for ? only for power dissipation purposes ?

I'll be glad to have answers for those question in comparison to the 317T MOSFET, since 317T datasheet has a different terminology :(

Appreciate any help, Guy

Why are you mentioning LM317T in the same sentence as MOSFET? It's adjustable linear voltage regulator. And about the voltages: gate is insulated from D-S channel by really tiny insulation layer with breaking voltage somewhere just over maximum voltages between G-S or G-D (doesn't matter which one). That also means gate has its capacity. Thats the only current to the gate - charge or discharge gate. — KIIV, May 24 '19 at 07:21
@KIIV - OK, so it is not :) can you please explain parameters not in comparison to 317 — guyd, May 24 '19 at 10:15
1b) Rds(on) is not only one nor two values. It's a function of Ids and Vgs. 2) Vgs(th) I'm not sure about this one, it might be voltage the transistor starts opening. It's voltage, only current you'll see is current needed to charge/discharge the gate capacitance. 3) yes, it's really huge difference if you have 10A on 20mOhms and 100mOhms. The first gives you voltage drop 200mV and therefore 2W of power dissipation and the second 1V voltage drop and 10W power dissipation (voltage drop times current). — KIIV, May 24 '19 at 10:39
regarding 1 - so even though 20v may be applied, 5v will supply max Vsource ? — guyd, May 24 '19 at 10:44
MOSFET aren't binary devices. They aren't just ON or OFF. Think of them like water taps. Vgs(th) is the voltage at with the mosfet starts to turn on. A bit like how much you have to turn a tap before it starts dripping. To turn it on more, you require a higher voltage. The amount of water flow also depend on what's attached to the tap. Like for example a dripper hose, sprinkler, or a kinked/blocked hose. — Gerben, May 24 '19 at 14:57
2a that just what they used during their measurements. That the downside of those datasheets. Sometimes the voltages or currents you use in your applications aren't specified in the datasheet. You'd have to interpolate from the values that are specified. Sometimes you can use the graph to lookup your case. — Gerben, May 24 '19 at 15:01
3 indeed. The lower the Rds(on), the less heat is being generated. This mostly applies to high current loads. This resistance will also generate a voltage drop. The voltage across the load therefor gets (a bit) lower. Depending on the application, this could have unwanted effects. — Gerben, May 24 '19 at 15:05

score 3 · Accepted Answer · answered May 24 '19 at 17:02

First of all, the LM317 and the are FDC855N different types of devices. The LM317 is a voltage regulator whereas the FDC855N is a discrete MOSFET.

A MOSFET will act as a fast switch allowing you to control the LED's brightness using Pulse Width Modulation.

For what you're trying to do, you can think of the MOSFET as a voltage-controlled switch. If the voltage between the gate and source (Vgs) is greater than the threshold (Vgs(th)), then the current will flow from the drain to the source (Ids).

1) Yes, anything over the typical Vgs(th) of 2.0V, will turn on the switch and cause current to flow; you need to provide a voltage across the drain and source (Vds).

1a) 20V is the maximum rating of Vgs, if you applying anything greater, you'll damage the device.

1b) In reality, the MOSFET is more complicated than just an on/off switch. The gate to source voltage (Vgs) determines how hard the switch is turned on, so increasing Vgs will turn the switch on harder and reduce the resistance (RDS(on)) between the gate and source.

2) Yes, this is essentially correct. The relationship follows a curve and Vds(th) represents an inflection point. This plot from page 3 in the datasheet shows the curve for different temperatures. You can see that the 25C line is basically zero until Vgs reaches about 2.75 volts and then it increases rapidly.

3) RDS(on) is the resistance between the drain and source when the MOSFET is on. You can treat the drain to source path like a resistor and use RDS(on) to determine the power dissipation. For example if your RDS(on) were 0.1 ohm and the current through the drain is 1.5A, then you would be dissipating I^2*R = (1.5)^2*(0.1) = 0.225 , which is below the maximum allowable steady-state power dissipation of the device (Pd) of 0.8 W.

Understanding MOSFET parameters - Dimming/Switching LED strip

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