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I thought that choosing the right MOSFET for my LED strip would be easy until I discovered how many different models there are.

Basically I want a MOSFET that allows me to control with PWM a 12V 6A (MAX) led strip, but every time I see the Vgs I get confused because of numbers like +-20V.. (I'm controlling it with an ATtiny13A or ATtiny85 - 5V pin output)

I done a lot of searching a came across a lot of different models: IRFZ44N, TIP120, STB36NF06L and a bunch more.. but I not sure if they will do the job

What MOSFET should I use and how do I read in the datasheet why this is a good choice?

I am new to hobby electronics.

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  • \$\begingroup\$ What is the actual average current that you will pulse into the LEDs? You need a transistor which has the dissipation and current handling for that, not for the maximum 6A current that the LED strip can take. \$\endgroup\$ – Kaz Jun 3 '13 at 5:38
  • \$\begingroup\$ I have one of these easy cut LED strips (cutting strip for each 3th RGB LED) and want to be able to cut into whatever length that fits my needs. \$\endgroup\$ – Norfeldt Jun 3 '13 at 20:02
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The 12V and 6A is a good starting point. This tells me you need a mosfet with a max drain-source voltage capability greater than 12V so 20V would be a minimum criteria for this.

You want to switch 6A and you'll want it to do so with minimum volt-drop - just like a relay contact so you are looking for Rds(on) below (say) 0.1 ohms. This means at 6A it will develop a small voltage across the device of 0.6V (ohms law).

However, that will produce a power disippation of 6 x 6 x 0.1 W = 3.6W so if you are looking for a surface mount device you would prefer a lower disippation of maybe 0.5W max.

This means Rds(on) would be more like 0.014 ohms.

So far, your application needs a 20V transistor, capable of switching 6A with an on resistance no more than 0.014 ohms.

Vgs is "like" the coil voltage on a relay - it's how much voltage you need to apply to the coil to get it to switch BUT for a FET it's a linear thing and, if you don't apply enough voltage, the mosfet will not turn on properly - its on-resistance will be too high, it'll get warm under load and have a volt or two across it when you want a nice low resistance.

You then need to inspect the details of the spec to see how much you need to apply to guarantee the low on-resistance you want. A bit more on this further down.

The IRFZ44N has on the front page of the data sheet: -

Vdss = 55V, Rds(on) = 17.5 milli ohms and Id = 49A

It's not a surface mount device therefore a little more heat generated isn't going to matter too much (with a heatsink) so it'll do what you want it to do but I'd research a device with smaller Vds (say 20V) and you'll probably find one with a lot less than 10 milli ohms on resistance.

If you look at the electrical characteristics on page 2 you'll see that the 17.5 milli ohms on resistance requires a 10V drive voltage on the gate (3rd line down in the table). Less than this drive level and the on-resistance rises as would the heat produced.

At this point I can't decide for you any more but I think you might be looking for a device that will operate from logic levels. In which case the IRFZ44N won't do.

The STB36NF06L is a little higher with the on-resistance but the spec does suggest it will work from a 5V drive on the gate - see electrical characteristics (ON) but i'd still be tempted to find one that is more suitable.

I'd be tempted by this. The PH2520U is a 20V, 100A, 2.7 milli ohm device when the gate voltage is 4.5V. If your logic levels are 3V3 check figure 9 to see it will work well at 3V3.

One last thought about things - you are wanting to PWM a load and if the frequency is high you'll find that the gate capacitance takes some drive current into the gate to get it moving up and down quickly. Sometimes it better to trade off on-resistance to find a device with lower Vgs capacitance. You're into horse-trading now. Keep as low as you can on switching frequency and it should drive ok from a 5V logic pin.

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    \$\begingroup\$ Thank you very much @Andy for taking your time to help me out. Your answer gave me a lot to think about and I just have to digest it. \$\endgroup\$ – Norfeldt Jun 3 '13 at 20:27
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    \$\begingroup\$ Any questions dude just ask \$\endgroup\$ – Andy aka Jun 3 '13 at 20:28
  • \$\begingroup\$ If the mosfet is about voltage and not current - could I then use a smaller transistor to control the mosfet? \$\endgroup\$ – Norfeldt Jun 4 '13 at 15:00
  • \$\begingroup\$ @Norfeldt For decent performance a push-pull circuit is my recommendation such as UCC27424D - it's a low-side mosfet driver - it'll work from 4V to 15V from memory. \$\endgroup\$ – Andy aka Jun 4 '13 at 15:11
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If you are going to use it with logic level outputs the first thing to mention is that the turn on voltage for most MOSFETs is a bit too high so you need to choose the ones especially designed for digital levels. Basically you are looking for a low GATE - SOURCE voltage that will provide the amount of DRAIN current for your application. Look for "Logic level power MOSFETs N-Channel" It then comes down to low Drain- Source resistance (remember Power lost = I^2 * R) and the ability to handle the amount of current you want to switch at the voltage you want to switch.

Look for a graph that shows you the Drain current for a particular Gate source voltage.

The other thing to remember about MOSFET switches is you need to actively turn them OFF - To switch ON you place a voltage onto the gate. To ensure the CHARGE is removed from the gate add a resistor (100k - 1M0) between the gate and ground or make sure your output pulls the input to ground rather than just become a high impedance.

As for a recommendation have a look at https://www.sparkfun.com/products/10213

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  • \$\begingroup\$ Thank you @JIm your suggestion pointed me to buy this datasheet.thaishopetc.com/d/RFP50N06.pdf MOSFET on ebay - meanwhile I wait for the shipment I'll try to learn some more about MOSFETs \$\endgroup\$ – Norfeldt Jun 3 '13 at 20:31
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First: Learn to use and love the "parametric search" feature of the Digi-Key catalog. It lets you search for common parameters (like Rdson, Vds, etc) across all manufacturers. It's awesome!

Second: MOSFETs often require 10V drive into the gate for best performance, and often require significant currents while switching (for very short amounts of time) to quickly drive them out of "isolating" to "fully conducting." If you keep them in the transition zone for too long, they will heat up, and fail.

Thus, you may want to look at a MOSFET plus a suitable driver chip. The IRS2301 is a MOSFET driver chip that can put 10V into the gate of a MOSFET from a 5V or 3V3 control signal in (assuming 5V or 3V3 Arduino is what you're using.) It can additionally provide 10V drive above the main voltage for high side switching, but you don't need that in this case, if you switch. Note that the full 12V should be put into the supply of the driver chip.

If you look up the data sheet, you don't need the high side drive if you're only switching on the low side; so you can skip the diode and bootstrap capacitor in the diagram.

Once you've found a number of MOSFETs that are sufficient for your load (largely meaning sufficiently low Rdson,) you can shop on price. However, another useful parameter is looking for a low gate charge, because this means the device will switch faster. It's typical that the lower the Rdson, the higher the required gate charge, though.

The Arduino itself is only rated for 25mA (absolute max 40mA) out of a single pin, which probably is not enough to drive the MOSFETs quickly enough. I've tried doing PWM without a driver chip at 6A loads, and it doesn't work that great. Either you burn out the pins of the Arduino, or you introduce a current limiting resistor, and end up not driving the MOSFETs quickly enough.

Another thing to worry about is maximum voltage. When the spec sheet says 20V, it means it. If you're driving an inductive load that may spike above the rated voltage, you will kill your MOSFETs. LEDs aren't very inductive, though, so a small capacitor to absorb the connecting wire inductance is probably enough to keep your switch safe.

Currently, the cheapest device in stock at DigiKey available in single quantities, with sufficiently low Rdson, is the NXP PSMN1R1-25YLC, at $1.50 in singles.

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  • \$\begingroup\$ Would using a bipolar junction before the MOSFET be considered to be a "driver"? I am already using a L7805 on a 12V power adapter to feed the ATTiny85 with some 5V power. So using an LM317 to get 10V to go to the MOSFET gate and while being controlled by an BC549 which then again is controlled by the ATtiny85 chip..? Am I crazy? \$\endgroup\$ – Norfeldt Jun 3 '13 at 20:40
  • \$\begingroup\$ You could feed the MOSFET or driver with 12V directly without any regulation. \$\endgroup\$ – Jon Watte Jun 5 '13 at 1:13
  • \$\begingroup\$ Okay, what parameter in the datasheet would tell me how much current the MOSFET would drain to be turned on. I would need to know this so I down burn out my bipolar NPN transistor. \$\endgroup\$ – Norfeldt Jun 5 '13 at 8:01
  • \$\begingroup\$ The three parameters are: - gate charge (how much charge to build up) or gate capacitance - driving voltage (what voltage you will drive it to) - gate resistance Note that the capacitance even for very big MOSFETs is measured in pico- or nanofarads. The biggest power MOSFETs may have a capacitance of 15 nF and a gate resistance of a few Ohms. Thus, the instant current for a 12 V voltage over 2 Ohm gate resistance could be 6A, assuming zero source impedance. This is all pulse current, though -- there's no continuous current once turned on. That's the good thing about MOSFETs! \$\endgroup\$ – Jon Watte Jun 7 '13 at 4:38
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Found this video which I found helpful I thought I wanted to share it with other people reading my question

http://www.youtube.com/watch?v=10R0Mrqwjuo

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