Choosing a transistor

[Luciano]

Hi,

The power dissipated in a MOSFET is the voltage across it times
the current going through it.

The voltage across the MOSFET when it is on will be the
resistance of the MOSFET, Rds(on) times the current going
thorough it.

A MOSFET IRLR/U2905 has a on-resistance of 0.027Ω.

Power dissipated by the MOSFET case when you have 2 Amps of
current going thorough it will be: (2A = 100 LED's 20mA).

P = 2A^2 * 0.027Ω = 0.108 Watts.

(No problem without heatsink).

Best regards,

Luciano

[The Master]

(No problem without heatsink).

Thats good news then. I dont even have room for TO220 packages. Theres no way i could get a heatsink to fit in there. Im not even using a heatsink for my voltage regulator since it never gets remotely warm with just a PIC chip and a MAX485 connected.

One thing i dont understand is how you know that 0.108W means you dont need a heatsink

[Luciano]

One thing i dont understand is how you know that 0.108W means you dont need a heatsink

Hi,

From the datasheet. The thermal resistance junction-to-ambient is for when
there is no heatsink.

IRLR/U2905 datasheet:
http://www.irf.com/product-info/data...a/irlr2905.pdf

The thermal resistance Junction-to-Ambient of an IRLR2905 is 110 °C/W.
Its operating junction temperature range is -55 to + 175 °C.

Example ambient air temperature = 45°C: 

175°C − 45°C
__________    =  1.18 Watts 
   110°C/W

With an ambient air temperature of 45°C, without 
heatsink an IRLR2905 can dissipate 1.18 Watts. (Absolute Maximum).

Best regards,

Luciano

[skimask]

The voltage across the MOSFET when it is on will be the
resistance of the MOSFET, Rds(on) times the current going
thorough it.
A MOSFET IRLR/U2905 has a on-resistance of 0.027Ω.

Of course that also assumes you are driving the MOSFET 'hard enough' (i.e. large enough Vgs vs. Vds) to get that low Rds(on).
If you come up a bit short on the Vgs, the Rds(on) could be much higher.

It's always something, ain't it?

[Luciano]

Of course that also assumes you are driving the MOSFET 'hard enough' (i.e. large enough Vgs vs. Vds) to get that low Rds(on).
If you come up a bit short on the Vgs, the Rds(on) could be much higher.

It's always something, ain't it?

The power MOSFET IRLR/U2905 has logic-level gate drive and
therefore you don’t have to worry about that. Just try and
measure the voltage drop.

[The Master]

Ive been looking at the datasheets again and as far as i can see the LEDs in total will use at most 200mA per color. Ive been trying to find a suitable mosfet to control them but im still having a bit of trouble. There is a category on rapid called "TO-220 Logic level power MOSFETs N-Channel". I dont want a TO220 package but i thought i would look at the datasheets anyway because they seem to be exactly what you recommended. I still cant seem to understand this gate voltage. Even the logic level one im looking at says "Gate-source voltage ±18V". There is also a "Gate Threshold Voltage" That says 2.5V. Is that what i should be looking at?

I almost got the heatsink bit. skimask confused me a bit though. I have a rough idea of how hot certain components will get with a certain load on them but usually i just put my finger on it to test. That seems to work well except for when i shorted a regulator out once... ouch :P

So, It looks like the only logic level mosfets that rapid do are in a TO220 package. I want to look through the rest of the mosfets to see if any will do the job but it looks like most of them are for audio circuits

Edit: Would the 2N7000 N CHANNEL MOSFET be suitable?

[Luciano]

Hi,

Use that:


Datasheet MPSA14 DARLINGTON TRANS NPN:
http://www.rapidonline.com/netalogue/specs/81-0142.pdf

Best regards,

Luciano

[The Master]

Thanx for pointing that one out. Its nice and cheap too.

I do still have a few questions though. I dont want to just use the one you picked, i want to understand why you picked that one.

I see in the datasheet Collector-Emitter Voltage: 30V and Collector Current: 500mA so i guess thats one of the first things i should check. Both of those value must be equal to or greater than the requirement for the load.

Is it the Base-Emitter On Voltage that says how many volts you need to put into the base to make it turn fully on?

Why did you use the 10K and 100K resistors? The 100K looks to be a simple grounding resistor that keeps the base grounded while there is no input from the PIC. The PIC im using (and i thought all pics) grounded output pins when you set them to low. In this case would i still need that resistor?

Where you have put the 10K i normally have a 1K or nothing at all. I assume its to limit the current that gets to the base but how do you know what value resistor to use?

[Luciano]

Hi,

The MPSA14 is a Darlington transistor. (See datasheet page 1).

http://www.rapidonline.com/netalogue/specs/81-0142.pdf


The Darlington transistor (often called a Darlington pair) is a semiconductor
device which combines two bipolar transistors in a single device so that the
current amplified by the first is amplified further by the second. This
configuration gives a high current gain and can take less space than two
separate transistors because the two transistors can use a shared collector.

* * *

The 100k resistor can be omitted if you can afford to have the LEDs in an
unpredictable state during the initialization of the microcontroller.

The 10k resistor must be present.

Best regards,

Luciano