Overview

Metal Oxide Semiconductor Field Effect Transistor 

"variable resistors controlled by voltage"

How they Work

MOSFETs work like BJT Transistors but trigger based on voltage instead of current.

When voltage is applied between Gate and Source, current is allowed to flow between DRAIN and SOURCE.

MOSFETs are variable resistors controlled by voltage. Depending on the voltage applied at the GATE, the resistance between the DRAIN and SOURCE will vary (known as RDS_on).

Do I Need a Heat Sink?

Calculate the power you are going to need to dissipate.

Where I is the current your application is going to pull.

Let's assume our little motor will pull 1A and R_{DS=0.035Omhs I}

P=R x I²

   ^{=R_DS x I2}

  = 0.035 * 1²

  ^{= 35 mWatts}

^{The formula to compute the amount of power our MOSFET can dissipate without a heat sink is as follows:}

^{P_D = ( (MAX(T_J) - (T_A) )/ R_0JA}

We will need a few more items from the data sheet.

^{MAX(T_J) - Maximum Junction Temperature}

^{T_A - Ambiant Temperature (25C)}

^{R_0JA - Thermal Resistance, Junction-to-Ambient, Max}

How many watts can we dissipate without a heat sink?

^{P_D = ( (MAX(T_J) - (T_A) )/ R_0JA}

  ^{=  (175C - 25C )/ 62.5}

  ^{= 2.4 Watts}

^NOTE:

^{The spec sheet will refer to a P_D  which is higher than our calculation. This is because it assumes that you can keep the MOSFET at 25C while operating which would mean you would need a heatsink.}

Testing For Fake Mosfets

https://www.youtube.com/watch?v=XXcEgddzjnI

Testing Procedure

Using a 12V source and a automotive 12v light bulb:

• connect up the bulb and record its current draw from the bulb and the voltage of the supply.

To measure the Voltage from Drain to Source

• connect negative to Source(S)
• connect voltmeter as shown
• connect the + side of the voltmeter to Drain (Bulb should be off)
• connect the + side of the voltmeter to Gate (will activate the mosfet)
• connect the + side of the voltmeter to Drain (Bulb should be ON)
• record voltage

Testing IRFP260

Measurements

Voltage: 12.09v

Current: 2.00A

Voltage Drain to Source: 0.42v

Calculated R(ds)

R(ds) = Vds / I = 0.42v / 2.00A = 0.210 ohms

Datasheet Specs

https://www.vishay.com/docs/91215/91215.pdf

R(ds) = 0.055 ohms

Result

FAKE!

Reference