N mosfet ....When can you not use a heatsink .... knowing Rdsₒₙ helps us here

Each Mosfet has a slight resistance at the drain-source main channel, Rds₍ₒₙ₎ We need to consider how much Power that will entail ......it will determine the relevant waste, heat dissipation of the mosfet. We can then compare that to the mosfet's relevant value of Power dissipation ...if our usage is below that value, then we do not need a heatsink on that mosfet. Pd .... Power dissipation without a heatsink: eg for a IRLZ34NPbF Logic Level mosfet 1) our usage: Rds₍ₒₙ₎ = 0.06Ω  ....datasheet  at Vgs ≈ 4v  Id=14A So at say 1A drain-source usage (not as for the attached schematic example) when P=I²R Pd= 60mW   ie this is a usage dissipation example. cp  : 2) the mosfet's value of Power dissipation: (for heat sink decision): Pd = (Tⱼ-Tₐ)/ Rᵩⱼₐ   eg say Tₐ = 25°C    ....ambient temp. Tⱼ = 175°C  ....datasheet    .....junction temp max Rᵩⱼₐ = 62°C/W      ....datasheet ..... thermal resistance for junction to Ambient So Pd for this mosfet ≈ 2.4W  ....(ie 150/62) a heatsink is needed if our usage dissipation is above this value. Thus 60mW is well below 2.4W  so no heatsink needed with this mosfet at 1A drain source usage example. If our Pd at 1) had been greater than this Pd at 2) then we would need a heatsink on the mosfet. NB: Although the mosfet may not need a heatsink as calculated above, we need to look also at the circuit Watts at any point to see if the wires and board are sturdy enough for the power being used at any point. ie...see the volts at any point and multiply that by the amps current flowing there...thus you have the power at that point....don't confuse a power heat dissipation with a power usage....both need considering. Now work out the Pd mosfet heat waste in this schematic....use the same datasheet values as above. Is it ok to use this schematic setup without a heatsink for the mosfet? With a total circuit usage of approx 30mA would a solderless breadboard be suitable? What is the Vgs here? What is the Vgs₍ₜₕ₎ set at here? Did you notice that the transconductance has been altered so that this EC mosfet works fully like a real one would....I have set the mosfet to have Rds₍ₒₙ₎ = 0.06Ω See: http://everycircuit.com/circuit/6359306484318208 and http://everycircuit.com/circuit/5927322113015808

published 8 years ago