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I would like to create an active load which draws current(4 to 6A) from a 12V battery. Unfortunately, the shipping costs and delivery times have prevented me from buying Linear FETs directly from well known manufacturers. Instead, I have found this interesting MOSFET in a local electronics shop; the SFH154 which is being advertised as an avalanche rugged advanced power FET. My question is, can this MOSFET be safely used as an active load ( with a feedback loop to prevent thermal runaway using a microcontroller DAC to provide VGS) or are there other factors to take into consideration? Also, would a CPU fan equipped heatsink be a suitable choice for cooling?

Note: this MOSFET is somewhat expensive and I would rather not see it turn to coal.

A.H.Z
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  • Since when is *Fairchild* of all companies not a "well known manufacturer"? – Hearth Apr 30 '22 at 22:40
  • @Hearth what I meant to say was that I can't buy directly from the manufacturers, I found this MOSFET in my local area's electronic shop's online catalog. – A.H.Z Apr 30 '22 at 22:41

3 Answers3

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The part is obsolete: no wonder it is expensive, you're buying old stock, or (worse) relabeled or recovered parts.

Yes, it could be used in linear mode, and your current and power are well within the listed limits. 14V*6A = 84W is less than the rated power (~100W). Less if the load voltage is more than ~ zero.

A high power FET is designed to spread the current across the FET body. Switching FETS are designed to do that only at full 'on' voltage, so their pulse current limit is much less when operated in linear mode. But when operated a low power with continuous current, the FET heats evenly, and the current spreads correctly. There is no indication that this old FET will fail at such low currents.

The junction-to-case thermal resistance is ~ 0.8. At 84W, that would be ~ 70C. Derating 204W at 1.34, the new power rating is around 100W, and you would need a large heatsink to keep the case temperature at 25C, and you need to be sure the FET mounting is good.

However, you still have a little room for junction-to-ambient. And if you aren't running the battery fully charged, it will be less than 14V, and if your load voltage is more than zero you have even more room.

david
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  • could you please suggest a way to mount the fet securely unto the heatsink? Would drilling a hole in the middle of the cpu heat sink be a good idea? – A.H.Z May 01 '22 at 03:33
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    @A.H.Z That looks like it should be a new question. Have a look for some heatsinks that can dissipate enough power (old CPU heatsinks with a fan might be worth considering, as you suggest), and ask a new question with a heatsink that you think will work, given the °C/W rating you need. (Also, google for "USB electronic load" to see how other people cool a MOSFET with a CPU heatsink - there will be reviews which take them apart to show the construction.) If the heatsink uses heatpipes, you must not drill into them. – Andrew Morton May 01 '22 at 18:58
  • @AndrewMorton Noted, i'll definetly check them out. – A.H.Z May 01 '22 at 21:11
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There's a DC SOA curve in the datasheet. Stay within the SOA and thermal limits and you should be fine.

enter image description here

John D
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You are within the DC SOA region, so the FET should be OK. However you will be dissipating 6*12 = 72 W (perhaps more if the battery is typically 14 V). and it will need good heatsinking.

A CPU fan & heatsink could work, but you need a way to mount it on the heatsink.

Also, connect a 10k R directly from G to S so that when the MCU powers up and perhaps its outputs are floating, the FET won't accidentally turn on.

To reduce the power in the FET, you could connect a power resistor in series with the drain. You can easily use this to reduce the FET's power dissipation by 50 % (that power will be in the resistor which is likely cheaper and more rugged). For 12V, 6 A, a R of 1 Ω would be suitable (remember it will dissipate 36 W). You can use a long wire (in water if necessary), or perhaps a 12V 50 W automotive bulb (which will need a heatsink...).

Note that FETs operated in a linear mode could be susceptible to 'thermal runaway'. This occurs when heat spreads unevenly through the device and happens at high VDS and low VGS (. Using a FET with "too good" on-resistance could result in this condition. Your case of 12 V is likely quite safe.

jp314
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