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LM317 Dummy load circuit
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Ian.M:
If you re going to use a 5V USB charger + boost converter, you don't need the rail splitter.  Connnect the 5V PSU between Gnd and the OPAMP's negative power pin for a -5V supply to it, and connect the boost converter input across the 5V PSU, and set it up for x2 boost to get your +5V supply for the OPAMP. 

My LTspice libraries are nothing special - the custom models are nearly all from LTwiki or the LTspice Yahoo group.  The very few I have customised myself (or cant remember where they came from) can all be found here zipped up with whichever sim I tweaked them for.
VEGETA:

--- Quote from: Ian.M on April 14, 2018, 06:39:09 pm ---If you re going to use a 5V USB charger + boost converter, you don't need the rail splitter.  Connnect the 5V PSU between Gnd and the OPAMP's negative power pin for a -5V supply to it, and connect the boost converter input across the 5V PSU, and set it up for x2 boost to get your +5V supply for the OPAMP. 

My LTspice libraries are nothing special - the custom models are nearly all from LTwiki or the LTspice Yahoo group.  The very few I have customised myself (or cant remember where they came from) can all be found here zipped up with whichever sim I tweaked them for.

--- End quote ---



So I will have 0 and -5v from the usb charger if I ground it's positive output, instead of 5 and 0v if I ground the negative output. Thus results in 5v and -5v from boost rather than 10v and 0v. So the 12v fan could be powered from the boost (10v diff) instead of 12v difference voltage? could be, but it will get less current... perhaps I should make the boost go up to 12v difference which will make it 7v-> -5v total output while the usb charger still the same. opamps won't argue with this.

Now I understood that the "floating" part is not what it matters here, but the mere splitting of the rail. unlike my linear low dropout design where I needed the floating part to be there.

Here the load power supply will be connected to the input of the LM317 and the -5v rail, right?

Ian.M:
No, the source under test goes between LM317 In and Gnd (which is the USB charger +V) so the LM317 load current does NOT pass through the auxiliary floating PSU.
VEGETA:
OK, I will test tomorrow and will update you on results.

Anyway, I will be doing the bigger version of it which is gonna be opamp+mosfet like everyone else, 30v 5A or so. I have ordered these mosfets:

IRL640A: https://www.aliexpress.com/item/Free-shipping-10pcs-lot-IRL640A-TO-220-200V-18A-new-original/32675986021.html?spm=a2g0s.9042311.0.0.u8tP2F
IRLIZ44N: https://www.aliexpress.com/item/10PCS-IRLIZ44N-IRLIZ44-TO220F/1000004693800.html?spm=a2g0s.9042311.0.0.u8tP2F

which are logic level ones, but total power dissipation is less than required... right? So I must parallel them to achieve 30v5A rating.

I also got these heatsinks:
https://www.aliexpress.com/item/2-x-Black-Aluminum-Radiator-Heat-Sink-Heat-Sink-40-x-40-mm-x-11-mm/32811370283.html?spm=a2g0s.9042311.0.0.u8tP2F

Can I drive both mosfets with one op-amp output without external parts? cuz I got 5 power resistors (1R) which can be used but I don't want to waste them if not required. I will get 12v fan to suck the air out to keep them cool.

So I need to know if I can parallel them easily or not. And in case I need a better mosfet, what is it? it has to be logic level one but with high power rating and, most importantly, available on aliexpress.

best regards
Ian.M:
MOSFETs typically have too much Vgs threshold (and gain) variation to be easy to parallel in linear mode without a lot of derating.   It would be preferable to give each its own OPAMP for gate drive and current sense resistor for feedback.   

However its easy enough to test your MOSFETs for matching at the max current you want to run them at: Start by writing an ID number on every one. Mount one on a heatsink (clip and silpad is good - you'll be changing it a lot!) and connect it to a PSU with a suitable drain resistor: 1R 10W would be a good choice.  Use a pot to bias the gate and ramp it up to the current you want to run at.   Measure the Vgs.   Repeat with the next one,  till you have them all.   Sort the results into Vgs order, then try the MOSFET with the highest Vgs for the desired current at the Vgs of the lowest and see what current you get - that's how much imbalance they'll have.   

Run one for a while starting from cold, initially at 80% of your desired current and monitor the current, Vgs and heatsink temperature to get some idea of their thermal behaviour.

If you are very lucky and they are all from the center region of the same die, they may all have very similar Vgs at the desired current and you'll be able to get away with directly paralleling them, though even then its better to keep the drain resistors separate if they are large enough to provide a voltage drop greater than the Vgs variation.
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