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DC dummy load circuit calibration
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VEGETA:

--- Quote from: Ian.M on May 12, 2018, 07:13:29 pm ---1.5A 30V with your 1R sense resistor is 42.75W dissipation in the MOSFETs.  There isn't a snowflake's chance in hell that one of those heatsinks with passive air cooling could keep the MOSFETs cool enough.   Two of them, each with a fan *may* be good enough but you'd have to do some testing to confirm that.

I cant comment about the panel meter as you haven't described it other than saying it draws 15-20mA.

--- End quote ---

This is the panel meter: https://www.banggood.com/0_28-Inch-Dual-Display-Red-Blue-LED-Panel-4_5-30V-Digital-Voltmeter-Ammeter-1-100A-p-1093413.html?rmmds=detail-left-hotproducts__1&ID=513878&cur_warehouse=CN

Ok, then what heatsink is gonna be enough? I meant, what size? I want to order the PCBs with components from JLCPCB and they offer this store: https://lcsc.com/products/Heat-Sinks_441.html

I could search locally but I am not confident I will find what I want.

It is worth mentioning that I will put the project in this box: https://www.banggood.com/Plastic-Electrical-Junction-Box-Instrument-Chassis-DIY-Black-Case-125X80X32mm-p-1141712.html

So, I guess determining heatsink size is important because I need to know where to cut for it and mount it.

I searched for heatsinks in aliexpress and banggood, but no one tells you the heatsink temperature raise per watt. How can I know then?
VEGETA:
I attached the data for the mosfet, what I understand is this:

It's junction to case temp increases by 3.3 degrees for each watt dissipated -> say 50W in our case = 3.3x50 = 165 degrees.
However, junction to ambient is way too much which I don't understand how to calculate.

So in my understanding, we need to dissipate 165 degrees in the heatsink to make the mosfet work better since it has 0.3 derating factor. Does this mean that it's power capability is decreased by 0.3 Watts per degree? that means 0.3*165 =  45.9 -> what does it mean?

I will get 2 mosfets to make it better but I need to understand what to do in terms of temperature. I need to understand how stuff works... I hope you can help.
VEGETA:
I forgot that I bought this one: https://www.aliexpress.com/item/Black-Extruded-Aluminum-Enclosures-PCB-Instrument-Electronic-Project-Box-Case-100x76x35mm/32813597400.html

What about connecting the two heatsinks to it (each mosfet on one heatsink)? Otherwise, my friend has promised to give me big heatsinks... in this case, will they be good with that plastic enclosure?
Ian.M:
Although that Aliexpress aluminum project box would be very nice for a lower powered device, its a poor choice when you need to dissipate over 40 watts.  Because the only flat sides are the ends, it would be difficult to mount a large enough heatsink to it neatly.  You cant just bolt the heatsink to the top of the box and fit the MOSFETs to its interior, you'd actually have to cut an opening under the heatsink so the MOSFETs could bolt direct to it.  The 76x35mm end plate size constrains the heatsink size you could mount to the back, and the restricted interior volume would make it difficult to mount a fan cooled heatsink internally.

However if the same box with a smooth top or if a taller version with ends large enough to take a CPU heatsink were available they would be very suitable.

Lets take a closer look at your IRILZ44N MOSFET thermal data:

The same basic information is presented twice in differing formats.
The first (highlighted in Tan) is the power dissipation at 25° C and the derating factor.   That assumes a heatsink capable of  keeping the MOSFET mounting surface under a particular temperature limit.  For every degree the mounting surface is above 25° C by, subtract 0.3W from its 45W @ 25° C rating.   That's convenient for quick back of the envelope calculations. e.g. if we can keep the mounting surface under 55° C, it can dissipate 45-0.3*30 = 36W, but is a PITA if you actually have or need data for the heatsink.

The second, (hilighted in Yellow) is the design data needed for a more formal solution.  TJ_max is 175°C, and the thermal resistance junction to case is RθJC of 3.3°C/W. (Ignore RθJA of 66°C/W - its only applicable if you are *NOT* using a heatsink.)

Lets assume a maximum ambient temperature of 45°C (as you have Jordan set as your location), and that we want a 10°C safety margin on TJ_max.   That means we can tolerate a temperature rise of 120°C.  With an infinite perfect heatsink, perfectly bonded to the mounting surface, that gives us a dissipation limit of 120/3.3 = 36.4W which closely matches the result for 55°C from the first method (55°C = 45°C + 10°C margin), as expected.  At this point we already know a single MOSFET cant handle your proposed usage - to get 42.75W dissipation without exceeding TJ_max, you'd need to keep the mounting surface under 33.9°C, with no margin, which is impractical without active cooling - its cheaper to add more MOSFETs.   

If you can split the power evenly between N MOSFETs,  the calculation for the thermal resistance (to ambient) for N separate heatsinks becomes:

   RθSA = (TJ-TA)/(Ptot/N) - RθJC - RθCS

Plug in N=2 , take RθCS as 0.5°C/W (typical for a TO220 screwed down on heatsink compound to an anodized heatsink), other figures as before, and you get an max individual heatsink  RθA  of 1.8°C/W - which is enough to start searching for heatsinks on and distributor's site that has a parametric search.   If you want to put them all on the same heatsink, you need to divide the result for individual heatsinks by N, which would give 0.9°C/W.

However, without separate driver OPAMPs and separate source resistors Rs, the power will *NOT* share evenly.   Lets guess that  it may be out of balance so one MOSFET is taking twice as much power as another and see what that does to the calculations.  We only need to calculate for the one that's hogging the power.     120/(42.75*2/3) - 3.3 -0.5 = 0.41°C/W  :-\ :scared:  That's only a 56.7°C maximum heatsink temperature - which may be possible with a big enough heatsink shared between all the MOSFETs, calculating its temperature rise from its RθSA and the *total* dissipation.

Heatsinks with RθSA below 2°C/W tend to be somewhat pricey and, apart from mass-produced CPU heatsinks, anything below 1°C/W tends to be really expensive, and for anything at all below 0.5°C/W you'll cry when you see the price.
VEGETA:
You mentioned that using multiple mosfets could help, I can use up to 4 mosfets. However, only one shunt resistor (1R). I can though use an opamp per mosfet with 100 ohm gate resistor. How about that?

a friend promised to get me a big heatsink so I will mount the mosfets and the resistor on it. I cannot use a fan since the project will be powered by 5v USB. I wonder why Dave's design seems way more easier despite the same spec (he has 1.3A and I have 1.5A while 1R is the same).

The problem is that I don't have the data for heat sinks (C\W). So one cannot determine if it is enough or not.
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