General > General Technical Chat

SOT-227(ISOTOP) vs TO-264 heat dissipation

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temperance:
The open loop output impedance of the OPA194 is 375 R. Driving a 23 nF load which increases towards 30 nF in the linear region with 375 R results in output stage bandwidth about 100 times lower in frequency the op amp. Or you have to add a high current buffer or properly compensate the op amp.

You also need an RC snubber circuit across the MOSFET or any wiring inductance will turn this circuit into a very good oscillator. 2.2...4.7 R with 100...470 nF will do up to some length of wire. (The oscillator is formed by the inductance connected to the drain and the MOSFET Cgd)

Wrong or faulty dummy load schematics are being posted on EEVblog almost every week.

A good post on the loop compensation subject made long ago by Jay_Diddy_B
https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/

Here I've done the same with a low cost op amp and some other low cost components:
https://www.eevblog.com/forum/beginners/constant-current-load-stability/25/
The schematic of a buffered op amp is in post 31
A test result is posted under reply 42

The remainder of what is being said is mostly people stating how I'm wrong and can be ignored. Except for what TopQuark has posted. (TopQuark, it somehow always makes me think of Dutch cheese? But the Top version.)

coppercone2:
The board on transistor concept you have is liable to get loose. I don't know how to solve that problem. I recommend copper cable to lead it to a PCB where its terminated to a  ring lug mount (its like a bracket soldered to the PCB you screw into with a short jumper).

If you use through hole ring lug mounts, with a hole on the bottom, it MIGHT work if you had trimmed screws and a funky assembly procedure of inserting a washer sideways underneath the ring terminal mount. They would need to be aligned very well.

Now IIRC the miller welder that did this direct2pcb, it had OK screw tension (after 15 years), a bit low, on the PCB they had. But this will depend on your PCB quality (how good the epoxy is, and probobly other factors). I don't like it, they should have used a small wire adapter. That welder did explode on someone for no reason, possibly because one of their dodgy junctions gave up.

If you MUST have the pcb there, its probobly better to use the leaded package, with a hole in the PCB for mounting the part, or a bracket that compresses the part from both sides (better) using two screws



So unless you plan on using cable, go with picture #1, and put holes on the PCB so you can screw down the transistor module. This is not a bad design IMO. The only problem is the transistor might twist and yank on the leads. Optimally you would put notches and spacing between the transistors so you can grip them with a jig or pliers (i.e. duck bill pliers) when tightening so the part does not twist. Its a problem because you only got 1 mounting hole. In the instruction manual I would expect there be a page on how to replace the transistor properly (show pliers being used to restrain it)






***
I am curious about how a large eyelet could work for picture #3 to prevent the board from experiencing tension and loosening. This would be special hardware that is staked and soldered to PCB and it would need to be beefy, probobly made from bronze, assembled on a arbor type press and soldered using resistance techniques (lumo)

Fauks:

--- Quote from: temperance on March 23, 2024, 10:46:13 pm ---The open loop output impedance of the OPA194 is 375 R. Driving a 23 nF load which increases towards 30 nF in the linear region with 375 R results in output stage bandwidth about 100 times lower in frequency the op amp. Or you have to add a high current buffer or properly compensate the op amp.

You also need an RC snubber circuit across the MOSFET or any wiring inductance will turn this circuit into a very good oscillator. 2.2...4.7 R with 100...470 nF will do up to some length of wire. (The oscillator is formed by the inductance connected to the drain and the MOSFET Cgd)

Wrong or faulty dummy load schematics are being posted on EEVblog almost every week.

A good post on the loop compensation subject made long ago by Jay_Diddy_B
https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/

Here I've done the same with a low cost op amp and some other low cost components:
https://www.eevblog.com/forum/beginners/constant-current-load-stability/25/
The schematic of a buffered op amp is in post 31
A test result is posted under reply 42

The remainder of what is being said is mostly people stating how I'm wrong and can be ignored. Except for what TopQuark has posted. (TopQuark, it somehow always makes me think of Dutch cheese? But the Top version.)


--- End quote ---

I appreciate the info. I found this schematic here:  https://github.com/kaktus85/MightyWattR3
It seems like a pretty popular open source electronic load I found when searching but it is now a few years old. I was planning on making changes and incorporating a small ESP32 instead of an old Arduino, among other changes.

It sounds like I have a lot more reading to do! 


--- Quote from: coppercone2 on March 23, 2024, 11:43:08 pm ---The board on transistor concept you have is liable to get loose. I don't know how to solve that problem. I recommend copper cable to lead it to a PCB where its terminated to a  ring lug mount (its like a bracket soldered to the PCB you screw into with a short jumper).

If you use through hole ring lug mounts, with a hole on the bottom, it MIGHT work if you had trimmed screws and a funky assembly procedure of inserting a washer sideways underneath the ring terminal mount. They would need to be aligned very well.

Now IIRC the miller welder that did this direct2pcb, it had OK screw tension (after 15 years), a bit low, on the PCB they had. But this will depend on your PCB quality (how good the epoxy is, and probobly other factors). I don't like it, they should have used a small wire adapter. That welder did explode on someone for no reason, possibly because one of their dodgy junctions gave up.

If you MUST have the pcb there, its probobly better to use the leaded package, with a hole in the PCB for mounting the part, or a bracket that compresses the part from both sides (better) using two screws



So unless you plan on using cable, go with picture #1, and put holes on the PCB so you can screw down the transistor module. This is not a bad design IMO. The only problem is the transistor might twist and yank on the leads. Optimally you would put notches and spacing between the transistors so you can grip them with a jig or pliers (i.e. duck bill pliers) when tightening so the part does not twist. Its a problem because you only got 1 mounting hole. In the instruction manual I would expect there be a page on how to replace the transistor properly (show pliers being used to restrain it)






***
I am curious about how a large eyelet could work for picture #3 to prevent the board from experiencing tension and loosening. This would be special hardware that is staked and soldered to PCB and it would need to be beefy, probobly made from bronze, assembled on a arbor type press and soldered using resistance techniques (lumo)

--- End quote ---
I appreciate the reply.

The mounting pattern is a standard CPU socket mounting pattern (75mmx75mm, my quick render may not be to scale but it is close) and most heat sinks include mounting hardware including a steel brace that goes on the under side of the board, and two brackets that screw into that brace from the top to which the heat sink attaches.  I have attached an image of the hardware included in the heatsink I had in mind. It clamps down with quite a bit of force, so assuming the thermal pad is thick enough I don't see why it would come loose. It has springs that keep constant tension over time.

Using the SOT-227 module, I would have to use threadripper/Epyc compatible coolers as the heat sink base plate is much larger on those but the same principle would still apply. The current spacing is for LGA1700 sockets which is just standard intel 12th+ gen CPUs. 

Here is a short video showing how the mounting would work. I apologize if I am completely missing your point though lol. 
https://youtu.be/Ioaon-11z24?t=50

T3sl4co1l:
Note that SOT-227 top side is not made to bear force; nuts are captive but loose in the package, and the leads are bent over tabs.  A mounting plate is best.  You might take a slab of hardware store extrusion and drill and tap it for the necessary features, or order a plate machined to the same.  I'm not sure offhand how that would work with the stock brackets; you may be better off finding some spots to drill and tap into the heatsink itself.

Tim

Fauks:

--- Quote from: T3sl4co1l on March 24, 2024, 06:45:19 pm ---Note that SOT-227 top side is not made to bear force; nuts are captive but loose in the package, and the leads are bent over tabs.  A mounting plate is best.  You might take a slab of hardware store extrusion and drill and tap it for the necessary features, or order a plate machined to the same.  I'm not sure offhand how that would work with the stock brackets; you may be better off finding some spots to drill and tap into the heatsink itself.

Tim

--- End quote ---

Interesting point. What about some aluminum spacers between the tabs and nut for bracing? Or like someone else mentioned, bending the tabs straight and mounting through the PCB so the nuts and making contact instead of the tabs.

Or are you referring to the top of the assembly in general should not bare weight?

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