Question: * I'm trying to build a controllable switch for a heavy load (3D printer), controlled by a microcomputer.
* I don't want to use relay because of slowness, sound and durability. So I've built a switch based on MOSFET IRLB3034 [datasheet](https://static.chipdip.ru/lib/300/DOC000300294.pdf).
* And here is my circuit diagram:
[![circuit diagram](https://i.stack.imgur.com/J92wC.png)](https://i.stack.imgur.com/J92wC.png)

The main issue is, the transistor getting more than 150 °C after 20 sec of working at full power.

1. Vgs = 4.5v or even 11v (that makes no difference) of constant DC
2. Vds = 1v and rising
3. Current is about 15A (max of power supply is 20A)

What could be my possible mistake?

Answer: 
As @Kripacharya [said](https://electronics.stackexchange.com/questions/449867/mosfet-is-overheating-when-running-on-a-20a-load#comment1129326_449867) the figure 8 in datasheet shows that I'm operating beyond transistor's capabilities. So that might be the main reason (as far I can't test with suitable MOSFET), so I need to find another one or another way to reach my goal.
[![fig. 8](https://i.stack.imgur.com/wnJ31.png)](https://i.stack.imgur.com/wnJ31.png)

P.S. Could anyone suggest transistor searching tool for similar cases?
The mosfet you picked should be able to handle 15A with no heatsink without any issues.  

The datasheet says that at Vgs=10V the Rdson should be 1.4mR. P = I2\*R = 15\*15\*0.0014 = 0.315W. At 20A that would be 0.56W  

The datasheet also says that Junction-To-Ambient thermal resistance is 62degC/W. So at 560mW that gives 35degC temperature rise over ambient.  

Did you measure the Vds over the MOSFET? At 15A and 1.4mR it should be only 21mV. If you see anything higher, the part is bogus.  

btw in your schematics you are running the load current through a jumper (J5). A standard jumper will have a resistance of 20-50mR. So at 15A that will dissipate at least 4.5W - that should be enough to melt the jumper.