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: 
Assuming you've actually **measured** Vgs **at the MOSFET pins** (it gets only about 90% of the drive voltage, and the drive voltage is heavily loaded by the LED), then one would tend to conclude that the MOSFET is not actually a genuine part of the type indicated. The certainty would increase to near 100% if the MOSFET was sourced on a platform that hosts bad actors. 

The dissipation should be no more than about 0.7W which **will** get hot without a heatsink, but not excessively so (150°C is excessive). If your traces or wires going to the MOSFET are thin you may also be getting heating from those sources. Tja is 62°C/W so the rise should be the order of 40-45°C. Rds(on) increases with increasing junction temperature, maybe 50%. But not 125°C rise worth.
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.