Hi all,

I am new to electronics and designed my first circuit. However I fried one of my PCB and trying to determine the root cause. What I am suspecting is caused by a diode polarity that I missed.

The PCB was working fine for about an hour or 2 without any issues. Then suddenly it stopped working. There are 3 power rails that supplies to the board. 24V, 5V and 3V. I realized 3V rail stopped working and 5V has a weak output. Also MOSFET was super hot, even though there were no load is connected and gate was off. D7 Indicator LED was also off so there were no voltage flowing.

I started checking all the diodes since some of them were connected reverse. I fixed all of them, or at least I thought I fixed them. Later I found out D8 were soldered in reverse.

I wonder due to diode was soldered in reverse it constantly supplied 24VDC to the drain side of the MOSFET instead of protecting it, which possibly leads to the failure of the PCB? Since I am absolutely new to electronics design I am trying to figure out if that is the cause of this failure.

Here is the MOSFET schematics below to control a load. It is controlled by esp32.

I also forgot to add a large pad at the bottom side of the PCB to cooldown the MOSFET properly. Is there any workaround for this like adding a separate heatsink on top of the MOSFET?

Thank you.

 

Welcome to AAC!

Voltage alone does not destroy a component or circuit board. You also need current.
In other words, power = current x voltage, is what causes heating.

You said that you have three power rails, 24V, 5V, and 3V. You did not state the current capacity of each supply voltage.

Also, you need to post the entire circuit schematic so that we can see the effect of reverse diodes in the circuit.

 

Thank you for the quick reply.

Here is the breakdown of the power supply capacity
24V is coming directly from 350W MEANWELL PSU
5V is rated for 3A - LM2596S-5.0
3V is rated 1A - AMS1117-3.3

ICs
1 ESP32-S3 - 3V 200mA
1 MAX6675 - 3V 50mA
1 74HC125D - 3V 70mA
1 CP2102N-A02-GQFN28 - 3V 13.7mA
1 ACS712ELCTR-20A-T - 5V 13mA
1 MKS-TFT35R DISPLAY - (3V/90mA) (5V/?)

So there should be a plenty of headroom for current limits.

Here the pdf file of the full schematics.

 

As a side note I designed this circuit based on MKS DLC32 board except some changes like usage of ESP32-S3 and others, which has a schematics available at this link:

https://github.com/makerbase-mks/MKS-DLC32/blob/main/hardware/MKS DLC32 V2.1_003/MKS DLC32 V2.1_003 SCH.pdf

 

Also only thing that MOSFET (Q3) does is to turning on and off an this external relay below:
https://www.digikey.com/en/products/detail/american-zettler/AZ2280-1A-24DEF/12171629