ESP32 Forum

We have ESP32 on Custom PCB Board. when it is powered up from Mains Supply of 230v/3.3v and Connect programming Header to PC to flash the firmware it fails with A fatal error occurred: Invalid head of packet (0x78) very frequently. Can someone please explain what can cause this error?

Programming header has : RX, TX, DTR, RTS pins to interface with external usb to uart bridge. program is uploaded by Automatic bootloader mode.

Last time this happened with me, it was a ground loop or open ground problem.
Make sure that the GND of programmer is connected to the GND of the ESP32 board if you are using an ISOLATED power supply.
Also, maybe try to power the ESP via the programmer and not use the SMPS while flashing it - that lowers ground noise and also is safer.

If your SMPS is custom made, that could be the cause.
The Y class cap between high voltage and low voltage side can cause this issue if the capacitor value is incorrect by a huge margin.