SMPS fail to supply Vcore, what could be happen?

Byann.1 · ‎2022-11-15

Hi,

I'm using a STM32H757XIH6 with SMPS. Everything (firmware & hardware) are working fine but after a switch off/on of the power supply, I could not connect to the microcontroler via SWD.

First I thought it was the Power configuration that had been corrupted. So I tried to connect in bootloader mode (BOOT0 pin at VDD) , but it doesn't work. I couldn't not connect. You could find in attachment the power supply schematic.

After some investigations, I feel that the SMPS was broken (400mV on Vcap) and the Vcore wasn't supply anymore. So I replaced the SMPS by an external LDO (1.2V). I succeed to connect to the microcontroler.

What I undertsand from the reference manual (chapter 7.4.1 - page 276) is that the microcontroler will stay in reset mode while the Vcore is not correct. I assume we stay in this state because Vcore was about 400mV. It's why, we could not connect anymore to the microcontroler.

=> the SMSP seems broken.

Do you already encountered such problem?

Do you have some suggestions to find the root cause of this failure (ESD, high voltage, high current, others, ...)? Which extra tests can be done to find the root cause?

According to you, is it an irreversible failure or can we recovery the SMPS?

How is it possible to break the SMPS?

Thanks in advances for your help

CYANG.1 · ‎2022-11-17

Hi

As you can access MCU by an external LDO (1.2V), you may check if SMPS is on or off in the MCU, or clean your codes in flash. After SMPS is set on (or clean codes), you can double check if VDD_SDC can provide 1.2V or not on your board. If output is 1.2V, that means SMPS is not broken.

baptor · ‎2022-11-17

Hi,

I am working with Byann.1 on this board. One point that I can add on this issue is that when the smps output voltage is at a steady 400mV, a short voltage pulse to 1.2V on VFB_SMPS ( using external LDO ) will make the smps output voltage to stay at 1.2V after we remove the extenal LDO voltage. Mean that the smps can still maintain and regulate voltage, but cannot rise from 0V to 1.2V at start.

Since we can acces the MCU with this trick, we erased all the flash ( mass erase ). But still same issue. At restart, smps output voltage do not rise more than 400mV...

Byann.1 · ‎2022-11-17

Hi,

We thought we found the root cause. A capacitor between the input of the SMPS (VDD SMPS) and the inductance (L4) is missing. Without this capacitor, the inrush current throught L4 create an overvoltage that could break the SMPS module. It seems it's an irreversible failure.