how to reduce inrush current when stm32 powering up?

JKW · ‎2024-05-07

Hi,

As the title described, how to mitigate the current behavior when stm32 is applied power? My commercial power system are quite sensitive when MCU is power on.

For example, let stm32 enters into low power mode once power is on and then control the chip enter into fromal user mode. Would that help?

thanks.

Andrew Neil · ‎2024-05-17

@JKW wrote:
Monitoring the current using a simple shunt needs PCB routing damage in our design, it's the least method we want to do.

So this "commercial power system (made according to uTCA system and spec)" is integrated on the same PCB as the microcontroller?

@JKW wrote:
Do you have any other cost saving idea?

Can you replicate your microcontroller setup on a dev board, and take measurements there?

You still haven't said what microcontroller it is.

As @Danish1 suggested, over 100mA seems a bit unlikely for just the microcontroller itself at startup... 🤔

Danish1 · ‎2024-05-17

I'm somewhat frustrated by what you tell us (or don't tell us). It's as if you've been given a theoretical* problem to sort out without the tools or knowledge rather than someone who was intimately involved with the design and engineering.

Has the power-supply tripped? Does it consistently trip, so that if you make a change and it doesn't trip after the change, you can be reasonably confident that the change you made genuinely helped?

My suspicion is tripping is associated with the inrush when charging up the power lines.

Have you had a look at the schematic? I can well understand commercial sensitivity so it wouldn't be right for you to share significant amounts of actual circuit details. I explicitly mentioned the capacitance of power-supply-decoupling capacitors you have on your +3.3V power-supply. What total do you have there in microfarads? Does the 3.3V regulator have soft-start? Is there explicit circuitry to hold the stm32 microcontroller in reset until the power-supply is up and stable?

Are there any other capacitors that need charging, that might also contribute to the inrush current?

But I could be wrong about this.

It could be something the microcontroller is doing. A way to test this could be to keep the microcontroller in reset (tie the RESET line low) then applying power.

Or it could be the time-window before the microcontroller starts doing what it should be doing. If the microcontroller drives some power-switches (e.g. transistors) but your design doesn't include resistors to pull the control lines into a "safe" state before the microcontroller starts up. In which case keeping the stm32 reset could make things worse.

*Maybe someone, perhaps in management rather than engineering, has asked "what if the power supply trips?"

JKW · ‎2024-05-17

Hi Andrew,

No , it is a independent power module for uTCA system, product weblink: https://nateurope.com/product/nat-pm-dc48/

There is big chassis to hold power module and customized cards. We used STM32F103VET6 as microcontroller on customized cards. While the power module is responsible to do power management and monitoring for all cards.

Andrew Neil · ‎2024-05-17

@JKW wrote:
No , it is a independent power module for uTCA system,

So you don't actually need to hack the PCB to measure current - you could tap it where it enters the board?

JKW · ‎2024-05-17

Hi Danish1,

I have to say I'm testing the hardware system but without enough hardware detail design information of the power system and power module(no schematic, only user manual). I tried to contact the technical support from manufacturer but failed.

I could provide some answer your questions:

decoupling cap for 3.3V about ~2uF

No 3.3V soft-start designed

No circuit to hold STM32 in reset until power supply is stable.

You mentioned to keep MCU in reset then applying power, it's good idea to give us right direction.

the last question you asked, I also want to know, but now only the original equipment manufacturer could answer this. I used some commercial system chassis and system as shown below:

Thanks.

Andrew Neil · ‎2024-05-17

@JKW wrote:
independent power module for uTCA system, product weblink: https://nateurope.com/product/nat-pm-dc48/

The Technical Reference Manual suggests that it shouldn't trip until 300mA:

So even less likely that's down to the microcontroller starting up!

The TRM also says there's a load of current sensors built in:

Could they help you?

JKW · ‎2024-05-17

Actually there is no information to guide me to measure current on power module.(we do not have the schematic or PCB)

But it is possible to measure current on customized cards by ourselves, while we need to cut the PCB trace off and add shunt resistor or current probe to find out what happened during powering up stage.

JKW · ‎2024-05-17

Thanks for your quick feedback.

Yes, it has current sensor built in power module, but it didn't say how I can read the current value it measured.

All I saw is the customized board designed by ourself easily triggered its over current protection.

Another information:

the customized board consumes ~60mA after running the application code(current value is read from the DC Bench power supply). But we don't know the current behavior during powering up, no current probe on hand.

Andrew Neil · ‎2024-05-17

@JKW wrote:
the customized board consumes ~60mA after running the application code (current value is read from the DC Bench power supply). But we don't know the current behavior during powering up, no current probe on hand.

So just use a simple shunt in this setup!

@JKW wrote:
Yes, it has current sensor built in power module, but it didn't say how I can read the current value it measured..

So contact the manufacturer for details!

Uwe Bonnes · ‎2024-05-17

"No circuit to hold STM32 in reset until power supply is stable."

I hope you have at least the recommended 100nF on RST to ground w/o any additional pullup. Otherwise I assume hickup is easy possible, especially with some "controlled" supply.