STM32F3 Discovery power circuit heating up

[st239955_stm1_st]

Posted on July 14, 2014 at 00:01

I was testing my STM32F3Discovery with an external 5V power supply (generic 5V 2A wall plug), and after about 30 seconds it stopped operating (LED flashing demo stopped flashing), and soon after I smelled something burning, and unplugged it.  

Now, when I plug it in via USB, I cannot debug it, although I can still program it, and it will seemingly run fine when being powered via USB, however the USB terminal becomes hot to the touch.  When I power it by connecting the power leads to the 5V and GND pins, the area around the U1 chip, which seems to be the power circuit according to the schematics, becomes very hot.  

Now, I assume I've probably irreversibly destroyed something in the board, and I'll have to order another one, however I have no idea what damaged it, and I'm afraid I'll just end up destroying another board.  

[frankmeyer9]

Posted on July 14, 2014 at 09:35

There is no obvious fail reason, judged by your description.

The problem might be your 5V/2A power supply. Especially the cheap ones do often not behave very well with small load conditions, i.e. very small currents drawn. And I expect a 5% or less load in your case. This might cause the output voltage to rise significantly, and damage/destroy the power supply circuitry on the F3 discovery.

Not sure if a repair of the F3 board is worth the effort, it probably costs more than a new one ...

However, I would investigate the load behavior of the power supply, and probably avoid the usage with discovery boards.

[st239955_stm1_st]

Posted on July 14, 2014 at 16:32

I've measured the voltage of the unloaded power supply, as well as while it is powering the STM32, and it was always between 5.2V and 5.4V (although I was not measuring it specifically the time that I smelled burning).  The datasheets for the voltage regulator (LD3985M33R) say it can handle up to 6V...  Is there some common known plug that works with it I could buy instead?

[frankmeyer9]

Posted on July 14, 2014 at 17:19

Well, this was just a guess, as I'm not having direct access to your 'burned' board.

But as example, I recently measured 34V for a (nominal) 24V power supply, at about 2..3% of the stated maximal current.

As a rule of thumb, I would not go below a 10% load

Your supply might exhibit some peculiar power-on characteristics with nonlinear/capacitive load, resulting in a voltage spike. That would be detectable with a scope.

Or, the damage might be caused by another transient event, like ESD. Those are rarely leaving any visible mark on the 'killed' object, and thus are hard to substantiate ...

[Tesla DeLorean]

Posted on July 14, 2014 at 17:29

Is there some common known plug that works with it I could buy instead?

Like a Mini-USB charger to USB USER?

[st239955_stm1_st]

Posted on July 14, 2014 at 18:36

What would that do differently than a plain adapter?  Do you think that USB chargers are more likely to be high quality or reliable?  Or just because they're usually lower amperage?

[Tesla DeLorean]

Posted on July 14, 2014 at 19:19

I was focusing on a ''common known plug'', 5V, available at 2.1A, likely designed with some better regulations than some arbitrary 5V DC wall wart (transformer, bridge rectifier, capacitor). People get rather teed-off when their $500-600 smart-phone combusts. Easy to find/buy at WalMart, or discount retailer of choice.

The USB USER also has some diode protection to permit other supplies, and to work cooperatively with the USB ST-LINK connector.

[frankmeyer9]

Posted on July 15, 2014 at 09:20

... likely designed with some better regulations than some arbitrary 5V DC wall wart (transformer, bridge rectifier, capacitor).

Most common ''wall wart'' power supplies don't come with transformer/rectifier anymore. Instead, they use buck converter, directly from the 230/120V main input. The cheaper the part, the simpler the circuitry, and the worse the behavior at the high-load and especially the low-load margins. With low loads, they get into discontinuous operation mode, and basically lose their ability to regulate the output voltage.