FemtoBuck blows up all the time 😭

Components & Accessories LEDs
1

I have blown up many FemtoBuck and also some PicoBuck and can’t figure out where the problem is…

There are 7 Osram Oslon HyperRed LED on a Metal PCB Module, and two are in series connected to one Femtobuck. My multimeter is total crap, the current messureing is gone, so I soldered a 10 Ohm Resistor between the two modules to measure the current over the the voltage of this resistor.

Every time I turn on the Power Supply (Meanwell UHP-500) at 36V the PicoBuck goes up in smoke… and with lower Voltage 33V to 35V the current through the Modules changes, sometimes it is 200mA, than 600mA, than 400mA.

Looks like the PicoBuck do not regulate the current, it also gets very hot.

I have soldered the FemtoBuck with a primitive 230V soldering iron, this could be one source of error.

  1. Does the soldering took to long and the BUCK may got some heat damage?

  2. Or may it be a ESD damage that happend from this soldering iron?

  3. Or maybe the UHP-500 is not worth what Meanwell is telling and I have a high ripple Voltage?

From seeing, that the Current is not regulated by the FemtoBuck I speculate that it possible Nr. 1 or Nr. 2 therefore I ordered a new Soldering station (ERSA i-CON NANO) with ESD protection that arrived this morning. Furthermore I will buy some 9V Block batteries connect them in series to cancel out the a ripple voltage from the Power Supply.

Does anyone have another Idea what could be the problem?

Thanks :smiley:

2

36 volts is the absolute maximum the FemtoBuck can handle, it’s very possible your power supply is exceeding that briefly at powerup and over volting the FemtoBuck.

You might consider a lower voltage like 24 volts so that you have some overhead in case there’s a small surge on the power input.

3

Thought about that, but even happend when the UHP-500 was set to 33V, 34V, 35V. It looks like that the current regulation do not take place at all.

And even the Datasheet of the AL8805 say it can handle up 40V as maximum input voltage, but it’s not recommended. According to this the AL8805 should be able the 36V even if the ripple voltage is 1V.

But yeah, I will try lower voltages. Just bought some 9V Block Batteries a few minutes ago, will later use 2-3 (maybe 4) in series to cancel out the possibility of damage from ripple voltages.

According do internal resistance of the Al8805, there should be only 25mW as heat losses in the IC itself, but it just blows up.

By the way, also the analog dimming do not work at all, doesen matter if CTLR is put to GND, the current in the LED String is just limited by the input voltage itself.

Crazy behavior, would be interesting if someone else also had this issues and could say that thisbis clearly a heat damage or ESD damage?

4

My multimeter is total crap

That is a serious problem, and one that is easy to correct.

It is a bad idea to run a component or module at close to any one of the manufacturer’s absolute maximum ratings. I stick to 70% or lower.

5

Yes I agree, the next thing I need to do is to get some better equipment to measure it.

So I measured a few of the defective ICs (AL8805) and compared them with the internal circuit diagram of Diodes. The unusual measurements caused me to disassemble some of the AL8805s to get to the bottom of the cause of these deviating measurements.

Under the microscope, ESD damage is clearly visible in many as burn holes in the silicon. So probably these are caused by bad handling or by the very bad soldering iron.

Likewise, defective bonding points can also be seen in some of them. Of course there are also damages caused by the disassembly, but I don’t mean these. I could see that the bondings, that is the solder joints on the silicon itself were dissolved by high temperatures. I also attribute this to my poor equipment, i.e. the unregulated soldering iron with too large a soldering tip.

The FemtoBuck I soldered with the new ESD safe soldering station (Ersa I-CON NANO) ran perfectly on 9V block batteries connected in series. The open circuit voltage was even 37,5V and did not damage the BUCKs. Under load the voltage of course dropped below 36V, but so the next source of error had to be the switching power supply from Meanwell.

Up to a voltage of about 34.6V, the UHP-500 from Meanwell did not cause any damage. However, at higher voltages, i.e. above 34.6V, the BUCK (AL8805 IC) was also destroyed here.

The cause was the power supply Meanwell UHP-500. It could be the long DC cable which I have connected to the UHP-500 to supply the BUCKs. But it could also have internal causes or by disconnecting under load, voltage peaks could have been caused here. Since I do not have an oscilloscope at hand, I can not investigate this.

All Shottky diodes of the destroyed PicoBUCK and FemtBUCK still block, presumably these have suffered thus no damage. But again I don’t have an oscilloscope to measure the exact temporal behavior of the diodes.

I can only advise everyone to leave the fingers of too cheap equipment, so you save a lot of money, trouble, frustration and especially time.

Thanks all for your attention and interest

6

I can only advise everyone to leave the fingers of too cheap equipment

Better advice: don’t stress the equipment, as you do.

7

The only way to figure this out will be with proper measurement equipment. A multimeter (even assuming one that works correctly) is too slow to see transient effects. You will either need to use a scope, or as you have discovered, set the voltage low enough that the transients don’t exceed the maximum operating voltage. You have a power supply that you have proven causes problems set close to the absolute max of 40v. You have a large load with a switching current regulator on the end of a long cable. Until you verify clean power, you can’t blame the chip.

/mike

8

Long leads to the power supply can introduce severe inductive spikes (3X the power supply voltage), as shown here: https://www.pololu.com/docs/0J16/all