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dannyboy

MOSFET failures I can't explain

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I've been playing around with an analogue LED strip - basically following this tutorial, but using a LaunchPad instead of an Arduino. I'm using STP16NF06 power MOSFETs rated for 16A/60V, which should be fine (the strip should draw no more than .8A at 12V for each colour). However, I've managed to induce failures into two of four of them - they allow drain-source current when the gate is held low, so two of my light channels never turn off.

 

I'm not sure how I've managed to cause that failure, since I'm playing well below their rating. I hadn't been doing anything too extreme with the microcontroller, either - I'd only gotten to the stage of giving each channel a 1s on time (followed by 2s off while the other two channels turned on), which appeared to be working fine. I noticed the breakdown after I tested with the micro, so it must have happened in between that test and when I next tried to just select the channels manually (by connecting jumper wires to ground/3.3V).

 

This page indicates that gate over voltage could cause 'hard on', which sounds like what I'm experiencing. (Gosh, that was a bit dirty in retrospect.) But I verified that the LaunchPad is giving 3.5V, and I had the grounds connected during operation. I did disconnect the LP from the circuit after that - could it have been something like disconnecting the ground in the wrong order? But I was pretty careful to always get rid of the 12V supply before touching the rest of the circuit.

 

Anyway. I figure there was some accident that applied over voltage to the gates; I haven't destroyed either of the other two transistors since, so it seems like my normal operations aren't harming them. I just want to know what I should be doing to avoid those kinds of mistakes in the future!

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Might not have been giving enough voltage to the gate... Vgs(threshold) is ~2-4V, but that's at Vds=Vgs, Id=250uA ...

 

What kind of voltage are you supplying to the LED strip, and about how much current do you expect to put through it?  Holding the gate active with a low voltage keeps the MOSFET in a "linear mode" where its resistance is higher, so it may dissipate more heat than it normally would if, say, you sent 9V into the gate using some sort of adapter hardware.

 

In adafruit's example, they're using an Arduino which typically runs at 5V ... that might be good enough not to fry it.

 

Could be something else though, just wanted to toss that out there as food for thought.

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----- and just a last thought, just in case :

 

double check that all the mosfets are wired correctly.

   -  specifically, the drain and source connection.

      if they are swapped, you will get current flowing through the forward biased (zener) diode

           this means current will always flow, source-to-drain-to-LED, and the LED turns on permanently

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Thanks, everyone!

 

@@RobG and @@veryalive, I'd show my connections if I had more than a crappy phone camera which really does not photograph breadboards well. Believe me, I've tried. But I've been over the schematic many times since I discovered the failure, and I'm as certain as I can be that the connections are the right way around. The sources are connected to ground, and the drains to 12V via the LED strip.

 

@@spirilis Thanks for pointing that out! I'd been reading that as needing 2-4V in any condition... silly me. I'm giving it 12V from a wall wart, and according to the Adafruit tutorial I can expect to see 600mA drawn if I have all the LEDs on. I wasn't up to the stage of verifying that yet. I forgot to mention that I tested the MOSFETS after running them a bit, and they didn't seem to have heated up - but you and @@oPossum are right, I was definitely driving them at way too low a voltage. It worked somewhat, but probably wasn't ideal. Given I don't need a ton of current, the Arduino's 5V should be fine according to figures 5 and 6, but not 3.5V.

 

So, it looks like RobG's booster would be an ideal solution, but I'm enthusiastic to make this work myself, and I eventually want to fit the circuit on a smaller footprint than the LaunchPad itself (I want this LED strip as a lighting fixture!). My naive first solution would be to include some sort of intermediate power supply at 5-6V, and connect non-power MOSFETs to the MSP to drive the gate voltages of the bigger MOSFETs. However, voltage dividing 12V to get 6V sounds like a bit of a waste. Must do more research. And think about MOSFET safety! A bit of research indicated I shouldn't need a heatsink for the MOSFETs, but better safe than sorry.

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i do understand your desire to do the job by yourself      = >  persistance.

 

- sounds like your wiring is ok, but the mosfets need higher drive voltage than the 430 output provides

 

- if you have bipolar transistors available, you can drive their base via a resistor (1 to 2 K ohm) from the 430, ground the emitters, LEDs to the collectors.  they only need about one volt at the base to turn on the collector current.

- in this case, a medium power NPN, darlington if you have them.   (there any many low cost options,  one is BD139)

 

- sticking with bipolar, due to the lower turn-on voltage, think of the uln2003 driver family. 

multiple drivers in one package for your stated 'compact size' requirement.

you may have to invert the logic level in your IO pin-driving software.

again, there's lots of similar drivers out there.

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That ULN2003 package looks convenient - am I right to think it'd be possible to power the entire LED strip through it rather than through the MOSFETs? That seems weird... what about heat dissipation? I guess my power requirements aren't very high (600mA at 12V makes 7W for the whole LED strip).

 

Thanks everyone for spending time on me - I'm slowly finding my way through practical electronics! I come from code-land where everything is just high or low :P.

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Without a picture i'm not quite sure what were dealing with.

 

Are the gates properly ESD protected & have the components been in a controlled ESD environment through shipping to your handling & soldering of them?

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