FETs and gate resistors - scope images

I’m liking what you’re doing - what equipment & s/w are you using?

SainSmart DDS120. http://www.sainsmart.com/sainsmart-dds-120-20m-50m-s-virtual-oscilloscope-silver.html

This is a properly-working 20DD flashed with STAR momentary, PWM levels of 2-6-18-54-130-255, using same gate/pulldown resistors of 130/12K as before.

PWM=2

PWM=6

PWM=18

PWM=54

PWM=130

PWM=255 (???)

that gives a good insight in how pulse-width and PWM percentages translate in amounts of light :-) . Reminds me of HKJ's review of the qlite driver and its problems with highfreq. modulation

But remember, this is the signal fed to the gate, not the output to the LED. I suppose since I have a second channel available I could grab both sides in the same picture...

Why is the voltage so low???

Edit:nevermind I guess it’s a 10:1 divider in line…

I would find it interesting if the negative bump goes away if you actually short it with a small wire or solder blob. Maybe the inductance of the tweezers cause it.

This is really great stuff. Well be following this thread closely. Thanks for reporting.

Also remember there's a threshold for the gate voltage before any current can get through the FET. The signals don't look all that different in duration between 2 & 6 & 18 if you look at the entire thing, but if you only count roughly the part above the '0.200' line the difference is about in line with what you'd expect with those PWM levels.

This is super info Comfy. Great work.

This is a different 20DD, same 70N02, same build of STAR momentary, but NO gate resistor (replaced with wire, soldered) and no pulldown resistor (left blank). This combo, this exact same driver, did not work using the clicky firmware. It works flawlessly with the momentary FW.

PWM=2

PWM=6

PWM=18

PWM=54

PWM=130

I do not understand what this is, but something tells me it is important!

20DD, 70N02, NO gate resistor (replaced with wire, soldered). When flashed with clicky firmware (either luxdrv or STAR) it occasionally goes goofy when changing modes - mostly when dropping from high to mid, other modes, if you can get to them, seem stable.

This is taken at the FET's gate pin, with everything normal:

Right. We've seen that before. But then...

With the polarity protection diode bypassed, no other changes, driver still running in the same mode as the first pic, not even shut off between the two pics:

How is... I mean... what? With the diode bypassed, all mode changing happens correctly, no more weirdness. Un-short the diode and it goes back to acting flaky. The voltage scale in the first pic is likely correct - yep, 6 volts at the gate when running thru the diode like normal. But bypass the diode and the gate voltage just happens to exactly match the actual Vin. Is this a diode issue, or a capacitor issue?? Where the hell is the extra 2 volts coming from?

That is real weird. Shouldn't the diode be using up some voltage and causing lower voltage to the MCU? I take it you made sure the screen shots were not mixed up?

The waveform with the diode operating normally matches the other ones taken earlier on the same driver (or other drivers while bridging the gate resistor). I saw it change from the top pic to the bottom pic when I grabbed across the diode with the tweezers. Did it multiple times while running the frame capture.

Does the location of the capacitor make a difference? Would it be 'better' on the topside of the board closer to pin #8? It just seems like it's a long way off from the thing it needs to filter.

Way back when these problems first showed up I replaced the Digikey diode with one from a 105C and it showed the same behavior, so I ruled out the diode. What I didn't ever do though was swap in a cap from a 105C.

Same no-resistor driver, low mode (PWM = 18). CH1 is the FET gate, CH2 is Vin measured between the B+ pad & ground ring (so, before the diode):

And this is the same but with a 470uF cap stuck between B+ and GND (also before the diode):

:|

Perfect example why an oscilloscope is such an invaluable tool.
One might have guessed something like that at some point, but seeing it is a quite different thing.
Leads inductance, diode and cap make a nice boost converter, don’t they? :wink:

Now we need one after the diode to check how the controller supply voltage reacts…

What do you want to see on the two channels, before diode & after diode, or after diode & gate pin?

The giant electrolytic cap I'm poking around onto various pads completely at random gives an interesting change when it's placed across the diode, too, but that's not a real-world solution to anything (it's way too big). Would a smaller SMD cap in parallel with the diode accomplish the same thing? I need to figure out a way to try that...

I also swapped our known-value 10uF/16v cap for the unknown-value part that comes on a 105C, no change... I also tried the cap on the top of the board straight from the MCU pin to ground (electrically the same, just different trace lengths) with no change, and then tried the two SMD caps at the same time, one in the original location on the battery side and a second one on top at the MCU, again no change.

Also tried the big electrolytic in the same location as the small SMD cap, no change. But putting it between B+ and GND, or in parallel with the diode, did get rid of the weird overvoltage spikes.

I am assuming here that this spike thing is the cause of the mode-change issues when there's no gate resistor present, since doing anything to eliminate the spike (bypassing the diode, or adding the cap between B+/GND) also eliminated the mode-change issues. Am I at least on the right track? That the solution is something practical that eliminates the voltage spike?

Like you know I also have often this problem, just put a normal nanjg in my t08 because my test driver with some smd FETs in parallel was gone crazy…
I also tried to add caps in some different capacities(sometimes stacked half a dozen), resistors and stuff everywhere but couldn’t get a valid explanation or solution. I am happy that you try to analyze these issues:)
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If I am right that is our hypothesis:
That the controller behaves sometimes strange and what often can get wiped out with a big cap is in my eyes also a problem with the controller input voltage, because every time the FET switches it is like a short….
The idea behind the diode is that the led can’t discharge the cap behind the diode and so the controller always has enough voltage to work also in the switching phase.
A cap between the bat- and bat+ buffers this also but because it gets discharged through the led with serious amps it needs to be a big one.
Now the only reasonable guess would be that this voltage protection isn’t working as we think.
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I just want to see if that diode and cap construct works to smoothen the power supply or if the controller also sees these spikes, because I think that has to be the problem…??? So both would be interesting controller voltage and FET signal, so that we can see if there is a correlation between the switching and the power. Also the before and after diode is interesting to see if it actually works like we think it will.

What has me confused is here:

(same pic from post #18 - trigger here is on the rising edge of CH1 (gate signal) at 1.00v, that's close to the FET's threshold voltage)

I can (mostly) understand the spike on CH2 after the PWM turns off. What I don't get is how the gate voltage rises to ~6v, and stays there, while the supply to the MCU is still down at battery voltage. If they tracked together more closely it wouldn't be much of a mystery. The ~6v being generated when it turns off, sure. Removing the load, it releases stored energy in the cap, and voltage spikes. But it's doing a constant, rock steady 6v output to the gate pin while input to the MCU is still down at ~3.9v.