FETs and gate resistors - scope images

Still learning what all the buttons do so this is very preliminary. Testing on a BLF-20DD, with the known-working combo of Vishay 70N02 FET, 130 ohm gate resistor, 12K gate pulldown resistor. Using a fast-PWM build of luxdrv, I'll get to other versions eventually to see if there's a difference.

In 'low' mode (PWM value of 12), this is the waveform at the FET's gate pin with the driver running normally:

...and then, with the gate resistor jumpered (actually, just grabbed on both pads with tweezers):

Interesting thing I wasn't expecting, the LEDs got dimmer with the gate resistor shorted. The peak at the gate is higher, but for less time. The resistor really drags it out and that leaves the FET on (or partially on) for longer, which makes more light.

I'm not into this electronics much, but it is nice to see you looking at the new drivers like this, it will help finding the right values for the components more accurately. Was the driver under load of a led? Or does that not matter in this case?

Yes, it was powered by a single ICR18650-28A in a battery box (with a switch on the side), and a triple XPG2 board mounted on my test heatsink.

One thing I'll have to figure out how to investigate is that on the drivers that don't work right, they most times just shut off completely after changing modes. So there's no signal there to measure. I think it happens too quick to get anything useful from a frame capture.

I still have lots of other stuff on the to-do list: like, what does the output side look like with various hardware/firmware, and one thing I really really want to see is the output side of the SRK-DD drivers comparing with inductor to without, to finally see for real what the inductor is doing to make the low modes lower.

Oh, and I forgot the most important part. That bounce at the end with the resistor shorted is likely the thing that makes the drivers act goofy with no resistor or the wrong value resistor.

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…