[retired] [WIP] 20mm single sided & 17mm double-sided ?-amp linear driver - surprisingly good!

I think a MOSFET of the type you desire is like Love - some things can’t be bought. :~

As for MOSFET selection from what we can buy: Start with the specs you do understand, those will narrow it down a lot. See what you come up with by focusing on these things:

  • I’d stick to 20-30v rated stuff
  • high current rating
  • high dissipation rating (of course)
  • a fairly low gate threshold voltage since it will be powered by no more than whatever the MCU supplies to the QX7136. This will be spec’d as Vgs(th) / gate-source threshold voltage, etc. Note that the spec’d number doesn’t necessarily apply to our case since it’s for something like 1mA or similar. Look for something which has graphs indicating good performance down to low Vgs.
  • gate charge - don’t let it be too high. Look at the other selections we’ve been discussing, but I think you should be fine with <50nC or <100nC or something.
  • “times” may matter, this has yet to be nailed down (see this discussion thread). turn-on delay time, rise time, fall time, turn-off delay time, etc.

Remember, both NXP PSMN3R0-30YLD and Vishay 70N02 failed to work with a standard firmware, but slowing the PWM down to 1khz (or whatever I said above) seemed to get them both working just fine.

Pinning down your voltages, gate charge, and current rating should quickly narrow the field.

Awesome thanks so much, that’s exactly the kind of explanation I was looking for! :slight_smile:

Don’t know if this helps you, I was just lurcking around datasheets of led-CC modules since I plan on modding my just purchased New117/TK35 alike driver.

Since the QX7136 datasheet doesn’t offer much info, I thought I’d share that find.

The somehow related QX9920(those with LEDA on top, same manufature) datasheet is somewhat more helpfull with stats, wich would be nice, but I can’t read Chineese. But it’s possible to squeeze out some stats by figuring it’s relations, since the numbers and units are latin digits.
It states a T_off time of 621ns in the sheet, that would -equal to a frequency of 1610.31khz -wich would just about equal to the frequency range you’d find out works good.
Might be a coincidence but I don’t think so.

nice work you do here, thanks a lot. :slight_smile:

EDIT: Crap, just turned on my brain. Was out of caffeine… T_off on QX9920 relates to the offtime memory cap. Nevermind still great work you do here.

T_off for the QX9920 (which is a step-down / buck controller) is supposed to refer to the fixed off-cycle time it uses. In other words it turns OFF for a fixed period of time, then turns ON for a variable period of time (approximately until the set current is reached), then repeats.

Whether the freq is related I’ll have to think about. :slight_smile:

On a driver like the A17DD-S08 would it be feasible to make a copper “heat sink” that attaches to the gate and wraps around to sit on top of the FET? Glued on with Arctic Alumina Thermal Adhesive as an isolation? This would be “live” I know, but as long as it wasn’t touching anything else it would be acting to dissipate heat from the MOSFET, right?

In some of the lights I use this driver in there is plenty of room in the pill for up to 3/4” tall sink. I can see it being possible to make quite a large heat sink in copper to connect to the gate and the LED negative lead could actually connect at the top of the sink as it would be live.

Would this do any good for pulling heat away from the MOSFET and keeping it nice and stable? Some of the triple and quad builds I’ve been doing are seeing in excess of 14A.

You mean attaches to the drain? The large heatsink pad on the bottom is the drain, the gate would be a pin equivalent of the pwm input on a 7135.

It’s that age old argument of having thermal mass/internal heatsinks vs an efficient thermal path to the outside of the light isn’t it.
I went for more of the thermal mass approach on my BTU build (even though I don’t actually have to worry about isolating the 7135s electrically) and neglected maximizing the thermal path to the outside of the light. Since that is the only area that can actually get rid of the heat generate from everything inside I was simply filling up the internal heatsink and when it was full the temps skyrocketed. The fact that this mosfet is always going to need an electrical and as a result also somewhat of a thermal isolation between it’s heatsink pad and the rest of the light is my primary concern.
It may well be an unfounded concern though. I have a bunch of parts on order to put this stuff to the test! :slight_smile:

I realize my application is somewhat extreme but if you’re dumping up to 15watts of heat into any internal heatsink that doesn’t have a great connection to the dissipating surface you’re probably not going to have a very long running, stable driver.

Thinking about it again now, I’m not really using the body of the flashlight as a ground conductor, I could probably isolate the “pill” from ground without too much hassle. That would certainly make heatsinking the fets more straight forward.

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Wight if I have found a mosfet in TO-220AB format that hits or exceeds all the other criteria but has a “Qg” total gate charge of 212nC @ 10v, is it not even worth trying?
I’ll keep looking for something that hits all the requirements but I guess I’m not too clear how gate charge affects the situation, does it take longer to “fill” by the MCU pwm signal before the fet is fully “On” or am I looking at that completely backwards?

On a related note, seeing as there may be quite a few people planning to run this driver in a 2s configuration. Wouldn’t a zener diode rated higher than 4.3v (but obviously lower than 6v) help in any of this stuff. Certainly looking at Rds On for these fets it seems a higher voltage on the gate is beneficial, wouldn’t that also help the rest of the circuit run at a higher frequency or allow for less specific fet requirements perhaps? Just thinking out loud here…

At 212nC I think you're going to have a hard time switching it at fast frequencies direct from the attiny like we usually do.

I built a light bar where I was driving 2x external TO-220 FETs mounted on the external LED heatsink at 36V and quite a few amps. On that setup I was running a ~5.5V zener diode because I figured I needed all of the voltage overhead I could get to switch those FETs and keep them turned on fully when needed. The zener diode was obviously very inefficient stepping down from 36V, but an extra 0.5 watt of loss wasn't a concern in that application, and it was a prototype setup anyways. For a momentary setup, a zener diode is obviously not the best solution.

Cheers Richard, I’ll keep looking for a better mosfet.
Good to get confirmation that higher voltages aren’t a bad idea in this type of thing, I’ll make sure to order in some 5.5v zeners just in case.

I never shared the 17mm v16 driver, here it is: OSH Park ~

I have some v06 boards on order, I assume the main difference here is the change to 0805 resistors and a bit of tidying?
Is the older version still fine for testing?

Cheers

Looks like it. I’m looking back now… Right, I see no major issues with v006. The changes are you described. The biggest functional issue with v006 may be that the DRV/GATE trace will be partially exposed VERY close to the GND ring on v006.

Still waiting on Osh so I can take a crack at this. How long does Osh usually take from Order>Delivery ?

Expect em to come in my late February, early March at the latest.

Oh, did I forget to add the :P? lol

Usually they come inside a couple of weeks, depends on what else is being made and how much as they fill a panel before running it. Then once it’s done they have guys separate everything and they ship immediately after that.

Thanks. I was notified that mine were sent to the fabricator the same day I ordered them, I just didn’t know how long it takes from there.

I know the feeling, I ordered mine on Dec 28th, shipped Jan 8th so hopefully they’ll be here soon.
It’s sad having all the components on hand but no boards in sight. :stuck_out_tongue:

I just got my shipping notification. 3 each of the 20mm and v06 17mm. Exciting stuff! I hate the look of all the 7135’s stacked, so hopefully this will be the solution. :party:

You say you hate the look of stacked chips, have a look at this board and tell me which ones are stacked….one is the moon mode, the others are the rest of the modes. Wights PZL driver.

Love how this looks in the tube when changing cells! :slight_smile:

That PZL has 15 chips, keeping it’s MT-G2 to a respectable 5.43A for 2500 lumens. And yes, that’s an Eagle Eye X6.

All of Wight’s boards are easy to assemble, with oversized pads and a nice layout that makes even hand soldering a piece of cake. :wink:

I’ll take a guess! The top left corner is not stacked.
a deeper pill with a retaining ring like the x6 makes it look fine, but sticking out the bottom of a P60 is ugly to me.