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

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wight
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LinusHofmann wrote:
Ah whoops. Sad I wasn’t paying attention and assumed the large pad underneath the fet was for ground/heatsink. Had thoughts of mounting those fets remotely and soldered directly to a copper insert or something in the driver cavity.
That makes it a little less ideal than I thought, I know from dealing with the 7135s that heat sinks making contact with just the top of the chip are next to useless in keeping the chip from going critical.

But again this is a massively bigger package so I’m keen to have a play around with this driver and see how it deals with the situation thermally.
Cheers

Technically the FET manufacturers expect you to heatsink these things through large pours on the PCB, but I think it’s reasonably common for that to not happen. I’m not too concerned about it, even at high power, as long as there’s heatsink contact somewhere on the FET package. If you look at the construction diagram (page 4) of an LFPAK56 MOSFET, it’s pretty serious in there. Big copper slabs on the inside. With the ones we are using being rated at 50-150W of dissipation (depending on the model), I think some fudging is possible.

What specifically are you referring to when you say “with the 7135s that heat sinks making contact with just the top of the chip are next to useless in keeping the chip from going critical. “ ?

aslpg wrote:
It may be useful to the wiring diagram M6.
IC U2 is something like: The ADM803 supervisory circuit monitors the power supply voltage
My M6 mod. v1 by RMM, works very well.

https://sepi1g.bn1302.livefilestore.com/y2pvqwzEz8WV-iTj47K8k6P3KwoPJvHV...

https://gahyvg.bn1302.livefilestore.com/y2pGgih2_gYTYlGAVtpKidiXULnpgfzl...

Good schematic making skills aslpg!

Still fine, still on a break. One day I’ll catch up with you folks! previous wight catchup Wink
list of my drivers & variants (A17DD, FET+1 stuff, WIP stuff, etc)

LinusHofmann
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wight][quote=LinusHofmann wrote:
What specifically are you referring to when you say “with the 7135s that heat sinks making contact with just the top of the chip are next to useless in keeping the chip from going critical. “ ?

Warning: I’m plugging in my Waffle Iron! Silly

-

Well if you expect a 7135 to dissipate quite a lot and it isn’t soldered directly to a copper pcb area then it won’t be long until the temperature get’s way too hot at the chip level and funky things start happening (i.e going critical Smile ). I came across this particularly badly in my BTU triple mt-g2 build where the 7135 chips at the top of the double stacks were getting particularly toasty and resulted in very unstable output/flicker not long after start up. These were the chips only connected to the heatsink/ground trace through a thin solder bridge between the two ground tabs.
In that case I tried having the top of the 7135 package in contact with a heatsink surface but that didn’t make much of a difference at all. I actually noticed that the grounding tab would be almost too hot to touch before the case of the chip was even warm.
I was just saying, that to shift a lot of heat it doesn’t make much sense to me to use the top surface of the device to do that. But maybe it’s sufficient here?

As an example in my mt-g2 light driven at 6A (with ~7v vF) from a relatively sag free 8.2-8.4v input, I believe thats over 7watts of heat dissipated per 17mm sized board, and in this case it’s all through a single component and into an electrically isolated copper trace that can’t be in direct contact with the flashlight host and to the outside of the light. Are these mosfets happy running at very high temperatures, like 100degC and up with no ill effects? I have no idea on this stuff.

I do know that with the 7135s as the temperature rises above a certain point output seems to drop off quite rapidly below the rated 350ma/380ma. Some kind of active throttling or just performance deterioration I don’t really know. If it get’s really bad then other interesting stuff like flickering and stuttering sets in. If you connect a 7135 up without any heatsink contact on the ground tab and run it for a bit you should see what I mean.

Maybe it’s not the tiny fet inside the chip that’s suffering under the heat but some of the other sense components that are also in there.
A problem solved by separating the sense parts of the circuit and the hot mosfet as with this driver design perhaps?

-

I can definitely see from studying your mosfet package that having the internals laid out like that is directly linked to the performance of the fet, both in terms of thermal and electrical resistance.
For my application in particular, but maybe for linear flashlight drivers in general a power mosfet with the large tab as the ground/source like the 7135 would seem like a far better option. Ultimately the body of the light is the primary heatsink and if the mosfet is electrically and thermally isolated from that it’s not ideal. Does something like this even exist though?

This fet may be able to dissipate silly amounts of heat like 50-150W but surely that’s not a realistic figure in this case and heavily dependent on an ideal heatsink path?

I’d be really interested in seeing a test showing how this driver responds when it has to dissipate ~3-5Watts for an extended amount of time. Seems that would be a useful power dissipation range to look at for driving an mt-g2 at over 5 amps.

I got stung by unexpected thermal issues with those 7135s so I’m probably a bit hyper sensitive to the issue at the moment Wink

Cheers
Linus

wight
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Issues with the 7135’s when they are treated like that are pretty well known….

The MOSFETs are happy at extremely high temps. For example, the NXP PSMN3R0-30YLD should be derated to 20% of it’s full power dissipation rating when the tab is at 140°C (so that gives us 18.2W of dissipation after derating from 91W). Your PCB should be kept under 120°C though I think, or risk delamination?

The more direct your thermal path, the better. N-channel has been king for a while and all n-channel have this heatsinking arrangement as far as I know. I’m sure heatsinking to GND would be more convenient for our application, but that’s just how it is. It’s not as convenient, but we don’t get a choice about it. If you don’t want to heatsink the top of the device you don’t have to, you just can’t connect LED- to GND so you’ll need electrical isolation. I recommended potting earlier in the thread and I’m recommending it again now. It’s also quite possible to use a PCB like I posted in Post #32 and just electrically isolate it from the heatsink. That’ll work effectively to allow the heat to move into the heatsink.

Don’t look to me to do a lot of testing. Sorry.

Still fine, still on a break. One day I’ll catch up with you folks! previous wight catchup Wink
list of my drivers & variants (A17DD, FET+1 stuff, WIP stuff, etc)

LinusHofmann
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That sounds promising and hopefully output will be stable up to a higher temperature compared with a 7135 board.
I have some of these boards and parts on order to test this kind of thing, was just wondering what experience you had with these fets at high dissipation levels.

Potting is something I try to avoid on my own builds just for ease of tweaking and maintenance, but if that’s the only way to get the heat out then so be it Smile
Cheers

wight
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I’m sure you could switch to TO220 with an insulator. That’s a bolt together solution. I’ve been ignoring TO220 because it’s huge compared to PowerSO8 / LFPAK56. Development probably has not kept pace, but I’m not really certain how much that matters with a linear driver like this.

TO220 – https://www.google.com/search?q=to220&es_sm=93&source=lnms&tbm=isch
TO220 insulator – https://www.google.com/search?q=to220+insulator&es_sm=93&source=lnms

Still fine, still on a break. One day I’ll catch up with you folks! previous wight catchup Wink
list of my drivers & variants (A17DD, FET+1 stuff, WIP stuff, etc)

LinusHofmann
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wight wrote:
I’m sure you could switch to TO220 with an insulator. That’s a bolt together solution. I’ve been ignoring TO220 because it’s huge compared to PowerSO8 / LFPAK56. Development probably has not kept pace, but I’m not really certain how much that matters with a linear driver like this.

TO220 – https://www.google.com/search?q=to220&es_sm=93&source=lnms&tbm=isch
TO220 insulator – https://www.google.com/search?q=to220+insulator&es_sm=93&source=lnms

Yep that looks like the kind of thing I was dreaming about, that form factor with source connected through the heatsink pad and I’d be in heaven. How many millions do I have to order to get a custom mosfet spun? :bigsmile: Wink

I actually have some silpads lying around in to-220 format salvaged from some old 7812 regulators, so I could experiment with those.
Problem is, when it comes to picking a mosfet I have no clue. Is there like a range of specs that you look for which are most critical in a driver like this?

Cheers
Linus

wight
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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
  • “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.

Still fine, still on a break. One day I’ll catch up with you folks! previous wight catchup Wink
list of my drivers & variants (A17DD, FET+1 stuff, WIP stuff, etc)

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

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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 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. 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.

wight
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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. Smile

Still fine, still on a break. One day I’ll catch up with you folks! previous wight catchup Wink
list of my drivers & variants (A17DD, FET+1 stuff, WIP stuff, etc)

DB Custom
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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.

LinusHofmann
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DBCstm wrote:
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?

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! 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.

-

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…

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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.  

Mountain Electronics : batteries, Noctigon, and much more! What's new? 

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RMM wrote:

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.  

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.

wight
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I never shared the 17mm v16 driver, here it is: https://oshpark.com/shared_projects/dOdfsgix

Still fine, still on a break. One day I’ll catch up with you folks! previous wight catchup Wink
list of my drivers & variants (A17DD, FET+1 stuff, WIP stuff, etc)

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wight wrote:
I never shared the 17mm v16 driver, here it is: https://oshpark.com/shared_projects/dOdfsgix

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

wight
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LinusHofmann wrote:
wight wrote:
I never shared the 17mm v16 driver, here it is: https://oshpark.com/shared_projects/dOdfsgix

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 fine, still on a break. One day I’ll catch up with you folks! previous wight catchup Wink
list of my drivers & variants (A17DD, FET+1 stuff, WIP stuff, etc)

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

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

Major Projects:  Illuminated Tailcap, TripleDown/TripleStack Driver

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

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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.

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DBCstm wrote:
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.

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

Major Projects:  Illuminated Tailcap, TripleDown/TripleStack Driver

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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. Silly

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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

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

Major Projects:  Illuminated Tailcap, TripleDown/TripleStack Driver

DB Custom
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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! Smile

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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

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DBCstm wrote:
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! Smile


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.

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

Major Projects:  Illuminated Tailcap, TripleDown/TripleStack Driver

DB Custom
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I sat there and looked at it, very pleased with how well the stacked ones blended in. Really it only shows in that center leg that isn’t soldered. The stacked ones show the center leg bent down. Wink

I hear ya, whenever possible I used to do all the stacking on the inside. Seems like I had 22 chips on a board before. Chips everywhere. No Coke, Pepsi….

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My boards were delivered today. You recommend 22.1 for R1?

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

Major Projects:  Illuminated Tailcap, TripleDown/TripleStack Driver

wight
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pilotdog68 wrote:
My boards were delivered today. You recommend 22.1 for R1?
IIRC I’ve been recommending 22k (22,000).

Don’t forget that you’ll need to significantly drop the PWM freq for this thing to work correctly. The last test I did was at ~1khz I think. I switched the firmware to Phase Correct PWM and used the :8 divider as per post #51 in this thread.

The divider should look something like this:

     TCCR0B = 0x02; // pre-scaler for timer (1 => 1, 2 => 8, 3 => 64...)

Still fine, still on a break. One day I’ll catch up with you folks! previous wight catchup Wink
list of my drivers & variants (A17DD, FET+1 stuff, WIP stuff, etc)

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I’m sure it hasn’t been tested, but do you have any expectation for what parasitic drain might be if used as momentary?

My Favorite Modded Lights: X6R, S8 , X2R , M6, SP03

Major Projects:  Illuminated Tailcap, TripleDown/TripleStack Driver

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