The Texas Buck driver series, Q8 / Skyray King 2S/4S buck driver RELEASED!

467 posts / 0 new
Last post
Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

Yes to first approximation( a 7135 -- edit, I meant to say PWM, was confused -- might be useful for moonlight if the Buck becomes innefficient that low but that seems like possibly a version 2 feature if it ever even works).  Anyway the input to iadj says it comes from 7135.  I think its just a stale label. It seems to match up sensibly with an out from the mcu.

 

By the way I would consider 4s battery to 2s led.  Problem is it will require twice the current handling but it should regulate longer at high power as batteries get low.

DB Custom
DB Custom's picture
Offline
Last seen: 9 hours 10 min ago
Joined: 01/13/2013 - 22:28
Posts: 20245
Location: Heart of Texas

I know what you mean about it being to easy, the light is well made and it’s simple to make tweaks for ultimate performance.

The Nitecore TM03 on the other hand, is not that kind of light! I feel like I’ve accomplished a lot just de-soldering the massive 14ga leads from the thick copper heat sink/proprietary mcpcb so I can slice the XHP-70. Complex light, this one! (the kind that kinda scares me, lol)

Texas_Ace
Texas_Ace's picture
Offline
Last seen: 1 day 11 hours ago
Joined: 03/24/2016 - 07:44
Posts: 8499
Location: Everything is brighter in Texas
Flintrock wrote:

Yes to first approximation( a 7135 might be useful for moonlight if the Buck becomes innefficient that low but that seems like possibly a version 2 feature if it ever even works).  Anyway the input to iadj says it comes from 7135.  I think its just a stale label. It seems to match up sensibly with an out from the mcu.


 


By the way I would consider 4s battery to 2s led.  Problem is it will require twice the current handling but it should regulate longer at high power as batteries get low.

Yeah, that is just an old label from the TA drivers. It is simply a PWM output from the MCU, I can/will swap that around later if we determine the best pin to use.

The buck IC should have a dimming range down to lower then that of a 7135, so i doubt we will need to go with a 7135.

If we get this working it could run any combo of leds from any voltage source we use with a swapping of the passive components. The first one will most likely be for xhp35’s and/or xhp50’s for the upcoming Q8.

If it works with those it should work fine for any other combo.

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

I was confusing myself and you but I did have point under the confusion.  Of course 7135 is useless.  What mght be useful for moonlight is true LED PWM. The buck will certainly go very low, but at some very low level it might be more efficient to buck down to say .1A or whatever and PWM down from there.  It's not obvious to me though which is better.   It adds complication.  The FET wouldn't have to be so big and expensive though to handle the lower currents.

 

And, yes I'm with you on the running any combination of things.  Amperage is the only issue really, both in handling the amps and setting the correct maximum amps.  Aiming for a high current scenario from the start sounds good.

Texas_Ace
Texas_Ace's picture
Offline
Last seen: 1 day 11 hours ago
Joined: 03/24/2016 - 07:44
Posts: 8499
Location: Everything is brighter in Texas

It is possible to use a second pin from the MCU to PWM the buck IC as well if that was desired. It could be difficult to make it work though. I have a feeling this will need a 4 layer PCB but I will see. I suppose I could start laying it out and make minor changes as we select the parts list.

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

Selecting parts is a fun part, especially if you want to be pretty systematic about it.  

 

A little brushing up.  This a reasonable reference for some of the basics, primarily for duty factor, and   ripple vs frequency and inductance.

http://rohmfs.rohm.com/en/products/databook/applinote/ic/power/switching_regulator/inductor_calculation_appli-e.pdf

But I'll cliffs notes.  (I don't like how casually it treats many of the linear approximations and averages, but ok)

 

 

It derives a detailed form of the duty factor including voltages losses in switches and diodes, but then I like the Ti approach of just calling it

V0/(eff*Vin).

 

The ripple current vs frequency and inductance is useful (and maybe helpful for choosing the inductor size and switching frequency) but also it's included in equation 10 here, a better document anyway:

http://www.ti.com/lit/an/slva390/slva390.pdf

 

Useful to go through all that, but equation 11 summarizes it.  But better to look at 7 and 8 instead.  7 is the mosfet loss and 8 is the freewheel diode loss except they use a mosfet there too.  For the diode I guess you have to look at the loss as Vf*I instead of I^2R, simple enough change. So that's losses for both of those components.  

 

Oh and equation 4 gives the inductor loss, which is almost stupidly just I_out^2*R_L.  So that's easy to figure too.

 

However you get to equation 12 and they say  blah "+ other losses"

 

Oops, some nastiness there to figure out, especially switching losses, that looks fun:

http://www.allaboutcircuits.com/technical-articles/switching-losses-effects-on-semiconductors/

 

Oh, why was Ti using a Mosfet instead of a diode?:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/652

 

So there's that too, but I don't think it's compatible with your IC.

 

Then there is discontinuous current mode to worry about at very low power probably.

 

But that's all just introductory for basic bucks.  This 3409 looks pretty fancy (aside from using a diode I guess), with variable frequency, off time control etc.  I'm just starting to look at its actual docs.  I prefer their math actually.  They don't make the approximations so casually, but then they don't start with -LdI/dt either, so that's too bad, unless you only want answers.

 

Anyway, the obvious things are obvious, keep Rdson low for the switch, diode Vf low, inductance high, frequency high (but not too high, see that link about switching losses, last figure), capacitor ESR matters significantly less, and here already are a few simple guidelines to put numbers on some of it.  I think it's really not so bad.

 

 

 

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

Now this is what's bad (as you know):

 

11.1 Layout Guidelines The performance of any switching converter depends as much upon the layout of the PCB as the component selection. Following a few simple guidelines will maximimize noise rejection and minimize the generation of EMI within the circuit.

Discontinuous currents are the most likely to generate EMI, therefore take care when routing these paths. The main path for discontinuous current in the LM3409/09HV buck converter contains the input capacitor (CIN), the recirculating diode (D1), the P-channel MOSFET (Q1), and the sense resistor (RSNS). This loop should be kept as small as possible and the connections between all three components should be short and thick to minimize parasitic inductance. In particular, the switch node (where L1, D1 and Q1 connect) should be just large enough to connect the components without excessive heating from the current it carries.

 

The IADJ, COFF, CSN and CSP pins are all high-impedance control inputs which couple external noise easily, therefore the loops containing these high impedance nodes should be minimized. The most sensitive loop contains the sense resistor (RSNS) which should be placed as close as possible to the CSN and CSP pins to maximize noise rejection. The off-time capacitor (COFF) should be placed close to the COFF and GND pins for the same reason. Finally, if an external resistor (REXT) is used to bias the IADJ pin, it should be placed close to the IADJ and GND pins, also.

 

 

 

Texas_Ace
Texas_Ace's picture
Offline
Last seen: 1 day 11 hours ago
Joined: 03/24/2016 - 07:44
Posts: 8499
Location: Everything is brighter in Texas

I should say that while I have a good 101 understanding of parts selection, picking them from scratch is something I prefer not to do. I tend to get bogged down in the details looking for the “best” possible option.

I spent spent some time looking at parts earlier, I think this mosfet would work ok, it is basically the p-channel version of the Sir800

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=2085762...

If that works it makes things simpler since I already have the pads and models for it.

For the diode I found this http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=2085766...

Only rated for 10A though but I think me might need 2 of them anyways.

The biggest things I need right now are footprints, I started looking at a very basic layout and it is gonna be a tight fit even n a 46mm board, it might have to be a 4 layer PCB.

So without the footprints I can’t do much of the layout.

The real issue I am running into are inductors, it is hard to find them with current ratings of 10-15A and enough inductance to work properly + be small enough to actually fit.

The spreadsheet calculator says that with a .75mhz switching frequency it needs at least a 25uh inductor. It could go up more but mentioned that above 1mzh can be difficult in the data sheet. The smallest I am seeing in that size are around 15×15mm, thats a lot of board space. They are only rated for 9A as well.

DB Custom
DB Custom's picture
Offline
Last seen: 9 hours 10 min ago
Joined: 01/13/2013 - 22:28
Posts: 20245
Location: Heart of Texas

Humorous to me all the technical jargon coming from “Flintrock”. Yabba dabba Dooooooo! ;

(sorry, haven’t had coffee yet, a bit delirious from a good nights sleep)

Texas_Ace
Texas_Ace's picture
Offline
Last seen: 1 day 11 hours ago
Joined: 03/24/2016 - 07:44
Posts: 8499
Location: Everything is brighter in Texas
Flintrock wrote:

Now this is what’s bad (as you know):


 


11.1 Layout Guidelines The performance of any switching converter depends as much upon the layout of the PCB as the component selection. Following a few simple guidelines will maximimize noise rejection and minimize the generation of EMI within the circuit.


Discontinuous currents are the most likely to generate EMI, therefore take care when routing these paths. The main path for discontinuous current in the LM3409/09HV buck converter contains the input capacitor (CIN), the recirculating diode (D1), the P-channel MOSFET (Q1), and the sense resistor (RSNS). This loop should be kept as small as possible and the connections between all three components should be short and thick to minimize parasitic inductance. In particular, the switch node (where L1, D1 and Q1 connect) should be just large enough to connect the components without excessive heating from the current it carries.


 


The IADJ, COFF, CSN and CSP pins are all high-impedance control inputs which couple external noise easily, therefore the loops containing these high impedance nodes should be minimized. The most sensitive loop contains the sense resistor (RSNS) which should be placed as close as possible to the CSN and CSP pins to maximize noise rejection. The off-time capacitor (COFF) should be placed close to the COFF and GND pins for the same reason. Finally, if an external resistor (REXT) is used to bias the IADJ pin, it should be placed close to the IADJ and GND pins, also.


 


 


 

Yep, I paid close attention to that section and it is part of why board space is at a premium and I think that a 4 layer PCB might be a must. To lay it out properly will need a fair amount of board space in order to keep things separated.

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

Texas_Ace wrote:
I should say that while I have a good 101 understanding of parts selection, picking them from scratch is something I prefer not to do. I tend to get bogged down in the details looking for the "best" possible option. I spent spent some time looking at parts earlier, I think this mosfet would work ok, it is basically the p-channel version of the Sir800 http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=2085762... If that works it makes things simpler since I already have the pads and models for it.

For the diode I found this http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=2085766... Only rated for 10A though but I think me might need 2 of them anyways. The biggest things I need right now are footprints, I started looking at a very basic layout and it is gonna be a tight fit even n a 46mm board, it might have to be a 4 layer PCB. So without the footprints I can't do much of the layout. The real issue I am running into are inductors, it is hard to find them with current ratings of 10-15A and enough inductance to work properly + be small enough to actually fit. The spreadsheet calculator says that with a .75mhz switching frequency it needs at least a 25uh inductor. It could go up more but mentioned that above 1mzh can be difficult in the data sheet. The smallest I am seeing in that size are around 15x15mm, thats a lot of board space. They are only rated for 9A as well.

 

Yeah, in the end, it may be fun to see if we can estimate the losses, but there probably aren't that many choices of components really that come close, and you just pick the best from the ones that can fit, if any and hope for the best.  

 

I found a couple of small buck converters on amazon that looked pretty nice.  Maybe a couple were upward of 10 A.   They were on the right size scale, single board, but not the right shape.  Maybe it's worth seeing what they use?  Then again, they might not really handle what they say they handle, or not well. I doubt they use any magic component that you don't see on digikey.  Another possibility is your standards are too high.  Maybe you can squeeze by with less than you think?

 

DB Custom
DB Custom's picture
Offline
Last seen: 9 hours 10 min ago
Joined: 01/13/2013 - 22:28
Posts: 20245
Location: Heart of Texas

And you checked the Coilcraft line-up on the inductors? Arrow Industries is a good place to look as well as DigiKey and Mouser, probably not telling you anything…

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

WIthout trying hard (first click) I found a mosfet with lower capacitance, but higher Rdson.  I don't know, maybe you can push harder on switching frequencies with lower capacitance?  Might require running numbers to  see what's a good tradeoff.

 

"In practice, switching frequencies higher than 1MHz may be difficult to obtain due to gate drive limitations, high input voltage, and thermal considerations." 

 

Ooh, "thermal considerations" sounds like power loss to me.

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

DB Custom wrote:
Humorous to me all the technical jargon coming from "Flintrock". Yabba dabba Dooooooo! ; (sorry, haven't had coffee yet, a bit delirious from a good nights sleep)

 

Well a flintrock makes a little more light than a Flinstone at least.

Texas_Ace
Texas_Ace's picture
Offline
Last seen: 1 day 11 hours ago
Joined: 03/24/2016 - 07:44
Posts: 8499
Location: Everything is brighter in Texas
Flintrock wrote:

WIthout trying hard (first click) I found a mosfet with lower capacitance, but higher Rdson.  I don’t know, maybe you can push harder on switching frequencies with lower capacitance?  Might require running numbers to  see what’s a good tradeoff.


 


“In practice, switching frequencies higher than 1MHz may be difficult to obtain due to gate drive limitations, high input voltage, and thermal considerations.” 


 


Ooh, “thermal considerations” sounds like power loss to me.

Very possible, I was looking for low resistance when I searched as the calculator was showing something like 30w of power would be dissipated by the default MOSFET at our drive currents.

Thermal considerations are something to think about but I was more worried about the gate drive limitations myself. Thermal can be dealt with, gate drive can not.

Like I said, picking parts from scratch is NOT my strong suite, I am simply trying to find footprints at this point.

This is the closest inductor I could find to our specs: http://www.digikey.com/product-detail/en/bourns-inc/SRP1770TA-220M/SRP17...

Still only rated for 12A but I figure we could push it a bit to ~15A since the saturation current is 18A.

The issue is it is huge at 17mm, it is also pricey at 3.50 each.

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

Seriously though, maybe the first component to check is the driver IC.

 

http://www.ti.com/lit/an/slyt358/slyt358.pdf

 

"A synchronous converter with an integrated low-side MOSFET offers benefits such as reduced size, lower parts count, and easier design. "  

I don't know why.

 

 

Look at that figure 3. Those moonlight efficiencies in the synchronous bucks look a bunch better. Synchronous means the diode is replaced with a switch.  There seem to be drawbacks at high curent though and that's probably not a good trade.  I haven't had time to dig into it.

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

Texas_Ace wrote:
Very possible, I was looking for low resistance when I searched as the calculator was showing something like 30w of power would be dissipated by the default MOSFET at our drive currents. Thermal considerations are something to think about but I was more worried about the gate drive limitations myself. Thermal can be dealt with, gate drive can not. Like I said, picking parts from scratch is NOT my strong suite, I am simply trying to find footprints at this point. This is the closest inductor I could find to our specs: http://www.digikey.com/product-detail/en/bourns-inc/SRP1770TA-220M/SRP17... Still only rated for 12A but I figure we could push it a bit to ~15A since the saturation current is 18A. The issue is it is huge at 17mm, it is also pricey at 3.50 each.

 

I was just about to suggest searching on price, highest to lowest.  I'd aim this project at quality.  

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

I wonder if the tradeoff from low to high current is Vf*I  vs I^2*Rds   That would make sense.  At low current I^2Rds wins.  At high current, Vf*I wins.  So.. why not put a switch and a diode in parallel (other than space and price obviously), then you get both?  A smart controller would turn the switch off at high current too. 

Texas_Ace
Texas_Ace's picture
Offline
Last seen: 1 day 11 hours ago
Joined: 03/24/2016 - 07:44
Posts: 8499
Location: Everything is brighter in Texas
Flintrock wrote:

Seriously though, maybe the first component to check is the driver IC.


 


http://www.ti.com/lit/an/slyt358/slyt358.pdf


 


“A synchronous converter with an integrated low-side MOSFET offers benefits such as reduced size, lower parts count, and easier design. “  


I don’t know why.


 


 


Look at that figure 3. Those moonlight efficiencies in the synchronous bucks look a bunch better. Synchronous means the diode is replaced with a switch.  There seem to be drawbacks at high curent though and that’s probably not a good trade.  I haven’t had time to dig into it.

If you have a suggestion for a better buck IC I am all ears, like I said finding parts from scratch is not my strong suite.

Texas_Ace
Texas_Ace's picture
Offline
Last seen: 1 day 11 hours ago
Joined: 03/24/2016 - 07:44
Posts: 8499
Location: Everything is brighter in Texas

Here is a VERY early first draft of the PCB, just put the components onto a 46mm pcb to see how fitment looked. All the small components are 0603 and I am sure some will need to be enlarged so as you can see it fits but there is not a lot of room to spare once you start running traces.

Once we nail down at least the footprints for the component list I will start routing traces.

DB Custom
DB Custom's picture
Offline
Last seen: 9 hours 10 min ago
Joined: 01/13/2013 - 22:28
Posts: 20245
Location: Heart of Texas

Not sure what y’all are finding wrong with the Vishay SIR800DP but it’s proven to be extremely hardy, Richard used a single one of these to run 12 XHP-50’s at 34,000 lumens. Just sayin.

I’m using one 17mm FET driver with Zener mod to run 4 of the 9V MT-G2’s at 15,000 lumens. Also running 3 9V MT-G2’s in a BTU Shocker at around 10,000 lumens. Same driver, again, for 3 XHP-70’s at 14,455 lumens in another BTU Shocker. Pretty sure this MOSFET works, tried and true.

Texas_Ace
Texas_Ace's picture
Offline
Last seen: 1 day 11 hours ago
Joined: 03/24/2016 - 07:44
Posts: 8499
Location: Everything is brighter in Texas

The sir800 is great for a driver that switches the ground but you can’t use an n-channel mosfet to switch the positive side, that is where the P-channel comes into play.

The one I linked to above is the P-channel version of the sir800 because we know that one works good. It has really low resistance which is great but the capacitance is higher then some which lowers the max frequency that you can run the buck driver at. This is turn means you have to use larger parts.

It is a trade off both ways, just a matter of figuring out the best options.

Texas_Ace
Texas_Ace's picture
Offline
Last seen: 1 day 11 hours ago
Joined: 03/24/2016 - 07:44
Posts: 8499
Location: Everything is brighter in Texas

Ok, I could not leave it alone, first run at running traces and not tweaked at all but the circuit itself should be good.

Lots of copper in the ground pour, I tried to get it between most traces to minimize interference.

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

Texas_Ace wrote:
If you have a suggestion for a better buck IC I am all ears, like I said finding parts from scratch is not my strong suite.

 

Not yet, spare time is tightening up.  But a bunch of those thoughts came from Ti docs, so I guess they make something, but don't know the specs.  Given enough time, I'm sure I'll take an interest to look (and understand the high current trade offs). At the rate you're going it may be irrelevant, which is great.

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

http://intl-outdoor.com/ld4b-24a-17mm-buck-driver-3v16v-p-817.html

 

hard to see exactly what's going on there.  It's "only" 2.4A. But it seems the inductor is lofted over some of the components, and they made use of the hole in the middle.  That might even be a nice driver.

Texas_Ace
Texas_Ace's picture
Offline
Last seen: 1 day 11 hours ago
Joined: 03/24/2016 - 07:44
Posts: 8499
Location: Everything is brighter in Texas
Flintrock wrote:

http://intl-outdoor.com/ld4b-24a-17mm-buck-driver-3v16v-p-817.html


 


hard to see exactly what’s going on there.  It’s “only” 2.4A. But it seems the inductor is lofted over some of the components, and they made use of the hole in the middle.  That might even be a nice driver.

That is a common way of mounting the inductors but it adds to the height and also those are not shielded coils. Both of which would cause issues that I think would be better avoided on the 46mm version of the driver. Once we have this size working that is an option to consider for the smaller versions if they are deemed a good option.

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

I haven't had much time but I did get to review that synchronous buck article more closely and the related IC's.  It seems not so exciting.  I had the right idea with I^Vf vs I^2Rdson as the main losses to compare, but that didn't give the picture.   The main difference, and it makes the synchronous worse, not better( saw that wrong), seems to be in the afterthought there, that in discontinuous mode, the inductor still reverses flow through the low side FET, but not through the diode. So that dumps energy and makes at least a too simplistic synchronous buck bad in moonlight. 

 

At reasonably higher current, I^2Rdson is apparently a better deal than  IVf and the synchronous buck can win a bit.  I suspect at some even more higher current this flips the other way again but that may be irrelevant.

 

Anyway, maybe kind of obvious, but diode losses (or low side FET) matters twice as much when driving 1S as driving 2S just because you spend twice as much time freewheeling for the low voltage output.

 

I guess the synchronous issues will get worked out with smarter switching, but anyway, the IC's I found for that are also much more complicated to implement and appear to be quite a big larger.  So this lead nowhere new.

 

I found an in interesting article about the importance of the input capacitor and having it very close to the FET.  I guess the IC doc covers that though. 

Texas_Ace
Texas_Ace's picture
Offline
Last seen: 1 day 11 hours ago
Joined: 03/24/2016 - 07:44
Posts: 8499
Location: Everything is brighter in Texas

Right now I am only worried about high current performance, that is what we have always had issues doing with buck drivers. Low current is easy and can always be taken care of with a 7135 if needed.

When you say input cap, you are talking about input for the LED or for the IC? Not sure we would have room for an input cap large enough to make any difference for the LED.

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

 Not 7135, now you're following my mispeaks.  If a 7135 reduces current from the buck, then it reduces current out of the buck and the buck has to operate at low current, and we're current regulating it anyway.  You've only raised the output voltage slightly, still with the same current, and dumping the extra dVI into resistive loss while not improving anything.  

 

But I made the same slip on the last page.  What you mean to say is for lower current you do LED PWM.  That needs and extra FET, but not a big one because you start with low current.  This was brought up by a couple of people in the "more efficient driver" thread.

 

Anyway,  the  cap I was talking about was input to the FET, parallel to the source.  In principle that does nothing since it's hard tied to the source (and the batteries probably have pretty enourmous capacitance themselves) but this note mentioned stray inductance and needing this cap very close to the FET because of that.  The claimed effect was very impressive.  I lost track of the link.  I'll try to find it again.

 

I started (not completed yet) my own calculator based mostly on this:

http://powerelectronics.com/site-files/powerelectronics.com/files/archive/powerelectronics.com/mag/606PET25.pdf

 

A very practical summary of many of the basics.  It's probably redundant with your spreadsheet but I'll compare later.

Anyway started looking at inductors.  It seems to me there's quite a bit in 22mm * 22mm * 22uH.  My initial impression is that 22uH would be nice, but size is still a pretty big problem there.  It looks like you've started with a bit smaller footprint, which is probably sensible if it can work.  Going shielded certainly takes more space.  Aparently shielding above 200khz can be conductive shielding (thin metal, maybe tape) and I don't think you need that much space to get away from the fields.  The irony is by requiring shielding you force things well up over 200khz anyway likely by not being able to get as much inductance.  An open inductor with shielding tape, maybe extra added, at 300 to 500 khz might not be a bad way to go. 

 

Anyway, I've only just started looking, and just started seeing how actual numbers fit in the math.  Math aside I've seen 22uH actually used in a cheap 10W driver. So it's not an absurd number, maybe just absurd for 15A.  I've noticed though that some of these inductors, like Vishay can keep going well above their saturation current.  They just don't go as well, so things will get more wild and less efficient, but this is pushing up into turbo modes anyway, so so what.

 

12A (plus ripple) will run 2S 2P at 6A per diode.  That's already a BUNCH.  And it will run 4p at 3A per diode which is also plenty high really and you can still crank higher with some slop and heat. I've been aiming for 15 saturation so far, but it's probably higher than needed.  This is why I would buck a Q8 at 4s batt 2s LED.  It's the only way that makes sense to me.  Many factors to trade off though.   

 

Flintrock
Offline
Last seen: 1 year 8 months ago
Joined: 09/10/2016 - 20:29
Posts: 1544

 http://www.eetimes.com/document.asp?doc_id=1273212

That meets more with my intuitive understanding of input capacitance, basically saying it shouldn't matter for a perfect source.  I'll try to hunt down the other note.

 

But even my link in the post above, page 49, bottom, calculates a value for the input cap somehow without any reference to the source impedance or other source qualities.  That's a bit puzzling to me but similarly to the other reference that I presently can't find. 

Pages