# 6 amp LED Driver Project



## HarryN (May 27, 2009)

Hi, I usually either buy regulators as needed or use a resistor to knock down the current flow for LED use, but I thought it would be fun to try to actually build one from scratch. This seemed like as good of place to post this as any.

I have never actually built a working circuit that contained more than a battery, diode, resistor and LED, so this will be a real test for me.

Constant current drivers require a bit more finess, so I thought I would start with a constant voltage one instead. I realize that they are some very sophisticated ones out there (Alan B versions) - this is just a simple version as a learning experience and for use in a project light I am thinking about. There is nothing "intelligent" or programmable in this project - think on/ off - maybe some dimming.

I am targeting a fairly wide voltage input range (about 5 - 30 volts) and approx 4 amps or so output at 3.3 volts. This is in the range where you can just about "cheat" and run a power LED direct drive. :naughty:

There are an amazing number of companies that offer controller chips - so it is almost a daunting task to pick from them all. I looked mostly at IRC (irf.com), TI, ON Semi, and National. They all have advantages, and I mostly wanted very simple and few parts, so I ended up at National. I don't know if it is the "best" decision, just a decision.

The main values their product brought to the table are
- 4.5 V to 42 volt input
- 5 amp output
- adjustable voltage output
- Built in mosfet (the on / off large power transistor)
- Very few parts needed to build (at least in theory)
- A very strong web design center that does all of the calculations and part selection for you.


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## HarryN (May 27, 2009)

*Re: Hobby voltage regulator*

I attended an LED lighting seminar in Sunnyvale CA recently and National gave a demosntration of how to use their on-line tools. I put in the values I was after and it recommended a design based on the "LM22679TJ-5.0", so I picked that one. The link is here http://www.national.com/pf/LM/LM22679.html

I assume they know what they are doing so I didn't try to guess against the web site.

I tried then to buy some of these parts:
- National - out of stock - 16 week lead time
- Future - on order
- Newark - non stock item
- Digikey - not in stock

I guess the part is either "too new" or something ? Suggestions ?


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## HarryN (May 27, 2009)

*Re: Hobby voltage regulator*

Inductors Question

The National Web Bench design software recommends an inductor / coil with about a 5 mH inductance. I was curious, with resistors and capacitors, you can just add them up to get the right resistance (yes, I know the parallel rule). Can you more or less do this with inductors as well ? The reason I ask, is that it would be handy to split the inductor size in 1/2 so they would be shorter.

Any idea if this works or not ?

Thanks

Harry


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## Mr Happy (May 27, 2009)

*Re: Hobby voltage regulator*

In theory, you can put inductors in series and the values add up. In practice, there is so much more to the specification of inductors than just the inductance value that this is unlikely to work very well. You are much better off finding a recommended part of the right specification and using that.


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## HarryN (May 27, 2009)

*Re: Hobby voltage regulator*

Hi Mr. Happy - Thank you for that information. I guess for some reason, I had intuitively thought that inductors in parallel would add up to a single larger inductor - I guess not.

I found more on the part - you can get them in the UK via Farnell. Here is the info from the Newark web site:

Farnell UK Direct Ship - Non-Cancelable/Non-Returnable
Handling Fee $20 per order
(Delivery within 7 business days) 

Basically, to get one part is $ 10 for the part + $ 20 more for shipping and handling and I am still just getting started. Now I know how the guys in the EU normally feel about parts costs.

That internal mosfet feature is a dang expensive feature.


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## Mr Happy (May 27, 2009)

*Re: Hobby voltage regulator*



HarryN said:


> Hi Mr. Happy - Thank you for that information. I guess for some reason, I had intuitively thought that inductors in parallel would add up to a single larger inductor - I guess not.


Here's a rule of thumb to help with the intuition:

Devices that work on voltage should go in parallel so they all see the same voltage.

Devices that work on current should go in series so they all see the same current.

Capacitors resist changes in voltage, so for more capacitance you put them in parallel.

Inductors resist changes in current, so for more inductance you put them in series.

Resistors resist the flow of current, so for more resistance you put them in series.


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## HarryN (May 28, 2009)

*Re: Hobby voltage regulator*

Thanks Mr. Happy - that is a good rule of thumb.


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## Steve K (May 28, 2009)

*Re: Hobby voltage regulator*



Mr Happy said:


> In theory, you can put inductors in series and the values add up. In practice, there is so much more to the specification of inductors than just the inductance value that this is unlikely to work very well. You are much better off finding a recommended part of the right specification and using that.



I have to admit that I've never considered using two power inductors instead of one. My guess is that it could be made to work, but wouldn't be ideal. The biggest risk might be that the flux from one coil might couple into the other coil in the wrong direction, and effectively reduce the total inductance. 

Other than that... I'd worry most about the current that the inductor is rated for (don't want to saturate the core), winding resistance, core losses, and the frequency that the inductor is rated for. I'd bet that the inductor manufacturer might have some useful application info on their website that would help the user understand the various qualities of a inductor, and how the user should select the appropriate inductor.

best wishes,
Steve K.


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## johnkan (May 28, 2009)

*Re: Hobby voltage regulator*

I think by using one inductor instead of two should make the switching regulator more stable too.


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## HarryN (May 28, 2009)

*Re: Hobby voltage regulator*

Hi - Any comments on the actual National switcher part ? Does this seem like a good selection or am I missing something in the big picture here ?


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## rmteo (May 28, 2009)

*Re: Hobby voltage regulator*

LED's are current driven devices. If your regulator puts out 3.3V, it will barely light up a high-power white LED as their forward voltage is typically in the 3.6-3.7V range. Look into CC - constant current drivers.


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## HarryN (May 28, 2009)

*Re: Hobby voltage regulator*



rmteo said:


> LED's are current driven devices. If your regulator puts out 3.3V, it will barely light up a high-power white LED as their forward voltage is typically in the 3.6-3.7V range. Look into CC - constant current drivers.



Hi - Thanks for the CC comment. I agree that CC drivers are ideal in general and I already own / have used a variety from taskled, the shoppe and leddynamics. They all work fine for what they are designed to do.

I would not pretend to even attempt a real CC driver as the next step in playing with electronics. I have watched experienced EEs with lots of diagnostic gear scratch their heads over CC circuits and fancy dimming methods. That is way beyond what I am trying to do.

This design is actually variable output voltage so I can tune to meet the needs of the LED (s) within my happiness level. It will certainly be no worse than my normal route of batteries + resistor (which works fine for me in general). 

I am not too worried about driving the LEDs at max current rating - the light for this project is planned for "bright" but mostly longer running time than my other lights.


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## Mr Happy (May 28, 2009)

*Re: Hobby voltage regulator*



HarryN said:


> It will certainly be no worse than my normal route of batteries + resistor (which works fine for me in general).


Actually it could be worse than a resistor. A resistor in series with batteries approximates a constant current supply. Whereas a voltage regulator is a true constant voltage supply. One reason constant voltage is not good for LEDs is that as they warm up they tend to draw more current. Which naturally makes them warmer still, and in the limit you can get a thermal runaway that destroys the LED.

A regulated current supply is not necessarily a whole lot different from a regulated voltage supply, though. To regulate current you can pass the current through a low value resistor, measure the voltage drop, and regulate that voltage.


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## zipplet (May 29, 2009)

*Re: Hobby voltage regulator*

I built a constant current regulator of that design (linear) to charge a 12V SLA. It is current limited to 300ma but also tops out at 13.8V to nicely float the battery. Such a design could be adapted to drive a high flux LED easily. I'd be willing to provide information on how to do this if useful to the OP. There is a lot of information on the internet about building simple regulators like that


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## HarryN (May 29, 2009)

*Re: Hobby voltage regulator*

Hi, Thanks for the offers about constant current supply designs. I really appreciate the offers for constant current designs but perhaps a bit later in my skill level development. 

I probably should tell you a little more about what I am up to and that will make it more clear why I chose this design path.

I am working on a home built light with at least 2 K2s and maybe some other LEDs in it. Since the various LEDs have different current demands, I cannot really power it with just one constant current supply. Instead, I am planning to use a constant voltage supply (approx 3 - 4 volts ) and very lightly resistor as needed for each LED.

It is relatively easy to set this up to dim using a rotary switch, which I much prefer over push button UI based setups. I could not find any CC supplies which:
- Fit inside this light body
- Had dimming without a push button UI
- Had a whole bunch of software code to drive them
- Could deal with the current demands without being massive

If this works out (which is of course questionable) then I can also integrate some more parts onto the board, making the overall light easier to build.

I did not set out to build this thing, I just could not find a driver that was going to work for this project.

If it is ok, I would like to ask everyone to help me focus on building the actual voltage regulator, not get into a discussion of the benefits of PWM vs other dimming methods or which LED is the "best".

I will see if I can find a way to post the output of National's web output.

Thanks

Harry


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## ctom (May 29, 2009)

*Re: Hobby voltage regulator*

I just checked Digikey's website and they do have them in stock:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=2556570&keywords=LM22679


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## HarryN (May 31, 2009)

*Re: Hobby voltage regulator*



ctom said:


> I just checked Digikey's website and they do have them in stock:
> 
> http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=2556570&keywords=LM22679



Thank you very much - I am not sure how I missed that one. Maybe I over specified or something.

I have had some boards fabricated by advanced circuits before ( www.4pcb.com) so I was playing with their cad software - trying to learn I guess. The last time I did a board, I designed in in excel draw, then Georges80 ( taskled.com ) helped me make it into a real gerber files board. I had hoped to avoid the two step process, but after several hours on their cad program, I am still not very far.

It would help if they actually had more parts or packages in their library. AFAIK, they don't even have Cree or Lumileds LEDs as a standard part. It also does not seem to run that well in Vista.


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## BillyNoMates (Jun 1, 2009)

*Re: Hobby voltage regulator*

Have you tried Pad2Pad (http://www.pad2pad.com/) for board layout?

I have used these guys before and found the process quite straight forward. They provide their own free software which is very easy to pick up and use - no need to mess around with Gerber files. Once your design is done, just submit it electronically and you're done......


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## HarryN (Jun 1, 2009)

*Re: Hobby voltage regulator*

Hi Billy - Thanks for the link to pad2pad. Do you happen to know where they make their boards ? I sort of ended up at advanced circuits because the boards were fabricated in North America vs China.

I have not used them (only did one board in my life and that was with 4pcb. Both firms offer similar setups - free software that ties you to them, which I guess is ok. 

I watched the video at pad2pad which actually helped me with the s/w at 4pcb, so that was helpful. I have kind of a strange shaped board, so that is atually a challenge for both firms, but I can work through that.

I am still debating about using a "new program" vs just drawing in in excel and doing a fully "home made" board. I am not too thrilled with having to learn how to draw up my own components, esp. for something as common as a K2 or a stock National Semi part.

BTW - I would like to put add in a way to help reduce the likelihood of "voltage spikes" on Vin. I think there are some parts that short these to ground? 

Any suggestions on where to find a part that will short to ground spikes over about 40 volts ?

Thanks

Harry


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## BillyNoMates (Jun 1, 2009)

*Re: Hobby voltage regulator*



HarryN said:


> Hi Billy - Thanks for the link to pad2pad. Do you happen to know where they make their boards ? I sort of ended up at advanced circuits because the boards were fabricated in North America vs China.


 
I'm based in UK and the boards I did were shipped from USA. I now use PCBPool in Ireland which works out slightly cheaper for me.

I'm pretty sure that Pad2Pad offer a routing service to get your board any shape you want.

I know it is a pain learn new software and to do all the component footprints from scratch, but it will be worth it - especically if your board gets quite complicated. These schematic/layout tools can cross check which can help avoid any embarrassing errors.......


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## ctom (Jun 2, 2009)

*Re: Hobby voltage regulator*

Since you are in the Bay Area, there are a lot of fab houses locally. I've used PCBFabExpress (www.pcbfabexpress.com) with good results as well as Sierra Proto Express (www.protoexpress.com). Both vendors are located in Sunnyvale and require Gerber files from programs like Eagle, Protel, Orcad, etc.

For hobbyist applications, Express PCB (www.expresspcb.com) will make three 3.8" X 2.5" PCB (2 sides, no soldermask or silkscreening) for $51. You have to use their proprietary CAD software but it is easy to use and creating footprints for K2 leds are straightforward. If you don't mind 3 to 4 week turnaround times, Sparkfun/BatchPCB (www.batchpcb.com) will make 2 sided PCBs with soldermasking and silkscreening for $2.50 per square inch. The boards come out of China and they require Gerber files.

If you plan on mounting the K2s on the PCB, you need to pay particular attention to thermal issues. K2s running at their max. rated current generate a lot of heat so large copper planes, soldered filled vias (use lots of small vias filled with solder to conduct heat to the backside of the PCB), thermal compound/epoxy (Arctic Silver, etc.), and heatsinks are in order!


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## HarryN (Jun 2, 2009)

*Re: Hobby voltage regulator*

Hi, Thanks for the additional information.

I have been looking around at the board design software options. While I can design this thing with draw functions in excel much faster (probably 1 day max) it is probably useful to learn a pcb software package if it does not take too long. 

If I am going to go through the bother of learning a s/w program, then getting tied to one pcb company is not that useful either, which is where wikipedia came in handy. (again). Probably you already know this, but there are some open source and free ware versions out there. (eagle lite, kicad, etc) I suppose learning one is not that much different than learning another, and then I actually obtain the gerber files, etc in case I actually want something made in more volume. (doubt it, but who knows)

The nice thing is if I can find the parts already in the library - such as a TO-263 package that this National part comes in - it does not appear that pcb artist had it, so that means I need to not only learn the basics, I would have to draw my own parts. I am hoping that products like eagle, etc have a large enough library to allow me to just cut and paste.

Thanks for the comments on thermal path - I have a pretty good method worked out for that from building up the " Breeze" light in my sig line. I am planning to build the 2 (or 3) LEDs into the same board, so it is pretty compact.

I have already worked out the layout I need with manual placement (graph paper) - it sure would be nice if I could just find someone that already knows how to work this s/w and just do some kind of pay or trade with them to teach me or have them do it.

Take care,

Harry


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## BillyNoMates (Jun 3, 2009)

*Re: Hobby voltage regulator*



HarryN said:


> Hi, Thanks for the additional information.
> 
> I have already worked out the layout I need with manual placement (graph paper) - it sure would be nice if I could just find someone that already knows how to work this s/w and just do some kind of pay or trade with them to teach me or have them do it.
> 
> ...


 

I may be able to help with some of the things you are looking to do - I've sent you a PM with more details in.


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## HarryN (Jun 3, 2009)

*Re: Hobby voltage regulator*

Hi, Thanks for the PM and the email conversation.

Harry


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## HarryN (Jun 25, 2009)

*Re: Hobby voltage regulator*

Hi, Thanks to BillyNoMates, good progess is being made, and just as importantly, I am learning a lot. He probably is sweating that his name is associated with my non standard approach to electronics, but he is being a kind and patient mentor.

We ended up making some significant changes in part selection to make it all fit, so we'll see how that goes.

The next step is to add reverse voltage protection. The board can be fed from 3 different sources ( 1 of 2 battery packs, as well as 12 VDC). I want to make sure that the higher voltage sources do not charge up the lower voltage cells by accident.

I am estimating that I need a 3 amp diode or eq. for each protected supply side. Are diodes the only way to obtain this kind of protection, or is there a lower Vf / smaller part method to obtain similar results? I cannot think of a way to electrically drive a FET to do this, but perhaps someone knows.

Thanks

Harry


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## HarryN (Jul 2, 2009)

*Re: 0402 size components*

Hi, well the design has evolved and is now solidly above 5 amps now, and current control might be possible with a feature trade off (dimming is harder)

One of the more interesting things is that to make it all fit, it will need some 0402 size parts. I wonder if these are a viable size for hand assy?

I have been told (and saw a video) showing some SMT resistors sort of "self align" during a hot plate re-flow process. Is anyone using a hot plate re-flow with SMT parts like this ?

Thanks

HarryN


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## Al Combs (Jul 3, 2009)

*Re: Hobby voltage regulator*

This might not be of much help to you. I don't know how far along in the process of making your own boards you are. But since you were considering just a voltage regulator instead of a constant current regulator anyway, here is an interesting hybrid from TI, PTR08060W. It has the inductor already built onto a tiny board they say is roughly the size of a TO-220. You only need to get two low-ESR capacitors for V-in and V-out. I like the way their site has purchase links half way down the page to Arrow, DigiKey and Mouser that show available inventory. They cost under 10 bucks.

It has an input range of 4.5 to 14 volts. And the output is from 0.6 to 5.5 volts. Just right for an LED. Well just right for 1 LED anyway. If you had a couple of LED's you'd have to wire them in parallel. Which is not really recommended. But if you weren't driving them to full power, even parallel would be OK. There is a 10 amp module in the series with a link on the same page.


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## HarryN (Jul 4, 2009)

*Re: Hobby voltage regulator*

Hi Al - Thank you for the information and for posting that here. I reallly appreciate the advice and guidance.

I looked at the data sheet in some detail - they did a nice job on that part, although the marketing description is a bit different than the actual data sheet. It still requires both an input and output capacitor, not just a set resistor, so building a board is still almost a requirement. It also has a fairly narrow V input range, and I really want this driver to handle from 2 - 4 cells, not just 2 - 3 cells.

I did find another controller chip which is a synch type, so I am in better shape than before. 

I will try to post some more info as it get further along.

Did you happen to notice my question about diodes vs using a FET to prevent reverse current?

Thanks

HarryN


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## mpf (Jul 13, 2009)

*Re: Hobby voltage regulator*



HarryN said:


> Did you happen to notice my question about diodes vs using a FET to prevent reverse current?



take a look at http://www.maxim-ic.com/appnotes.cfm/an_pk/636
and http://electronicdesign.com/Articles/ArticleID/9945/9945.html
or just goole "fet reverse polarity protection"


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## HarryN (Jul 13, 2009)

Hi Mathew, Thank you for the reply and links. I also read through your light builds in your sig line - nice work.

The solutions I have found so far, similar to your suggestions, are for the case where the goal is to protect the circuit from putting in the cells backwards in a 1 - 4 cells in series setup.

This case is a little different, at least I think so. In this project, there will be at least 2 battery packs in parallel, typically ranging from 2S Li Ion to 4S Li Ion in voltage. There is no ability for the designer to decide or control this selection, nor to really know the discharge state of the cells.

Each pack can be activated to "on" with a mechanical switch, but the current design does not easily allow only 1 pack to be exclusively selected. If the end user were to turn on 2 packs in parallel, then the real risk is that the higher voltage pack will start reverse charging into the lower voltage pack. This would clearly be a disaster.

If I use a diode for each pack, then this will protect each pack from being reverse changed by a higher voltage supply, but it will represent the most wasteful part of the entire circuit. (even with schottky's, at least 0.4V x 3 amps) If I used the FET based solutions suggested for single pack use, then either or both battery packs will charge the FET to open and I think allow completely unprotected reverse charging.

I am very open to suggestions about methods to reduce risk or improve the circuit protection. It follows standard synchronous buck designs.


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## bshanahan14rulz (Jul 13, 2009)

*Re: 0402 size components*



HarryN said:


> I have been told (and saw a video) showing some SMT resistors sort of "self align" during a hot plate re-flow process. Is anyone using a hot plate re-flow with SMT parts like this ?



If your solder pads are accurate (I.E. not obscenely crooked or something crazy) then yes, your parts should align themselves. I have experienced this when soldering xp-e to an 8mm board. It was so hard to get straight that I just gave up and started soldering anyways, and the darned thing just adjusted itself perfectly! but I was using a 25W radiohut iron to heat up the mcpcb. 

Subscribed, awesome info, can't wait to see what you come up with. 

And happy, thanks for that condensed rule of thumbs regarding voltage and current devices in series and/or parallel. A nice little treat that comes along with bookmarking this thread =)


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## Mr Happy (Jul 13, 2009)

HarryN said:


> Each pack can be activated to "on" with a mechanical switch, but the current design does not easily allow only 1 pack to be exclusively selected. If the end user were to turn on 2 packs in parallel, then the real risk is that the higher voltage pack will start reverse charging into the lower voltage pack. This would clearly be a disaster.


Just a note on terminology here: in this case the lower voltage pack would not be reverse charged, it would just be charged in the normal direction. While this could potentially be a problem due to uncontrolled charging current, it would not necessarily be a disaster.


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## HarryN (Jul 13, 2009)

Hi Mr. Happy - Thank you for the terminology correction.

I guess I am thinking that if 2 x 18650s that are largely charged, are then further "charged" by 4 x 123s, that would be a significant problem. At a minimum, I am assuming substantial cell damage.

In any event, is a diode on each "pack" my only real option here?


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## Mr Happy (Jul 13, 2009)

HarryN said:


> Hi Mr. Happy - Thank you for the terminology correction.
> 
> I guess I am thinking that if 2 x 18650s that are largely charged, are then further "charged" by 4 x 123s, that would be a significant problem. At a minimum, I am assuming substantial cell damage.
> 
> In any event, is a diode on each "pack" my only real option here?


Yes, if you have parallel batteries of different voltages, then letting them be connected is going to be a significant problem.

I suspect a diode on each pack is your only realistic option. A MOSFET by itself cannot be used as a diode replacement except in certain very special cases. The trouble is the required gate voltage needed to fully turn the MOSFET on and off. In many configurations this gate voltage is not naturally available without some extra circuitry to generate it and control it.


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## Nos (Jul 14, 2009)

im sorry for bringing something into play while this thread has just started. But what about a 9A driver for the sst-90 series.

I think once these will be selling in the marketplace there will be a huge public demand for this kind of driver


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## mpf (Jul 15, 2009)

HarryN said:


> This case is a little different, at least I think so. In this project, there will be at least 2 battery packs in parallel, typically ranging from 2S Li Ion to 4S Li Ion in voltage.



LTC4412HV http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1037,C1134,P2476
seems to be just what you want, see Figure 5 in the data sheet.


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## HarryN (Jul 15, 2009)

mpf said:


> LTC4412HV http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1037,C1134,P2476
> seems to be just what you want, see Figure 5 in the data sheet.



Hi - Thank you for the suggestion. I will see if it can be used in this application. (if there is room)


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## HarryN (Jul 15, 2009)

Nos said:


> im sorry for bringing something into play while this thread has just started. But what about a 9A driver for the sst-90 series.
> 
> I think once these will be selling in the marketplace there will be a huge public demand for this kind of driver



Hi, yes I have seen the SST-90s, and it would be nice to have a driver like that. 9 amps x 3ish volts = 27+ watts. IMHO, that is possibly even over the line for hand held, but there probably is a way to do it. I am busy enough with this project right now, and you can see it is already taking inputs from solid EEs to help pull me through it. Of course, everyone likes multiple LEDs in a light, so maybe we really need a 20 or 30 amp driver. Sounds like a good project for georges80 / taskled.com.

This particular driver board is being designed specifically for my "Project Tornado" light, so it is not entirely generic. I figured that maybe someone might want this board for other hobby projects as well, so I am trying to accomodate them in the design. The only way that it all sort of fits on one board in the design goal size (20 x 40mm) is to use pretty small parts and actually "cheat" a bit. I am hoping it can also be used in some 3 cell generic light mods by end users, but we will see.

As a practical matter, since the controller chip max is 6 amps, I probably should re-name the thread 5 amp LED Driver to be on the safe side when end users ignore the power ratings.


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## mpf (Jul 18, 2009)

HarryN said:


> ... 9 amps x 3ish volts = 27+ watts. IMHO, that is possibly even over the line for hand held, but there probably is a way to do it. ...


I have a 45W hand held and currently working on a 37W hand held. Its all in the heatsinking.


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## HarryN (Jul 18, 2009)

mpf said:


> LTC4412HV http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1037,C1134,P2476
> seems to be just what you want, see Figure 5 in the data sheet.




Hi Mathew, thank you for the suggestions and comments. You have built some impressive projects. Maybe you would like to take on the SST-90 driver project. 

I looked at the Linear data sheet and have more questions. I guess that power supply protections like this are referred to as OR’ing?

Perhaps a bit more about my project details would help.

In the light I am working on, it will have 2 "packs" in a side x side configuration. 
- Pack “A” will most often be 2S 18650, and be the primary / normal source for power. It is possible that this one could also be 4 x R123 or 4 x primary CR123.
- Pack “B” will be more of a "back up" source. It could be 2 x 18650 as well, but I think more often, it will be 4 x CR123 primaries. 
- Pack “C” (could drop this if I had to / does not fit) is a plug to a 12 v auto supply.

Each "pack" has its own advantages and limits:
- Pack A - normal use pack with low operating cost R 2 x 18650 with approx 15 watt hours of power, but with self discharge and commercial flight limitations.
- Pack B - back up use with long shelf life, relatively common cells, but higher operating cost and similar power
- Pack C - Unclear how often I would use this, but sometimes this is handy and more or less unlimited power

If we take pack “C” out of the equation for now, then my most often used pack will be the lower voltage pack. (A)

I read through the literature for the Linear part you suggested (Thank you very much for that). If I correctly understand it, it makes the assumption that when you switch from battery mode to aux A/C adapter mode, you will naturally want to switch to the higher voltage source, which will be less sensitive to losses from the in-line reverse protection diode. This makes perfect sense.

The switch from source “A” to “B” for that device is based on the Vsense pin detecting that the voltage is higher in one than the other, and it will pull from the higher voltage source, no matter what, which is probably not that often for this light. That is a good trick though.

I am starting to think that perhaps I should be focusing on a ways to just select / switch between voltage sources and not worry about a diode protection. Is that a beginner mistake?

I am starting to think I should use an SMT FET of some kind that comes full on at 5 volts, but still can accept up to 20 volts on the gate. Some of the FET specs are still unclear to me, as they seem to have the equivalent of a Vf which is actually higher than a diode - something like 1 - 2 volts. I would like to be able to activate the FET with the same Vbat as the rest of the system, but perhaps this is not possible.


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## mpf (Jul 18, 2009)

HarryN said:


> ...
> I am starting to think that perhaps I should be focusing on a ways to just select / switch between voltage sources
> I am starting to think I should use an SMT FET of some kind that comes full on at 5 volts, but still can accept up to 20 volts on the gate..



Look at fig 4 in the LTC4412HV for controlled way to switch between supplies. Also check out irf7328, dual P channel 20VGS max 30VDS max
It is speced at 4.5V drive (thats what you should look for is you want to drive it with 5V)
At 4.5V fig 13 show the on resistance is 0.025 at 10A that is at 6A the voltage drop will be 0.15V over each fet or 0.3V for the pair.
The other figures to look at are fig 1 and 2 which show the voltage drop versus current for given gate drives.

The problem with high current low voltage led systems is that the any resistance in the circuit has a marked effect percent wise, on available voltage. e.g 27W torch using 4.5V is 6A and the drop across two fets is 0.3V/4.5V = 6.7% loss, on the other hand 27W torch using 14.4V (typical of my torches) is 1.67A and the drop across the two fets is now only 2 x 0.025ohms x 1.67A = 0.083V or 0.083V/14.4V = 0.58% loss

So lower battery voltages get you both ways, the current goes up so the voltage drops go up AND the voltage drops represent a larger % loss


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## HarryN (Jul 19, 2009)

Thank you for the pointers and tips. I really appreciate the help and advice.

That IRF part looks pretty interesting. I have not seen anything yet with lower "on" resistance. I had assumed that IRC would have made a smaller package (bottom pads only) version of that part, but so far I have not seen it.

You are right about the challenges of working with lower voltage to drive the light. I considered using 4S 18650 as the "main" and 8S 123 as the secondary, but this had its own complications on part selection.

I will play some more with the design and see what can be made to fit on the board.


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## mpf (Jul 20, 2009)

Looking over the posts again I noticed that 6A x 0.3v for the dual fets is 1.8W, the package is rated at 1.3W at 70C. However you could split the dissipation between two packages so that the paired fets (back to back) are actually one from on package and one from the other package. The left over fet each package would be paired to form the back to back fets for then second battery supply. This would give you 0.9W per package, well within the ratings.


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## HarryN (Jul 20, 2009)

Hi Mathew - Thank you for pointing that out.

Sorry for these silly questions / comments, but they have been nagging me. 

a) If I were to use a diode to prevent reverse current, the lowest Vf I have found is approx. 0.3 - 0.4 V Vf silicon Schottky types. Even then, I would still need a way to switch from one battery pack to the other, either mechanically or electrically. This Vf loss is a lot as you point out - something approaching 2 watts of a 20 watt driver.

b) The GaAs and SiC type diodes seem to be more RF or higher current capable, but actually had higher Vfs than the Si type.

c) The Si MOSFETs are more or less a solid state switch. When the gate is "off", then it acts as a diode, but still seems to allow for some reverse current, or is it really zero if V bat is less than the 20 V rating?

When the gate is "on", it still has the Vf of a Si diode plus the resistance (Rds) ? Is that right? For some reason, I though a FET in the "on" position had more or less zero Vf, but I think I am confused on this point.

Thanks

HarryN


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## HarryN (Jul 20, 2009)

I just realized that you had calculated the voltage drop just using the Rds of the FET at 6 amps, so I guess it only has this resistance loss and not the Vf loss in addition.

From 2 LiIons, even pretty drained, the Vbat will be at least 5 volts, so the current through those FET packages will be more like 4 amps, not 6. That puts the power in the range of the package I think. I have some extra heat dissipation methods available on this board anyway, so removing a little more might not be that problematic. 

It is amazing how hard it is to make an efficient driver, especially when you start adding on a few protection aspects. Even using the best parts I can find, I am not sure that this will hit 85% efficiency across the whole voltage range.


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## mpf (Jul 21, 2009)

HarryN said:


> so I guess it only has this resistance loss and not the Vf loss in addition.


Correct. Also diodes don't usually have Vf of 0.3V at 6A, check the curves and you will find most are more like 0.55 to 0.65V


HarryN said:


> That puts the power in the range of the package I think.


My suggestion was to spit the pair of fets between the two dual fet packages so at any one time only one fet in each package is passing current and the power dissipation is half per package. When you switch to the other battery pack, the other half of each dual fet package is used, again splitting the dissipation between the packages. Using this approach you will not need extra heatsinking.


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## mpf (Jul 21, 2009)

HarryN said:


> c) The Si MOSFETs are more or less a solid state switch. When the gate is "off", then it acts as a diode, but still seems to allow for some reverse current, or is it really zero if V bat is less than the 20 V rating?
> 
> When the gate is "on", it still has the Vf of a Si diode plus the resistance (Rds) ? Is that right? For some reason, I though a FET in the "on" position had more or less zero Vf, but I think I am confused on this point.


Think of a FET as voltage controlled resistor in parallel with a zener diode.

When the Fet is 'off' in this case for the dual P-fets, off is 0V gate to source, the current though the fet is 25uA with 25V across the Drain to source. That is very much off. However due to the way the fets are constructed there is a parasitic diode in the reverse direction. You can see it drawn in as a zener in the diagram at the top of the spec. The zener part is the Drain to Source breakdown voltage. But in the other direction it looks like a normal diode and if you put +V on the Drain and -V on the Source the Fet will conduct just like a diode with a voltage drop of 1.2V (for this part).
This diode is why you need a pair of back to back fet to isolate the battery packs. With just one fet, when the lower battery voltage was turned off the high battery would feed current to the lower battery voltage via the diode. With the back to back pair there is always atleast one diode blocking regardless of which battery has the higher voltage.

The trick is how these back to back fets ever turn on. Look at bottom pair of fet in fig 3 of LTC4412HV the Sources are tied together in the middle. It is the Vgs that turns the fet on so when the gate is pulled down both the fets turn on. The source voltage Vs is supplied from the battery via the fet diode on the left and is at least Vbattery - 1.2V. As the gate is pulled to gnd the Vgs goes negative and starts to turn both fets on and the Vs rises to battery voltage (less IxR drop) which increases the Vgs and turns the fet on harder.

The above is for P-fets, N-fet have the diode in the other direction and require the gate to go +Ve with respect to the Source in order to turn on. For both types of fets (N and P) 0V gate to source is OFF


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## HarryN (Jul 24, 2009)

Hi Mathew, thank you very much for all of the pointers and suggestions. Sorry for all of the elementary questions, but I am learning a lot here.

Of course, now that I have been buried in data sheets and searches on mosfets for the past week, I could use a double check here on what I am finding. 

I guess you must use that irf7328, dual P channel 20VGS max 30VDS max? I looked through many data sheets from siliconix, irf, on semi, diodes inc, and fairchild and could not beat the specs on it. The only one I wonder about is this one from On Semi MMSF7PO3HD. 

http://www.onsemi.com/PowerSolutions/product.do?id=MMSF7P03HD

The reason I am tempted by it, is that my input voltage sources will be physically coming into the board opposite from each other, so I was thinking of using 2 each of these, back to back to, for each input. I really cannot use the two mosfets in them efficiently by themselves, but I can now see why they double with 2 mosfets in one package.

I did not realize that N channel mosfets had such better characteristics - Rds seems to be 10 - 20%. I would like to ask a question related to the P vs N channel mosfets in this application, and their practical aspects of each.

Suppose we have the "typical application" of 2 x LiIon feeding V bat connection A, and 4 x Li Ion feeding Vbat connection B.

I want to turn on the light, so I activate a switch which feeds Vbat A to both the gate of the mosfet pair, and "through" them - going into the source in one case, and the drain in the other. Since there is no voltage on the gate of the other mosfet pair for Vbat B, then it will be completely shut off. This works as long as Vgs is equal to Vbat, and I stay with P channel mosfets - right?

In order to switch to using Vbat B, then the switch moves away from the gates of the Vbat A, and activates the 2 back to back mosfets in B - right?

Since the charge on the A mosfet pair needs to be discharged, I just connect a very high impedence resistor, say 10 mega ohms to it? I realize that the Linear part is more elegant, but there just is not room on the board, unless perhaps I use the smaller N channel mosfets.

The othe idea I wondered about is sort of wiring the gates in reverse. If the FETs turn on when the gate is at ground, then I just need hold a small charge on the gate of the "off state" FETs to keep them from conducting. Does this makes sense, or am I off the deep end here? I guess the downside of a FET that is only off when it is charged up would be the very tiny continuous drain on the battery through the resistor that drains away Vgs?

Guys, MPF does not need to be the only one who answers here. Feel free to chime in.

Thanks to anyone who is still following along here.

HarryN


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## mpf (Jul 25, 2009)

HarryN said:


> I guess you must use that irf7328, dual P channel 20VGS max 30VDS max? I looked through many data sheets from siliconix, irf, on semi, diodes inc, and fairchild and could not beat the specs on it. The only one I wonder about is this one from On Semi MMSF7PO3HD.
> 
> http://www.onsemi.com/PowerSolutions/product.do?id=MMSF7P03HD


These irf7328 has lower Ron but higher thermal resistance. Also irf7328 is a dual p-fet in one package. I would use it, provided the wattage is suitable.


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## mpf (Jul 25, 2009)

Here is a circuit that does what I think were describing. There is no drain on the battery that is off. R1 and R2 are about 100K or so and the fets are what ever you end up choosing.


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## mpf (Jul 25, 2009)

HarryN said:


> I guess the downside of a FET that is only off when it is charged up would be the very tiny continuous drain on the battery through the resistor that drains away Vgs?



Fets (both N and P) are always OFF when Vgs == 0V.


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## HarryN (Jul 25, 2009)

Hi Mathew - WOW - yes, that is exactly what I am referring to. Thank you very much.

Harry


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## HarryN (Jul 27, 2009)

Hi, just curious, does it make sense as well to consider doing this using N channel mosfets, such as the IRF6726M, or is there something inherent about N vs P that I should know?

https://ec.irf.com/v6/en/US/adirect...e&domSendTo=byID&domProductQueryName=IRF6726M

Thanks

Harry


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## mpf (Jul 28, 2009)

looks like a good one.
In the previous circuit, turn the batteries upside down, replace the P fets with these N fets and the GND becomes +V and Supply A and Supply B become GND A and GND B


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