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Breaktru Forum  |  eCigarette Forum  |  Modding  |  Topic: Fun with DC-DC converters
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Author Topic: Fun with DC-DC converters  (Read 6391 times)

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Offline CraigHB

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Fun with DC-DC converters
« on: July 23, 2014, 02:46:51 AM »
This is pretty cool, it's a simulation of a new design I'm working on using a Linear LTC3786 boost controller in a buck-boost configuration.  The simulation shows what this design can do when taken to the limit.  The chart shows output wattage.  That's at 8 Volts though the range is 2.5 to 8.0 Volts.  It can do the same thing at 2.5 Volts, but switching currents are ridiculously high, like 60 Amps.


Offline Breaktru

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Re: Fun with DC-DC converters
« Reply #1 on: July 23, 2014, 07:06:45 AM »
Absolutely cool.
That is an amazing output.

Can you explain how a 4.5V to 38V input voltage but can operate as low as 2.5V after start-up works?
Does this mean two series batteries are necessary?
« Last Edit: July 23, 2014, 07:12:20 AM by Breaktru »

Offline CraigHB

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Re: Fun with DC-DC converters
« Reply #2 on: July 23, 2014, 02:52:04 PM »
This is a single cell design, input voltage 2.7 to 4.2 Volts.  I use a charge pump to provide the power supply for the controller, this one specifically; http://www.ti.com/lit/ds/symlink/lm2665.pdf 

The LTC3786 uses a high side NMOS (in lieu of a PMOS).  Any of these converter controllers that use a high side NMOS require a supply voltage at minimum 4.5 Volts, independent of input voltage.  That's because they have to drive high side gate voltage at least that much higher than output to turn the high side NMOS on and off.  The controller incorporates a simple circuit to do that.  If you look at the data sheet for that part, you'll see a pin labeled boost with a diode and capacitor tied to it. 

A cool little hack is employed that uses the step voltage at the switches to generate the voltage required to drive the high side NMOS.  It's really quite ingenious and a recent innovation in converter controller design.  It complicates design with a high side NMOS, but there are considerable advantages there.

Charge pumps are really cool little devices, they just use a capacitor and a set of on-chip switches to increase or decrease input voltage.  The one I'm using is a simple doubler, but you can get ones that are regulated as well.  They have limited output so one couldn't power an atomizer or anything, but they're great for powering other electronics that need higher or lower voltages than what you have available from the power supply.

It's possible to use a small DC-DC converter in lieu of a charge pump, they can actually get quite small for low current outputs, but the charge pumps have a lower part count and lower pin count so they're better for minimizing footprint.  They're also a more simple device and easier to incorporate into a design.

It would be quite possible to power this circuit with dual cells instead of a single cell and get the same output results, but I use the single cell to make it a simple affair to incorporate a USB charger.  The down side is I need a charge pump to power the converter controller.  There is some advantage in using buck-boost with series cells, you don't run into that limitation in output voltage when the cells are depleted like you do with buck only.


« Last Edit: July 23, 2014, 03:21:55 PM by CraigHB »

Offline Breaktru

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Re: Fun with DC-DC converters
« Reply #3 on: July 23, 2014, 03:14:43 PM »
Ingenious indeed. Thanks buddy

Offline CraigHB

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Re: Fun with DC-DC converters
« Reply #4 on: July 23, 2014, 03:28:02 PM »
Well, Ill see how ingenious it is when I actually build the circuit.  I have the PCB design almost finished, not a complete controller board, just the converter.  I tried the same circuit already with a TPS43061 which is very similar to the LTC3786.  It worked well, but I had an issue with "dead time" that I was not able to resolve.  TI support had no help to offer me on that.   It's a showstopper since it causes a big efficiency hit.  I'm hoping the Linear controller does not have the same problem, just have to try it and see.

Offline Breaktru

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Re: Fun with DC-DC converters
« Reply #5 on: July 23, 2014, 04:50:53 PM »
I hope the LTC3786 works out better for you Craig

Offline Visus

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Re: Fun with DC-DC converters
« Reply #6 on: July 24, 2014, 08:09:13 AM »
Cant stop can ya,  that circuit looks awesome and off of one batt makes it more awesome. 

Its the best hobby I have hobbied too, r/c is fun, adventurous, etc.  but theres something about making an ecig lol...

Supposedly new batt tech is incoming and they are producing a 5000mah 18650 with over 50A continuous by swapping around the chemistry that is now in a battery, nothing new just changing the carbon arrangement around.  That will easily power these high powered regs for a long days worth of vaping.  They also take up to 8A+ and an instant charge function of charging so whoa.  You could run a single 18650 and shrink footprints to almost ego size..  Technology just keeps roaring in..
Makes pipe mechs much more interesting looking too...

No worries about over discharging is the best feature I hope they hurry up with em to market..  I am trying to get a 14500 sample set lol...  Just think 2000mah 14500's 10A or so lol awesome..

Some quotes:
Quote
The battery is stable enough to be discharged completely without harming the longevity of the product. Currently, the Ryden battery is rated for 3,000 charge/discharge cycles before the function of the battery begins to diminish.
http://revolution-green.com/new-ryden-dual-carbon-battery-real-breakthrough/

They just released the press conference in may so it should be soon..
A group of college kids here U.S.  found the same tech at almost the same time so one or the other come on..  :laughing2:

Offline CraigHB

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Re: Fun with DC-DC converters
« Reply #7 on: July 24, 2014, 12:56:45 PM »
I'm always up for a better battery.  Li-Ions are kind of a pain really, sounds like this battery will eliminate most of the issues in using them.  Though I have to take these claims with a grain of salt.  Seen too many of them come and go already.  Hopefully this one is not just another one of those.  They've stated a production run is scheduled, but will it happen and will this battery deliver on the claims they've made.

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