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Topic: P01-109 BLiVIT (Read 8195 times) previous topic - next topic

P01-109 BLiVIT

Reissue as the previous post needed updating, and is more appropriate here.

One of the things I have noticed missing, for a pretty long time, is a good *safe* lithium cell charger that also supplies a load with a good efficient regulator. My answer is the BLiVIT. This is a moderate-sized board that incorporates charging from either USB or a wall wart, efficiently manages a single Li+ or LiFePO4 cell, has a power-on/off soft (using an elevated Powah-1), a fully isolated load supply (no need to blow up my scope when the wall wart is connected), and an efficient boost/buck regulator that supplies either 3.3V or 5V.

The BLiVIT optionally incorporates a POQiRX receiver (Project P01-114) which enhances the BLiVIT with wireless power charging capability, per the Qi specification of the Wireless Power Consortium.  A corresponding transmitter (Project P01-113) is also in work.

I am currently awaiting the rev. L pcb, and when that happens I will get parts on it soonest and post a photo. One of the things I have been needing to do is decide whether to custom fab the enclosure for this.

[Update - I am not.  It would cost too much, so I have changed over to a DP6037 form-factor in a Sick-Of-Beige enclosure.  It will also allow me to potentially use a larger Li+ cell.]

Elsewhere, in the OSHW blogosphere and marketplace, I have been seeing not a few LIFePO4 chargers. Most of the designs out there have been intended for standalone charging function only. Mostly these were for use on cell form-factors identical to AA and C type alkaline batteries.

The deficiencies of this design are serious, and I will enumerate some of them here.

The common AA cell chemistry does not allow for a great deal of charge density. There are also a lot of thermal problems, mostly having to do with the shape of the cells. Another problem is that the typical charger device has about 4 cell slots, and the charging circuitry simply is not designed properly to handle multiple-cell geometries.

That is why I have elected to support a single cell in the BLiVIT project. Many characteristics and compositions of cell chemistry are now available. I have chosen a cell with 950mAH charge capacity, while taking up only about 1.5 x 2 x .25 inch volume.

The next major deficiency of typical charger systems is that they rely on physically isolating the cell from the power load during the charging cycle. Aside from design life issues with mechanical connectors there _are_ safety issues with handling a fully charged lithium cell. That is why for the BLiVIT I required the capability of supporting the power load during the charging cycle while also striving for preserving cell working life. There are numerous articles and books published which warn very strongly that ineffective charging cycle management will have severe effect on the working life of the cell.

The necessity of overcoming some of these limitations of previous charger designs required much research. Not being a power management engineer, I was not likely to use jellybean parts alone. So I opted to just start at the "front" and work my way back.
I exhaustively worked over many datasheets for power management devices and found many that "almost" fit my needs. Finally I came across the Maxim MAX1874. The first criterion for the device was to have the outside circuitry be as simple as possible. The second being that at least one of a DC adapter or USB power be permissible for sourcing the charger. Happily, the MAX1874 can be used with both sources, the DC power taking precedence when connected.

My next requirement was to be able to drive the load or charge the cell with automatic switch-over. It was nice to find out that the MAX1874 could do that as well, as well as drive the load circuit, also automatically.

Thermal issues with the cell are important. Lithium ion cells generate a lot of heat, which can be slow to dissipate depending on the geometry of the cell. The MAX1874 has a thermistor sensor feature that I have chosen to employ with an NTC thermistor that has resistance 10K at 25 C. The combination of constant current sensing and temperature sensing means the charging cycles can be constantly monitored for best efficiency.

That accounts for most of the front-end of the BLiVIT. Next we must look at downstream requirements.

Even if the power load is miniscule it is not zero, so a soft-toggle switch using two schmitt inverters and a PFET is put downstream of the cell. I wanted the charger to be functioning and keeping the cell trickle-charged even if the load is disconnected or turned off.
The switch is activated with the TACT switch on-board, or any momentary NO switch connected to a header.

One of my particular issues is that the power be fully isolated from any connected source (the USB or DC input). It would be a very bad day if I was powering a test circuit and a ground loop happened and blew out my oscilloscope. I am using an isolation transformer and driver chip to protect the load from the upstream.

The voltage range of a lithium cell can range from 3.1 to 4.2V, which are not altogether useful for digital logic, so I use a switching boost/buck regulator to drive any of the input voltage to a locked output voltage. I am using an adjustable regulator so that a jumper can be used to select 3.3V or 5V output, which is available at a header or the terminal block.

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Re: P01-109 BLiVIT

Reply #1
This project has been accepted for publication in Elektor magazine in the Summer projects issue.

Re: P01-109 BLiVIT

Reply #2
This photo is a rendering of the new case design for the BLiVIT.  It should have enough vertical clearance for the entire BLiVIT maximum assembly, that being the BLiVIT board, the P01-114 POQiRX transmitter daughter board, the Li+ cell, and the POQiRX transmitter coil assembly.  There should also be sufficient airflow vertically through the enclosure since the Li+ cell and coil assemblies need to be regulated thermally.

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Re: P01-109 BLiVIT

Reply #3
for your latest case, how do you plan to keep the sides from falling off?
In the other type of lasercut case, the tops hold the sides on.
I don't see cross bolts on the sides, so you'll probably have to glue the sides together...

Re: P01-109 BLiVIT

Reply #4
Correct.  The sides and bottom plate are to be cemented together, while the top plate is affixed with mechanical fasteners.  Those will extend via threaded standoff to similar screws on the bottom plate, which will also aid centering of the BLiVIT on the POQiTX transmitter plate.

Re: P01-109 BLiVIT

Reply #5
I have recently gotten my enclosure kits back from the fab.  Here are a few photos of what the unit's going to look like.
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BLiVIT prototype as assembled

Reply #6
Testing continues.  I have validated the power flow switches and the battery charger/management.
The measured idle current draw from the Li+ cell is 23.4uA.  With the Powah-1 module and the 10Meg load resistance inherent in my scope the current flow is 3.5mA.

With the BLiVIT idle and the USB plugged in the flow back into the cell is approx 770mA (initial charge only).  This current winds down as the cell is charged, until the maintenance mode is achieved.  I am validating that as of this time.  The isolation section also works perfectly, creating a +/- 5V square wave at the isolation transformer secondary, which is rectified into a constant +5V.  The downstream switching regulator has to be tweaked, but when that is correct I will call it good all around.

Here are some assembly photos.
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Re: P01-109 BLiVIT

Reply #7
Here is another update for this project, at some danger of being guilty of responding to my own posts :)  The datasheet for
the Analog Devices ADP2503 boost/buck regulator used in the BLiVIT project includes this formula for deciding the adjustment settings
for the output:

  Vout = (R1 + R2) * Vref / R2

Vref is a fixed 500mA voltage provided by the ADP2503.  R1 is the smaller values at either R11 or R12 in the BLiVIT board.  R2 is the larger resistance R14.  In the current thinking, for example, R14 is 300K while R11 is 33.33K.  This is shown in the OpenOffice spreadsheet included here.