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'[EE] Need advice on a boost converter for charging'
2008\08\14@122905 by Matthew Miller

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face
Hello,

I posted about this a while back and this is a bit of an update. I have an
electric golf cart that has solar panels mounted on top the canopy. The
solar panels produce about 15V under load and the lead acid battery pack has
a voltage of 48V. My problem is to raise the panel voltage in order to
charge the batteries.

My first attempt was to build a boost converter using the LT1680
controller. I built the typical application circuit that is in the device's
datasheet and under workbench tests it operated fine, with the exception of
a bit of audible noise. The web page for the LT1680 is here:
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1042,C1031,C1115,P1597

My trouble is that I connected the converter ground to the battery pack and
then when I touched the positive wire to the battery terminal there was a
big blue spark and now the converter is no longer working. :( I haven't
repaired the converter yet, but I'm wondering if the converter design I used
is appropriate for this application?

I would appreciate any suggestions, insight, or questions.

2008\08\14@123857 by peter green

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face

> My trouble is that I connected the converter ground to the battery pack and
> then when I touched the positive wire to the battery terminal there was a
> big blue spark and now the converter is no longer working. :( I haven't
> repaired the converter yet, but I'm wondering if the converter design I used
> is appropriate for this application?
>
> I would appreciate any suggestions, insight, or questions.
>  
My first throught would be you probablly need something to limit current
flow into the battery and a diode to prevent backflow from the battery
to the converter.


2008\08\14@124743 by M. Adam Davis

face picon face
I haven't analyzed the circuit, but in general most power supplies
don't like it when there is voltage on the output and nothing on the
input side.

You might follow the step up circuit with a charging circuit, or
perhaps a simple diode would be sufficient if you don't want to do
charge control.

-Adam

On 8/14/08, Matthew Miller <spam_OUTnamiller2TakeThisOuTspamnaxs.net> wrote:
{Quote hidden}

> -

2008\08\14@131644 by gardenyu

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part 1 1883 bytes content-type:text/plain; charset="gb2312" (decoded quoted-printable)

 My little question is: do the solal panel and the battery share the same ground? Or is there anywhere connected to earth ground by fault?
  I just had this problem this early morning.



> Date: Thu, 14 Aug 2008 12:28:55 -0400> From: .....namiller2KILLspamspam@spam@naxs.net> To: piclistspamKILLspammit.edu> Subject: [EE] Need advice on a boost converter for charging batteries.> > Hello,> > I posted about this a while back and this is a bit of an update. I have an> electric golf cart that has solar panels mounted on top the canopy. The> solar panels produce about 15V under load and the lead acid battery pack has> a voltage of 48V. My problem is to raise the panel voltage in order to> charge the batteries.> > My first attempt was to build a boost converter using the LT1680> controller. I built the typical application circuit that is in the device's> datasheet and under workbench tests it operated fine, with the exception of> a bit of audible noise. The web page for the LT1680 is here:> http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1042,C1031,C1115,P1597> > My trouble is that I connected the converter ground to the battery pack and> then when I touched the positive wire to the ba
ttery terminal there was a> big blue spark and now the converter is no longer working. :( I haven't> repaired the converter yet, but I'm wondering if the converter design I used> is appropriate for this application?> > I would appreciate any suggestions, insight, or questions.> > -- > http://www.piclist.com PIC/SX FAQ & list archive> View/change your membership options at> mailman.mit.edu/mailman/listinfo/piclist
_________________________________________________________________
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part 2 35 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)

2008\08\14@132838 by olin piclist

face picon face
Matthew Miller wrote:
> I posted about this a while back and this is a bit of an update. I
> have an
> electric golf cart that has solar panels mounted on top the canopy.
> The
> solar panels produce about 15V under load and the lead acid battery
> pack has
> a voltage of 48V. My problem is to raise the panel voltage in order to
> charge the batteries.
>
> My first attempt was to build a boost converter using the LT1680
> controller.

My first attempt would be a dsPIC controlled boost converter that separately
calculated the parameters for each boost pulse.  It sounds like there is no
way the solar panel can put out enough power to hurt the battery, even when
it is fully charged.  If so, the only choice is when to start a boost pulse
based on what is available from the solar panel.  I would "fill" the
inductor each pulse, meaning its current is always brought to a little under
saturation.

You need to measure the solar panel voltage and the battery voltage.  The
rest is math knowing the capacitance accross the panel and the boost
switcher inductance.  The capacitance and inductance will vary, but will be
easy to calibrate out at manufacturing time.  You can measure the panel
power output by watching how it charges up the cap.  The rest is firmware,
although beware of some patents in the area of solar panel maximum point
tracking.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2008\08\14@133056 by olin piclist

face picon face
peter green wrote:
> My first throught would be you probablly need something to limit
> current
> flow into the battery

Unless the solar panel in full sunlight doesn't put out enough power to hurt
the battery.

> and a diode to prevent backflow from the battery
> to the converter.

Think about it.  That's inherent to a boost converter.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2008\08\14@135041 by Marcel Duchamp

picon face
Matthew Miller wrote:
> www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1042,C1031,C1115,P1597
>
> My trouble is that I connected the converter ground to the battery pack and
> then when I touched the positive wire to the battery terminal there was a
> big blue spark and now the converter is no longer working. :( I haven't
> repaired the converter yet, but I'm wondering if the converter design I used
> is appropriate for this application?
>
> I would appreciate any suggestions, insight, or questions.
>

Your cells provide 15V "under load"; thus some amount of current flows
and your cells provide some amount of power (V*I).  If the charger and
battery draw more current than is available, the solar cell voltage will
drop requiring even more current (it's a switcher: think constant
power).  Depending on how the switcher reacts (shuts down until power
cycled?, current limits?, foldback limits?, oscillates?, etc.) anything
can happen.

To proceed, here's some things to check.

Repair the supply noting what went wrong. Try to figure out why it
failed.  Connecting the battery to the supply with no current limiting
could easily induce transient conditions that will prevent proper operation.

Characterize both the supply and the battery.  Either get a bunch of
resistors or build a constant current sink and load the supply taking
V*I data points.

Do the same for the battery.  If it's 48V and you connect a 48V charger
to it, it should draw very little current.  How much does it draw when
it is 47V?  46V? 40V?  At some point, a heavily discharged battery may
draw more than your supply can deliver.

Think about a current limit scheme to put between the supply and the
battery.  Limit the current to something the supply can handle.

Diode isolation (or FET isolation) may be needed also.  What happens
when clouds block the sun during high power charging?  The cells can't
deliver the power, the voltage drops, the supply draws more current, bang.

Good luck!

2008\08\14@150540 by alan smith

picon face
First I would replace the fuse so the circuit is working again, per benchtop mode.  Oh, you *did* put a fuse in there right?  On both sides?

I believe it was suggest about putting a current limit, or maybe even a soft start feeding it from the solar panel.  Linear Tech makes a nice 48V hotswap controller that acts like a soft start, and will shut off the FET when the voltage drops below a set threshold.  

It wasn't clear about what wire you attached where, so assumption is you are taking the input from the solar panel to boost for the battery, so was it connected to the solar panels and then it went sparkly when connecting to the batteries?  I think Olin or someone mentioned about monitoring the current, you will want to do that so you don't cook the batteries, and also monitor the voltage on them as well.  Your first attempt is a rather brut force method, but there is alot more to doing a charging circuit than just giving the batteries 52V for a period of time.


--- On Thu, 8/14/08, Matthew Miller <.....namiller2KILLspamspam.....naxs.net> wrote:

{Quote hidden}

> --

2008\08\14@160035 by Dr Skip

picon face
While I'm no expert on this, the first thing I see is that without an output
diode in series, the battery voltage would upset the feedback voltage to the
chip. That changes everything. Perhaps it caused it to drive the fet on 100%
and it fried. An output diode, past any feedback point, is always good for
switchers as chargers IMHO.



{Quote hidden}

>> --

2008\08\14@210608 by Matthew Miller

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face
Everyone, thanks for your replies! I'll try to answer a few questions: no
fuse is present in my converter, mistake 1; no blocking is present to
isolate the feedback point from the batteries, mistake 2.

The converter died (the spark I mentioned in the OP) when I attached the
positive output of the converter to the positive terminal of the
battery. The solar panel positive was not hooked up at this time to the
converter. The spark could have been because of the output caps charging,
but that shouldn't have killed the converter...

Tomorrow I'm going to take the convter apart and try to determine what is
wrong. Thanks for the comments. I've never built a high current boost
converter before, so this has been interesting and a bit frustrating...

Matthew

2008\08\14@211824 by Marcel Duchamp

picon face
Matthew Miller wrote:
> Everyone, thanks for your replies! I'll try to answer a few questions: no
> fuse is present in my converter, mistake 1; no blocking is present to
> isolate the feedback point from the batteries, mistake 2.
>
> The converter died (the spark I mentioned in the OP) when I attached the
> positive output of the converter to the positive terminal of the
> battery. The solar panel positive was not hooked up at this time to the
> converter. The spark could have been because of the output caps charging,
> but that shouldn't have killed the converter...
>
> Tomorrow I'm going to take the convter apart and try to determine what is
> wrong. Thanks for the comments. I've never built a high current boost
> converter before, so this has been interesting and a bit frustrating...
>
> Matthew

If your output caps (large if I remember correctly) were tantalum, one
of them could easily have shorted due to inrush current from the
battery.  Once shorted, they usually begin to burn and often explode.
This is far more common with lithium batteries but I have seen it from
bench supplies being connected to boards with tantalums.

2008\08\14@215317 by Matthew Miller

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On Thu, Aug 14, 2008 at 06:18:00PM -0700, Marcel Duchamp wrote:
>
> If your output caps (large if I remember correctly) were tantalum, one
> of them could easily have shorted due to inrush current from the
> battery.  Once shorted, they usually begin to burn and often explode.
> This is far more common with lithium batteries but I have seen it from
> bench supplies being connected to boards with tantalums.

No, the caps are Al electrolytics. I've got about the whole day tomorrow to
investigate this and find out what is wrong. I wish something had visibly
burned, my job would be easier!

Matthew

2008\08\15@035200 by Apptech

face
flavicon
face
>> and a diode to prevent backflow from the battery
>> to the converter.
>
> Think about it.  That's inherent to a boost converter.

I'll be picky for completeness.
In most boost converter applications, yes.
I happen to be adapting a product at present with a boost
converter that has (or had) no output diode!
It's a LED driver and they feed AC pulses to the LED. This
is not especially uncommon and has some advantages IF the
LED can tolerate the peak to average ratio.

The existing circuit uses energy dump open loop control -
they charge the inductor to a selected current and then dump
it and control the repetition rate. Not good for regulation
wit the IC used - so I'm adding Olin's diode and a smoothing
cap and sensing the LED current. The IC concerned isn't
meant to be regulated in this way but it can be persuaded to
be. (SC6601D which you won't see on many Western shelves.
Operates down to 0.6V battery voltage !!!)
(http://www.super-ic.com/product.html).

A very useful IC - 1 cell to white LEDs to about 50 mA LED
current. LEDs must be in parallel, alas. Costs about $US0.10
in volume (!).





       Russell


2008\08\15@042553 by Alan B. Pearce

face picon face
>The existing circuit uses energy dump open loop control -
>they charge the inductor to a selected current and then
>dump it and control the repetition rate.

I believe this is essentially what Olin was proposing for the solar panel
charger, but he was selecting a current that is close to saturation for the
inductor.

2008\08\15@083808 by olin piclist

face picon face
Apptech wrote:
> I'll be picky for completeness.
> In most boost converter applications, yes.
> I happen to be adapting a product at present with a boost
> converter that has (or had) no output diode!
> It's a LED driver and they feed AC pulses to the LED.

OK, that's a very different case since your final load has no energy storage
so there is nothing to flow backwards when the inductor is not dumping
current to the load.

Just to be clear to the OP, this is NOT the case in your design.

> Not good for regulation
> wit the IC used - so I'm adding Olin's diode and a smoothing
> cap and sensing the LED current.

Why not keep the diode away, but add a small current sensing resistor
between the LED and ground?  You can low pass filter that a little to get
smoother current feedback as long as you make sure the response of the
regulator is even slower, else the system will become unstable.

> LEDs must be in parallel, alas.

Why?  Does this thing use such a low voltage process that the switch can't
tolerate the voltage of two LEDs in series?  And I guess you don't want to
spend the extra $.03 for a discrete switch?


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2008\08\15@091943 by Apptech

face
flavicon
face
> Apptech wrote:
>> I'll be picky for completeness.
>> In most boost converter applications, yes.
>> I happen to be adapting a product at present with a boost
>> converter that has (or had) no output diode!
>> It's a LED driver and they feed AC pulses to the LED.

> OK, that's a very different case since your final load has
> no energy storage
> so there is nothing to flow backwards when the inductor is
> not dumping
> current to the load.

> Just to be clear to the OP, this is NOT the case in your
> design.

Yes & yes.
My example is quite  special case.

{Quote hidden}

That could be made to work. Saves a high side diode drop and
Schottky diode cost which is good. Probably a little more
complex at bottom end but low cost parts. I do have a low
side current sense resistor. It is wasteful of efficincy as
it drops 0.5V (transistor Vbe). it's possible to actually
use a lower sense voltage by using a transistor as a
comparator (see eg Richards circuit which morphed into the
Black regulator) but you want something like a stable
voltage to make the reference, 'which we have not got'*.
It's OK enough as is.
* Naming of parts.

>> LEDs must be in parallel, alas.

> Why?  Does this thing use such a low voltage process that
> the switch can't
> tolerate the voltage of two LEDs in series?

Apparently but not certainly. Real data on the IC is
extremely hard to come by. I will get more but it's not a
trivial process. The available data sheets are more like
application notes. In none of the circuits given do they
ever serial connect LEDs. I do have a version working on the
bench at present with 2 LEDs in series but there is no
guarantee that this is "in spec". Also, see below, doing
this increases the Vdd to the same value !!!! :-)

> And I guess you don't want to
> spend the extra $.03 for a discrete switch?

It is a lower cost product than certain others that are
publicly known so far, but it's more complicated than that.
The IC has NO Vdd connection per se. It draws power from the
Inductor connection during flyback and internally rectifies
it. There is an external Vdd pin with a capacitor only
connected to filter this supply. If you double the LED
voltage you also approx double this Vdd :-). This may or may
not be what the manufacturers allow. It works for me but as
above, is in NO application circuits. If you add more series
diodes it stops working. Can't remember now how many it was.
But a bad idea [tm] :-). The app note says (sort of) supply
voltage is 0.6V to 1.6V. The above powering arrangement
makes using an external switch an interesting challenge.
I've drawn up possible circuits but not tried them. Despite
all this arcanery the IC works very well when not pushed too
far outside the app note examples. My regulation scheme is
in none of the app notes but does not do violence to the
general principles.

Why, they chorused, use this IC ??? OR its relatives.
The ability to drive white LEDs from a single cell, the sub
10 cent price and the fact that the manufacturer uses it in
other equipment already, helps.
Nothing much else comes close on price, which is reasonably
important. I can get MC34063 for $US0.08 but Vdd is too
high. Bootstrapping is possible but not nice. And 34063
output drive wants an external switch for efficiency due to
nasty non sat darlington output. None of which is too
important here. There are other ICs in the same family that
I am looking at now. Data sheets are about as (un)available
and useful.



           Russell


2008\08\18@221758 by Martin K

face
flavicon
face
Apptech wrote:
> Nothing much else comes close on price, which is reasonably
> important. I can get MC34063 for $US0.08 but Vdd is too
> high. Bootstrapping is possible but not nice. And 34063
> output drive wants an external switch for efficiency due to
> nasty non sat darlington output. None of which is too
> important here. There are other ICs in the same family that
> I am looking at now. Data sheets are about as (un)available
> and useful.
>
>
>
>             Russell

Where do you get an MC34063 for $.08?

-
Martin

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