"piglet" wrote in message news:oumboe$5ap$1@dont-email.me...
> The body diodes are pointing in the right direction, they always were.
> What was wrong is the channel or substrate diodes are shown reversed. For
> instance you have Q22 correct - it is a P-channel and so the arrow points
> out of the substrate and towards the source. It is in fact just another
> way of showing the parasitic body diode. But now look at the BSS84
> symbols - those too are P-channel but the symbol on your design has their
> substrate diode arrow pointing from source towards the back of the gate.
> In other words the channel diode direction should point the same way as
> the body diode (if drawn) the two arrows are showing the same thing and so
> should point the same way. Likewise the symbols on your design for the
> BSS138 which is an N-channel device have the body diode drawn right but
> have the channel arrow pointing out the channel pointing to source - the
> arrow should point into the channel. Look at how the On-Semi datasheets
> for those parts correctly depict them. I think sometimes people get
> confused between FETS and BJTs, on an NPN BJT the emitter arrow points
> towards the outside world, not so with an N-channel fet!
Whew! I found out that my PADS Logic symbol editor was somehow corrupted. I
would edit the symbol, but when I applied it to the schematic, it kept the
old one. Now I think I finally have it right, and a made the arrow open
rather than closed. I still like the body diode, but I see your point.
Anyway, I have arranged the parts on a 2"x 6" board, and that isn't really
very dense. I will be getting the prototypes made by www.eiconnect.com,
originally zoompcb.com, and they have a deal where I have gotten 5" x 5"
boards (maximum is 25 in^2) for $25 each. I am considering adding more
circuitry to fill up a 4" x 6" board - perhaps my design using TL222 optos
and one using one or two DG408 or DG409 analog switches.
Thanks,
Paul
Reply by piglet●November 17, 20172017-11-17
On 16/11/2017 23:39, P E Schoen wrote:
> "piglet"� wrote in message news:ouji62$u62$1@dont-email.me...
>> (Apart from Q22 the mosfet symbol arrows have still got conflicting
>> arrow directions - see the datasheet symbols for BSS84 and BSS138 at
>> ON-Semi or Diodes Inc. A solid line vs broken line channel is used to
>> distinguish depletion from enhancement. Explicity drawing the
>> parasitic diode adds clutter and is redundant if the substrate diode
>> arrowhead is correctly placed.)
>
> I have corrected that. My schematic symbol editor is quirky and somehow
> I got them reversed. I have redrawn the symbols, but I have left the
> body diodes in place, because it is easier for me to follow the current
> flow to see possible problems like the sneak current paths you found.
>
> Thanks for your help. I hope to commit this to copper and FR4 pretty
> soon. Should be fun!
>
> Paul
The body diodes are pointing in the right direction, they always were.
What was wrong is the channel or substrate diodes are shown reversed.
For instance you have Q22 correct - it is a P-channel and so the arrow
points out of the substrate and towards the source. It is in fact just
another way of showing the parasitic body diode. But now look at the
BSS84 symbols - those too are P-channel but the symbol on your design
has their substrate diode arrow pointing from source towards the back of
the gate. In other words the channel diode direction should point the
same way as the body diode (if drawn) the two arrows are showing the
same thing and so should point the same way. Likewise the symbols on
your design for the BSS138 which is an N-channel device have the body
diode drawn right but have the channel arrow pointing out the channel
pointing to source - the arrow should point into the channel. Look at
how the On-Semi datasheets for those parts correctly depict them. I
think sometimes people get confused between FETS and BJTs, on an NPN BJT
the emitter arrow points towards the outside world, not so with an
N-channel fet!
piglet
Reply by piglet●November 17, 20172017-11-17
On 16/11/2017 23:39, P E Schoen wrote:
> "piglet"� wrote in message news:ouji62$u62$1@dont-email.me...
>
>> It is possible R5 will introduce measurement errors by charging up the
>> flying capacitor C3.
>
> Yes, it looks to be from the pack voltage through R5, R4, and D14. A
> diode in series with R4 should take care of that.
>
Simpler still is remove R5 then no diode (D21) needed either :)
piglet
Reply by P E Schoen●November 16, 20172017-11-16
"piglet" wrote in message news:ouji62$u62$1@dont-email.me...
I will be updating this schematic as I fine tune the design:
> Looking good. D21,D19,D7,D8 are not doing much useful work as drawn but
> would if moved slightly - e.g. if D19 moved to the Q4/Q6 gate path then
> blocks a sneak error current that can flow via R1 and R2.
I can see why the diodes are probably unnecessary, and I see where current
can flow from MUX+ and MUX- through Q1 and Q5 body diode when sampling, but
I don't understand how a diode can block that. Also I think you mean Q2/Q6?
> It is possible R5 will introduce measurement errors by charging up the
> flying capacitor C3.
Yes, it looks to be from the pack voltage through R5, R4, and D14. A diode
in series with R4 should take care of that.
> I don't know the charge/measurement switching strategy you use but it may
> be possible to merge R19 and R16 (and R23/R20,R14/R15) into one?
I think that would be good. I might use a polyswitch fuse but a 200 mA
device has 7.5 ohms resistance, and that severely limits the average charge
current. I will probably use 1 ohm resistors, and the one at the bottom will
be used for charge current monitoring.
> Buck converter D9 and D10 puzzle me but seem mostly harmless?
They are probably superfluous and I have removed them.
> (Apart from Q22 the mosfet symbol arrows have still got conflicting arrow
> directions - see the datasheet symbols for BSS84 and BSS138 at ON-Semi or
> Diodes Inc. A solid line vs broken line channel is used to distinguish
> depletion from enhancement. Explicity drawing the parasitic diode adds
> clutter and is redundant if the substrate diode arrowhead is correctly
> placed.)
I have corrected that. My schematic symbol editor is quirky and somehow I
got them reversed. I have redrawn the symbols, but I have left the body
diodes in place, because it is easier for me to follow the current flow to
see possible problems like the sneak current paths you found.
Thanks for your help. I hope to commit this to copper and FR4 pretty soon.
Should be fun!
Paul
Reply by piglet●November 16, 20172017-11-16
On 16/11/2017 06:04, P E Schoen wrote:
> I think you are correct. But I have now made some major changes in the
> circuit. It uses back-to-back P-channel MOSFETs which can sample
> voltages of the plus side of the lowest cell and above. It is much
> simplified. I also added a buck converter which can charge the low cell
> (which is used for the PIC power supply) from the total pack. This
> accomplishes balancing without wasting power in shunt resistors.
>
> http://enginuitysystems.com/pix/electronics/BMS_MOSFET.pdf
>
Looking good. D21,D19,D7,D8 are not doing much useful work as drawn but
would if moved slightly - e.g. if D19 moved to the Q4/Q6 gate path then
blocks a sneak error current that can flow via R1 and R2.
It is possible R5 will introduce measurement errors by charging up the
flying capacitor C3.
I don't know the charge/measurement switching strategy you use but it
may be possible to merge R19 and R16 (and R23/R20,R14/R15) into one?
Buck converter D9 and D10 puzzle me but seem mostly harmless?
(Apart from Q22 the mosfet symbol arrows have still got conflicting
arrow directions - see the datasheet symbols for BSS84 and BSS138 at
ON-Semi or Diodes Inc. A solid line vs broken line channel is used to
distinguish depletion from enhancement. Explicity drawing the parasitic
diode adds clutter and is redundant if the substrate diode arrowhead is
correctly placed.)
piglet
Reply by P E Schoen●November 16, 20172017-11-16
"piglet" wrote in message news:ou6v8h$iot$1@dont-email.me...
> Is the wiring around R26 - R34 correct, looks like you have a continuous
> 30uA drain on the battery?
"piglet" wrote in message news:ou6ukf$e8b$1@dont-email.me...
> A lot of resistors and zener diodes in your circuit :)
> Your PMOS symbol for Q8 (and possibly others) is wrong, for a pmos device
> the substrate arrow points away from the channel so opposite to an nmos
> device where arrow points into the channel. Think of the arrow as being
> another way of indicating the intrinsic substrate D-S diode.
That is probably an artifact of the screen-shot of the PDF. The symbols are
correct, but they are rather small.
> Here is my sketch of some universal bidrectional switches using inverse
> series nmos devices :-
> In your BMS application the logicside world actually has a galvanic
> connection to the batteryside world so the (B) circuit may be of more
> interest and since the flying capacitor is very large compared to the HF
> coupling capacitors measurement errors should not be a worry.
> Battery state of charge tends not to change rapidly so why not slow right
> down to just a few samples per minute and use real relays? Relays are
> cheap, robust against misuse and come with multiple poles or changeover
> contacts at minimal extra cost.
I would agree that solid state relays would be good, and I may go that
route. But electromechanical relays are bulky, not cheap, and draw a lot
more current. What might be interesting would be a miniature stepper switch,
perhaps like the movement in battery powered clocks and watches for the
second hand.
Paul
Reply by piglet●November 11, 20172017-11-11
On 10/11/2017 09:04, P E Schoen wrote:
> My latest project is a battery management system (BMS) that can
> accurately measure cells in a lithium battery pack, and also perform
> shunt balancing and possibly charge shuttling to balance the pack. I
> would also like to be able to use it for a pack of four 12V SLA
> batteries. A DG408 could be used to select up to 8 cell voltage taps
> from nominal 3.2V (LiFePO4) or 3.7V (Li-Ion), with a maximum voltage of
> 8 * 4.2 or 33.6V, which is within the maximum voltage of 40V for the
> DG408. But the voltage would need a 12:1 voltage divider to read using a
> PIC with a 3V power supply (from the bottom cell), and this renders the
> precision of the 10 bit ADC to 33.6/1024=0.032V, which is really not
> accurate enough for lithium cell monitoring.
>
> I did a preliminary design using a DG409 which can read any one of four
> cells differentially, or two DG408s for 8 cells. The outputs can be fed
> to a "flying capacitor", which will hold a voltage sample taken from a
> cell, and then a pair of MOSFETs can translate that charge to ground
> level so the ADC can read it using the full range allowed by the reference.
>
> Another method, used by some BMS chips, utilizes a high CMRR
> differential amplifier to translate the samples to the ADC.
>
> It is also desired to implement shunt charge balancing, where selected
> cells that have a higher voltage can be discharged to match lower cells
> in the pack. This is wasteful, but is a commonly used method, and can be
> done with MOSFETs and resistors.
>
> It is also possible to use charge shuttling, where the flying capacitor
> can be connected to lower voltage cells so that its charge adds to that
> of the low cell. But this requires a fairly large capacitor and a low
> resistance MUX. The DG408/9 have about 50 ohms per element, so that
> severely limits the practicality of such charge shuttling.
>
> So, I endeavored to design a multiplexer using discrete MOSFETs. My
> first attempt used MOSFET opto-isolators, but they are somewhat costly
> and use a fair amount of current for the LED. Then I tried several
> designs using discrete MOSFETs, and it looks to be successful, although
> it's a bit complex. Here is the LTSpice file for a simulation:
>
> http://enginuitysystems.com/pix/electronics/Analog_Mux_1.asc
>
> Here is an image of a complete schematic (as it stands now):
>
> http://enginuitysystems.com/pix/electronics/BMS_MOSFET_1.png
>
> And a PDF that may be easier to view:
>
> http://enginuitysystems.com/pix/electronics/BMS_MOSFET.pdf
>
> I think this should work pretty well, and the discrete MOSFETs should be
> able to carry at least 100 mA to perform shunt balancing. A larger
> sampling capacitor might be able to provide charge shuttling. The
> circuit seems pretty efficient and should work on cell voltages of 2.5
> to 4.5 volts, and 12V nominal batteries.
>
> If anyone has a schematic for the DG408 or similar analog MUX, I'd
> appreciate a link to the design. What I found from searching turned up
> NMOS and PMOS devices in anti-parallel, which won't work because of the
> body diodes. In series should work, but providing the correct gate
> voltages is a challenge.
Is the wiring around R26 - R34 correct, looks like you have a continuous
30uA drain on the battery?
piglet
Reply by piglet●November 11, 20172017-11-11
On 10/11/2017 09:04, P E Schoen wrote:
> My latest project is a battery management system (BMS) that can
> accurately measure cells in a lithium battery pack, and also perform
> shunt balancing and possibly charge shuttling to balance the pack. I
> would also like to be able to use it for a pack of four 12V SLA
> batteries. A DG408 could be used to select up to 8 cell voltage taps
> from nominal 3.2V (LiFePO4) or 3.7V (Li-Ion), with a maximum voltage of
> 8 * 4.2 or 33.6V, which is within the maximum voltage of 40V for the
> DG408. But the voltage would need a 12:1 voltage divider to read using a
> PIC with a 3V power supply (from the bottom cell), and this renders the
> precision of the 10 bit ADC to 33.6/1024=0.032V, which is really not
> accurate enough for lithium cell monitoring.
>
> I did a preliminary design using a DG409 which can read any one of four
> cells differentially, or two DG408s for 8 cells. The outputs can be fed
> to a "flying capacitor", which will hold a voltage sample taken from a
> cell, and then a pair of MOSFETs can translate that charge to ground
> level so the ADC can read it using the full range allowed by the reference.
>
> Another method, used by some BMS chips, utilizes a high CMRR
> differential amplifier to translate the samples to the ADC.
>
> It is also desired to implement shunt charge balancing, where selected
> cells that have a higher voltage can be discharged to match lower cells
> in the pack. This is wasteful, but is a commonly used method, and can be
> done with MOSFETs and resistors.
>
> It is also possible to use charge shuttling, where the flying capacitor
> can be connected to lower voltage cells so that its charge adds to that
> of the low cell. But this requires a fairly large capacitor and a low
> resistance MUX. The DG408/9 have about 50 ohms per element, so that
> severely limits the practicality of such charge shuttling.
>
> So, I endeavored to design a multiplexer using discrete MOSFETs. My
> first attempt used MOSFET opto-isolators, but they are somewhat costly
> and use a fair amount of current for the LED. Then I tried several
> designs using discrete MOSFETs, and it looks to be successful, although
> it's a bit complex. Here is the LTSpice file for a simulation:
>
> http://enginuitysystems.com/pix/electronics/Analog_Mux_1.asc
>
> Here is an image of a complete schematic (as it stands now):
>
> http://enginuitysystems.com/pix/electronics/BMS_MOSFET_1.png
>
> And a PDF that may be easier to view:
>
> http://enginuitysystems.com/pix/electronics/BMS_MOSFET.pdf
>
> I think this should work pretty well, and the discrete MOSFETs should be
> able to carry at least 100 mA to perform shunt balancing. A larger
> sampling capacitor might be able to provide charge shuttling. The
> circuit seems pretty efficient and should work on cell voltages of 2.5
> to 4.5 volts, and 12V nominal batteries.
>
> If anyone has a schematic for the DG408 or similar analog MUX, I'd
> appreciate a link to the design. What I found from searching turned up
> NMOS and PMOS devices in anti-parallel, which won't work because of the
> body diodes. In series should work, but providing the correct gate
> voltages is a challenge.
A lot of resistors and zener diodes in your circuit :)
Your PMOS symbol for Q8 (and possibly others) is wrong, for a pmos
device the substrate arrow points away from the channel so opposite to
an nmos device where arrow points into the channel. Think of the arrow
as being another way of indicating the intrinsic substrate D-S diode.
Here is my sketch of some universal bidrectional switches using inverse
series nmos devices :-
<https://www.dropbox.com/s/ye7fdkkagydge9g/poormans_ssr.pdf?dl=0>
In your BMS application the logicside world actually has a galvanic
connection to the batteryside world so the (B) circuit may be of more
interest and since the flying capacitor is very large compared to the HF
coupling capacitors measurement errors should not be a worry.
Battery state of charge tends not to change rapidly so why not slow
right down to just a few samples per minute and use real relays? Relays
are cheap, robust against misuse and come with multiple poles or
changeover contacts at minimal extra cost.
piglet
Reply by Jan Panteltje●November 11, 20172017-11-11
On a sunny day (Fri, 10 Nov 2017 15:51:51 -0500) it happened "P E Schoen"
<paul@pstech-inc.com> wrote in <ou53hf$5e7$1@dont-email.me>:
>"Jan Panteltje" wrote in message news:ou44at$5dc$1@adenine.netfront.net...
>
>> My idea is n small switchers with each one PIC and 1 MOSFET, so 1 for each
>> cell, isolated. and fed from a 1 turn on a big transformer driven by 2
>> MOSFETS push pull oscillator
>
>> Big charge currents no problem.
>
>> Avoids disspiating power in parallel MOSFETS, no heat.
>
>> Usenet patent.
>
>> When mass produced the small PCBs are possibly cheaper. And greener.
>
>I have also considered such a design. Small PICs are like 50 cents each. But
>the problem is with communication of the cell voltages to a master processor
>for display and control. It is possible (and has been done) to daisy-chain
>multiple units with communication up and down the chain, but that involves
>opto-isolators or MOSFETs which increases complexity and cost.
Yes, for fun give each small PIC board a photo-diode and IR LED and a fixed serial address
8 bits data, bit 9 set is address mode, old proved system.
Case should be light proof.
Master PIC (also opto and IR LED) addresses each slave board in turn and
gets reply, can set parameters and read voltages and currents.
:-)
Using optos is safer :-)
There are many ways.
But serial commienukeation is easy.
Yes, it all depends on the power levels and voltages too for course.
Low speed optocouplers are not that expensive anymore either:
https://www.ebay.com/itm/401093464932
50 for 1$51 free shipping