I'm using a THS4281 to make a general purpose High-Side currentsensor....
J
Joerg
You could consider current monitor chips that have a helper voltage and where common mode includes ground. But it's not as cheap. If each charge channel has its own switcher, can't you sense current there?
Regards, Joerg
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J
Joerg
If the battery terminals short out you do get a total loss of voltage. But it's no big deal if there's a helper voltage that, for example, provides a negative supply if the opamp CM range doesn't include VSS.
Regards, Joerg
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J
Jamie
Operate the sensing circuit from a separate supply that does not get effected from the main supply drop.
In your circuit, I assume you're using the +/- inputs of the opamp to form a dif shift via a shunt R ? The op-amp and Fet you're using can be operating from a secondary supply that can also share the same common.
of course, if you throttle back the drive using the current sense, you wouldn't get a total loss of voltage!.
It is the normal op-amp based constant current source but the op-amp doesn't have to take the large common mode and you don't need the 0.1% resistors.
The down side is you needenough voltage for the op-amp.
T
TTman
Protect power on with 'battery good' detect first ?
A
Adam Seychell
What are your desired current, voltage and measurement accuracy ?
R
Raveninghorde
Thanks.
That looks good and appears to get around the short circuit problem.
It adds 3 resistors to my component count. Under half a cent extra component cost plus placement time.
R
Raveninghorde
I'm happy with 3% accuracy on current. Who cares if a battery charges at 970mA rather than 1A.
I'm charging lithium ion so voltage is more important. On a 4 cell pack I expect to be between 16.70V and 16.75V so that's +/- 0.15% after adjustment.
R
Raveninghorde
In this application I am using National simple switchers so they don't help with the current measurement. They do provide an internal current limit which does provide short circuit protection.
I don't like the short circuit current being higher than the charging current. In the other job I've got to tidy up for production I'm using a sepic which does need a proper current limit.
I like to use common building blocks and MooseFET's circuit looks good at the moment.
R
Raveninghorde
I have battery good detection in the micro. The charger has to be safe even if the micro dies.
The customer checks short circuit by putting a switch across the output and a then a fuse in line to the battery. They then start charging and throw the switch.
Similarly for reverse battery protection they fit a reversing switch into the wiring. Again they start charging and throw the switch.
J
Joerg
On a SEPIC that's easy. You can sense the current at the base of the second coil and run that into an inverting opamp.
So you owe him a beer then :-)
Regards, Joerg
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D
dagmargoodboat
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 !
-/\/\--- =A0 =A0 =A0 =A0 =A0!
=A0 =A0 =A0 =A0 =A0 =A0 ! =A0 =A0 =A0 =A0!
=A0 =A0 =A0!
=A0>--+--/\/\--+--!! =A0 =A0!
=A0 =A0 !!----
=A0! =A0 =A0 =A0 =A0 --!! P-MOSFET
=A0 =A0 =A0 =A0 =A0 =A0 =A0 --------+--/\/\--GND
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0!
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0V =3D K*I
Or, if the op amp's inputs can go slightly below the rail:
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0R1
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0\
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0/ R2
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0\
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =3D=3D=3D
>
> > Two precision resistors instead of six.
>
> > (I used an LM324 for John. =A0He loves them.)
>
> If the load is shorted, the circuit doesn't work.
> This was the problem that my circuit was aimed at solving.
I guess I'm not visualizing Ravenhorde's setup. I'm assuming Vcc to
the op amp higher than the raw battery charging voltage, just as you
did...
Ah, I see. So use mine ahead of the regulator, if that's feasible.
If not, yours wins.
Nice work.
--
Cheers,
James Arthur
R
Raveninghorde
Your circuit is the same as what I use at the moment except + and - inputs of the op amp are swapped.
The op amp does come from a higher supply, but as you relaized that doesn't help.
I'm using a switching regulator so measuring in front doesn't work.
It trades an op-amp for the transistor. By making the first stage increase the voltage on the sense resistor to a higher value and then using the simple diff stage, we can get a ground referenced current sense.
You could also make the measurement at the catch diode of the bucker if you don't need it to be accurate.
J
Joerg
[...]
What kind? Any chance to add a C and an L and make it a SEPIC? Then current sense would become easy.
Regards, Joerg
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R
Raveninghorde
I'm using an LM2675 simple switcher in a buck configuration. Changing it to a sepic doesn't make sense as the extra cost would outweigh the cost of high side current sensing.
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