Battery charger current sense
Battery charger current sense
(OP)
hello,
I am doing a switch mode, lead acid battery charger with UC3909 Charger IC.....
Datasheet.....
http://focus.ti.com/lit/ds/symlink/uc3909.pdf
..on pg 7 of the datasheet for UC3909, it shows the charge current sense resistor being placed at the non load side of the buck converter.....can any reader understand why it is placed here?
I thought that in this position it is having to take the ripple current through the output capacitor aswell as the charging current,....and that surely is bad ?
..surely this current sense resistor should be on the battery side of the output caspacitor, then it will only experience the DC charging current, which i thought would be right?
------------------------
Relevant to this, the following app note on this UC3909...
http://focus.ti.com/lit/an/slua058/slua058.pdf
...on page 4 it shows a flyback charger circuit, and again shows the sense resistor the "seemingly wrong" side of the output capacitor...(they then use Rsf and Csf to filter out the flyback ripple current).
I cannot see the point of this...why bother having to filter it?...why not i think just put the sense resistor the other side of the output capacitor ?
In other words, surely its best to keep the sense resistor outside the rectifier loop and in the output loop , where its nice smooth DC?
grateful for any thoughts.
I am doing a switch mode, lead acid battery charger with UC3909 Charger IC.....
Datasheet.....
http://focus.ti.com/lit/ds/symlink/uc3909.pdf
..on pg 7 of the datasheet for UC3909, it shows the charge current sense resistor being placed at the non load side of the buck converter.....can any reader understand why it is placed here?
I thought that in this position it is having to take the ripple current through the output capacitor aswell as the charging current,....and that surely is bad ?
..surely this current sense resistor should be on the battery side of the output caspacitor, then it will only experience the DC charging current, which i thought would be right?
------------------------
Relevant to this, the following app note on this UC3909...
http://focus.ti.com/lit/an/slua058/slua058.pdf
...on page 4 it shows a flyback charger circuit, and again shows the sense resistor the "seemingly wrong" side of the output capacitor...(they then use Rsf and Csf to filter out the flyback ripple current).
I cannot see the point of this...why bother having to filter it?...why not i think just put the sense resistor the other side of the output capacitor ?
In other words, surely its best to keep the sense resistor outside the rectifier loop and in the output loop , where its nice smooth DC?
grateful for any thoughts.





RE: Battery charger current sense
One reason may be for biasing. Not a lot of head room.
The other is; Many of these controllers use the current thru the inductor to control the turn off of the power elements each cycle. Heavily filtering it may cause complete failure of the control, whereas some filtering may be needed for winnowing out the noise.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Battery charger current sense
RE: Battery charger current sense
RE: Battery charger current sense
There is an overcurrent failure mode that is not obvious, in fact you need a storage scope to identify it.
This happens when the regulator is recovering from a short-circuit on the output. The output capacitor voltage is 0 (due to the short) and the short is then removed. If you are sensing the output current after the cap, the resistor will not see the charging current going into the cap to bring it up to the required output voltage.
Worst case, your inductor saturates and your switch will have to try to survive a large current spike going into the cap.
By placing the resistor before that cap, the IC will see the spike and turn off the switch accordingly.
Benta.
RE: Battery charger current sense
Thanks for looking into this.
-Actually , ive just noticed that on pages 8 and 9 (figures 3 and 4) of the datasheet for UC3909.....
DATASHEET:
http://focus.ti.com/lit/ds/symlink/uc3909.pdf
...they actually do have the current sense resistor outside of the rectifier loop. Do you belive these are mistaken figures?
RE: Battery charger current sense
The IC is powered by default from the bias winding, with battery backup through that diode close to "Vcc".
RE: Battery charger current sense
Benta.
RE: Battery charger current sense
actually by measuring the AC H.F current attached to DC output current a better pk-to-pk fast regulation can be achieved.