13-07-2017, 09:00 PM
Some further thoughts.
It appears that the capacity is sensibly independent of voltage, including a small amount of reverse bias.
With the two capacitors in back to back in series, in the steady state, the reverse biased capacitor only has to supply the leakage current of the correctly biased capacitor.
(Of course it also has to supply the initial charging current as well.)
Even if the reversed capacitor behaved as a good semiconductor diode (in parallel with a capacitor) it's slope resistance would be several hundred ohms assuming that the leakage current is low.
e.g. 50uA gives >500 ohm (26/I)
As this is much greater than the reactance (100uF @ 100Hz is 16ohm) it has little effect.
Extending this to the case where the back to back capacitors are subjected to significant AC, each capacitor will charge to the corresponding peak of the AC and only leakage current will flow thereafter.
I've simulated this using 'perfect' capacitors, schottky diodes to minimise reverse voltage and resistors to crudely model leakage.
As can be seen both the capacitors are correctly biased over almost the whole of the cycle, polarity reversal occurring only at the peaks of the waveform.
It also shows the initial reverse bias of C1 as C2 charges.
My conclusion is that the overall capacity is always the simple series value.
I am concerned as to whether repeatedly passing the initial charging current through the reversed biased capacitor will lead to degradation in the long term. I think I would be tempted to use a diode in parallel with each capacitor to avoid this possibility. This may well increase the 'hardness' of any distortion at the peaks but feels safer to me.
Jim
It appears that the capacity is sensibly independent of voltage, including a small amount of reverse bias.
With the two capacitors in back to back in series, in the steady state, the reverse biased capacitor only has to supply the leakage current of the correctly biased capacitor.
(Of course it also has to supply the initial charging current as well.)
Even if the reversed capacitor behaved as a good semiconductor diode (in parallel with a capacitor) it's slope resistance would be several hundred ohms assuming that the leakage current is low.
e.g. 50uA gives >500 ohm (26/I)
As this is much greater than the reactance (100uF @ 100Hz is 16ohm) it has little effect.
Extending this to the case where the back to back capacitors are subjected to significant AC, each capacitor will charge to the corresponding peak of the AC and only leakage current will flow thereafter.
I've simulated this using 'perfect' capacitors, schottky diodes to minimise reverse voltage and resistors to crudely model leakage.
As can be seen both the capacitors are correctly biased over almost the whole of the cycle, polarity reversal occurring only at the peaks of the waveform.
It also shows the initial reverse bias of C1 as C2 charges.
My conclusion is that the overall capacity is always the simple series value.
I am concerned as to whether repeatedly passing the initial charging current through the reversed biased capacitor will lead to degradation in the long term. I think I would be tempted to use a diode in parallel with each capacitor to avoid this possibility. This may well increase the 'hardness' of any distortion at the peaks but feels safer to me.
Jim
