thank you for your reply Mr Al . I have a doubt if i want to charge a capacitor at 40A which is initially at 0V i will have to apply 1V right. even if it is curent mode mode controlled also wouldn't the converter indirectly regulate the voltage to 1V. hence I thought instead of controlling the current i thought regulating the voltage would be a easier one. kindly correct me if am wrong.
Hi,
I was under the impression that the current the capacitor could take was 40 amps to charge or discharge. If that is not true, then you have to find out what it really is because that will affect the design. If the cap can only take 10 amps for example, then all you have to do is design a buck circuit that can put out 10 amps and then add current regulation.
Adding current regulation to a lower power regulator like this is easy. Depending on the type of voltage control, either the voltage control pin gets pulled higher or gets pulled lower as the current is measured. For example, for a buck chip that has a 2.5v reference as the output voltage goes above 2.5v the chip starts to cut back the output pulse width, which lowers the output power. So to get this to regulate current a circuit that measured current then pulls the voltage up higher when the current goes up too high, and that lowers the output power too and so the current gets regulated.
For a chip like the LM317, the ADJ pin is pulled lower instead of higher. This decreases the output and thus lowers the current. And this can be done using a single transistor with collector tied to the ADJ pin, and current from the output is made to flow through a current sense resistor that is in parallel with the base emitter. As the current increases eventually the transistor collector starts to pull down the ADJ pin voltage and that lowers the output current. So in this case it is a simple transistor and a couple resistors.
But the main idea is to move to current regulation with voltage regulation on top of that. It's never about pure voltage regulation, where you regulate to 1v or 2v when the cap is at 0v, and 4v or 5v when the cap is at 1v, etc. It is about regulating the current itself.
However, if you dont mind wasting a bit of power you can use a resistor in series with the capacitor and regulate the voltage on the other side of the resistor based on what the capacitor voltage is. I think this is what you had in mind. In this way you only have to regulate voltage, but not the voltage on the cap itself. What this means however is that we basically are regulating the voltage across the resistor, and when we regulate that voltage that means we are really just regulating current because I=E/R, and R is fixed. And what this also means is we end up wasting more power because P=I*I*R, R fixed. So this is not the most efficient way to do it even if you use a switching buck circuit unless the resistor value can be kept small, but then we have a current regulator again not a voltage regulator
And when you have a capacitor with 0v across it you do not apply 1v to charge it. That's because to apply 1v across a capacitor in zero time requires an infinite current to flow, which would blow the capacitor
What you do is apply a current of maybe 1 amps or 10 amps or whatever it will take without damage, and wait for the voltage to rise. Once the voltage rises to say 50v, you then turn to voltage regulation, and that keeps the voltage at 50v until it is discharged. Also, because of the current regulation, you do not need to disconnect the charge circuit when discharging if you dont want to because the current will be limited anyway.