# Controlled supercapacitor charging with buck converter

I'm building a charging circuit for two supercaps (2.7V, 3000F each) in series. I've got a 12V supply and a buck converter rated to 15A, which I've set to provide 4.8V. I'm using balance resistors to evenly distribute the voltage between the two caps. I'm using an Arduino to monitor and display the charge across the capacitor bank through another potential divider, and the current with a 20A ACS712 module. I can charge the capacitors by using a current limiting ~0.5 Ohm resistor (actually quite a few power resistors in parallel) in series with the capacitor but this is quite inefficient in terms of both volume and power. Can I use a PWM signal from the Arduino to control a power MOSFET in place of the resistor to limit the current to the capacitor, basing the duty cycle on the ACS712 current signal? I'm guessing I'd need to smooth the input signal to the Arduino to get an average current reading, but my question is would this rapid switching of current damage the buck converter or capacitor?

• Why not make your buck converter a switch mode current source that shuts off when your cap reaches the desired end voltage? – John D May 30 '15 at 2:21
• You want to use the buck topology, but sensing current instead of voltage. You can probably find a buck LED driver IC that is specifically designed to sense current with a small resistor. Also, what buck converter are you using? Some of them are designed to limit current (by sensing Rds on high-side internal FET). If so, you can possibly omit the current limiting resistors and just let the Buck limit the current. – mkeith May 30 '15 at 2:25
• @JohnD, that could work, to reconfigure the buck, but most likely the reference voltage will be too high for that solution to be practical (will lose too much power in the current sense resistor). To the OP, please specify what you are using for a Buck. – mkeith May 30 '15 at 2:26
• @mkeith You could always use an op-amp to gain up the current sense voltage to whatever you need it to be. Or, like you mentioned you could also use a buck that limits output current to a fixed value rather than holdback or hiccup mode. A switch mode LED driver that regulates current would be a good solution too. – John D May 30 '15 at 2:33
• @JohnD, I have never used an amp that way. It seems like a good idea. Do you think there could be any worries about stability if you gain up the sense voltage from a low value resistor? – mkeith May 30 '15 at 3:38