# CC/CV mode of Buck Converter used for Battery Charging

I was trying to understand the CV/CC charging of the battery using a buck converter. I came across this website, where it described the addition of sensor resistors to measure current and get the reading in voltage using op amp and send it as feedback. https://www.ti.com/lit/an/snva829/snva829.pdf

I have a couple of questions regarding its implementation

1. I have closed-loop DC to DC voltage regulator working, can I use voltage obtained from measuring current in the load as feedback and control the DC to DC converter?
2. If I use current sensor feedback voltage, how does the buck converter will be controlled to keep constant current at the output with varying voltage in constant current mode?

I couldn't find good material explaining the control block diagram of the whole system, so didn't understand it. Could someone please help me with that?

A CC-CV control is a classic in battery management systems. Basically, you have two loops:

1. a voltage loop for constant-voltage operation (CV) which observes the output voltage and maintains it to the regulated level by generating a frequency-compensated error voltage.

2. a current loop for constant-current operation (CC) which senses the output current - usually via a resistive shunt - and keeps it constant while the output voltage goes down.

In a practical implementation, the two loops are ORed meaning that when one takes the lead, the other one is silent. For instance, if you want to regulate at a 12-V level with a 1-A CC current, then if you draw 650 mA, the CC loop remains silent while the CV takes the leads. If you increase the current and reaches 1 A, then the CC will take the lead, making the output voltage drop as the current increases (the output is the load resistance by the CC current). As voltage regulation is lost, the CV loop becomes silent and the CC loop leads the regulation effort. Above is a typical configuration for two ORed op-amps with two different reference voltages. A 2.5-V source for the CV and a much lower value, e.g. 100 mV, for the current sense (you want a low-value shunt resistance). When the CV op-amp takes the lead, e.g. with an output current less than the CC value, the $$\D_V\$$ diode conducts and drives $$\V_{err}\$$. Because the current is below the CC level, op-amp CC rails up and thus $$\D_I\$$ is blocked. When you now reach the CC target and $$\V_{out}\$$ starts dropping, $$\D_V\$$ blocks and $$\D_I\$$ conducts with the CC op-amp taking the lead.

In your case, you have to identify how the buck is controlled and build the CC-CV circuitry. Ac compensation differs for the two loops but, usually, the CC section Bode plot is the CV section scaled down by the shut element value. I have covered a compensation example in my blue book on control loops.