Help Understanding Active Current Control and Its Implementation - Page 1
Forum section: electronics / general

author: Krishh
author_rank: Newbie
timestamp: January 19, 2025, 02:43:36 pm
content: Hello everyone,I'm attempting to understand the idea of active current control and how power electronics accomplishes it. I'm specifically dealing with a dual active bridge (DAB), which controls current while keeping the output voltage steady.How the current can be dynamically raised while maintaining a steady voltage is what baffles me. It seems contradictory, but up until now, I've always been taught that voltage (V) is proportional to current (I).To be clear, I am referring to the current delivered to the load rather than the current drawn by the load.Could someone please explain what active current control is?How is it accomplished in DAB-like systems?In this situation, how can current be increased without affecting voltage?Any explanations or examples that could help me understand this would be greatly appreciated. Thank you ahead of time!

author: Slh
author_rank: Regular Contributor
timestamp: January 19, 2025, 03:51:07 pm
content: You're right that the DAB current won't change without changing the output voltage. Of course this is assuming that the load current stays constant.Typically, there's a voltage control loop that sets the current demand. The DAB will be constantly increasing/decreasing the current through it to maintain the output voltage. The output voltage will be fluctuating slightly depending on the load.If the load current increases then the voltage will drop and the DAB controller will demand more current to get back to the voltage target. It will then reduce the current to match the load current demand. The opposite will happen when the load current drops.

author: mtwieg
author_rank: Frequent Contributor
timestamp: January 19, 2025, 05:02:13 pm
content: In addition to what Slh said above, keep in mind that controller types (current mode, voltage mode, etc) are independent of the power circuit topology (buck, boost, flyback, DAB, etc). If you want to understand the control side of things, best to start out looking at it in very general terms, like a set of black box transfer functions instead of actual circuits.Reading datasheets for PWM controller chips is also a good way to learn, I got my start reading Linear Tech white papers.

author: Krishh
author_rank: Newbie
timestamp: January 19, 2025, 05:53:30 pm
content: Thank you for the explanation! From what I understand now:The DAB controller modifies the current drawn from the source to maintain a constant output voltage in the event that the load current demand fluctuates. For example, the output voltage decreases when the load requests more current; the controller then raises the delivered current to restore the voltage. Am i right in this sense or is there a bigger picture?

author: Krishh
author_rank: Newbie
timestamp: January 19, 2025, 05:55:35 pm
content: Thanks for the advice will look into them soon!!

author: mtwieg
author_rank: Frequent Contributor
timestamp: January 20, 2025, 01:55:45 am
content: Quote from: Krishh on January 19, 2025, 05:53:30 pmThis description could apply to either voltage mode or current mode controllers.Also the term "active current control" is, AFAIK, not a standard term in power electronics (as opposed to current mode control or voltage mode control). So impossible to say whether you're right or wrong.Here's a youtube video giving an overview of current mode vs voltage mode controllers (in particular, voltage mode vs peak current mode control):The basic idea is that with a voltage mode converter the plant has a order transfer function (due to the LC), which can be difficult to stabilize. Using an inner current feedback loop effectively turns the inductor L into a controlled current source. Thus the plant behaves like a first order transfer function which is easier to stabilize. Of course there are caveats to this, but in most cases it does offer a net benefit in transient response.