This article presents a practical implementation of the CC-CV control algorithm using a Full-Bridge LLC Resonant Converter, known for its high efficiency, soft-switching characteristics, and suitability for high-frequency isolated DC-DC applications such as battery chargers.
System Parameters Used for Simulation
The simulation parameters considered for the system as follows:
- Battery Specifications:
- LLC Resonant Converter Parameters:
Design parameters include resonant inductance, resonant capacitance, magnetizing inductance, switching frequency range, and transformer turns ratio.
(Note: A complete parameter file is provided along with the simulation model.)
Controller Design
Unlike traditional converter control loops which may require cascaded current and voltage loops, this system employs a single PI controller based on smart control principles. The same controller is reused for both voltage and current regulation by dynamically switching the feedback source based on the selected charging mode. This simplifies the control architecture without compromising performance. The PI gains are carefully tuned and found to perform effectively for both CC and CV operations.
CC-CV Mode Logic Implementation
The core logic of the CC-CV control is summarized below:
- CC Mode Operation:
During the initial charging phase, the battery receives a constant current. The controller compares the sensed battery current against a reference value. The error signal is processed by the PI controller to generate a modulation factor (K_rel_freq), which in turn adjusts the switching frequency of the LLC converter to regulate the output. - CV Mode Operation:
Once the battery terminal voltage reaches a predefined threshold, the system transitions to CV mode. Here, the controller maintains a constant terminal voltage. The sensed voltage is compared with the reference, and the corresponding PI controller output again defines K_rel_freq to sustain constant voltage while allowing the current to naturally reduce as the battery reaches full charge (SOC = 1).
The following figure gives the idea to implement this logic.
Smooth CC-CV Transition with Shared PI Controller:
By using a single PI controller for both constant current (CC) and constant voltage (CV) modes, the control system maintains the integral (I) state across the transition point. This approach allows the controller output (e.g., modulation factor or switching frequency command) to evolve smoothly even when the control mode changes, avoiding sudden jumps or discontinuities.
Specifically, during CC mode, the PI controller regulates current (error = Iref − Imeasured). Once the battery voltage approaches the CV threshold, the mode switches to voltage regulation (error = Vref− Vmeasured), but the integrator term in the PI controller retains its accumulated state. This continuity ensures a seamless transition between modes, minimizing overshoot, avoiding control glitches, and reducing stress on the power stage and battery.
This is particularly effective when using mode selection logic with hysteresis, as the shared controller avoids transients at the mode-switching boundaries.
Mode Selection Circuit
A key element of the implementation is the mode transition logic. The system monitors the scaled battery voltage and selects the operating mode based on a hysteresis window. For instance, with a normalized reference voltage of 1.0 p.u., the system enters CV mode when the battery voltage exceeds 0.98 and maintains CV mode up to 1.01, providing a stable band for mode switching. This logic ensures a seamless and robust transition between modes without oscillation or instability.
Simulation Results
The simulation results validate the proper functioning of the CC-CV charging strategy. As depicted in the waveform figures, the battery initially charges with a constant current, followed by a smooth transition to constant voltage mode. The Full-Bridge LLC Resonant Converter successfully adjusts its switching behavior to maintain the desired charging profile, ensuring efficient energy transfer, reduced switching stress, and reliable operation.
Simulation Files:
Learn More:
- Seamless Transition Control in CC-CV Battery Charging Using PSIM - Altair Community
- FIT Model-Based System Simulation of LLC Battery Chargers - Altair Community
- Energy Storage & Generation - Batteries| PV | Wind & Generators - Altair Community
