1 LLC Converter Control

The primary objective of the control strategy in power converters is to achieve a fast dynamic response, characterized by high bandwidth. This ensures stable and accurate output performance across the full range of operating conditions. Additionally, the chosen control methodology must maintain overall system stability and reliability, which are critical for both safety and long-term operation.

Applying current mode control to LLC resonant converters can significantly improve dynamic response. However, the alternating nature of the resonant tank current—closely tied to the switching frequency—introduces implementation challenges, especially at higher switching frequencies. These challenges often complicate direct software-based control and may require specialized hardware or algorithms.

Average current control is widely used due to its simplicity and effectiveness in many applications. Nevertheless, the inherent filtering process in this method restricts the achievable system bandwidth, potentially limiting the converter’s ability to respond rapidly to load or input changes.

Advanced control methodologies, such as Hybrid Hysteretic Control (HHC) and Bang-Bang control, leverage the resonant tank capacitor current or voltage to directly regulate the tank current. Since the capacitor voltage is determined by the charging current, these approaches can achieve high bandwidth and superior dynamic response, making them ideal for demanding applications.

Charge control constructs an equivalent current loop by integrating the resonant tank current, rather than directly controlling it. This approach introduces a 90-degree phase shift between the resonant current and capacitor voltage and adds a pole to the system, both of which can negatively affect dynamic response and system stability (Qiu et al. [1]).

As an alternative to these methods, Direct Peak Current Control methods are introduced by Qiu et al. [1]. This method directly captures the peak current during the positive half-cycle using a dedicated detection circuit. This approach offers the benefits of HHC and charge control—such as high bandwidth and dynamic response—while reducing complexity by eliminating the need for an integrator.

Despite its advantages, Direct Peak Current Control at high switching frequencies requires precise controller behavior. Specifically, the ON time of the high-side switch must be accurately and seamlessly replicated for the low-side switch to maintain a proper sinusoidal waveform. This demands very specific behavior from the controller.