Isolated Resonant DC-DC Converter Charge Control for Fast Transients
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Solution Overview
Problem
Conventional isolated resonant DC-DC converters face challenges in achieving wide-bandwidth control loop stability, leading to poor transient response to input voltage and load current changes, and insufficient suppression of low-frequency output voltage ripple.
Innovation Solution
The proposed solution combines charge control with input voltage feedforward control and/or output current feedforward control, enhancing the transient response and reducing low-frequency output voltage ripple. This approach is implemented in a full-bridge LLC converter with a controller located on the secondary side of the isolation transformer, allowing for cost-effective digital control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If direct frequency control is used for output voltage regulation, then the converter can operate efficiently, but the control loop bandwidth is limited and transient response to input voltage and load current changes is poor
Solution Approach 1:
The control system is segmented into multiple independent control loops: an inner charge control loop that operates at high bandwidth for fast transient response, and an outer voltage control loop that provides steady-state regulation. This segmentation allows each loop to be optimized independently, achieving fast response without excessive overall complexity.
Solution Approach 2:
Charge control is introduced as an intermediary control mechanism between the voltage error signal and the switching frequency. Instead of directly controlling frequency based on voltage error (which limits bandwidth), the charge control loop processes the error signal through charge integration, enabling wider bandwidth and faster transient response while maintaining system stability.
2Speed
If the control loop bandwidth is increased to improve transient response, then the response to input voltage and load changes improves, but stability margins deteriorate
Solution Approach 1:
The charge control loop employs dynamic compensation techniques where the control parameters adapt based on operating conditions. The charge integration process naturally provides frequency-dependent phase compensation, allowing the loop to maintain stability margins across a wide bandwidth range while achieving fast transient response.
Solution Approach 2:
Multiple feedback paths are implemented: voltage feedback for steady-state regulation, charge feedback for transient response, and current feedback for disturbance rejection. These feedback mechanisms work together to maintain stability while enabling wide bandwidth operation, with each feedback path contributing to different aspects of system stability.
3Object-affected harmful factors
If conventional control methods are used, then the device structure remains simple, but low-frequency output voltage ripple is insufficiently suppressed
Solution Approach 1:
The charge control loop performs preliminary action by anticipating and correcting voltage deviations before they manifest as significant output ripple. By controlling the charge transfer in advance, the system proactively compensates for low-frequency disturbances such as rectifier ripple, preventing them from appearing as output voltage variations.
Solution Approach 2:
The control method changes the operating parameters dynamically by adjusting the charge control reference based on the voltage error signal. This parameter modulation enables the converter to actively suppress low-frequency ripple components without requiring additional passive filtering components, achieving ripple reduction through active parameter control.
Data Source
AI summary
An isolated DC-DC converter is provided, including a full-bridge switching stage, a resonant network, a transformer, an output stage, an output stage, an error amplifier, a feedforward controller, a charge sensor and a switch controller. The output stage generates an output signal, wherein the output signal is an output voltage or output current. The error amplifier generates an error signal based on the output signal and a reference signal, wherein the reference signal is a reference voltage or reference current. The feedforward controller senses an input voltage of the full bridge switching stage, and generates a feedforward control signal based on the sensed signal. The charge sensor generates at least one charge sensing signal. The switch controller generates and provides switch control signals to the plurality of active switches in charge control mode based on at least the error signal, the feedforward control signal and the charge sensing signal.