Digital Controller for Full-Bridge Power Converter Volt-Second Imbalance
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Solution Overview
Problem
Full-bridge power converters face challenges in maintaining volt-second balance without using large and costly capacitors, particularly when employing digital controllers, as conventional control methods like voltage mode control can lead to imbalances and analog current mode control is difficult to implement digitally.
Innovation Solution
A digital controller for full-bridge power converters that uses a sigma-delta modulator with two integrators and decimation filters to detect and adjust for volt-second imbalances by modulating the integrated signal into a bitstream and controlling pulse-width modulation to balance currents, eliminating the need for capacitors and achieving balance without high-speed analog-to-digital converters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If voltage mode control is used in full-bridge power converters, then the control implementation is simple, but volt-second imbalance occurs leading to transformer saturation and potential damage
Solution Approach 1:
The patent implements a feedback mechanism using a current transformer to sense the current through the common node, converting it to a voltage signal. This sensed voltage is integrated and compared between phase 1 and phase 2, with the difference fed back to the PWM generator to adjust pulse widths and eliminate volt-second imbalance automatically
Solution Approach 2:
The patent replaces the conventional voltage mode control mechanism with a digital control system that uses a sigma-delta modulator and decimation filters to process the current signal. This substitution of control methodology enables precise digital measurement and adjustment of volt-second balance without requiring analog high-speed ADCs
2Reliability
If current mode control is used to avoid volt-second imbalance, then volt-second balance is maintained, but implementation with digital controllers becomes difficult
Solution Approach 1:
The patent substitutes complex high-speed analog current mode control with a digital implementation using sigma-delta modulation. The current signal is integrated and modulated into a bitstream, which is then decimated to produce digital words that represent the volt-second balance error, simplifying digital controller implementation while maintaining reliability
Solution Approach 2:
The patent changes the parameter representation from direct high-speed current measurement to integrated voltage signals that are modulated and decimated. This parameter transformation allows digital controllers to achieve current mode control functionality at lower bandwidths, reducing implementation complexity
3Reliability
If a capacitor is added in series with the primary side to compensate for volt-second imbalance, then magnetic flux build-up is avoided, but the capacitor size and cost increase significantly
Solution Approach 1:
The patent extracts the volt-second balance compensation function from the passive capacitor approach and implements it actively through digital control. By sensing the current and using digital signal processing to generate corrective PWM signals, the system eliminates the need for large compensation capacitors while maintaining magnetic flux balance
Solution Approach 2:
The patent replaces the passive electrical compensation method (using large capacitors) with an active digital control system. The digital controller processes current signals through integration and decimation, then adjusts PWM duty cycles to maintain volt-second balance, substituting complex passive components with intelligent control logic
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution effectively balances volt-second imbalances without capacitors, reducing costs and improving reliability by using a digital controller that can maintain balance within a lower bandwidth than the switching frequency, enhancing the stability and efficiency of full-bridge power converters.
Implementation Method 1
A current transformer, the primary windings of which are coupled to the common node so that they conduct the first and second currents
Implementation Method 2
A rectifying circuit is coupled to the current transformer's secondary windings, and provides a DC voltage Vin at an output when either of the first or second currents is greater than zero
Data Source
AI summary
A digital controller for use with a full-bridge power converter which includes an isolation transformer that conducts first and second currents of opposite polarity during respective power-transfer phases. A current transformer senses the currents, and a non-zero current Iin is generated when either of the first or second currents is >0. The controller includes a sigma-delta modulator arranged to integrate a current applied to its input and to modulate the integrated signal to a bitstream. Iin is integrated by a first integrator and the bitstream is decimated to a digital word by a first decimation filter during the first power-transfer phase, and is integrated by a second integrator and decimated with a second decimation filter during the second phase. The difference between the digital values is used to adjust the pulses that operate the full-bridge switches as necessary to reduce any imbalance between the first and second currents.