Synchronous Rectifier Control for Boost Converter Bursting
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Solution Overview
Problem
Conventional synchronous boost converters experience bursting behavior and significant output voltage ripple when the input voltage approaches the output voltage, leading to inefficiencies and large inductor current excursions, as the minimum on-time of the power switch becomes too large to maintain the required duty cycle.
Innovation Solution
A control circuit adjusts the average impedance of the synchronous rectifier's conduction path by incrementally increasing it as the input voltage approaches and exceeds the output voltage, reducing the conduction period of the synchronous switch and disabling it for durations necessary to reset the inductor current, thereby optimizing efficiency and reducing ripple.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the synchronous rectifier is kept conducting to maintain low impedance path, then power conversion efficiency is improved, but inductor current cannot be sufficiently reset leading to bursting behavior and large output voltage ripple
Solution Approach 1:
The synchronous rectifier's conduction state is dynamically adjusted based on operating conditions. When Vin approaches Vout, the control circuit selectively disables the synchronous rectifier during portions of the discharge phase to allow sufficient inductor current reset, while keeping it conducting during other portions to maintain efficiency. This dynamic switching between conducting and non-conducting states resolves the contradiction between efficiency and current reset requirements.
Solution Approach 2:
The control circuit changes the impedance parameter of the synchronous rectifier based on the voltage difference between Vin and Vout. When the voltage difference is small (Vin approaches Vout), the synchronous rectifier is disabled for sufficient time periods to increase impedance and allow inductor current reset. When the voltage difference is larger, the synchronous rectifier remains conducting to maintain low impedance and high efficiency. This parameter change resolves the contradiction adaptively.
2Reliability
If the minimum on-time of the power switch is increased to ensure sufficient switching, then reliable switching operation is achieved, but the duty cycle becomes too large when Vin approaches Vout causing bursting behavior
Solution Approach 1:
The patent applies minimum on-time enforcement from the power switch control to the synchronous rectifier control. By disabling the synchronous rectifier for sufficient time periods, a copy of the minimum on-time constraint is created in the discharge phase, ensuring that the inductor current has adequate time to reset regardless of the power switch's minimum on-time requirements. This copying approach allows the power switch to maintain its minimum on-time for reliable operation while the synchronous rectifier provides complementary timing for current reset.
3Reliability
If the synchronous rectifier is completely disabled to ensure sufficient inductor current reset, then bursting behavior is eliminated, but power conversion efficiency drops drastically
Solution Approach 1:
Instead of completely disabling the synchronous rectifier, the control circuit applies partial action by disabling it only for sufficient time periods within the discharge phase when Vin approaches Vout. This partial disabling provides just enough impedance increase to allow inductor current reset while keeping the synchronous rectifier conducting during other portions of the discharge phase to maintain low impedance and high efficiency. This partial action approach resolves the contradiction between current reset sufficiency and efficiency.
Data Source
AI summary
A synchronous boost DC/DC conversion system comprises an input for receiving a DC input voltage, an output for producing a DC output voltage, a power switch controllable to adjust an output signal of the conversion system, and an inductor coupled to the input. A synchronous rectifier is configurable to create a conduction path between the inductor and the output to provide the inductor discharge. A control circuit is provided for controlling the synchronous rectifier as the input voltage approaches the output voltage, so as to adjust average impedance of the conduction path over a discharge period of the inductor.