DC-to-DC Converter Pre-bias Startup Control
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Solution Overview
Problem
Voltage converters employing synchronous rectifiers face reverse current flow during start-up, particularly when pre-biased, which can cause malfunction or damage to downstream devices, and existing solutions either fail to completely eliminate this issue or require additional components increasing costs.
Innovation Solution
A DC-to-DC converter with a voltage-second (VS) controller that selectively controls a second duty cycle for a switch at the secondary side based on a first duty cycle at the primary side, ensuring a monotonic startup behavior by maintaining the output voltage at a pre-biased level until a threshold is reached, thereby preventing reverse current flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If synchronous rectifiers are used in a DC-to-DC converter, then power efficiency is improved, but reverse current flow occurs during start-up causing malfunction or damage to downstream devices
Solution Approach 1:
The patent applies preliminary action by detecting the output voltage level before the converter starts up and comparing it with a reference voltage. Based on this pre-detection, the controller pre-configures the duty cycle of the synchronous rectifier switch to ensure it remains off when the output voltage exceeds the reference, thereby preventing reverse current flow before it can occur. This proactive approach eliminates the harmful reverse current while maintaining the efficiency benefits of synchronous rectification.
2Device complexity
If the synchronous rectifier switch is controlled with a fixed duty cycle during start-up, then control simplicity is improved, but reverse current cannot be completely prevented when output voltage exceeds reference voltage
Solution Approach 1:
The patent implements dynamics by making the duty cycle of the synchronous rectifier switch variable rather than fixed. The controller dynamically adjusts the duty cycle based on the detected output voltage level relative to a reference voltage. When the output voltage exceeds the reference, the duty cycle is reduced to zero, turning off the synchronous rectifier switch to prevent reverse current. This dynamic control strategy maintains reliability while managing complexity through intelligent adaptation.
Solution Approach 2:
The patent applies feedback by continuously detecting the output voltage level and using this information to adjust the synchronous rectifier switch control. The controller monitors the output voltage and compares it with a reference voltage, then feeds this information back to modulate the duty cycle accordingly. This closed-loop feedback mechanism ensures reverse current prevention while maintaining efficient operation during normal converter operation.
3Reliability
If additional discharging circuits are added to prevent reverse current, then reverse current prevention is improved, but device complexity and cost increase
Solution Approach 1:
The patent applies self-service by enabling the synchronous rectifier switch to serve dual functions: it acts as a rectifier during normal operation and as a self-controlled switch that can be turned off to prevent reverse current during start-up. The controller utilizes the existing switch and its control circuitry to monitor output voltage and automatically disable the synchronous rectifier when conditions warrant, eliminating the need for separate discharging circuits or additional protection components.
Data Source
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AI summary
A method includes comparing, by a voltage-second (VS) controller (120)，a first duty cycle (D1) used to control a first switch (M1) at a primary side of a power transformer (102) of a DC-to-DC converter (100) with a threshold. The method further includes if a value of the first duty cycle (D1) is less than the threshold, controlling, by the VS controller (120), a second duty cycle (D2) used to control a second switch (M3) at a secondary side of the power transformer (102), and maintaining a voltage level at an output voltage node (V O) at a non-zero value, and if the value of the first duty cycle (D1) is greater than the threshold, controlling, by an output voltage loop (110), the second duty cycle (D2) based on the first duty cycle (D1), and monotonically increasing the voltage level at the output voltage node (V O) from the non-zero value to a predetermined value.