Isolated DC-DC Converter Soft-Start Control
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Solution Overview
Problem
Isolated DC-DC converters experience excessive in-rush current during startup, leading to potential damage of load circuits due to voltage overshoot, as they fully drive the primary side without controlled power ramping across the isolation barrier.
Innovation Solution
The solution involves providing power to the primary side of the transformer in a predetermined increasing rate until the secondary side reaches a threshold, enabling feedback communication to control the power supply and ramp the voltage across the output terminals, thereby preventing overshoot. This is achieved by using a power supply that increments current in a stair-stepping fashion and employing a closed-loop controller to adjust the power delivery based on feedback signals from the secondary side.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the primary side is fully driven through startup, then the output power is maximized, but excessive in-rush current is generated causing voltage overshoot that can damage load circuits
Solution Approach 1:
The patent applies preliminary action by implementing a soft-start mechanism that gradually ramps up the output voltage from zero to the target voltage level. The controller progressively increases the duty cycle of the primary side switching, allowing the output capacitor to charge in stages rather than immediately applying full power. This prevents in-rush current spikes and voltage overshoot that would occur with direct full-power startup, while still achieving maximum output power once stabilized.
Solution Approach 2:
The patent employs dynamics by making the power delivery adaptive and variable during startup. The controller dynamically adjusts the switching duty cycle and frequency based on the instantaneous output voltage level and load conditions. This dynamic control allows the system to transition smoothly from zero power to full power, optimizing the balance between delivering sufficient output power and preventing harmful in-rush current effects throughout the startup process.
2Loss of time
If the output voltage is ramped up quickly, then the startup time is reduced, but the in-rush current increases causing potential damage to load circuits
Solution Approach 1:
The patent applies periodic action through a staged voltage ramping scheme where the controller increments the output voltage in discrete steps or pulses rather than continuous linear ramping. The duty cycle is adjusted in periodic intervals, allowing the output capacitor to charge in controlled bursts. This periodic adjustment enables faster startup compared to slow continuous ramping while limiting peak current draw by pausing between voltage increments, thus reducing in-rush current effects.
3Loss of energy
If the primary side is driven at high current levels, then the power transfer efficiency is improved, but the in-rush current damages the isolation barrier and load circuits
Solution Approach 1:
The patent applies preliminary action by pre-charging the output capacitor through a controlled current limiting circuit before enabling full power transfer. During the preliminary phase, the primary side switch operates at reduced current levels to safely charge the isolation barrier capacitance and output capacitor. Once the barrier is pre-charged and the output voltage reaches a safe threshold, the controller transitions to high-current operation for efficient power transfer, thereby protecting the isolation barrier from damage while achieving high efficiency in the steady state.
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 approach allows for rapid and controlled startup of isolated DC-DC converters, preventing excessive in-rush current and voltage overshoot, enabling safe and efficient power transfer across the isolation barrier while allowing for bidirectional communication and adaptive power management.
Implementation Method 1
Isolated DC-DC converters comprise a transformer, with a power generating input side, typically called the primary side, and a power output side, typically called the secondary side
Data Source
AI summary
Methods and systems for providing electrical power using an isolated DC-DC converter include: using a power supply to provide multiple amplitudes of open loop current to a primary side of a transformer. The amplitudes are selected to increase stepwise, at predetermined times, to ramp a voltage across output terminals connected to a secondary side of said transformer so that after said power supply provides a maximum open loop current amplitude, said voltage reaches a threshold sufficient to enable a closed loop controller connected to said output terminals to send a feedback signal to said primary side. The threshold voltage is insufficient to fully power said output terminals. The feedback signal is selected to control said power supply to increase current to said primary side at closed loop current levels until said output terminals are fully powered.