DC-DC Converter Startup Control for Backflow Current Reduction
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Solution Overview
Problem
Conventional DC-DC converters experience significant backflow currents when starting up or transitioning to synchronous rectification, leading to inefficiencies and increased costs due to the need for high-rated FETs to handle large current surges, especially when the output voltage is near the target voltage or during changes in load power consumption.
Innovation Solution
A DC-DC converter design that includes a first switching element and a second switching element, an inductance element, and a diode, with a PWM control circuit that sets a high target voltage initially to reduce backflow by increasing the on-off ratio of the first switching element and keeping the second switching element off, then transitions to normal operating voltage once stable, allowing for lower-rated components and reduced cost.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional DC-DC converter control methods are used during startup or transition to synchronous rectification, then the converter can operate, but large backflow currents occur causing inefficiency and requiring high-rated expensive FETs
Solution Approach 1:
The control method performs preliminary actions by detecting startup conditions and transition conditions before they become problematic. When startup is detected, the controller proactively controls the second FET to remain off until the output voltage reaches a predetermined level, preventing backflow current before it occurs. Similarly, when transition to synchronous rectification is detected, the controller proactively controls the second FET's on-off timing based on the output voltage relative to the target voltage, preventing regenerative braking current before it occurs. This preliminary detection and control approach eliminates the need for high-rated expensive FETs while maintaining operational reliability.
2Reliability
If high-rated FETs are used to handle large backflow currents, then operational reliability is maintained, but device cost increases
Solution Approach 1:
The control method changes the operating parameters of the FETs dynamically based on the converter's operational state. During startup, the controller adjusts the second FET's on-off state based on whether the output voltage has reached the predetermined level. During transition to synchronous rectification, the controller adjusts the second FET's on-off timing based on whether the output voltage is below or above the target voltage. These parameter changes allow the use of lower-rated, less expensive FETs while maintaining operational reliability by preventing excessive current flow through intelligent control.
3Loss of energy
If the second switching element is kept off during startup to prevent backflow, then current backflow is reduced, but the converter cannot perform efficient boost operation during stable operation
Solution Approach 1:
The control method makes the second FET's state dynamic rather than static. During startup, when the output voltage is below the predetermined level, the second FET is kept off to prevent backflow current. Once the output voltage reaches the predetermined level, the second FET transitions to normal alternating on-off operation to enable efficient boost operation. During transition to synchronous rectification, the controller dynamically adjusts the second FET's on-off timing based on the output voltage relative to the target voltage. This dynamic control approach resolves the contradiction by adapting the second FET's state to the converter's operational phase.
4Use of energy by moving object
If synchronous rectification is started immediately during transition, then power consumption can be reduced, but regenerative braking current occurs causing voltage drop and instability
Solution Approach 1:
The control method uses feedback by continuously monitoring the output voltage and comparing it to the target voltage during transition to synchronous rectification. When the output voltage is below the target voltage, the controller controls the second FET to prevent regenerative braking current, maintaining voltage stability. When the output voltage reaches or exceeds the target voltage, the controller allows synchronous rectification to proceed, reducing power consumption. This feedback-based control resolves the contradiction by using voltage level information to dynamically adjust the rectification mode.
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 design effectively minimizes backflow currents by setting a higher target voltage at startup, reducing the on-time of the second switching element and synchronizing rectification, thereby enabling the use of lower-rated components and stabilizing output voltage efficiently, thus lowering overall costs and improving operational reliability.
Implementation Method 1
an inductance element for receiving a voltage from the DC voltage source and storing energy therein when the first switching element is in an ON state, and for charging the energy to the energy storage when the second switching element is in an ON state
Data Source
Figure 1
Figure 2A~2E
Figure 3
AI summary
A DC-DC converter includes a control circuit (19) which can change the target voltage of the reference voltage source (15). At the start-up of the DC-DC converter, the PWM control circuit (21) controls so that the first switching element (9) gradually increases the on-off ratio from a minimum on-off ratio; that the second switching element (11) is placed in the OFF state, and that the target voltage is set higher than the normal operating voltage. When the start-up operation is completed, the PWM control circuit (21) controls so that the second switching element (11) starts to turn on and off, and that the target voltage is returned to the normal operating voltage.