Biased Current-Limit Circuit for Power Converter Output Control
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Solution Overview
Problem
Existing current limit schemes for power converters either result in power loss due to the need for an input resistor or cause sub-harmonic oscillation when limiting switching current, especially at high flux density in transformers.
Innovation Solution
A biased current-limit circuit that includes an oscillator, waveform generator, sample-hold circuit, and current comparator to generate a pulse signal and current-limit threshold, allowing for the comparison of current-sensing signals to limit the maximum on-time of switching signals without the drawbacks of existing methods.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If an input resistor is connected to the input voltage for current limiting, then the output power can be limited, but power loss occurs at the input resistor
Solution Approach 1:
The patent extracts the current sensing function from the input voltage path by using a separate current-sensing resistor connected in series with the switching transistor. This allows current limiting to be achieved without placing a resistor in the input voltage path, thereby eliminating the power loss issue while maintaining the output power limit function.
Solution Approach 2:
The patent introduces a current-sensing resistor as an intermediary element that measures the switching current without being part of the main power path. This intermediary component enables indirect current measurement and limiting, avoiding the direct power loss associated with input resistor-based limiting methods.
2Device complexity
If a saw-limiter is used for output power limit without sensing input voltage, then the circuit complexity is reduced, but sub-harmonic oscillation occurs when switching current is limited at high flux density
Solution Approach 1:
The patent implements feedback by continuously monitoring the switching current through the current-sensing resistor and using the sensed signal to control the switching transistor's on-time. This feedback mechanism ensures stable operation by dynamically adjusting the switching parameters based on actual current conditions, preventing sub-harmonic oscillation while maintaining simple circuit architecture.
Solution Approach 2:
The patent employs dynamic current limiting by adjusting the maximum on-time of the switching transistor based on the sensed current signal. This dynamic control allows the system to adapt to varying load conditions and flux density levels, maintaining stability and preventing oscillation throughout the operating range rather than using fixed limiting thresholds.
Data Source
AI summary
A biased current-limit circuit for limiting a maximum output power of a power converter includes an oscillator for generating a pulse signal and an oscillation signal. A waveform generator generates a waveform signal in response to the oscillation signal. A sample-hold circuit is used to sample the waveform signal to generate a hold signal in response to a switching signal. The sample-hold circuit further samples the hold signal to generate a current-limit threshold in response to a second-sampling signal. A current comparator is utilized to compare a current-sensing signal with the current-limit threshold to limit a maximum on-time of the switching signal.