Adaptive Minimum On-Time Control for Power Converter Reflected Voltage
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Solution Overview
Problem
Traditional power converters face challenges in accurately measuring reflected voltage at light load conditions due to distortion caused by parasitic capacitors and resistors, leading to inaccurate detection of discharge time and output voltage.
Innovation Solution
A control circuit with a sampling circuit that generates a voltage signal from the reflected voltage, an adaptive signal correlated to the input voltage, and a switching circuit that adjusts the minimum on-time of the switching signal to maintain constant discharge time, thereby improving reflected voltage detection accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the on time and discharge time are short in light load conditions, then the power converter operates efficiently at light loads, but the parasitic capacitor and resistors form a low pass filter that distorts the voltage-sense signal waveform
Solution Approach 1:
The control circuit preemptively extends the minimum on-time of the switching signal based on the input voltage level before the reflected voltage measurement occurs. This preliminary adjustment ensures that sufficient energy is stored in the transformer during the on-time, compensating for the distortive effects of parasitic elements during the subsequent discharge phase, thereby maintaining accurate reflected voltage detection at light loads.
Solution Approach 2:
The invention dynamically changes the on-time parameter of the switching signal based on the input voltage level. By establishing a minimum on-time that is inversely proportional to the input voltage, the system adapts the energy storage duration to compensate for the fixed parasitic time constants, ensuring accurate reflected voltage measurement across varying load conditions without requiring hardware modification.
2Device complexity
If a parasitic capacitor and resistors form a low pass filter, then the circuit is simple to implement, but the waveform of the voltage-sense signal is distorted and reflected voltage is inaccurately detected
Solution Approach 1:
Instead of attempting to eliminate or bypass the parasitic capacitor and resistors, the invention accepts their presence and converts their harmful filtering effect into a beneficial design parameter. By designing the minimum on-time to be inversely proportional to the input voltage, the system ensures that sufficient energy is stored before discharge, making the measurement robust against the fixed parasitic time constants. This approach maintains circuit simplicity while achieving accurate measurement.
3Measurement precision
If the minimum on-time is extended to improve reflected voltage sampling, then the light load operation efficiency decreases, but the measurement accuracy improves
Solution Approach 1:
The invention makes the on-time dynamic by establishing a minimum on-time that varies inversely with the input voltage level. This dynamic adjustment ensures that the on-time is extended only when necessary (at higher input voltages where the parasitic filtering effect is more pronounced), rather than using a fixed extended on-time that would always reduce efficiency. The system adapts the energy storage duration to the specific operating conditions, maintaining efficiency while ensuring adequate sampling accuracy.
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
The solution ensures accurate detection of reflected voltage and maintains constant discharge time, even at light loads, by inversely proportioning the minimum on-time of the switching signal to the input voltage, thus enhancing the regulation of the power converter's output.
Implementation Method 1
The energy is stored into the transformer 10 during a switch 20 is on... The energy will be discharged to the output terminal of the power converter when the switch 20 is switched off
Implementation Method 2
Once the switch 20 is turned off, the auxiliary winding NAUX will produce a reflected voltage VF correlated to the output voltage VO
Implementation Method 3
a parasitic capacitor 23 and resistors 21, 22 forms a low pass filter that will distort the waveform of the voltage-sense signal VDET
Data Source
AI summary
A control circuit includes a switch coupled to a transformer of a power converter for switching the transformer. A sampling circuit is coupled to the transformer to sample a reflected voltage of the transformer to generate a voltage signal. A switching circuit generates a switching signal to control the switch in response to the voltage signal. The minimum on time of the switching signal is changed in response to the change of an input voltage of the power converter. Because the pulse width of the reflected voltage is narrower at light load, the minimum on time of the switching signal helps the reflected voltage detection.