LED Driving Circuit Parasitic Capacitance Compensation
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Solution Overview
Problem
Existing solutions for controlling the current to LEDs in isolated converters, such as flyback converters, are hindered by parasitic capacitances, which lead to inaccurate current adjustment due to delayed switching off times and varying load conditions, resulting in unintended current levels at the secondary side.
Innovation Solution
The driving circuit employs a primary side choke and a secondary side choke, with an auxiliary winding to monitor the switch-off time through voltage thresholds, allowing for real-time correction of switch-on and switch-off times to accurately control the current delivered to the LED, thereby compensating for parasitic capacitance effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional current control methods are used in isolated converters, then the circuit structure remains simple, but the current control precision deteriorates due to parasitic capacitance effects
Solution Approach 1:
An auxiliary winding is introduced as an intermediary element to sense the switch-off time indirectly through voltage monitoring. This mediator allows the control circuit to detect parasitic capacitance effects without directly measuring the secondary side current, thereby improving measurement precision while avoiding complex additional sensing circuits on the isolated side.
Solution Approach 2:
The control circuit uses feedback from the auxiliary winding voltage signal to detect the actual switch-off time and compensate for delays caused by parasitic capacitance. By continuously monitoring the voltage across the auxiliary winding and comparing it against threshold values, the system adjusts the switch-on time to maintain accurate current control despite the parasitic effects.
2Reliability
If switch-on time is extended to compensate for parasitic capacitance delays, then the current delivery to LED is improved, but the switching losses increase
Solution Approach 1:
The switch-on time is made dynamic rather than fixed. The control circuit continuously adjusts the switch-on time based on real-time detection of the switch-off time from the auxiliary winding voltage. This dynamic adjustment ensures the minimum necessary duration is applied to compensate for parasitic capacitance effects without excessive extension, thereby maintaining current delivery accuracy while minimizing switching losses.
3Speed
If the switching frequency is increased to improve current response, then the current control speed is improved, but the parasitic capacitance effects become more significant
Solution Approach 1:
The patent replaces direct mechanical/electrical measurement of secondary side current with an electromagnetic field-based sensing method using the auxiliary winding. By monitoring the voltage induced in the auxiliary winding during the switch-off transition, the system can detect timing information without being directly affected by the high-frequency parasitic capacitance effects that plague direct measurement methods at higher switching frequencies.
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 enables precise control of the current to LEDs by accounting for parasitic capacitance delays, ensuring the desired current is maintained across varying load conditions and dimming levels, improving the accuracy of current adjustment and reducing the influence of parasitic capacitances.
Implementation Method 1
The determining means comprises an auxiliary winding coupled to a secondary side choke and the determining means is configured to monitor the voltage across the auxiliary winding for determining a switch-off time
Data Source
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AI summary
The invention is related to a driving circuit for provision of an operating current for at least one lighting means, the driving circuit comprising an isolated switched converter having a switch (4) controlled by a control circuit (10), wherein a primary side choke (3) is charged when the switch (4) it is in its conducting state and the primary side choke (3) is discharged when the control circuit (10) controls the switch (4) in its non-conducting state, wherein the circuit comprises first determining means (13) for monitoring the current through the primary side choke (3), wherein the circuit comprises second determining means (11) for determining a switch-off time, wherein the second determining means (11) comprises an auxiliary winding (12) coupled to a secondary side choke (6) and the second determining means (11) is configured to monitor the voltage across the auxiliary winding (12) for determining a switch-off time.