Capacitor-Less LED Driver Using Transformer Magnetizing Inductance
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Solution Overview
Problem
Conventional offline LED drivers with input electrolytic capacitors suffer from poor power factor due to distorted input current, requiring bulky and unreliable components, and face issues with voltage regulation and feedback loop stability.
Innovation Solution
A control circuit and method for an LED driver that eliminates the need for an input capacitor by using a primary-side controlled flyback power converter with a switching controller that generates a switching signal based on feedback signals from voltage and current detection circuits, ensuring constant current output and improved power factor without an input capacitor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an input electrolytic capacitor is used for energy storage in the LED driver, then the power conversion can operate properly with sufficient voltage, but the input current becomes distorted and the power factor deteriorates
Solution Approach 1:
The patent removes the input electrolytic capacitor from the circuit by implementing a capacitor-less PFC topology. The energy storage function is transferred to the transformer's magnetizing inductance, eliminating the source of current distortion while maintaining proper power conversion operation.
Solution Approach 2:
The transformer's magnetizing inductance serves dual functions: providing galvanic isolation between primary and secondary sides and simultaneously acting as the energy storage element that replaces the input capacitor. This multi-functionality eliminates the need for separate capacitor components.
2Object-generated harmful factors
If the capacitance of the input electrolytic capacitor is reduced to improve power factor, then the power factor improves, but the input voltage VDC becomes low causing feedback open loop
Solution Approach 1:
By removing the input capacitor entirely and using the transformer's magnetizing inductance for energy storage, the patent eliminates the trade-off between capacitor size and power factor. The PFC circuit achieves high power factor without compromising feedback loop stability through primary-side regulation.
3Reliability
If an input electrolytic capacitor is used for energy storage, then sufficient voltage can be maintained, but the LED driver requires bulky and low reliability components
Solution Approach 1:
The patent eliminates the bulky input electrolytic capacitor by transferring the energy storage function to the transformer's magnetizing inductance. This removal significantly reduces the overall size and weight of the LED driver while eliminating the reliability issues associated with electrolytic capacitors.
Solution Approach 2:
The transformer serves multiple functions including galvanic isolation, voltage transformation, and energy storage. By utilizing the magnetizing inductance for energy storage, the patent eliminates the need for separate capacitor components, reducing overall device size and improving reliability.
4Power
If the feedback loop is significantly on/off in response to input line voltage changes, then voltage regulation can be achieved, but overshoot and undershoot signals are generated at the output
Solution Approach 1:
The patent implements anticipatory control by detecting the zero-crossing point of the input current and提前 adjusting the switching duty cycle. This preliminary action prevents the feedback loop from reacting too aggressively to voltage changes, thereby eliminating overshoot and undershoot while maintaining proper voltage regulation.
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 solution enhances the power factor, reduces the size and cost of the LED driver, and maintains stable feedback loops, preventing overshoot and undershoot while eliminating the need for bulky electrolytic capacitors, thus improving reliability.
Implementation Method 1
A transformer 10 has a primary winding NP, a secondary winding NS and an auxiliary winding NA. When the transistor 20 is turned on, a switching current IP will flow through the transformer 10.
Implementation Method 2
The rectifier 12 receives an input line voltage VAC and rectifies the input line voltage VAC. A voltage VDC is provided by the input electrolytic capacitor 40.
Implementation Method 3
Another terminal of the primary winding NP is coupled to a transistor 20. The transistor 20 is utilized to switch the transformer 10. The switching controller 50 generates a switching signal SW. The switching signal SW controls the transistor 20 to switch the transformer 10
Implementation Method 4
A terminal of the auxiliary winding NA is coupled to an anode terminal of a diode 41. The auxiliary winding NA charge the capacitor 45 through the diode 41 to generate a power source VCC for a switching controller 50.
Data Source
AI summary
A control circuit of a LED driver according to the present invention comprises an output circuit, an input circuit and an input-voltage detection circuit. The output circuit generates a switching signal to produce an output current for driving at least one LED in response to a feedback signal. The switching signal is coupled to switch a transformer. The input circuit samples an input signal for generating the feedback signal. The input signal is correlated to the output current of the LED driver. The input-voltage detection circuit generates an input-voltage signal in response to an input voltage of the LED driver. The input circuit will not sample the input signal when the input-voltage signal is lower than a threshold. The control circuit can eliminate the need of the input capacitor for improving the reliability of the LED driver.