Flyback LED Power Supply with Schmitt Trigger Control
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Solution Overview
Problem
Existing LED power supplies with flyback converters face challenges in minimizing component count while maximizing efficiency, particularly due to the high cost of transformers with multiple windings and inefficiencies in switching losses at high input voltages.
Innovation Solution
The design incorporates a feedback Schmitt trigger clock oscillator, a comparator with a voltage divider network, and an optocoupler for cost-effective control, along with a burst mode oscillator and overvoltage protection using a series transistor and Zener diode, to achieve efficient switching with minimal additional components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a transformer with three separate windings (two center-tapped) is used for LED power supply control, then constant output current regulation is achieved, but production cost increases significantly
Solution Approach 1:
The patent extracts the center-tap requirement from the transformer design by using a single winding configuration. The controlled switch and diode rectifier are positioned to utilize the same winding for both switching and rectification functions, eliminating the need for separate center-tapped windings while maintaining constant output current regulation capability.
Solution Approach 2:
The single transformer winding serves multiple functions: it acts as the primary switching winding for the controlled switch, provides the rectification path through the diode, and enables both voltage transformation and current regulation. This multi-functional design replaces the specialized three-winding structure with a more economical single-winding configuration.
2Ease of operation
If conventional switching power supply design is used with FET and sawtooth voltage control, then basic LED driving function is achieved, but switching losses increase at high input voltages
Solution Approach 1:
The patent implements feedback control by monitoring the LED current through a sensor and comparing it with a reference value. The error signal is fed back to the control circuit, which adjusts the controlled switch timing and duty cycle to maintain optimal switching conditions. This feedback mechanism enables the system to adapt to high input voltages by optimizing the switching instant to minimize voltage across the switch during turn-on, thereby reducing switching losses.
Solution Approach 2:
The controlled switch is designed with dynamic control capabilities, allowing the switching instant and duty cycle to be adjusted in real-time based on input voltage conditions. The control circuit modifies the switching parameters dynamically to ensure the switch turns on at the optimal moment when voltage stress is minimized, adapting the switching behavior to varying input conditions and reducing energy losses during switching transitions.
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 allows for efficient switching with reduced turn-on losses and effective overvoltage protection, maintaining high efficiency and low production costs, thereby addressing the need for a cost-effective and efficient LED power supply.
Implementation Method 1
the clock oscillator is designed as a feedback Schmitt trigger
Implementation Method 2
the output of the control amplifier is connected to the input of an optotransistor of the optocoupler
Data Source
Figure 1(a)~2(b)
Figure 3
Figure 4~5
AI summary
An LED power supply, comprising a flyback converter (1), wherein the primary winding of a transformer (6) is connected in series with a controlled switch (7) designed as a power transistor to an input DC voltage (UE), and a diode rectifier (8) and a charging capacitor (9) are connected downstream of the secondary winding, as well as a control circuit for the controlled switch, which has a clock oscillator (13) and a driver stage (12) and is configured to switch on the controlled switch at least approximately at the voltage minimum of the parasitic oscillations of the drain-source voltage, and with a control of the secondary current (iD) by an LED arrangement with at least one LED, wherein the output signal (iLED) of a current sensor (17) of the secondary side and a reference signal (iLED,ref) are supplied to the inputs of the control amplifier (18), the output of which is supplied to the control circuit.wherein the clock oscillator (13) is configured as a feedback Schmitt trigger, the drain-source voltage of the power transistor (7) is supplied to the input of a comparator (26) via a voltage divider (Rd, Rm), and the output of the comparator (26) is connected to the feedback point (A) of the clock oscillator (13) via a coupling network (28), the output signal of a control amplifier (18) is added to a signal proportional to the switch current (iT), and a voltage value corresponding to this sum signal is likewise supplied to the feedback point of the clock oscillator (13).