Opportunity
The widespread adoption of LEDs in displays, signage, and general lighting is driven by their long lifespan, safety, and efficiency. However, a significant challenge arises when multiple LED strings are connected in parallel for high-power applications. Due to manufacturing tolerances, aging, and temperature variations, individual LEDs within parallel strings exhibit different forward voltage characteristics. This inherent mismatch leads to a critical current imbalance among the strings when powered by a common voltage source. Consequently, some strings draw more current than others, resulting in uneven luminous intensity, color variation (chromaticity shift), and the risk of exceeding the absolute maximum current rating for certain strings, which accelerates failure and reduces system reliability. Existing solutions to enforce current sharing, such as adding series ballast resistors or using linear current regulators for each string, suffer from substantial power losses and poor efficiency. More advanced architectures employing separate, independently controlled power converters for each string achieve good current balance but at the cost of increased circuit complexity, component count, and expense, making them less viable for cost-sensitive commercial applications. Therefore, there is a clear need for an LED driver topology that can provide precise current balancing for parallel LED strings while maintaining high efficiency, simplicity, and low cost.
Technology
This patent discloses an innovative LED driver architecture designed to solve the current imbalance problem in parallel LED strings efficiently. The core innovation is a hybrid power delivery system consisting of a single "master" power converter and multiple "slave" power converter modules, all working in concert. The master power converter, such as a switch-mode forward or flyback converter, generates a common DC output voltage (Vm) that supplies the majority (e.g., around 90%) of the required voltage for all connected LED strings. This master voltage is connected to a common node shared by all strings. In series with this master source, each individual LED string is connected to its own dedicated "slave" power converter. Each slave converter generates a smaller, independently regulated residual DC voltage (Vs1, Vs2,... Vsn). The total driving voltage for any given LED string is the sum of the common master voltage and its specific slave voltage. The key technological advancement lies in using these low-power slave converters solely for fine-tuning and current regulation. They actively adjust their small output voltage to compensate for the forward voltage differences among the LED strings, thereby forcing an equal current through each string. This is achieved through feedback control, where the current in each string is sensed and compared to a reference; the slave converter's output (e.g., via a magnetic amplifier or a semiconductor switch like a MOSFET) is then adjusted to maintain the set current. The topology can be efficiently implemented using a transformer with only two secondary windings: one for the high-power master source and one that feeds all the low-power slave post-regulators, simplifying magnetics. The invention also supports efficient dimming via a single PWM switch for all strings or a phase-shifted PWM (PSPWM) scheme per string.
Advantages
- Achieves precise current balancing across parallel LED strings, ensuring uniform brightness and color.
- Delivers high overall system efficiency because the majority of power is processed by a single, efficient master converter, and the slave converters handle only a small portion of the power.
- Reduces component count and cost compared to architectures using a full, independent converter for each string.
- Simplifies transformer design by requiring only two secondary windings to generate multiple independently regulated outputs.
- Enables the use of smaller, lower-cost components (e.g., Schottky diodes, smaller filter inductors) in the slave circuits due to their low operating voltage.
- Offers flexible dimming capabilities (conventional PWM or PSPWM) with minimal added loss.
- Improves reliability by preventing any LED string from operating beyond its current rating.
- The modular slave design allows for easy scalability to drive more parallel LED strings.
Applications
- LED backlighting units for liquid crystal displays (LCDs), including televisions, monitors, and laptops.
- Large-area LED lighting fixtures for commercial, industrial, and architectural applications requiring multiple parallel strings.
- Dynamic color-mixing lighting systems using parallel strings of Red, Green, and Blue (RGB) LEDs.
- LED signage and advertising displays.
- General-purpose high-power LED lighting solutions where consistent output and efficiency are critical
