How to drive LED light string small trick
Some people may know how to drive an LED string, and this might be a widely accepted method. However, there are many hidden tricks behind this popular technique that most people aren't aware of. Today, Xiaobian will take you through some alternative approaches to better drive LED strings.
In mechanical and electrical systems, power and frequency have a critical relationship when operating near resonance (see Figure 1). Resonance can sometimes be problematic—if too much energy is injected into a single mode, it could damage the system. But it can also be beneficial. For example, resonance is commonly used to adjust frequency while maintaining enough power to keep the system oscillating at resonant frequencies, like in mechanical or electrical clocks. What many don’t realize is that resonance can also be used to control power levels, adapting them to variable loads—such as lighting arrays. This makes it ideal for achieving cost-effective and reliable solid-state lighting (SSL) systems.
Figure 1. This plot shows normalized power for a typical resonance (center frequency 30 kHz, bandwidth 20 kHz). Note that there's no overlap with line frequency.
LED applications are particularly interesting because LEDs are becoming more affordable for lighting purposes and offer better reliability compared to traditional DC drives. LEDs are inherently low-voltage DC devices, and their current-voltage (IV) curve is very steep at certain operating points. While a constant voltage source can drive LEDs, most designers prefer using a constant current DC driver. To match typical power distribution levels (like 120/240 VAC), luminaires often include multiple LED strings. These must be closely matched since the brightness of each LED depends on the current flowing through the string. A single LED failure, such as a short circuit or wiring issue, can cause the entire string to fail.
Distributed Reactance Components
Using resonance to control the power of an LED array helps overcome the limitations of AC LED drivers. In the simplest case, resonance can regulate the power of a single load. Verdi Semiconductor has effectively used resonance to create a current driver suitable for LED strings with high efficiency.
However, a more advanced approach is to distribute reactive components across the arrays. This allows not only overall power control but also individual sub-network adjustments without adding semiconductor devices. Distributed reactive components provide powerful new control capabilities at high efficiency and low cost. Reactive components can be capacitors or inductors, and they’re typically small and inexpensive, especially in the kilohertz to megahertz range. They can be implemented as discrete components or integrated on-chip.
Adding series and parallel reactive components opens up new ways to control power. These components can form a resonant tank where the main power loss comes from the resistive load of the LED. Near-lossless reactances can replace energy-consuming resistors used in basic DC circuits for current regulation.
Unit and Array
Imagine an illumination network made up of multiple lighting units, each containing one or more lighting elements, such as LEDs connected in pairs with series and shunt capacitors. There are various topologies, but Figure 2 shows a basic unit design. Any number of these units, along with hybrid configurations, can be connected in series and/or parallel to form a resonant network of reactive strings. We refer to this network as "Solid-State Lighting Reactance Strings" (RSSL).
Figure 2. Circuit showing two reactive string units
For example, in Figure 3, an energy storage circuit consists of 10 reactive strings. Assuming all LEDs are the same and all capacitors have the same value C, the total capacitance per unit is 2C, and the total capacitance of the string is C/5. The resonant frequency is √(5LC). The reactance of a unit is 1/(2ωC). As long as X >> R (where R is the actual resistance of the LED), the reactance string behaves as pure reactance, which means the resonant circuit has a high Q factor.
Figure 3. The reactive circuit consists of 10 A-type cells
A detailed analysis of a resonant network can be done using a circuit simulator, but even rough estimates can be made by selecting component values based on the desired operating frequency. The relationship between inductance and capacitance is fixed for a given frequency. Capacitors should be chosen so that their reactance is large enough to ensure a sufficiently high Q. The current through each cell is distributed between the LED and the shunt capacitor, and limited by the series capacitor, which acts like a resistor in a DC circuit. Ohm’s law for reactance can help find the desired value. The bypass capacitor stores the recirculating current when no current flows through the LED. In fact, not only is the current through the whole string controlled by resonance, but each LED also experiences local resonance control.
Multi-channel and Line Frequency Suppression
While the entire RSSL system can operate at a single frequency using the same capacitance, it's not necessary. Instead, we can think of the two-wire lighting bus as supporting multiple channels. Since each reactive string responds only to its specific frequency band, multiple bands can operate on the same wiring if they are spaced apart. Each center frequency can also be modulated for data communication between sensors and controllers.
If the line frequency is separated from the resonant frequency used for the reactance string, the response to the line frequency becomes negligible, and even without explicit filtering, cable frequency flicker won't occur. Therefore, an electrolytic capacitor isn't needed in the driver.
The RSSL system itself is electromagnetically quiet and resistant to noise spikes. Energy outside the narrow passband quickly dissipates. Cells and cell strings can be hot-swapped or switched without affecting other parts of the network. This allows many luminaires to share the same high-power drive. For example, a residential or commercial space can be powered by a two-wire bus using a single drive mounted on a power strip, with LEDs and capacitors, but without active semiconductor components. Dimming and switching can be done separately (see Figure 4).
Figure 4. A complete RSSL network consisting of drivers, various fixtures, dimming groups, and programmable or local dimmers.
The Larger the Array, the Higher the Reliability of RSSL
With DC drives, increasing the number of LEDs is often seen as a reliability concern, especially due to the sensitivity of individual components or connections. This is where the RSSL system shines. Failure analysis shows that the system’s reliability and lifespan actually improve as the array size increases. Even with 50% component failure, the remaining components can still function well.
Additionally, most high-power LEDs experience a significant drop in lumen output at the upper end of their rated current, reducing efficiency. The RSSL system allows for a low-cost design that minimizes this drop, making the lumen output less sensitive to maximum ratings.
Cost savings and reliability improvements can also be achieved through a COB (Chip-on-Board) architecture with multi-junction chips. Instead of building large-area devices on a single chip, you can choose the device area, power level, and cooling strategy to maximize efficiency. Then, place as many of these efficient devices as possible on a single chip to meet performance requirements. By driving the illumination array with one or more resonant strings, you can create a product line that can be scaled to any desired lumen output.
Figure 5. The upper curve shows the waveform when current flows through the array of serial illumination cells. The lower curve represents the current waveform through a pair of LEDs. The lumen waveform is the absolute value of the lower half of the curve. Note the short non-illuminated interval at the start of each half cycle.
Using resonance to control the power of LEDs in a reactance string is a powerful new method for driving LED arrays in any lighting application. This article only scratches the surface of the features and benefits of the RSSL system. Resonant drives offer a wide range of innovative tools that can be used to build advanced, low-cost, and multifunctional lighting systems.
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