Some people may be familiar with driving LED strings, which is a widely accepted method. However, there are many lesser-known techniques behind this popular approach. Today, we’ll explore some clever tricks to better drive LED strings and improve their performance.
In mechanical and electrical systems, power and frequency have a critical relationship when operating near resonance (see Figure 1). While resonance can sometimes be harmful—like when too much energy enters a single mode and damages the system—it can also be beneficial. For example, resonance is often used in devices like clocks to maintain oscillation at specific frequencies. What many people don’t realize is that resonance can also be used to control power and adapt it to varying loads, such as in lighting arrays. This makes it ideal for solid-state lighting (SSL) systems, offering both cost-effectiveness and reliability.
Figure 1 shows the normalized power of a typical resonant circuit with a center frequency of 30 kHz and a bandwidth of 20 kHz. Importantly, there is no overlap with the line frequency, which helps prevent interference.
LEDs are particularly interesting because they are becoming more affordable and efficient for lighting applications. However, traditional DC drivers come with challenges like cost and reliability. LEDs operate on low-voltage DC and have a steep current-voltage curve, making constant current sources more common than constant voltage. To match standard power distribution levels (like 120/240 VAC), multiple LED strings are often used in luminaires. These strings must be well-matched, as the light output of each LED depends on the current flowing through it. A single failure, such as a short circuit or wiring issue, can cause the entire string to fail.
**Distributed Reactance Components**
Using resonance to control power in an LED array offers a solution to these issues. In its simplest form, resonance can control power for a single load. Verdi Semiconductor has applied this technique to create efficient current drivers suitable for LED strings. But a more advanced approach is to distribute reactive components across the array. This allows not only overall power control but also individual sub-network adjustments without adding extra semiconductor devices. Distributed reactance components provide high efficiency and low cost, using capacitors or inductors that are small and inexpensive.
Adding series and parallel reactive elements creates new ways to manage power. These components can form a resonant tank where the main energy loss comes from the LED's resistance. Near-lossless reactances can replace resistors typically used as current regulators in simple DC circuits.
**Unit and Array**
Imagine a lighting network composed of multiple units, each containing one or more LEDs along with capacitors. These units can be connected in series or parallel to form a resonant network. We refer to this as "solid-state lighting reactance strings" (RSSL).
Figure 2 illustrates a basic unit design. With multiple units connected, you can build a larger resonant system. For instance, in Figure 3, a network of 10 reactive strings uses identical LEDs and capacitors. The total capacitance per unit is 2C, and the total for the string is C/5. The resonant frequency is calculated as √(5LC). If the reactance is large enough compared to the LED’s resistance, the system behaves like a pure reactance, achieving a high Q factor.
A detailed analysis can be done with a circuit simulator, but even rough estimates can help. Choosing the right inductor and capacitor values ensures a high Q resonance. The current through each unit is controlled by the series capacitor, acting like a resistor in a DC circuit. Bypass capacitors store current when LEDs aren't conducting, enabling local resonance control for each LED.
**Multi-channel and Line Frequency Suppression**
The RSSL system doesn't need to run at a single frequency. Instead, it can use multiple channels over the same two-wire bus. Each reactive string responds to its own frequency band, allowing multiple frequencies to coexist without interference. Line frequency is kept separate from the resonant frequency, preventing flicker and eliminating the need for electrolytic capacitors.
The RSSL system is also immune to noise and electromagnetic interference. Energy outside the narrow passband quickly dissipates, and components can be hot-swapped without affecting the rest of the network. This allows multiple luminaires to share a single driver, with dimming and switching handled separately.
Figure 4 shows a complete RSSL network, including drivers, fixtures, dimming groups, and programmable dimmers.
**Larger Arrays Mean Higher Reliability**
Unlike traditional DC drives, the RSSL system becomes more reliable as the array size increases. Even with 50% component failure, the system remains functional. High-power LEDs also benefit from RSSL, as it reduces lumen output drop at higher currents, improving efficiency.
Using a COB (Chip-on-Board) architecture with multi-junction chips further enhances cost and reliability. By optimizing device placement and cooling, RSSL enables flexible lumen outputs tailored to any application.
Figure 5 shows the current waveform through an LED array. The lower curve represents the current through a pair of LEDs, with the lumen output based on the absolute value of the lower half. There is a brief non-illuminated interval during each cycle.
Using resonance to drive LED strings is a powerful and innovative method that works well in various lighting applications. This article only scratches the surface of the RSSL system’s potential. Resonant drives offer a wide range of design possibilities, helping to create advanced, cost-effective, and multifunctional lighting solutions.
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