How to drive LED light string small trick

Some people may know how to drive the LED string, which may be a popular method that most people agree with, but in fact there are many tricks that many people don't know behind this popular method. Today, Xiaobian will take you from other places to better drive the LED string.

In mechanical and electrical systems, there is a critical relationship between power and frequency when operating at or near resonance (Figure 1). Sometimes resonance is a bad thing, if too much energy enters the single mode state, it may damage the system. But resonance can also be good. Resonance is typically used to adjust the frequency by keeping enough power to keep the system oscillating at resonant frequencies (eg, mechanical and electrical clocks). Many people may not know that resonance can be used to adjust power and can adjust the power to a variable-size array of variable loads. For example, this can be applied to lighting arrays such as to achieve cost effectiveness and reliability of solid state lighting (SSL) systems.

Figure 1. This plot depicts the normalized power for a typical resonance (center frequency 30 kHz and bandwidth 20 kHz). Note that there is no overlap in line frequency.

LED applications are particularly interesting because LEDs are becoming more economical in lighting applications and are also cost and reliability issues due to conventional DC drives. LEDs are inherently low voltage DC devices, and the current-voltage (IV) curve is very steep at certain operating points. Although a constant voltage source can be used to drive the LEDs, in practice most designers use a constant current DC driver design. To be closer to working at typical power distribution levels (eg, 120/240 VAC), luminaires are typically equipped with many LED strings. These LEDs must be closely matched because the light output of each LED is proportional to the current flowing through the string. Failure of a single LED (such as a short circuit or wiring fault) can result in failure of the entire string.

Distributed reactance component

The use of resonance to control the power of the LED array overcomes these shortcomings of AC LED drivers. In the simplest case, resonance can be used to control the power of a single load. Verdi Semiconductor has effectively utilized resonance to create a current driver that is less suitable for LED strings and has high efficiency.

However, a more powerful approach is to distribute the reactive components between the arrays. In this way, not only can the overall power of the lighting elements be controlled, but in large networks, it is also possible to separately adjust the sub-network without adding a semiconductor device. Distributed reactive components deliver powerful new control capabilities with high efficiency and low cost. Typically, the reactive component can be a capacitor or an inductor. Between kilohertz to megahertz (or even gigahertz, if needed), suitable components are very small and inexpensive, and can be implemented as discrete devices or on-chip devices. Specifically, we assume that the capacitors are distributed throughout the network and use a smaller number of discrete inductors, but can also create a low cost inductor design.

Adding series and parallel reactive components (capacitors and / or inductors) opens up a whole new approach to power control. The reactive components can form a resonant tank where the primary dissipation mechanism is the resistive load of the LED. At the same time, near-lossless reactances can replace energy-consuming resistors, which are commonly used as current regulators in the simplest DC circuits.

Unit and array

Imagine an illumination network consisting of a set of lighting units, each unit containing one or more lighting elements, such as a pair of anode-connected cathode LEDs, and series and shunt capacitors. There are many variations in topology, but Figure 2 shows a basic lighting unit design. Any number of such units, as well as units of the actual hybrid topology, may be connected in series and/or in parallel to form a resonant network of reactive reactance strings. More generally, we refer to the network of reactive reactance strings as "solid-state lighting reactance strings" (RSSL).

Figure 2. The circuit shows two reactive string units

For example, in Figure 3, an energy storage circuit consists of 10 reactive strings. Assume that all LEDs are of the same type and all capacitors have the same value C. The total capacitance of each unit is 2C. The total capacitance of the string is C/5. The resonant frequency is √(5LC). The reactance of a unit is 1/2 ωC. As long as X? R, where R is the actual resistance of the LED, then the reactance string is expressed as pure reactance, which is equivalent to the requirement to use the resonant circuit to reduce the damping, Q? 1.

Figure 3. The reaction circuit consists of 10 A-type cells

A detailed analysis of a particular resonant network can be performed using a circuit simulator, but it is also easy to make a rough estimate, roughly selecting the values ​​of the components. For a given operating frequency, the relationship between inductance and capacitance is deterministic. The capacitor should be chosen so that the reactance is large enough to ensure a sufficiently high Q resonance. The current flowing through each cell is distributed by the LED and the shunt capacitor in parallel and is limited by the series capacitor, which acts much like a resistor in a DC circuit to control the current. You can find the desired value by simply using Ohm's law for the reactance. Note that the function of the bypass capacitor is to locally store the recirculating current when current does not flow through the LED. In fact, in addition to the resonant control of the current through the entire string, there is actually a local resonance control for each LED current.

Multi-channel and line frequency suppression

Although the entire RSSL system can be operated at a single frequency using the same capacitance value, it is not necessary to do so. In fact, we can think of the two-wire lighting bus as supporting spectrum, including a lot of available channels. Since any one of the reactive strings only responds to the frequency band, as long as they have sufficient space between the frequency bands to operate, the multiple individual frequency bands can operate on the same wiring. Each center frequency can be further modulated as a data path between the sensor and the controller.

As long as the line frequency is separated from the resonant frequency used for the reactance string, the response to the line frequency is negligible, and even if there is no explicit line frequency filtering, the cable frequency flicker will not occur. Therefore, an electrolytic capacitor is not required in the driver.

The RSSL system itself has electromagnetic quietness and is resistant to noise spikes. Any energy that goes beyond the narrow passband will quickly disappear. Cells and cell strings can be hot swapped or switched, with no effect on other parts of the network. With this property, many luminaires can 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 and dimming groups, and programmable and local dimmers.

The larger the array size, the higher the reliability of RSSL

For DC drives, the use of more LEDs is often considered to pose serious reliability and lifetime issues, especially considering the sensitivity of individual components (or connections) and drive failures. This is another point that makes the RSSL system shine. The failure analysis of the RSSL shows that the overall reliability and lifetime of the system will actually increase as the size of the array increases. This is because the adjustment of the remaining components is acceptable even with 50% component failure.

In addition, most high-power LEDs show a significant drop in lumen output at the upper end of their rated current, resulting in a loss of net wattage and radiant watt conversion efficiency. The RSSL system allows the low-cost design to make the lumen output drop less noticeably than its rated maximum.

In addition, cost savings and reliability improvements can be achieved through a COB architecture that includes multi-junction chips. Instead of building several large-area devices on a single chip, you can choose the device area, power level, and cooling strategy to achieve maximum single device efficiency, then place as many of these devices as possible on a single chip to achieve Required performance specifications. By driving the illumination array with one or more resonant strings, we can get a product line that can be arbitrarily cropped to any desired lumen output.

Figure 5. The upper curve is the waveform formed when current flows through the array of serial illumination cells. The lower curve is the current waveform formed by a pair of LEDs flowing through the cell. The lumen waveform is given by the absolute value of the lower half of the curve. Note that there is a very short non-illuminated interval at the beginning of each half cycle.

Using resonance to control the power of the LEDs in the reactance string is a powerful new LED driving method that can be used in any array application including illumination. This article only touches on the features and benefits of the RSSL system. Resonant drives offer a rich and powerful set of innovative design tools that can be used further to create advanced, low-cost, multi-functional lighting systems.