What is the LED SMD silicone, the maximum posture of the LED hardware dimming!

How to adjust the brightness of the LED circuit? Wide-range high-brightness LED circuit dimming method

High-brightness LEDs are an ideal alternative to traditional incandescent lamps because they have much higher lifetimes and efficiencies than the latter, and unlike compact fluorescent bulbs, these LEDs can operate at low temperatures. However, like cold cathode fluorescent lamp (CCFL) bulbs, high-brightness LEDs have not been able to get rid of the triac TRIAC dimmers that are common in many homes and applications. This article will introduce a cost-effective high-brightness LED (HBLED) dimming method.

Design challenge

Basic TRIAC dimmer switches are widely used in home improvement stores and most homes. To understand the design challenges of using such dimmer switches, we must deepen the basic working principles of TRIAC circuits and the basic design principles of high-brightness LED circuits. Research.

High-brightness LEDs are generally driven by a constant-current power supply, because as the LEDs heat up, their voltage drop will decrease; and, if the LED strings are powered by a constant-voltage power supply, the power supply will continue to provide excessive current to the output voltage. Increase until the power supply reaches the current limit or the LED fails.

Basic LED dimming method

High brightness LEDs have two basic dimming methods. The first is a PWM dimming method that turns the LED on and off at a certain percentage of on-time (duty cycle) from 0% to 100% at certain frequencies greater than 200 Hz. The LED operates at full current during turn-on and no current flows through the LED during turn-off. This ensures color consistency.

Another method is to control the amount of current flowing through the LED string. This can cause the voltage of the LED string to drop and cause a slight chromatic aberration. However, if you observe an incandescent lamp that works with the dimmer turned on, you will see noticeable color changes.

How the TRIAC dimmer works:

Most dimmers have a simple TRIAC circuit, the core of which is shown in Figure 1. The focus of our discussion is on the two attributes of TRIAC. That is, the current is allowed to flow once the gate is triggered, and the TRIAC remains on if sufficient current flows. To design the correct dimming circuit, you must understand these two currents, the trigger current and the holding current. Take the example of the 3A800VTRIAC device FKPF3N80 commonly used in dimmer switches. The trigger current of this device is 20mA and the holding current is 30mA. When the gate current is close to 20 mA, the TRIAC is turned on, and when the current flows through at least 30 mA, the TRIAC remains in an on state.

When the user unscrews the dimmer knob, he is actually changing the resistor divider. The voltage divider sets different trigger current points in the AC cycle to set the trigger point of the TRIAC. By selecting the set point of the TRIAC, the user actually selects the duty cycle of the AC voltage required for the load supply, which is the information required by the LED driver to adjust the brightness of the LED.

In order to dim the LED, the 60 Hz duty cycle needs to be converted to a value that can be used for any of the above dimming methods. Once the turn-on is triggered, it must be ensured that the TRIAC has sufficient current. The first part is easy to do and can be implemented using the circuit shown in Figure 2. In the figure, the TRIAC dimmer and bidirectional optocoupler obtain duty cycle information from the AC line input to power the simplified circuit.

The 120 Hz signal is processed by a resistor/capacitor filter to represent the voltage of the AC voltage duty cycle and supplied to the power supply via a TRIAC dimmer. This voltage can be utilized to control the LED current in a number of ways. In the circuit shown, two resistors are used to bias the bipolar junction transistor (BJT) to the required maximum load current, and the optocoupler is fully turned on (duty ratio = 100%). The capacitor is charged to the maximum potential energy. Since VCC is generally low, less than ~24V, the size of the capacitor is small, even though its value is often quite large enough to be a 120Hz filter.

In the above method of implementing the dimmer and the LED current regulator, it is preferable to have a constant voltage circuit. In this way, a simple BJT can be used to regulate the current. The designer needs to fully bias the BJT to the maximum current allowed by the LED string and set the output voltage to the desired value for the cold LED temperature at that current, allowing the BJT to control the current with a 100% duty cycle. Note that VCE is as low as ~0.2V and the maximum current is typically between 350mA and 1.35A.

Therefore, for a 1.35A load current design, the power consumption is VCE(SAT)*Ic~0.27W. As the duty cycle drops, the BJT begins to limit the current and its VCE will rise, so with 50% duty cycle saturation, the LED current will be half of the maximum design point. Therefore, the power consumption is VCE*Ic, and it is easy to design a low enough range for management.

Another key part of this solution is the AC-DC power supply that operates as a constant voltage source. This typically consumes a large amount of current, causing the TRIAC in the dimmer switch to latch once triggered.

Since we have a voltage representing the duty cycle of the AC input (the output of the RC filter), we can use this information to control the brightness of the LEDs driven by other circuits. To use pulse-by-pulse current limiting or PWM current limiting techniques in the circuit, the basic AC-DC circuit must be a constant current circuit, as shown in Figure 3. Therefore, we only need to load the voltage representing the duty cycle in the circuit to the comparator, so that we can add another duty cycle voltage. For example, in the circuit shown in Figure 3, we can have IPEAK set the impedance R8 in parallel with a small MOSFET that is biased in the linear region and controlled by the optocoupler circuit shown above.

Many high-brightness LED driver circuits have a comparator that can be used as a dimming LED. Some of these current outputs are small and can read the voltage on the pin to control the primary side switch or low frequency duty cycle. In either case, the key is to convert the AC duty cycle to a usable value. The optical coupling circuit does this very well and provides isolation so that it can be used anywhere in the primary or secondary circuit.

What is the LED silicone patch?

SMD LED silicone is mainly used in the packaging technology of high power LED. Due to the different fields and spaces of the application, there are many LED products of various specifications to meet the specifications of various products. Nowadays, almost all of the SMD LED packages are replaced by SMD LEDs. Well-known brands at home and abroad include: Gaoyang Chemical in the UK, HITACHEM in Japan, Dow Corning, Shin-Etsu, Toshiba, etc.

First, the appearance of patch LED silicone

SMD LED silica gel is one of many LED silicone materials. It is usually available in single-component and two-component packaging. It is colorless and transparent liquid, non-toxic, curable at room temperature or high temperature, and has certain elasticity after curing. good.

Second, the patch LED silicone properties

The basic structural unit of the SMD LED silica product is composed of silicon-oxygen chain links, and the side chains are connected to various other organic groups through silicon atoms.

Therefore, in the structure of the SMD LED silica product, both the "organic group" and the "inorganic structure" are contained, and this special composition and molecular structure make it combine the characteristics of the organic substance with the function of the inorganic substance.

Compared with other polymer materials, the most outstanding performance of SMD LED silicone products is:

1. Temperature resistance

The SMD LED silica gel product is based on a silicon-oxygen (Si-O) bond, the bond energy of the C-C bond is 82.6 kcal/g molecule, and the bond energy of the Si-O bond is in the SMD LED silica gel. It is 121 kcal / gram, so the thermal stability of the SMD LED silica product is high, and the chemical bonds of the molecules at high temperature (or radiation irradiation) are not broken or decomposed. SMD LED Silicone is not only resistant to high temperatures, but also resistant to low temperatures and can be used over a wide temperature range. Whether it is chemical or physical and mechanical properties, it varies little with temperature.

2. Weather resistance

The main chain of the SMD LED silica product is -Si-O-, which has no double bonds and is therefore not easily decomposed by ultraviolet light and ozone. SMD LED silica gel has better thermal stability and resistance to radiation and weathering than other polymer materials. The lifetime of the SMD LED silica gel in the natural environment can reach several decades.

3. Electrical insulation performance

SMD LED silicone products have good electrical insulation properties, and their dielectric loss, withstand voltage, arc resistance, corona resistance, volume resistivity and surface resistivity are among the best in insulating materials, and their electrical properties are affected by temperature. The effect of frequency and frequency is small. Therefore, they are a stable electrical insulating material and are widely used in the electronics and electrical industries. In addition to excellent heat resistance, SMD LED silica gel also has excellent water repellency, which is a guarantee of high reliability of electrical equipment under wet conditions.

4. Physiological inertia

Polysiloxanes are one of the most inactive compounds known. They are very resistant to biological aging, have no rejection reaction with animals, and have good anticoagulant properties.

5. Low surface tension and low surface energy

The main chain of the SMD LED silica gel is very compliant, and its intermolecular force is much weaker than that of hydrocarbons. Therefore, it has lower viscosity than the same molecular weight hydrocarbon, weak surface tension, small surface energy, and strong film forming ability. . This low surface tension and low surface energy are the main reasons for its multi-faceted application: hydrophobic, defoaming, foam stabilization, anti-sticking, lubrication, glazing and other excellent properties.

Third, the application of patch LED silicone

There are many types of SMD LEDs, which can be described as various types. The properties of organic silicone materials required by different product shapes are also different. Some focus on the hardness of colloids, some focus on the adhesion of colloids, and some emphasize The surface viscosity of the colloid is required and the like. The main function of the SMD LED silica gel is to seal the gold wire after sealing the gold wire in the production of the SMD LED to protect the chip. The specifications of the SMD LED are 3528, 5050, 1210, etc., which are both front-illuminated and the most used type of SMD LED. The 3528 SMD LED requires a silicone body to be cured. Can not be sticky, otherwise it will stick the nozzle when splitting and producing the light bar, which will affect the production. Because the 5050 SMD LED has a large area of encapsulation, it requires the bonding ability of the silica gel to the metal material and PPA. The white-light patch LED is used to mix the phosphor in the silica gel and then packaged on the surface of the blue chip. This requires the silica gel to have a certain viscosity to prevent the phosphor from precipitating too quickly, but the viscosity of the silica gel used in the automatic dispenser is not too high. Otherwise, it will affect the amount of silica gel when dispensing.

Fourth, the market for SMD LED silicone

The demand for SMD LED silica gel will be very large, and chemical research institutions all over the world are working hard to develop and expand production. At present, domestically produced silica gel products have matured.