Lighting devices, where superbright LEDs are used as light sources, will not surprise anyone. The demand for such devices is constantly growing, this is directly related to the low power consumption of these devices. Considering that about 25-35% of consumed electricity is spent on lighting, the savings will be very noticeable.
But given the relatively high cost of super-bright LEDs, due to their design features, it is not yet timely to talk about a complete transition to this type of lighting. According to experts, this process will take from 5 to 10 years, it will be so much needed for debugging and implementation of new technologies.
Content
- Performance Brief
- Influence of design features on cost
- Mounting Features
- Power Features
- Homemade LED Driver
Performance Brief
The efficiency of the lighting device is considered to be the ratio of the generated light flux (measured in lumens) to the consumed electricity (watts). A quality filament lamp has an efficiency of about 16 lumens per watt, fluorescent (energy-saving) - four times more (64 lm / W), for long daytime lamps this indicator is in the region 80 lm / W.
The efficiency of superbright LEDs, mass-produced at the moment, is about the same as that of fluorescent lamps. Please note that we are talking about mass production. As for the theoretical limit for superbright LED sources, it is defined by a threshold of 320 lm / W.
As many manufacturers promise, in the next few years, the efficiency can be increased to 213 lm / W.
Influence of design features on cost
For the manufacture of superbright LED light sources, one of two methods can be used:
- To obtain light close in spectrum to white, three crystals are used installed in one housing. One is red, the second is blue and the third is green;
- a crystal emitting in the blue or ultraviolet spectrum is used; it illuminates the lens coated with a phosphor; as a result, the radiation is converted into light that is close in spectrum to natural light.
Despite the fact that the first option is more effective, its implementation is somewhat more expensive, which negatively affects prevalence. In addition, the spectrum of light emitted by such a source is different from natural.
The devices manufactured by the second technology have less efficiency. It should also be borne in mind that the phosphor contains a composite composite of cerium and yttrium based on composition, which are not cheap in and of themselves. Actually, this explains the relatively high cost of superbright white LEDs. The design of such a device is shown in the figure.
Designations:
- A - printed conductor;
- B - base with increased thermal conductivity;
- C - protective housing of the device;
- D - solder paste;
- E - LED crystal emitting ultraviolet or blue light;
- F - phosphor coating;
- G - glue (can be replaced by a eutectic alloy);
- H is the wire connecting the crystal and the output;
- K is the reflector;
- J is the heat sink base;
- L - power output;
- M is the dielectric layer.
Mounting Features
The operation of superbright LEDs is affected by the degree of heating of the crystal and the pn junction itself. The life of the device directly depends on the first, and the level of luminous flux on the second. Therefore, for the long service life of superbright LEDs, it is necessary to organize a reliable heat sink, this is done using a radiator.
It should be noted that the heat-conducting bases of these semiconductors, as a rule, conduct electricity. Therefore, when several elements are installed on one radiator, care should be taken about reliable electrical insulation of the bases.
The rest of the installation rules are almost the same as with conventional diodes, that is, the polarity must be observed, both when installing the part itself and when connecting the power.
Power Features
Given the relatively high cost of superbright LEDs, it is very important to use for their work reliable and high-quality power supplies, since these semiconductor elements are critical to current overload.
After an abnormal mode, the device may remain operational, but the power of the emitted light flux will be significantly reduced. In addition, such an element is likely to cause damage to other, jointly connected LEDs.
Before talking about drivers for super-bright LEDs, we will briefly talk about the features of their power supply. First of all, it is necessary to take into account the following factors:
- the power of the light flux emitted by these elements directly depends on the magnitude of the electric current flowing through them;
- superbright LEDs are characterized by a non-linear I-V characteristic (volt-ampere characteristic);
- temperature has a strong influence on the I – V characteristics of these semiconductor devices.
The change in the I – V characteristic at the temperature of the semiconductor element (superbright SMD LED) of 20 ° C and 70 ° C is shown below.
As can be seen from the graph, when a stable voltage of 2 V is applied to the semiconductor, the electric current passing through it changes depending on the temperature. When the crystal is heated to 20 ° C, it will be equal to 14 mA, when the temperature rises to 70 ° C, this parameter will correspond to 35 mA.
The result of such a difference will be a change in the power of the light flux at the same supply voltage. Based on this, it is necessary to stabilize not the voltage, but the electric current passing through the semiconductor.
Such power supplies are called LED drivers, they are ordinary current stabilizers. This device can be purchased ready-made or assembled on your own, in the next section we will provide some typical driver schemes.
Homemade LED Driver
We bring to your attention several options for drivers based on specialized chips of the Monolithic Power System company, the use of which greatly simplifies the design. The schemes are given as an example, a full description of the typical inclusion can be found in the datasheet on the microcircuit.
Option one based on the MP4688 buck converter.
This driver can work with voltages from 4.5 to 80 V, the maximum output current threshold is 2 A, which allows the luminaire to be powered by ultra-bright high-power LEDs. The level of electric current passing through the LEDs is regulated by the resistance RFb. The implementation of PWM dimming with a frequency of 20 kHz allows you to smoothly change the electric current flowing through the LED.
The second version of the driver is based on the MP2489 chip. Its compact housing (QFN8 or TSOT23-5) makes it possible to place the driver in the MR16 base used by halogen lamps, which allows replacing the latter with LED ones. A typical MP2489 connection diagram is shown in the figure.
The above circuit allows you to turn on two parallel LEDs, each of which has an operating current of 350 mA.
The latest driver based on the MP3412 chip, which can be used in portable flashlights. A distinctive feature of such a scheme is the ability to work from a AA AA battery.
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