This article main discussion based on micro controller’s LED driver. It inspected has taken each kind of different topology which by the micro controller the system core could use. It also in detail discussed each kind of analysis situs balance, emphatically to theirs principal characteristic and limitation: Communication, voltage and current capacity, accent light technology, as well as shutter speed and so on.
What is high luminance LED, what does it need to use to actuate?
High luminance light emitter diode (HI-LED) is one kind of semiconductor equipment, only allows the electric current to be mobile according to a direction. It is after two kind of semiconducting material union forms the PN structure becomes. High luminance LED and the standard LED difference lies in their output. The traditional LED output defines generally in 50 milliwatts, but high luminance LED may reach 1-5 tiles.
Figure 1 has demonstrated the HI-LED internal voltage and the electric current model relations. Before direct voltage (VF) surpasses the internal threshold voltage, on HI-LED does not have forward current (IF) to wind through nearly. If VF further elevates, the curve will rise fast suddenly by the linear slope, forms one to take the form of knee’s curve.
The LED output brightness and the forward current are proportional, therefore, if has not been under the suitable control, output brightness possibly presents unacceptable the change. Moreover, if surpasses the manufacturer stipulation the biggest IF limit, but also possible to reduce LED seriously the service life.
High luminance LED should carry on the control by the electronic driver, these electronic driver’s major function constitutes a constant current supply. Uses technology which behind this article said that these electric circuits may provide the luminosity control, may also carry on the compensation in the certain circumstances to the temperature change.
In order to guarantee that the system provides the color the uniformity, the HI-LED manufacturer suggested carries on the beam control by the constant nominal current’s pulse output to LED.
 |
Simple analysis situs and balance
The design high luminance LED driver faces the challenge is constructs a control good, programmable, the stable current supply, moreover also has the high efficiency.
1st, use series resistor (linear law)
Regulating current’s simple way adds a series connected resistor, like chart 2A shows. Its merit lies in the cost to be low, the implementation is simple, will not have the noise as a result of the switch. Unfortunately, this kind of analysis situs has two main flaws: First, on resistor’s massive losses cause the system efficiency to reduce; Next, it cannot change the luminosity. Moreover, this kind of plan needs to use the constant voltage source to obtain the constant electric current. Cites an example, if our supposition VDD is 5 volts, but LED VF is 3.0 volts, if then needs to produce 350 milliampere constant currents, you will need: R=V/I, this time R=(5V-3.0V)/350mA=5.7Ω.
 |
May see that will use these values, R will consume R×I2 namely 0.7 tile (to be equal nearly to the LED power), therefore the overall efficiency will inevitably be lower than 50%.
This method hypothesis has constant VDD and constant VF. In fact, VF will change along with the temperature change, enables the electric current also to change. Uses high VDD to be possible the overall fluctuation of current which causes by VF to drop to lowly, but will have the huge loss on the resistor, will thus further cut the efficiency.
After we have constructed winds through LED the constant current, needs to find some method to establish the different luminosity. We knew that these LED always needs to actuate by its nominal current, therefore we may use the programmable dutyfactor to come the make-and-break electric current, thus realizes to the luminosity the control. This needs a switch, like chart 2B shows.
2nd, uses the linear current supply
In addition a transistor and/or an operational amplifier, may the electric current establishment be 350 milliamperes very precisely. Unfortunately, overall efficiency and R power loss question as before.
3rd, uses the low end switch (switch pattern law)
Chart 2C has demonstrated this concept. As shown in Figure 3, through permission inductor L on electric current when switch breakover rises, when the switch separates drops, we may adjust flow through LED the electric current. Is the same with any perceptual load, when the switch separates, we need to provide a circuit for the electric current. This may realize through the chart 2D in after flow diode, in the chart we use N channel MOSFET to replace the switch, and adds on resistor R to use to survey flows through LED the electric current.
 |
When the electric current drops to the low electric current threshold value (for example 300mA), switch breakover, but when the electric current climbs to the high electric current threshold value (for example 400mA), the switch will separate.
In this example the switch puts in the low end (this method therefore to acquire fame), realizes the method to be simple. Breakover FET only need on add the 5V voltage extremely in its, this may provide directly by a micro controller’s outlet. Moreover, this kind of analysis situs no longer needs the constant VDD voltage, even if input voltage in fluctuation, can also maintain the regulating current.
Electroresponse resistance R must be located at the electric circuit ” the high-end ” the part. If links it arrives MOSFET the source, can only obtain when the switch breakover on LED the electric current, could not use for to adjust another threshold value, see also Figure 3.
This kind of analysis situs looks like is pressure-rise switch’s front end likely, it has uses the N channel, the low cost FET superiority, but needs to carry on the voltage difference survey in the R both sides, gains flows through LED the electric current.
Please note the switch in fact to provide two kind of functions: First, it causes the electric current which produces on the inductor may adjust; Next, it permits the luminosity adjustment.
4th, uses the high-end switch
Exchange the position except the load and the transistor, this electric circuit and front identical. In chart 2E demonstrated the switch is located at “the high-end”. We also turn FET from the N channel the P channel. N channel FET requests VGS>5V by the complete breakover: In this analysis situs, the N channel’s source voltage will change unceasingly, moreover in 3 will respectively consider frequently on, therefore on will need 8 volts voltages at least extremely in the gate. This needs a similar sufficient electric pump’s gate driving circuit, causes the entire electric circuit a little to be more complex. If uses P channel FET, moreover may provide - 5V VGS directly from the micro controller’s out-port for it, that simply were many. This kind of analysis situs is similar to voltage dropping switch’s front end.
Its main superiority is can carry on the current measurement directly in the R both sides, therefore no longer needs the difference survey the method.
Beam control technology
Has many technologies to be possible to carry on the beam control to LED, many are the patented technology. Here to several kinds carries on the brief introduction. In all methods, the average luminosity is through (avoids twinkle) by the very quick speed lightening completely (by its nominal current) closes LED to obtain again, moreover lightens time with LED the percentage to be proportional.
1st, pulse-duration modulation
This kind of technology uses the cycle is the T fixed frequency, as shown in Figure 4. Brightness adjustment realizes through the change pulse width. Figure 4 had demonstrated three kind of different luminosity ranks, its dutyfactor respectively is 6%, 50% and 94%.
 |
2nd, frequency modulation
This kind of technology announces by Artistic Licence, it uses the fixed width control pulse the concept, as shown in Figure 5. The pulse A always same width, the luminosity controls by the pulse A repetition interval.
 |
3rd, position angle modulation
This is one item by Artistic the Licence invention new technology, it contains the luminous intensity based on a string the binary pulse row. Pulse row each presses its value proportion delaying. If the most low position b0 duration is 1, then the b1 position’s duration is 2, b2 to the b7 position’s duration respectively is correspondingly 4, 8, 16, 32, 64 and 128, as shown in Figure 6.
Communication protocol
1st, DMX512
DMX512 is a standard which (American theater Technical research institute) announces by U.S.I.T.T. This agreement uses for to control the illumination light modulator at first, now already extended to controls the lamps and lanterns to move, the slide film projector and many other illumination facilities. DMX512 movement in the EIA-485 standard. The data carries on the transmission in 8 asynchronous serial communication’s foundations, 1 starts the position, 2 stop positions, and non-parity check. It has 256 beam control rank.
2nd, DALI (digital addressing illumination connection)
DALI is for the electronic ballast message center development one kind of standard, it takes the appendix to contain in ECG standard IEC in 929. DALI is designed uses in the standard package and the simple wiring, namely low cost application.
The application domain possibly is adjusts the light and the initialization different illumination environment value, the basis sunshine direction and the energy conservation factor and so on suitable adjustment light establishment.
The DALI foundation is main - from the principle: The user (main engine) carries on the operation through the controller to the system, the controller (from machine) transmits to all ballasts contains the address and the order news. The address was deciding whether the ballast should obey the instruction. Each ballast is the digital addressing, therefore it insensitive (surpasses to the electromagnetism noise simulates 1-10 volt light modulator switching system).
3rd, ZIGBEE
Zigbee is the communication protocol which becomes by Home RF lite and the IEEE 802.15.4 specification unions. Zigbee movement in 2.4GHz and 868/915MHz in ISM wave band. Because it can obtain the low power loss by the low cost, the illumination applies into one of its staple markets. Zigbee provides the network function is also useful in the lighting system, moreover it also has the wireless control superiority.
Uses the micro controller’s limitation
1st, voltage and electric current
If VDD is LED and the micro controller’s common power source, then this voltage can only actuate LED. We had already discussed the simple analysis situs does not allow the LED voltage to be higher than VDD, please see also Figure 2 and Figure 7. If connects uses LED, then all LED has the same electric current, this is a merit, but VDD must be higher, moreover the micro controller needs an independent power source.
2nd, support correspondence physical interface
The micro controller only provides simple synchronized (SPI) or the asynchronous (SCI) correspondence. Must want to implement DALI, DMX, the LIN communication protocol and so on, it also needs the extra hardware and the software.
3rd, constant flow adjustment and shutter speed
In this application’s key parameter is the shutter speed. The shutter speed is slower, the inductor is bigger, the cost is also higher. The majority micro controllers may complete a/D transformation in about 15 microseconds. In addition some comparison reading and the internal threshold value’s instruction, now, we may say that a complete switching period is from 30 to 40 microseconds, adds on 15 microsecond indefinite time again. This error has defined the smallest inductance value which in Figure 8 shows. Other plan is establishes the breakover and the shutdown duration willfully, then basis actual situation reset these values, attempt and achieve two electric current threshold values. This kind of indirect plan permission uses is smaller, a cost lower inductor, but the accuracy is bad.
4th, adjusts the light and the modulating velocity
Does not need in 100% luminosity to modulate the transistor. In other extreme, to the lowest luminosity rank (for example 1%), needs to open the transistor 1% time. The supposition beam control must or in a higher frequency completes in 100 Hz, avoids the scintillation, then the PWM frequency must be 10 kHz or is higher. But the naked eye may distinguish the slight change in the low luminosity sector, therefore 100 levels are not enough. If needs 4000 levels (12 resolution), then the PWM frequency must achieve above 400 kHz, this to a simple micro controller is nearly impossible.
In the future will forecast
Now, we already saw designs one based on the micro controller’s high luminance LED driver is how simple. Three main limitations lie in the processing speed and inductor’s size and adjust the light resolution the influence, to have the profession standard correspondence function, as well as to multi-outputs and/or LED string driving force.
