The following figure shows the relationship between forward voltage drop (VF) and forward current (IF). It can be seen from the curve that IF is proportional to VF when the DC voltage exceeds a certain threshold (about 2V), which is the so-called on voltage. At present, the electrical characteristics of the main ultra-bright LEDs are shown in the table. As can be seen from the table, at present, the highest IF of ultra-bright LED can reach 1A, while VF is usually 2 ~ 4V.
Because the light characteristics of LED are usually described as a function of current, not voltage, the relationship curve between luminous flux (Φ V) and IF can be better controlled by driving with a constant current source. In addition, the forward voltage drop of LED changes greatly (up to 1V). As can be seen from the VF-IF curve in the above figure, a small change in VF will cause a big change in IF, thus causing a big change in brightness. Therefore, the use of constant voltage source driving can not guarantee the consistency of LED brightness, which affects the reliability, life and light attenuation of LED. Therefore, ultra-bright LEDs are usually driven by a constant current source.
The following figure shows the relationship between LED temperature and luminous flux (φV). As can be seen from the figure below, the luminous flux is inversely proportional to the temperature. The luminous flux at 85℃ is half that at 25℃, while the light output at 40℃ is 1.8 times that at 25℃. The change of temperature also has a certain influence on the wavelength of LED, so good heat dissipation is the guarantee for LED to keep constant brightness.
The following figure shows the relationship between LED temperature and luminous flux.
Introduction of General LED Driving Circuit
Due to the limitation of LED power level, it is usually necessary to drive multiple LEDs at the same time to meet the brightness requirements. Therefore, a special driving circuit is needed to light the led. Let's briefly introduce the LED concept drive circuit.
The resistor current limiting circuit is shown in the following figure. The resistance current-limiting drive circuit is the simplest drive circuit, and the current-limiting resistance is calculated according to the following formula.
Where: Vin is the input voltage of the circuit; VF is the forward current of IED; VF is the voltage drop of LED when the forward current is 0 IF; VD is the voltage drop of the anti-reflection diode (optional); Y is the number of LEDs in each string; X is the number of parallel LED strings.
From the above figure, we can get the linear mathematical model of LED as follows.
Where: Vo is the turn-on voltage drop of a single LED; Rs is the linearized equivalent series resistance of a single LED. The calculation of the current limiting resistance of the above formula can be written as
When selecting a resistor, the relationship between IF and VF of the resistor current-limiting circuit is as follows.
As can be seen from the above formula, the resistance current limiting circuit is simple, but when the input voltage fluctuates, the current flowing through the LED will also change accordingly, so the regulation performance is poor. In addition, because the power lost by the connection of the resistor R is xRIF, the efficiency is low.
Brief introduction of linear regulator
The core of the linear regulator is to control the load by using the power transistor or MOSFFET working in the linear region as a dynamically adjustable resistor. There are two kinds of linear regulators: parallel type and series type.
Figure A below shows a parallel linear regulator, also known as shunt regulator (only one LED is shown in the figure, but actually the load can be multiple LEDs in series, the same below), which is connected in parallel with the LEDs. When the input voltage increases or the led decreases, the current through the shunt regulator will increase, which will increase the voltage drop across the current limiting resistor to keep the current through the led constant.
Because the parallel regulator needs a resistor in series, it is inefficient, and it is difficult to achieve constant regulation when the input voltage changes greatly.
Figure B below shows a series regulator. When the input voltage increases, the dynamic resistance of the regulator increases to keep the voltage (current) on the LED constant.
Because the power transistor or MOSFET has a saturated turn-on voltage, the minimum input voltage must be greater than the sum of the saturated voltage and the load voltage before the circuit can work normally.
Brief introduction of switching regulator
The above driving technology is not only limited by the input voltage range, but also inefficient. When used to drive ordinary low-power LED, the current is only a few milliamps, and the loss is not obvious. When it is used to drive high-brightness LED with a current of several hundred milliamps or more, the loss of power circuit becomes a serious problem. Switching power supply is the highest energy conversion efficiency at present, which can reach more than 90%. Buek, Boost and Buck-Boost power converters can all be used to drive LED, but in order to meet LED driving, output current is detected instead of output voltage for feedback control.
The following figure (a) shows the LED driving circuit with buck converter. Different from the traditional Buek converter, the switch tube S is moved behind the inductor L, so that the source of S is grounded, which facilitates the driving of S. The LED is connected in series with L, and the freewheeling diode D is connected in anti-parallel with this series circuit. This driving circuit is not only simple, but also does not need output filter capacitor, thus reducing the cost. But the buck converter is a buck converter, which is not suitable for low input voltage or multiple LEDs connected in series.
Figure (b) above shows the LED driving power supply with boost converter. The output voltage is raised to a higher expected value than the input voltage by inductive energy storage, and the LED is driven at a low input voltage. Its advantage is that the output of this driving IC can be used in parallel, thus effectively improving the power of a single LED.
Figure (c) above shows the LED driving circuit with buck-boost converter. Similar to Buek circuit, the source of circuit S can be directly grounded, which facilitates the driving of S. Although Boost and Buck-boostl converters have one more capacitor than Buck converter, they can both improve the absolute value of output voltage, so they are widely used when the input voltage is low and multiple LEDs need to be driven.
Introduction of PWM dimming knowledge
In consumer electronic products such as mobile phones, white LEDs are increasingly used as backlights for display screens. Recently, many product designers hope that the brightness of white LED can be changed correspondingly in different applications. This means that the driver of white LED should be able to support the adjustment function of LED brightness. At present, there are three main dimming technologies: PWM dimming, analog dimming and digital dimming. Many drivers on the market can support one or more dimming technologies. This paper will introduce the characteristics of these three dimming technologies, and product designers can choose the corresponding technologies according to specific requirements.
PWM dimming (pulse width modulation) dimming mode-This is a dimming technique that uses simple digital pulses to repeatedly turn on and off the white LED driver. The user's system can simply change the output current by providing digital pulses with different widths, thus adjusting the brightness of white LED. The advantage of PWM dimming is that it can provide high-quality white light, simple application and high efficiency! For example, in the mobile phone system, a special PWM interface can simply generate a pulse signal with arbitrary duty ratio, which is connected to the driver's EN interface through a resistor. Most driver manufacturers support PWM dimming.
However, PWM dimming has its disadvantages. Mainly reflected in: PWM dimming is easy to make the driving circuit of white LED produce audible noise (or chattering noise). How did this noise come about? White LED drivers generally belong to switching power supply devices (buck, boost, charge pump, etc. ), and their switching frequencies are all around 1MHz, so there will be no audible noise in the typical application of drivers. However, when the driver performs PWM dimming, if the frequency of the PWM signal falls between 200Hz and 20kHz, the inductance and output capacitance around the white LED driver will generate audible noise. Therefore, the low frequency band below 20kHz should be avoided in the design.
As we all know, when the low-frequency switching signal acts on the common winding coil, the coils in the inductor will generate mechanical vibration with each other. The frequency of mechanical vibration falls at the above frequency, and the noise emitted by the inductor can be heard by human ears. Inductance produces part of noise, and the other part comes from output capacitance. Nowadays, more and more mobile phone designers use ceramic capacitors as the output capacitors of drivers. Ceramic capacitors have piezoelectric characteristics, which means that when the low-frequency voltage ripple signal acts on the output capacitor, the capacitor will make a squeaky hum. When the PWM signal is low, the white LED driver stops working, and the output capacitor is discharged through the white LED and the lower resistor. Therefore, in PWM dimming, the output capacitor will inevitably produce large ripple. In short, in order to avoid audible noise during PWM dimming, the white LED driver should be able to provide dimming frequency beyond the audible range of human ears!
Compared with PWM dimming, if the resistance of RS can be changed, the current flowing through white LED can also be changed, thus changing the brightness of LED. We call this technique analog dimming.
The biggest advantage of analog dimming is to avoid the noise caused by dimming. When using analog dimming technology, the forward turn-on voltage drop of LED will decrease with the decrease of LED current, which will also reduce the energy consumption of white LED. However, different from PWM dimming technology, the white LED driver is always in working mode during analog dimming. With the decrease of output current, the power conversion efficiency of the driver drops rapidly. Therefore, adopting analog dimming technology will often increase the energy consumption of the whole system. Another disadvantage of analog dimming technology is the luminous quality. Because it directly changes the current of white LED, the white quality of white LED has also changed!
In addition to PWM dimming and analog dimming, some manufacturers currently support digital dimming. The white LED driver with digital dimming technology will have corresponding digital interface. The digital interface can be SMB, I2C or single-wire digital interface. As long as the system designer gives the driver a string of digital signals according to a specific communication protocol, the brightness of the white LED can be changed.