LED lamps are widely used in various fields of the lighting industry. Stage LED lights such as LED moving head lights, LED flood lights, LED wall washers, LED pattern effect lights, etc. have been recognized and loved by users. .

LED as a light source, dimming is very important. Not only to get a more comfortable environment in the home, today, the purpose of reducing unnecessary electric light to further achieve energy saving and emission reduction is even more important. Moreover, for LED light sources, dimming is easier to implement than other fluorescent lamps, energy-saving lamps, high-pressure sodium lamps, etc., so it is necessary to add dimming function to various types of LED lamps.

 

First, the dimming technology using DC power led

Adjusting the brightness by adjusting the forward current is very easy to achieve by changing the brightness of the LED. The first thing that comes to mind is to change its drive current because the brightness of the LED is almost directly proportional to its drive current.

 

1.1 Method of adjusting forward current

The easiest way to adjust the LED current is to change the current sense resistor in series with the LED load. Almost all DC-DC constant current chips have an interface for detecting current, which is the comparison between the detected voltage and the internal reference voltage of the chip. The current is constant. However, the value of this sense resistor is usually very small, only a few ohms. If you want to install a potentiometer of zero ohms on the wall to adjust the current is not possible, because the lead resistance will also have a few ohms. Therefore, some chips provide a control voltage interface, and the output constant voltage can be changed by changing the input control voltage.

 

1.2 Adjusting the forward current will cause the chromatogram to shift

However, adjusting the brightness by adjusting the forward current has a problem, that is, it also changes its spectral and color temperature while adjusting the brightness. Because white LEDs are currently generated by blue LEDs to excite yellow phosphors, when the forward current is reduced, the brightness of the blue LEDs increases and the thickness of the yellow phosphors is not scaled down, thereby increasing the dominant wavelength of the spectrum. For example, when the forward current is 350mA, the color temperature is 5734K, and when the forward current is increased to 350mA, the color temperature is shifted to 5636K. When the current is further reduced, the color temperature will change to a warm color.

Of course, these problems may not be a big problem in general actual lighting. However, in the RGB LED system, color shift is caused, and the human eye is very sensitive to color deviation, and therefore is not allowed.

 

1.3 Adjusting the current will cause serious problems that make the constant current source unable to work.

However, in the specific implementation, dimming by using the forward current method may cause a more serious problem.

We know that LEDs are usually driven by DC-DC constant current drive power, which is usually divided into boost or buck (of course, buck-boost, but due to low efficiency , the price is expensive and not commonly used). Whether the boost type or the step-down type is used is determined by the relationship between the power supply voltage and the LED load voltage. If the power supply voltage is lower than the load voltage, the boost type is used; if the power supply voltage is higher than the load voltage, the step-down type is used. The forward voltage of the LED is determined by its forward current. From the volt-ampere characteristics of the LED, it is known that the change in the forward current causes a corresponding change in the forward voltage. Specifically, the decrease in the forward current also causes a decrease in the forward voltage. Therefore, when the current is turned down, the forward voltage of the LED is also reduced. This will change the relationship between the supply voltage and the load voltage.

For example, in an LED luminaire with an input of 24V, eight 1W high-power LEDs are connected in series. At a forward current of 350 mA, the forward voltage of each LED is 3.3 V. Then 8 series is 26.4 V, which is higher than the input voltage. Therefore, a boost type constant current source should be used. However, in order to dim, the current is reduced to 100 mA. At this time, the forward voltage is only 2.8 V, and 8 in series is 22.4 V, and the load voltage becomes lower than the power supply voltage. This step-up constant current source does not work at all, but should be a step-down type. For a boost type constant current source, it must not work for buck, and finally the LED will flicker. In fact, as long as a boost-type constant current source is used, when dimming with a forward current is regulated, it is almost always caused to flicker as long as the brightness is adjusted to a very low level. Because the LED load voltage at that time must be lower than the supply voltage. Many people do not understand the problems, but also always go to the dimming circuit to find the problem, it is useless.

There is less problem with the buck constant current source, because if the original supply voltage is higher than the load voltage, when the brightness is low, the load voltage is reduced, so a buck constant current source is still needed. However, if the very low forward current is adjusted, the load voltage of the LED becomes very low. At that time, the step-down ratio is very large, and it may exceed the normal working range of the buck constant current source, and it will also make it Can't work and produce flicker.

 

1.4 Long-term operation at low brightness may reduce the efficiency of the step-down constant current source and increase the temperature rise without working.

The average person may think that downward dimming is to reduce the output power of the constant current source, so it is impossible to cause the power consumption of the buck constant current source to increase and the temperature rise to increase. It is not known that the decrease in the forward voltage caused by the reduction of the forward current causes the step-down ratio to be lowered. The efficiency of the buck constant current source is related to the buck ratio. The larger the buck ratio, the lower the efficiency, and the greater the power consumption of the loss on the chip.

 

1.5 Adjusting the forward current does not result in precise dimming

Because the forward current and light output are not completely proportional, and different LEDs have different forward current and light output curves. Therefore, it is difficult to achieve accurate light output control by adjusting the forward current.

The LED is a diode that enables fast switching. Its switching speed can be as high as microseconds or more. It is unmatched by any light-emitting device. Therefore, as long as the power supply is changed to a pulse constant current source, the brightness can be changed by changing the pulse width. This method is called pulse width modulation (PWM) dimming. If the period of the pulse is tpwm and the pulse width is ton, then the working ratio D (or the ratio of the holes) is ton/tpwm. Changing the working ratio of the constant current source pulse can change the brightness of the LED.

 

2.1 How to implement PWM dimming

The specific method of implementing PWM dimming is to string a MOS switch tube in the load of the LED. The anode of the string is powered by a constant current source.

A PWM signal is then applied to the gate of the MOS transistor to quickly switch the string of LEDs to achieve dimming. There are also a number of constant current chips that have a PWM interface that can directly accept the PWM signal and then output the control MOS switch. So what are the advantages and disadvantages of this PWM dimming method?

 

2.2 Advantages of pulse width modulation dimming

1. Does not produce any chromatographic shift. Because the LED always works between full-scale current and zero.

2. Can have extremely high dimming accuracy. Because the pulse waveform is fully controllable to very high precision, it is easy to achieve one-tenth of the accuracy.

3. Can be combined with digital control technology for control. Because any number can be easily converted into a PWM signal.

4. Flicker does not occur even if it is dimmed over a wide range. Since the operating conditions of the constant current source (boost ratio or step-down ratio) are not changed, it is less likely that overheating and the like occur.

 

2.3 Pulse width dimming should pay attention to the problem

1. Pulse frequency selection: Because the LED is in the fast switching state, if the working frequency is very low, the human eye will feel flicker. In order to make full use of the visual residual phenomenon of the human eye, its operating frequency should be higher than 100 Hz, preferably 200 Hz.

2. Eliminate the whistling caused by dimming: Although the human eye can't detect it above 200Hz, it is the range of human hearing until 20kHz. At this time, it is possible to hear the sound of the silk. There are two ways to solve this problem. One is to increase the switching frequency to above 20 kHz and jump out of the human ear. However, too high a frequency can cause some problems, because the influence of various parasitic parameters will cause the pulse waveform (front and rear edges) to be distorted. This reduces the accuracy of dimming. Another method is to find out the sounding device and handle it. In fact, the main sounding device is the ceramic capacitor at the output, because ceramic capacitors are usually made of high dielectric constant ceramics, which have piezoelectric properties. Mechanical vibration occurs under the action of a 200 Hz pulse. The solution is to use a tantalum capacitor instead. However, high-voltage tantalum capacitors are difficult to obtain, and the price is very expensive, which will increase some costs.

 

Second, the use of AC power LED dimming

3, dimming the LED with thyristor

Conventional incandescent and halogen lamps are typically dimmed with thyristors. Because incandescent and halogen lamps are purely resistive devices, it does not require that the input voltage be a sine wave, because its current waveform is always the same as the voltage waveform, so regardless of how the voltage waveform deviates from the sine wave, just change the effective value of the input voltage. You can dim. The use of thyristors is to cut the sine wave of the alternating current to achieve the purpose of changing its effective value. The load is in series with the thyristor switch.

Changing the voltage division ratio of the variable resistor changes its conduction angle, thereby achieving the purpose of changing its effective value. Usually this potentiometer has a switch that is connected to the input of n for switching the lamp. In addition to thyristors, there are transistor trailing edge dimming techniques, etc., because their basic problems are the same, they are not introduced here.

 

3.1 SCR dimming shortcomings and problems

However, thyristor dimming has a number of problems.

1. The thyristor destroys the waveform of the sine wave, which reduces the power factor value. Usually, the PF is lower than 0.5, and the smaller the conduction angle is, the worse the power factor is (only 0.25 for 1/4 brightness).

2. Similarly, non-sinusoidal waveforms increase the harmonic coefficients.

3. Non-sinusoidal waveforms can cause severe interference signals (EMI) on the line.

4. It is easy to be unstable at low loads, and a drain resistor must be added for this purpose. This drain resistor consumes at least 1-2 watts of power.

5. An unexpected problem arises when the ordinary thyristor dimming circuit outputs the driving power to the LED. That is, the LC filter at the input causes the thyristor to oscillate. This oscillation does not matter for the incandescent lamp. Because the thermal inertia of incandescent lamps makes the human eye see this oscillation at all. However, the audible noise and flicker are generated by the driving power of the LED.

 

3.2 Advantages of thyristor dimming

Although there are so many shortcomings and problems with thyristor dimming, it has certain advantages, that is, it has formed an alliance with incandescent halogen lamps, occupying a large dimming market. If the LED is intended to replace the position of the thyristor dimming incandescent and halogen lamps, it is also compatible with thyristor dimming.

Specifically, in some places where thyristor dimming incandescent or halogen lamps have been installed, thyristor dimmer switches and knobs have been installed on the wall, and two wires leading to the luminaire have been installed in the wall. Connection line. It is not so easy to replace the thyristor switch on the wall and increase the number of connecting wires. The easiest way is to keep nothing. Just unscrew the incandescent lamp on the lamp cap and replace it with compatible thyristor. Light-enabled LED bulbs are fine. This strategy is like a led fluorescent lamp. It is best to be the same size as the current T10 and T8 fluorescent lamps. It does not require a professional electrician. The ordinary people can directly replace it, and it can be popularized very quickly. Therefore, many foreign manufacturers of LED driver ICs have developed ICs that are compatible with existing thyristor dimming.

The difference from the general flyback IC is that they can detect the conduction angle of the thyristor to determine the current of the LED for dimming. We are not going to detail their working principle and performance, because we are not Think this is the direction of LED dimming.

 

3.4 Compatible with thyristor dimming problems and shortcomings

Although a number of multinational big chip companies have introduced chips and solutions compatible with existing thyristor dimming. However, such solutions are not recommended, the main reasons are as follows:

1. Thyristor technology is an old technology with more than half a century. It has many shortcomings as mentioned above, and it is a technology that should be eliminated. It should exit the historical stage at the same time as incandescent and halogen lamps.

2. Many of these chips claim to have PFCs that can improve power factor. In fact, it only improves the power factor as a thyristor load, making them look like pure-resistance incandescent and halogen lamps without improving them. The power factor of the entire system, including the thyristor.

3. The overall efficiency of all thyristor-compatible LED dimming systems is very low. Some have not considered the loss of the bleeder resistors needed for stable operation, completely damaging the high energy efficiency of the LEDs.

4. All thyristor LED dimming systems also regulate the forward current of the LED, which has the disadvantages of the chromatographic offset described above.

5. The proportion of incandescent and halogen lamps installed with thyristor dimming is less than one in ten thousand, while the proportion of thyristor switches installed in the wall is even one in ten thousand in thyristor dimming lamps. Not all, because most of the installation of thyristor dimming are table lamps, bedside lamps, standing lamps. What's more, there are dozens of different types of thyristor and transistor dimmer switches on the market. In fact, the developed ICs are simply not compatible with all thyristors, but only a small part of them.

6. LED is a brand new Genesis technology with unparalleled superiority. There is absolutely no need to sacrifice the advantages of LEDs in order to take care of the underlying thyristors. It is even more important to go to the newly installed thyristor switch on the wall to achieve dimming of the LED.

 

4, the future LED dimming system

So what kind of dimming system should LED use?

 

4.1 PWM dimming

It has been said that LED dimming is preferably PWM dimming. When PWM dimming is used, a simple PWM generator can be installed in the wall switch, and then the potentiometer is used to control the PWM working ratio to achieve dimming. But if you want to turn the lights on and off, you need to add a pair of lines. Therefore, it is not compatible with the leads of the thyristor switch in the original wall. The original thyristor switch has only two leads, which can be dimmed and switched. This advantage is difficult to be compatible. However, in fact, the most commonly used dimming fixtures are desk lamps or stand-up lamps. Those dimmer switches are installed on the power line and not in the wall. It does not matter if you want to use the two leads in the wall. In other words, PWM dimming can be directly applied to dimming lamps.

 

4.2 Segmented Switch Dimming

A Taiwanese company has introduced a four-stage switch dimming called EZ-Dimming's GM6182, which is a good solution. It can realize 4 sections of dimming only by using the ordinary light switch on the wall. The first time is full light, the second time is 60% brightness, the third time is 40% brightness, the fourth time is open.