Implementation of LED dimming and discussion of related issues

1 LED Market and Technology Progress

Due to the low luminous efficiency of incandescent lamps, governments around the world have formulated a timetable for phasing out incandescent lamps as shown in Figure 1. However, the high luminous efficiency and long service life of LEDs have led to their widespread use. The performance and luminous efficiency of LEDs are constantly improving. The progress forecast of relevant LED technical indicators is shown in Table 1.

2 Lighting dimming

Dimming has the advantage of energy saving. There is a good market for lighting dimming in applications. Compared with incandescent lamps/gas discharge lamps, LED dimming has a more obvious energy saving effect. The working efficiency of incandescent lamps/gas discharge lamps is significantly reduced when dimming, while the luminous efficiency of LED dimming will increase. This is mainly because the forward current passing through the LED is small at low luminous brightness, and the loss in the relevant loop resistance component is reduced. Lighting dimming is not only energy-saving, but also can change the visual effect of the space, thereby affecting people's behavior patterns.

However, the design of LED dimming circuit has a great influence on the performance of LED dimming. High-performance dimmers are popular among users. The following discusses the relevant issues of LED thyristor dimming.

2.1 LED Dimming

LED has been widely used due to its good color rendering, luminous efficiency, small size, low operating voltage and long service life. However, standardization of dimming control is very important for the promotion of LED dimming application. Since the thyristor dimming of incandescent lamps has been widely used, LED dimming should be well matched with thyristor dimming to utilize the existing thyristor dimmer of incandescent lamps.

LEDs are used to replace general incandescent lighting. LED dimming is related to factors such as the LED's luminous color, LED's light output, and LED's luminous efficiency (dimming curve). Factors such as dimming control methods (such as 0-10V, DALI and other related control protocols), heat dissipation management, drive solutions, topology architectures, and existing infrastructure have a great impact on the working state and dimming control performance of LED dimming.

2.2 LED dimming performance

Since people are used to using thyristor dimmers for incandescent lamps, LED dimmers should be compatible with thyristor dimmers and the following technical indicators should be considered:

(1) Dimming range

The dimming range is usually expressed as a percentage of the maximum brightness value, such as X%-XX%. In practice, as the brightness of the lamp decreases, the pupil of the human eye will dilate, so the light received by the human eye does not decrease quickly with the decrease of the brightness of the lamp. If it is expressed as a percentage of the maximum brightness level, the light received by the human eye is the root mean square value of the measured light percentage. For example, if the measured dimming level is 10% of the maximum luminous value, the light received by the human eye is the maximum luminous value, which is the so-called law of psychophysics.

(2) Dimming changes should be smooth over the entire dimming range.

(3) When the lamp is started at the lowest dimming level, the light should quickly reach the predetermined light value.

(4) After reaching the predetermined dimming level, the LED light should emit a stable light without flickering.

(5) The LED lamp, LED driver power supply and dimming controller should not have audible noise throughout the dimming range.

(6) Motion control/presence detection/photoelectric detection and other components can control the operation of LED lights.

2.3 Working reliability

Dimming can extend the service life of most light sources because the power consumption and heat generation of the lamp are reduced at low luminous brightness, which is beneficial to increasing the service life of the light source.

Temperature has an important impact on the life of electronic components. Low temperature helps to extend the life of electronic components. LED lamps are integrated lamps and electronic components, so the life of the entire lamp is restricted by the life of the electronic components with the shortest life. Dimming helps to reduce the temperature of LED lamps, which is beneficial to the luminous flux maintenance rate of LED lamps and extend the service life of LED lamps.

3 Phase control dimming

Phase-controlled dimming was first used for dimming incandescent lamps. The main advantage of phase-controlled dimming is that it is easy to use and install. The frequency of LED phase-controlled dimming should be no less than 100Hz to avoid human eyes from feeling flickering. The larger the phase-controlled conduction angle, the greater the brightness of the LED. Phase-controlled semiconductor devices can be thyristors, field-effect transistors or related semiconductor devices.

Since existing dimmers are designed for incandescent lamps (approximately resistive loads, power is mostly between 20W and 50W), some additional circuitry is needed to allow them to be used in LED lighting systems. Currently, there is no standard for incandescent lamp dimmers on the market, so there are a large number of dimmers with different performance and parameter values ​​on the market.

3.1 Leading-edge phase-controlled dimming

Leading edge phase dimming (front phase dimming) has been widely used due to its simple circuit implementation. The switching device is usually a thyristor. The output AC effective value is controlled by controlling the conduction angle of the thyristor, thereby achieving dimming control. The schematic diagram of a typical thyristor leading edge phase dimming circuit is shown in Figure 2. In Figure 2, the adjustable resistor VR4 and the capacitor C23 form a phase shift circuit. When the voltage across C23 reaches or exceeds the breakdown voltage of DIAC (DB3), the charge of C23 is injected into the bidirectional thyristor T2 through the DB3 part, and the bidirectional thyristor T2 is triggered to conduct.

The minimum gate trigger current (IGT) of the bidirectional thyristor T2 turns on the bidirectional thyristor. It also requires a minimum holding current to maintain the conduction of the bidirectional thyristor. When the current is lower than the holding current, the bidirectional thyristor T2 is turned off.

The dimming waveform of the leading edge phase-controlled dimming of thyristor is shown in Figure 3. Leading edge phase-controlled dimming is often used in applications where incandescent lamps and halogen tungsten lamps are dimmed. Since the current through the thyristor decreases when the dimming brightness is low, if the dimming brightness is very low and the current through the thyristor is lower than the thyristor's maintenance conduction current, dimming is no longer possible, otherwise the thyristor is turned off, which may be a problem for low-brightness LED dimming.

4 Issues to be noted when evaluating thyristor phase-controlled LED dimmable systems

The following are some common issues that need to be paid attention to when evaluating thyristor phase-controlled LED dimmable systems.

4.1 100% light output

How much does the light output decrease when the dimmer is at 100% position and when the light output is not dimmable?

4.2 Maximum dimming level

What is the maximum dimming level of a dimmable LED lighting system?

4.3 Physical adjustment range and light output characteristics of dimmers

(1) What is the physical adjustment range of the dimmer? What is the dimming range of the corresponding LED?

(2) What is the physical adjustment range within which the dimming light output remains unchanged in the low light output adjustment range before the dimmer is turned off?

(3) Is there any relationship between the physical adjustment position of the dimmer, the light output, and the effective value of the voltage applied to the LED lamp load (phase angle, conduction time, duty cycle, etc.)?

4.4 Range of LED quantity that can be dimmed by the dimmer/minimum load

(1) What is the range of LED loads that can be dimmed normally by a dimmer?

(2) The minimum load that a thyristor dimmer can carry is related to the thyristor's holding current. When the LED is dimmed at the lowest brightness, can the minimum operating current of the LED meet the thyristor's holding current requirement?

4.5 Surge Current

For incandescent thyristor dimming, its surge input current is related to the cold resistance of the incandescent filament. For LED dimming load, this surge input current value should be smaller than that of incandescent thyristor dimming to ensure reliable operation of the thyristor dimming circuit.

4.6 Repetitive Peak Voltage

The voltage resistance of thyristors is limited. When using them, you should ensure that the repetitive peak voltage in LED applications is lower than the repetitive peak voltage of the thyristor dimmer for incandescent lamps to ensure that the thyristor dimming circuit works reliably.

4.7 Minimum dimming range

Is the LED still turned on at the lowest dimming position of the thyristor dimmer?

4.8 Dimming position of LED not emitting light

In practice, when the thyristor dimmer is dimming from maximum to minimum or from minimum to maximum, it is possible that the LED does not work at a certain position in the middle of the dimmer. Does this LED thyristor dimmer have this phenomenon?

4.9 Turn-on delay time

(1) What is the maximum turn-on delay time of an LED thyristor dimmer?

(2) Where does the turn-on delay of an LED thyristor dimmer occur?

(3) Is there a relationship between turn-on delay and dimming level?

(4) Does this turn-on delay time vary when different LED products are connected to the LED thyristor dimmer? Does the dimming "popcorn" effect (flickering of the dimming light output) occur during dimming?

4.10 Dimming and flickering

Is there a flickering effect when dimming with a thyristor dimmer? Is there a stroboscopic effect when dimming with a thyristor dimmer?

4.11 Color Change

Does the LED light color change when the LED thyristor dimmer is dimming? If so, how?

4.12 Audible Noise

Is there any audible noise when the LED thyristor dimmer is dimming? Is the audible noise when the LED thyristor dimmer is dimming related to certain factors (for example, the number of LED lamps connected to the LED thyristor dimmer and the dimming position of the LED thyristor dimmer)?

5 Summary

Since thyristor dimming is easy to use and in line with people's usage habits, the promotion of LED thyristor dimming has a good market prospect. However, in the application of LED dimming, thyristor dimming needs to make further efforts in terms of circuit power factor, dimming flicker and other dimming performance and cost, so as to expand the application scope of thyristor dimming in LED lighting dimming.