How to Make LED Bulbs Dimmable

灞波儿奔

How to Make LED Bulbs Dimmable [Copy link]

Over the years, manufacturers have introduced LED lamps to the market with the ultimate goal of replacing incandescent and compact fluorescent lamps (CFLs). The design of these lamps has evolved from very simple non-dimmable solutions to advanced but expensive dimmable solutions to more cost-effective dimmable solutions.
　　Many LED lamps are advertised as dimmable, but in reality, many LED lamps do not perform very well and the performance varies depending on the dimmer used and the circuit load. Sometimes, when LED lamps are installed in a room with a dimmer, the LED lamps will flicker and not adjust the light level evenly.
　　These defects are due to the fact that most dimmers currently used in the United States are based on two-wire leading-edge phase-cut circuits of bidirectional thyristors (TRIACs), which were developed in the 1960s for use with resistive incandescent lamps. A TRIAC is a bidirectional semiconductor power switch that is triggered by a pulse generated by a variable timing circuit and remains on when the conducting current is higher than the holding current. There are many different types of dimmer circuits, using devices with different characteristics and different control circuits and filtering components.
　　The LED lamp driver circuit converts the AC input power to a low-voltage DC power supply and maintains a regulated current to drive the high-brightness LED load for constant light output. To adjust the basic LED driver circuit with a bidirectional thyristor-based dimmer, additional components must be added to achieve stable dimmer operation and adjust the output current according to the dimmer phase angle.
　　Because dimmers vary widely, the performance of the connected LED dimming circuit will also vary. This problem is further complicated by the fact that there is no clear standard to classify the performance of LED bulbs with dimmers. At most, some bulb manufacturers provide a list of dimmers that they believe are compatible with their products. The
　　driver circuit used in most LED bulbs includes a buck, buck-boost, or flyback converter. In each case, the basic circuit can be modified to achieve acceptable dimming performance without increasing the cost and complexity of the components. This can improve the performance of dimmable drivers and meet the cost-saving requirements of the consumer lighting market. The
　　compatibility issue lies in how the TRIAC dimmer circuit interacts with the LED driver input circuit.
　　The single-stage LED driver example circuit (Figure 2) replaces the resistive load that represents the incandescent lamp in Figure 1. Although this circuit simulates a resistive load due to its high power factor during stable operation, it also includes the capacitors necessary for EMI filtering on the front end. In addition, the LED bulb consumes less than 25% of the power of an equivalent incandescent bulb. As a result, the dimmer is primarily subject to a capacitive load during the AC line half cycle before the TRIAC is triggered.

　　Figure 1: Typical dimmer schematic

　　Figure 2: Basic LED driver circuit block diagram
　　The bidirectional trigger circuit shown in Figure 1 requires a resistive path to neutral to operate as designed. If the load is capacitive, the circuit will not function properly and will result in unstable triggering during cycle transitions, which is manifested as flickering of the light output. EMI filters in dimmers and LED drivers can also cause ringing oscillations due to the high dv/dt at TRIAC startup.
　　When the oscillations reach a certain level, the current drops below the "holding current", causing the TRIAC to turn off and not be able to maintain conduction until the next line zero crossing. This is usually caused by the trigger circuit re-triggering the TRIAC, causing it to turn on and off multiple times in a single line half-cycle. In addition to stressing the components and possibly destroying the dimmer or LED driver, this will cause severe flickering and unpleasant noise.
　　Assuming that replacing the dimmer with a dimmer suitable for LED lamps is not an ideal solution, the above problems can be solved by modifying the LED driver so that the LED driver can be used with a standard dimmer.

　　Figure 3: Schematic diagram of a dimmable LED driver.
　　The example circuit (Figure 3) is a single-stage LED flyback converter, but the same techniques can be used for a buck-boost or adaptive buck converter. First, the input capacitor must be kept to a minimum when designing the input filter, which also helps achieve the best power factor.
Next, the active attenuator and passive bleeder circuits are introduced. The attenuator circuit limits the inrush current when the TRIAC is triggered, thereby greatly suppressing the ringing so that the TRIAC remains in the on state. After a short delay, the attenuator resistor is bypassed by a small MOSFET to prevent power loss during the remaining on period. To minimize the cost of low-power drivers, the bypass MOSFET and its associated driver circuitry can be omitted, but this will result in heat dissipation in the resistor and the associated efficiency loss. A
　　passive bleeder circuit can be used instead of the active bleeder used in some dimming solutions. This series RC network conducts current from the trigger point long enough for the switching converter to start drawing current, which helps ensure that the current does not drop below the holding current during this period. A flyback or buck-boost converter operating with constant on-time acts as the primary resistive load to the DC bus to keep the dimmer TRIAC on until the next line zero crossing. The converter needs to draw enough current to stay above the TRIAC holding current. A single-stage PFC flyback or buck-boost converter can usually achieve this. The
　　circuit described here uses the IRS2983 controller IC, which operates in voltage mode. The DC voltage level at the COMP input determines the on-time of the switching cycle. Because the controller IC is often used with primary-side regulation to maintain constant output power, a Zener diode must be added to this input to clamp the COMP voltage. This sets a limit on the maximum on-time so that when the DC bus voltage drops during dimming, the on-time cannot be increased to compensate.
　　As a result, the output current decreases as the dimmer setting is reduced and the DC bus voltage drops. This allows the light level to be dimmed to less than 20% by adjusting the dimmer control without the need for more complex circuitry to detect the dimmer phase angle or adjust the output. At the same time, the controller VCC supply must be released during the dimming off period to ensure that the IC operates only during the required period. A high-voltage diode is used to connect VCC to the DC bus for this purpose.
　　Active attenuator and passive bleeder circuits can also be used with buck converters, but the results depend on the LED voltage. Since the converter cannot draw current when the line voltage is lower than the output voltage, the phase dimming operating range will be limited. For this reason, the LED voltage is best kept low, but not too low, otherwise the circuit will become ineffective and an oversized inductor will be required. For a 120VAC system to maintain a reasonable regulation range, the LED voltage is best between 20V and 40V. CCM buck LED controller ICs (such as the IRS2980) can maintain average current regulation of the LED lamp while the unsmoothed bus voltage is always higher than the total LED output voltage.
　　These simple techniques described in this article, when used with the LED converter described, can achieve smooth dimming without flickering with most TRIAC-based dimmers.

okhxyyo

Thank you for sharing, I'm learning from it.

luck_gfb

These are all dimmed using dimmers. Are there any other dimming methods?

qwqwqw2088

TRIAC is also a kind of analog dimming

Mainly phase control