LED thyristor dimming principle and solution

There are three main dimming methods: analog dimming, PWM dimming, and thyristor dimming. The use of thyristor dimming for LED replacement lamp dimming, the existing dimmer circuit can be unchanged, so the dimming method is generally optimistic, so there is an AC-DC control chip suitable for thyristor dimming. Infineon's ICL8002G LED driver chip supports TRIAC dimming and features single-stage PFC and primary measurement control.

The principle of thyristor dimming

The potentiometer RV2 adjusts the phase angle of the thyristor (TRIAC). When VC3 exceeds the breakdown voltage of the DIAC, the thyristor turns on. When the thyristor current drops below its holding current (Iholding) (see Figure 2 below), the TRIAC turns off and must wait until C3 is recharged in the next half cycle before turning it back on. The voltage and current in the filament of the bulb are closely related to the phase angle of the dimming signal, and the phase angle varies from 0 degrees (nearly 0 degrees) to 180 degrees (depending on the dimmer).

LED dimming problems

In order for the LED to be dimmable, its power supply must be able to detect the variable phase angle output of the thyristor controller to adjust the current flowing to the LED. It is very difficult to do this while maintaining the dimmer's normal operation, which often leads to poor performance. Problems can be manifested as problems such as flicker and audio noise. These undesirable phenomena are usually caused by factors such as false triggering or premature shutdown of the thyristor. The root cause of false triggering is current oscillations when the thyristor is turned on. Figure 3 illustrates this effect graphically.

When the thyristor is turned on, the AC mains voltage is applied almost instantaneously to the LC input filter of the LED lamp power supply. A voltage step applied to the inductor can cause oscillation. If the dimmer current is below the thyristor hold current during oscillation, the thyristor will stop conducting. The thyristor trigger circuit charges and then turns on the thyristor again. This irregular multiple thyristor restart (Figure 3) allows the LED driver to generate audio noise or LED flicker. Designing a simpler EMI filter helps reduce such unwanted oscillations. For excellent dimming, the input EMI filter inductor and capacitor must be as small as possible.

For thyristors, the holding current required to maintain conduction is typically between 8 mA and 75 mA. Incandescent lamps are easier to maintain at this current level, but for LED lamps that consume only 10% of the equivalent incandescent lamp, this current can be reduced below the thyristor holding current, causing the thyristor to turn off prematurely. This will cause flicker or limit the dimming range.

Slightly flickering problem

It can be seen from Table 1 that due to the characteristics of DIAC, there is an error in the positive and negative breakdown voltages. The asymmetry of the breakdown voltage causes the conduction angles of the positive and negative half cycles of the thyristor to be different (see Figure 4A). Especially obvious in the dimmer, the output current will also follow the input change (Figure 4b), causing the LED light to flicker and dim, especially at low output.

Infineon's practical LED driver dimming solution

Based on the above issues, Infineon has introduced a quasi-resonant PWM controller designed for efficient off-line LED dimming drive applications, the ICL8002G, which can be used as a flyback converter or buck converter design and application. Its quasi-resonant mode of operation, primary side control, integrated PFC and phase-cut dimming control, and various protection features make it an excellent system solution for dimmable LED bulbs. Compared to the ICL8001G, the new ICL8002G offers significant improvements in diopter performance and output current stability. The compatibility of the TRIAC-based phase-cut dimmer can be improved by adding a damper circuit and a bleeder circuit, and the output current is stabilized over a wide input voltage range through an additional linear adjustment circuit.

TRIAC-based dimmer compatibility

TRIAC-based dimmers are ideal for resistive loads such as incandescent lamps. When they are used for non-linear loads such as switched LED drivers, there may be flicker problems, mainly due to insufficient holding current (the current consumed by the LED is less than the thyristor's holding current) and current oscillations—especially in TRIAC Caused during the conduction period. Therefore, in order to improve compatibility with TRIAC-based dimmers, bleeder and damper circuits are typically added to the LED driver. The passive bleeder circuit (consisting of C1, C2, R4, R5) included in this design allows the input current to be above the TRIAC's hold current threshold. The two resistors R1 and R2 are used to suppress oscillation and reduce inrush current.

Slightly flashing solution and experimental data

Circuit A consists of a circuit network consisting of R6, R7, R8, C4, Q2, and ZD1 designed for deep dimming and improved dimming (slightly flickering), where ZD1 is a protection diode to prevent V2 overvoltage breakdown of Q2, R6 R7 and R8 form a voltage divider detector. Due to the large capacity of C4, the C4 terminal is a smooth voltage. When the output current is small and the C5 voltage waveform of the above circuit is not increased (two adjacent half-wave inputs are asymmetrical). If this circuit is added, when the voltage at the C4 terminal is as shown in the red line of the figure below, the voltage of C5 will be clamped by Q2 to obtain a relatively uniform voltage as shown in Fig. 6B. At the same time, the output current is determined by the voltage of the VR terminal. An uneven VR terminal voltage will cause the LED to flicker, and a more uniform VR terminal voltage will improve the slight flicker of the output LED. The voltage of C5 changes with the change of the voltage of C4 terminal through Q2. If the voltage of C5 is low at the conduction angle, the voltage of C5 will be reduced to a lower voltage with the voltage of C4 to reduce the output current and increase the dimming range. Dimming.

Protective function

Output open circuit protection

During operation, if the output is open, the output voltage will rise and the voltage generated by the VCC winding will also rise when the MOSFET is turned off. The pin ZCV of ICL8002G detects the VCC winding voltage through R15 and R16. When the ZCV voltage reaches the OVP threshold (Vzcovp = 3.7V), the output overvoltage protection is triggered and the IC enters the latched shutdown mode. On the other hand, the voltage generated by the VCC winding will supply Vcc, and if Vcc reaches the threshold (Vvccovp = 25V), Vcc overvoltage protection will be triggered. In this demo board design, when the output is open, the ZCV pin voltage will reach the OVP threshold and be triggered, and the IC will also enter the latched shutdown mode. The power consumption in the latched shutdown mode is less than 0.5W.

Output short circuit protection

If the output is shorted, the IC will switch to auto-restart mode via VCC undervoltage protection. The total input power consumption in this mode will remain below 1W.