Based on Infineon LED thyristor dimming solution

The principle of thyristor dimming

Figure 1 shows a typical leading-edge thyristor dimmer schematic and the voltage and current waveforms it produces. The loop voltage/current are in phase (the load is an incandescent lamp).

Potentiometer RV2 adjusts the phase angle of the TRIAC, and when VC3 exceeds the breakdown voltage of the DIAC, the TRIAC will turn on. When the TRIAC 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 it can turn on again. The voltage and current in the bulb filament are closely related to the phase angle of the dimming signal, which can vary between 0 degrees (close to 0 degrees) and 180 degrees (depending on the dimmer).

Problems with LED dimming

For an LED lamp to be dimmable, its power supply must be able to sense the variable phase angle output of the thyristor controller in order to adjust the current to the LED. Doing this while maintaining proper dimmer operation is difficult and often results in poor performance. Problems can manifest as flicker and audible noise. These undesirable phenomena are often caused by factors such as false triggering or premature shutdown of the thyristor. The root cause of false triggering is current oscillation when the thyristor is turned on. Figure 3 illustrates this effect in graphical form.

Figure 2. SCR conduction operating conditions

Figure 3. SCR current (the SCR is triggered multiple times but cannot maintain conduction)

When the thyristor turns on, the AC mains voltage is applied almost instantaneously to the LC input filter of the LED lamp power supply. The voltage step applied to the inductor causes oscillation. If the dimmer current falls below the thyristor holding current during the oscillation, the thyristor stops conducting. The thyristor trigger circuit charges and then turns the thyristor on again. This irregular multiple thyristor restart (as shown in Figure 3) can cause audible noise or LED flickering in the LED driver. A simpler EMI filter design can help reduce this unwanted oscillation. To achieve excellent dimming performance, the input EMI filter inductance and capacitance must be as small as possible.

For a thyristor, the holding current required to maintain conduction is typically between 8 mA and 75 mA. Incandescent lamps can easily maintain this current, but for LED lamps, which consume only 10% of the power of an equivalent incandescent lamp, this current can drop below the thyristor holding current, causing the thyristor to shut down prematurely. This can cause flickering or limit the dimming range.

Minor flickering issue

Table 1 DIAC characteristics

From Table 1, it can be seen that due to the error in the description of the forward and reverse breakdown voltages by the characteristics of DIAC, the asymmetric breakdown voltage will cause the conduction angles of the thyristor to be different in the positive and negative half cycles (see Figure 4A). This is especially obvious in low-cost dimmers. The output current will also change with the input (as shown in Figure 4b), causing the LED light to flicker, especially at low output.

Infineon's practical LED driver dimming solution

Based on the above problems, Infineon has launched a quasi-resonant PWM controller designed for high-efficiency offline LED dimming drive applications - ICL8002G, which can be used for the design and application of flyback converters or buck converters. Its quasi-resonant working mode, primary side control, integrated PFC and phase-cut dimming control, and various protection functions make it an excellent system solution for dimmable LED bulbs. Compared with ICL8001G, the new ICL8002G has great improvements in dimming performance and output current stability. The compatibility with TRIAC-based phase-cut dimmers can be improved by adding damping circuits and bleeder circuits, and the output current can be kept stable over a wide input voltage range through additional linear adjustment circuits.

The following table shows the design specifications of the ICL8002G demo board.

Schematic

Figure 5 Schematic diagram of the application of ICL8002G 12W thyristor dimming LED bulb

Compatibility with TRIAC-based dimmers

TRIAC-based dimmers work perfectly with resistive loads such as incandescent lamps. When they are used with nonlinear loads such as switched LED drivers, flickering problems may occur, which is mainly caused by insufficient holding current (the current consumed by the entire LED lamp is less than the holding current of the thyristor) and current oscillations - especially during the TRIAC conduction period. Therefore, in order to improve compatibility with TRIAC-based dimmers, bleeder circuits and damping circuits are usually added to the LED driver. The passive bleeder circuit included in this design (composed of C1, C2, R4, and R5) can make the input current greater than the holding current threshold of the TRIAC. The two resistors R1 and R2 are used to suppress oscillations and reduce inrush current. Solutions and experimental data for slight flicker

Circuit A in Figure 5 consists of a circuit network composed of R6, R7, R8, C4, Q2, and ZD1, which is specially designed for deep dimming and improving flickering (slight flickering). Among them, ZD1 is a protection diode to prevent Q2's Vbe from overvoltage breakdown. R6, R7, and R8 form a voltage divider detector. Since C4 has a large capacity, the C4 terminal is a smooth voltage. Figure 6A below is the voltage waveform of C5 when the output current is small and the above circuit is not added (the two adjacent half-wave inputs are asymmetric). If this circuit is added, when the voltage at the C4 terminal is the voltage of the red line in the figure below, the voltage of C5 will be clamped through Q2 to obtain a more uniform voltage as shown in Figure 6B. At the same time, since the voltage at the VR terminal determines the output current, an uneven VR terminal voltage will cause the LED to flicker. On the contrary, a more uniform VR terminal voltage will improve the slight flickering of the output LED. The voltage of C5 changes through Q2 following the change of the voltage at the C4 terminal. If at a low conduction angle, the voltage at the C5 terminal will follow the voltage at the C4 terminal to drop to a lower voltage through Q2 to reduce the output current, thereby increasing the dimming range and achieving deep dimming.

Figure 7 shows the actual test waveforms of C4 and C5, where the brown line is the voltage at the C5 terminal and the blue line is the voltage at the C4 terminal. Actual measurements have shown that adding circuit A can improve the asymmetry of the VR terminal voltage.

FIG8A shows the LED output current waveform without adding circuit A. The difference between the positive and negative half cycles is too large, resulting in a slight flickering of the output. FIG8B shows the output LED current waveform with adding circuit A. It can be seen that the flickering problem of the LED is improved after adding this circuit.

Line Regulation

Figure 9 shows the measured LED current vs. supply voltage. The maximum current deviation is limited to ±3% over the entire input voltage range (180Vac-265Vac).

Figure 9. Input voltage vs output current

Dimming curve

Figure 10 shows the measured LED dimming range versus thyristor conduction angle. The dimming range can be reduced to less than 1%.

Figure 10. LED dimming curve (LED dimming range vs. thyristor conduction angle)

Protective function

Output open circuit protection

During operation, if the output is open circuit, the output voltage will increase, so the voltage generated by the VCC winding will also increase when the MOSFET is turned off. The ZCV pin of the ICL8002G detects the VCC winding voltage through R15 and R16. Once the ZCV voltage reaches the OVP threshold (Vzcovp = 3.7V), the output overvoltage protection will be triggered and the IC will enter the latched shutdown mode. On the other hand, the voltage generated by the VCC winding will power Vcc, and if Vcc reaches the threshold (Vvccovp = 25V), the Vcc overvoltage protection will be triggered. In this demo board design, when the output is open circuit, the ZCV pin voltage will reach the OVP threshold and be triggered, and the IC will 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 short-circuited, the IC switches to auto-restart mode via VCC undervoltage protection. The total input power consumption in this mode is kept below 1W.