“The LED driving power supply for general lighting generally adopts a flyback converter circuit topology based on a PWM controller. This solution has a simple structure, but it is generally not possible to use a triac (TRIAC) dimmer for dimming of traditional incandescent lamps. This is because the incandescent lamp is a pure resistive load, and the AC/DC power supply system and the incandescent lamp The situation is completely different.

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The LED driving power supply for general lighting generally adopts a flyback converter circuit topology based on a PWM controller. This solution has a simple structure, but it is generally not possible to use a triac (TRIAC) dimmer for dimming of traditional incandescent lamps. This is because the incandescent lamp is a pure resistive load, and the AC/DC power supply system and the incandescent lamp The situation is completely different.

Use the iW3610 AC/DC digital power controller to construct a flyback LED driver, which can be compatible with all types of dimmers, with a dimming range of 2% to 10%, and no flicker. When there is no dimmer The power factor is 0.9, and the coefficient efficiency is 85%.

Structure and characteristics of iW3610

iW3610 adopts 8-pin SOIC package, and the pin configuration is shown in Figure 1.
　　

The iW3610 chip integrates start-up and input voltage detection circuit, feedback signal adjustment circuit, A/D converter, D/A converter, dimmer detection and phase measurement circuit, constant current control circuit, overcurrent protection comparator, peak current Limit comparator, chopping (chopping) circuit MOSFFT gate driver above the main power supply MOSFET gate driver, etc., as shown in Figure 2.
　

The functions of each pin of iW3610 are as follows.

Pin 1 (OUTPUT (TR)): Chopper circuit MOSFFT switch gate drive output.

Pin 2 (VSENSE): Transformer auxiliary winding sense signal input, used for secondary side voltage feedback to adjust the output.

Pin 3 (VIN): Rectified output voltage detection signal input, used for dimmer phase detection, input undervoltage/overvoltage protection, and provides power supply current for the chip during startup.

Pin 4 (VT): External close control terminal. If the shutdown control is not used, a resistor should be connected to the ground.

Pin 5 (GND): Ground pin.

Pin 6 (TSENSE): Primary current sensing input, used for cycle-by-cycle peak current control.

Pin 7 (OUTPUT): Flyback converter MOSFET switch gate drive output.

Pin 8 (VCC): The controller power supply, the startup threshold is 12V, and the undervoltage shutdown threshold level is 7.5V.

iW3610 adopts digital control technology, including: chopper circuit, its function is to improve the power factor, provide dynamic impedance for the dimmer; isolated flyback circuit topology, provide a low-cost solution, allowing the use of traditional incandescent lamp dimmer The LED is dimmed. iW3610 can detect the type of wall dimmer and measure the phase. iW3610 switches in valley mode, and the efficiency can reach 85% without dimmer. iW3610 uses primary-side feedback constant current control technology to obtain LED current regulation with a tolerance of ±5%.

Dimmable LED drive power supply based on iW3610

The dimmable LED drive power circuit using iW3610 is shown in Figure 3. Appropriate selection of components in the circuit, the output power can reach 45W.

1 Circuit composition

One is the input EMI filter. L1, L2, and C1 form an EMI filter circuit, and R1 and R2 are used to dampen the LC resonance oscillation.

The second is the bridge ballast. BR1 is a full bridge bridge rectifier.

The third is the chopper circuit. VD1～VD3, C2 and C4, L3, VT2, R6 and R7 form a chopper circuit, which is used as a dimmer to provide dynamic impedance.

The fourth is a flyback converter. U1, VT1, converter T1, etc. constitute a flyback converter. R8, C5, and VD4 on the primary winding of T1 form an RCD-type primary clamping capacitor. VD6 and C7 on the secondary side of T1 form the output rectifier filter circuit, R14 is the preload, T1 auxiliary (or bias) winding, VD5 and C6 form the bias power supply on the U1 pin VCC. The auxiliary winding also provides output feedback, eliminating the sensing and photoelectric coupling feedback circuit on the secondary side.

The dimmer is connected in series to the input phase line L of the AC line. U1 can detect the dimmer type (such as leading edge dimmer, trailing edge dimmer, etc.) and detect the dimmer phase. When U1 detects that the dimmer is not present, the circuit can still operate, and it has a high power factor.

2 The working principle of the circuit

(1) The circuit starts

After the AC power supply is turned on, the rectified DC high voltage charges the capacitor C6 through the diode connected between the pin VIN and the pin VCC through the resistors R3, R4 and U1. As long as the voltage on the U1 pin VCC exceeds the 12V threshold, the control logic in U1 is enabled and U1 enters the normal operation mode. During the first 3 AC half-cycles at the beginning, the U1 pin OUTPUT (TR) remains high and VT2 is turned on. After the dimmer type and AC line cycle are detected, the constant current circuit is enabled and the output voltage starts to rise. When the output voltage is higher than the total forward voltage on the LED string, U1 starts to operate in constant current mode.

After U1 starts, the U1 pin VCC is powered by the bias power supply.

(2) Dimmer detection and phase measurement

Dimmer detection and dimmer phase measurement are realized through the internal circuit of resistors R3, R4 and U1 pin VIN.

Dimmer detection is divided into two steps: the first step is to determine whether the dimmer is stored; the second step is to determine the type of dimmer (is it a leading edge dimmer or a trailing edge dimmer if the dimmer is detected) Optical device). The dimmer detection occurs in the third cycle after the system starts. When the voltage VIN on the U1 pin ③
In the second cycle when the dimmer is found, the VIN cycle is detected and locked for standby. When VIN exceeds 0.1V and count the input voltage samples, start measuring the dimmer phase. If the thyristor conduction time is ton, the dimming period is t, and the dimmer phase is ton/t. The larger the conduction angle of the thyristor in the dimmer, the greater the output power of the power supply, and the brighter the LED; conversely, the smaller the conduction angle of the dimmer, the darker the brightness of the LED.

(3) Chopper circuit

The function of the chopper circuit is to provide dynamic impedance for the dimmer and build energy for the flyback converter. When the voltage Vc4 on the circuit C4 of VD2 is lower than the input voltage, it provides a path for the charging C4. When the TRIAC is triggered, the surge current can be reduced. During the chopping cycle, when VT2 turns on and L3 turns on, L3 stores energy; when VT2 turns off, L3 releases energy to make VD3 turn on.

The circuit composed of L3, VT2, VD3, and C4 is similar to a conventional power factor correction (PFC) boost converter. When the dimmer is not connected, the average current through L3 is in phase with the input AC voltage, so a power factor higher than 0.9 is produced.

Design of dimmable LED drive power supply based on iW3610 chip

(4) Primary side feedback and constant LED current operation

iW3610 adopts primary side feedback, without secondary side sensing and optocoupler. The voltage VAUX on the auxiliary winding of T1 (the number of turns is NAUX) is the result of the output voltage emission. The forward voltage drop on VD6 is only about 0.5V. If this forward voltage drop is ignored, when the number of turns of the secondary winding of T1 is Ns, the voltage on the auxiliary winding is VAUX=Uo×(NAUX/NS). The voltage on the auxiliary winding of T1 is fed to the U1 pin VSENSE through R9 and R10, and the output current is adjusted to a constant level by the internal constant current control circuit, regardless of the output voltage.

The primary side current is detected by VT1 source resistor R13 to implement peak current limit (PCL) and over current protection (OCP).

(5) Bottom mode switch

During the constant current output operation, U1 adopts valley mode switching, that is, VT1 switches at the lowest point of drain-source resonance voltage, so it has the smallest switching loss and EMI.

(6) LED temperature drift compensation

U1 pin VT connects an NTC thermistor RNTC externally to provide temperature drift compensation for the LED. RNTC can sense the LED temperature. When the temperature is high, U1 can dim the LED. If the LED temperature reaches the limit threshold, U1 will be turned off.

Concluding remarks

iW3610 is a flyback power controller that adopts advanced numerical control technology. The dimmable LED driver based on iW3610 can detect the existence of dimmer, dimmer type, and measure dimmer phase, with a flicker-free dimming range of 2% to 100%. iW3610 adopts primary-side sensing technology, without secondary feedback circuit and loop compensation components, and realizes LED constant current adjustment through pulse-to-pulse waveform analysis. iW3610 operates in quasi-resonant mode, providing 85% efficiency when there is no dimmer. The iW3610 combines a chopper circuit with dimming, and the power factor reaches 0.9 without a dimmer. The full range of protection functions of iW3610 make the system highly reliable.