introduction

Dimming LED Driver Based on Controller IC usually adopts two dimming methods, namely digital PWM dimming and analog DC voltage dimming.

If triac traditional incandescent lamp and halogen lamp dimmer based on phase control is used for LED dimming control, it will produce 100Hz or 120Hz flicker, and the dimming range is very narrow.An off-line AC / DC Buck constant current LED controller lm3445 with triac dimming decoder launched by National Semiconductor (NS) of the United States,It allows the use of standard triac dimmer for a wide range of stable and flicker free dimming of LED, breaking a bottleneck in the application of traditional triac dimmer and LED energy-saving lighting.

Main features of lm3445:

The lm3445 is compared with the previous similar off-line AC / DC step-down constant current LED Driver IC,Its main feature is that the triac dimming decoder circuit is designed on the chip,It can sense triac dimming waveform of AC line and convert it into dimming signal controlling LED current,It can almost achieve flicker free LED brightness regulation in the dimming range from 0% to 100%.

Other features of lm3445 include:

(1) The AC input voltage range is 80 ~ 270V, which is suitable for international general AC lines;

(2) Able to control LED current greater than 1A;

(3) It is suitable for configuring passive (passive) power factor correction (PFC) circuit to meet the commercial application requirements of Energy Star solid state lighting (SSL);

(4) Multi chip solution supporting master / slave control function,

Using a triac dimmer and a master lm3445,

Multiple strings of LEDs driven by multiple slave buck converters based on lm3445 can be controlled;

(5) Provide thermal shutdown protection and current limit with VCC under locking and 165 ℃ threshold;

(6) Fixed off time programmable, switching frequency adjustable;

(7) 10 pin MSOP package is adopted, and the junction temperature range is – 40 ℃ ~ + 125 ℃.

Triac dimming off-line LED driving circuit based on lm3445

1) Basic circuit

The internal structure of lm3445 and the triac dimming off-line LED driving circuit composed of lm3445 are shown in Figure 1.This AC-DC constant current LED driving circuit mainly consists of five parts,That is, triac dimmer, bridge rectifier BR1, rectifier line voltage detection and dimming decoder circuit, passive power factor correction (PFC) circuit and buck DC / DC converter circuit. The core of the whole system is lm3445.

2) Circuit working principle

(1) Triac dimmer

In Fig. 1, the triac dimmer connected in series at the input of bridge rectifier BR1 adopts a traditional circuit based on phase control, as shown in the figure.

The R1, R2 and C1 values determine the delay time before the voltage on C1 reaches the bidirectional trigger diode (DIAC) trigger voltage (about 32V).When the load is incandescent lamp, the R1 value decreases, the conduction delay of triac shortens, the conduction angle increases, and the lamp brightness increases;On the contrary, if the R1 value increases, the triac conduction angle will decrease and the light will dim.In Figure 1, triac dimmer is connected in series at the input end of AC line. Through the dimming decoder circuit of lm3445, the current of LED string can be controlled to realize brightness regulation.

(2) Triac dimming decoding circuit

Triac dimming decoding circuit consists of rectifier line voltage sensing circuit, triac conduction angle detection circuit and dimming decoder circuit (see Fig. 1).

Principle analysis of triac dimming off-line LED driving circuit based on controller lm3445

Principle analysis of triac dimming off-line LED driving circuit based on controller lm3445

① Line voltage sensing

The R1 and 15V zener diodes VD1 and VT1 behind the bridge rectifier form a series path rectifier, which converts the rectified line voltage to an appropriate level and is sensed by the pin bldr of IC (lm3445).

Since the VT1 source is not connected to the capacitor, when the line voltage drops below 15V, the voltage on the IC pin bldr is allowed to increase and decrease with the rectified voltage.

R5 has two functions:

One is used to discharge the charge of parasitic capacitance of IC pin bldr node; The second is to provide the dimmer with the required holding current when operating on the low current output.

The function of diode (Schottky type) VD2 and capacitor C5 is to maintain the voltage on IC pin VCC when the voltage on IC pin bldr becomes low, so that the IC can operate normally.

② Angle detection and dimming decoder

Triac on angle detection circuit uses a comparator with a threshold of 7.2V to monitor IC pin bldr to determine whether triac is on or off.

Comparator output via 4 μ S delay line controls a drain circuit and drives a buffer.

The swing of buffer output (pin ASNs) is limited to 0 ~ 4V, filtered by low-pass filter composed of R1 and C3, and input to slope comparator (inverting end) through IC pin fltrl,

Compared with the sawtooth wave of 5.88khz and L ~ 3V generated by the ramp generator, the ramp comparator outputs the drive pin dim and an n-channel MOSFET.

The signal on the MOSFET drain is filtered by the (second) low-pass filter composed of internal 370k Ω and capacitor C4 on IC pin fltr2 and transmitted to the internal PWM comparator.

The dimming decoder outputs a DC voltage with an amplitude varying from 0 to 750 MV, the corresponding duty cycle of the dimmer varies from 25% to 75%, and the triac conduction angle ranges from 45 ℃ to 135 ℃,

Thus, the peak current of the LED is directly controlled to obtain a dimming range from almost 0% to 100%.

(3) Passive PFC circuit

Capacitors C7 and C9 and diodes VD4, vd8 and vd9 form a filter valley filling passive (i.e. passive) PFC circuit.

Replacing a traditional high-capacity filter capacitor with it can improve the line power factor.

Capacitor C10 (10nf) can attenuate voltage ripple when C7 and C9 are charged. The output voltage ubuck of the passive PFC circuit is used as the DC bus voltage of the step-down converter.

Without triac dimmer access, when the AC line voltage is higher than 1 / 2 of its peak value, VD3 and vd8 are on, VD4 and vd9 are off, capacitors C7 and C9 are charged in series, and the current will flow into the load.

When the AC line voltage is lower than 1 / 2 of its peak value, VD3 and vd8 are reverse biased, while VD4 and vd9 are forward biased. C7 and C9 discharge in parallel, and the current flows into the load.

Fig. 3 shows the waveforms of AC line voltage UAC, rectifier voltage ubr1 and PFC circuit output voltage ubuck without triac dimmer.

As can be seen from Figure 3, although the ubuck waveform is not smooth,

However, in the half cycle of AC line, the current conduction angle is up to 120 ° (i.e. from 30 ° to 150 °), and the line power factor is more than 0.9.

Although only a single high-capacity capacitor filter can obtain a relatively smooth DC voltage,

However, the current flow angle is only about 60 ° (i.e. from 60 ° to 120 °), and the line power factor does not exceed 0.6.

Principle analysis of triac dimming off-line LED driving circuit based on controller lm3445

The relevant voltage waveform when triac dimmer is added is shown in Figure 4, where θ Is the conduction angle of triac.

(4) DC-DC buck converter

The controller lm3445, power MOSFET (vt2), inductor L2, diode vd10, resistor R3 and capacitor C12 form a switching DC-DC step-down converter to drive the LED string.

When the PWM signal on the gate of lm3445 pin drives vt2 to turn on, the current through L2 and LED string increases linearly and is sensed by R3.

When the voltage on R3 is equal to the reference voltage on IC pin fltr2, vt2 is turned off, L2 releases energy storage, vd10 is turned on, and the current passes through LED string and L2 and decreases linearly from its peak.

C12 is used to eliminate the ripple current of most inductors L2, and R4, C11 and VT3 provide a linear current ramp signal for setting a fixed off time.

Principle analysis of triac dimming off-line LED driving circuit based on controller lm3445

Principle analysis of triac dimming off-line LED driving circuit based on controller lm3445

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