Most of the dimming circuits of traditional white heat bulbs use a simple two-way AC triac (Triac) phase control method. The white heat bulb uses high-temperature illumination of tungsten wire, and uses the phase control method of the bidirectional AC trigger triode, so there is no flash phenomenon during the no voltage period. Conversely, when the light source becomes the LED mode, the same bidirectional AC trigger triode position control circuit of the frequency is typically two times the commercial frequency, by no impact voltage period, prone to flicker.
     Recently, National Semiconductor has developed a direct-coupled bidirectional AC-triggered triode dimmer, which is almost completely free of flickering LED driver IC LM3445 and evaluation substrate. Then the author combines the evaluation substrate and the simple two-way AC trigger triode dimming circuit to illustrate the focus of the LM3445 's evaluation of the substrate and circuit design.
     Evaluation of the substrate package LM3445 , power supply circuit, and peripheral circuits, evaluation of the substrate using a bidirectional AC trigger triode dimming circuit, input the voltage that has been controlled by the phase, use the high frequency switch to provide LED current, LED driver can be controlled The buck converter that flows into the peak of the LED current sets the OFF time to more than a certain value during operation . The action first accepts the output voltage of the bidirectional AC trigger triode dimming circuit, and then detects the ON period of the bidirectional AC triode , and then converts this signal into an inflow LED current command value, at which time the inflow LED current and the bidirectional AC trigger The triode ON time is proportional, and the dimming circuit of the conventional white heat bulb can be used. In addition, the evaluation board supports the master-slave structure, which can dim the complex LEDs with the same current .
Evaluation and overall circuit architecture
     Figure 1 (a) is a block diagram of the evaluation circuit; Figure 1 (b) is a dimming circuit of the bidirectional AC trigger triode, which can be seen from the figure by the " Anode fire" mode, using two-way AC trigger triodes The terminal voltage is used as the driving voltage. After the variable resistor VR is passed, the capacitor C 1 is charged with a positive voltage or a negative voltage. At this time, regardless of the polarity, once the voltage of the capacitor C 1 exceeds a certain level, the trigger diode is energized to cause the bidirectional AC trigger. The triode is in the arc and flows into the bidirectional AC to trigger the current of the triode. Even if it exceeds a value, it continues to be energized, and the current flows into the load.
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     The diodes D 1 to D 4 and the 15kΩ resistors in the figure are connected to the two ends of the bidirectional AC triode. The main purpose is to maintain the starting charging voltage of the capacitor C 1 regardless of the polarity , and to avoid the commercial power supply. Polarity effects, so deliberately integrate this into a circuit with the same arc phase. Since the bidirectional AC trigger triode circuit is OFF , the current is not completely interrupted, and there is approximately 15kΩ impedance. To reduce the influence on the evaluation substrate, this circuit is inserted with a 1kΩ dummy resistor. Figure 1 (c) is a voltage waveform of the supply evaluation substrate, taking a sine wave of the power supply.
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