The innovative H-bridge-based LED lighting technology presents a modern take on a classic circuit design, traditionally used to control DC motors. By leveraging the principles of the H-bridge, which involves manipulating four switches or relays to manage motor speed and direction, this technology has been adapted to drive white LED arrays directly from an AC power source. Unlike conventional lighting systems, this approach operates in a full-wave current-limiting mode, ensuring flicker-free, energy-efficient illumination. The system achieves this by alternating the electrical switching mode during the positive and negative phases of the input voltage, maintaining a consistent LED excitation current throughout both halves of the cycle. This method effectively rectifies the AC voltage, converting it into a stable DC supply for the LED string, resulting in minimal ripples and significantly enhanced power factor.
As illustrated in Figure 1, the H-bridge consists of two arms formed by transistors Q1, Q3, Q5, and diode D4 on one side, and transistors Q2, Q4, Q6, and diode D3 on the other. Diodes D1 and D2 complete the bridge structure. The LED string is connected between the midpoints of the bridge, labeled VLED+ and VLEDGND. The AC input is applied through a current-limiting PTC resistor R5, series capacitors C4 and C5 (forming a non-polar capacitor CEFF), and an inductor L1. The neutral line of the main AC supply connects to the circuit's ground via inductor L2.
During the positive half-cycle, the AC power bus is more positive, allowing transistor Q1 to receive the appropriate base bias through resistor R1. Current flows through diode D4, transistor Q1, and resistor R3, indicated by arrow A1, before passing through the LED string (LED1 to LED12) and exiting via diode D2 as shown by arrow A2. Conversely, during the negative half-cycle, the AC power bus becomes relatively negative, activating transistor Q2 via resistor R2. Current flows through diode D3, transistor Q2, and resistor R4 (arrow A3), then through the LED string and out via diode D1 into the AC power bus (arrow A4). This ensures that current flows through the LED string in the same direction during each cycle, mimicking the functionality of a full-wave rectifier bridge. However, the current through the LED string remains constant due to regulation by the corresponding switch acting as a voltage-controlled current source.
Transistors Q3 and Q4 monitor the current flow through R3 and R4. When the voltage drop across these resistors exceeds the base-emitter voltage thresholds of Q3 and Q4, the transistors turn on, pulling the bases of Q1 and Q2 low. This action disrupts the current flow through Q1 and Q2 during their respective half-cycles, ensuring the current remains constant and does not surpass a predetermined threshold. Resistors R3 and R4 are selected to set this threshold, while Q5 and Q6 limit the base current of Q1 and Q2 to a safe level (approximately 150μA). Additionally, when the base-emitter voltages of Q1 and Q2 exceed the voltage drops across R6 and R8 in series with R1 and R2, a significant portion of the base currents is diverted to R3 and R4 through Q5 and Q6.
The AC current entering the bus is limited by the reactance of CEFF, which can be adjusted by choosing appropriate values for C4 and C5. Alternatively, the circuit can be powered by a resistive supply by replacing CEFF with a high-power resistor ranging from 50Ω to 200Ω. This adjustment improves the power factor but increases the power dissipation in the current-limiting resistor. Resistors R3 and R4 can be fine-tuned to achieve the desired constant current level. Diode D5 provides reverse voltage protection for the LED string, while R5 limits inrush current during startup. Inductors L1 and L2, along with capacitor C1, help reduce electromagnetic interference (EMI)/radio-frequency interference (RFI) and enhance the power factor. A metal oxide varistor can be added alongside the AC power supply to safeguard against voltage spikes.
In the given circuit, the 12 medium-power LEDs (0.5W each) operate at 120mA DC (135mA RMS), with current sense resistors R3 and R4 set to 1Ω. The number of LEDs can be increased to 18 if the total forward voltage of the string exceeds the combined forward voltages of the individual LEDs (ranging from 3.3V to 4V). The circuit's self-limiting voltage (around 42V) eliminates the need for additional adjustments since the series-connected LEDs act like a high-power Zener diode in forward bias. At 230V AC, the circuit consumes 11.5W of power with a power factor of 0.93, eliminating flickering. Adding a 220μF capacitor C2 between VLED+ and VLEDGND can further reduce ripple. Furthermore, the LED string can be divided into six parallel strings, each containing 12 to 18 high-brightness LEDs rated at 20mA. To prevent thermal runaway, transistors Q1 and Q2 should be mounted on heat sinks.
This advanced H-bridge LED lighting system offers numerous advantages, including improved efficiency, reduced flicker, and enhanced power factor, making it an ideal solution for modern lighting applications.
We started to manufacture traffic signal pole from 2001 and exported from 2002.
Our firm introduced whole set of good-sized numerical control hydraulic folding equipment(1280/16000) as well as equipped with a series of good-sized professional equipments of armor plate-flatted machine, lengthways cut machine, numerical control cut machine, auto-closed up machine, auto-arc-weld machine, hydraulic redressing straight machine, etc. The firm produces all sorts of conical, pyramidal, cylindrical steel poles with production range of dia 50mm-2250mm, thickness 1mm-25mm, once taking shape 16000mm long, and large-scale steel components. The firm also is equipped with a multicolor-spayed pipelining. At the meantime, for better service to the clients, our firm founded a branch com. The Yixing Jinlei Lighting Installation Com, which offers clients a succession of service from design to manufacture and fixing.
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