"New technology of LED lighting based on H-bridge H-bridge is a classic circuit that drives DC motor in a user-defined way, such as forward / reverse direction or PWM auxiliary control RPM through four discrete / integrated switches or electromechanical relays. It is widely used in machines and power electronics. This design example is a new implementation method of this technology, which can directly drive the white LED array from the AC power supply in the full wave current limiting mode, so as to realize a flicker free and high-energy-efficient solid-state lighting lamp. During the positive and negative offset of the excitation voltage, the circuit will adopt the alternating electric switching mode to control and maintain the LED excitation current in the negative half cycle and positive half cycle of the excitation voltage at a constant level. This method can make current controlled rectification of AC voltage, make it become the DC power supply voltage of series LED, have clean and almost ripple free current, and greatly improve the power factor.
The principle is shown in Figure 1. Transistors Q1, Q3 and Q5 and diode D4, as well as transistors Q2, Q4 and Q6 and diode D3 are configured as voltage controlled current switches in series to form the two arms of H bridge; Diodes D1 and D2 form the other two arms of the bridge. The LED string is connected between the midpoint of the bridge. The two points are called VLED + and vledgnd respectively. The alternating current is applied to the circuit through a current limiting PTC resistor R5, series capacitors C4 and C5 (forming a non-polar capacitor ceff), and inductance L1. Similarly, the neutral line of the main AC is connected to the ground of the circuit through inductance L2.
In the positive half cycle, the AC power bus is positive relative to the ground, and the transistor Q1 obtains an appropriate base bias through the resistor R1. The current flows through diode D4, transistor Q1 and resistor R3, as shown by arrow A1, then flows through the LED string composed of 12 medium power LEDs (LED1 ~ led12), and enters the earth through diode D2, as shown by arrow A2. Similarly, in the negative half cycle, the AC power bus is negative relative to the ground, and the transistor Q2 obtains the base bias through the resistance R2. The current flows through diode D3, transistor Q2 and resistor R4, as shown by arrow A3, then flows through the LED string and enters the AC power bus through diode D1, as shown by arrow A4. In this way, the current flows through the LED string in the same direction in a complete cycle, and the effect of a full wave rectifier bridge is obtained. However, the amplitude of the current iled remains constant because it is regulated by the corresponding switch as a voltage controlled current source.
Since the base emitter junctions of transistors Q3 and Q4 are connected to current detection resistors R3 and R4 respectively, when the voltage drop on R3 and R4 rises above the base emitter voltage of Q3 and Q4, the two transistors turn on. At this point, the bases of Q1 and Q2 are pulled down, disturbing the current passing through them in the half cycle of the corresponding AC power supply. The current flowing through the transistor remains constant in this way and will never exceed a certain threshold, which is set by selecting the values of R3 and R4. Q5 and Q6 limit the base current of Q1 and Q2 to a safe value (about 150 μ A) , make sure they never drive too far. When the base emission voltage of Q1 and Q2 exceeds the voltage drop on R6 and R8 connected in series with R1 and R2 respectively, a considerable part of the base current of Q1 and Q2 is shunted to R3 and R4 through Q5 and Q6.
The AC current amplitude entering the bus is limited by the ceff reactance (1 / 2 π fceff) at the main frequency and can be changed by selecting C4 and C5. The two capacitors form a non-polar capacitor. The circuit can also be driven by a resistive power supply by replacing ceff with a suitable high-power resistor of 50 Ω ~ 200 Ω. This helps to obtain excellent power factor, but the cost is that the current limiting resistor has very large power consumption. R3 and R4 can be appropriately selected according to the required constant current level. D5 provides high back voltage protection for the LED string, while R5 limits the surge current at power on. Inductors L1 and L2 and capacitor C1 help minimize EMI / RFI while improving power factor. A metal oxide varistor can be inserted side by side with the AC power supply to protect the circuit from transient.
Twelve 0.5W LEDs in the circuit work under 120madc (135m arms), and the corresponding current detection resistors R3 and R4 are selected as 1 Ω. However, the number of LEDs can be increased to 18 as long as the voltage applied on the LED string exceeds the sum of the forward voltage of a single LED (the forward voltage of white LED varies in the range of 3.3V ~ 4V). The voltage on the LED is self limiting (in this case, about 42V) and does not require any additional adjustment, because when working in positive bias mode, the series LED behaves like a high-power zener diode. The power consumption of the circuit is 11.5w under 230V AC power supply, the power factor is 0.93, and there is no flicker on the LED. A 220 can be optionally connected between VLED + and vledgnd μ The capacitance C2 of F further suppresses the ripple, as shown in Figure 2. In addition, six parallel LED strings can be used to replace the existing lamp strings, and each string has 12 ~ 18 high brightness LEDs with nominal 20mA. Transistors Q1 and Q2 must be equipped with heat sinks to avoid thermal runaway., Read the full text, technical section
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