H-bridge paves new way for LED lighting

The H-bridge is a classic circuit for driving a DC motor in a user-defined manner, such as forward/reverse direction or RPM controlled by PWM assistance of four discrete/integrated switches or electromechanical relays. It is widely used in machinery and power electronics. This Design Idea is a new implementation of this technology that can drive a white LED array directly from an AC power source in full-wave current-limited mode, thereby realizing a flicker-free, energy-efficient solid-state lighting. The circuit uses alternating electrical switching during the positive and negative excursions of the excitation voltage to control and maintain the LED excitation current at a constant level during the negative and positive half-cycles of the excitation voltage. This method can perform current-controlled rectification of the AC voltage to make it a DC supply voltage for the series LEDs with clean, almost ripple-free current, greatly improving 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, forming two arms of the H bridge; diodes D1 and D2 form the other two arms of the bridge. The LED string is connected between the midpoints of the bridge, and the two points are called VLED+ and VLEDGND. AC power is applied to the circuit through a current-limiting PTC resistor R5, series capacitors C4 and C5 (forming a non-polarized capacitor CEFF), and inductor L1. Similarly, the neutral line of the main AC power is connected to the ground of the circuit through inductor L2.

During the positive half cycle, the AC power bus is positive relative to the ground, and transistor Q1 obtains appropriate base bias through resistor R1. The current flows through diode D4, transistor Q1, and resistor R3, as shown by arrow A1, and then flows through the LED string consisting of 12 medium-power LEDs (LED1~LED12), and enters the ground through diode D2, as shown by arrow A2. Similarly, during the negative half cycle, the AC power bus is negative relative to the ground, and transistor Q2 obtains base bias through resistor R2. The current flows through diode D3, transistor Q2, and resistor R4, as shown by arrow A3, and then flows through the LED string and enters the AC power bus through diode D1, as shown by arrow A4. In this way, during a complete cycle, the current flows through the LED string in the same direction, achieving the effect of a full-wave rectifier bridge. 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 across current-sense resistors R3 and R4, respectively, both transistors conduct when the voltage drop across R3 and R4 rises above the base-emitter voltage of Q3 and Q4. At this point, the bases of Q1 and Q2 are both pulled low, disrupting the current through them during the respective half-cycles of the AC source. In this way, the current through the transistors is kept constant and never exceeds a certain threshold, which is set by the choice of the values ​​of R3 and R4. Q5 and Q6 limit the base current of Q1 and Q2 to a safe value (approximately 150 μA), ensuring that they are never overdriven. When the base-emitter voltage of Q1 and Q2 exceeds the voltage drop across R6 and R8, which are in series with R1 and R2, respectively, a significant portion 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 reactance of CEFF (1/2πfCEFF) at the main frequency, which can be changed by selecting C4 and C5, which form a non-polarized capacitor. The circuit can also be driven by a resistive source by replacing CEFF with a suitable high-power resistor of 50Ω~200Ω. This helps to achieve excellent power factor, but the price paid is that the current limiting resistor has very large power dissipation. R3 and R4 can be appropriately selected according to the required constant current level. D5 provides high reverse voltage protection for the LED string, while R5 limits the inrush current at power-up. Inductors L1 and L2 and capacitor C1 help minimize EMI/RFI while improving power factor. A metal oxide varistor can be inserted in parallel with the AC source to protect the circuit from transients.

The circuit has 12 0.5W LEDs operating at 120mAdc (135mARMS), and the corresponding current sensing resistors R3 and R4 are selected as 1Ω. However, the number of LEDs can be increased to 18, as long as the voltage applied to the LED string exceeds the sum of the forward voltages of the individual LEDs (the forward voltage of white LEDs varies from 3.3V to 4V). The voltage across the LEDs is self-limited (in this case, it is about 42V) and does not require any additional regulation because the series LEDs behave like high-power Zener diodes when operating in forward bias mode. The circuit consumes 11.5W of power from a 230V AC supply, with a power factor of 0.93 and no flickering on the LEDs. A 220μF capacitor C2 can be optionally connected between VLED+ and VLEDGND to further suppress the ripple, see Figure 2. Alternatively, the existing light string can be replaced with six parallel LED strings, each with 12 to 18 high-brightness LEDs rated at 20mA. Transistors Q1 and Q2 must be equipped with heat sinks to avoid thermal runaway.