H-bridge enables novel ways for LED lighting

The H-bridge is a classic circuit employed for driving DC motors in a user-defined manner, such as in forward/reverse direction or PWM-assisted controlled RPM with the help of four discrete/integrated switches or electromechanical relays. It is widely employed in robotics and power electronics. This Design Idea is a novel implementation of this technique for driving white-LED arrays directly from the AC mains in full-wave current-limited mode to realise an excellent flicker-free, energy-efficient solid-state lamp. The circuit controls and maintains the LED excitation current in both negative and positive half cycles of the excitation voltage to a constant level by way of electronic switches operating alternately during the positive and negative excursion of the excitation voltage. This approach facilitates current-controlled rectification of AC voltage into a DC voltage for energizing series-connected LEDs with clean DC current with negligible ripple and substantially enhances the power factor.

As 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 series-connected voltage-controlled current switches to form 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 designated as V_LED+ and V_LED GND, respectively. The AC is applied to the circuit through a current-limiting PTC resistor, R₅; series-connected capacitors, C₄ and C₅ (configured as a nonpolar capacitor, C_EFF); and inductor, L₁. Likewise, the neutral side of the mains is connected to the circuit ground through an inductor, L₂.

During the positive half cycle, the AC power bus becomes positive with respect to the ground, and transistor Q₁ gets appropriate base bias through resistor R₁. Current flows through diode D₄, transistor Q₁, and resistor R₃, as illustrated by arrow A₁, and then through the LED string comprising 12 medium-power LEDs (LED₁ to LED₁₂) to the ground through diode D₂, as shown by arrow A₂. In a similar fashion, during the negative half cycle when the ac power bus becomes negative with respect to ground and transistor Q₂ gets base bias through resistor R₂, the current flows through diode D₃, transistor Q₂, and resistor R₄, as illustrated by arrow A₃, and then through the LED string to the ac power bus through diode D₁, as shown by arrow A₄. In this way, during a complete cycle the current flows through the string in the same direction and gets added up like you would get in a full-wave bridge rectifier. However, the magnitude of current I_LED remains constant as regulated by the respective switches serving as voltage-controlled current sources.