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H-bridge enables novel ways for LED lighting

Posted: 31 Jul 2013     Print Version  Bookmark and Share

Keywords:H-bridge  PWM  DC motors  DC current  transistors 

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 VLED+ and VLED 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, CEFF); and inductor, L1. Likewise, the neutral side of the mains is connected to the circuit ground through an inductor, L2.

Figure 1: Current-limiting transistors and diodes route alternate AC half cycles to the series LED string.

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 ILED remains constant as regulated by the respective switches serving as voltage-controlled current sources.

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