Subodh Johri and Prateek Johri, India
EDN
The H-bridge is a classic circuit used 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 realize 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 non-polar capacitor, CEFF); and inductor, L1. Likewise, the neutral side of the mains is connected to the circuit ground through an inductor, L2.
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| 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.
As the base emitter junctions of transistors Q3 and Q4 are connected across current-sensing resistors R3 and R4, respectively, they turn on when the voltage drop across R3 and R4 increases beyond Q3 and Q4’s base emitter voltages. At this point, Q1’s and Q2’s bases are pulled down, disrupting the flow of current through them during respective half cycles of the ac mains. In this way, the current flowing through the transistors is kept constant and never allowed to go beyond a threshold, set by appropriately choosing the R3 and R4 values. Q5 and Q6 limit the base current of Q1 and Q2 to a safe value (around 150 µA) to ensure that they are never overdriven. The substantial parts of the base currents of Q1 and Q2 are shunted to R3 and R4 by means of Q5 and Q6 when their respective base emitter voltages exceed the potential drop across R6 and R8 connected in series with R1 and R2, respectively.
The magnitude of ac current flowing into the bus is limited by the reactance of CEFF(1/2πfCEFF) at the mains frequency and can be altered by appropriately choosing C4 and C5, configured as a nonpolar capacitor. The circuit can also be driven by a resistive supply by replacing CEFF with a suitable high-power resistor of 50 to 200 Ohm. This may facilitate an excellent power factor, but at the expense of very high power losses in the current-limiting resistor. R3 and R4 can be chosen appropriately as per the required constant-current magnitude. D5 protects the LED string from high reverse voltage, and R5 limits the inrush of current at turn-on. Inductors L1 and L2 and capacitor C1 help in minimizing the EMI/RFI besides improving the power factor. A metal oxide varistor can also be inserted in parallel with the ac mains to protect the circuit from transients.
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| Figure 2. | VLED+ without capacitor C2 has ripple (a); VLED+ with capacitor C2 has reduced ripple (b). |
In the circuit, 12 0.5 W LEDs operate at 120-mA dc (135-mA RMS) with respect to current-sensing resistors R3 and R4, chosen as 1 Ohm. You can, however, increase the number of LEDs to 18 as long as the voltage being applied across the string is more than the sum of the forward voltage of the individual LEDs. (White LEDs’ forward voltage varies from 3.3 to 4 V.) The voltage appearing across the string is self-limiting (in this case, it is around 42 V) and does not require any additional regulation, since series-connected LEDs behave like high-power zener diodes when operated in forward-biased mode. The circuit draws 11.5 W power at 230 V ac RMS and exhibits a power factor of 0.93 without any perceptible flicker in the LEDs. You may optionally connect a 220-uF capacitor, C2, between VLED+ and V LED GND to further suppress ripple, as shown in Figure 2. Alternatively, the given string can be replaced by six parallel-connected strings of LEDs, each having 12 to 18 20-mA rated high-brightness LEDs. You must mount transistors Q1 and Q2 on heat sinks to avoid thermal runaway.
