Keep LEDs lit when AC voltage drops.
TA Babu, India
LEDs need power when rectified ac-mains voltage drops during its cycle. The circuit in Figure 1 lets you use an inductor-less, switching, offline power supply as an LED driver for emergency-exit signs and neon-light replacements. The design uses off-the-shelf components, offers efficient operation without an inductor in the dc side of the circuit, has no high-voltage capacitors, operates directly from either 120 or 230V ac, has minimal power dissipation, and has adjustable output voltage.
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| Figure 1. | The transistor and MOSFET provide current to keep the LEDs lit. |
The circuit operates by controlling the conduction angle of MOSFET Q2. When the rectified ac voltage is below the high-voltage threshold, VTH, which D1 sets, the series pass transistor turns on. The series pass transistor turns off when the output storage capacitor, C2, charges up to the regulation point.
The circuit’s output voltage decays when Q2 is off and when the rectified ac is below the output voltage (Figure 2). The load and the value of C2 determine the amount of decay. The switch conducts only when it has low voltages across it, minimizing power dissipation. The output capacitor charges on the rising edge of a sine wave, which achieves reasonable efficiencies. Fusible resistor R1 provides catastrophic-failure protection and limits input inrush when you first apply ac power. A 15V diode, D2, limits the voltage to the gate of Q2 and limits the voltage across transistor Q1.
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| Figure 2. | Switch Q2 turns on when the rectified ac input voltage drops below a threshold. |
The current interruption in the MOSFET causes ringing on the drain-to-source voltage of Q2, creating conducted EMI (electromagnetic interference). The 2.2-mH choke, L1, and capacitor C1 suppress EMI. This design maintains a fairly constant illumination over a wide voltage variation in the input. If necessary, you can add a few more such strings to suit your requirements.
Note that this circuit does not provide galvanic isolation. Touching any part of the circuit during operation can give you an electric shock.
