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Efficient LED power supply has battery backup

Zhihong Yu, Juno Lighting Group, Des Plaines, IL; Edited by Martin Rowe and Fran Granville -April 08, 2010

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LEDs find wide use in emergency lighting because of their high efficiency and control simplicity. The circuit in Figure 1 provides a highly efficient and reliable design for emergency LED lighting at 3 to 6W. The circuit's input is 12V ac, which the full-wave bridge rectifies and one or two capacitors filter into dc. The battery (not shown) is a 12V lead-acid type. IC1 compares the battery voltage to the supply voltage. When the rectified voltage drops below the battery voltage, the battery takes over to provide LED power.


Figure 1
This circuit converts ac voltage to dc voltage, charges a battery, and drives LEDs from the ac source or the battery.

The circuit has some small switching losses, which should be acceptable as long as IC2, a 12V PB137 battery-charging circuit from STMicroelectronics, keeps the battery from draining. If this switch-over is unacceptable, add a 470-µF electrolytic capacitor to filter the input voltage to maintain a certain level above the battery voltage. Note that adding this capacitor lowers the power factor.

To get 12V ac, you can use an electronic transformer. These transformers provide 12V at a higher frequency, so a 10-µF capacitor can hold the voltage high as well as provide a high power factor.

IC1, a Linear Technology LTC4412, controls two external PFETs that create a near-ideal diode function for switching between ac and battery output. The PFETs' voltage drop is only about 20 mV compared with a normal 0.7V diode-voltage drop. Pin 5 is low when ac power is off, so you can use this pin to turn on a warning LED through another PFET. IC2 has an internal current limit of 1.5A. Resistor R1 limits IC2's input; when the current reaches a certain level, Q4 turns off the charging circuit. This IC does not require reverse-diode protection.

IC3, an LT3517 LED driver from Linear Technology, acts as an inverting buck-boost converter because the input can range from 8 to 17V for rectified ac. R10 sets up the LEDs' current. Because the voltage drop from each of the three LEDs varies from 3 to 4V, the IC's output voltage can be higher or lower than its input voltage if all 300-mA LEDs connect in series.

By connecting a resistor divider, including a photocell, to the analog-dimming pin, Pin 8, you can achieve some dimming, which results in some power savings at higher ambient light. You can use IC1's Pin 5 to turn on a transistor or an optoisolator to pull IC3's control-pin voltage lower if you need to dim the LED when ac power is out. Resistor R7 programs IC1 to operate at 1 MHz. The circuit's efficiency is 82% when you power it directly from the ac power supply and about 70% from an electronic transformer.

With a few minor changes to the circuit, you can add LEDs. For example, you can use Linear Technology's LT3518, which is a pin-to-pin-compatible version of the LT3517 but with a higher switching-current limit. You may need to adjust the feedback-resistor pair R8 and R9 for higher output voltage. You may also need more input-filtering capacitance to hold up the voltage.

Tests show that the circuit can power as many as six LED in series. Figure 2 shows the circuit in operation.


Figure 2
LEDs provide enough light for emergency lighting.

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