Electronic Design

Novel Series-Boost Circuit Uses Flying Inductor

The power supply that's described here boosts the voltage from a 1.5-V flashlight cell to 3.5 V across an LED, while the LED and flashlight cell are in series with the power supply. What spawned this circuit was the need to make an LED retrofit kit for a flashlight. A voltage boost circuit would take the place of one cell in a two-cell flashlight, and an LED assembly would replace the incandescent lamp (Fig. 1).

The result is an LED flashlight that operates for a very long time on a single flashlight cell. The hard part was boosting the voltage when the power supply only made contact with one terminal of the flashlight cell and one contact in the lamp.

The power-supply circuit shown in Figure 2 relies on a trick to work. In the LED assembly, there's a diode connected across the LED such that when the power supply draws current, it does so through the diode. During that time, the LED is reverse-biased. After a short period of time, the power supply stops drawing current and sources current instead. In doing so, it also forward-biases the LED.

The "working part" is a blocking oscillator that uses two transistors instead of the usual one transistor. While the transistors conduct, the pnp transistor connects one tap on the inductor to the positive supply voltage, and the npn transistor connects the other tap to the negative supply voltage. Current builds up in the inductor until one transistor comes out of saturation. The resulting reversal in the base voltages ensures that both transistors turn off quickly.

After the transistors switch off and the voltage in the inductor windings reverses as the inductor keeps the current flowing, the inductor sources current through diodes D1 and D2. In effect, the inductor has flipped whereby the end that was driven positive is now connected to battery negative through D2. Additionally, the end that was negative is connected to battery positive through D1. The inductor continuously flies between these two sets of connections (current buildup through the transistors and current decay through D1 and D2) as the circuit oscillates.

A 4.7-kΩ resistor can be optionally placed in parallel with the LED assembly to slightly lower the turn-on voltage. In the prototype, the startup voltage was lowered from about 1.3 V to under 1.1 V. This is significant because the time it takes a fresh alkaline flashlight cell to discharge to 1.1 V can easily be twice the time to discharge to 1.3 V.

The 100-kΩ resistor across the 2N4401 was added to ensure that there's always enough leakage current to get the transistors to turn on, thereby starting the oscillator. The inductor was made with a continuous winding of 45 turns of #32 single-strand magnet wire on a Ferroxcube TC8.2/3.7/4-3E7 toroid core. Taps were taken at 15 and 30 turns, making three sections of 15 turns each. The oscillation frequency is 17 kHz with a 1.6-V battery voltage.

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