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To drive multiple high-brightness white LEDs, engineers must decide between series or parallel configurations. Each approach has its own trade-offs. In a parallel setup, each LED requires only a low voltage, but balancing brightness across them can be challenging. Ballast resistors or current sources are often used to maintain uniform brightness, which may reduce battery life due to increased power consumption.
On the other hand, a series connection ensures consistent current flow through all LEDs, which simplifies brightness control. However, this configuration demands a higher voltage to power the entire string. A typical white LED needs around 3.6V and up to 20mA of current. For example, a circuit using seven white LEDs in series would require a voltage exceeding 26V to operate effectively.
The circuit shown in Figure 1 is a cost-effective inductive boost design that adjusts the brightness of the LED string. It consists of two main parts: a boost circuit with transistors Q1 and Q2, and a control circuit involving Q3 and JFET1.
When Q1 is off, and the battery voltage exceeds the VBE of Q2, current flows into the base of Q2, turning it on. This allows the inductor L1 to charge, storing energy in its magnetic field. As the current increases, it flows through Q2’s saturation resistor (RSAT), while the LED string remains off. When the collector voltage of Q2 rises enough, it turns on Q1, which then grounds the base of Q2, turning it off and releasing the stored energy from L1 into the LED string.
This rapid discharge creates a forward bias voltage over 26V, causing the LEDs to emit light. The human eye is not sensitive to the high-frequency flicker, so the light appears steady. Once L1 discharges, Q1 turns off, and the cycle repeats until the battery voltage drops below the threshold needed to keep Q2 conducting.
The oscillation frequency and duty cycle depend on the resistance of RSAT, the inductor, and the switching characteristics of Q1 and Q2. With four alkaline batteries, the voltage is boosted above 26V to power the seven-LED series.
Q3 regulates the channel resistance of JFET1 using a small DC current through R4, reducing battery leakage and extending battery life. The gate voltage of JFET1 is about 0.9V higher than the battery voltage, and since it's a p-JFET, it operates in depletion mode—turning on when VGS is zero.
July 29, 2025