JFET-based dc/dc converter operates from 300-mV supply

You use a JFET's self-biasing characteristics to build a dc/dc converter that operates from power sources such as solar cells, thermopiles, and single-stage fuel cells, all of which deliver less than 600 mV and sometimes as little as 300 mV. Figure 1 shows the drain-to-source characteristics of an N-channel JFET under zero-bias conditions, which you can produce by connecting its gate and source together. Applying 100 mV causes a current of 10 mA to flow through the device, increasing to 30 mA at 350 mV. Exploiting the JFET's ability to conduct significant current at zero bias makes it possible to design a self-starting, low-input-voltage converter.

The circuit can supply 5V at currents as large as 2 mA—enough to serve many micropowered applications or to provide auxiliary bias for a higher power switched-mode voltage regulator. At 300-mV input, the circuit starts up at load currents of 300 µA. A load current of 2 mA requires an input of 475 mV.

In Figure 2, Q₁, a parallel-connected pair of Philips Semiconductor's (www.semiconductors.philips.com) BF862 JFETs, and Coiltronics' (www.coiltronics.com) Versa-Pac transformer, T₁, form an oscillator in which T₁'s secondary winding provides feedback to Q₁'s gate. When you first apply power, Q₁'s gate rests at 0V, and drain current flows through T₁'s primary winding. T₁'s phase-inverted secondary winding responds by delivering a negative voltage to Q₁'s gate, which turns off Q₁ and interrupts current flow through T₁'s primary winding. In turn, T₁'s secondary voltage collapses, and sustained oscillations begin. Although the BF862's published specifications do not cover the device's internal geometry, the device has a low on-resistance and maintains a low gate-turn-on threshold voltage. Using a pair of parallel-connected JFETs for Q₁ ensures the low saturation voltage for operation at low power-supply voltages.

Rectifying and filtering the positive-going flyback-voltage impulses on Q₁'s drain produce a dc voltage across capacitor C₁. To assist the circuit's start-up, a P-channel MOSFET, Q₂, which requires a gate-to-source voltage of approximately 2V for conduction, initially isolates the output load from the rectifier. When Q₂ conducts, the output voltage increases toward 5V. Comparator IC₁, a Linear Technology (www.linear.com) LTC-1440, draws power from Q₂'s source and imposes output-voltage regulation by comparing its internal voltage reference with a sample of the output voltage. The output from IC₁ varies Q₁'s on-time through Q₃ to close the control loop and maintain output-voltage regulation. Figure 3 shows the ripple voltage present at the power supply's output. When the output voltage decays, comparator IC₁ switches (Trace B, middle) and allows Q₁ to oscillate. The resulting flyback events at Q₁'s drain (Trace C, bottom) restore the output voltage.