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 300 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.

Figure 1.	At 100 mV between drain and source (horizontal axis), the drain current reaches 10 mA (vertical axis) and increases to 30 mA at 300 mV.

The circuit can supply 5 V 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.

Figure 2.	A pair of parallel-connected JFETs allows this dc/dc converter to operate from power sources that supply as little as 300 mV.

In Figure 2, Q₁, a parallel-connected pair of NXP's BF862 JFETs, and Coiltronics' 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 0 V, 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.

Figure 3.	The dc output (Trace A), comparator IC1’s output (Trace B), and the voltage at Q1’s drain (Trace C) have a horizontal-deflection factor of 5 msec.

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 2 V for conduction, initially isolates the output load from the rectifier. When Q₂ conducts, the output voltage increases toward 5 V. Comparator IC₁, a Linear Technology LTC1440, 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.

Figure 4.	Adding Q3, Q4, and the bootstrapped negative-bias generator comprising D2, D3, and C4 reduces the circuit’s quiescent current from 25 mA to 1 mA.

Using Q₃ as a simple but effective shunt control for Q₁'s gate voltage results in a 25-mA quiescent-current drain from the power source. A modification reduces the quiescent drain to 1 mA (Figure 4). Inserting switch Q₄ in series with T₁'s secondary winding more efficiently controls Q₁'s gate. Bootstrapping the voltage across T₁'s secondary winding produces negative-turn-off-bias voltage for Q₄. Figure 5 illustrates how to connect T₁'s windings. When Q₄ switches off, it interrupts the current flowing in T₁'s secondary winding and drives T₁'s Pin 5 positive. Without diodes D₄ and D₅, the peak voltage would approach 15 V and reverse-bias Q₄, an undesirable condition. Under normal operating conditions, excursions of approximately 0.8 V appear at Pin 5, necessitating the use of two series-connected diodes to clamp the voltage at a safe level. Zener diode D₂ holds off bias-supply loading to aid start-up during initial power application.

Figure 5.	Comprising six independent windings that offer more than 500 configurations, Coiltronics’ VP1-1400 serves as a combination feedback and flyback transformer in this application. Connect the windings as shown.

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