AC-to-DC converter. Output from RF antenna is given to bridge rectifier to convert it into constant DC supply voltage. A diode bridge rectifier can be used for rectification purpose here. But output of an AC-to-DC power converter would be as low as 600mV, which is not sufficient to turn the diodes that have 700mV forward-voltage drop across these. To reduce forward-voltage drop, we can use MOSFET bridge rectifier circuit, which works perfectly under very-low-voltage input conditions (Fig. 4).
DC-to-DC converter. The converter used here is not a normal step-up converter. It has to step up very low input voltage autonomously. This self-powered converter is based on a self-oscillating circuit like Armstrong oscillator. Advantage of this circuit is ultra-low-input voltage and high-voltage stepping-up capability.
An oscillating circuit is mainly used as a kick-start of a main converter because of the need to skip external-regulated voltage. Most self-oscillating circuits work at mill-watt power levels. Armstrong oscillator based converter topology is shown in Fig. 5. For such a system, energy source, which is an antenna, has a large internal impedance (up to several kilo-ohms).
Working of the self-oscillating Armstrong converter topology is described below:
Oscillator. It is formed by junction field effect transistor (JFET) and amplifies gate input signal. Oscillator feedback loop is formed with two inductors, which are coupled, and gate-source capacitor Cgs.
Rectifier. To rectify JFET’s gate-oscillating voltage, gate-source PN junction of JFET is used. When JFET’s gate voltage becomes greater than diode-threshold voltage, the diode is turned on and circuit supplies output to load. When the power converter is connected to an energy source, current rises in the primary winding. Secondary winding provides a positive gate voltage on n-channel JFET.
Here, it is assumed that JFET is already on. So Cgs connected to gate terminal of JFET is charged to a positive (Vgs) value. Gate source PN junction of JFET is forward-biased because of enough gate supply voltage given by the capacitor; output capacitor (Cout) is charged with a negative voltage.
Output voltage is therefore negative. When primary current reaches saturation, primary winding voltage gets cancelled and output capacitor delivers a negative voltage to JFET gate to make it pinch off.