Electronic Design

Simple Isolated Supply Design Suits Fixed-DC-Voltage Systems

Most high-voltage systems that include low-voltage ICs must inject isolated supplies into their setups. The design and implementation of these supplies is usually quite involved. However, a simple approach is possible for systems in which the potential difference is a fixed dc voltage (see the figure).

The circuit for such an approach is based on a digital oscillator, U1. The oscillator contains a MOSFET driver whose complementary outputs drive a pair of capacitors (C1 and C2). These capacitors connect to a full-wave rectifier formed by diodes D1­D4, and the rectifier outputs connect to a filter capacitor (C3).

As shown in the figure, additional diode-capacitor networks can be added to create a multitude of isolated voltages. The additional voltages are limited mainly by the maximum total load current of all outputs and the maximum drive available from U1 (over 1 A for VCC = +18 V).

The circuit's output voltages are two diode drops below the input supply voltage. If the rectifiers are built with silicon small-signal diodes, such as 1N914 diodes, the circuit provides up to 4 mA at 3.3 V from a +5-V input. Higher currents are available with larger diodes, and substituting Schottky for silicon diodes allows for higher outputs by lowering the burden voltage. As previously mentioned, this circuit is suitable for systems with fixed dc potentials, but it's less suitable for those in which the potential between low-voltage and high-voltage sections differ.

ifd winners
Moshe Gerstenhaber and Chau Tran, Analog Devices Inc., "High-Voltage Monitor Features High Accuracy," August 19, 2002.

Monika Sardana and Saurav Gupta, NIST, "Use Multiplier Core For FM Stereo Multiplexed Transmission System," September 2, 2002.

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