Friday, May 4, 2012

Voltage Booster – High Voltage from a 5V Supply

Digital circuits operating from 5V regulated supplies are common but occasionally a higher voltage is required, perhaps for a bio-medical circuit, or for liquid level measurement or for monitoring high resistance contacts. For such circuits a means of generating a high voltage from the 5V supply can be a solution. Diode/capacitor multipliers can offer advantages over switched-mode circuits, since they do not use inductors, are easier to design and troubleshoot and often generate less radiated interference.

The principle of the voltage multiplier is fairly well known. A capacitor is used with a square wave drive signal to “pump” current through a pair of diodes, roughly doubling the supply voltage. A series of such stages can be cascaded to raise the voltage in multiples of the supply, but it is possible to improve efficiency and reduce the number of stages by using two driving signals with opposite phases. However, each diode incurs a drop of about 0.6V so with two diodes per stage, and with an initial supply of just 5V this becomes significant, leading to poor efficiency and an impractical number of stages.
Voltage Booster
These problems are overcome in the Voltage Booster circuit design of Fig.1 by increasing the voltage before multiplication with IC1, an SI7660 “negative rail generator” (not the ICL7660 – ARW). The additional negative supply is generated very efficiently since switching is performed not by diodes but by CMOS switches in the IC, which cause almost no voltage loss at low currents.

Pin 2 of this IC also drives a diode multiplier which, even though it does suffer from diode voltage drops, still generates about +8.8V. With a minimum of components this device generates a pair of supply rails with an overall potential of more than 13.5V.

This voltage is supplied to IC2, a 4047B used as an oscillator whose frequency is set to about 400Hz by resistor R1 and capacitor C6. The oscillator employs an internal divide-by-two stage so that the outputs at pin 10 and pin 11 are complementary 200Hz square waves with perfect 1:1 duty cycles, ideal for multiplier driving.

With just the five diodes D3 to D7 shown, the circuit generates 60 volts with no output load. The output may be treated as having a fairly linear source resistance of about 200k since a drop of about 10V results for each 50uA of current drawn. In other words it will sustain 50V across a 1M resistor, or about 40V across one of 390k.

At no load the quiescent current was measured at just 650uA rising to about 1.9mA at 100uA load. The capacitors used were all resin-dipped ceramic types, though other non-electrolytic types with suitable voltage ratings could be used. The diodes are inexpensive 1N4148 signal types.

Increased output power could be obtained by using a higher frequency for IC2 and larger capacitors for C2 and C3, though this would increase the standby current. More (or less) stages in the multiplier chain could be used to obtain different output voltages if needed.

Finally, the value of capacitor C11 could be increased to allow the build-up of a larger charge, but this would obviously cause a time lag at switch-on or discharge. The circuit reaches full output voltage in well under a second.

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